4tH, the friendly Forth compiler
J.L. Bezemer
Table of Contents
Chapter 1 What's new
1.1 What's new in version 3.5d, release 3
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Library reorganization
New reserved words
1.2 What's new in version 3.5d, release 2
Words
Functionality
Bugfixes
Developer
Documentation
Hints
1.3 What's new in version 3.5d
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Changed words
Dropped words
New reserved words
1.4 What's new in version 3.5c, release 3
Words
Functionality
Bugfixes
Developer
Documentation
Hints
1.5 What's new in version 3.5c, release 2
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Library reorganization
Interpreter
Table search
Reserved words
1.6 What's new in version 3.5c
Words
Functionality
Bugfixes
Developer
Documentation
Hints
New reserved words
1.7 What's new in version 3.5b, release 2
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Renamed words
New reserved words
1.8 What's new in version 3.5b
Words
Functionality
Bugfixes
Developer
Documentation
Hints
New reserved words
1.9 What's new in version 3.5a, release 2
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Renamed words
New reserved words
1.10 What's new in version 3.5a
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Strings
PAD
Arrays of string constants
Deferred words
Library files
Dropped words
New reserved words
1.11 What's new in version 3.3d, release 2
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Dropped words
New reserved words
1.12 What's new in version 3.3d
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Using files
Using 'INPUT' and 'OUTPUT'
New reserved words
1.13 What's new in version 3.3c
Words
Functionality
Bugfixes
Developer
Documentation
Hints
Programs using '+UNDER'
New reserved words
1.14 What's new in version 3.3a
Words
Functionality
Bugfixes
Developer
Documentation
Hints
New reserved words
Changed words
1.15 What's new in version 3.2e
Words
Functionality
Bugfixes
Developer
Documentation
Hints
New reserved words
Dropped words
Unsigned characters
1.16 What's new in version 3.1d
Words
Functionality
Bugfixes
Developer
Documentation
Hints
AT
New reserved words
Using 'VALUE' with 'ALLOT'
Using 'VARIABLE' with 'ALLOT'
ANS-Forth compatibility
Part I Getting Started
Chapter 2 Overview
2.1 Introduction
2.2 History
2.3 Applications
2.4 Architecture
2.4.1 The 4tH language
2.4.2 H-code
2.4.3 H-code compiler
2.4.4 Error handling
2.4.5 Interfacing with C
Chapter 3 Installation Guide
3.1 About this package
3.1.1 Example code
3.1.2 Main program
3.1.3 Unix package
3.1.4 Linux package
3.1.4.1 /etc/magic
3.1.4.2 Using binfmt_misc
3.1.4.3 DIR4TH environment variable
3.1.5 MS-DOS package
3.1.5.1 DIR4TH environment variable
3.1.6 MS-Windows package
3.1.6.1 DIR4TH environment variable
MS-Windows 9x
MS-Windows NT
3.2 Now what?
3.3 Pedigree
3.4 Questions
3.4.1 4tH Website
3.4.2 4tH Google group
3.4.2.1 Conditions of use
3.4.2.2 What to discuss?
3.4.3 Newsgroup
Chapter 4 A guided tour
4.1 4tH interactive
4.2 Starting up 4tH
4.3 Running a program
4.4 Starting an editing session
4.5 Writing your first 4tH program
4.6 A more complex program
4.7 Advanced features
4.8 Suspending a program
4.9 Calculator mode
4.10 Epilogue
Chapter 5 Frequently asked questions
Part II Primer
Chapter 6 Introduction
Chapter 7 4tH fundamentals
7.1 Making calculations without parentheses
7.2 Manipulating the stack
7.3 Deep stack manipulators
7.4 Passing arguments to functions
7.5 Making your own words
7.6 Adding comment
7.7 Text-format of 4tH source
7.8 Displaying string literals
7.9 Creating variables
7.10 Using variables
7.11 Built-in variables
7.12 What is a cell?
7.13 What is a literal expression?
7.14 Declaring arrays of numbers
7.15 Using arrays of numbers
7.16 Declaring and using constants
7.17 Built-in constants
7.18 Using booleans
7.19 IF-ELSE constructs
7.20 FOR-NEXT constructs
7.21 WHILE-DO constructs
7.22 REPEAT-UNTIL constructs
7.23 Infinite loops
7.24 Including source files
7.25 Getting a number from the keyboard
Chapter 8 4tH arrays
8.1 Aligning numbers
8.2 Creating arrays of constants
8.3 Using arrays of constants
8.4 Using values
declaration:
fetching:
storing:
8.5 Creating string variables
8.6 What is an address?
8.7 String literals
8.8 String constants
8.9 Initializing string variables
8.10 Getting the length of a string variable
8.11 Printing a string variable
8.12 Copying a string variable
8.13 The string terminator
8.14 Slicing strings
8.15 Appending strings
8.16 Comparing strings
8.17 Finding a substring
8.18 Replacing substrings
8.19 Deleting substrings
8.20 Removing trailing spaces
8.21 Removing leading spaces
8.22 Upper and lower case
8.23 String literals and string variables
8.24 Printing individual characters
8.25 Distinguishing characters
8.26 Getting ASCII values
8.27 Printing spaces
8.28 Fetching individual characters
8.29 Storing individual characters
8.30 Getting a string from the keyboard
Chapter 9 Character Segment
9.1 The Character Segment
9.2 What is the TIB?
9.3 What is the PAD?
9.4 How do I use TIB and PAD?
9.5 Simple parsing
9.6 Converting a string to a number
9.7 Controlling the radix
9.8 Pictured numeric output
9.9 Converting a number to a string
9.10 Aborting a program
9.11 Opening a file
9.12 Reading and writing from/to a file
9.13 Closing a file
9.14 Writing text-files
9.15 Reading text-files
9.16 Reading long lines
9.17 Reading binary files
9.18 Writing binary files
9.19 Reading and writing block files
9.20 Parsing textfiles
9.21 Parsing binary files
9.22 Parsing comma-delimited files
9.23 Advanced parsing
9.24 Appending to existing files
9.25 Using pipes
9.26 Opening a file in read/write mode
9.27 Using random access files
9.28 The layout of the I/O system
9.29 Speech synthesis
9.30 Using a printer
9.31 The layout of the Character Segment
Chapter 10 Integer Segment and Code Segment
10.1 The Code Segment
10.2 The address of a colon-definition
10.3 Vectored execution
10.4 The Integer Segment
10.5 A portable way to access application variables
10.6 Returning a result to the host program
10.7 Using commandline arguments
10.8 The layout of the Variable Area
10.9 The stacks
10.10 Saving temporary values
10.11 The Return Stack and the DO..LOOP
10.12 Other Return Stack manipulations
10.13 Altering the flow with the Return Stack
10.14 Leaving a colon-definition
10.15 The layout of the Stack Area
10.16 Booleans and numbers
10.17 Using ' with other names
10.18 Assertions
10.19 Breakpoints
10.20 Random numbers
10.21 Timers
10.22 Time & date
10.23 What is not implemented
10.24 Known bugs and limitations
Chapter 11 Advanced programming
11.1 Compiletime calculations
11.2 Conditional compilation
11.3 Checking the environment at compiletime
11.4 Checking a definition at compiletime
11.5 Exceptions
11.6 Mixing character and number data
11.7 Enumerations
11.8 Dynamic memory allocation
11.9 Tweaking dynamic memory
11.10 Application stacks
11.11 Forward declarations
11.12 Recursion
11.13 Lookup tables with integer keys
11.14 Lookup tables with string keys
11.15 Lookup tables with multiple keys
11.16 Lookup tables with duplicate keys
11.17 Interpreters
11.18 Adding your own library
11.19 Adding templates
11.20 Private declarations
11.21 Aliases
11.22 Changing behaviour of data
11.23 Multidimensional arrays
11.24 Binary string constants
11.25 Records and structures
11.26 Fixed point calculation
11.27 Double numbers
11.28 Floating point numbers (basic)
11.29 Floating point numbers (full)
11.30 Forth Scientific Library
11.31 Complex control structures
11.32 Sorting
11.33 Tokenizing strings
11.34 Regular expressions
11.35 Escape characters
11.36 Running 4tH programs from the Unix shell
11.37 Embedding 4tH programs in a batch file
11.38 This is the end
Part III Reference guide
Chapter 12 Glossary
Chapter 13 Editor manual
13.1 Introduction
13.2 Selecting a screen and input of text
13.3 Line editing
13.4 Line editing commands
13.5 Screen editing commands
13.6 Cursor control and string editing
13.7 Commands to position the cursor
13.8 String editing commands
13.9 Saving and exiting
13.10 Calculator mode
Chapter 14 Shell manual
14.1 Introduction
14.2 Loading and saving
14.3 Task management
14.4 Scripting
14.5 Stack, I/O and arithmetic
Chapter 15 Preprocessor manual
15.1 Introduction
15.2 Macros
15.3 Invocation
15.4 Preprocessor commands
15.5 Error messages
Chapter 16 ANS Forth statement
16.1 ANS-Forth Label
16.2 Unsupported CORE words
16.3 Supported ANS Forth word sets
16.3.1 Core Extensions word set
16.3.2 Block word set
16.3.3 Block Extensions word set
16.3.4 Double number word set
16.3.5 Double number Extensions word set
16.3.6 Facility Extensions word set
16.3.7 File-Access word set
16.3.8 File-Access Extensions word set
16.3.9 Floating-Point word set
16.3.10 Floating-Point Extensions word set
16.3.11 Programming-Tools word set
16.3.12 Programming-Tools Extensions word set
16.3.13 String word set
Chapter 17 Errors guide
17.1 How to use this manual
17.2 Interpreter (exec_4th)
17.3 Compiler (comp_4th)
17.4 Loader (load_4th)
17.5 Saver (save_4th)
Chapter 18 Library dependencies
Chapter 19 Porting guide
19.1 Introduction
19.2 General guidelines
19.3 Differences between 4tH and ANS-Forth
19.3.1 Strings
19.3.2 Double numbers
19.3.3 Booleans
19.3.4 CREATE..DOES>
19.3.5 HERE
19.3.6 Interpretation and compilation mode
19.3.7 BEGIN..WHILE..REPEAT
19.3.8 CASE..OF..ENDOF..ENDCASE
19.3.9 DO..LOOP
19.3.10 I/O
19.4 Easy 4tH
19.4.1 Disabling DOES>
19.4.2 Enabling the String Space
19.4.3 The structure of Easy 4tH
19.5 Converting ANS-Forth programs to 4tH
Part IV Development guide
Chapter 20 Compiling the source
20.1 Introduction
20.2 Recommended and preferred compilers
20.3 Compiling 4th
20.4 Compiling the library
20.5 Using the library
20.6 Shared library
20.7 64-bit platforms
20.8 Generating the editor
20.9 Optimizations
Chapter 21 Using the 4tH API
21.1 Introduction
21.2 A sample program
21.3 A first look at open_4th()
21.4 A closer look at H-code
21.5 A closer look at HX-code
21.6 A first look at comp_4th()
21.7 A first look at exec_4th()
21.8 A first look at free_4th()
21.9 A first look at save_4th()
21.10 A first look at load_4th()
21.11 A first look at error-trapping
21.12 A first look at dump_4th()
21.13 A first look at cgen_4th()
21.14 Converting HX-files
21.15 A first look at fetch_4th()
21.16 A first look at store_4th()
21.17 Examples of embedded HX code
21.18 Suspended execution
21.19 Useful variables
Chapter 22 Modifying 4tH
22.1 Introduction
22.2 A closer look at comp_4th()
22.3 Adding a constant
22.4 Adding a word
22.5 A closer look at exec_4th()
22.6 A first look at name_4th()
22.7 Extending the compiler
22.8 Making aliases
22.9 Giving a name to an application variable
22.10 Adding new variables
22.11 Resizing the 4tH environment
22.12 Tuning pipe failure detection
22.13 Adding new error messages
22.14 Sizing the Code Segment
22.15 Adding inline macros
22.16 Adding string words
22.17 Adding words with arguments
22.18 Adding conditionals
22.19 Extending the I/O subsystem
22.20 Using the symbol table
22.21 Using variables and datatypes
22.22 Other tools
22.23 Patching 4tH
22.23.1 Tokens
22.23.2 Words
22.23.3 The virtual machine
22.23.4 Immediate words
22.23.5 Applying the patches
22.23.6 Error messages
List of Figures
1. Integer segment layout
2. Character segment layout
3. Hcode structure
4. Editor architecture
5. Character segment
6. The 4tH I/O system
7. Integer segment
8. Double, mixed and floating point word dependencies
9. Hcode structure
List of Tables
1. Forth-79 to ANS conversion
2. Character typing words
3. NELL equivalents
4. Fraction words
5. Examples of single and double number counterparts
6. Range and digits of precision
7. Examples of single and floating point number counterparts
8. IEEE 754 FP math errors
9. Supported control characters
10. DC commands
11. 4tsh commands
12. Dumb words
13. List of compilers
14. API functions
15. HX type-byte encoding
16. comp_4th() variables
17. exec_4th() basic API
18. comp_4th() basic API
19. Examples of aliases
20. Mapping between 4tH and C variables
21. Mapping between 4tH and C variable names
22. Accessing 4tH data from C
23. exec_4th() data access API
24. Example execution plan
25. Branch resolving API
26. Members of Stream[] structure
27. Device status macros
28. Symboltable API
29. Table search API
What's new
1.1 What's new in version 3.5d, release 3
Words
• The words '[/]' and '[SIGN]' have been added.
Functionality
• The preprocessor was expanded and now takes the DIR4TH
environment variable into account.
• The library files now support ANS Forth compatible versions of
all floating point input and output words.
Bugfixes
• None.
Developer
• The library files now support ANS Forth compatible versions of
all floating point input and output words.
• The library file getenv.4th was rewritten.
• The library file row.4th was changed.
Documentation
• All documentation now reflects the functionality of the current
version. A chapter on library dependencies was added.
Hints
Porting your V3.5d release 2 programs to V3.5d release 3
shouldn't be any problem. Most of them will only need
recompilation. There are two things to consider:
Library reorganization
If you have used the ANS floating point wordset in your programs
you will have to add an extra include file. Just add ansfpio.4th
to the list of include files, right after ansfloat.4th, e.g.:
include lib/ansfloat.4th
include lib/fsinfcos.4th
Becomes:
include lib/ansfloat.4th
include lib/ansfpio.4th
include lib/fsinfcos.4th
The reason for this change is that a set of fully ANS compatible
floating point I/O words has been added. If you still experience
problems, you might want to consult chapter [cha:Library-dependencies]
to resolve more complex dependency issues.
The row.4th library member was awkward to use and syntactically
not very clean. This has been fixed. The following snippet:
:this keyword does>
['] skey= is key= 2 row
if cell+ @c execute else drop type then
;
Can now be expressed like this:
:this keyword does>
2 string-key row
if cell+ @c execute else drop type then
;
Numeric keys can be searched by using the num-key word. Both
num-key and string-key are execution tokens, so you can still
replace them with your own versions. The words key=, nkey= and
skey= are now private and cannot be called directly anymore.
The getenv.4th library member has been enhanced. Before it was up
to you to decide which OS your program was intended to support,
e.g.:
256 env-buffer
/env-buffer string env-buffer
s" $PATH" env-buffer /env-buffer getenv
The getenv.4th library member now determines automatically at
runtime which OS is used. You only have to remove any prefixes or
postfixes, e.g.:
256 env-buffer
/env-buffer string env-buffer
s" PATH" env-buffer /env-buffer getenv
No other changes are required.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are '[/]' and '[SIGN]'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.2 What's new in version 3.5d, release 2
Words
• None.
Functionality
• A preprocessor was added to the toolchain.
• The library version is shown during 4th startup.
Bugfixes
• A few bugs in the ANS floating point library were fixed.
Developer
• Another floating point library called ZEN float was added.
• REPRESENT is now supported.
• A preprocessor was added to the toolchain.
• The patch4th.4th script allows more complex modifications.
• 4tH can be compiled as a shared library under Linux.
• 4tsh was almost completely rewritten.
Documentation
• All documentation now reflects the functionality of the current
version. A chapter on the preprocessor was added.
Hints
Porting your V3.5d programs to release 2 isn't a problem. All
executables will run without recompilation. All sources will
compile properly without modification.
This release is dedicated to my father.
1.3 What's new in version 3.5d
Words
• The words 'BUFFER:' and 'ERROR?' have been added.
• The word 'OPEN' has been changed.
• The word 'AS' has been dropped.
Functionality
• The editor can now save text files.
Bugfixes
• A few bugs in the floating point library were fixed.
• The circular buffer library was rewritten.
Developer
• The library files now support a subset of the Forth Scientific
Library.
• Some recent Forth200x submissions were added to the library.
• The license was changed from LGPL v2+ to LGPL v3.
Documentation
• All documentation now reflects the functionality of the current
version.
• The Development Guide was expanded.
Hints
Porting your V3.5c release 3 programs to V3.5d shouldn't be any
problem. Most of them will only need recompilation. There are
three things to consider:
Changed words
Previously, 'OPEN' returned zero on error. This was not according
to the 4tH standard, which uses '(ERROR)' to signal a problem.
Although there were sound reasons at the time to deviate from the
standard (see section [sec:What's-new-in]), this deficiency has
been corrected in the current version.
In order to facilitate the transition, the word 'ERROR?' has been
added. It converts the 4tH convention to the ANS-Forth convention
by adding an additional error flag. Consequently, 'ERROR?' can
also be used for other 4tH words that already return '(ERROR)',
but its use is not required and existing programs will continue
to work correctly.
This is a typcal file opening construction in version 3.5c:
s" Hello.txt" input open
dup 0= abort" Cannot open file"
You can easily convert it to version 3.5d with 'ERROR?':
s" Hello.txt" input open
error? abort" Cannot open file"
Note that the stackdiagrams of both constructions are exactly the
same.
Dropped words
The word 'AS' has been dropped since it seems very unlikely that
this syntactic sugar will ever be used again. Just replace 'AS'
with 'TO'.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are 'ERROR?' and 'BUFFER:'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.4 What's new in version 3.5c, release 3
Words
• None.
Functionality
• None.
Bugfixes
• A few bugs in the mixed numbers library were fixed.
Developer
• The library files now support most of the FLOAT and FLOAT EXT
wordsets.
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.5c release 2 programs to release 3 isn't a
problem. All executables will run without recompilation. All
sources will compile properly without modification.
1.5 What's new in version 3.5c, release 2
Words
• None.
Functionality
• A default 4tH directory can be defined by setting an
environment variable.
• Support for creating custom 4tH implementations.
• 4tsh is scriptable now.
Bugfixes
• None.
Developer
• The library files concerning ANS Core Extensions, table
searching and interpretation have been rewritten or replaced.
• A superfluous #define was removed from 4th.h.
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.5c programs to release 2 shouldn't be any
problem. All executables will run without recompilation. However,
you might have to change a few source files in order to make them
compile properly.
Library reorganization
Some words have been moved to another library file, so you might
have to change your includes according to the following table:
+----------------+--------------+------------------+
| Word | v3.5c | v3.5c, release 2 |
+----------------+--------------+------------------+
+----------------+--------------+------------------+
| WITHIN | anscext.4th | ranges.4th |
+----------------+--------------+------------------+
| BETWEEN | comus.4th | ranges.4th |
+----------------+--------------+------------------+
| SAVE-INPUT | anscext.4th | evaluate.4th |
+----------------+--------------+------------------+
| RESTORE-INPUT | anscext.4th | evaluate.4th |
+----------------+--------------+------------------+
Interpreter
The inclusion of interprt.4th has to be done at the very
beginning of the program, like all other include files. ”NotFound”
now always uses the same stack diagram: it leaves the
address/count string on the stack that could not be interpreted. ”
NotFound” is now a deferred word with default behaviour, so
defining it is optional. Either remove the definition or change
it from, e.g.:
: NotFound type ." is not defined" cr ;
To:
:noname type ." is not defined" cr ; is NotFound
The ”dictionary” table used to be mandatory. Change it from e.g.:
create dictionary
To:
create wordlist
After you've completely defined the table add this line:
wordlist to dictionary
Your program should compile and run correctly now.
Table search
Both find.4th and lookup.4th have been superseded by row.4th.
Since ”ROW” works slightly different, you might have to do some
rewriting. Please consult the primer if you're unsure how. If
you're not willing to do that, there are two options:
1. Use the find.4th and lookup.4th from a previous version of
4th;
2. Use the following definitions:
: find
['] skey= is key= >r row
if nip nip r> cells + @c true
else r> drop drop false
then
;
: lookup
['] nkey= is key= >r row
if nip r> cells + @c true
else r> drop drop false
then
;
Reserved words
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.6 What's new in version 3.5c
Words
• The words 'C,' and 'OFFSET' have been added.
Functionality
• Binary string constants can be defined.
Bugfixes
• None.
Developer
• MakeSymbol() has been added to comp_4th().
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.5b, release 2 programs to V3.5c shouldn't be any
problem. Most of them will only need recompilation. There is one
thing to consider:
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are 'C,' and 'OFFSET'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.7 What's new in version 3.5b, release 2
Words
• Renamed 'FIELD' to '+FIELD'. The words '[NEGATE]', 'CHOP' and
'/STRING' have been added.
Functionality
• None.
Bugfixes
• The word '->' allocated slightly more memory than needed. This
has been fixed.
Developer
• The function hgen_4th() has been removed from the API.
• The library files have been updated and expanded.
Documentation
• All documentation now reflects the functionality of the current
version.
• A section on the 4tH shell (4tsh) has been added.
Hints
Porting your V3.5b programs to release 2 shouldn't be any
problem. All executables will run without recompilation. However,
you might have to change a few source files in order to make them
compile properly. There are two things to consider:
Renamed words
If you used 'FIELD' in your programs, you'll have to replace it
by '+FIELD'. No other changes are necessary.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are '[NEGATE]', 'CHOP' and '/STRING'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.8 What's new in version 3.5b
Words
• The words '.|' and 'SYNC' have been added.
Functionality
• Output buffers can be flushed.
Bugfixes
• None.
Developer
• The CODE() and NEXT macros have been added to allow easy
modification of exec_4th().
• The library files now support most of the CORE and DOUBLE
wordsets.
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.5a release 2 programs to V3.5b shouldn't be any
problem. Most of them will only need recompilation. There is one
thing to consider:
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are '.|' and 'SYNC'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.9 What's new in version 3.5a, release 2
Words
• Renamed 'SLEEP' to 'PAUSE'. The word 'FILES' has been added.
Functionality
• None.
Bugfixes
• A bad mode string disabled pipes in the Unix version. This has
been fixed.
Developer
• None.
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.5a programs to release 2 shouldn't be any
problem. All executables will run without recompilation. However,
you might have to change a few source files in order to make them
compile properly. There are two things to consider:
Renamed words
If you used 'SLEEP' in your programs, you'll have to replace it
by 'PAUSE'. No other changes are necessary.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved word is 'FILES'
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.10 What's new in version 3.5a
Words
• The words 'WORD', '”', 'TOKEN', 'COPY', 'TEXT' and 'WAIT' have
been discarded.
• The words 'NUMBER', 'ARGS', 'IS', 'REPEAT', 'AGAIN' and 'UNTIL'
have been changed.
• Renamed '@'' to '@C', 'SKIP' to 'OMIT' and 'RESULT' to 'OUT'.
• The words '@GOTO', '+CONSTANT', 'SOURCE-ID', 'CIN, 'COUT',
'PARSE-WORD', 'IMMEDIATE', 'NOT', 'INCLUDE', '[UNDEFINED]',
'4TH#', 'SLEEP', ',”', '2DUP', '2DROP', '2SWAP', '2>R', '2R>',
'S|', ',|', '+PLACE', '-ROT', 'BOUNDS', '2R@', 'R'@', 'UNLOOP',
'SOURCE', 'SOURCE!', 'DEFER@', 'DEFER!', '>BODY', 'SCONSTANT',
':THIS', 'DOES>', 'STRUCT', 'END-STRUCT', '->', 'FIELD',
'ENUM', 'SEEK', 'TELL', 'AKA', 'ALIAS' and 'HIDE' have been
added.
Functionality
• The execution of a 4tH program can be suspended.
• A suspended 4tH program can be saved and reloaded.
• A 4tH program can be embedded in a MS batch file.
• User defined words can be made private.
• User defined words can be aliased.
• User defined terminal input buffers are supported.
• Complete, ANS-Forth compatible redesign of all string handling
words.
• Multiple WHILEs are supported with REPEAT, AGAIN and UNTIL.
• Support for structures and enumerations has been added.
• Files can now be opened in read/write mode.
• File pointers can be interrogated and repositioned.
• Limited DOES> support has been added.
• More ANS-Forth, COMUS and TOOLKIT words have been added.
Bugfixes
• Several small bugs in the editor were fixed.
• A small bug in 'FILL' was fixed.
• A bug in hgen_4th.c that caused SEGFAULT was fixed.
• A security vulnerability in 4th.c was fixed.
Developer
• Several changes in exec_4th(), comp_4th(), save_4th() and
load_4th() to support suspension.
• The function inst_4th() has been renamed to fetch_4th().
• The function store_4th() has been added.
• The Hcode structure has been expanded with the members CellSeg,
UnitSeg and Offset.
• PAD has been converted to a circular buffer for temporary
strings.
• Most of the string handling and all file functions in
exec_4th() have been rewritten.
• The entire virtual machine was rewritten and its performance
significantly improved.
• All internal 4tH variables are now located in a hidden area of
the Integer Segment.
• The performance of 'MOVE' has been significantly improved.
• The library files have been updated and significantly expanded.
Documentation
• All documentation now reflects the functionality of the current
version.
• There is now one single manual.
Hints
Porting your V3.3d release 2 programs to V3.5a may require some
effort. In previous versions, string support was quite a mess
(IMHO), requiring awkward words like 'COPY'. Some words returned
or expected an address, others an address/count pair. With
version 3.5a string support was completely redesigned.
Consequently, source files using strings or arrays of string
constants will have to be partially rewritten in order to make
them compile and run properly. There are several things to
consider:
Strings
There has been a conversion to the format recommended by the
ANS-Forth standard. All strings are now represented by an
address/count pair, with the exception of string variables and
string addresses returned by '@C'. For this purpose, 'WORD' has
been replaced by 'PARSE-WORD'. 'NUMBER' and 'ARGS' now return an
address/count pair. Parsed strings are no longer copied to PAD,
but remain in TIB and are not zero-terminated. However, since
parsed strings are now represented by an address/count pair this
should not be a problem.
Most programs we examined used constructions like this:
[char] ; word count type
s" 567" drop number
1 args count my_variable place
Those can easily be converted to:
[char] ; parse-word type
s" 567" number
1 args my_variable place
As a rule of the thumb, we advise you to use 'COUNT' only on
string variables and string addresses returned by '@C'. You might
find after a while, that these are the only situations where
'COUNT' is actually required. In all other situations, you use
the count on the stack. Special operators like '2DUP', '2DROP'
and '2SWAP' have been added to make manipulation of address/count
pairs easier.
Please note that 'OPEN' already required an address/count pair,
but simply discarded the count. In version 3.5a the count is
required. If you didn't program properly, this might cause errors
now. Well designed programs will continue to function properly.
We advise against the use of 'MOVE' or 'CMOVE' for moving
strings. Most of these constructions will continue to work, but
some may fail. In any case, they are not portable. Use 'PLACE'
and '+PLACE' wherever you can.
PAD
PAD has been converted into a circular string buffer. Because
some routines directly interface with their C counterparts,
temporary zero-terminated strings are stored in PAD. When the
buffer overflows it wraps around, overwriting whatever is there.
Some previously correctly running programs may corrupt the PAD
this way. If this happens, you can solve this by storing the
overwritten string into a string variable. The reason for all
this is that this now works:
s" This is not overwritten" s" By this string" compare
Number representations are not clobbered unless you use extremely
long number formats.
Arrays of string constants
Consider this construction:
16 string weekday
create weekdays
" Monday" ,
" Tuesday" ,
" Wednesday" ,
" Thursday" ,
" Friday" ,
" Saturday" ,
" Sunday" ,
weekdays 4 th @' weekday copy count type cr
'@'' returns an address in the String Segment. 'COPY' is the only
word in pre-3.5a versions that can access the String Segment. It
copies the string from the String Segment to an address in the
Character Segment and returns that address . In version 3.5a and
up, '@'' has been replaced by '@C'. '@C' is a lot smarter. It
copies the zero-terminated string from the String Segment to PAD
and returns that address. A 'COUNT' is still needed, but since
all temporary strings in PAD are zero-terminated, this can safely
be done:
create weekdays
," Monday"
," Tuesday"
," Wednesday"
," Thursday"
," Friday"
," Saturday"
," Sunday"
weekdays 4 th @c count type cr
Note that the string variable is no longer needed and the
resulting code is much cleaner! String constants are now declared
by a simple ',"'. '”' has been discarded. '@C' also works for
integer constants and behaves like '@''.
Deferred words
Deferred words are now fully COMUS compatible. You have to change
your programs only slightly:
defer my-vector
: do-nothing ;
' do-nothing is my-vector
my-vector execute
Just remove the 'EXECUTE':
defer my-vector
: do-nothing ;
' do-nothing is my-vector
my-vector
Please note that 'IS' is no longer an alias for 'TO'. If you have
used illegal constructions like that, you'll have to correct
them.
Library files
Note the library files have been revised and expanded. Some words
have been renamed or placed into another file. Note that the
'toolbelt.4th' and 'comus.4th' library files are primarily
intented for porting purposes. The 'easy.4th' library file is
intended to port 4tH programs to other Forth compilers. Note that
this only works for ANS-Forth compliant programs.
Dropped words
'WAIT' has been dropped and replaced by 'MS'. You'll have to
'INCLUDE' the library file 'ansfacil.4th' in order to use it.
Note that this implementation is very crude and may vary between
0 and +1999 milliseconds[footnote:
The ”Forth Programmers Handbook” states that 'MS' should be at
least the duration plus twice the resolution of the system (which
is one second in 4tH).
].
'TOKEN' has been replaced by 'PARSE', which returns an
address/count pair. 'WORD' has been replaced by 'PARSE-WORD',
which returns an address/count pair. 'COPY' has been incorporated
into '@C'.
'TEXT' has been dropped. If you treat a file as a text file, it
will be handled as a text file. Just remove 'TEXT':
s" textfile.txt" input text + open
So now it reads:
s" textfile.txt" input open
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are '@C', 'OMIT', 'OUT', '@GOTO', '+CONSTANT',
'SOURCE-ID', 'CIN, 'COUT', 'PARSE-WORD', 'IMMEDIATE', 'NOT',
'INCLUDE', '[UNDEFINED]', '4TH#', 'SLEEP', ',”', '2DUP', '2DROP',
'2SWAP', '2>R', '2R>', 'S|', ',|', '+PLACE', '-ROT', 'BOUNDS',
'2R@', 'R'@', 'UNLOOP', 'SOURCE', 'SOURCE!', 'DEFER@', 'DEFER!',
'>BODY', 'SCONSTANT', ':THIS', 'DOES>', 'STRUCT', 'END-STRUCT',
'->', 'FIELD', 'ENUM', 'SEEK', 'TELL', 'AKA', 'ALIAS' and 'HIDE'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list, TOOLBELT list or (proposed[footnote:
A proposed ANS-Forth standard is usually published on
comp.lang.forth (usenet) by an ANS-Forth committee member.
]) ANS-Forth standard, except for porting purposes.
1.11 What's new in version 3.3d, release 2
Words
• The word 'C”' has been discarded. The words '[NEEDS' and
'[DEFINED]' have been added.
Functionality
• Source files can be included at compile time.
• The existence of words in the dictionary can be checked at
compile time.
• More COMUS words have been added.
• The 4tH program allows you to enter parameters in the menu.
• The Linux module 'binfmt_misc' is supported.
Bugfixes
• None.
Developer
• Function open_4th() has been rewritten.
• The parser in comp_4th() has been changed significantly and is
now much more transparant.
• There is an extra option in the menu of 4th.c
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.3d programs to release 2 shouldn't be any
problem. All executables will run without recompilation. However,
you might have to change a few source files in order to make them
compile properly. There are two things to consider:
Dropped words
If you used 'C”' in your programs, you'll have to replace it by '”
'. No other changes are necessary.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are '[NEEDS' and '[DEFINED]'
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
COMUS list or ANS-Forth standard, except for porting purposes.
1.12 What's new in version 3.3d<sec:What's-new-in>
Words
• The words 'FILE' and 'TTY' have been discarded. The words
'FILE', 'AS', 'USE', 'DEFER', 'IS', 'STDIN' and 'STDOUT' have
been added. The words 'INPUT', 'OUTPUT', 'OPEN' and 'CLOSE'
have been changed.
Functionality
• Multiple files can be opened concurrently.
• More COMUS words have been added.
Bugfixes
• A segment violation was caused in 4th.c when an invalid
sequence of commands was issued. This has been fixed.
• Better errorhandling when a pipe cannot be opened.
Developer
• The file support in function exec_4th() has been rewritten.
• Added DoInitValue().
Documentation
• All documentation now reflects the functionality of the current
version.
Hints
Porting your V3.3c programs to V3.3d shouldn't be any problem.
Most of them will only require recompilation, except when files
are manipulated. There are three things to consider:
Using files
The new 4tH file handling module adds the concepts of streams and
channels. You have two channels, an input channel and an output
channel. In (standard) 4tH you have eight streams (you can
increase this when you compile 4tH), two are already taken by the
system (stdin and stdout). At startup the stdin stream is
connected to the input channel and the stdout stream is connected
to the output channel.
You can open additional streams by using the 'OPEN' word:
OPEN (a n fmod -- handle)
E.g.
s" ls" input pipe + open
This is not a significant deviation from V3.3c in which 'OPEN'
returned only a flag. You can still interpret the handle as a
flag since 'OPEN' returns zero when it failed.
To use the handle you only have to connect it to the appropriate
channel. In V3.3c this was done by using:
input file
In V3.3d, you use the word 'USE'. 'USE' takes a handle and
connects the stream to the appropriate channel.
file ls
s" ls" input pipe + open dup as ls
0= abort" Cannot open pipe"
ls use
In V3.3c you had to close a file by closing the channel, while
the stream was still connected:
s" ls" input pipe + open
0= abort" Cannot open pipe"
input file
input close
In V3.3d you have to close the stream:
file ls
s" ls" input pipe + open dup as ls
0= abort" Cannot open pipe"
ls use
ls close
The default stream is reconnected to the channel, even if another
stream was currently connected to that channel. We give you an
example how 4tH now handles files in respect to the previous
version:
Version 3.3c
s" hello.txt" output text + open
0= abort" Cannot open file"
output file
." Hello world" cr
output close
Version 3.3d
file hello
s" hello.txt" output text + open dup as hello
0= abort" Cannot open file"
hello use
." Hello world" cr
hello close
I hope you can appreciate the extended possibilities of 4tH and
the way we tried to minimize breaking existing code.
Using 'INPUT' and 'OUTPUT'
Two new constants have been added to 4tH: 'STDIN' and 'STDOUT'.
Before you could use 'INPUT' and 'OUTPUT' as follows:
input file
output tty
Using 'INPUT' and 'OUTPUT' this way is depreciated and should be
replaced by:
stdin use
stdout use
Please use 'INPUT' and 'OUTPUT' only as flags for OPEN.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are 'AS', 'USE', 'DEFER', 'IS', 'STDIN' and
'STDOUT'
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
ANS-Forth standard, except for porting purposes.
1.13 What's new in version 3.3c
Words
• The word '+UNDER' has been discarded. The words 'PIPE',
'PLACE', 'TOKEN', 'SKIP', 'PARSE', '/CELL', '/CHAR', 'ABORT”',
'[ABORT]' and '[=]' have been added.
Functionality
• A complete mini-IDE has been added.
• Parsing has been enhanced significantly.
• The Unix version now supports pipes.
• More CORE words implemented.
• Some environmental dependancies can be checked at compiletime.
Bugfixes
• Reentry of several 4tH functions was seriously flawed, most
notoriously in 'comp_4th()'. This has been fixed.
Developer
• Several new functions have been added, most significantly in
the area of C source generation.
• The loading of sourcefiles is now done by open_4th(); fload()
can still be used, but is no longer supported.
• The function save_4th() has been optimized. HX files are up to
50% smaller compared to those created by previous versions.
• The function dump_4th() has two extra arguments, allowing
partial decompilation.
• The file support in function exec_4th() has been rewritten and
now supports popen() and pclose().
• The demonstration program 4th.c has been completely rewritten.
Documentation
• All documentation now reflects the functionality of the current
version.
• A document describing a sample session in 4tH interactive mode
has been added.
• Several documents have been merged.
Hints
Porting your V3.3a programs to V3.3c shouldn't be any problem.
Most of them will only require recompilation. There are two
things to consider:
Programs using '+UNDER'
Which is no longer supported. If you have such programs, just add
this definition at the top:
: +UNDER ROT + SWAP ;
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are 'PIPE', 'PLACE', 'TOKEN', 'PARSE', 'SKIP',
'/CELL', '/CHAR', 'ABORT”', '[ABORT]', and '[=]'.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
ANS-Forth standard, except for porting purposes.
1.14 What's new in version 3.3a
Words
• The words 'APPEND', 'TEXT', 'S"', '[*]', '[+]', '[NOT]' and
'#!' have been added.
• The word 'OPEN' has been changed.
Functionality
• An output file can now be opened in "append" and "text" mode.
• A 4tH program can now be run from the shell.
• More CORE words implemented.
• Compiletime calculation is possible now.
Bugfixes
• When reallocation of the segments during compilation fails,
resources are freed.
• When memory allocation of the header during the loading of an
HX file fails, the file is closed.
Developer
• Dropped the EasyC syntax.
• Added the proper 'int main()' declarations.
• Modern prototypes, local include files and no stricmp()
function are now the default behaviour.
• Dropped stricmp() from the distribution and added MatchName()
to comp_4th().
• Added CompileString().
Documentation
• All documentation now reflects the functionality of the current
version.
• The Developers Guide has been enhanced.
Hints
Porting your V3.2e programs to V3.3a shouldn't be any problem.
Most of them will only require recompilation. There are two
things to consider.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are '#!', 'S"', 'APPEND', '[*]', '[+]', '[NOT]'
and 'TEXT'.
Changed words
The word 'OPEN' now takes an extra value from the stack. If you
used an construction like this:
64 string filename
" myfile.dat" filename copy
input open
Change it to this:
s" myfile.dat" input open
If you used a construction like this:
refill drop
bl word
input open
Change it to this:
refill drop
bl word count
input open
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
ANS-Forth standard, except for porting purposes.
1.15 What's new in version 3.2e
Words
• The words 'I'', 'R', 'QUERY', 'ENDIF', 'END', 'MINUS', 'NOT',
'ASCII', '2+' and '2-' have been discarded.
• The words ':NONAME', '?DO', 'BLANK', 'ERASE', 'CMOVE>', 'NIP',
'TUCK', '+UNDER' 'REFILL', 'D>S', 'RSHIFT', 'CATCH' and
'MAX-RAND' have been added.
• Renamed '-TRAIL' to '-TRAILING', 'STACK' to 'STACK-CELLS',
'#PAD' to '/PAD', '#TIB' to '/TIB' and 'LIMIT' to 'MAX-N'.
Functionality
• The Character Segment is now unsigned, so no more negative
characters.
• Vectored execution has been enhanced.
• Better implementation of 'RECURSE'.
• Better ANS-Forth compatibility by adding some commonly used
words.
• Compatibility with Forth-79 has been dropped.
Bugfixes
• load_4th() closes the file when memory allocation failed.
• An error in GetImmediate(), GetConstant() and GetWord() has
been fixed.
• DoRecurse() can now detect the use of 'RECURSE' outside a colon
definition.
Developer
• Complete redesign of the parser. The whole parser now consists
of the functions: ParseText(), ParseString() and
ParseDirective(). Inline macros are supported.
• MoveString() does not require any arguments anymore.
• A textmode has been added to accept().
• Removed and added several tokens.
• The names of all internal words are now pointers instead of
sized arrays, which means name can have any length now.
Documentation
• All documentation now reflects the functionality of the current
version.
• The Developers Guide has been enhanced.
• The Porting Guide has been enhanced.
Hints
Porting your V3.1d programs to V3.2e shouldn't be any problem.
There are three things to consider.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are ':NONAME', '-TRAILING', '?DO', 'BLANK',
'ERASE', 'CMOVE>', 'NIP', 'TUCK', '+UNDER', 'REFILL', 'D>S',
'RSHIFT', 'CATCH', '/PAD', '/TIB', 'STACK-CELLS', 'MAX-N' and
'MAX-RAND'. Most likely you have used these names for
compatibility purposes, e.g.:
: rshift negate shift ;
In that case you can simply remove these definitions.
Dropped words
The Forth-79 words 'R', '2-', '2+', 'QUERY', 'ENDIF', 'END',
'MINUS', 'NOT', 'ASCII' and 'I'' are no longer supported.
'-TRAIL', '#PAD', '#TIB', 'LIMIT' and 'STACK' have been renamed.
If you have programs that use these words then either modify them
or add the following definitions:
: 2+ 2 + ;
: 2- 2 - ;
: i' r> r> r> dup >r rot rot >r >r ;
: r r> r> swap over >r >r ;
: query input tty refill drop ;
: minus negate ;
: not invert ;
: -trail -trailing ;
: #pad /pad ;
: #tib /tib ;
: limit max-n ;
: stack stack-cells ;
Unfortunately, you still have to replace the following words by
their ANS-Forth equivalent, since there is no colon definition
available for them:
[float Table:
+----------+--------------+
| Change: | To: |
+----------+--------------+
+----------+--------------+
| ASCII | CHAR, [CHAR] |
+----------+--------------+
| END | AGAIN |
+----------+--------------+
| ENDIF | THEN |
+----------+--------------+
[Senseless!!!
Forth-79 to ANS conversion
]
]
Unsigned characters
If a character with an ASCII value greater than 127 was fetched
from the Character Segment, it was converted to a negative value.
'C@' will now return a positive value. This means that you can
remove patches like these:
: c@' c@ dup 0< if 256 + then ;
On the other hand, if you have programs that rely on this
negative value (e.g. by storing "-1" in a character), then you
have to modify them.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
ANS-Forth standard, except for porting purposes.
1.16 What's new in version 3.1d
Words
• The words 'AT' and 'ALLOT' have been discarded
• The words 'ARRAY', 'TABLE', '.(', 'ABORT', 'S>D', '"',
'RECURSE', '[IF]', '[THEN]', 'ARGS' and 'ARGN' have been added.
Functionality
• Better ANS-Forth compatibility (thank you, Wil Baden)
• Commandline arguments are now supported
• Nested assertions are now supported
• Conditional compilation is now supported.
Bugfixes
• ASCII bug has been fixed
• Several bugs in ParseText() and ParseStrings() have been fixed.
Developer
• Added SkipSource(), DecodeSymbol() and DecodeLiteral()
• Added two more arguments to exec_4th()
• Added two more tokens to cmds_4th.h
• Moved <limits.h> to 4th.h
• Added an extra compilation option "LOCAL_H" for those who
cannot access /usr/include.
Documentation
• All documentation now reflects the functionality of the current
version
• A 'Porting Guide' has been added
• A 'What's New' bulletin has been added
• The 'Developers Guide' has been enhanced
• The 'Primer' has been enhanced.
Hints
Porting your V3.1c programs to V3.1d shouldn't be any problem.
There are five things to consider.
AT
'AT' has been discarded. Simply replace all occurences of 'AT' by
'STRING'. If you used the 'CHARS' keyword, you can leave right
there since it doesn't have any effect, except when you are
porting your program to Forth.
New reserved words
If you used the any of the new reserved words in your program as
a name, you should replace those names by another. The new
reserved words are 'ARRAY', 'TABLE', '.(', 'ABORT', 'S>D', '"',
'RECURSE', '[IF]', '[THEN]', 'ARGS' and 'ARGN'.
Using 'VALUE' with 'ALLOT'
If you ALLOTted any space to a VALUE, you should rewrite your
code. Note that this is bad practice anyway. Example:
10 value room 10 cells allot \ allotting space to a VALUE
20 to room \ changing ROOM
5 ' room first + 4 th ! \ accessing allotted space
Change this to:
11 array room \ define an ARRAY
10 room 0 th ! \ init 1st element of ROOM
20 room 0 th ! \ change 1st element
5 room 4 th ! \ accessing allotted space
Using 'VARIABLE' with 'ALLOT'
This should be common practice to define cell arrays. However, as
Wil Baden pointed out, this is not a common practice in
ANS-Forth. Therefore, the word 'ARRAY' has been added, which can
easily be implemented in both Forth-79 and ANS-Forth. All the
programs using the old syntax have to be modified, though.
Example:
variable room 15 cells allot
Change this to:
16 array room
Special care must be taken of arrays that are sized using a
constant, e.g. when the same constant is used to check the range.
Example:
15 constant size
variable room size cells allot
: room? \ is it a valid variable?
dup ( n n)
size not and ( n f)
if \ exit program
drop ." Not an element of ROOM" cr quit
then
;
Change this to:
16 constant size
size array room
: room? \ is it a valid variable?
dup ( n n)
size 1- not and ( n f)
if \ exit program
drop ." Not an element of ROOM" cr quit
then
;
ANS-Forth compatibility
Sometimes it proved to be impossible to port a program to
ANS-Forth since some constructions could not be implemented.
There is no such thing as a 'state' in 4tH, which means that
compilation- and interpretation semantics are completely the
same, e.g.
c" This is a string" value addr
." String address has been stored in ADDR" cr
This is perfectly valid in 4tH, but cannot be ported in any way
to ANS-Forth. With the new version, you can write:
" This is a string" value addr
.( String address has been stored in ADDR) cr
So if you have a 4tH program which you wanted to port to
ANS-Forth, but couldn't, study the Porting Guide and try again.
Note that no change is required if you do not intend to port your
program to Forth. Apart from the modifications already mentioned
you do not have to change a single line.
Our apologies for any inconvenience caused. It is certainly not
our policy to change the syntax with every single version, but we
found the arguments in favor of this change so strong that we
didn't see any other way.
In order to prepare your programs for other changes, we strongly
advise you not to use any names which are also mentioned in the
ANS-Forth standard, except for porting purposes.
Getting Started
Overview
2.1 Introduction
Like Forth, 4tH is a compiler and a interpreter. Unlike Forth you
cannot switch between the two. Like Forth, 4tH runs
Forth-programs. Not all of them but some. But in a quite
different way.
Most things have already been written. There have been Forths
written in a high level language. There have been portable
Forths. There have been Forths that could interface with C.
Different architectures have been used to implement Forth. There
have been Forths that were 16 kB or even less.
Well, all of that has been done. But here is a
compiler/interpreter that's all of the above. And none of them
either. It sounds like an ancient Greek riddle, but it isn't.
It's 4tH.
2.2 History
To understand 4tH you have to know how it came to be. As most
things in life, 4tH developed slowly. Its predecessor is a
C-function called strcalc(). This function is an implementation
of a RPN calculator in one very compact function (about 6 kB
source). It works with signed 32 bits integers and has about 20
commands and 20 variables. The C-programmer can add additional
variables.
Using it in a C-program is very easy too. Just pass the source as
a string and add any variables you need. It will return the
result of that calculation.
Well, although primitive it can still be very useful. You can
implement an interactive RPN calculator in less than 5 lines of
C. It can also be used to make calculations from sources stored
elsewhere, like in a file or an environment-variable. If you can
store a string there, you can store strcalc() source.
But we were not satisfied. We wanted to create some successor to
strcalc() that could be used to create applets, small
applications that can be embedded in an application. Like
strcalc() it had to be fast and compact and easy to use. All
these requirements and 'Reverse Polish Notation'. What language
comes to mind first? Forth.
There were a few advantages and disadvantages to that approach.
First, if it looked like Forth, it had to be compatible with
Forth up to a certain point. Second, if it looked like Forth, we
wouldn't have to write thick manuals and explain how to use the
language. Third, if it looked like Forth, could we make it
crash-proof?
A user can easily crash a Forth-system. Store something at a
wrong address and your system hangs. We don't like that, even
when the user is at fault. So we had to make a few concessions
somewhere, since adding checks means the program will be less
compact and slower.
For a very long time we just didn't get the right idea. Then on a
dark night in October 1994, it happened. The baby was called 4tH
and could do everything strcalc() did.
It took quite a while before 4tH had successfully got away from
its strcalc() roots. The very first version was very buggy and
little more than an RPN calculator with (incompatible)
flowcontrol and some string facilities. It required two passes to
compile a source and the resulting bytecode could not be saved.
The I/O was C-based and very primitive. There was no Character
Segment.
The second version got string and file facilities. The I/O and
flowcontrol was completely rewritten, so they now were fully
Forth-compatible. The second pass was discarded and H-code could
finally be saved. The first move to ANS-Forth was made.
The third version came to be when the H-code eXecutable was
created. This fileformat made it possible to port bytecode across
platforms. At the same time, 4tH moved more and more toward
ANS-Forth. Exception-handling and assertions were introduced. And
in the spring of 1997, version 3.1c was released to the general
public.
Of course, 4tH didn't stop there. Since then, conditional
compilation, enumerations, structures, forward declarations,
inline-macros, pipes, source file inclusion, threads, private
declarations and a small IDE have been added. The compatibility
with ANS-Forth has been significantly improved. Neither the
compactness nor the speed of 4tH have been compromised. It uses
less memory than previous versions and is 50% faster.
2.3 Applications
4tH is an excellent platform to learn Forth. It looks and behaves
like a conventional compiler, but essentially is Forth. A Forth
that detects virtually every error and reports what was wrong and
where it went wrong, but still is quite fast and compact.
But like any good teacher 4tH is quite strict. Forth allows
constructions that should be avoided. 4tH on the other hand,
either does not implement these words or restricts their usage.
Other Forth concepts are hard to handle, like the different
wordsets for different kinds of numbers. 4tH only uses signed 32
bit integers, which enables the programmer to make a wide range
of applications without being bothered by overflow. Pointers,
integers and characters are transparently converted.
That doesn't mean that 4tH cannot be used as a scripting language
anymore. There are still excellent facilities in 4tH to do just
that. They are just modified in order to allow programmers to use
4tH as a stand-alone language. If you wonder how we did all that,
here is the answer.
2.4 Architecture
4tH is a segmented Forth. There are different segments for
constant strings, characters, cells and tokens. This shows you
where each data-type is located:
• Return stack (Integer Segment)
• Data stack (Integer Segment)
• Variables & values (Integer Segment)
• String variables (Character Segment)
• Temporary storage (Character Segment)
• Compiled code (Code Segment)
• Compiled constants (Code Segment)
• String constants (String Segment)
The return-stack, data-stack and variables are allocated in one
large array of signed 32 bit integers. On top of that 4tHs
primitives check all parameters. This makes 4tH a very safe
environment.
4tH also propagates clean programming. E.g. storing and fetching
of the data-stack is not allowed. You can only store and fetch in
the Variable Area.
In effect, as far as we know 4tH cannot be crashed by a
user-program. The memory layout of the Integer Segment looks like
figure [cap:Integer-segment-layout].
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/intseg.eps>
[Senseless!!!
<cap:Integer-segment-layout>Integer segment layout
]
]The allocation of variables is totally transparent to the
C-programmer. He can also transfer C-variables to the
user-program (application variables). These variables can be used
like any other variable.
Combining return- and data-stack means the C-programmer only has
to worry about the size of the stack and not the sizes of both
stacks, thus allowing a wider range of user-applications with
different requirements.
The Code Segment contains words. A word is a structure that
contains a unsigned byte (the token) and a signed long integer
(the argument). Only the argument can be accessed by the 4tH
programmer. He cannot change the program in memory, since we
never really liked self-modifying code.
True, this scheme has some redundancy, but a more elaborate
scheme means a more code to encode and decode the tokens and
arguments. That means the memory-space we saved by compacting the
program-code will make the compiler and interpreter less compact.
And it certainly won't run any faster!
The String Segment contains all string constants. The words which
use strings contain an offset to the ASCIIZ strings in the String
Segment. The 4tH programmer can copy strings from this segment,
but cannot write any. Constants are constants.
Finally there is a chunk of memory the user can manipulate at
will. It contains the TIB, the PAD and all string variables (if
any). The memory layout of the Character Segment looks like
figure [cap:Character-segment-layout].
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/charseg.eps>
[Senseless!!!
Character segment layout<cap:Character-segment-layout>
]
]The 4tH programmer can store and fetch anything here. Since 4tH
uses some C-functions ASCIIZ strings are used. The words that act
on counted strings take the same parameters and deliver
functionally the same results.
File I/O is supported too in a more Forth-like way than Forth
itself. You can have six concurrently open files and/or pipes.
4tH has threads too. A thread can be saved to disk and reloaded.
The only restriction is that all files are closed when the
execution of a thread is suspended.
2.4.1 The 4tH language
Most Forths use four different datatypes: signed 16 bit numbers,
unsigned 16 bit numbers, signed 32 bit numbers and unsigned 32
bit numbers. The latter two are usually called "double numbers".
Unlike C they all have their own operators. On top of that there
are mixed operators too. Highly confusing!
We never liked that in the first place. Application programmers
want to make an application. They don't want to worry whether any
intermediate result could possibly be larger than 32767. So 4tH
gets rid of most data- types and operators. It uses signed 32 bit
numbers. That's it. No mixed, double or unsigned operators.
Second, a Forth programmer has to know how much address-units a
cell takes. Since every data-type in 4tH has its own segment, the
address-unit of a segment is always one, regardless the
data-type. Consequently, ANS- Forth words like 'CELLS' and
'CHARS' are 'NOOP's. Which fits 4tH nicely.
Although 4tH has different words for storing and fetching
different data- types, most of its vocabulary is still compatible
with Forth. E.g. the word "C!" takes an address in the Character
Segment and "!" takes an address in the Integer Segment. Since
the Code Segment and String Segment do not allow any writing,
there is no need for such operators.
Each segment has its own allocation operators too. 'VARIABLE',
'ARRAY' and 'VALUE' allocate space in the Integer Area. 'STRING'
allocates space in the Character Area. Other words like ''' and
'CREATE' have restricted functionality and compatibility with
Forth.
4tH was originally loosely based on the Forth-79 standard, but
now it supports most of the CORE wordset of ANS-Forth. Note that
compatibility never had the highest priority. 4tH was designed to
write applets, not to be the next "fully ANS-Forth compatible
compiler with a little difference". If that is what you want, 4tH
is not for you.
2.4.2 H-code
Long before the dawn of the original IBM-XT there was a language
called UCSD Pascal. Like Forth, it was a compiler and an
interpreter. In fact, it didn't compile source into object-code
for some silicon-based processor. Instead it made P-code. So if
you wanted to execute it, you needed a P-code interpreter for
your system.
Such an interpreter can run faster than an ordinary interpreter
since it doesn't interpret source-statements with all of its
symbolic labels intact, but optimized P-code. It seems to have
been discovered again, since Java and previous versions of Visual
Basic work the same way. Visual Basic hides the interpreter in a
DLL, but basically it doesn't work any different.
The 4tH uses the same basic architecture. First the source is
compiled into H-code. Then the H-code interpreter is run. A token
is a very simple structure. It's got a single byte instruction
and an argument. Here's a sample of disassembled H-code:
[62] CR (0)
[63] VARIABLE (2)
[64] @ (0)
[65] 1- (0)
[66] DUP (0)
[67] VARIABLE (2)
[68] ! (0)
[69] 0BRANCH (62)
BTW, building a decompiler for tokenized code is quite simple.
There is one for Visual Basic and it seems like one emerged for
Java too. The H- code was the result after compiling this little
piece of source code:
cr begin times @ 1- dup times ! until
You can clearly see that everything is actually compiled.
Flow-statements are compiled into BRANCH and 0BRANCH instructions
pointing to addresses in the Code Segment.
Compiled H-code can be used on its own. It can be kept in memory,
loaded, saved, decompiled and executed. H-code is a combination
of the String Segment, the Code Segment and a header (figure [cap:Hcode-structure]
). The header contains all the information to set up the runtime
environment and some information on the String- and the Code
Segments. The Integer Segment and the Character Segment are
created at runtime.
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/hcode.eps>
[Senseless!!!
Hcode structure<cap:Hcode-structure>
]
]Although speed was an issue when 4tH was designed, it is beaten
by some other Forths. There are several possible explanations.
• 4tH uses 32 bit numbers, while most other Forths use only 16
bit numbers
• 4tH checks all parameters, while other Forths depend on signals
or don't do any checking at all
• 4tH is written in C, while some other Forths are written in
assembler
When 4tH is compiled with a 32-bit compiler it outruns Python,
Perl and most other C-based Forths (upto 4 times) or has a
comparable performance (with the possible exception of GCC
optimized Forth compilers). In real life applications the
difference is barely noticeable.
To make compiled H-code portable, a separate scheme was
developed: the Hcode-eXecutable. Or HX-file for short. It
contains all the information in the header, a compacted Code
Segment, the String Segment and some additional information on
compatibility and integrity. Numbers are stored in an
architecture-independant way.
2.4.3 H-code compiler
The H-code compiler looks a lot like any conventional compiler or
assembler. Basically it is a simple one-pass compiler. In order
to understand the workings of 4tH you have to know that not all
H-code instructions are equal:
• Immediate words (flow control, declarations, etc.)
• Predefined constants (addresses, aliases, etc.)
• Simple words (do not require an argument)
• Symboltable entries (user-definitions)
To determine the initial size of both the Code Segment and the
symbol- table the source is parsed first and the actual number of
words counted. This determines the initial size of the Code
Segment with a high degree of accuracy, so extending the Code
Segment is never necessairy. After compilation the Code Segment
will be shrunk to its actual size.
The parser can distinguish between directives and string
constants. The size of the symbol-table is determined by simply
counting all definitions. Every definition needs one symbol-table
entry. That makes determining the size of the symbol-table very
easy.
During compilation all simple words are compiled into tokens
without a valid argument. When a definition is encountered, like
a colon-definition or a variable-declaration, a symbol is added
to the symbol-table.
There are four compiler directives which determine how a number
is interpreted. '[BINARY]' interprets numbers as binary numbers,
'[HEX]' interprets them as hexadecimal numbers. '[DECIMAL]' and
'[OCTAL]' are available too. The "simple words" 'HEX', 'DECIMAL'
and 'OCTAL' only act during execution and do not determine how a
number is interpreted during compilation.
During compilation the compiler also resolves all flow words. It
simply matches the correct instruction and enters the
jump-address into the argument of the 'BRANCH', '?DO', 'LOOP',
'+LOOP', 'CALL' or '0BRANCH' word. The way 4tH handles flow
control is almost completely identical to Forth.
It may sound strange, but colon-definitions are also treated like
flow-words. The colon simply compiles into a 'BRANCH' instruction
that skips the colon definition.
When the user calls a colon definition, it simply compiles into a
'CALL' instruction that puts the current address on the
return-stack and jumps inside the colon definition, after the
'BRANCH'. The semi-colon works like a RETURN instruction that
pops the return address from the return-stack. Yes, like a
subroutine in BASIC or assembler!
2.4.4 Error handling
When 4tH finds an error during compilation or execution it stops
and sets the H-code member ErrNo. It works like "errno" in C. You
can optionally link in an array of error-messages. ErrNo is an
index to this array, which makes issuing the correct error
message very simple. The instruction pointer is frozen at the
point where the error occured, so it is very easy to find out
where the error occured.
2.4.5 Interfacing with C
A minimal compiler would take only a few lines of C-code. The
C-programmer can send C-variables to the interpreter, just like
strcalc().
E.g. a compile takes a string-pointer as argument and returns a
pointer to H-code:
object = comp_4th (source);
Executing H-code is easy too:
ReturnVal = exec_4th (object, argc, argv, 3, Var1, Var2, Var3);
Which would preload variables Var1, Var2 and Var3. You must
specify how many variables are preloaded. Also 'argc' and
'**argv' are available from the 4tH program.
The value returned by exec_4th() and stored into ReturnVal is the
value of the 4tH variable 'OUT', which initially contains
CELL_MIN. If an error occurs exec_4th() will always return
CELL_MIN, regardless the value stored in 'OUT'.
Installation Guide
3.1 About this package
4tH will compile ordinary text-files (MS-DOS and Unix) as well as
block-files produced by the 4tH editor. The user-interface of
this line-editor is highly compatible with conventional Forth
block-editors.
4tHs special architecture almost forces you to write "clean"
code, so you will learn Forth the proper way. This does not mean
that you can't write portable code with 4tH. In fact, because
Forth is so flexible you can usually write a small interface to
your well-written 4tH-code in a matter of minutes.
You can use 4tH in virtually every environment, from Linux to
MS-Windows. You don't even have to recompile your applications
since 4tH uses a special executable format, that is interpreted
by the 4tH virtual machine.
3.1.1 Example code
There are a lot of example programs, written in 4tH. From
line-editors and calculators to adventure-games. Not all have
been especially written for 4tH. There are quite a few programs
from the hand of people like Professor C.H. Ting and Leo Brodie
that started their existence as Forth-programs.
Most are available in source. That means they have the extension
'.4th'. You can examine or edit them like any other source-file.
Source-files written with the 4tH editor get the extension
'.scr'. They can only be edited with the 4tH editor or other
Forth blockfile editors. Executables have the extension '.hx'
(Hcode eXecutable).
3.1.2 Main program
You will find a binary program within this package called 4tH.
You can copy this binary to any directory. 4tH is a small
development system by itself. When you start it, it will
automatically enter interactive mode and show you a menu not
unlike early versions of Turbo Pascal. You can edit, compile, run
and debug programs from the 4tH prompt. Please read chapter [GuidedTour]
for more details.
You can also use 4tH from the commandline:
4th <commands> <file> [file | argument .. argument]
It takes most combinations of these ten commands:
m enter interactive mode
e edit a 4tH screenfile
c load a sourcefile (.4th) and compile it
l load an objectfile (.hx)
d decompile a 4tH program
g generate a C sourcefile (default: out.c)
s save a 4tH program (default: out.hx)
x execute a 4tH program
v enter verbose mode
q suppress copyright message
A few examples:
• To compile a 4tH program and save the object code: 4th csv
<source.4th> [object.hx]
• To compile a 4tH program and execute it: 4th cx <source.4th>
• To decompile object code: 4th ld <object.hx>
• To convert object code to C source: 4th lg <object.hx>
[source.c]
• To load and execute object code: 4th lx <object.hx> [arguments]
• To load and execute object code without arguments: 4th
<object.hx>
• To edit a 4tH screenfile: 4th e <source.scr>
• To enter interactive mode: 4th m <source.scr>
• To enter interactive mode (without loading a screenfile): 4th
Note: don't include the "[]" and "<>" in your commandline. They
are just there to show whether an argument is optional ([arg]) or
mandatory (<arg>).
3.1.3 Unix package
It is not possible for us to provide Unix binaries for all
possible platforms, not now and not in the future, simply because
we don't have access to them all. Here is a list of the Unix
(like) platforms that are known to compile 4tH:
• Intel - FreeBSD
• Intel - Coherent
• Intel - Linux
• Intel - BeOS
• RS/6000 - AIX
• Zaurus - Linux
• Sun - Solaris
• ARM - RISC/OS
• Apple - Linux
• Apple - OS/X
If your platform is not listed, give it a try anyway. The chances
are it will compile flawlessly, since we've never had a report of
a Unix platform that refused to compile or run 4tH. Please send
us an email with your results, so we can add it (or remove it)
from our list.
You have to compile 4tH yourself, which is not difficult if you
read the 'Developers Guide'. Usually this will do the trick:
make
make install
If you have any special needs, feel free to edit the makefile.
3.1.4 Linux package
You will find Linux binaries in this package. They will run under
most modern Linux distributions for Intel. If the Linux binary
doesn't run, you can easily recompile it. Just enter:
make
make install
You don't have to run './configure'. If you have any special
needs, feel free to edit the makefile, e.g. compiling for the
Zaurus means you have to add the '-DZAURUS' option.
You'll also find some icons for KDE or GNOME and a 'man' page.
However, you have to install them manually. If you want to embed
4tH in KDE or GNOME you have to do that manually as well. Please
consult your KDE or GNOME documentation.
3.1.4.1 /etc/magic
If you want Linux to recognize your 4tH files, you have to add
the following lines to your /etc/magic file:
# From [email protected]
# These are the magic numbers for 4tH HX files
0 belong 0x01020400 4tH eXecutable
>9 leshort x \b, version %x
E.g. if you enter:
file editor.hx
It will respond:
editor.hx: 4tH eXecutable, version 35d
3.1.4.2 Using binfmt_misc
There is a module in Linux that will allow you to execute 4tH
programs from the prompt without explicitly calling the 4tH
interpreter. It is called 'binfmt_misc'. 4tH has built-in support
for this module. Just add the following lines to your
'boot.local'[footnote:
On SuSE 'boot.local' is located in the /sbin/init.d directory.
] file:
insmod binfmt_misc
cd /proc/sys/fs/binfmt_misc
echo ':HX:M::\x01\x02\x04\x00\xff\xff\xff\x7f\x04\x5d\x03\x08:
:/usr/local/bin/4thx:' >register
If you use a kernel version later than 2.4.13 you have to add
these lines:
insmod binfmt_misc
mount -t binfmt_misc none /proc/sys/fs/binfmt_misc
cd /proc/sys/fs/binfmt_misc
echo ':HX:M::\x01\x02\x04\x00\xff\xff\xff\x7f\x04\x5d\x03\x08:
:/usr/local/bin/4thx:' >register
You can find out whether 4tH support has been properly installed
by issuing:
cd /proc/sys/fs/binfmt_misc
cat HX
And Linux should answer:
enabled
interpreter /usr/local/bin/4thx
offset 0
magic 01020400ffffff7f045d0308
Finally, you should go to the directory where 4tH has been
installed (usually /usr/local/bin) and enter:
ln -s 4th 4thx
Now, after you've compiled a program you should make it
executable and it will run like it is a native executable, e.g.:
4th cs asc2html.4th asc2html
chmod 755 asc2html
asc2html ascii7.4th ascii7.html
Note you have to be root in order to run some of these commands!
3.1.4.3 DIR4TH environment variable
This variable is used to indicate where 4tHs default directory
is. If a sourcefile cannot be found in the current directory, the
compiler will try to get it here. You can set this environment
variable in your .profile or .bashrc file. Simply login into your
default user account and type:
cd
vi .profile
or:
cd
vi .bashrc
This will launch the editor and allow you to edit the appropriate
file. In this example your default 4tH directory is
/home/joe/4th:
export DIR4TH=/home/joe/4th/
If 4tH is unable to find a sourcefile, e.g. lib/anscore.4th, it
will try to load /home/joe/4th/lib/anscore.4th. Do not forget to
add the trailing slash. If you do, it will not work properly.
3.1.5 MS-DOS package
The "4th.exe" that is included in the MS-DOS package is a 32-bit
MS-DOS version of the main Unix utility. It will only run on
80386 class machines and up. It allows you to compile and run
very large 4tH programs. It requires CWSDPMI.EXE somewhere in
your path. It is also available as "4th86.exe", which will run on
any IBM-PC with 256 KB memory. This version is a bit slower and
you may experience some memory restrictions.
3.1.5.1 DIR4TH environment variable
This variable is used to indicate where 4tHs default directory
is. If a sourcefile cannot be found in the current directory, the
compiler will try to get it here. You can set this environment
variable in your autoexec.bat file. In this example your default
4tH directory is C:\4th:
set DIR4TH=C:\4th\
If 4tH is unable to find a sourcefile, e.g. lib/anscore.4th, it
will try to load C:\4th\lib\anscore.4th. Do not forget to add the
trailing backslash. If you do, it will not work properly.
3.1.6 MS-Windows package
Run "setup.exe" to install the package. It runs with Windows 95
OSR2 and up, Windows NT 4.0, Windows 2000, Windows XP and Windows
Vista.
You can launch Explorer and double-click an HX-file. Windows will
complain it doesn't recognize the file and tell you what to do.
Browse to "4th.exe" and select it. After that you can click on an
HX-file and it will be executed. You can even add HX-files to
your desktop where they will start and run like ordinary Windows
applications.
This is a true 32-bit version, so it does take long filenames,
but you can't run it with Windows V3.x and early versions of
Windows 95. It is a console application, so you'll need an MS-DOS
box to run and use it. Note that it will exit immediately once a
program has halted. We recommend you run 4tH from the MS-DOS
prompt when you're using 4tH as a development environment.
3.1.6.1 DIR4TH environment variable
This variable is used to indicate where 4tHs default directory
is. If a sourcefile cannot be found in the current directory, the
compiler will try to get it here. In this example your default
4tH directory is C:\4th:
set DIR4TH=C:\4th\
If 4tH is unable to find a sourcefile, e.g. lib/anscore.4th, it
will try to load C:\4th\lib\anscore.4th. Do not forget to add the
trailing backslash. If you do, it will not work properly.
MS-Windows 9x
While it is possible to set environment variables in the same way
as for MS-DOS by editing autoexec.bat, it is easier to use
msconfig. First run msconfig from the task bar by selecting ”Run ”
.
Select the "Autoexec.bat" pane, then go to the bottom of the
window, select the last entry and click the "New" button. A small
input window appears below the last entry, and in this you should
type a new entry with the exact syntax as shown in the example
above. Then click "OK" and a small pen appears against the entry,
indicating that autoexec.bat will be modified. You may have to
reboot afterwards.
MS-Windows NT
Click on the ”My computer” icon or the ”Start” menu, then click
on the ”Control panel”. Click on the "System" icon to get the
"System Properties" dialog box. For Windows NT use the
"Environment" tab instead of the "Advanced" tab. Click on the
"Environment Variables" button and select ”New”. Enter the DIR4TH
and its value in the boxes and then click "OK".
If there are several users on the PC, it is probably better to
set the variables as "System variables", rather than "User
variables" since they will then automatically be accessible for
all users. You will need to have Administrator rights to do this.
3.2 Now what?
After you've installed and played around with the utilities, we
suggest you either click the 4tH icon on your desktop or start an
interactive session by entering:
4th m session1.scr
And start reading the Primer. When you've thoroughly read and
understood the very first section you're ready to go on. Start up
your favourite editor (or use the built-in editor if you don't
have one) and make your own very first 4tH program. If you don't
know how to use the built-in editor, read chapter [GuidedTour].
If you encounter an error during compilation or execution, refer
to the 'Errors Guide' for a detailled description what it means,
what probable causes are and how you can fix it.
3.3 Pedigree
4tH is basically an original work. However, some concepts have
been derived from the work of other, much smarter people.
• The interpreter is taken from strcalc() and modified.
• The pictured numeric output and flow-control routines are based
on Abersoft Forth.
• The exception handler is based on the dpANS-6 implementation.
• The enumerations are based on the Swift-Forth implementation.
• The structures are based on the GForth implementation.
• The 'ASSERT(' and ')' words are based on an idea implemented in
GForth.
• The implementation of '[DECIMAL]', '[HEX]', '[OCTAL]' and
'[BINARY]' was suggested by William Tanksley.
• The HX-format was suggested by Mikael Cardell.
4tH was discussed in Volume XVIII, Number 3 of Forth Dimensions.
Thank you, Marlin Ouverson for giving me that opportunity.
3.4 Questions
We tried to provide you with all the documentation you'll
probably ever need. That doesn't mean that you'll never have any
questions. NEVER EMAIL THE PEOPLE WHOSE SITE YOU GOT THIS FROM!
THEY DON'T KNOW EITHER! INSTEAD, MAIL TO:
[email protected]
You'll usually get fast answers, although when your question is
very complex we'll probably give you just some general
directions. We have to stress that any comment is welcome,
always.
3.4.1 4tH Website
You can visit our website, which is dedicated to 4tH. You will
find all the latest information there, including additions and
bugfixes (service packs):
http://hansoft.come.to
3.4.2 4tH Google group
We've got a Google group for discussions about 4tH. If you want
to interact with other 4tH users, we recommend you subscribe to
this group. You will also have to become a Google member if you
are not already, e.g. when you already have a gmail account:
http://groups.google.com/group/4th-compiler
Important! Your posts will not be accepted by the server if you
don't subscribe first!
3.4.2.1 Conditions of use
This group has been created as a service to, and in support of,
the 4tH (and Forth) community. As in most discussion groups,
there are a few rules to ensure the survivability of the group
for the future.
1. This group is for discussions of 4tH problems, 4tH questions
and answers. It is not to be used for non-4tH discussions.
2. This is not an 4tH advocacy group. Stick to 4tH questions and
problem-solving or move your discussion to an appropriate
channel. i.e. alternative site or private e-mail.
3. Flames, insults, foul language will not be tolerated. You will
be unsubscribed and barred from re-subscribing under your
present e-mail address.
3.4.2.2 What to discuss?
Well, Problems, wishes, needs, solutions (how you did something)
basically anything 4tH related.
3.4.3 Newsgroup
There is no special newsgroup for 4tH. However, comp.lang.forth
will prove to be able to answer most of your questions.
A guided tour<GuidedTour>
4.1 4tH interactive
4tH's interactive mode was introduced with version 3.3c, but it
is still fully compatible with previous versions, so you can
still use all your external IDE's and script files. The
interactive mode is especially useful when you are using an
environment where other tools are not available or impossible to
use. This document shows you how to use interactive mode and get
the most out of it.
4.2 Starting up 4tH
You can enter 4tH's interactive mode by just clicking the icon
(when you are using MS-Windows) or by issuing this command on the
Unix or MS-DOS commandline:
4th
4tH will respond by showing you this screen:
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>_
This is the main menu. It is slightly reminiscent to the earlier
versions of Turbo Pascal. At the bottom is the prompt. Just press
the appropriate key and hit enter, e.g. "S", which stands for the
name of the screenfile. 4tH will now prompt you for the name of
the screenfile. Note that 4tH is not case sensitive, so both "s"
and "S" will do.
4.3 Running a program
We assume you've installed 4tH according to the instructions. If
not, this might not work. Now press ”S” and hit enter. 4tH will
prompt you for the name of a screenfile:
Screen file name:
Answer by typing ”examples/romans.scr”[footnote:
This works for both Windows and Unix type Operating Systems.
] and hit answer. 4tH will return to the menu:
Screen file name: examples/romans.scr
(S)creen file: examples/romans.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>
Now hit ”R” and press enter. What now appears is your program
that is actually running:
>r
Enter number: 2005
Roman number: MMV
After the program has ended, you will return to the menu. Well,
that wasn't too hard, was it?
4.4 Starting an editing session
We start by entering the editor mode. Just type ”e” and hit
enter. Ignore any file opening errors. The ”OK” prompt shows you
you're now in the editor. Now type:
0 clear
This will erase the first screen and select it for editing. 4tH's
editor is a typical Forth editor. Forth organizes its mass
storage into "screens" of 1024 characters. Forth may have one
screen in memory at a time for storing text. The screens are
numbered, starting with screen 0.
Each screen is organized as 16 lines with 64 characters. The
Forth screens are merely an arrangement of virtual memory and do
not correspond to the screen format of the target machine.
Depending on memory model and operating system, you have either
28, 32 or 64 screens available. This will be sufficient in most
situations. These screens correspond to a region in memory, which
acts like a RAM drive.
The actual editing is done in an area that is called the
'workspace'. With the word 'clear' you wipe all information in
the workspace. With the word 'list' you can select a certain
screen for editing and load its information from the RAM disk
into the workspace. The figure below shows you how to transfer
information between the screenfile, the RAM disk and the
workspace (figure [cap:Editor-architecture]).
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/Workspace.eps>
[Senseless!!!
Editor architecture<cap:Editor-architecture>
]
]When you enter the editor the file is automatically loaded into
the RAM disk. With 'list' you transfer the source from a screen
in the RAM disk into the workspace. Since we started a new file
(that's why you got the error message) all screens are empty.
That why we cleared screen 0 and selected it for editing. You can
quit the editor without changes by pressing ”q” and hitting the
enter key.
4.5 Writing your first 4tH program
We start our program by giving it a name. Press ”s” and enter ”
hello.scr”. Now we're going to enter the source text, so we start
up the editor by pressing ”e” (you know by now you have to press
the enter key afterwards). Then we select screen 0 for editing by
entering:
0 clear
If you want to know what you've entered so far you can list the
editing screen by entering:
l
The editor will now show you a full listing:
Scr # 0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
^
0 OK
The first line will tell you which screen you're working on,
which is screen 0. Then all sixteen lines are listed, all blank
of course. Finally it will show you the current line, which is
line 0. The ”^” is the cursor, which is at the beginning of the
line. You can move the cursor around with the ”m” command. Try:
10 m
The editor will respond with:
^
0 OK
And shows you this way that the cursor has moved 10 positions. If
you want to move the cursor backwards, you can do that too. Just
enter a negative value, like:
-5 m
And the cursor will move back five positions:
^
0 OK
If you enter a larger value, that is perfectly acceptable too:
128 m
Note that every line is 64 characters long, so the editor will
tell you you've just moved to line 2:
^
2 OK
Don't be afraid that you'll do something wrong and lose your
source. Note that this is 4tH, not Forth. If you try something
funny like entering a very large value, the editor will just
issue an error message:
1024 m
Off screen OK
You just tried to go beyond the workspace and the editor won't
allow you to do that. Okay, we've moved around enough. How about
writing that program? You can enter text with the ”p” command,
which stands for ”PUT”. Just provide the editor with the
appropriate linenumber and the text:
0 p ." Hello world!" cr
Let's list our screen:
l
Scr # 0
0 ." Hello world!" cr
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
^." Hello world!" cr
0 OK
That's it. That's it? What about all that red tape like ”Program
Hello” or ”int main()”, opening parenthesis or closing braces?
Hey, this is Forth[footnote:
If you are not familiar with Forth and want to learn it, please
read the primer. Everything you want to know is explained there
in detail.
], not C or something. You've just told the compiler it has to
print the text ”Hello world!” and write a newline. Isn't that
what you wanted?
According to the figure in section 3, we first have to save the
workspace in the RAM disk by entering ”flush”, then save it to
disk by entering ”write” and subsequently leave the editor by
entering ”q”. Although perfectly correct, it is a lot of typing
for just saving and exiting. You can do that a lot faster by just
entering ”wq”, which stands for ”Write and Quit”.
Now we're back in the main menu and we want to see our program
run. Just hit ”R” and press enter. Don't we have to compile it
first? Sure, but 4tH will notice your program hasn't been
compiled yet and will compile it automatically for you. If you
get an error message like this:
Compiling; Word 0: Undefined name
Then you know you've just made a classical beginners error: there
is a space between .” and the text. You'll have to go back to the
editor to correct it. Reload screen 0 by entering:
0 list
Scr # 0
0 ."Hello world!" cr
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
OK
Now let's see where our cursor is:
0 m
^."Hello world!" cr
0 OK
Now we know we have to move our cursor two positions and enter a
space. Entering text at the cursor position is done by the ”c”
command, which stands for ”COPY”. Note that you have to add a
space after each command, so adding a space at the cursor
position is done by entering a ”c” with two spaces:
2 m
."^Hello world!" cr
0 OK
c
." ^Hello world!" cr
0 OK
Now we can exit the editor again and rerun our program. Yes, 4tH
will know you've changed the text and recompile your program
automatically:
wq
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>r
Hello world!
That's it! You've just successfully entered, compiled and ran
your very first 4tH program!
4.6 A more complex program
Note that this is not a tutorial on Forth. If you do not know the
language you'll probably won't understand the statements we're
going to enter. You don't have to, but if you need to please
refer to our highly acclaimed 4tH primer.
Okay, let's presume you're looking at your 4tH prompt. We want to
write a program which converts Unix ASCII files to DOS ASCII
files. Unix ASCII files use a single linefeed to signify the end
of a line while DOS ASCII files use an carriage return/linefeed
pair for that purpose.
First, we need to name our program, so we press ”s” to enter the
name of the screen file. We'll call it ”convert.scr”. Then we
enter the editor by pressing ”e” and are greeted by the ”OK”
prompt. First we'll define a word (that's what a subroutine is
called in Forth) that converts a file:
0 clear
0 p : ProcessFile
1 p begin
2 p refill
3 p while
4 p 0 parse-word
5 p type 13 emit 10 emit
6 p repeat
7 p ;
Note that 4tH confirms you after each line that everything is ”OK”
, but we left those messages out. When we list our program it
looks like this:
l
Scr # 0
0 : ProcessFile
1 begin
2 refill
3 while
4 0 parse-word
5 type 13 emit 10 emit
6 repeat
7 ;
8
9
10
11
12
13
14
15
^: ProcessFile
0 OK
It is a good custom to start each screen with a comment line, so
others will know what we've been doing. However, line 0 is
already taken. To insert a blank line we use the ”s” command,
which stands for ”SPREAD”. All lines following it will move down.
If you happen to use line 15 you're in trouble since that one
will be lost:
0 s 0 s l
Scr # 0
0
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11
12
13
14
15
^
0 OK
Yes, as long as you're not entering a command with a trailing
text parameter, you can enter multiple commands on a single line.
So this one tells the editor ”spread at line 0, spread at line 0,
list”. Now we're going to enter our comment line:
0 p ( Conversion from UNIX ASCII files to DOS ASCII files - I)
OK
l
Scr # 0
0 ( Conversion from UNIX ASCII files to DOS ASCII files - I)
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11
12
13
14
15
^( Conversion from UNIX ASCII files to DOS ASCII files - I)
0 OK
That will do nicely. Although this word will do the job, we still
have to open the input- and the output file. Since we want to
test our program quickly we make a quick and dirty word that will
do the job:
11 p : test s" code.txt" inpud open s" out.txt" outpud open
12 p error? rot error? rot or abort" Error!" use use;
13 p ttest ProcessFile
wq
When we try to compile this program by entering ”c”, it doesn't
work:
Compiling; Word 17: Undefined name
Oops, we've obviously made an error, but where? Word 17? Where is
word 17? We can find that out by decompiling the program and see
where it went wrong. Just press ”d”:
Object size: 17 words
String size: 9 chars
Variables : 0 cells
Strings : 0 chars
Reliable : No
[ 8] type (0)
[ 9] literal (13)
[ 10] emit (0)
[ 11] literal (10)
[ 12] emit (0)
[ 13] branch (0)
[ 14] exit (0)
[ 15] branch (0)
[ 16] s" (0) code.txt
The last thing it compiled was the start of the 'TEST'
definition. It must have gone wrong right after that one. So we
go back to the editor and find out. Sure, ”inpud” must be ”input”
. We can even find out it we made more errors like this:
f pud
: test s" code.txt" inpud^ open s" out.txt" outpud open
11 OK
n
: test s" code.txt" inpud open s" out.txt" outpud^ open
11 OK
n
Not found OK
And yes, we did. On lines eleven and twelve to be exact. With the
”f” command (which stand for ”FIND”) we can find a string. By
entering ”n” (which stands for ”NEXT”) we can find the same text
again. Now we have to correct it. We'll get back to the top of
the screen and find the offending word:
top f pud
: test s" code.txt" inpud^ open s" out.txt" outpud open
11 OK
Note that the cursor is positioned at the end of ”input”. We only
have to wipe one character and insert the correct one:
1 w c t
: test s" code.txt" inpu^ open s" out.txt" outpud open
11
: test s" code.txt" input^ open s" out.txt" outpud open
11 OK
With the command ”1 w” we destructively backup the cursor by one
position. Then we enter the 't' at the cursor position by using
the ”c” command. However, there is a quicker way to do this:
x pud
: test s" code.txt" input open s" out.txt" out^ open
11 OK
c put
: test s" code.txt" input open s" out.txt" output^ open
11 OK
The ”x” command works very much like ”f”, but it does not only
find the string, it also deletes it. Still, there are other
errors left in the source:
f test
ttest^ ProcessFile
13 OK
b
t^test ProcessFile
13 OK
1 w
Yes, ”test” has an extra ”t”. So we find the next occurrance of ”
test”. Note that a search is always performed from the cursor
position, so the definition of ”test” is not found. The ”b”
command will move the cursor backwards up to the point where ”
test” begins and we can delete the superfluous ”t” with the
command ”1 w”. The final typo we have to correct is a lacking
space between ”then” and the semicolon. That can be fixed pretty
quickly:
top f use
error? rot error? rot or abort" Error!" use^ use;
12 OK
n
error? rot error? rot or abort" Error!" use use^;
12 OK
till ;
error? rot error? rot or abort" Error!" use use^
12 OK
c ;
error? rot error? rot or abort" Error!" use use ;^
12 OK
Now the cursor is positioned right after then. The ”till” command
deletes everything from the current cursor position (indicated by
the caret, remember?) to the end of the following string. In this
case the semicolon but you can use any string. Finally, we copy
the correct string into the text, which is a space followed by a
semicolon. Four errors corrected. Let's write the screen back to
RAM disk and see what we have got:
flush l
Scr # 0
0 ( Conversion from UNIX ASCII files to DOS ASCII files - I)
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11 : test s" code.txt" input open s" out.txt" output open
12 error? rot error? rot or abort" Error!" use use ;
13 test ProcessFile
14
15
^test ProcessFile
13 OK
Seems to be okay. Let's go back to the main 4tH screen by issuing
the ”wq” command. We recompile the source by pressing ”c” and
presto: we got a program! Simply hit ”r” to run it. After we've
run the program we find a file named ”out.txt” in our working
directory and examine it with a hex editor:
4261 6445 7865 6375 7465 2028 202d 2d20
2920 2d34 2045 5845 4355 5445 203b 0d0a
3a20 4261 6441 6464 7265 7373 2028 202d
2d20 2920 2d34 2040 2044 524f 5020 3b0d
0a3a 2042 6164 416c 6967 6e20 2820 2d2d
2029 2031 2040 2044 524f 5020 3b0d 0a
It seems our program is working perfectly. However, it doesn't
seem very practical to copy a textfile to your working directory,
rename it, start up 4tH, load your program and finally run it.
That can be fixed. How? Well, you'll read that in the next
section.
But first we have to stress that you don't have to use 4tHs
editor. You can use any editor you like. Shame you've already
entered and saved your source. But there is a way out. And you
don't have to go too far. Just start up the editor again and
enter:
OK
export convert.4th
OK
You'll find an ordinary file called ”convert.4th” in your working
directory hat you can modify with any text editor you like.
4.7 Advanced features
What we actually want is a program we can run from the prompt,
something like:
convert in.txt out.txt
And if you do not provide the required parameters it has to issue
an error message:
Usage: convert infile outfile
We will get there, but we still have some coding to do. First of
all, we have to structure our program. We already have a working
word[footnote:
A subroutine in Forth is called a ”word”, remember?
] called ”ProcessFile”. It seems like a good idea to define two
others, one that opens the files and one that closes the files.
And we have to get rid of our ”test” word. So let's fire up the
editor and take care of that right now:
OK
0 list
Scr # 0
0 ( Conversion from UNIX ASCII files to DOS ASCII files - I)
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11 : test s" code.txt" input open s" out.txt" output open
12 error? rot error? rot or abort" Error!" use use ;
13 test ProcessFile
14
15
OK
11 d l
Scr # 0
0 ( Conversion from UNIX ASCII files to DOS ASCII files - I)
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11 error? rot error? rot or abort" Error!" use use ;
12 test ProcessFile
13
14
15
^( Conversion from UNIX ASCII files to DOS ASCII files - I)
0 OK
You can remove lines with the ”d” command, which stands for ”
DELETE”. This will remove the line and move all remaining lines
up. Line 15 becomes blank. But there is another way to get rid of
unwanted lines:
11 e l
Scr # 0
0 ( Convert UNIX ASCII files to DOS ASCII files - I)
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11
12 test ProcessFile
13
14
15
^( Convert UNIX ASCII files to DOS ASCII files - I)
0 OK
The ”e” command, which stands for ”ERASE”, will leave every line
at exactly the same position. It just blanks that line. Let's
finish this:
11 p : Convert OpenFiles ProcessFile ;
OK
12 e
OK
13 p Convert
OK
l
Scr # 0
0 ( Convert UNIX ASCII files to DOS ASCII files - I)
1
2 : ProcessFile
3 begin
4 refill
5 while
6 0 parse-word
7 type 13 emit 10 emit
8 repeat
9 ;
10
11 : Convert OpenFiles ProcessFile ;
12
13 Convert
14
15
^( Convert UNIX ASCII files to DOS ASCII files - I)
0 OK
Seems neat enough, but we still haven't got a ”OpenFiles” word.
This has to be defined before ”Convert”, but do we have still
have room for that on screen 0? No, we haven't. Fortunately, you
can insert screens with the 4tH editor[footnote:
Note that this command is usually not available in other Forth
editors!
]. Don't forget to flush. That is not only a good practice when
you've visited the bathroom, but also when you're working with a
Forth editor:
flush 0 insert
OK
We start our screen with a comment of course. We'll use the same
comment as in our previous screen, so why not copy it?
1 list 0 h 0 list 0 r l
Scr # 0
0 ( Convert UNIX ASCII files to DOS ASCII files - I)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
^( Convert UNIX ASCII files to DOS ASCII files - I)
0 OK
What did we do here? First, we switched to screen 1, which is our
previous screen 0. Then we used the ”h”[footnote:
In case you wondered, ”h” stands for ”HOLD”.
] command, which copied line 0 into PAD. PAD is a buffer, which
is able to hold the contents of a single line. Note that line 0
of screen 1 remains intact. It is only copied.
Then we switched back to screen 0 and issued the ”r” command,
which stands for ”REPLACE”. It replaces whatever is there with
the contents of the PAD. Finally, we listed the screen. Let's
play around a little with this PAD thing:
1 r 1 t
^( Convert UNIX ASCII files to DOS ASCII files - I)
1 OK
Yes, the line we copied was still in PAD! We also used the
command ”t” to ”TYPE” line 1. This command is very similar to ”h”
, since it copies line 1 to PAD. But is also moves the cursor to
the beginning of the line and types it. Let's see if you can
explain this one:
1 d 2 r 2 t
^( Convert UNIX ASCII files to DOS ASCII files - I)
2 OK
Sure, the ”d” command not only deletes the line, it also copies
it to PAD. So when the ”r” command is issued, it replaces line 2
with the contents of the line we deleted. Let's do one final
test:
0 i l
Scr # 0
0 ( Convert UNIX ASCII files to DOS ASCII files - I)
1 ( Convert UNIX ASCII files to DOS ASCII files - I)
2
3 ( Convert UNIX ASCII files to DOS ASCII files - I)
4
5
6
7
8
9
10
11
12
13
14
15
^
2 OK
Here we used the ”i” command, which stands for ”INSERT”. It
inserted the contents of PAD at line 0 and moved all the
remaining lines down. Note that the cursor didn't move a bit.
That's enough play for one day, let's get back to work:
1 e 3 e 2 p : OpenFile
OK
3 p args 2dup 2>r rot open error?
OK
4 p if
OK
5 p drop ." Cannot open " 2r> type cr abort
OK
6 p else
OK
7 p dup use 2r> 2drop
OK
8 p then
OK
9 p ;
OK
l
Scr # 0
0 ( Conversion from UNIX ASCII files to DOS ASCII files - I)
1
2 : OpenFile
3 args 2dup 2>r rot open error?
4 if
5 drop ." Cannot open " 2r> type cr abort
6 else
7 dup use 2r> 2drop
8 then
9 ;
10
11
12
13
14
15
^( Conversion from UNIX ASCII files to DOS ASCII files - I)
0 OK
Hmm, it seems like we're going to need another screen. It is
always wise to leave some room for future extensions, so this
screen is full enough. But don't forget the commentline. We don't
want to enter that one again, so let's store it in PAD:
0 h flush 1 insert 0 r l
Scr # 1
0 ( Convert UNIX ASCII files to DOS ASCII files - I)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
^
2 OK
Hold the line in PAD, flush the screen, insert screen 1 and
replace line 0 with the contents in PAD. But the commentline is
not entirely correct, so let's fix it:
top x I)
( Convert UNIX ASCII files to DOS ASCII files - ^
0 OK
c II)
( Convert UNIX ASCII files to DOS ASCII files - II)^
0 OK
The cursor is still on line 2, so we move it to the top again.
Then we find and delete ”I)”. Finally we copy in ”II)”. We can do
that since the cursor is at the right position. Now let's enter
our final word:
2 p : OpenFiles
OK
3 p argn 3 < abort" Usage: convert infile outfile"
OK
4 p input 1 OpenFile
OK
5 p output 2 Openfile
OK
6 p ;
OK
l
Scr # 1
0 ( Conversion from UNIX ASCII files to DOS ASCII files - II)
1
2 : OpenFiles
3 argn 3 < abort" Usage: convert infile outfile"
4 input 1 OpenFile
5 output 2 Openfile
6 ;
7
8
9
10
11
12
13
14
15
^( Conversion from UNIX ASCII files to DOS ASCII files - II)
0 OK
Almost there! We just have to fix the commentline in screen 2:
flush 2 list
OK
top x I)
( Convert UNIX ASCII files to DOS ASCII files - ^
0 OK
c III)
( Convert UNIX ASCII files to DOS ASCII files - III)^
0 OK
The current screen is flushed, then screen 2 is listed. We
position the cursor at the top, find and delete ”I)” and copy ”
III)” in at the cursor position. Done! Let's leave the editor and
see what we have got. It compiles cleanly and when we run it it
answers:
Usage: convert infile outfile
Sure, but what we actually want is to convert a file. Well, you
can do that too without leaving 4tH. Just press ”a” and enter the
filenames, just like you would do at the prompt:
(S)creen file: convert.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>a
Arguments: code.txt out.txt
When you press ”r” now, the arguments entered will be passed to
your 4tH program, just like they would at the prompt. To clear
the arguments, press ”a” again and just hit enter when prompted
for arguments.
But how do we run it from the prompt? Easy, just press ”o” and
enter ”convert.hx” at the prompt. Now press ”b”:
(S)creen file: convert.scr
(O)bject file: convert.hx
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>b
If 4tH has nothing to complain about, it doesn't complain, so you
can safely assume that everything is okay. Now we can go to the
prompt[footnote:
Windows users can do this by starting an MS-DOS session.
] and run it:
user@linux:~ > 4th lxq convert.hx code out.txt
Cannot open code
user@linux:~ >
That was to be expected. Our file was called ”code.txt”, not ”
code”. It is always a good idea to test all exceptions as well.
There could be a bug in that code too.
user@linux:~ > 4th lxq convert.hx code.txt out.txt
user@linux:~ >
Well, it seems to work.. But what we really want is a standalone
program. One that can be run without invoking 4tH and shared with
our friends and families. Why this ”.hx” thing? HX-files do have
their merits. First of all, it is very small, less than 200
bytes. But most importantly, you can take this file and run it on
a Windows NT, MS-DOS or other Unix machine without modification
or recompilation, provided a 4tH is available for that platform.
If you still want a standalone program, startup 4tH and reload ”
convert.scr”. Then press ”o” and enter ”convert.c”. Isn't that
the extension of a C-program? Yes, it is. 4tH is able to generate
C code. Just press ”g” and you've created a C program. You don't
even have to know C. If you know how to compile a C program
that's more than enough[footnote:
Windows users need to consult the documentation that came with
their C compiler. Some Windows compilers may not be able to
compile standard C programs. MS-DOS users are encouraged to use
the 'DJGPP' compiler, which is free.
]. We assume you've installed the 4tH library and header files,
since those are needed to compile ”convert.c”[footnote:
Read the ”Developers Guide” if you are not sure how to do this.
]:
user@linux:~ > cc -o convert convert.c -l4th
user@linux:~ > convert
Usage: convert infile outfile
user@linux:~ >
Is that all? No that's not all 4tH can do. We have a few
surprises left.
4.8 Suspending a program
We've entered this program:
Scr # 0
0 ." Is everybody in? The ceremony is about to begin.." cr
1 44596 36 base !
2 pause
3 ." Wake up! Do you remember where it was?" cr
4 ." Has this dream stopped? " . cr
5
6
7
8
9
10
11
12
13
14
15
The first line is simply a string we print to screen. The next
line, we push a number on the stack and we change the radix. Then
we go to sleep. After that, we wake up again, print a few lines
and retrieve the number on the stack. Let's run it:
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>r
Is everybody in? The ceremony is about to begin..
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>
At first, it seems like 'PAUSE' is nothing more than an alias for
'ABORT', but that is not entirely true. Let's save the executable
and enter ”r” one more time:
>b
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>r
Wake up! Do you remember where it was?
Has this dream stopped? YES
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>
Now the second part of the program is run, that is the part after
'PAUSE'. Note that both the stack and the radix have remained
intact. Every time 'PAUSE' is invoked, it will return you to the
prompt. When you enter ”r” again, it will continue where it left
off, until it meets one of the following three conditions:
1. It encounters another 'PAUSE'; entering ”r” will continue
where it left off.
2. It encounters 'ABORT', 'QUIT' or 'ABORT”'; entering ”r” will
restart the program.
3. There are no more instructions to execute; entering ”r” will
restart the program.
But why did we save an executable? We'll have to go back to the
shell to show you:
user@linux:~> 4th lxq out
Wake up! Do you remember where it was?
Has this dream stopped? YES
user@linux:~>
Entering ”b” during suspension will save the program in its
suspended state. When you run the resulting executable, it will
behave like you've entered ”r”. That's neat, isn't it?
4.9 Calculator mode
Startup 4tH and enter the editor. We're going to show you this
baby can do a lot more than just editing:
OK
.( Hello world!) cr
Hello world!
OK
Hey, that is a lot like the very first program we ran! Yes, it
is. You can enter a subset of the 4tH language at the editor
prompt, so you can test simple programs like this without getting
into the ”edit-compile-run” cycle. You can even make some simple
calculations:
OK
23 45 + .
68 OK
Simple? Aren't the operators and operands entered in the wrong
order? No, they aren't. 4tH uses Reverse Polish Notation, which
is also used by HP calculators and the Unix ”dc” command. 4tH has
even eight built-in variables in which you can store numbers:
23 45 + A. !
OK
A. ?
68 OK
It even understands binary, hexadecimal and octal numbers:
23 45 + binary .
1000100 OK
1000011111 hex FACE octal 765 + + decimal .
65250 OK
This is called the ”calculator mode” and you don't have to do
anything if you want to use it. It is part of the editor command
set. You can mix editor commands and calculations[footnote:
The full calculator command set is listed in the ”Editor
reference guide”.
] as you like. Nice extra, isn't it?
4.10 Epilogue
This concludes our tour of the 4tH interactive mode. We hope
we've shown you what you can do with it and how to use it. Of
course, you don't have to use 4tH's interactive mode. It will
happily reside and cooperate with existing external IDE's,
editors and the like. But if memory is tight and you have nothing
else, 4tH will prove to be a completely selfcontained
environment.
If you're still wondering what you can do with Forth and 4tH in
particular, let me tell you this: if you worked your way through
this tour, you've been working with Forth all the time. The
entire editor is a 4tH program, embedded in the 4tH executable,
taking up less than 3 KB. It is run by the very same interpreter
as your initial ”Hello world!” program. Have fun!
Frequently asked questions
-------------------------------------------
Question: Why has exec_4th() this enormous switch() statement?
Why wasn't a structure used with pointers to functions?
Answer: That one was built too, but it proved to be upto four
times slower on all platforms. Which is perfectly understandable,
because every time you evaluate a token, you have to take the
overhead of calling a function into account.
-------------------------------------------
Question: Why are the tokens of exec_4th() listed in a random
order?
Answer: We have statistically analyzed which tokens are used more
often. They are up front. Some C-compilers generate a jumptable.
Others generate a repeated "if .. elif .. endif" construction.
Compilants produced by the latter perform better when tokens are
ordered this way.
-------------------------------------------
Question: Do I have to use 4tHs builtin editor?
Answer: No. You can use any editor you like. 4tH will happily
compile all vanilla MS-DOS, MS-Windows and Unix text files and
most block files as well. Just use your favorite editor to create
your source and compile it at the command line. Note the editor
is capable of exporting vanilla text files.
-------------------------------------------
Question: I get ”I/O error” all the time. What am I doing wrong?
Answer: If you're working with Windows and you're trying to
compile some example programs, type 'S' and enter the relative
path to the example file, e.g. ”examples/romans.4th”. If you're
working on some other Operating System, be sure you've also
installed the library files. Now go to the directory where you
have installed them, e.g. ”cd /home/john/4th/lib” and type ”cd ..”
. From this working directory use the relative or absolute path
to your source file when you compile it. Be sure the path of the
environment variable DIR4TH is correct.
-------------------------------------------
Question: When I use 4tHs builtin editor I get ”Cannot open file”
all the time. What am I doing wrong?
Answer: Probably nothing. 4tH just informs you it cannot load the
file you issued at the main menu or the command line. This is
always the case when you start a new file.
-------------------------------------------
Question: When I try to load a screen file or execute an .hx
file, 4tH doesn't seem to take the DIR4TH environment variable
into account.
Answer: The DIR4TH environment variable is used by the compiler
when it tries to pull all source files from their different
locations at compiletime. Note that it only reads files. When the
editor would start writing files where you don't expect it to,
things might get very dangerous. The .hx files are executables
and your Operating System already offers several ways to find
them.
-------------------------------------------
Question: When I open up the editor in 4tH, it takes most 4tH
code like an actual Forth compiler, but not my colon definitions.
Why?
Answer: The 4tH editor mimics Forth, that's true. But it is
actually a Forth like environment on top of 4tH. It may seem like
you're working on a Forth prompt, but you're not. You can use the
editor only for editing or some quick calculations, but if you
want to use the full capability of 4tH, you're stuck to the menu.
-------------------------------------------
Question: I defined a word called mymultiplications and it
compiles fine. But if I want to use it, the compiler complains it
is an undefined word.
Answer: There is a maximum length to names. You can find that
length by compiling and executing this small program: ”WIDTH . CR”
. It compiles fine, because 4tH automatically clips names when
compiling. But when you start to use it, it can't be found in the
symbol table. Hence, 4tH reports it is an undefined word. Just
trim down your name to an acceptable length and it will compile
just fine.
Primer
Introduction
Don't you hate it? You've just got a new programming language and
you're trying to write your first program. You want to use a
certain feature (you know it's got to be there) and you can't
find it in the manual.
I've had that experience many times. So when I wrote 4tH I
promised myself, that would not happen to 4tH-users. In this
manual you will find many short features on all kind of topics.
How to input a number from the keyboard, what a cell is, etc.
I hope this will enable you to get quickly on your way. If it
didn't, email me at '[email protected]'. You will not only get
an answer, but you will help future 4tH users as well.
You can use this manual two ways. You can either just get what
you need or work your way through. Every section builds on the
knowledge you obtained in the previous sections. All sections are
grouped into levels. We advise you to use what you've learned
after you've worked your way through a level.
There are five levels. First, 4tH fundamentals. It assumes a
working knowledge of programming and covers the basics. Second,
4tH arrays. We'll try to explain to you what an address is and
teach you basic string handling.
Third, 4tHs Character Segment. We'll explain you how it is laid
out and what you can do with it. Fourth, 4tHs Integer Segment and
Code Segment. We'll explain you how it is laid out and what you
can do with it.
Finally, advanced programming techniques. We'll teach you how to
program multilevel exits, write interpreters, use jump-tables,
emulate floating point calculation and a lot more!!
I don't think it is enough to teach you Forth, from which 4tH was
derived, but you can always get a good textbook on Forth, like
"Starting Forth" by Leo Brodie. Have fun!
4tH fundamentals
7.1 Making calculations without parentheses
To use 4tH you must understand Reverse Polish Notation. This is a
way to write arithmetic expressions. The form is a bit tricky for
people to understand, since it is geared towards making it easy
for the computer to perform calculations; however, most people
can get used to the notation with a bit of practice.
Reverse Polish Notation stores values in a stack. A stack of
values is just like a stack of books: one value is placed on top
of another. When you want to perform a calculation, the
calculation uses the top numbers on the stack. For example,
here's a typical addition operation:
1 2 +
When 4tH reads a number, it just puts the value onto the stack.
Thus 1 goes on the stack, then 2 goes on the stack. When you put
a value onto the stack, we say that you push it onto the stack.
When 4tH reads the operator '+', it takes the top two values off
the stack, adds them, then pushes the result back onto the stack.
This means that the stack contains:
3
after the above addition. As another example, consider:
2 3 4 + *
(The '*' stands for multiplication.) 4tH begins by pushing the
three numbers onto the stack. When it finds the '+', it takes the
top two numbers off the stack and adds them. (Taking a value off
the stack is called popping the stack.) 4tH then pushes the
result of the addition back onto the stack in place of the two
numbers. Thus the stack contains:
2 7
When 4tH finds the '*' operator, it again pops the top two values
off the stack. It multiplies them, then pushes the result back
onto the stack, leaving:
14
The following list gives a few more examples of Reverse Polish
expressions. After each, we show the contents of the stack, in
parentheses.
7 2 - (5)
2 7 - (-5)
12 3 / (4)
-12 3 / (-4)
4 5 + 2 * (18)
4 5 2 + * (28)
4 5 2 * - (-6)
7.2 Manipulating the stack
You will often find that the items on the stack are not in the
right order or that you need a copy. There are stack-manipulators
which can take care of that.
To display a number you use '.', pronounced "dot". It takes a
number from the stack and displays it. 'SWAP' reverses the order
of two items on the stack. If we enter
2 3 . . cr
4tH answers:
3 2
If you want to display the numbers in the same order as you
entered them, you have to enter:
2 3 swap . . cr
In that case 4tH will answer:
2 3
You can duplicate a number using 'DUP'. If you enter:
2 . . cr
4tH will complain that the stack is empty. However, if you enter:
2 dup . . cr
4tH will display:
2 2
Another way to duplicate a number is using 'OVER'. In that case
not the topmost number of the stack is duplicated, but the number
beneath. E.g.
2 3 dup . . . cr
will give you the following result:
3 3 2
But this one:
2 3 over . . . cr
will give you:
2 3 2
Sometimes you want to discard a number, e.g. you duplicated it to
check a condition, but since the test failed, you don't need it
anymore. 'DROP' is the word we use to discard numbers. So this:
2 3 drop .
will give you "2" instead of "3", since we dropped the "3".
The final one I want to introduce is 'ROT'. Most users find 'ROT'
the most complex one since it has its effects deep in the stack.
The thirdmost item to be exact. This item is taken from its place
and put on top of the stack. It is 'rotated', as this small
program will show you:
1 2 3 \ 1 is the thirdmost item
. . . cr \ display all numbers
( This will display '3 2 1' as expected)
1 2 3 \ same numbers stacked
rot \ performs a 'ROT'
. . . cr \ same operation
( This will display '1 3 2'!)
7.3 Deep stack manipulators
No, there are no manipulators that can dig deeper into the stack.
A stack is NOT an array! So if there are some Forth-83 users out
there, I can only tell you: learn Forth the proper way. Programs
that have so many items on the stack are just badly written. Leo
Brodie agrees with me.
If you are in 'deep' trouble you can always use the returnstack
manipulators. Check out that section.
7.4 Passing arguments to functions
There is no easier way to pass arguments to functions as in 4tH.
Functions have another name in 4tH. We call them "words". Words
take their "arguments" from the stack and leave the "result" on
the stack.
Other languages, like C, do exactly the same. But they hide the
process from you. Because passing data to the stack is made
explicit in 4tH it has powerful capabilities. In other languages,
you can get back only one result. In 4tH you can get back
several!
All words in 4tH have a stack-effect-diagram. It describes what
data is passed to the stack in what order and what is returned.
The word '*' for instance takes numbers from the stack,
multiplies them and leaves the result on the stack. It's
stack-effect-diagram is:
n1 n2 -- n3
Meaning it takes number n1 and n2 from the stack, multiplies them
and leaves the product (number n3) on the stack. The rightmost
number is always on top of the stack, which means it is the first
number which will be taken from the stack. The word '.' is
described like this:
n --
Which means it takes a number from the stack and leaves nothing.
Now we get to the most powerful feature of it all. Take this
program:
2 ( leaves a number on the stack)
3 ( leaves a number on the stack on top of the 2)
* ( takes both from the stack and leaves the result)
. ( takes the result from the stack and displays it)
Note that all data between the words '*' and '.' is passed
implicitly! Like putting LEGO stones on top of another. Isn't it
great?
7.5 Making your own words
Of course, every serious language has to have a capability to
extend it. So has 4tH. The only thing you have to do is to
determine what name you want to give it. Let's say you want to
make a word which multiplies two numbers and displays the result.
Well, that's easy. We've already seen how you have to code it.
The only words you need are '*' and '.'. You can't name it '*'
because that name is already taken. You could name it 'multiply',
but is that a word you want to type in forever? No, far too long.
Let's call it '*.'. Is that a valid name? If you've programmed in
other languages, you'll probably say it isn't. But it is! The
only characters you can't use in a name are whitespace characters
(<CR>, <LF>, <space>, <TAB>). Note that 4tH is not
case-sensitive!
So '*.' is okay. Now how do we turn it into a self-defined word.
Just add a colon at the beginning and a semi-colon at the end:
: *. * . ;
That's it. Your word is ready for use. So instead of:
2 3 * .
We can type:
: *. * . ;
2 3 *.
And we can use our '*.' over and over again. Hurray, you've just
defined your first word in 4tH!
7.6 Adding comment
Adding comment is very simple. In fact, there are two ways to add
comment in 4tH. That is because we like programs with a lot of
comments.
You've already encountered the first form. Let's say we want to
add comment to this little program:
: *. * . ;
2 3 *.
So we add our comment:
: *. * . ; This will multiply and print two numbers
2 3 *.
4tH will not understand this. It will desperately look for the
words 'this', 'will', etc. However the word '\' will mark
everything up to the end of the line as comment. So this will
work:
: *. * . ; \ This will multiply and print two numbers
2 3 *.
There is another word called '(' which will mark everything up to
the next ')' as comment. Yes, even multiple lines. Of course,
these lines may not contain a ')' or you'll make 4tH very
confused. So this comment will be recognized too:
: *. * . ; ( This will multiply and print two numbers)
2 3 *.
Note that there is a whitespace-character after both '\' and '('.
This is mandatory! However the closing paren ) does not have to
have a leading blank space. It is optional.
7.7 Text-format of 4tH source
4tH source is a simple ASCII-file. And you can use any layout as
long a this rule is followed:
All words are separated by at least one whitespace character!
Well, in 4tH everything is a word or becoming a word. Yes, even
'\' and '(' are words! And you can add all the empty lines or
spaces or tabs you like, 4tH won't care and your harddisk
supplier either.
7.8 Displaying string literals
Displaying a string is as easy as adding a comment. Let's say you
want to make the ultimate program, one that is displaying "Hello
world!". Well, that's almost the entire program. The famous
'hello world' program is simply this in 4tH:
." Hello world!"
Compile this and it works. Yes, that's it! No declaration that
this is the main function and it is beginning here and ending
there. May be you think it looks funny on the display. Well, you
can add a carriage return by adding the word 'CR'. So now it
looks like:
." Hello world!" cr
Still pretty simple, huh?
7.9 Creating variables
One time or another you're going to need variables. Declaring a
variable is easy.
variable one
The same rules for declaring words apply for variables. You can't
use a name that already has been taken. A variable is a word too!
And whitespace characters are not allowed. Note that 4tH is not
case-sensitive!
7.10 Using variables
Of course variables are of little use when you could not assign
values to them. This assigns the number 6 to variable 'ONE':
6 one !
We don't call '!' bang or something like that, we call it
'store'. Of course you don't have to put a number on the stack to
use it, you can use a number that is already on the stack. To
retrieve the value stored in 'ONE' we use:
one @
The word '@' is called 'fetch' and it puts the number stored in
'one' on the stack. To display it you use '.':
one @ .
There is a shortcut for that, the word '?', which will fetch the
number stored in 'ONE' and displays it:
one ?
7.11 Built-in variables
4tH has only three built-in variables. They are called 'BASE',
'>IN' and 'OUT'. 'BASE' controls the radix at run-time, '>IN' is
used by 'WORD' and 'OUT' returns a value to the host program.
7.12 What is a cell?
A cell is simply the space a number takes up. So the size of a
variable is one cell. The size of a cell is important since it
determines the range 4tH can handle. It also helps make code
portable across machines with different cell sized, for example
16 bit and 32 big systems. We'll come to that further on.
7.13 What is a literal expression?
A literal expression is simply anything that compiles to a
literal. All numbers, all defined constants and some expressions
are compiled to a literal. In the glossary you can find what
compiles to a literal, but we list them here too:
' <name>
['] <name>
CHAR <char>
[CHAR] <char>
<literal> [NOT]
<literal> <literal> [*]
<literal> <literal> [+]
<literal> <literal> [=]
[DEFINED] <name>
[UNDEFINED] <name>
7.14 Declaring arrays of numbers
You can make arrays of numbers very easily. It is very much like
making a variable. Let's say we want an array of 16 numbers:
16 array sixteen
That's it, we're done! You must omit the word 'CELLS', since
'ARRAY' implicates that you want an array of numbers, not
characters. The size is a literal expression. You can't take it
from the stack or calculate it, so this is invalid:
3 5 * 1+ array sixteen
4tH will let you know that this is not a valid construction, but
in case you wonder.. By the way, 4tH allows you size an array
just like that as we will learn later on.
7.15 Using arrays of numbers
You can use arrays of numbers just like variables. The array
cells are numbered from 0 to N, N being the size of the array
minus one. Storing a value in the 0th cell is easy. It works just
like a simple variable:
5 sixteen 0 th !
Which will store '5' in the 0th cell. So storing '7' in the 8th
cell is done like this:
7 sixteen 8 th !
Of course when you want to store a value in the first, second or
third cell you have to use 'TH' too, since it is a word. If you
don't like that try defining 'ST', 'ND' and 'RD' yourself:
: st th ;
: nd th ;
: rd th ;
4 sixteen 1 st !
5 sixteen 2 nd !
6 sixteen 3 rd !
Isn't 4tH wonderful? Fetching is done the same of course:
sixteen 0 th @
sixteen 4 th @
Plain and easy.
7.16 Declaring and using constants
Declaring a simple constant is easy too. Let's say we want to
make a constant called 'FIVE':
5 constant five
Now you can use 'FIVE' like you would '5'. E.g. this will print
five spaces:
five spaces
The same rules for declaring words apply for constants. You can't
use a name that already has been taken. A constant is a word too!
And whitespace characters are not allowed. Note that 4tH is not
case-sensitive. By the way, '5' is a literal expression. You
can't take it from the stack or calculate it.
7.17 Built-in constants
There are several built-in constants. Of course, they are all
literals in case you wonder. Here's a list. Refer to the glossary
for a more detailed description:
/PAD
/TIB
/HOLD
/CELL
/CHAR
MAX-N
MAX-RAND
(ERROR)
BL
FALSE
LO
APP
PAD
STACK-CELLS
TIB
TRUE
VARS
WIDTH
INPUT
OUTPUT
STDOUT
STDIN
TEXT
APPEND
PIPE
FILES
4TH#
7.18 Using booleans
Booleans are expressions or values that are either true or false.
They are used to conditionally execute parts of your program. In
4tH a value is false when it is zero and true when it is
non-zero. Most booleans come into existence when you do
comparisons. This example will determine whether the value in
variable 'VAR' is greater than 5. Try to predict whether it will
evaluate to true or false:
variable var
4 var !
var @ 5 > .
No, it wasn't! But hey, you can print booleans as numbers. Well,
they are numbers. But with a special meaning as we will see in
the next section.
7.19 IF-ELSE constructs
Like most other languages you can use IF-ELSE constructs. Let's
enhance our previous example:
variable var
4 var !
var @ 5 >
if ." Greater" cr
else ." Less or equal" cr
then
So now our program tells you when it's greater and when not. Note
that contrary to other languages the condition comes before the
'IF' and 'THEN' ends the IF-clause. In other words, whatever path
the program takes, it always continues after the 'THEN'. A tip:
think of 'THEN' as 'ENDIF'..
7.20 FOR-NEXT constructs
4tH has FOR-NEXT constructs as well. The number of iterations is
known in this construct. E.g. let's print the numbers from 1 to
10:
11 1 do i . cr loop
The first number represents the limit. When the limit is reached
or exceeded the loop terminates. The second number presents the
initial value of the index. That's where it starts off. So
remember, this loop iterates at least once! You can use '?DO'
instead of 'DO'. That will not enter the loop if the limit and
the index are the same to begin with:
0 0 ?do i . cr loop
'i' represents the index. It is not a variable or a constant, it
is a predefined word, which puts the index on the stack, so '.'
can get it from the stack and print it.
But what if I want to increase the index by two? Or want to count
downwards? Is that possible. Sure. There is another construct to
do just that. Okay, let's take the first question:
11 1 do i . cr 2 +loop
This one will produce exactly what you asked for. An increment by
two. This one will produce all negative numbers from -1 to -10:
-11 -1 do i . cr -1 +loop
Note that the step is not a literal expression. You can change
the step if you want to, e.g.:
32767 1 do i . i +loop
This will print: 1, 2, 4, 8, all up to 16384. Pretty flexible, I
guess. You can break out of a loop by using 'LEAVE'. Note that
'LEAVE' only sets the index to the value of the limit: it doesn't
branch or anything. Make sure that there is no code left between
'LEAVE' and 'LOOP' that you don't want to execute. So this is
okay:
10 0 do i dup 5 = if drop leave else . cr then loop
And this is not:
10 0 do i dup 5 = if drop leave then . cr loop
Since it will still get past the '.' before leaving. In this case
you will catch the error quickly, because the stack is empty.
7.21 WHILE-DO constructs<sec:WHILE-DO>
A WHILE-DO construction is a construction that will perform zero
or more iterations. First a condition is checked, then the body
is executed. Then it will branch back to the condition. In 4tH it
looks like this:
BEGIN <condition> WHILE <body> REPEAT
The condition will have to evaluate to TRUE in order to execute
the body. If it evaluates to FALSE it branches to just after the
REPEAT. This example does a Fibbonaci test.
: fib 0 1
begin
dup >r rot dup r> > \ condition
while
rot rot dup rot + dup . \ body
repeat
drop drop drop ; \ after loop executed
You might not understand all of the commands, but we'll get to
that. If you enter "20 fib" you will get:
1 2 3 5 8 13 21
This construct is particularly handy if you are not sure that all
data will pass the condition.
7.22 REPEAT-UNTIL constructs<sec:REPEAT-UNTIL>
The counterpart of WHILE-DO constructs is the REPEAT-UNTIL
construct. This executes the body, then checks a condition at
'UNTIL'. If the expression evaluates to FALSE, it branches back
to the top of the body (marked by 'BEGIN') again. It executes at
least once. This program calculates the largest common divisor.
: lcd
begin
swap over mod \ body
dup 0= \ condition
until drop . ;
If you enter "27 21 lcd" the programs will answer "3".
7.23 Infinite loops<sec:Infinite-loops>
In order to make an infinite loop one could write:
begin ." Diamonds are forever" cr 0 until
But there is a nicer way to do just that:
begin ." Diamonds are forever" cr again
This will execute until the end of times, unless you exit the
program another way.
7.24 Including source files
4tH has a vocabulary of over 200 words. If you use them in one of
your 4tH programs 4tH will recognize them instantly. These words
are internal.
But if you take a look at the glossary, you'll find that there
are a lot of other words too. Words that 4tH will not recognize;
they have to be included first. These words are external.
These words are defined in an include file. An include file is
just an ordinary ASCII file with 4tH source. You can read them if
you want. In order to use these words, you have to tell 4tH where
it can find the include file.
This is done by the '[NEEDS' directive, which is equivalent to
the COMUS word 'INCLUDE' (which 4tH also supports). Everything up
to the next ”]” is considered to be a filename, so the path may
contain embedded spaces. You can use absolute paths or relative
paths, just make sure that you're starting 4tH from the proper
directory. E.g. this one includes additional ANS-Forth CORE-words
from the directory just above 'lib'[footnote:
If you're not sure where that is, enter the 'lib' directory and
execute “cd ..”.
]:
[needs lib/anscore.4th]
Or:
include lib/anscore.4th
4tH comes with a rich library of words, which covers a large part
of ANS-Forth and COMUS[footnote:
In case you wonder, COMUS stands for COMmon USage.
] standard words and beyond. They are all located in the 'lib'
directory. In the next level we're going to need a lot of these
words, so you'd better know how to include them.
7.25 Getting a number from the keyboard
The word to enter a number from the keyboard can be found in the
'lib' directory and is defined in the 'enter.4th' file. To
include it you have to tell 4tH. We assume your working directory
is just above the 'lib' directory[footnote:
As a matter of fact, we will always assume that! If you don't
know what we mean, execute ”cd <path to lib directory>” and then ”
cd ..”. Now you're there for sure!
]:
[needs lib/enter.4th]
That's all! Now you can use 'ENTER' just like any 4tH word. This
will allow you to enter a number and print it:
[needs lib/enter.4th]
enter . cr
By the way, this is the end of the first level. Take our advise
and give it a try!
4tH arrays
8.1 Aligning numbers
You may find that printing numbers in columns (I prefer
"right-aligned") can be pretty hard. That is because the standard
word to print numbers ('.') prints the number and then a trailing
space. That is why '.R' was added.
The word '.R' works just like '.' but instead of just printing
the number with a trailing space '.R' will print the number
right-aligned in a field of N characters wide. Try this and you
will see the difference:
140 . cr
150 5 .r cr
In this example the field is five characters wide, so '150' will
be printed with two leading spaces.
8.2 Creating arrays of constants
Making an array of constants is quite easy. First you have to
define the name of the array by using the word 'TABLE' or
'CREATE' (which is ANS-Forth). Then you specify all its elements.
Note that every element is a literal expression. All elements
(even the last) are terminated by the word ','. An example:
create sizes 18 , 21 , 24 , 27 , 30 , 255 ,
Please note that ',' is a word! It has to be separated by spaces
on both ends.
8.3 Using arrays of constants
Accessing an array of constants is very much like accessing an
array of numbers. In an array of numbers you access the 0th
element like this:
sixteen 0 th @
When you access the first element of an array of constants you
use this construction:
sizes 0 th @c
The only difference is the word '@C', which is exclusively used
to access arrays of constants.
8.4 Using values
A value is a cross-over between a variable and a constant. May be
this example will give you an idea:
declaration:
variable a ( No initial value)
1 constant b ( Literal expression assigned at
compiletime)
2 b + value c ( Expression assigned at runtime)
fetching:
a @ ( Variable throws address on stack)
b ( Constant throws value on stack)
c ( Value throws value on stack)
storing:
2 b + a ! ( Expression can be stored at runtime)
( Constant cannot be reassigned)
2 b + to c ( Expression can be stored at runtime)
In many aspects, values behave like variables and can replace
variables. The only thing you cannot do is make arrays of values.
A value is not a literal expression either, so you can't use them
to size arrays. In fact, a value is a variable that behaves in
certain aspects like a constant.
Why use a value at all? Well, there are situations where a value
can help:
• When converting Forth programs (replacing constants)
• When a constant can change during execution
Note that although 'VALUE' and 'TO' are aliases, it is more
portable and more readable to use 'VALUE' for declaration and
'TO' for reassignment. Note that each 'TO' or 'VALUE' consumes a
little memory when compiling, so reassignments have to be rare.
It is certainly not a good idea to replace all variables by
values.
8.5 Creating string variables
In 4tH you have to define the maximum length of the string, like
Pascal:
10 string name
You cannot add the 'CHARS' keyword, since 'STRING' already
implies that you are creating an array of characters. Note that
the string variable includes the terminator. That is a special
character that tells 4tH where the string ends (see section [StringTerm]
). You usually don't have to add that yourself because 4tH will
do that for you. But you will have to reserve space for it.
That means that the string "name" we just declared can contain up
to nine characters AND the terminator. These kind of strings are
usually referred to as ASCIIZ strings.
E.g. when you want to define a string that has to contain
"Hello!" (without the quotes) you have to define a string that is
at least 7 characters long:
7 string hello
8.6 What is an address?
An address is a location in memory. Usually, you don't need to
know addresses, because 4tH will take care of that. But if you
want it, you can retrieve them as we will show you later. Think
of memory like a city. It has roads and houses and inhabitants.
There are three roads in 4tH city:
1. Integer Segment, that is where the cells live;
2. Character Segment, that is where the strings live;
3. Code Segment, that is where the instructions that form your
program live.
If you want to visit a certain person, you go to the city where
he lives, find the right street and knock on the door. If you
want to retrieve a certain string or integer, you do the same.
When you define a string, you actually create a constant with the
address of that string. When you later refer to the string you
just defined its address is thrown on the stack. An address is
simply a number that refers to its location. As you will see you
can work with string-addresses without ever knowing what that
number is. But because it is a number you can manipulate it like
any other number. E.g. this is perfectly valid:
16 string hello
hello \ address of string on stack
dup \ duplicate it
drop drop \ drop them both
Later, we will tell you how to get "Hello!" into the string.
8.7 String literals
In 4tH a string literal is created by the word 'S”'. The word
'S"' is very much like '."', but instead of printing it to the
screen you will just be defining a string literal.
s" This is a string"
4tH is a stack oriented language, so what does 'S”' leave on the
stack? In 4tH, a string is usually represented by on the stack by
its address and its count. So in order to get its length, you
only have to get the first value on the stack. In order to get
its address you have to get the second value on the stack, which
is demonstrated by this small program:
s" This is a string" \ create a temporary string
." Length : " . cr \ show the length
." Address: " . cr \ show the address
And what about string literals with quotes. Easy, there is an
equivalent to 'S”' that does the same thing:
s| "This is a string with quotes"|
." Length : " . cr \ show the length
." Address: " . cr \ show the address
Instead of a quote, the string is delimited by a bar. And what
about string literals that include them both? Bad luck? Well,
almost but not quite. Just take a look at section [sec:Escape-characters]
.
8.8 String constants
String constants work the same way as numeric constants:
10 constant ten \ define a string constant
ten . cr \ equivalent to: 10 . cr
In fact, you give a name to a literal value. After that, you can
refer to that literal throughout your program by using its name.
String constants do the same thing. Take a look at this little
piece of code:
s" This is a string" \ create a temporary string
." Length : " . cr \ show the length
." Address: " . cr \ show the address
Now we do the same thing, but this time we define a string
constant by using 'SCONSTANT':
s" This is a string" sconstant mystring
\ define a string constant
mystring \ now we use the string constant
." Length : " . cr \ show the length
." Address: " . cr \ show the address
Why use string constants? Well, first of all, if you use a string
constant throughout your program, it will save you some editing
when you have to change your program for one reason or another.
Second, it will make your program a little smaller.
8.9 Initializing string variables<InitStringVars>
You can initialize a string with the 'S"' word. If you want the
string to contain your first name use this construction:
s" Hello!" name place
The word 'PLACE' copies the contents of a string literal into a
string-variable.
If you still don't understand it yet, don't worry. As long as you
use this construction, you'll get what you want. Just remember
that assigning a string literal to a string that is too short
will result in an error or even worse, corrupt other strings.
8.10 Getting the length of a string variable
You get the length of a string variable by using the word
'COUNT'. It will not only return the length of the string
variable, but also the string address. It is illustrated by this
short program:
32 string greeting \ define string greeting
s" Hello!" greeting place \ set string to 'Hello!'
greeting count \ get string length
." String length: " . cr \ print the length
drop \ discard the address
Most string handling words return or take an address/count pair.
One of the exceptions is the string variable itself (see section [InitStringVars]
). To copy the contents of an address/count pair represented
string into a string variable, we use 'PLACE'. In order to
convert a string variable back to an address/count pair
represented string, we use 'COUNT':
32 string my-string \ create a string variable
\ create an address/count
s" This is a string" \ pair represented string
my-string place \ copy it into the variable
my-string count \ convert it into an address/count
pair
." Length : " . cr \ show the length
." Address: " . cr \ show the address
Note that the contents of the string variable do not change by a
'COUNT' conversion!
8.11 Printing a string variable
Printing a string variable is pretty straight forward. The word
that is required to print a string variable is 'TYPE'. It
requires an address/count pair. Yes, that are the values that are
left on the stack by 'COUNT'! So printing a string means issuing
both 'COUNT' and 'TYPE':
32 string greeting \ define string greeting
s" Hello!" greeting place \ set string to 'Hello!'
greeting count type cr \ print the string
If you don't like this you can always define a word like
'PRINT$':
: print$ count type ;
32 string greeting \ define string greeting
s" Hello!" greeting place \ set string to 'Hello!'
greeting print$ cr \ print the string
8.12 Copying a string variable
You might want to copy one string variable to another. Let's take
a look at this example:
32 string one \ define the first string
32 string two \ define the second string
s" Greetings!" one place \ initialize string one
one count \ get the length of string one
two place \ and copy it into string two
two count type cr \ print string two
First we place the string ”Greetings!” into a string variable. 'S”
' will put an address/count pair on the stack, that is consumed
by 'PLACE'. Variable ”ONE” only puts its address on the stack,
that is converted into an address/count pair by 'COUNT'. After it
has been consumed again by 'PLACE' we need 'COUNT' again to
provide 'TYPE' with an address/count pair.
8.13 <StringTerm>The string terminator
In order for 'COUNT' to work, it has to know where the string
stops. So a special character at the end of the string, the
string terminator, is used to indicate the end of an ASCIIZ
string. It has nothing to do with Arnold Schwarzenegger
obliterating innocent strings! It is simply a character, having
the ASCII value zero. It may also be referred to as the
NULL-character. Although most strings in 4tH will be terminated
automatically it is considered bad style to rely on that.
8.14 Slicing strings
Slicing strings is just like copying strings. We just don't copy
all of it and we don't always start copying at the beginning of a
string. We'll show you what we mean:
[needs lib/anstring.4th]
32 string one \ define string one
s" Hans Bezemer" one place \ initialize string one
one count 2dup type cr \ duplicate and print it
1 /string \ move one character forward
2dup type cr \ duplicate and print it again
1 /string \ move one character forward
2dup type cr \ duplicate and print it again
1 /string \ move one character forward
type cr \ print it for the last time
First it will print "Hans Bezemer", then "ans Bezemer", then "ns
Bezemer" and finally "s Bezemer". The word '/STRING' adjusts the
address/count pair by a given number of characters, in this case
one character. It is part of the library member 'anstring.4th',
so we have to include that one. The word '2DUP' is much like
'DUP', but it copies the top two values on the stack. It is
functionally equivalent to:
over over
If we want to discard the first name at all we could even write:
[needs lib/anstring.4th]
32 string one \ define string one
s" Hans Bezemer" one place \ initialize string one
one count 5 /string type cr \ print sliced string
The five characters we want to skip are the first name (which is
four characters) and a space (which adds up to five). There is a
special word for slicing strings in the library member
'slice.4th'. You call it with:
address count position-to-start position-to-end
Both positions start counting at zero. So this will copy the
first name to string "two" and print it:
[needs lib/slice.4th]
32 string one \ declare string one
32 string two \ declare string two
s" Hans Bezemer" one place \ initialize string one
one count 0 3 slice \ slice the first name
two place \ copy it to string two
two count type cr \ print string two
This will slice the last name off and store it in string "two":
[needs lib/slice.4th]
32 string one \ declare string one
32 string two \ declare string two
s" Hans Bezemer" one place \ initialize string one
one count 5 11 slice \ slice the last name
two place \ copy it to string two
two count type cr \ print string two
Since the last name is seven characters long and starts at
position five (start counting with zero!).
8.15 Appending strings
The word '+PLACE[footnote:
There is a COMUS word called 'APPEND' which works exactly the
same.
]' appends two strings. In this example string "one" holds the
first name. The second string literal is appended to string "one"
to form the full name. Finally string "one" is printed.
32 string one \ define string one
s" Hans " one place \ initialize first string
s" Bezemer" one +place \ append 'Bezemer' to string
one count type cr \ print first string
8.16 Comparing strings
<ComparingStrings>If you ever sorted strings you know how
indispensable comparing strings is. As we mentioned before, there
are very few words in Forth that act on strings. Here is a word
that can compare two strings. It is located in the library member
'compare.4th'.
[needs lib/compare.4th]
\ compare two chars
32 string one \ define string one
s" Hans Bezemer" one place \ initialize string one
32 string two \ define string two
s" HANS BEZEMER" two place \ initialize string two
one count two count compare \ compare two strings
if
." Strings differ" \ message: strings ok
else
." Strings are the same" \ message: strings not ok
then
cr \ send CR
Simply pass two strings (represented by their address/count
pairs) to 'COMPARE' and it will return a TRUE flag when the
strings are different. This might seem a bit odd, but strcmp()
does exactly the same. If you don't like that you can always add
'0=' to the end of 'COMPARE' to reverse the flag.
You'll soon find out that ANS-Forth's 'COMPARE' is case
sensitive. Lucky for you, you can modify the behaviour of 4tH's
'COMPARE'. Just define this before the '[NEEDS' directive:
false constant ignorecase
[needs lib/compare.4th]
\ compare two chars
32 string one \ define string one
s" Hans Bezemer" one place \ initialize string one
32 string two \ define string two
s" HANS BEZEMER" two place \ initialize string two
one count two count compare \ compare two strings
if
." Strings differ" \ message: strings ok
else
." Strings are the same" \ message: strings not ok
then
cr \ send CR
Now 'COMPARE' will do a case sensitive comparison.
8.17 Finding a substring
Sometimes you need to find a string within a string. ANS-Forth
has defined a word for that too. It is called 'SEARCH'. You need
to include 'search.4th' in order to use it. Now lets find ”the”
in this string:
[needs lib/search.4th]
s" How the cow catches the hare"
s" the" search \ search for 'the'
0= if ." not " then ." found: "
type \ print the result
'SEARCH' always returns a flag and a address/count pair. If it
returns true, the substring was found; if it returns false, the
substring was not found. Now that's pretty straightforward, isn't
it? That means that the small program above will print:
found:
When the substring was found and:
not found:
When the substring was not found. But what kind of string does it
return when the substring was not found? Well, the entire string
you fed it, so this would have been its output if we had been
looking for the substring ”now” instead of ”the”:
not found: How the cow catches the hare
But in this specific example we are looking for ”the”. When
found, 'SEARCH' returns the string after the first occurrence of
the substring we were looking for:
found: the cow catches the hare
Why that? Why not a position? Well, first of all, you can look
for the same substring again:
[needs lib/search.4th]
s" How the cow catches the hare"
s" the" search drop \ drop the flag
2dup type \ print the string
s" the" search drop \ now search again
type \ print the string
This will print:
the cow catches the hare
the hare
But if you still want to see a position instead of a string, you
can simply define this:
[needs lib/search.4th]
: position
2>r over swap 2r> search 0= >r drop swap - r> if 1- then
;
s" How the cow catches the hare"
s" the" position . cr
That will take care of your problems. If the substring was found,
”POSITION” will return a positive number. If it wasn't found, it
will return a negative number. Note that 'SEARCH' can be
persuaded to do a case-sensitive comparison, just like 'COMPARE':
false constant ignorecase
[needs lib/search.4th]
Now 'SEARCH' will do a case sensitive comparison, just like
'COMPARE'.
8.18 Replacing substrings
Sometimes finding is not enough. You have replace it by something
else. You can do that very easily with 4tH. Just include ”
replace.4th”. It contains a word that will do all that. Take this
example:
[needs lib/replace.4th]
s" How the cow catches the hare" s" the" s" a"
replaceall type cr
It will print:
How a cow catches a hare
Yes, this one replaces all occurrences of ”the” by ”a”. Note that
like 'COMPARE' and 'SEARCH' this one can be made case sensitive
too:
false constant ignorecase
[needs lib/replaceall.4th]
8.19 Deleting substrings
Yes, we even got a word for 'search-and-destroy' missions. You
only have to include ”replace.4th”:
[needs lib/replace.4th]
s" How the cow catches the hare" s" the"
deleteall type cr
This will print:
How cow catches hare
Yes, it deletes all occurrences of ”the”. Note that like
'COMPARE', 'SEARCH' and 'REPLACEALL' this one can be made case
sensitive too:
false constant ignorecase
[needs lib/replaceall.4th]
8.20 Removing trailing spaces
You probably know the problem. The user of your well-made program
types his name and hits the spacebar before hitting the
enter-key. There you go. His name will be stored in your datafile
with a space and nobody will ever find it.
In 4tH there is a special word called '-TRAILING' that removes
the extra spaces at the end with very little effort. Just paste
it after 'COUNT'. Like we did in this example:
32 string one \ define a string
s" Hans Bezemer " \ string with trailing spaces
one place \ now copy it to string one
one dup \ save the address
." [" \ print a bracket
count type \ old method of printing
." ]" cr \ print bracket and newline
." [" \ print a bracket
count -trailing type \ new method of printing
." ]" cr \ print a bracket and newline
You will see that the string is printed twice. First with the
trailing spaces, second without trailing spaces.
8.21 Removing leading spaces
And what about leading spaces? Patience, old chap. You've got a
lot of ground to cover. There is no built-in word for that, but
we can use a library member like we did in this example:
[needs lib/scanskip.4th]
32 string one \ define a string
s" Hans Bezemer" \ string with leading spaces
one place \ now copy it to string one
one dup \ save the address
." [" \ print a bracket
count type \ old method of printing
." ]" cr \ print bracket and newline
." [" \ print a bracket
count -leading type \ new method of printing
." ]" cr \ print a bracket and newline
You will see that the string is printed twice. First with the
leading spaces, second without leading spaces. Happy?
8.22 Upper and lower case
Sometimes you will have to convert a string to upper or lower
case. 4tH has a library member for that too. Just include:
[needs lib/ulcase.4th]
This will define several easy to use conversion words. E.g. in
order to convert a string to upper case, just enter:
s" Convert this!" s>upper \ convert addr/count string to
uppercase
type cr \ type the string
Its lower case counterpart is:
s" Convert this!" s>lower \ convert addr/count string to
lowercase
type cr \ type the string
Like most string words it takes and returns an address/count
pair. Note that the string in question is modified, so if you
still need the original, copy it first. You can also convert an
individual character:
char A char>lower emit \ convert a character and show it
And consequently, its counterpart is:
char a char>upper emit \ convert a character and show it
These words take an ASCII value from the stack, convert it and
put the converted ASCII value back on the stack. If the value
does not represent a alphabetic character, it is left unchanged.
8.23 String literals and string variables
Most computer languages allow you to mix string literals and
string variables. Not in 4tH. In 4tH they are two distinct
datatypes. To print a string literal you use the word '."'. To
print a string variable you use the 'COUNT TYPE' construction.
There are only three different actions you can do with a string
literal. First, you can define one using 'S"'. Second, you can
print one using '."' Finally, you can compile a string into your
program using ',"'.
This may seem a bit mind-boggling to you now, but we'll elaborate
a bit further on this subject later.
8.24 Printing individual characters
"I already know that!"
Sure you do. If you want to print "G" you simply write:
." G"
Don't you? But what if you want to use a TAB character (ASCII 9)?
You can't type in that one so easily, huh? You may even find it
doesn't work at all!
Don't ever use characters outside the ASCII range 32 to 127
decimal. It may or may not work, but it won't be portable anyway.
the word 'EMIT' may be of some help. If you want to use the
TAB-character simply write:
9 emit
That works!
8.25 Distinguishing characters<sec:Distinguishing-characters>
Like in a novel, not all characters are created equal. There are
upper case characters, lower case characters, control characters,
whitespace, etc. Sometimes it is necessary to find out what kind
of character we are dealing with. Of course, 4tH can help you
there. You need to include 'istype.4th' in order to use it:
char a is-lower . cr
char a is-upper . cr
4tH will first print a TRUE value (because 'a' is a lower case
character) and then a FALSE value. This table tells you what
words 4tH offers and the ranges of valid characters:
[float Table:
+------------+----------------------------------+--------------------------------------+
| Word | Range (ASCII) | Description |
+------------+----------------------------------+--------------------------------------+
+------------+----------------------------------+--------------------------------------+
| IS-ASCII | 0 - 127 | All 7-bit ASCII characters |
+------------+----------------------------------+--------------------------------------+
| IS-PRINT | 32 - 127 | As above, without control characters |
+------------+----------------------------------+--------------------------------------+
| IS-WHITE | 0 - 32 | All control characters plus space |
+------------+----------------------------------+--------------------------------------+
| IS-DIGIT | '0' - '9' | All digits |
+------------+----------------------------------+--------------------------------------+
| IS-LOWER | 'a' - 'z' | All lower case characters |
+------------+----------------------------------+--------------------------------------+
| IS-UPPER | 'A' - 'Z' | All upper case characters |
+------------+----------------------------------+--------------------------------------+
| IS-ALPHA | 'a'-'z', 'A' - 'Z' | All alphabetic characters |
+------------+----------------------------------+--------------------------------------+
| IS-ALNUM | '0' - '9', 'a' - 'z', 'A' - 'Z' | All alphanumeric characters |
+------------+----------------------------------+--------------------------------------+
[Senseless!!!
Character typing words
]
]
8.26 Getting ASCII values
Ok, 'EMIT' is a nice addition, but it has its drawbacks. What if
you want to emit the character "G". Do you have to look up the
ASCII value in a table? No. 4tH has another word that can help
you with that. It is called 'CHAR'. This will emit a "G":
char G emit
The word 'CHAR' looks up the ASCII-value of "G" and leave it on
the stack. You can also use '[CHAR]'. It does exactly the same
thing. It is included for compatibility with ANS-Forth versions.
Note that 'CHAR' only works with printable characters (ASCII 33
to 127 decimal).
8.27 Printing spaces
If you try to print a space by using this construction:
char emit
You will notice it won't work. Sure, you can also use:
." "
But that isn't too elegant. You can use the built-in constant
'BL' which holds the ASCII-value of a space:
bl emit
That is much better. But you can achieve the same thing by simply
writing:
space
Which means that if you want to write two spaces you have to
write:
space space
If you want to write ten spaces you either have to repeat the
command 'SPACE' ten times or use a DO-LOOP construction, which is
a bit cumbersome. Of course, 4tH has a more elegant solution for
that:
10 spaces
Which will output ten spaces. Need I say more?
8.28 Fetching individual characters
Take a look at this small program:
32 string one \ define string one
s" Hans" one place \ initialize string one
What is the second character of string "one"? Sure, its an "a".
But how can you let your program determine that? You can't use
'@' because that word can only access variables.
Sure, you can do that in 4tH, but it requires a new word, called
'C@'. Think of a string as an array of characters and you will
find it much easier to picture the idea. Arrays in 4tH always
start with zero instead of one. So accessing the first character
might be done with:
one 0 th c@
We do not recommend using this construction, although it will
work perfectly. If you never want to convert your program to
Forth you might even choose to keep it that way. We recommend the
construction:
one 0 chars + c@
Which is slightly more wordy. 4tH will compile both constructions
in exactly the same way. Anyway, accessing the second character
is easy now:
one 1 chars + c@
This is the complete program:
32 string one \ define string one
s" Hans" one place \ initialize string one
one 1 chars + c@ \ get the second character
emit cr \ print it
8.29 Storing individual characters
Storing individual characters works just the same. Keep that
array of characters in mind. When we want to fetch a variable we
write:
my_var @
When we want to store a value in a variable we write:
5 my_var !
Fetching only requires the address of the variable. Storing
requires both the address of the variable and the value we want
to store. On top of the stack is the address of the variable,
below that is value we want to store. Keep that in mind, this is
very important.
Let's say we have this program:
32 string one \ define string one
s" Hans" one place \ initialize string one
Now we want to change "Hans" to "Hand". If we want to find out
what the 4th character of string "one" is we write:
32 string one \ define string one
s" Hans" one place \ initialize string one
one 3 chars + c@ \ get the fourth character
Remember, we start counting from zero! If we want to store the
character "d" in the fourth character, we have to use a new word,
and (yes, you guessed it right!) it is called 'C!':
32 string one \ define string one
s" Hans" one place \ initialize string one
one 3 chars + \ address of the fourth char
char d \ we want to store 'd'
swap \ get the order right
c! \ now store 'd'
If we throw the character "d" on the stack before we calculate
the address, we can even remove the 'SWAP':
32 string one \ define string one
char d \ we want to store 'd'
s" Hans" one place \ initialize string one
one 3 chars + \ address of the fourth char
c! \ now store 'd'
We will present the very same programs, but now with
stack-effect-diagrams in order to explain how this works. We will
call the index 'i', the character we want to store 'c' and the
address of the string 'a'. By convention, stack-effect-diagrams
are enclosed by parenthesis.
If you create complex programs this technique can help you to
understand more clearly how your program actually works. It might
even save you a lot of debugging. This is the first version:
32 string one ( --)
s" Hans" one place ( --)
one 3 chars ( a i)
+ ( a+i)
char d ( a+i c)
swap ( c a+i)
c! ( --)
Now the second, optimized version:
32 string one ( --)
char d ( c)
s" Hans" one place ( c)
one 3 chars ( c a i)
+ ( c a+i)
c! ( --)
8.30 Getting a string from the keyboard
Of course, you don't want to initialize strings all your life.
Real applications get their input from the keyboard. We've
already shown you how to get a number from the keyboard. Now we
turn to strings.
When programming in BASIC, strings usually have an undefined
length. Some BASICs move strings around in memory, others have to
perform some kind of "garbage-collection". Whatever method they
use, it takes up memory and processor-time.
4tH forces you to think about your application. E.g. when you
want to store somebodies name in a string variable, 16 characters
will be too few and 512 characters too many. But 64 characters
will probably do.
But that poses a problem when you want to get a string from the
keyboard. How can you prevent that somebody types a string that
is just too long? And how do you terminate it?
The word 'ACCEPT' takes two arguments. First, the string variable
where you want to save the input and second, the maximum number
of characters it can take. It automatically terminates the string
when reading from the keyboard. But there is a catch. This
program can get you into trouble:
64 constant #name \ length of string
#name string name \ define string 'name'
name #name accept \ input string
name swap type cr \ swap count and print
Since 64 characters plus the terminator add up to 65 characters.
The word 'ACCEPT' always returns the number of characters it
received. You will find that you won't need that information most
of the time.
This is the end of the second level. Now you should be able to
understand most of the example programs and write simple ones. I
suggest you do just that. Experience is the best teacher after
all.
Character Segment
9.1 The Character Segment
Wonder where all these strings are created? I bet you do. Well,
when you define a string, memory is allocated in the Character
Segment. When you define another one, space is allocated after
the first string. That means that if you go beyond the boundaries
of the first string, you'll end up in the space allocated to the
second string.
After the second string there is a void. If you end up there your
program will end with an error-message. And what about the space
before the first string? Well, take a look at figure [cap:Character-segment]
.
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/charseg.eps>
[Senseless!!!
Character segment<cap:Character-segment>
]
]The lower memory is at the bottom. Yes, before your strings
there are two other areas, the TIB and the PAD. We'll elaborate
on that in the next section.
The Character Segment is created at run-time. That means that it
isn't there when you compile a program. The compiler just keeps
track of how much memory would be needed to create such a
Character Segment and stores that information in the header.
When you run the program the header is read first. Then the
Character Segment is created, so it is already there when your
program starts executing. When you exit the program, the
Character Segment is destroyed and all information stored there
is lost (unless you save it first).
9.2 What is the TIB?
The TIB stands for "Terminal Input Buffer" and is used by one
single, but very important word called 'REFILL'. In essence,
'REFILL' does the same thing as 'ACCEPT', except that it has a
dedicated area to store its data and sets up everything for
parsing. Whatever you type when you call 'REFILL', it is stored
in the TIB.
9.3 <WhatIsPAD>What is the PAD?
The PAD is short for "scratch-pad". It is a temporary storage
area for strings. It is heavily used by 4tH itself, e.g. when you
print a number the string is formed in the PAD. Yes, that's
right: when you print a number it is first converted to a string.
Then that string is 'COUNT'ed and 'TYPE'd. You can even program
that subsystem yourself as we will see when we encounter
formatted numbers (see section [PicturedNumbers]).
Furthermore, string constants (compiled by 'S”' or ',”') are
temporarily stored in the PAD. Finally, 'NUMBER' and 'ARGS' also
use the PAD. The PAD is actually a circular buffer. That means
that strings are stored in the PAD until it runs out of space.
Then it starts to overwrite the oldest strings. Usually, they
have turned into garbage that is no longer used, but sometimes
they still have some significance to your program. In that case,
you'll have to save the string that was overwritten into a
variable. Don't rely on the PAD to keep your strings alive!
9.4 How do I use TIB and PAD?
In general, you don't. The TIB is a system-related area and it is
considered bad practice when you manipulate it yourself. The PAD
can be used for temporary storage, but beware! Temporary really
means temporary. A few words at the most, provided you don't
generate any output or do any parsing.
Think of both these areas as predefined strings. You can refer to
them as 'TIB' and 'PAD'. You don't have to declare them in any
way. This program is perfectly alright:
s" Hello world" pad place \ store a string in pad
pad count type cr \ print contents of the pad
If you want to know how big TIB and PAD are, you can use the
predefined constants '/TIB' and '/PAD':
." Size of TIB: " /TIB . cr \ print sizeof TIB
." Size of PAD: " /PAD . cr \ print sizeof PAD
Note, this does not print the length of a string stored in the
area, but the maximum size of the string that can be stored
there. Some space of the PAD is reserved for number generation
(see section [WhatIsPAD]). You can get the size of this area by
the predefined constant '/HOLD'. This will print the size of this
area and the size of PADs circular buffer:
." Size of HOLD : " /HOLD . cr \ print sizeof HOLD
." Size of buffer: " /PAD /HOLD - . cr
If that area did not exist even printing a number could corrupt
the circular buffer. In some unusual circumstances, the PAD can
get corrupted. If so, identify the temporary string that gets
corrupted and store it explitly into a string variable.
9.5 Simple parsing
We have already discussed 'REFILL' a bit. We've seen that it is
closely related to 'ACCEPT'. 'REFILL' returns a true flag if all
is well. When you use the keyboard it usually is, so we can
safely drop it, but we will encounter a situation where this flag
comes in handy.
If you want to get a string from the keyboard, you only have to
type:
refill drop \ get string from keyboard
Every next call to 'REFILL' will overwrite any previously entered
string. So if you want to do something with that string you've
got to get it out of there, usually to one of your own strings.
But if accessing the TIB directly is not the proper way, what is?
The use of 'REFILL' is closely linked to the word 'PARSE-WORD',
which is a parser. 'PARSE-WORD' looks for the delimiter, whose
ASCII code is on the stack.
If the string starts with the delimiter, it will skip this and
all subsequent occurrences until it finds a string. Then it will
look for the delimiter again and slice the string right there. It
then returns its address and count.
This extremely handy when you want to obtain filtered input. E.g.
when you want to split somebodies name into first name, initials
and lastname:
Hans L. Bezemer
Just use this program:
." Give first name, initials, lastname: "
refill drop \ get string from keyboard
bl parse-word \ parse first name
." First name: " \ write message
type cr \ type first name
bl parse-word \ parse initials
." Initials : " \ write message
type cr \ type initials
bl parse-word \ parse last name
." Last name : " \ write message
type cr \ write last name
You don't have to parse the entire string with the same
character. This program will split up an MS-DOS filename into its
components:
." DOS filename: " refill \ input a DOS filename
drop cr \ get rid of the flag
char : parse-word \ parse drive
." Drive: " type ." :" cr
\ print drive
begin
char \ parse-word \ parse path
dup 0<> \ if not a NULL string
while \ print path
." Path : " type cr
repeat \ parse again
drop drop \ discard string
If 'PARSE-WORD' reaches the end of the string and the delimiter
is still not found, it returns the remainder of that string. If
you try to parse beyond the end of the string, it returns a NULL
string. That is an empty string or, in other words, a string with
length zero.
Therefore, we checked whether the string had zero length. If it
had, we had reached the end of the string and further parsing was
deemed useless.
9.6 Converting a string to a number
We now learned how to parse strings and retrieve components from
them. But what if these components are numbers? Well, there is a
way in 4tH to convert a string to a number, but like every
number-conversion routine it has to act on invalid strings. That
is, strings that cannot be converted to a valid number.
4tH uses an internal error-value, called '(ERROR)'. The constant
'(ERROR)' is a strange number. You can't negate it, you can't
subtract any number from it and you can't print it. If 4tHs
number-conversion word 'NUMBER' can't convert a string it returns
that constant. 'ERROR?' checks the return value and leaves an
additional true flag if an error occured (which means: '(ERROR)'
was returned). Let's take a look at this program:
." Enter a number: " \ write prompt
refill drop \ enter string
bl parse-word \ parse string
number \ convert to a number
error? \ test for valid number
if \ if not valid
." You didn't enter a valid number!" drop cr
else \ print if valid
." The number was: " . cr
then
You first enter a string, then it is parsed and 'PARSE-WORD'
returns the address and count. 'NUMBER' tries to convert it. If
'NUMBER' returns '(ERROR)' it wasn't a valid string. Otherwise,
the number is right on the stack, waiting to be printed. That
wasn't so hard, was it?
9.7 Controlling the radix
If you are a programmer, you know how important this subject is
to you. Sometimes, you want to print numbers in octal, binary or
hex. 4tH can do that too. Let's take the previous program and
alter it a bit:
." Enter a number: " \ write prompt
refill drop \ enter string
bl parse-word \ parse string
number \ convert to a number
error? \ test for valid number
if \ if not valid
." You didn't enter a valid number!" drop cr
else \ print if valid
hex
." The number was: " . cr
then
We added the word 'HEX' just before printing the number. Now the
number will be printed in hexadecimal. 4tH has a number of words
that can change the radix, like 'DECIMAL' and 'OCTAL'. They work
in the same way as 'HEX'.
4tH always starts in decimal. After that you are responsible.
Note that all radix control follows the flow of the program. If
you call a self-defined word that alters the radix all subsequent
conversion is done too in that radix:
: .hex hex . ; \ print a number in hex
." Enter a number: " \ write prompt
refill drop \ enter string
bl parse-word \ parse string
number \ convert to a number
error? \ test for valid number
if \ if not valid
." You didn't enter a valid number!" drop cr
else \ print if valid
." The number was: " .hex cr
then
In this example not only that single number is printed in hex,
but also all subsequent numbers will be printed in hex! A better
version of the ".HEX" definition would be:
: .hex hex . decimal ;
Since that one resets the radix back to decimal. Words like 'HEX'
do not only control the output of a number, but the input of
numbers is also affected:
." Enter a number: " \ write prompt
refill drop \ enter string
bl parse-word \ parse string
hex \ convert hexadecimal
number \ convert to a number
error? \ test for valid number
if \ if not valid
." You didn't enter a valid number!" drop cr
else \ print if valid
dup
." The number was: " decimal . ." decimal" cr
." The number was: " hex . ." hex" cr
then
'NUMBER' will now also accept hexadecimal numbers. If the number
is not a valid hexadecimal number, it will return '(ERROR)'. You
probably know there is more to radix control than 'OCTAL', 'HEX'
and 'DECIMAL'. No, we have not forgotten them. In fact, you can
choose any radix between 2 and 36. This slightly modified program
will only accept binary numbers:
: binary 2 base ! ;
." Enter a number: " \ write prompt
refill drop \ enter string
bl parse-word \ parse string
binary \ convert hexadecimal
number \ convert to a number
error? \ test for valid number
if \ if not valid
." You didn't enter a valid number!" drop cr
else \ print if valid
dup \ both decimal and hex
." The number was: " decimal . ." decimal" cr
." The number was: " hex . ." hex" cr
then
'BASE' is a predefined variable that enables you to select any
radix between 2 and 36. This makes 4tH very flexible. However,
this won't work:
hex 02B decimal . cr
4tH will try to compile "02B", but since it isn't a word or a
valid decimal number, it will fail. Words like 'HEX' and the
'BASE' variable work only at run-time, not at compile-time! Isn't
there a way to compile non-decimal numbers?
Sure, there is, although it is not that flexible. There are four
words that control the interpretation of numbers at compile-time:
1. [BINARY]
2. [OCTAL]
3. [DECIMAL]
4. [HEX]
They work fundamentally different than their run-time
equivalents. First, they only work at compile-time. Second, they
are interpreted sequentially and do not follow the flow of the
program. Let's take a look at these two programs:
[binary] 101 . cr
[octal] 101 . cr
[decimal] 101 . cr
[hex] 101 . cr
This will print the decimal numbers "5", "65", "101" and "257",
since each one of them is compiled with a specific radix.
: binary 2 base ! ;
binary 101 . cr
octal 101 . cr
decimal 101 . cr
hex 101 . cr
Now the decimal number "101" is printed in four different
radixes, since at compile-time the radix was set to decimal
(which is the default). Now take a look at this program:
: do_binary [binary] ;
: do_decimal [decimal] ;
do_binary 101 decimal . cr
do_decimal 101 decimal . cr
The program will print "101" two times! Haven't we selected
binary at compile-time? No, both '[BINARY]' and '[DECIMAL]' are
interpreted sequentially!
When '[BINARY]' is encountered at the first time, it will set the
radix at compile-time to binary. When '[DECIMAL]' is encountered
in the second line, it will set the radix to decimal. When the
third line it compiled, the radix is still set to decimal. If you
want to make this program work, try this:
[binary]
101 decimal . cr
[decimal]
101 decimal . cr
When the first line is encountered, it sets the radix (at
compile-time) to binary. So the number "101" at line two is
compiled as a binary number. 'DECIMAL' will just be compiled. It
will only influence the radix at run-time. The third line sets
the radix at compile-time to decimal. So the number "101" at line
four is compiled as a decimal number.
Since the run-time of 4tH starts up in decimal, both occurrences
of 'DECIMAL' have little value. We can even eliminate 'DECIMAL'
from the program altogether without affecting the result:
[binary] 101 . cr
[decimal] 101 . cr
Note that both the compile-time radix control words and the
run-time radix control words stay in effect until they are
superseded by others:
[binary] \ compile-time binary
101 \ first binary number
1011 \ second binary number
[decimal] \ compile-time decimal
5 \ decimal 5
do \ set run-time radix
i base ! \ to loop-index
dup . cr \ print number
loop
drop \ clean stack
9.8 <PicturedNumbers>Pictured numeric output
You probably have used this before, like when writing Basic.
Never heard of "PRINT USING.."? Well, it is a way to print
numbers in a certain format. Like telephone-numbers, time, dates,
etc. Of course 4tH can do this too. In fact, you've probably used
it before. Both '.' and '.R' use the same internal routines. They
are called just before a number is printed.
This numeric string is created in the PAD and overwritten with
each new call. But we'll go into that a bit later on.
What you have to remember is that you define the format reverse.
What is printed first, is defined last in the format. So if you
want to print:
060-5556916
You have to define it this way:
6196555-060
Formatting begins with the word '<#' and ends with the word '#>'.
A single number is printed using '#' and the remainder of the
number is printed using '#s' (which is always at least one
digit). Let's go a bit further into that:
: print# <# #s #> type cr ;
256 print#
This simply prints a single number (since only '#S' is between
the '<#' and the '#>' and goes to a new line. There is hardly any
difference with '.'. You can try any (positive) number. Note that
the values that '#>' leaves on the stack can directly be used by
'TYPE'.
This is a slightly different format:
: print3# <# # # # #> type cr ;
256 print3#
1 print3#
1000 print3#
This one will print "256", "001" and "000". Always the last three
positions. The '#' simply stands for 'print a single digit'. So
if you want to print a number with at least three digits, the
format would be:
#s # #
That is: print the remainder of the number (at least one digit)
and then two more. Now reverse it:
# # #s
Enclose it by '<#' and '#>' and add 'TYPE CR':
<# # # #s #> type cr
And that's it! Is it? Not quite. So far we've only printed
positive numbers. If you try a negative number, you will find it
prints garbage. This behavior can be fixed with the word 'SIGN'.
'SIGN' simply takes the number from the stack and prints a "-"
when it is negative. The problem is that all other formatting
words can only handle positive numbers. So we need the same
number twice. One with the sign and one without. A typical signed
number formatting word looks like:
: signed# dup abs <# #s sign #> type ;
Note the 'DUP ABS' sequence. First the number is duplicated (for
'SIGN') and then the absolute value is taken (for the other
formatting words). So we got the number on the stack twice. First
with sign (for 'SIGN'), second without sign (for the other
formatting words). Does that make sense to you?
We can place 'SIGN' wherever we want. If we want to place the
sign after the number (like some accountants do) we would write:
: account# dup abs <# sign #s #> type ;
But that is still not enough to write "$2000.15" is it? Well, in
order to do that there is another very handy word called 'HOLD'.
The word 'HOLD' just copies any character into the formatted
number. Let's give it a try:
$2000.16
Let's reverse that:
61.0002$
So we first want to print two numbers, even when they are zero:
# # .0002$
Then we want to print a dot. This is where 'HOLD' comes in.
'HOLD' takes an ASCII code and places the equivalent character in
the formatting string. We don't have to look up the ASCII code
for a dot of course. We can use 'CHAR':
# # char . hold 0002$
Then we want to print the rest of the number (which is at least
one digit):
# # char . hold #s $
Finally we want to print the character "$". Another job for
'HOLD':
# # char . hold #s char $ hold
So this is our formatting word:
: currency <# # # char . hold #s char $ hold #> type cr ;
And we call it like this:
200016 currency
You can do some pretty complex stuff with these formatting words.
Try to figure out this one from the master himself, Leo Brodie:
: sextal 6 base ! ;
: :00 # sextal # decimal 58 hold ;
: time# <# :00 :00 #S #> type cr ;
3615 time#
Yeah, it prints the time! Pretty neat, huh? Now try the
telephone-number we discussed in the beginning. That shouldn't be
too hard.
9.9 Converting a number to a string
Since there is no special word in 4tH which will convert a number
to a string, we'll have to create it ourselves. In the previous
section we have seen how a numeric string is created in the PAD.
We can use this to create a word that converts a number to a
string.
Because the PAD is highly volatile, we have to save the string
immediately after its creation. So we'll create a word that not
only creates the string, but places it directly in its proper
location:
: >string >r dup abs <# #s sign #> r> place ;
( n a -- )
It takes a number, the address of a string and returns nothing.
Example:
16 string num$
-1024 num$ >string
num$ count type cr
9.10 Aborting a program
Some conditions are so grave you can consider them to be fatal
errors. In such cases the only thing you can do is abort the
program as soon as possible. Of course, there is a way in 4tH to
do just that. You can use either 'ABORT' or 'QUIT'. Same thing.
Both will terminate your program immediately. This small program
prints nothing:
abort
." This will never be printed." cr
But there is more. Let's say you only want to exit a program when
a certain condition is met, e.g. a word left a non-zero value on
the stack. In that case you would have to write something like
this:
if
." We have an error condition!" cr quit
then
You can write that much shorter by using the word 'ABORT"':
abort" We have an error condition!"
'ABORT"' will print the message following it and abort, but only
when there is a non-zero value on the stack. So this program does
not abort:
false abort" This will not be printed!"
." This will be printed!"
You will find that 'ABORT"' is a very handy tool when processing
error conditions.
9.11 Opening a file
You probably don't want to write programs that only write to the
screen and read from the keyboard. So 4tH has a few words that
allow you to work with files. Since 4tH is a scripting language,
its capabilities are limited. But you will find that you can
perform most common operations.
One of the limitations is that you can have a limited number of
open files, but it will do in most situations.
Opening a output-file is pretty simple. Just throw the address
and length of a filename and a file access mode on the stack and
execute the word 'OPEN'. The value 'OPEN' returns is a simple
number which bears little significance. However, you have to save
it to a variable or value, for you will need it later. We'd like
to use values for storing file pointers, so we created the word
'FILE'. 'FILE' simply creates a value and initializes it, so if
you use it prematurely 4tH will issue an error message.
file myfile
s" outfile.dat" output open error? ( a1 n1 fam -- h f)
abort" File could not be opened" ( h)
to myfile ( --)
'OUTPUT' is a file access mode and will open a file for writing.
'OPEN' leaves a value on the stack. If it equals '(ERROR)',
something was not quite right. If not, the file was successfully
opened. 'FILE' is nothing but an initialized value, so you can
assign it with 'TO'. 'ERROR?' leaves the handle intact, but
leaves an additional true flag if an error occurred, which makes
it much easier to evaluate.
The syntax for opening an input file is the same, except for the
read-flag 'INPUT' of course:
file myotherfile
s" infile.dat" input open error?
abort" File could not be opened"
to myotherfile
9.12 Reading and writing from/to a file
There are no special words to read from or write to a file. You
can use all the words you used for keyboard-input and
screen-output.
But if you open a file and do some I/O you will notice nothing
has changed. Of course not. You should be able to determine
whether you write to a file or to the screen. There are special
words to do just that:
file OutFile \ file variable
s" outfile.dat" output open error?
abort" File could not be opened"
to OutFile \ open the file
OutFile use \ write to file
." This is written to disk" cr
stdout use \ write to screen
." This is written to screen" cr
After you've opened the file, the program will still write to the
screen. When 'USE' executes, all output will be redirected to the
file. When 'USE' executes again, but this time with the 'OUTPUT'
flag, all output will go to the screen again, but the output-file
will not be closed! Both words take the same read/write-flags as
'OPEN'.
You can call 'USE' again and again, without closing or opening
any files. Here is an example using an input-file:
file OutFile
s" outfile.dat" output open error?
abort" File could not be opened"
to OutFile \ open output file
OutFile use \ write to file
." This is written to disk" cr
stdout use \ write to screen
." This is written to screen" cr
OutFile close \ close file
s" outfile.dat" input open error?
abort" File could not be opened"
to OutFile
OutFile use \ read from disk
pad dup 32 accept \ read 32 characters
type \ write string to screen
stdin use \ read from keyboard
OutFile close \ close file
The output of this program is:
This is written to screen
This is written to disk
Microsoft-users, note that files are opened in binary mode (no
CR/LF translations). If you issue 'CR' the line will be
terminated by a linefeed. Don't worry. You can fix that as we
will see later on.
9.13 Closing a file
There is usually no need to close any files. When you quit the
program all files are closed. It seems like there is no need at
all to close files manually, but that is a mistake.
If you want to open a file for reading to which you've just
written, you will find it doesn't work. Of course, you can open a
file only once.
No, there is a word which closes either the input- or the
output-file, using the same read/write-flags. You've already seen
it, it is called 'CLOSE'. When you close an active file, the
input (or output) is redirected to the keyboard (or screen).
9.14 Writing text-files
Writing text to a file is just as easy as writing text to screen.
Open the file, redirect the output, and write like you would
write to the screen:
file OutFile \ value for file
s" outfile.dat" \ put the filename on the stack
output \ add the modifier
open error? \ open the file
abort" File could not be opened"
to OutFile
OutFile use \ write to file
." This is written to disk" cr
That's all! Note that if you execute your program on a Microsoft
Operating System, it will write a Microsoft text file. If you do
so on a Unix Operating System, it will write a Unix text file. If
you want to override that you'll have to issue the end-of-line
sequences yourself using 'EMIT'.
9.15 Reading text-files
Reading text-files is pretty straightforward. You don't even have
to open a file in text-mode contrary to other languages. Just
open the file and call 'REFILL' until it signals end-of-file
(EOF):
\ Example program. It reads a file line by line
\ and prints it to the screen.
file InFile
s" readln.4th" input open error?
abort" Could not open file" \ open file
to InFile \ save handle
InFile use \ read from file
begin
refill \ read a line
while \ while EOF not found
0 parse-word \ parse the entire line
type \ print it
cr \ terminate line
repeat \ read next line
You will find that if you run this program, it will print itself
to the screen.
'REFILL' will return a non-zero value if EOF was not detected. By
using the word '0=' you can invert this value. Finally, it will
read Unix ASCII-files as well as DOS ASCII-files, no matter where
your program is executed.
9.16 Reading long lines
The TIB is only /TIB characters long. If you read a line that is
longer than that, only /TIB - 1 characters are read. The rest of
the line is read when you invoke 'REFILL' again. Although you
don't lose any information that way, it might not be what you
want. Fortunately, you can define your own TIB:
2048 constant /mytib \ length of your TIB
/mytib string mytib \ define your own TIB
mytib /mytib source! \ tell the system about your TIB
The next time you invoke 'REFILL', it will use your TIB instead
of the system TIB, so it will now read lines up to 2047
characters. 'SOURCE!' takes an address/count pair and makes it
the current TIB. So if you want to use the system TIB again you
issue:
tib /tib source!
And if you have forgotten which TIB you're using try this:
source . . cr
'SOURCE' will return the address/count pair of the TIB you're
currently using. In fact, this definition does absolutely
nothing:
: doesnothing source source! ;
For the simple reason that it reassigns the TIB it is already
using.
9.17 Reading binary files
If you process binary files, you won't get far reading it line by
line. You want to read chunks of data. 4tH can do that too by
using 'ACCEPT'. You feel there must be a catch, since 'ACCEPT'
terminates strings automatically. Well, there isn't. When
'ACCEPT' does not read from the keyboard, it won't add that extra
byte.
Reading blocks of data usually means defining buffers. If
maintainability is an issue, define a constant for the sizes of
these buffers. You cannot only use this constants when defining
buffers, but also when calling 'ACCEPT'.
Furthermore, 'ACCEPT' returns the number of characters actually
read. If this value is compared to the number of characters we
actually wanted to read, we can determine whether a reading error
or EOF occurred:
1024 constant bufsize \ actual buffersize
bufsize string buffer \ define buffer
file InFile \ value for file
\ open input file
s" infile.dat" input open error?
abort" File could not be opened"
to InFile \ save handle
InFile use \ redirect input
begin \ using bufsize
bufsize ( n1)
buffer over ( n1 a n1)
accept ( n1 n2)
<> ( f) \ make EOF flag
until \ until EOF
Note that "BUFFER" is actually not a string, but a chunk of
memory. But since a character in 4tH takes up a single
address-unit (=byte), raw chunks of memory are allocated in the
Character Segment. This is not an uncommon practice in both Forth
and C.
9.18 Writing binary files
Writing binary files is very easy. Of course you need a buffer,
like we discussed in the previous section. The program is not
much different than the previous one:
1024 constant bufsize \ actual buffersize
bufsize string buffer \ define buffer
file OutFile \ value for file
buffer bufsize char H fill \ fill the buffer
\ open output file
s" infile.dat" output open error?
abort" File could not be opened"
to Outfile \ save handle
OutFile use \ redirect input
buffer bufsize type \ write to file
This will write 1024 "H"s to "infile.dat". The actual command
that does all writing is 'TYPE'. The word 'TYPE' does not return
anything. You can be assured that everything was alright, since
if it wasn't, 4tH would have caught the error itself.
9.19 Reading and writing block files
Block files are a special kind of files used by Forth compilers.
In the old days Forth controlled the entire computer and directly
communicated with all peripherals, including disks. To Forth, a
disk is just a bunch of numbered blocks. Each block is divided
into 16 lines of exactly 64 characters. A block file simply
mimics that layout.
Before we can begin, you need to create a block file. Well,
that's easy, an empty file will do:
s" blocks.scr" output open close
Then we have to load the ANS Forth wordset and tell 4tH which
file to use:
include lib/ansblock.4th
s" blocks.scr" use-block
Note that apart from creating the file, we haven't performed any
I/O yet. First, we have to request a block. When a block is
requested, its contents are transferred to a memory buffer. You
can manipulate this buffer any way you want with the standard
words. If you request another block its contents are transferred
to the buffer too, overwriting whatever is there. All changes you
have made are lost, unless you have flagged the block as dirty,
which means its contents are different from the block on disk. If
a block is dirty, it is written to disk before the next block is
read. 'CLEAR' is a special word, assigning an empty buffer to a
block without reading it first. The buffer is BLANKed. 'UPDATE'
will flag the buffer as dirty. 'FLUSH' writes the dirty buffer to
disk and unassigns the buffer. So, first we clear block 0 and
write it to disk:
0 clear update flush
Then we clear block 1, copy a string to it and flag it as dirty:
1 clear
s" Hello world!" >r 1 block r@ cmove update
'BLOCK' returns the address of the buffer assigned to that block[footnote:
That is particularly handy if your implementation can handle
multiple buffers. In this implementation we have only one buffer,
so we always return the same address.
]. If the block is not present, it is read into the buffer. If
the buffer is dirty, it is FLUSHed first. We can also write the
dirty buffer to disk, without unassigning it:
save-buffers
Note that the buffer is not dirty anymore, since it has been
synchronized. Let's write something to another block:
s" Goodbye cruel world!" 0 block swap cmove update
It is always a nice game to figure out what will happen now. The
current block is block 1. Since we haven't UPDATEd it since
'SAVE-BUFFERS', it is clean. That means that 4tH won't perform a
write. Since block 0 isn't current, it is read into the buffer.
The 'UPDATE' will flag the buffer as dirty.
1 block r> type cr
This is fun! The current block is block 0. It is dirty, so it is
written to disk. Since block 1 isn't current, it is read into the
buffer. You catch my drift? If you want to print the contents of
a block, you can use 'LIST'. Of course, 'LIST' uses 'BLOCK' and
applies to the same rules:
0 list
." This block has been listed: " scr ? cr
'SCR' is a variable containing the last screen LISTed. Note that
is not the same thing as the current block! Finally, we can
discard all our changes:
empty-buffers
'EMPTY-BUFFERS' does not perform any I/O nor does it change the
contents of the buffer. It just unassigns the buffer and flags it
as clean. Note that you don't have to close a block file since
all I/O is block-oriented. You can use different block files
within the same program, but you'll lose the changes in any dirty
buffers.
9.20 Parsing textfiles
As we've already seen, it is very easy to enter a line using
'REFILL' and parse it. You can also use 'REFILL' to read lines
from a text-file. It is quite similar to reading lines from the
keyboard, except that you have to open a file first. This little
program prints all the words of a textfile on a new line:
file InFile \ value for file
s" file.txt" input open error?
abort" File could not be opened"
\ open the file
to InFile \ save handle
InFile use \ redirect input to file
begin
refill \ get a line from file
while \ check if EOF
begin
bl parse-word \ if not, parse line
dup 0<> \ check if zero length
while
type cr \ if not, print word
repeat \ parse next word
drop drop \ drop address/count
repeat \ get next line
Now that flag left by 'REFILL' makes sense! If it is zero, we
have reached the end of the line. Note that you don't have to
open a file in text-mode and both Microsoft ASCII and Unix ASCII
files are supported.
9.21 Parsing binary files
And what about binary files, like classic Forth blockfiles? Well,
you could use 'REFILL' in that context too, but it would probably
break up words since it can't find an end-of-line marker and its
buffer is smaller than 1024 characters. Does that mean it can't
be done? No! But 'REFILL' makes it easier for you, because it
handles a few tasks automatically.
First, it has its own buffer (TIB). When you're not using
'REFILL' you have to define one yourself. Second, it terminates
the string for you. You don't want 'PARSE-WORD' to wander into
new territory, do you? Third, it sets '>IN' for you every time
its receives new input. You have to take care of that one too.
Never heard of '>IN'? Well, the only way for 'PARSE-WORD' to know
on what position the previous scan ended is to store that
information into a variable. This variable is called '>IN'.
Not all internal 4tH variables are accessible, mostly because we
can't imagine what use they could have to you. Some variables are
just better left alone. But '>IN' is available for some very
obvious reason: you can reset it and make 'PARSE-WORD' work for
you. Note that for '>IN' to work, you have to make the buffer the
parsing area by using 'SOURCE!'
The following program will read the first screen of a block-file
for you and print out all the words. You will see that all spaces
are eliminated and every word is printed on a new line, just the
behavior you would expect from 'PARSE-WORD'.
1025 constant /buffer \ screensize + terminator
/buffer -1 [+] constant c/scr \ size of the block
file InFile \ value for file
/buffer string buffer \ 1: our own buffer
: openfile \ open the block file
s" romans.blk" input open error?
abort" Cannot open file"
to InFile \ save handle
InFile use \ read from file
;
: readfile \ fill the buffer
buffer c/scr 2dup \ address and count
bl fill \ clear the buffer
accept drop \ fill the buffer
input close \ close the file
;
: initparse \ configures parsing
0 buffer c/scr chars + c! \ 2: terminate screen
buffer /buffer source! \ 3: make buffer the parse area
0 >in ! \ 4: reset >IN
;
: parseblock
begin
bl parse-word \ get word
dup 0<> \ length zero?
while
type cr \ if so, print it
repeat
2drop \ else drop addr/cnt
." End of block" cr \ signal "End of block"
;
: parsefile \ do it all
openfile \ open the file
readfile \ read it
initparse \ set up parsing
parseblock \ parse it
;
parsefile
Note there is no need to reset '>IN' if you use 'REFILL', since
it will be reset automatically. In this case, if you want to
parse another block, you will have to reset '>IN' again.
9.22 Parsing comma-delimited files
'PARSE-WORD' is a powerful and very useful word, but it is less
than useful when parsing comma-delimited files. Why? Well,
because 'PARSE-WORD' skips leading delimiters. So when you have a
file like this it doesn't work:
FIRSTNAME,NAME,EMAIL,TELEPHONE,HOMEPAGE,FAX
Hans,Bezemer,[email protected],,http://hansoft.come.to,
Again, 'PARSE-WORD' skips leading delimiters, so instead of an
empty string we get the homepage when we're trying to read the
(non-existant) telephone number. Fortunately, we got a word like
'PARSE'. 'PARSE' also takes a delimiter from the stack, just like
'PARSE-WORD', but it acts on leading delimiters. Take a look at
this program:
file OutFile \ value for output file
file InFile \ value for input file
: WriteCommaFile ( --)
s" address.csv" output open error?
abort" Could not write CSV file"
to OutFile \ save handle
OutFile use \ redirect output to file
." FIRSTNAME,NAME,EMAIL,TELEPHONE,HOMEPAGE,FAX" cr
." Hans,Bezemer,,,http://hansoft.come.to," cr
OutFile close \ close file
stdout use \ redirect output to screen
;
: ProcessLine ( --)
refill \ get line
0= abort" Read error"
[char] , parse type cr \ parse first name
[char] , parse type cr \ parse name
[char] , parse type cr \ parse email
[char] , parse type cr \ parse telephone
[char] , parse type cr \ parse homepage
[char] , parse type cr cr \ parse fax
;
: ReadCommaFile ( --)
s" address.csv" input open error?
abort" Could not read CSV file"
to InFile \ save handle
InFile use \ redirect input to file
." _Headerline_" cr \ this is the headerline
ProcessLine \ now process headerline
." _First record_" cr \ this is the first record
ProcessLine \ now process first record
InFile close \ close file
;
WriteCommaFile \ write the CSV file
ReadCommaFile \ read the CSV file
With "WriteCommaFile" we write a simple comma-delimited file to
disk. We got to read something, don't we? Then we read the file
we've just written with "ReadCommaFile". "ProcessLine" does the
actual job. Since we have six fields we use 'PARSE' six times. We
cannot do this with a loop. Why not? 'PARSE-WORD' can do it that
way.
Well, 'PARSE' not only returns a NULL-string when we've reached
the end of a line, but also when a field is empty. So we've got
to know how many fields we actually want to read. Of course, you
could parse the headerline with 'PARSE-WORD' to find that out,
but you already know how to do this.
9.23 Advanced parsing
Let's think of something difficult to parse, e.g.:
;;;;;;;;;.........Can you parse me.
;;;;,,,,,And me too, huh?
If we would use 'PARSE' we would have to know how many semicolons
to skip and there is a different number of them on each line. If
we would use 'PARSE-WORD' we'd lose all the semicolons, but the
parsed string would have all these nasty leading dots.
Even worse, if we were able to skip the semicolons and use
'PARSE-WORD' with the leading delimiter we'd get "Can you parse
me" and "And me too" instead of "Can you parse me." and "And me
too, huh?". What can we do?
Fortunately, 4tH doesn't really know about 'PARSE-WORD' but
translates it into a sequence of words[footnote:
If you're really curious, 'PARSE-WORD' is equivalent to 'DUP OMIT
PARSE'.
]. We can also use them directly. 'OMIT' is very handy. It
doesn't actually do anything, it just skips leading delimiters
and sets '>IN' accordingly. It takes an ASCII value from the
stack as its delimiter. This will correctly parse the first line:
char ; omit \ omit the semicolon
char . omit \ omit the dot
0 parse \ parse the remainder of the line
This will correctly parse the second line:
char ; omit \ omit the semicolon
char , omit \ omit the comma
0 parse \ parse the remainder of the string
Please note that this are special 4tH words! Unfortunately you
cannot port this to ANS-Forth, where only a limited version of
'PARSE-WORD' and 'PARSE' are available.
9.24 Appending to existing files
You can use a so-called modifier to signal 4tH, it shouldn't
overwrite the file it opens, but append to it:
file OutFile \ value for output file
s" outfile.dat" output open error?
abort" Cannot open file" \ open the file
to OutFile \ save handle
OutFile use \ now write to disk
10 0 do i . loop \ write 0 to 9
OutFile close \ close the file
s" outfile.dat" output append + open error?
abort" Cannot open file" \ reopen in append mode
to OutFile \ save handle
OutFile use \ now write to disk
20 10 do i . loop \ write 10 to 19
OutFile close \ close the file
Take a look at the contents of this file after you've run the
program and you'll find it contains the number 0 to 19.
9.25 Using pipes
If you're using Windows 95 OSR2 (and up), Linux or another Unix
system you're in for a treat! With Unix you can do neat tricks
like this:
ls | mail root
Which means you can redirect the output of 'ls' to 'mail', so in
effect you send an email to root with the contents of your
current working directory. Yes, 4tH can do this too, but you can
do even more. You can start 'ls' and read its output line by line
as if it were a file. You can also start 'mail' and write the
output of a 4tH program to it. We do that by opening a pipe to a
program.
If you've ever written a program using C, you know this is a bit
cumbersome, since you've got to use special functions to use
pipes. In 4tH you don't. Just let 4tH know it's a pipe that
you're opening and not a file:
file InFile \ value for input file
file OutFile \ value for output file
s" ls" input pipe + open error?
s" mail hans" output pipe + open error? rot or
abort" Cannot open pipe"
to OutFile to InFile
The only thing you have to do to signal 4tH that you're using a
pipe is add the word 'PIPE', just like 'APPEND'. The filename is
replaced by the command you want to execute. That's all. If one
of the pipes in this program fails, the program aborts.
InFile use OutFile use
." These are the contents of my current working directory:"
cr cr
Now we can treat our pipes just as if they are ordinary files. We
redirect input and output and write a nice header to our email.
Now we can start to process the output of 'ls':
begin
refill
while
0 parse-word type cr
repeat
Note that we don't have to signal that we're reading the pipe to
'ls' as a text file. We just read it line by line until 'REFILL'
returns zero. Then we can parse the line and 'TYPE' it to 'mail'.
InFile close OutFile close
Of course you don't have to close the pipe, but it won't harm
when you don't. 4tH knows what to do. After executing this
program, Hans will receive this email:
From: Hans Bezemer <[email protected]>
Message-Id: <[email protected]>
To: [email protected]
Status: RO
These are the contents of my current working directory:
4th.c
4thc.c
4thd.c
4thg.c
4thx.c
Well, that wasn't too hard, was it?
9.26 Opening a file in read/write mode
For special purposes you might want to open a file in read/write
mode. That's quite easy:
file InOutFile \ value for output file
s" outfile.dat" output input + open error?
abort" Cannot open file" \ open the file
to InOutFile \ save handle
Just add both together like you adding a modifier. Note that once
you 'USE' this file, you're both reading and writing to this
file. Furthermore, the file has to exist otherwise you get an
error. If you want to write to a new file, you first have to open
it in write mode:
file InOutFile \ value for output file
\ create a new file
s" outfile.dat" output open close
s" outfile.dat" output input + open error?
abort" Cannot open file" \ open the file
to InOutFile \ save handle
9.27 Using random access files
Upto now we've always accessed a file sequentially, but it is
also possible to use random access files. Two words are crucial
here, 'SEEK' and 'TELL'. 'SEEK' will seek for the desired file
position and 'TELL' will tell you that you're there. It is as
simple as that..!
Let's take a look at this example. We've got a block-file called
"Messages.scr" with the following contents:
Scr # 0
0 (0) No errors
1 (1) Out of memory
2 (2) Bad object
3 (3) Stack overflow
4 (4) Stack empty
5 (5) Return stack overflow
6 (6) Return stack empty
7 (7) Bad string
8 (8) Bad variable
9 (9) Bad address
10 (A) Divide by zero
11 (B) Bad token
12 (C) Bad radix
13 (D) Undefined name
14 (E) I/O error
15 (F) Assertion failed
First, let's define a word that reads a message and then displays
it:
: next-msg pad dup 64 accept -trailing type cr ;
Since this is a simple program we can safely use the 'PAD' to
store our messages. Every message has the length of a standard
block-file line, which is 64 characters. Trailing spaces are
stripped by '-TRAILING'. Now we need a word that tells us what
our file position is:
: tell-msg cr ." Current position: " dup tell . cr ;
'TELL' needs a file pointer and leaves the current position of
that file pointer on the stack. This word assumes that the top of
the stack contains a valid file pointer. Finally we need a word
that sets the file position:
: seek-msg over seek abort" Seek failed" ;
'SEEK' needs a file position and a file pointer. If it returns
false, it was successful; if it returns true there was an error.
This word assumes that the top of the stack contains a valid file
pointer. We're ready now, let's play. First we open the file and
use it:
s" Messages.scr" input open dup use
This leaves a file pointer on the top of the stack, assuming
everything went OK. Now let's read some messages:
next-msg next-msg next-msg tell-msg
You'll see these messages appear on the screen:
(0) No errors
(1) Out of memory
(2) Bad object
Current position: 192
After reading three messages we've obviously reached position 192
in the file. That makes sense, since 3 lines of 64 characters
makes 192 characters in total. Let's see what 'SEEK' does:
0 seek-msg tell-msg next-msg
This should take us back to the very beginning of the file, as if
we've freshly opened it. And yes, it does:
Current position: 0
(0) No errors
After executing 'SEEK', 'TELL' confirms that we've actually
returned to the very beginning of the file. Reading the next
message reconfirms that again. When you feed 'SEEK' positive
values, it always starts seeking from the beginning of the file.
When you feed 'SEEK' negative values, it seeks from the end of
the file. So this one takes you to the last line:
-64 seek-msg tell-msg next-msg
On screen it looks like this:
Current position: 960
(F) Assertion failed
Finally, we clean up the mess we made:
close
This will consume the file pointer we left on the stack and close
the file. Note that 'SEEK' and 'TELL' come with a few
restrictions. Pipes are out of the question and so are the
standard streams 'STDIN' and 'STDOUT'. Apart from that you can
pretty much do with them what you want.
9.28 The layout of the I/O system
You're probably quite confident manipulating files now, so I
guess it is time to offer you a view under the hood. 4tH has two
channels, an input channel and an output channel. All words read
from the input channel or write to the output channel. At
startup, the input channel is connected a stream that reads from
the keyboard ('STDIN') and the output channel is connected to a
stream that writes to the screen ('STDOUT').
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/4tHIO.eps>
[Senseless!!!
The 4tH I/O system
]
]
With 'OPEN' you can open additional streams, which are connected
to a file or a pipe. The return value of 'OPEN' points to the
stream that was opened. There are few words that directly handle
streams, 'USE', 'CLOSE', 'TELL' and 'SEEK' being the exceptions.
'USE' attaches a stream to one or both channels, which results in
redirecting all in- and/or output to that stream. E.g. if a file
is opened in read/write mode using 'OPEN', a stream is returned.
If we 'USE' that stream, both the input and the output channel
are connected to that stream. If it had only been opened in read
mode, only the input channel would have been connected to the
stream.
'CLOSE' closes a stream, even if it is still attached to a
channel. If that is the case, the appropriate default streams
('STDIN', 'STDOUT') are reattached. We can find out which streams
are currently used by 'CIN' and 'COUT'. 'CIN' returns the stream
that is currently attached to the input channel, 'COUT' returns
the stream that is currently attached to the output channel.
9.29 Speech synthesis
If you're using Unix[footnote:
There might still be an MS-Windows package available at
http://flame.cs.dal.ca/~lalita/festival/festivalXP.htm. This has
not been tested.
], you're in for a treat. 4tH can talk! All you need is the ”
Festival” speech synthesis package[footnote:
Homepage: http://www.cstr.ed.ac.uk/projects/festival/.
] and a small 4tH interface. If you want to imitate old Arnold,
this will do:
include lib/say.4th
s" I'll be back!" say abort" Festival not available"
Well, that's cool, isn't it?
9.30 Using a printer
How you access a printer depends on the operating system you're
working on. That is not a flaw of 4tH, you will encounter this
problem with every programming language. If you're working with
MS-DOS or MS-Windows it is quite basic:
file printer \ value for printer
s" lpt1" output open error?
if
drop
else
to printer
printer use
." This will be printed." cr
stdout use
then
Just open the port as a file and print to it. Unix isn't that
different, but instead of opening a file, you open a pipe:
file printer \ value for printer
s" lp" output pipe + open error?
if
drop
else
to printer
printer use
." This will be printed." cr
stdout use
then
If you're using a different Operating System, you may have to
check your manual.
9.31 The layout of the Character Segment
The final topic of this chapter again. You already know that 4tH
checks whether an operation is still within the Character
Segment. However, sometimes you want to check this yourself.
You already know how you can obtain the size of TIB and PAD. Yes,
you can using '/TIB' and '/PAD'. But TIB and PAD have their
addresses too. And when you query them, you will find that PAD
comes after TIB:
." Address of TIB: " tib . cr
." Size of TIB : " /tib . cr
." Address of PAD: " pad . cr
." Size of PAD : " /pad . cr
And beyond PAD, what is there? Well, allocated memory of course.
Things you defined using 'STRING'. There are two words which can
give you information about allocated memory. First, 'LO'. 'LO'
gives you the lowest address of allocated memory. Second, 'HI'.
'HI' gives you the highest valid address of the Character
Segment. That means that:
0 hi c!
Is always valid and:
0 hi char+ c!
Is always invalid. It you try it, 4tH will stop executing the
program with an error-message. 'LO' and 'HI' are addresses.
Addresses are just numbers, so you can print and compare them.
E.g.
hi char+ lo - . cr
Will print how much memory as allocated to your strings. And:
lo hi >
Will indicate whether you allocated any memory at all. If 'LO' is
greater than 'HI', you didn't. If 'HI' is greater or equal to
'LO' you did. Experiment a bit with the knowledge you obtained in
this chapter and continue with the next one where we will go much
deeper into the secrets of the Integer Segment and Code Segment.
Integer Segment and Code Segment
10.1 The Code Segment
It is know by designers of microprocessors that a processor can
run much faster when every instruction has the same length. In
fact, 4tH has his own virtual microprocessor. The compiler is
nothing more than an assembler and the interpreter nothing more
than an emulator on top of the real microprocessor.
In order to speed up 4tH, all instructions have the same length.
They consist of a token (which is the real instruction) and an
argument. The argument is a value that gives meaning to the
instruction, e.g. the 'LITERAL' token means that a number is
compiled here. The argument is the actual number.
Some instructions wouldn't need an argument, but for speeds sake,
they have: it is always zero. Isn't that a lot overhead? Not
really. Half the instructions in an actual program need an
argument. Decoding a more elaborate scheme would need more
processor time and more programming. So in the end, it would make
hardly any difference. Except for the speed.
A token with its argument is called a word. And the Code Segment
is one large array of words. Each of these words has an address
and can be accessed by the word '@C'. In fact, '@C' throws the
argument on the stack. Where have we seen '@C' before?
Yes, when fetching from an array of constants. These arrays are
compiled into the Code Segment. How come that 4tH isn't confused
by these arrays? Because they have the token 'NOOP', which does
absolutely nothing.
10.2 The address of a colon-definition
You can get the address of a colon definition by using the word
''' (tick):
: add + ; \ a colon definition
' add . cr \ display address
Very nice, but what good is it for? Well, first of all the
construction "' ADD" throws the address of "ADD" on the stack. In
fact, it is a literal expression. You can assign it to a
variable, define a constant for it, or compile it into an array
of constants:
' add constant add-address
variable addr
' add addr !
create addresses ' add ,
Are you with us so far? If we would simply write "ADD", "ADD"
would be executed right away and no value would be left on the
stack. Tick forces 4tH to throw the address of "ADD" on the stack
instead of executing "ADD".
Note this only works for your own colon-definitions. It doesn't
work for 4tHs built-in words. If you try to, you'll get an
error-message. What you can actually do with it, we will show you
in the next section.
10.3 Vectored execution
<VectoredExecution>This is a thing that can be terribly difficult
in other languages, but is extremely easy in Forth. Maybe you've
ever seen a BASIC program like this:
10 LET A=40
20 GOSUB A
30 END
40 PRINT "Hello"
50 RETURN
60 PRINT "Goodbye"
70 RETURN
If you execute this program, it will print "Hello". If you change
variable "A" to "60", it will print "Goodbye". In fact, the mere
expression "GOSUB A" can do two different things. In 4tH you can
do this much more comfortable:
: goodbye ." Goodbye" cr ;
: hello ." Hello" cr ;
variable a
: greet a @ execute ;
' hello a !
greet
' goodbye a !
greet
What are we doing here? First, we define a few colon-definitions,
called "HELLO" and "GOODBYE". Second, we define a variable called
"A". Third, we define another colon-definition which fetches the
value of "A" and executes it by calling 'EXECUTE'. Then, we get
the address of "HELLO" (by using "' HELLO") and assign it to "A"
(by using "A !"). Finally, we execute "GREET" and it says
"Hello".
It seems as if "GREET" is simply an alias for "HELLO", but if it
were it would print "Hello" throughout the program. However, the
second time we execute "GREET", it prints "Goodbye". That is
because we assigned the address of "GOODBYE" to "A".
The trick behind this all is 'EXECUTE'. 'EXECUTE' takes the
address of e.g. "HELLO" from the stack and calls it. In fact, the
expression:
hello
Is equivalent to:
' hello execute
This can be extremely useful as we will see in the next chapter
when we build a full-fledged interpreter. We'll give you a little
hint:
create subs ' hello , ' goodbye ,
Does this give you any ideas?
10.4 The Integer Segment
Wonder where all these variables are created? Or where that
infamous stack really is? I bet you do. Well, when you define a
variable, memory is allocated in the Integer Segment. When you
define another one, space is allocated after the first variable.
That means that if you go beyond the boundaries of the first
variable, you'll end up in the space allocated to the second
variable.
After the second variable there is a void. If you end up there
your program will end with an error-message. However, if you
define an 'ARRAY', single variable is created with a number of
additional cells. You can only access these additional by
referring to the array itself.
And what about the space before the first variable? There are
other variables and they are not defined by you. Well, take a
look at figure [cap:Integer-segment].
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/intseg.eps>
[Senseless!!!
Integer segment<cap:Integer-segment>
]
]Lower memory is at the bottom. The user variables are the
variables you defined yourself. The application variables can
differ from host program to host program. Refer to your
documentation on that subject. You have already seen the 4tH
variables, which are 'BASE' and '>IN'. There are also variables
you cannot access. These variables are hidden and only used by
the system. All these variables are located in the Variable Area.
There is also a Stack Area, which contain the datastack and the
returnstack. If you enter a number like "5", it is thrown on the
datastack. Most words in 4tH take or put numbers on the
datastack. It is very heavily used. We'll come to the returnstack
later on.
The datastack and the returnstack share the same memory space.
The datastack grows upward and the returnstack downward. If they
clash the stack is full and 4tH will issue an error-message.
10.5 A portable way to access application variables
A host program can add special variables to the 4tH environment.
If 4tH is used as a scripting language in e.g. a printer program,
the programmer can "send" variables to 4tH. These variables are
called "application variables". Do not confuse them with 4tH
variables, like 'BASE' or '>IN' which are used internally by 4tH.
4tH doesn't do anything with application variables.
If the creator of the host program provided special names for
each of these variables, he will probably have documented them.
However, even if he didn't there is another way to access these
variables.
They are stored in a predefined array called 'APP' and its values
can be fetched like any other array, e.g.:
app 1 th @
Which fetches the value of the second element in the array. This
also enables you to write programs that can be compiled and run
under all "standard" versions of 4tH.
10.6 Returning a result to the host program
The 'APP' array can feed values from the host program to yours,
but it can't return any. For that you need 'RESULT', the third
4tH variable. Returning a value is very easy. Just store it in
'RESULT'. Let's assume the host program has send two values to
the 'APP' array and you want to return the sum. All you have to
do is add them and store the result in 'RESULT':
app 0 th @ app 1 th @ + result !
Nothing to it..
10.7 Using commandline arguments
A host program can also transfer an array of strings to the 4tH
environment. Usually, commandline arguments will be transferred
this way, although any string array with the correct format can
be used. If so, you will probably find it in the documentation of
the host program.
If you are familiar with C, the concept is probably quite easy to
understand. There are two words, 'ARGN' and 'ARGS'. 'ARGN' will
leave the number of commandline arguments on the stack. The
commandline arguments itself are numbered from 0 to (ARGN - 1),
e.g.
argn 0> / test if there are
if / any arguments
argn 0 do / loop through them
i args type cr / print them
loop
then
First, we test if there are any commandline arguments. Second, if
that is the case we loop through them with 'ARGN' as upper limit.
Why? Since "ARGN 1- ARGS" is always the last valid commandline
argument!
Third, when 'ARGS' executes, it takes a number from the stack as
index. Then it leaves the address of the Character Segment (where
it is temporarily stored, usually PAD) and its count on the
stack.
Using the expression "TYPE CR" we can print that string. Because
it is already stored in the Character Segment we can treat it
like any other string. Remember, that if you don't save it
anywhere else it won't last long!
10.8 The layout of the Variable Area
There are special words that allow you to get information about
the layout of the Variable Area. They are called 'VARS', 'APP',
'FIRST' and 'LAST'.
'VARS' is the address of the very first variable. Before that is
the Stack Area and other variables you are not allowed to touch.
'APP' is the address of the first application variable. All
variables before that are 4tHs own built-in variables. 'FIRST' is
the address of the first user-variable, a variable you defined
yourself in your 4tH program. 'LAST' is the address of the last
accessible variable, so
last ?
will never fail. The first question that will pop in your mind
is, what can I do with them. Well, you can use it to see how many
variables there are of a certain kind, so you can prevent runtime
errors:
." number of 4tH variables: " app vars - . cr
." number of application variables: " first app - . cr
." number of user variables: " last first - 1+ . cr
These tests are possible too:
app vars - 0= if ." No 4tH variables" cr then
first app - 0= if ." No application variables" cr then
last first - 1+ 0= if ." No user variables" cr then
This is a general test to see whether the address of any variable
is within range:
dup 0<
dup last >
or
if ." Out of range" cr then
You can use this check on numeric arrays too, of course.
10.9 The stacks
The Stack Area contains two stacks. So far we've talked about one
stack, which is the Data Stack. The Data Stack is heavily used,
e.g. when you execute this code:
2 3 + .
Only the Data Stack is used. First, "2" is thrown on it. Second,
"3" is thrown on it. Third, '+' takes both values from the stack
and returns the sum. Fourth, this value is taken from the stack
by '.' and displayed. So where do we need the other stack for?
Well, we need it when we want to call a colon-definition. Before
execution continues at the colon-definition, it saves the address
of the currently executed token in the Code Segment on the other
stack, which is called the Return Stack for obvious reasons.
Then execution continues at the colon-definition. Every
colon-definition is terminated by ';', which compiles into
'EXIT'. When 'EXIT' is encountered, the address on top of the
Return Stack is popped. Execution then continues at that address,
which in fact is the place where we came from.
If we would store that address on the Data Stack, things would go
wrong, because we can never be sure how many values were on that
stack when we called the colon-definition, nor would be know how
many there are on that stack when we encounter 'EXIT'. A separate
stack takes care of that.
Try and figure out how this algorithm works when we call a
colon-definition from a colon-definition and you will see that it
works (4tH is proof of that).
It now becomes clear how 'EXECUTE' works. When 'EXECUTE' is
called, the address of the colon-definition is on the Data Stack.
All 'EXECUTE' does is copy its address on the Return Stack, take
the address from the Data Stack and call it. 'EXIT' never knows
the difference..
But the Return Stack is used by other words too. Like 'DO' and
'LOOP'. 'DO' takes the limit and the counter from the Data Stack
and puts them on the Return Stack. 'LOOP' takes both of them from
the Return Stack and compares them. If they don't match, it
continues execution after 'DO'. That is one of the reasons that
you cannot split a 'DO..'LOOP'.
However, if you call a colon-definition from within a
'DO'..'LOOP' you will see it works: the return address is put on
top of the limit and the counter. As long as you keep the Return
Stack balanced (which isn't too hard) you can get away with quite
a few things as we will see in the following section.
10.10 Saving temporary values
We haven't shown you how the Return Stack works just for the fun
of it. Although it is an area that is almost exclusively used by
the system you can use it too.
We know we can manipulate the Data Stack only three items deep
(using 'ROT'). Most of the time that is more than enough, but
sometimes it isn't.
In 4tH there are special words to manipulate stack items in
pairs, e.g. '2DUP' ( n1 n2 -- n1 n2 n1 n2) or '2DROP' ( n1 n2
--). Although they are already part of 4tH, we could easily
define those two ourselves:
: 2dup over over ;
: 2drop drop drop ;
You will notice that '2SWAP' ( n1 n2 n3 n4 -- n3 n4 n1 n2)
becomes a lot harder. How can we get this deep? You can use the
Return Stack for that..
The word '>R' takes an item from the Data Stack and puts it on
the Return Stack. The word 'R>' does it the other way around. It
takes the topmost item from the Return Stack and puts it on the
Data Stack. Let's try it out:
: 2swap ( n1 n2 n3 n4) \ four items on the stack
rot ( n1 n3 n4 n2) \ rotate the topmost three
>r ( n1 n3 n4) \ n2 is now on the Return Stack
rot ( n3 n4 n1) \ rotate other items
r> ( n3 n4 n1 n2) \ get n2 from the Return Stack
;
And why does it work in this colon-definition? Why doesn't the
program go haywire? Because the Return Stack is and was perfectly
balanced. The only thing we had to do was to get off "n2" before
the semi-colon was encountered. Remember, the semi-colon compiles
into 'EXIT' and 'EXIT' pops a return-address from the Return
Stack. Okay, let me show you the Return Stack effects:
: 2swap ( r1)
rot ( r1)
>r ( r1 n2)
rot ( r1 n2)
r> ( r1)
; ( --)
Note, these are the Return Stack effects! "R1" is the
return-address. And it is there on top on the Return Stack when
'EXIT' is encountered. The general rule is:
"Clean up your mess inside a colon-definition"
If you save two values on the Return Stack, get them off there
before you attempt to leave. If you save three, get three off.
And so on. This means you have to be very careful with looping
and branching. Otherwise you have a program that works perfectly
in one situation and not in another:
: this-wont-work ( n1 n2 -- n1 n2)
>r ( n1)
0= if ( --)
r> ( n2)
dup ( n2 n2)
else
1 2 ( 1 2)
then
;
This program will work perfectly if n1 equals zero. Why? Let's
look at the Return Stack effects:
: this-wont-work ( r1)
>r ( r1 n2)
0= if ( r1 n2)
r> ( r1)
dup ( r1)
else ( r1 n2)
1 2 ( r1 n2)
then
;
You see when it enters the 'ELSE' clause the Return Stack is
never cleaned up, so 4tH attempts to return to the wrong address.
Avoid this, since this can be very hard bugs to fix.
10.11 The Return Stack and the DO..LOOP
We've already told you that the limit and the counter of a
DO..LOOP (or DO..+LOOP) are stored on the Return Stack. But how
does this affect saving values in the middle of a loop? Well,
this example will make that quite clear:
1 ( n)
10 0 do ( n)
>r ( --)
i . ( --)
r> ( n)
loop ( n)
cr ( n)
drop ( --)
You might expect that it will show you the value of the counter
ten times. In fact, it doesn't. Let's take a look at the Return
Stack:
1 ( --)
10 0 do ( l c)
>r ( l c n)
i . ( l c n)
r> ( l c)
loop ( --)
cr ( --)
drop ( --)
You might have noticed that it prints ten times the number "1".
Where does it come from? Usually 'I' prints the value of the
counter, which is on top of the Return Stack.
This time it isn't: the number "1" is there. So 'I' thinks that
"1" is actually the counter and displays it. Since that value is
removed from the Return Stack when 'LOOP' is encountered, it
doesn't do much harm.
We see that we can safely store temporary values on the Return
Stack inside a DO..LOOP, but we have to clean up the mess, before
we encounter 'LOOP'. So, this rule applies here too:
"Clean up your mess inside a DO..LOOP"
But we still have to be prepared that the word 'I' will not
provide the expected result (which is the current value of the
counter). In fact, 'I' does simply copy the topmost value on the
Return Stack. Which is usually correct, unless you've manipulated
the Return Stack yourself.
Note that there are other words beside 'I', which do exactly the
same thing: copy the top of the Return Stack. But they are
intended to be used outside a DO..LOOP. We'll see an example of
that in the following section.
10.12 Other Return Stack manipulations
The Return Stack can avoid some complex stack acrobatics. Stack
acrobatics? Well, you know it by now. Sometimes all these values
and addresses are just not in proper sequence, so you have to
'SWAP' and 'ROT' a lot until they are.
You can avoid some of these constructions by just moving a single
value on the Return Stack. You can return it to the Data Stack
when the time is there. Or you can use the top of the Return
Stack as a kind of local variable.
No, you don't have to move it around between both stacks all the
time and you don't have to use 'I' out of its context. There is a
well-established word, which does the same thing: 'R@'. This is
an example of the use of 'R@':
: delete ( n )
>r #lag + ( a1)
r@ #lag ( a1 a2 n2)
r@ negate ( a1 a2 n2 n3)
r# +! ( a1 a2 n2)
#lead + ( a1 a2 n2 a3)
swap cmove ( a1)
r> blanks ( --)
;
'R@' copies the top of the Return Stack to the Data Stack. This
example is taken from the 4tH-editor. It deletes "n" characters
left of the cursor. By putting the number of characters on the
Return Stack right away, its value can be fetched by 'R@' without
using 'DUP' or 'OVER'. Since it can be fetched at any time, no
'SWAP' or 'ROT' has to come in.
10.13 Altering the flow with the Return Stack
The mere fact that return addresses are kept on the stack means
that you can alter the flow of a program. This is hardly ever
necessary, but if you're a real hacker you'll try this anyway, so
we'd better give you some pointers on how it is done. Let's take
a look at this program. Note that we comment on the Return Stack
effects:
: soup ." soup " ; ( r1 r2)
: dessert ." dessert " ; ( r1 r6)
: chicken ." chicken " ; ( r1 r3 r4)
: rice ." rice " ; ( r1 r3 r5)
: entree chicken rice ; ( r1 r3)
: dinner soup entree dessert ; ( r1)
dinner cr ( --)
And this is the output:
soup chicken rice dessert
Before we execute "DINNER" the Return Stack is empty. When we
enter "DINNER" the return address to the main program is on the
Return Stack (r1).
"DINNER" calls "SOUP". When we enter "SOUP" the return address to
"DINNER" is on the Return Stack (r2). When we are done with
"SOUP", its return address disappears from the Return Stack and
execution continues within "DINNER".
Then "ENTREE" is called, putting another return address on the
Return Stack (r3). "ENTREE" on its turn, calls "CHICKEN". Another
return address (r4) is put on the Return Stack. Let's take a look
on what currently lies on the Return Stack:
- Top Of Return Stack (TORS)
r4 returns to ENTREE
r3 returns to DINNER
r1 returns to main program
As we already know, ';' compiles an 'EXIT', which takes the TORS
and jumps to that address. What if we lose the current TORS? Will
the system crash?
Apart from other stack effects (e.g. too few or the wrong data
are left on the Data Stack) nothing will go wrong. Unless the
colon-definition was called from inside a DO..LOOP, of course.
But what DOES happen? The solution is provided by the table: it
will jump back to "DINNER" and continue execution from there.
: soup ." soup " ; ( r1 r2)
: dessert ." dessert " ; ( r1 r6)
: chicken ." chicken " r> drop ; ( r1 r3 - r4 gets lost!)
: rice ." rice " ; ( r1 r3 r5)
: entree chicken rice ; ( r1 r3)
: dinner soup entree dessert ; ( r1)
dinner cr ( --)
Since "CHICKEN" gets rid of the return address to "ENTREE",
"RICE" is never called. Instead, a jump is made to "DINNER" that
assumes that "ENTREE" is done, so it continues with "DESSERT".
This is the output:
soup chicken dessert
Note that this is not common practice and we do not encourage its
use. However, it gives you a pretty good idea how the Return
Stack is used by the system.
10.14 Leaving a colon-definition
You can sometimes achieve the very same effect by using the word
'EXIT' on a strategic place. We've already encountered 'EXIT'. It
is the actual word that is compiled by ';'.
What you didn't know is that you can compile an 'EXIT' without
using a ';'. And it does the very same thing: it pops the return
address from the Return Stack and jumps to it. Let's take a look
at our slightly modified previous example:
: soup ." soup " ; ( r1 r2)
: dessert ." dessert " ; ( r1 r6)
: chicken ." chicken " ; ( r1 r3 r4)
: rice ." rice " ; ( is never reached)
: entree chicken exit rice ; ( r1 r3)
: dinner soup entree dessert ; ( r1)
dinner cr ( --)
After "CHICKEN" has been executed by "ENTREE", an 'EXIT' is
encountered. 'EXIT' works just like ';', so 4tH thinks the
colon-definition has come to an end and jumps back to "DINNER".
It never comes to calling "RICE", so the output is:
soup chicken dessert
'EXIT' is mostly used in combination with some kind of branching
like IF..ELSE..THEN. Compare it with 'LEAVE' that leaves a
DO..LOOP early.
But now for the big question: what is the difference between
'EXIT' and ';'? Both compile an 'EXIT', but they are not aliases.
4tH will try to match every ';' with a ':'. If it doesn't
succeed, it will issue an error message. This matching is not
performed by 'EXIT'.
10.15 The layout of the Stack Area
Before we tell you how to obtain information on the Stack Area,
we first have to explain you how it is laid out. We've already
seen that there are two stacks: the Data Stack and the Return
Stack. We also know what they are used for.
The next question is what part of the Stack Area is used by the
Data Stack and what part is used by the Return Stack. In fact,
both stacks share the very same Stack Area.
The Data Stack grows upward from the bottom and the Return Stack
grows downward from the top. When they meet, you're in trouble.
If the Return Stack causes the overflow, 4tH will report that the
Return Stack overflowed. If it was the Data Stack, it will report
that the Data Stack overflowed.
If an overflow happens, you can't say which stack actually
overflowed. If the Data Stack filled up the Stack Area and a
colon-definition tries to put a return address on the Return
Stack, the Return Stack will get the blame.
Now for the good news. Because of this shared stack space,
programs with different requirements can run without having to
modify stack sizes (you can't do that; only the programmer of
your application can). It can be a program that heavily uses the
Return Stack (recursive colon-definitions) or a program that
needs lots of data on the Data Stack.
What you can check is how big the Stack Area actually is. It is a
constant named 'STACK'. It will report the size in cells. Every
value on any stack (address or value) takes up a single cell.
You can also ask 4tH how many values are on the Data Stack using
'DEPTH'. It will report the number of values, before you executed
'DEPTH'. Let's elaborate on that a little more:
." Begin" cr \ no values on the stack
10 \ 1 value on the stack
5 \ 2 values on the stack
9 \ 3 values on the stack
depth \ 4 values on the stack
. cr \ 4tH reports "3"
If you want to know what values the actual stack pointers have,
you have to use 'SP@' and 'RP@'. By subtracting 'SP@' from 'RP@'
you can see how much space is left in the Stack Area:
rp@ sp@ -
." Space left: " . ." cells" cr
10.16 Booleans and numbers
You might have expected we had discussed this subject much
earlier. But we haven't and for one very good reason. We've told
you a few chapters ago that 'IF' branches if the top of the stack
is non-zero. Any number will do. So you would expect that this
program will print "I'm here":
1 2 and
if
." I'm here"
then
In fact, it doesn't! Why? Well, 'AND' is a BINARY operator, not a
LOGICAL operator. That means it reacts on bit-patterns. Given two
numbers, it will evaluate bits at the same position.
The number "1" is "01" in binary. The number "2" is "10" in
binary. 'AND' will evaluate the first bit (binary digit, now you
know where that came from!). The first bit is the rightmost bit,
so "0" for the number "2" and "1" for the number "1".
'AND' works on a simple rule, if both bits are "1" the result
will be "1" on that position. Otherwise it will be "0". So "1"
and "0" are "0". The evaluation of the second bit has the same
result: "0". We're stuck with a number that is "0". False. So
'IF' concludes that the expression is not true:
2 base ! [binary] \ set radix to binary
10 \ binary number "2"
01 AND \ binary number "1"
. cr \ binary result after AND
It will print "0". However, "3" and "2" would work just fine:
2 base ! [binary] \ set radix to binary
10 \ binary number "2"
11 AND \ binary number "3"
. cr \ binary result after AND
It will print "10". The same applies to other binary operators as
'OR' and 'INVERT'. 'OR' works just like 'AND' but works the other
way around. If both bits are "0" the result will be "0" on that
position. Otherwise it will be "1":
2 base ! [binary] \ set radix to binary
10 \ binary number "2"
01 OR \ binary number "1"
. cr \ binary result after OR
It will print "11". We do not encourage the use of 'INVERT' for
logical operations. You should use '0=' instead.
'0=' takes the top of the stack and leave a true-flag if it is
zero. Otherwise it will leave a false-flag. That means that if a
condition is true (non-zero), it will leave a false-flag. Which
is exactly what a logical NOT should do.
Take a look at his brother '0<>'. '0<>' takes the top of the
stack and leaves a true-flag if it is non-zero. Otherwise it will
leave a false-flag.
The funny thing is 'AND' and 'OR' work perfectly with flags and
behave as expected. '0<>' will convert a value to a flag for you.
So this works:
1 0<>
2 0<>
and if
." I'm here" cr
then
Of course, you don't have to use '0<>' when a word returns a
flag. You should check the glossary for details on that.
10.17 Using ' with other names
So far we've only used ''' (tick) with colon-definitions, but you
can also use it with all constants, variables, values, strings,
vectors (see section [Vectors]) and constant arrays. However, the
information it provides is not always useful. E.g. the
expression:
10 constant ten
' ten
Does not compile differently from:
10 constant ten
ten
The same applies to constant arrays and strings. It will give you
possibly information on the address of variables, vectors, arrays
and values, e.g.:
variable ten
' variable ." Relative address of ten: " . cr
Yes, relative address! What does that mean? When a 4tH program is
compiled it has no idea how many application variables a host
program will provide. So it stores a relative address. This
address is relative to the address returned by 'FIRST'. You might
call it an offset if you want to. 4tH provides a word which will
convert the relative address of vectors, variables, values and
numeric arrays to an absolute address, called '>BODY'. So this
piece of code does exactly the same thing:
variable ten
ten \ throw address of 'ten' on stack
dup \ duplicate address
10 swap ! \ store 10 at address
? cr \ show value stored at address
As this piece of code:
variable ten
' ten >body \ calculate address
dup \ duplicate address
10 swap ! \ store 10 at address
? cr \ show value stored at address
There are not too many occasions where this is useful, but it
let's take a look at this one:
0 value ten \ define a value
' ten >body \ calculate address of value
dup \ duplicate address
10 swap ! \ store 10 at address
? cr \ show value stored at address
We already know that values, numeric arrays and vectors are
stored in the very same area of the Integer Segment. This
construction makes it possible to access them as variables.
You can access string constants or arrays of string constants
with tick, but they will return a value which only has a meaning
to 4tH itself. You won't be able to do anything useful with those
values.
You should avoid these kind of constructions, but there might be
some situations out there where it might come in handy. Note that
you can only tick your own names. All of 4tHs built-in variables,
strings, words, etc. cannot be accessed by tick.
10.18 Assertions
You have probably seen this before: you've made a program,
compiled it and it doesn't work. Then you start putting code at
strategic places, trying to pinpoint the error. And when you're
finally done, you've got to revisit all of these places to remove
that code. And you probably forget a few..
4tH has a built-in facility which allows to put that code there,
debug your program and remove the debugging code from your
program by changing a single line.
It is called "assertion" and those of you who have ever worked
with C probably know what we're talking about.
An assertion is a line of code that will evaluate an expression.
If the expression evaluates to false, it will exit the program
with an error message. Let's take a look at this simple
colon-definition:
: add \ expects two numbers on the stack
+
;
If we call add by writing:
1 add
it will fail. Now we add this assertion:
assert( depth 2 >= )
It will evaluate to false when there are less than two items on
the stack. The program will be terminated and the appropriate
error message will be issued. You may think that this is nice,
but you still have to remove all assertion manually.
Not true! If you tried this out already you will see that you
won't find an assertion anywhere. It's gone! True, if you want to
use assertions you have to enable them. You do that with the word
'[ASSERT]':
[assert]
: add
assert( depth 2 >= )
+
;
1 add
Now assertions will compile and work. If you remove the word
'[ASSERT]' all assertions will disappear like they were comment.
'[ASSERT]' works just like '[DECIMAL]', '[HEX]', etc. They work
linear and do not follow the program flow. If you put '[ASSERT]'
halfway your source-file you will notice that assertions work
from that point:
: add
assert( depth 2 >= ) \ assertions disabled
+
;
[assert] \ enable assertions
: print-hex
base @ >r hex
assert( depth 1 >= ) \ assertions enabled
. cr r> base !
;
Assertions are only enabled in "PRINT-HEX". The assertion inside
"ADD" will be removed and thus be disabled. But there is more to
'[ASSERT]' than the eye meets. It doesn't enable assertions, it
toggles them. When the 4tH compiler starts, assertions are
disabled. The first '[ASSERT]' enables them. A second '[ASSERT]'
will disable them again:
[assert] \ enable assertions
: add
assert( depth 2 >= ) \ assertions enabled
+
;
[assert] \ disable assertions
: print-hex
base @ >r hex
assert( depth 1 >= ) \ assertions disabled
. cr r> base !
;
There are many possibilities:
• You can start testing low level colon-definitions and move your
way up to the high level definitions by moving '[ASSERT]' down.
• You can enable assertions on certain parts of your code by
enclosing them with an '[ASSERT]' pair.
• You can switch the entire context of '[ASSERT]'s by adding a
single '[ASSERT]' to the top of your source.
You are not limited to range-checking when using 'ASSERT('. Any
expression that evaluates to TRUE is allowed:
[assert]
: add
assert( ." ADD starts at " here . cr true )
assert( depth 2 >= )
assert( ." Values: " over over . . cr true )
+
;
We're sure you can come up with more useful ideas. We did too.
10.19 Breakpoints
4tH also offers you the possibility to set breakpoints. It's
quite easy to enable this facility. Just add this to the very
beginning of your source:
[needs lib/debug.4th]
Setting a breakpoint is quite easy too, e.g. this piece of code
malfunctions:
32 string argument
1 args argument place
Change it to:
32 string argument
1 args argument ~~ place
Now the breakpoint is enabled. It will enter a Forth-like shell
just before 'PLACE' is executed. Now a host of words are at your
disposal. You can examine any region of the Character Segment
with ”DUMP” or print any string variable with ”TYPE”. 4tHs
internal variables and regions are known by name, like ”PAD”, ”
TIB”, ”>IN”, ”BASE” and ”OUT”. You can examine them or any other
variable by using ”?”, ”@” and ”.”.
You have a small calculator, that you can use to multiply,
substract, add. You can change 'BASE' by executing ”OCTAL”, ”HEX”
, ”BINARY” or ”DECIMAL”. It also has a host of binary operators
like ”OR”, ”AND”, ”XOR”, ”INVERT”, ”LSHIFT” and ”RSHIFT”. It also
has stack operators like ”DUP”, ”DROP”, ”OVER” and ”SWAP”. ”CLEAR”
will clear the stack for you.
You can examine both stacks. ”.S” will show you the data stack
(including any rubbish you put there yourself during the
debugging session) and ”R.S” will show you the return stack. ”
DEPTH” and ”RDEPTH” will tell you how many items there are on the
stack. When you're done, you may leave the debugger by typing ”
BYE”. Your program will continue as usual.
A word of caution: since the debugger is a 4tH program itself, it
doesn't actually freeze the virtual machine. It just seems like
it is frozen. The contents of PAD may be slightly different than
you expected. If you really need to examine the PAD as it was,
don't examine it directly, but use ”SPAD” to examine the ”shadow
PAD”. ”SPAD” leaves the address for the ”shadow PAD” on the
stack. The same goes for ”>IN”, ”BASE” and ”OUT”: never examine
these by address, but always by name. Although every effort has
been made to catch any errors, some extreme stress tests might
fail. It is not recommended to use the debugger when stack space
is very tight.
10.20 Random numbers
If you want to program a game or a simulation, you'll probably
need random number generation. Of course, you can do that too
with 4tH. It generates a number between 0 and 'MAX-RAND', but
we'll teach you how to generate a number for virtually any range
below that.
There are two things important when you want to do that: the
range limit and the lower limit. Say, we want to simulate a dice
with numbers in a range from 1 to 6. The lower limit is 1. We
subtract that from the upper limit to get the range limit.
So: upper limit minus lower limit gives a range limit of five (6
- 1 = 5). The general formula looks like this:
<range-limit> 1+ random * max-rand 1+ / <lower limit> +
When we apply this to the dice-example, the complete formula is:
5 1+ random * max-rand 1+ / 1 +
This will give you a dice-simulation, that produces random
numbers between 1 and 6. Happy? Then thank Wil Baden for the
algorithm!
10.21 Timers
There is a very low level word in 4tH that keeps track of time.
It has several uses. Like a timer that measures how long certain
operation takes, like the execution of a colon-definition
("DO-SOME-WORD" in this case):
time do-some-word time
swap -
." Do-Some-Word took " . ." seconds." cr
There is a somewhat more elaborate library member that does it
all for you:
[needs lib/timer.4th]
timer-reset
do-some-word
.elapsed
This always prints the number of seconds that have elapsed. If
you want to create your own display, you can define one easily:
[needs lib/timer.4th]
:noname <# # 6 base ! # decimal 58 hold # #> type ." mins" ;
is timer-stop
You define ”TIMER-STOP” after inclusion of ”time.4th”, but before
the first usage of ”.ELAPSED”.
10.22 Time & date
There is also a word in 4tH that will tell you what time and what
date it is. With a little trouble ;). The word is called 'TIME'
(again) and it will tell you how many seconds have gone since
January 1st, 1970. That is the Posix time format. You can also
find out quickly how late it is:
[needs lib/time.4th]
now ." hours:" . ." minutes:" . ." seconds:" . cr
Note that it doesn't know about daylight-saving! It does know
about timezones, which may be neccesary on some systems. You can
determine your timezone by looking at an email message from a
local friend. It will probably say somewhere:
Date: Mon, 25 Feb 2002 22:28:59 +0100 (CET)
The '+0100' means that you're in timezone CET, which is one hour
later than GMT. If it said:
Date: Sun, 16 Dec 2001 02:19:40 -0800 (PST)
This indicates that you're in timezone PST, which is eight hours
earlier than GMT. In that case 'tz' would be:
-8 3600 [*] +constant tz \ Pacific Standard Time
If you need it, define it accordingly before the inclusion of ”
time.4th”. There are also several words that will allow you to
convert any Posix time:
time posix>time . . . cr
Which will return the number of seconds (TOS), the number of
minutes and the number of hours. ”POSIX>JDAY” will convert any
Posix time to a Julian day. The day of the week is another thing
you can easily calculate:
[needs lib/time.4th]
: Weekdays
dup 0 = if drop s" Monday" exit then
dup 1 = if drop s" Tuesday" exit then
dup 2 = if drop s" Wednesday" exit then
dup 3 = if drop s" Thursday" exit then
dup 4 = if drop s" Friday" exit then
dup 5 = if drop s" Saturday" exit then
dup 6 = if drop s" Sunday" exit then
;
today weekday Weekdays type cr
By the way, didn't you hate the way we had to define "Weekdays"?
Ugly, isn't it? Well, there is a better way to do it. You'll
learn that in the next chapter (see section [LookupTables])! You
can also print the full date:
[needs lib/time.4th]
today ." year:" . ." month:" . ." day:" . cr
Just don't ask me how this thing works, Everett F. Carter figured
this one out. ”TODAY” does the easy work. ”JDATE” converts the
Julian day to the Gregorian date. There is also a way to convert
a Gregorian date to a Julian day, called ”JDAY”:
[needs lib/time.4th]
26 02 2002 jday 64 - jdate
today ." year:" . ." month:" . ." day:" . cr
This can be quite handy if you want to calculate which date it
was 64 days ago. ANS-Forth also defines a word that does it all
called ”TIME&DATE”. This word throws seconds, minutes, hours,
day, month and year (TOS) on the stack, but always returns GMT:
[needs lib/ansfacil.4th]
time&date . . . . . . cr
And finally, we even got a word that returns the date of easter:
[needs lib/easter.4th]
2005 easterSunday ." year:" . ." month:" . ." day:" . cr
Well, tell me, isn't that kind of neat?
10.23 What is not implemented
When writing a product like 4tH that is modelled after an
existing programming language like Forth one has to cut a few
corners somewhere.
Forth has a fundamentally different architecture, which allows
you to extend the compiler with ease. 4tH is much more like
conventional programming languages and many still wonder how we
got this far.
When you're learning 4tH to learn Forth you will find there are
things you can't do in 4tH. This section sums up most of the
restrictions 4tH has in comparison to Forth and other languages.
Datatypes There are no words that allow you to define your own
datatypes, although you can change the behaviour of individual
variables.
Interpreter Since 4tH is a conventional compiler, you won't find
a built-in interpreter. There is a library-source, which will
enable you to make an interpreter for specific applications with
ease. Next chapter we will show you how to use it.
If you have more questions concerning the functionality of 4tH,
please read the ANS-Forth document. This describes the compliance
of 4tH to the ANS-Forth standard. Further information can be
obtained by studying the glossary.
10.24 Known bugs and limitations
Like every software product, 4tH has bugs. Because a work-around
is available, fixing these bugs has no high priority.
• When you use '\' without any actual comment in a Unix ASCII
file the complete next line will be marked as comment. With
MS-DOS ASCII files 4tH will correctly detect a null string and
terminate with an error. Use "\ ." just to be safe.
• There can be only one space between '[DEFINED]', '[UNDEFINED]',
'[CHAR]', 'CHAR' and the string following it. If you don't
comply, 4tH will complain about empty string constants.
• You cannot comment out '[THEN]' or ')' This is bad practice
anyway.
• You cannot use 'HIDE' conditionally. If you specify a 'HIDE' it
must always be executed in 4tH.
Advanced programming
11.1 Compiletime calculations
When you've reached this chapter, you must have quite some
experience with 4tH. This chapter will help you to use 4tH to its
full capacity. You'll be able to use software exceptions,
conditional compilation, compiletime calculation, lookup tables,
fixed point calculation and much, much more.
We'll start with something wich may seem difficult at first, but
is extremely handy in some circumstances. We've already explained
that when you define a string, it has to be preceded by a literal
expression. So you cannot define something like this:
64 constant name
16 constant #names
name #names * string name_space
When you want to do this in 4tH you first have to calculate the
size of "name_space" by hand and then insert it into your
program:
64 constant name
16 constant #names
1024 string name_space
But this has a serious drawback when maintaining your code,
because when you change either "name" or "#names", you have to
remember that you have to recalculate "name_space"! Is there no
solution to this problem? Of course there is:
64 constant name
16 constant #names
name #names [*] string name_space
The word '[*]' takes two subsequent literal expressions and
multiplies them to a new single literal expression as if you'd
written "1024" yourself! The difference between:
10 10 *
and:
10 10 [*]
is essentially that the first expression will compile to two
literals and the word '*' and the second expression will compile
to just a literal.
In other words: the first expression will just compile and
quietly wait until it is evaluated at runtime, while the second
is already evaluated at compiletime. Which means that everything
that is evaluated, must already be known at compiletime, thus a
literal expression.
There are other compiletime calculations possible too. The first
one is '[+]', which adds two literal expressions. This will
compile to the literal "12":
5 7 [+]
The second one is '[/]', which compiles a quotient. This will
compile to the literal "5":
35 7 [/]
You can even mix and chain compiletime calculations. This will
compile to the literal "500":
5 25 75 [+] [*]
Just as if you'd just written "500" in the sourcecode yourself.
You can also write:
25 75 [+] 5 [*]
Because it's just simple postfix notation. Note that there must
be two subsequent literal expressions available at any time, so
this doesn't work:
5 5 [*] dup [*]
Since 'DUP' isn't a literal expression, but a word which is
simply compiled. But don't worry: 4tH will notify you when you
make an error like this. Another useful word is '[NEGATE]', e.g.
when you need to assign a negative value to a constant:
16 constant +range
+range [negate] constant -range
In this example the value of ”-RANGE” is -16. The final word,
we'd like to present you is '[NOT]', which logically inverts a
flag at compiletime, just like '0=' at runtime. This expression
will compile to a true flag:
false [not]
You might wonder why we included this one, but that will become
clear when you read the next section.
11.2 Conditional compilation
This is something which can be very handy when you're designing a
4tH program for different environments or even different Forth
compilers. Let's say you've written a general ledger program in
4tH that is so good, you can sell it. Your customers want a demo,
of course. You're willing to give one to them, but you're afraid
they're going to use the demo without ever paying for it.
One thing you can do is limit the number of entries they can
make. So, you copy the source and make a special demo version.
But you have to do that for every new release. Wouldn't it just
be easier to have one version of the program and just change one
single constant? You can with conditional compilation:
true constant DEMO
DEMO [if]
256 constant #Entries
[then]
variable CurrentEntry
DEMO [not] [if]
limit constant #Entries
[then]
#Entries array Entries
We defined a constant, called "DEMO", which is true. So, when the
compiler reaches the "DEMO [if]" line, it knows that it has to
compile "256 constant Entries", since "DEMO" is true. When it
comes to "DEMO [not] [if]", it knows it has to skip everything up
to "[then]" since "[not] DEMO" is evaluated at compiletime to
false. So, in this case the compiler behaves like you've written:
256 constant #Entries
variable CurrentEntry
#Entries array Entries
Would you change "DEMO" to false, the compiler would behave as if
you wrote:
variable CurrentEntry
limit constant #Entries
#Entries array Entries
The word '[IF]' only works at compile time and is never compiled
into the object. '[IF]' takes a literal expression. If this
expression is true, the code following the '[IF]' is compiled,
just as '[IF]' wasn't there. Is this expression false, everything
up to '[THEN]' is discarded as if it wasn't there.
That also means you can discard any code that is superfluous in
the program. E.g. when you're making a colon-definition to check
whether you can make any more entries. If you didn't use
conditional compilation, you might have written it like this:
: CheckIfFull ( n -- n)
dup #Entries = ( n f)
if ( n)
drop ( --)
DEMO ( f)
if ( --)
." Buy the full version"
else \ give message and exit program
." No more entries"
then ( --)
cr quit
then ( n)
;
But his one is nicer and will take up less code:
: CheckIfFull ( n -- n)
dup #Entries = ( n f)
if ( n)
drop ( --)
DEMO [if] ( n f)
." Buy the full version"
[then]
DEMO [not] [if]
." No more entries"
[then]
cr quit
then ( n)
;
You can also use conditional compilation to discard large chunks
of code. This is a much better way than to comment all the lines
out, e.g. this won't work anyway:
(
: room? \ is it a valid variable?
dup ( n n)
size 1- invert and ( n f)
if \ exit program
drop ." Not an element of ROOM" cr quit
then
;
)
This is pretty cumbersome and prone to error:
\ : room? \ is it a valid variable?
\ dup ( n n)
\ size 1- invert and ( n f)
\ if \ exit program
\ drop ." Not an element of ROOM" cr quit
\ then
\ ;
But this is something that can easily be handled:
false [if]
: room? \ is it a valid variable?
dup ( n n)
size 1- invert and ( n f)
if \ exit program
drop ." Not an element of ROOM" cr quit
then
;
[then]
Just change "false" to "true" and the colon-definition is part of
the program again. Note that '[IF] .. [THEN]' can be nested!
Conditional compilation is very powerful and one of the easiest
features a language can have. And it's ANS-Forth compatible!
11.3 Checking the environment at compiletime
Let's say you've written something which works perfectly on your
own machine and you want to use it on the mainframe at work. It
turns out to be it doesn't work. Why? Because your program
assumed that a cell was four address units wide. And it didn't
turn out to be that way.
You could have prevented that if you had used a check at
compiletime. You can do that this way:
/cell 4 [=] [NOT] [IF]
( do something)
[THEN]
'/CELL' is a constant which holds the number of address units in
a cell. '/CELL' has got a little brother called '/CHAR', which
will tell you how many address units there are in a character.
'[=]' will check whether a cell has four address units and
'[NOT]' will reverse that flag. Neat huh?
But then again, what do we do if it doesn't turn out to be that
way. Any action will first be executed at runtime so a message or
'ABORT' won't do. Further compilation will be useless, so we
actually want to stop. You're in luck, since we have a special
word that will stop the compiler regardless. It's called
'[ABORT]'. So this is our complete snippet:
/cell 4 [=] [NOT] [IF]
[ABORT]
[THEN]
But suppose you want to check whether a cell is at least 4
address units. '[=]' won't do in that case. Of course, you can
check every imaginable cellsize, but that is not very pretty.
That is where '[SIGN]' comes in. '[SIGN]' will take a previously
compiled literal expression and compile -1 if the number was
negative, zero if the number was zero and 1 if the number was
positive. You may wonder how that does help. Well, consider this
one:
/cell -4 [+] [SIGN] -1 [=] [IF]
[ABORT]
[THEN]
What have we been doing here? First, we substract the minimal
cellsize from the actual cellsize. If the sign of the sum is -1,
compilation is aborted. The sum can only be negative when '/CELL'
is three or smaller. Get it? By using '[SIGN]' you can make all
kinds of compiletime comparisons, which makes it a real asset.
11.4 Checking a definition at compiletime
We've already encountered 'COMPARE' in section [ComparingStrings]
. 'COMPARE' is word that compares two strings. It can do that
both case sensitive and case insensitive. If you define a
constant called ”IGNORECASE” before the '[NEEDS' directive and
set it to FALSE, it will perform a case sensitive comparison. If
you don't, it will do a case insensitive comparison by default.
Most approaches would require the definition of ”IGNORECASE”,
regardless which mode you select. This one doesn't:
[DEFINED] ignorecase [NOT] [IF]
true constant ignorecase \ default ignore case
[THEN]
: compare ( a1 n1 a2 n2 -- f )
rot over over swap - >r ( a1 a2 n2 n1)
min 0 tuck ( a1 a2 0 n 0)
?do ( a1 a2 f)
drop ( a1 a2)
over i + c@ ( a1 a2 c1)
ignorecase [IF]
dup [char] A - max-n and 26 < if bl or then
[THEN]
over i + c@ ( a1 a2 c1 c2)
ignorecase [IF]
dup [char] A - max-n and 26 < if bl or then
[THEN]
- dup ( a1 a2 f f)
if leave then ( a1 a2 f)
loop
>r drop drop r> r> swap dup ( f1 f2 f2)
if swap then drop ( f)
;
'[DEFINED]' checks whether the word following it has been defined
and leaves a TRUE flag if it was. It doesn't matter whether the
word is built-in, included or defined in your program. It can be
a variable, a word, a constant, anything you like.
In this case it checks whether ”IGNORECASE” has been defined. If
it wasn't it will define it for you. Later, it checks whether ”
IGNORECASE” is TRUE. If it is, a line of code is compiled. If it
is FALSE, it will compile that line.
If an 'IF' had been used, the code would always be compiled with
the added overhead of testing a constant at runtime. This
construction allows for tighter and faster code.
Note that '[DEFINED]' has also a counterpart called
'[UNDEFINED]'. It is equivalent to ”[DEFINED] [NOT]” and leaves a
true flag when the word following it has not been defined.
11.5 Exceptions
You know when you violate the integrity of 4tH, it will exit and
report the cause and location of the error. Wouldn't it be nice
if you could catch these errors within the program? It would save
a lot of error-checking anyway. It is quite possible to check
every value within 4tH, but it takes code and performance, which
makes your program less compact and slower.
Well, you can do that too in 4tH. And not even that, you can
trigger your own errors as well. This simple program triggers an
error and exits 4tH when you enter a "0":
[needs lib/enter.4th] \ get a number
\ if non-zero, return it
\ if zero, throw exception
: could-fail ( -- n)
enter dup 0=
if 1 throw then
;
\ drop numbers and
\ call COULD-FAIL
: do-it ( -- )
drop drop could-fail
;
\ put 2 nums on stack and
\ execute DO-IT
: try-it ( -- )
1 2 ['] do-it execute
." The number was" . cr
;
\ call TRY-IT
try-it
"TRY-IT" puts two numbers on the stack, gets the execution token
of "DO-IT" and executes it. "DO-IT" drops both numbers and calls
"COULDFAIL". "COULD-FAIL" gets a number and compares it against
"0". If zero, it calls an exception. If not, it returns the
number.
The expression "1 THROW" has the same effect as calling 'QUIT'.
The program exits, but with the error message "Unhandled
exception". You can use any positive number for 'THROW', but "0
THROW" has no effect. This is called a "user exception", which
means you defined and triggered the error.
There are also system exceptions. These are triggered by the
system, e.g. when you want to access an undefined variable or
print a number when the stack is empty. These exceptions have a
negative number, so:
throw -4
Will trigger the "Stack empty" error. You can use these if you
want but we don't recommend it, since it will confuse the users
of your program.
You're probably not interested in an alternative for 'QUIT'.
Well, 'THROW' isn't. It just enables you to "throw" an exception
and exceptions can be caught by your program. That means that 4tH
won't exit, but transfers control back to some routine. Let's do
just that:
[needs lib/enter.4th]
: could-fail ( -- n)
enter dup 0=
if 1 throw then
;
: do-it ( -- )
drop drop couldfail
;
: try-it ( -- )
1 2 ['] do-it catch
if drop drop ." There was an exception" cr
else ." The number was" . cr
then
;
try-it
The only things we changed is a somewhat more elaborate "TRY-IT"
definition and we replaced 'EXECUTE' by 'CATCH'.
'CATCH' works just like 'EXECUTE', except it returns a
result-code. If the result-code is zero, everything is okay. If
it isn't, it returns the value of 'THROW'. In this case it would
be "1", since we execute "1 THROW". That is why "0 THROW" doesn't
have any effect.
If you enter a nonzero value at the prompt, you won't see any
difference with the previous version. However, if we enter "0",
we'll get the message "There was an exception", before the
program exits.
But hey, if we got that message, that means 4tH was still in
control! In fact, it was. When "1 THROW" was executed, the
stack-pointers were restored and we were directly returned to
"TRY-IT". As if "1 THROW" performed an 'EXIT' to the token
following 'CATCH'.
Since the stack-pointers were returned to their original state,
the two values we discarded in "DO-IT" are still on the stack.
But the possibility exists they have been altered by previous
definitions. The best thing we can do is discard them.
So, the first version exited when you didn't enter a nonzero
value. The second version did too, but not after giving us a
message. Can't we make a version in which we can have another
try? Yes we can:
[needs lib/enter.4th]
: could-fail ( -- n)
enter dup 0=
if 1 throw then
;
: do-it ( -- )
drop drop could-fail
;
: retry-it ( -- )
begin
1 2 ['] do-it catch
while
drop drop ." Exception, keep trying" cr
repeat
." The number was " . cr
;
retry-it
This version will not only catch the error, but it allows us to
have another go! We can keep on entering "0", until we enter a
nonzero value. Isn't that great? But it gets even better! We can
exhaust the stack, trigger a system exception and still keep on
going. But let's take it one step at the time. First we change
"COULD-FAIL" into:
: could-fail ( -- n)
enter dup 0=
if drop ." Stack: " depth . cr 1 throw then
;
This will tell us that the stack is exhausted at his point. Let's
exhaust is a little further by redefining "COULD-FAIL" again:
: could-fail ( -- n)
enter dup 0=
if drop drop then
;
Another 'DROP'? But wouldn't that trigger an "Stack empty" error?
Yeah, it does. But instead of exiting, the program will react as
if we wrote "-4 THROW" instead of "DROP DROP". The program will
correctly report an exception when we enter "0" and act
accordingly.
This will work with virtually every runtime error. Which means we
won't have to protect our program against every possible
user-error, but let 4tH do the checking.
We won't even have to set flags in every possible
colon-definition, since 4tH will automatically skip every level
between 'THROW' and 'CATCH'. Even better, the stacks will be
restored to the same depth as they were before 'CATCH' was
called.
You can handle the error in any way you want. You can display an
error message, call some kind of error-handler, or just ignore
the error. Is that enough flexibility for you?
11.6 Mixing character and number data<NCoding>
Sometimes you have to mix character and number data, e.g. when
you're porting a Forth program or when the need complex
datastructures arises. Since 4tH gives each datatype its own
segment this is not easy. However, there is a library that can
help you. Let's have a look at this program:
16 constant /my \ size of array
/my array my \ define array
0 \ set up counter
begin
dup dup \ duplicate counter
cells my + ! \ store counter in array
1+ \ increment counter
dup /my = \ limit reached?
until drop \ drop the counter
my \ set up index
begin
dup @ . cr \ print the value
cell+ \ next element
dup my /my cells + = \ limit reached
until drop \ drop the index
This simple program defines a small array, fills and displays it.
Now, this little thing does the same thing, but is located in the
Character Segment:
include lib/ncoding.4th
\ load the library
16 constant /my \ size of array
/my nell [*] string my \ define array
0 \ set up counter
begin
dup dup \ duplicate counter
nells my + n! \ store counter in array
1+ \ increment counter
dup /my = \ limit reached?
until drop \ drop the counter
my \ set up index
begin
dup n@ . cr \ print the value
nell+ \ next element
dup my /my nells + = \ limit reached
until drop \ drop the index
You see that the code is very similar. The 'STRING' declaration
clearly indicates that the array is allocated in the Character
Segment. But as you can see it is not an array of cells, but an
array of nells. 'NELL' holds the size of a single nell, so we
multiply it by the number of nells we want to get the proper size
of the array. After that, it is just replacing the Integer
Segment words with nell equivalents:
[float Table:
+--------+-------+
| NELL | CELL |
+--------+-------+
+--------+-------+
| /nell | /cell |
+--------+-------+
| nells | cells |
+--------+-------+
| n@ | @ |
+--------+-------+
| n! | ! |
+--------+-------+
| nell+ | cell+ |
+--------+-------+
| nell- | cell- |
+--------+-------+
[Senseless!!!
NELL equivalents
]
]
Note that although you can replace every cell with a nell, you do
pay a penalty in execution speed, so use with caution.
11.7 Enumerations
Sometimes you need a lot of constants:
0 constant Monday
1 constant Tuesday
2 constant Wednesday
3 constant Thursday
4 constant Friday
5 constant Saturday
6 constant Sunday
A little error here may ruin your program. This does the very
same thing, except it is easier to maintain:
0 enum Monday enum Tuesday enum Wednesday
enum Thursday enum Friday enum Saturday
enum Sunday drop
'ENUM' is much like a 'CONSTANT', but increments and leaves a
value after the constant has been created. That is why we need to
add 'DROP' after the final enumeration. To show you that 'ENUM'
and 'CONSTANT' are much alike, you could also write the
declaration above as:
0 enum Monday enum Tuesday enum Wednesday
enum Thursday enum Friday enum Saturday
constant Sunday
Since 'CONSTANT' just consumes the value, you don't need the
final 'DROP'.
11.8 Dynamic memory allocation
If you don't know what this is, you probably shouldn't bother.
Sometimes you don't know how much memory you will actually need,
sometimes you know how much you need, but you won't need it
during the entire execution of the program. In these cases, you
can temporarily allocate a chunk of memory and release it when
you no longer need it.
4tH has similar facilities. E.g. if you want to allocate 600
bytes, you simply include "ansmem.4th" and allocate it:
[needs lib/ansmem.4th]
600 allocate
'ALLOCATE' leaves two items on the stack. The first one is a
flag. If it is true, memory allocation has failed, so we can
easily add some error checking to our little program:
[needs lib/ansmem.4th]
600 allocate abort" Out of memory"
It it returns false, memory has been allocated. Its address is
the second item on the stack. You can pretty much do what you
want with it, but remember that memory is always allocated in the
Character Segment, so if you want to store numbers over there,
read section [NCoding] again. Anyway, this is completely valid:
[needs lib/ansmem.4th]
600 allocate abort" Out of memory"
s" Hello temporary world!" rot place
Let's change that one a little bit to prove we've actually stored
anything:
[needs lib/ansmem.4th]
600 allocate abort" Out of memory"
>r s" Hello temporary world!" \ Let's save the address
r@ place \ Now store the string
r> count type cr \ Let's print the string
Let's allocate another 100 bytes and free all memory afterwards:
[needs lib/ansmem.4th]
600 allocate
abort" Out of memory" >r \ First allocation
s" Hello temporary world!"
r@ place \ Now store the string
r@ count type cr \ Let's print the string
100 allocate
abort" Out of memory" >r \ Second allocation
s" I'm a little crammed!"
r@ place \ Store another string
r@ count type cr \ Let's print the string
r> free
abort" Cannot free memory" \ Now free the first block
r> free
abort" Cannot free memory" \ Now free the second block
Yes, that's right: you feed 'FREE' the address that 'ALLOCATE'
returned and it returns a flag. If it is a true flag, an error
occurred; if not, everything is hunky dory. Let's try to free it
twice:
[needs lib/ansmem.4th]
600 allocate
abort" Out of memory" >r \ First allocation
s" Hello temporary world!"
r@ place \ Now store the string
r@ count type cr \ Let's print the string
r@ free
abort" First attempt" \ Now let's free the block
r> free
abort" Second attempt" \ And try to free it again..
Yes, now 4tH terminated with the error message ”Second attempt”.
You can not free a block twice..! But you can reallocate it if
you happen to change your mind. You can increase or decrease its
size, without losing any data. When the new block is too small to
hold all the data, the data is truncated. Let's see it in action:
[needs lib/ansmem.4th]
50 allocate
abort" Out of memory" >r \ First allocation
s" Hello temporary world!"
r@ place \ Now store the string
r@ count type cr \ Let's print the string
r> 100 resize
abort" Out of memory" >r \ Now resize the block
r@ count type cr \ Here is your string again
r> free
abort" Cannot free memory" \ Now free it
You'll see that your precious string is still alright. Apart from
a flag, 'RESIZE' also returns the address of the reallocated
block. If 'RESIZE' fails, your original data is still alright, so
in some circumstances you might want to save the old address.
Sometimes 'RESIZE' fails, even when you're decreasing the size of
a block. Well, 'RESIZE' always allocates a new block, so when
memory is low or fragmented it may not succeed.
11.9 Tweaking dynamic memory
You might find that 4tH doesn't reserve much memory for dynamic
allocation. Dynamic memory is allocated at the heap, which is 16
kB. You can increase it, but first you have to know how dynamic
memory works. You can determine how much memory has been
allocated by using the word 'ALLOCATED':
[needs lib/ansmem.4th]
50 allocate
abort" Out of memory" >r \ First allocation
r@ . ." allocates "
r@ allocated . ." bytes." cr
r> free
abort" Cannot free memory" \ Now free it
And it will print something like:
768 allocates 64 bytes.
64 bytes? I thought we allocated 50 bytes! Let's try another one:
[needs lib/ansmem.4th]
500 allocate
abort" Out of memory" >r \ First allocation
r@ . ." allocates "
r@ allocated . ." bytes." cr
r> free
abort" Cannot free memory" \ Now free it
This time it prints something like:
768 allocates 512 bytes.
As a matter of fact, 'ALLOCATED' will always return multiples of
64 bytes. That is a consequence of how 4tH handles dynamic
memory. 4tH divides dynamic memory into fragments. When you
allocate memory, 4tH allocates as much fragments as it needs to
provide you with the memory you requested. Then these fragments
are marked as 'taken'. This marking is done in the Heap
Allocation Table, which is located in the Integer Segment. Every
fragment is represented by a cell in the HAT.
You can fine-tune this mechanism by defining some constants
before including "ansmem.4th". This will create a heap with 512
fragments of 256 bytes, which is 128 kB:
512 constant #heap \ 512 fragments
256 constant /heap \ each fragment is 256 bytes
[needs lib/ansmem.4th]
500 allocate
abort" Out of memory" >r \ First allocation
r@ . ." allocates "
r@ allocated . ." bytes." cr
r> free
abort" Cannot free memory" \ Now free it
Try to keep the number of fragments low. 1024 seems like a nice
upper limit. If you need that much memory, it is much better to
handle it in larger chunks. This avoids fragmentation and keeps
the time to search the HAT within acceptable limits.
11.10 Application stacks
Did you ever feel like a second return stack would be nice? Well,
you can. As a matter of fact you can have several dedicated
stacks. It's quite easy to use:
[needs lib/stack.4th]
16 array mystack \ allocate some space
mystack stack \ convert it into a stack
234 mystack >a \ push 234 on the stack
456 mystack >a \ push 456 on the stack
mystack a@ . cr \ examine top of stack
mystack a> \ pop 456 from the stack
mystack a> \ pop 234 from the stack
. . cr \ show the values
Wouldn't it be nice to have a string stack too? Yes, 4tH provides
that one too! It works the same way:
[needs lib/stsstack.4th]
1024 string mystack \ allocate some space
mystack string-stack \ convert it to a string stack
s" Hello" mystack >s \ push string 'Hello' on stack
s" World" mystack >s \ push string 'World' on stack
mystack s@ type cr \ examine top of stack
mystack s> \ pop 'World' from the stack
mystack s> \ pop 'Hello' from the stack
type cr type cr \ show the values
Note there is a catch: when you've popped a string from the
string stack, the string itself is untouched, so the
address-count pair is still valid. However, if you push another
string onto the same stack, the popped string is clobbered. There
is another way to create a string stack without these
disadvantages, but it is slightly larger and slower. It is
initialized by:
[needs lib/strstack.4th]
1024 constant /mystack \ allocate some space
/mystack string mystack
mystack /mystack string-stack \ convert it to a string stack
All other words work the same way.
11.11 Forward declarations<Vectors>
It doesn't happen very often, but sometimes you have a program
where two colon-definitions call each other. When you look at
4tHs source you find several examples. The throw() function calls
the rpop() function, because 'THROW' takes items from the Return
Stack. On the other hand, when the Return Stack underflows, it
has to call 'THROW'.
There is a special instruction in 4tH to do this, called 'DEFER'.
'DEFER' doesn't create an executable word, but a vector
containing an execution token, which is executed when called. You
might want to consult section [VectoredExecution] first to see
how this works. But for all purposes you might consider it to be
an executable word, because it behaves the same way.
defer Step2
Now we can create "STEP1" without a problem:
: Step1 1+ dup . cr Step2 ;
But "STEP2" does not have a body yet. Of course, you could create
a new colon-definition, tick it and assign the execution token to
"STEP2" manually, but it is much neater to use ':NONAME'.
':NONAME' can be used like a normal ':', but it doesn't require a
name. Instead, it pushes the execution token of the
colon-definition it created on the stack. No, ':NONAME' does not
create a literal expression, but it is just what we need:
:noname 1+ dup . cr Step1 ; is Step2
Now we are ready! We can simply execute the program by calling
"STEP1":
1 Step1
Note that if you run this program, you'll get stack errors!
Sorry, but the example has been taken from a Turbo Pascal manual
;-). If you have forgotten what a deferred word actually
executes, you can retrieve the execution token by using 'DEFER@':
defer thisword \ create a vector
: plus + ; \ define a word
' plus is thisword \ assign the word to the vector
' thisword defer@ \ retrieve the execution token
2 3 rot execute \ execute the deferred word
. cr \ display the result
As a matter of fact, this expression:
' thisword defer@ execute
Is equivalent to this one:
thisword
You can also reassign a vector without using 'IS'. 'IS' is a
parsing version. That means the actual vector to which a certain
behaviour is assigned is determined at compiletime. 'DEFER!' can
be used to assign a certain behaviour at runtime. 'DEFER!' takes
two execution tokens:
defer thisword \ create a vector
: plus + ; \ define a word
' plus ' thisword defer! \ assign it to a vector
This is equivalent to this:
defer thisword \ create a vector
: plus + ; \ define a word
' plus is thisword \ assign it to a vector
I guess you'll agree with me that this creates countless
possibilities.
11.12 Recursion
Yes, but can she do recursion? Of course she can! It is even very
natural and easy. Everybody knows how to calculate a factorial.
In 4tH you can do this by:
: factorial ( n1 -- n2)
dup 2 >
if
dup 1-
factorial *
then
;
10 factorial . cr
Which is exactly as one would expect. Unfortunately, this is not
the way it is done in ANS-Forth. In order to let a
colon-definition call itself, you have to use the word 'RECURSE'.
4tH supports this word too:
: factorial ( n1 -- n2)
dup 2 >
if
dup 1-
recurse *
then
;
10 factorial . cr
It will even compile to the same code. If you use the word
'RECURSE' outside a colon-definition, the results are undefined.
Note that recursion lays a heavy burden on the return stack.
Sometimes it is wiser to implement such a routine differently:
: factorial
dup
begin
dup 2 >
while
1- swap over * swap
repeat
drop
;
10 factorial . cr
So if you ever run into stack errors when you use recursion, keep
this in mind.
11.13 <LookupTables>Lookup tables with integer keys
No CASE construct, huh? Now how are we supposed to make those
complex decisions? Well, do it the proper way. Leo Brodie wrote:
"I consider the case statement an elegant solution to a misguided
problem: attempting an algorithmic expression of what is more
aptly described in a decision table". And that is exactly what we
are going to teach you.
Let's say we want a routine that takes a number and then prints
the appropriate month. In ANS-Forth, you could do that this way:
: Get-Month
case
1 of ." January " endof
2 of ." February " endof
3 of ." March " endof
4 of ." April " endof
5 of ." May " endof
6 of ." June " endof
7 of ." July " endof
8 of ." August " endof
9 of ." September" endof
10 of ." October " endof
11 of ." November " endof
12 of ." December " endof
endcase
cr
;
This takes a lot of code and a lot of comparing. In this case
(little wordplay) you would be better of with an indexed table,
like this:
create MonthTable
," January "
," February "
," March "
," April "
," May "
," June "
," July "
," August "
," September"
," October "
," November "
," December "
: Get-Month ( n -- )
12 min 1- MonthTable @c count type cr
;
Which does the very same thing and will certainly work faster.
True, you can't do that this easily in ANS-Forth, but in 4tH you
can, so use it! The word ',"' compiles a string, whose address
can be retrieved by '@C' as if it were a numeric constant. Note
that '@C' just returns the address of the string, so you have to
use 'COUNT' to obtain an address/count pair. Of course, there is
also an equivalent to ',"' called ',|'. The latter is delimited
by a bar instead of a quote, but essentially works the same way.
But can you use the same method when you're working with a random
set of values like "2, 1, 3, 12, 5, 6, 4, 7, 11, 8, 10, 9". Yes,
you can. But you need a special routine to access such a table.
Of course we designed one for you. It is called ”ROW” and you can
use it by adding this directive:
[needs lib/row.4th]
This routine takes three values. The first one is the value you
want to search. The second is the address of the table you want
to search. And on top of the stack you'll find the number of
fields this table has. The first field must be the "index" field.
It contains the values which have to be compared. That field has
number zero.
This routine can search zero-terminated tables. That means the
last value in the index field must be zero. Finally, it can only
lookup positive values. It returns the value you searched, the
address of the row where it was found and a flag. If the flag is
false, the value was not found.
Now, how do we apply this to our month table? First, we have to
redefine it:
create MonthTable
1 , ," January "
2 , ," February "
3 , ," March "
4 , ," April "
5 , ," May "
6 , ," June "
7 , ," July "
8 , ," August "
9 , ," September"
10 , ," October "
11 , ," November "
12 , ," December "
NULL ,
Note that this table is sorted, but that doesn't matter. It would
work just as well when it was unsorted. Let's get our stuff
together: the address of the table is "MonthTable", it has two
fields and we want to return the address of the string, which is
located in field 1. Field 0 contains the values we want to
compare. We can now define a routine which searches our table:
: Search-Month ( n1 -- n2 f)
MonthTable 2 num-key row \ search the table
dup >r \ save flag
if nip cell+ @c else drop then
r> \ if found get value
; \ if not drop address
Because ”ROW” is able to search integer tables and string tables,
you have to define which one it is by using either num-key or
string-key. Now, we define a new "Get-Month" routine:
: Get-Month ( n --)
Search-Month \ search table
if \ if month is found
count type \ print its name
else \ if month is not found
drop ." Not found" \ drop value
then \ and show message
cr
;
Is this flexible? Oh, you bet! We can extend the table with ease:
3 Constant #MonthFields
create MonthTable
1 , ," January " 31 ,
2 , ," February " 28 ,
3 , ," March " 31 ,
4 , ," April " 30 ,
5 , ," May " 31 ,
6 , ," June " 30 ,
7 , ," July " 31 ,
8 , ," August " 31 ,
9 , ," September" 30 ,
10 , ," October " 31 ,
11 , ," November " 30 ,
12 , ," December " 31 ,
NULL ,
Now we make a slight modification to "Search-Month":
: Search-Month ( n1 -- n2 f)
MonthTable #MonthFields num-key row
dup >r \ search table, save flag
if nip cell+ @c else drop then
r> \ if found get value
; \ if not drop address
This enables us to add more fields without ever having to modify
"SearchMonth" again. If we add another field, we just have to
modify "#MonthFields". We can now even add another routine, which
enables us to retrieve the number of days in a month:
: Search-#Days ( n1 -- n2 f)
MonthTable #MonthFields num-key row
dup >r \ search table, save flag
if nip cell+ cell+ @c else drop then
r> \ if found get value
; \ if not drop address
Of course, there is room for even more optimization, but for now
we leave it at that. Do you now understand why 4tH doesn't have a
CASE construct?
11.14 Lookup tables with string keys
But what if the table we're using looks like this:
create MonthTable
," January" 31 ,
," February" 28 ,
," March" 31 ,
," April" 30 ,
," May" 31 ,
," June" 30 ,
," July" 31 ,
," August" 31 ,
," September" 30 ,
," October" 31 ,
," November" 30 ,
," December" 31 ,
NULL ,
Sure, 4tH compiled some kind of integer value there, but an
address to a string is less than helpful. We have to compare
strings in order to find the correct entry, not addresses. So, we
need a word that searches the table and returns the contents of
the field that follows the appropriate string. Well, of course
there is such a word. It is ”ROW” again. You can use it by
entering:
[needs lib/row.4th]
At the beginning of your program. ”ROW” takes an address/count
pair of the string that has to be found, the address of the table
it has to search for that string and the number of fields the
table has. It returns the original address/count pair of the
string, the address of the row where the search stopped and a
flag. That makes it quite a useful word, e.g. how many days has
June:
: GetDays ( a n --)
MonthTable 2 string-key row \ search the table
if
cell+ @c . drop drop \ if found, display the number of
days
else \ else an error message
drop type ." is not a month!"
then
cr
;
s" June" GetDays
If ”ROW” returns true, the value was found. If it returns false,
it wasn't. Note you have to indicate which datatype ”ROW” has to
deal with. ”ROW” is quite versatile, but that is not the only
merit of ”ROW” as we will see in the next sections.
11.15 Lookup tables with multiple keys
Some tables have multiple keys to search them, e.g. by name or by
number. So far all tables we've seen dealt with a single key in
the first column. It would be a shame if you had to split a table
into two tables, simply because you had two different ways to
access it. Fortunately, ”ROW” can handle this kind of tables as
well as long as you put the key columns up front and add a NULL
at the end of the table for every key, e.g.
create mytable
," Monday" 1 ,
," Tuesday" 2 ,
," Wednesday" 3 ,
," Thursday" 4 ,
," Friday" 5 ,
," Saturday" 6 ,
," Sunday" 7 ,
NULL , NULL ,
This table consists only of key fields. You can search for the
name and get a number or search for the number and get the
equivalent name. The trick is to keep in mind what the key field
is and the relative position of the datafield. In this case we
want to search on number, so the corresponding name is the field
before the key field. When you start the search the pointer you
pass to ”ROW” has to point to the key field you want to search.
In this case that is equivalent to:
mytable cell+
Let's assume we want to search this table both ways:
: day>num ( a1 n1 -- a1 n1 -f | n2 f)
mytable 2 string-key row dup >r
if nip nip cell+ @c else drop then r>
;
: num>day ( n1 -- n1 -f | a1 n2 f)
mytable cell+ 2 num-key row dup >r
if nip cell- @c count else drop then r>
;
The first word doesn't hold any surprises. It is a vanilla search
word. The second one passes a slightly modified pointer to ”ROW”
and decrements the address it returns, so it now points to the
name field. We can use both words quite easily and transparently:
s" Friday" day>num if . else type ." not found" then cr
s" New yearsday" day>num if . else type ." not found" then cr
5 num>day if type else . ." not found" then cr
8 num>day if type else . ." not found" then cr
You will see they work as expected.
11.16 Lookup tables with duplicate keys
Although most tables come with unique keys you may find yourself
in a situation where you have to resume a search. ”ROW” can
handle that situation as well. Let's examine this table:
create people
," Ritchie" ," Lionel"
," Dijkstra" ," Edsger"
," Moore" ," Henri"
," Ritchie" ," Dennis"
," Wirth" ," Nick"
," Hopper" ," Grace"
," Moore" ," Chuck"
," Hopper" ," Dennis"
NULL ,
There is one key field, since the table is terminated with only
one NULL. We also find multiple Hoppers and Moores, so we can't
be sure we've found the right one right away. In order to get
that one we might have to continue our search. That is exactly
what this program does:
: >surname 2 string-key row ; ( a n x1 -- a n x2 f)
: first? rot cell+ @c count compare 0= ;
: >next cell+ cell+ >surname ;
: .name type space type ; ( a1 n1 a2 n2 --)
: >first ( a1 n1 x a2 n2 --)
2>r ( a n x)
if ( a n x)
dup 2r@ first? ( a n x f)
if ( a n x)
drop ." Found " 2r> .name cr
else ( a n x)
>next 2r> recurse ( a n x a n)
then ( --)
else ( a n x)
drop 2r> .name ." not found" cr
then ( --)
;
: >name -rot 2>r >surname 2r> >first ;
: demo
s" Ritchie" s" Dennis" people >name
s" Moore" s" Chuck" people >name
s" Lovelace" s" Ada" people >name
;
demo \ run the demo
”>NAME” is a wrapper around this programs most important words ”
>SURNAME” and ”>FIRST”. ”>SURNAME” simply searches for a given
surname in the table and returns its address. ”>FIRST” takes over
and compares the first name. If it checks out we're done, if not
it calls ”>NEXT”. ”>NEXT” increments the pointer, so it now
points to the next row. Then it calls ”>SURNAME” again,
effectively continuing the search. Finally ”>FIRST” calls itself
to check the first name again. Depending on the contents of the
table, this process can be repeated several times.
11.17 Interpreters
Those of you who know Forth will be very surprised to see that
4tH doesn't have a Forth prompt. Some will be even more surprised
to see that 4tH does have an interpreter. It is a library
routine, written in 4tH, that can easily be adapted and expanded.
If you can write 4tH and maintain a table, you can use it. The
next question you have to ask yourself, is do you want your
interpreter to be case sensitive or not? If it is, "id" will
work, but "Id" or "ID" will not. If you want it to be case
sensitive, change the constant "ignorecase" to "false". Example:
false constant ignorecase \ don't ignore case
[needs lib/interpret.4th]
: _+ + ;
: _. . ;
: id ." This is 4tH" cr ;
Well, that isn't very hard, is it. Now we add a table to all
that:
create wordlist
," +" ' _+ ,
," ." ' _. ,
," id" ' id ,
NULL ,
Remember to terminate your table with "NULL"! Every entry
consists of a string and an address to your routine. What will
happen is that your user enters the string and the appropriate
routine will be called. In this case, your interpreter has three
commands: "+", "." and "id". We're a hair away from a real
interpreter. We just have to assign our table to the dictionary.
These lines do the job:
wordlist to dictionary
refill drop interpret
Now you can compile your application and run it. Enter:
45 12 + .
And it will print:
57
Yes, it's just as easy as that! If you enter something the
interpreter doesn't recognize it will try to convert it to a
number and throw it on the stack. But you will also see that it
exits after you've entered that single line. That is because the
interpreter is called just once. If you change that to:
begin refill drop interpret again
It will return with an new prompt. In that case it is wise to add
a routine like:
: _quit quit ;
And add it to your interpreter, because otherwise your user will
not be able to leave the application. Note that you have to do
all the error-checking. E.g., if your user calls "_+" without
putting sufficient items on the stack, 4tH will exit with an
error. Of course, you can catch any exceptions. ”INTERPRET” has a
builtin word, ”NotFound”, that deals with any unrecognized
strings. You can define your own if you want to. The only thing
you have to do is to write a word which takes an address/count
string and returns nothing, e.g.:
:noname 2drop ." I don't understand this!" cr ; is NotFound
Or more elaborate:
:noname ." I don't what '" type ." ' means!" cr ; is NotFound
You could even integrate it with the exception trapping, if you
defined one:
1 constant #UndefName
:noname #UndefName throw ; is NotFound
When a word is not found, a user exception is thrown. This
example is taken from "dc.4th":
: dc
begin \ main interpretation loop
." OK" cr \ print prompt
refill drop \ get input from use
['] interpret \ interpret it
catch dup \ catch any errors
if \ if one occurred
ShowMessage \ show a message
else \ otherwise
drop \ drop the throw code
then
again \ loop back
;
You can still see the basic structure, but this one is much more
advanced. You can also remove the code from the interpreter that
decodes numbers. In that case, if a word is not found in the
"dictionary" table it will exit immediately and report an error:
true constant ignorenumbers
Don't let anybody ever tell you you can't make interactive
applications with 4tH. As you have seen, you can with very little
effort.
11.18 Adding your own library
This is a lot easier than you might think! As a matter of fact,
almost any program can be turned into a specialized library. A
well-written program contains a lot of definitions and only one
executable word. Take that word away and you've got a library!
A library may contain word defintions, variables, constants,
almost anything you like. And a program that includes that
library will have all these definitions at its disposal. As a
matter of fact, the resulting program will behave like you
entered the contents of the entire library file at the position
of the '[NEEDS' directive, e.g. these are the contents of ”
null.4th”:
-1 constant NULL
When it is included in this file:
\ This is a sample table using NULL
[needs lib/null.4th]
create sample
," First entry"
," Second entry"
," Third entry"
NULL ,
It will compile to the same code as this:
\ This is a sample table using NULL
-1 constant NULL
create sample
," First entry"
," Second entry"
," Third entry"
NULL ,
So it is not a good idea to make your library files too big,
since there will be a lot of superfluous code included in the
compilant which 4tH will not dispose of automatically.
You can nest '[NEEDS' directives, so one library file may include
other library files. This helps to prevent duplicate code, which
can be a serious maintenance problem. You can nest them as deep
as you want, available memory being the only restriction.
However, when nesting inclusions you always have the problem of
multiple inclusions. Don't think that all 4tH users know by heart
which library files calls which. Multiple inclusions will lead to
errors, unless you take precautions. We have '[DEFINED]' and
'[UNDEFINED]' to prevent that:
[UNDEFINED] 2drop [IF]
: 2drop drop drop ;
: 2dup over over ;
: 2swap rot >r rot r> ;
[THEN]
If you have included this file before, '2DROP' is already
defined, so in fact all definitions are skipped when the file is
included for the second time. Of course, it will take up some
extra memory, but at least it won't generate any errors.
If you want to port your library file, it might be a good idea to
hide specific 4tH constructions, e.g.:
[DEFINED] 4TH# [IF] ( a n --)
: string! chars + 0 swap c! ;
[THEN] \ make an ASCIIZ string
[UNDEFINED] 4TH# [IF]
: string! swap 1- c! ; ( a n --)
[THEN] \ make a counted string
Since '4TH#' is a 4tH specific constant, it will not be defined
in other Forth compilers. This way the compiler will
automatically select the correct definition.
Where you place your library files is up to you. You can add them
to the library files that come with 4tH, you can put them in
another directory, whatever pleases you.
11.19 Adding templates
When you include a library file you add some words to your
program. When you include a template you add some words to an
existing program. That is the major difference between a library
file and a template file. We've included a template with 4tH
which allows you to create conversion program pretty quickly. The
template is called "convert.4th” and it allows you to create a
conversion program by defining just three words.
A standard conversion program takes an input file and creates an
output file in a different format. When it can't open a file it
will issue an error message, e.g.
Cant open input.txt
When you don't supply an input file and an output file, it will
issue an error message e.g.:
Usage: myconversion input output
And of course, it will read and process the input file. And
that's all you have to tell 4tH:
• The usage message
• How to read the file
• How to process the file
So, let's create a program that will convert a block file to a
regular text file. How do we do that? First of all we've got to
issue a usage message, like:
Usage: blk2txt blockfile textfile
Well, that is easy. If it comes to that we've got to abort the
program, so this will do:
: Usage abort" Usage: blk2txt blockfile textfile" ;
Then we've got to read the file. A block file contains lines of
64 characters, always. So, we've got to create a buffer and read
64 characters. This will do:
64 string buffer
: Read-file buffer 64 accept ;
Finally, we've got to write the output file. Adding a 'CR' after
typing the line will do, but we don't want any trailing spaces,
so we need to strip those trailing spaces:
: Process buffer 64 -trailing type cr ;
Now we need to include the template and we're done:
[needs lib/convert.4th]
Wow! Do you know how much coding we need to do when we try to do
this in C? This source code takes less than 256 bytes! Compile it
and we're done! So how does it work? Well, the template expects
us to define ”Usage”, ”Read-file” and ”Process”. If you don't it
will abort compilation:
\ Has Usage been defined? If not, abort!
[DEFINED] Usage [NOT] [IF]
[ABORT] [THEN]
\ Has Read-File been defined?
[DEFINED] Read-File [NOT] [IF]
[ABORT] [THEN]
\ Has Process been defined?
[DEFINED] Process [NOT] [IF]
[ABORT] [THEN]
Furthermore, you can optionally define ”PreProcess” and ”
PostProcess” if you need anything at the top of the file or the
bottom of the file:
: ProcessFile \ process the input file
line by line
[DEFINED] PreProcess [IF]
PreProcess \ do any preprocessing
[THEN]
begin
Read-file \ read the file
while
Process \ process the line or
buffer
repeat
[DEFINED] PostProcess [IF]
PostProcess \ do any postprocessing
[THEN]
;
If you don't define it, it won't include it. You can use such
templates for many different programs, e.g. this will convert a
Unix text file to an MS-DOS text file:
: Usage abort" Usage: udc infile outfile " ;
: Read-file refill ;
: Process 0 parse-word type 13 emit 10 emit ;
You can make them as sophisticated or as simple as you like. You
can create other words as well, as long as those three words have
been defined. Templates can be handled like any other library
file. You can place them where you want, they can hold anything
you want. Amaze your collegues by writing programs in a fraction
of the time they should need!
11.20 Private declarations
Sometimes you want to hide some definitions from other
programmers. This is especially true when you're writing
libraries or templates. The Application Programmers Interface
must be public of course, but you don't want anyone else to
tinker with the internals of your library. And there is the
problem of cluttering your name space.
Relax, 4tH has a way to get rid of these internal words. It's
easy, just tell 4tH to hide them:
VARIABLE #emits \ private
: SHOW emit 1 #emits +! ; \ public
: NL CR 0 #emits ! ; \ public
hide #emits
After that the name "#EMITS" is no longer recognized and can be
reused if you want to, e.g. this is completely valid:
: dummy ;
hide dummy
: dummy ." I am no longer a dummy!" cr ;
As a matter of fact, the previous declaration of "DUMMY" has been
turned into a ':NONAME' declaration by the use of 'HIDE'. Note
that 'HIDE' is meant to make definitions private; not to
optionally override previously defined definitions in order to
achieve a Forth-like behavior:
[DEFINED] myoption [IF]
hide myoption
[THEN]
true CONSTANT myoption
If you want to achieve that, use the ”first come, first served”
rule:
[UNDEFINED] myoption [IF]
true CONSTANT myoption
[THEN]
The constant "MYOPTION" is only defined if it hasn't been defined
before. You may have to change a few things here and there to
make it work, but there are obvious advantages to this
construction.: it saves memory and is much easier to understand
and maintain. And you won't scratch your head why your perfectly
valid 4tH program doesn't compile[footnote:
The tokenizer evaluates 'HIDE' in order to estimate the size of
the symboltable while conditional compilation is first evaluated
by the parser.
].
11.21 Aliases
Sometimes you want to make an alias for a word. Of course you can
embed the word you want to alias in a new definition:
: noop ;
: nop noop ;
Although this approach works perfectly under all circumstances it
has its disadvantages, because calling a word is relatively slow.
Unless you're trying to make an alias for an internal word, you'd
better use an 'ALIAS ':
: noop ;
' noop alias nop
This is completely equivalent to:
defer nop
: noop ;
' noop is nop
Although the vector takes up a little space, it will save you
from most of the calling overhead. Since you can only alias
self-defined executable words, 'ALIAS' is quite limited. 'AKA'
does not have that disadvantage:
: noop ;
aka noop nop
Both words are now completely equivalent and compile to exactly
the same code. Even better, you can use 'AKA' with every
self-defined word, including variables, vectors, files, values,
fields and constants. 'AKA' is also known as ”also known as”.
11.22 Changing behaviour of data
One of the most ingenious things Forth can do, is change the
behaviour of data at runtime. With 4tH, you cannot do this for an
entire datatype, but you can do it for individual 'VARIABLE's,
'CREATE's, 'STRING's, 'ARRAY's and 'CONSTANT's. Just use:
:THIS <name> DOES> <definition> ;
Where <name> is a previously defined 'VARIABLE', 'STRING', etc.
The 'DOES>' word is optional. The definition will behave as if
the 'VARIABLE', etc. has just been thrown on the stack, e.g. to
make a 'VARIABLE' behave as a 'CONSTANT 'you define:
variable me
10 me !
:this me does> @ ;
The body of the definition will behave as if it said:
me @
Which boils down to a (rather slow) constant. You cannot change
the contents of the variable anymore if you haven't taken
precautions, because there is no way to address it. Here is
another, more elaborate example:
create life \ create an array of string
constants
," This is my life!"
," This is your life!"
0 constant my \ create two constants
1 constant your \ to address the elements
\ now change the behaviour of LIFE
:this life does> swap th @c count type cr ;
my life \ use it!
your life
At runtime, this will print:
This is my life!
This is your life!
Wording has always been very important to Forth. Using this
technique, you can make your programs even more readable.
11.23 Multidimensional arrays
We've seen two dimensional arrays with 'ARRAY' and 'STRING', but
what about multi-dimensional arrays. Well, it's the same thing
all over again. C doesn't actually have multi-dimensional arrays
either. When you define one, just a chunk of memory is allocated.
In 4tH you can do the same thing, but now you have to do it
yourself. E.g. when you want to define a matrix of cells of 4
rows by 5 elements, you have to multiply those and allocate an
array of that size:
4 5 [*] array my_array
But what if you want to reference the fourth element of the third
row? You cannot write something like:
2 3 my_array @
That's right. But you can change the behaviour of ”MY_ARRAY”
accordingly:
:this my_array ( n1 n2 -- a)
does>
rot 5 * \ calculate row offset
rot + \ calculate element offset
cells \ calculate number of cells
+ \ add to address of my_array
;
This word calculates the correct offset for you. Note that the
third row is row number two (since we start counting from 0) and
the fourth element is element number three:
2 3 my_array @
You can also use "MY_ARRAY" to initialize an array, since it
simply calculates the correct address for you:
5 2 3 my_array ! \ sets 3rd row 4th element to 5
You can add more dimensions if you want. This works basically the
same way: create an array of a size that equals the products of
its dimensions and design a word that calculates the correct
address.
11.24 Binary string constants
A binary string constant is an unterminated string that doesn't
necessarily contain characters. Creating binary string constants
is easy. Just compile them by their ASCII value into the String
Segment with 'C,':
char H c, char i c, char ! c, 0 c,
The fun of it all is that 4tH doesn't allow you to access the
String Segment directly, so you can never retrieve them. You need
'OFFSET' to define a word which does all the hard work for you.
At runtime it takes an index and leaves the ASCII value of the
character in question on the stack. 'OFFSET' is used just before
you compile the ASCII values:
offset greet char H c, char i c, char ! c, 0 c,
Note that you have to terminate a binary string constant manually
if you need to, although it is perfectly legal to create binary
string constants with no termination at all. Retrieving
characters is easy. This will print ”Hi!”:
0 greet emit
1 greet emit
2 greet emit cr
And so will this:
0 begin \ setup index
dup greet dup \ retrieve character
while \ if not terminated
emit 1+ \ emit and increase index
repeat drop drop \ clear stack
You can use binary string constants for compact tables,
bitstrings or any other raw data as long as each element doesn't
exceed the size of a single character.
11.25 Records and structures
The easiest way is to allocate a structure in the Character
Segment. Just define the structure like this:
struct
32 +field Name
64 +field Address
32 +field City
12 +field Age
end-struct /Person
This might be a familiar example to you. We'll store information
on a single person in this structure. Now we got the fields, the
length of the fields and the length of the entire structure,
stored in ”/Person”. Both the fields and the entire structure are
nothing more than a set of constants, e.g. he offset of the field
”Name” is stored in a CONSTANT named ”Name”. However, we still
haven't allocated any memory. We can allocate room for the
structure we've just defined by just using the word 'STRING'.
Note that you can also create a cell-based structure. Then you
need the word 'ARRAY' to allocate the memory required.
/Person string Person
Now we can define a word which initializes the fields:
: InitRecord \ initialize fields
s" Hans Bezemer" Person -> Name place
s" Lagendijk 79" Person -> Address place
s" Den Helder" Person -> City place
s" 44" Person -> Age place
;
Of course, you can also use 'ACCEPT' to enter the contents of the
fields. Fields act like ordinary strings. Note that numbers are
stored as strings as well. This is not too much of a problem
since 'NUMBER' can convert them back to numbers anyway.
This is a very simple use of structures. You can also use
structures within structures:
struct
64 +field Address
32 +field City
end-struct /Location
struct
32 +field Name
/Location field Location
end-struct /Person
/Person string Person
s" Delft" Person -> Location -> City place
If you want to make an array of structures, that can be done as
well:
struct \ create structure
32 +field Name
64 +field Address
32 +field City
12 +field Age
end-struct /Person
32 constant #Person \ size of array of structs
\ now allocate the room
#Person /Person [*] string Persons
\ make it behave properly
:this Persons does> swap /Person * + ;
\ initialize the first record
s" Hans Bezemer" 0 Persons -> Name place
s" Lagendijk 79" 0 Persons -> Address place
s" Den Helder" 0 Persons -> City place
s" 44" 0 Persons -> Age place
You can also extend an already existing structure:
struct \ create structure
32 +field Name
64 +field Address
32 +field City
12 +field Age
end-struct /Person
\ now extend the structure
/Person
32 +field Job
16 +field Emp-number
end-struct /Employee
You now got two different structures, ”/Person” and ”/Employee”,
that share the first four fields. Well, if that isn't a complete
implementation, I don't know what is..
11.26 Fixed point calculation
We already learned that if we can't calculate it in dollars, we
can calculate it in cents. And still present the result in
dollars using pictured numeric output:
: currency <# # # [char] . hold #s [char] $ hold #> type cr ;
In this case, this:
200012 currency
Will print this:
$2000.12
Well, that may be a relief for the bookkeepers, but what about us
scientists? You can do the very same trick. We have converted
some Forth code for you that gives you very accurate results. You
can use routines like SIN, COS and SQRT. A small example:
[needs lib/math.4th]
45 sin . cr
You will get "7071", because the result is multiplied by 10000.
You can correct this the same way you did with the dollars: just
print the number in the right format. You can also use a
delightful little library created by Leo Brodie called ”
fraction.4th”. This library allows you to do arithmetic in the
range of -13.1072 and 13.1071 with a precision of 0.0001! It
seamlessly integrates with the ”math.4th” library:
[needs lib/math.4th]
[needs lib/fraction.4th]
45 sin s>v v. cr
This one will actually print:
0.7071
Of course, you can also use it with other math words, as long as
they are properly scaled. The scaling constant is called '10K'
and is available from ”constant.4th”. The nice thing about
fractions is that you can do all basic math (within its range, of
course) without bothering about the decimal point. To convert a
fixed point number to a fraction, you have to multiply it by
'10K', which is 10,000 and call 'S>V', e.g. in order to convert
'0.7071' to a fraction you need to do this:
7071 s>v
You can also define a fraction yourself, e.g. this converts
two-thirds to a fraction:
2 3 v/
You can add or subtract fractions, e.g.:
7071 s>v 2 3 v/ - v. cr
This is equivalent to:
PRINT 0.7071 - 0.6666
You can multiply or divide fractions, e.g.:
7071 s>v 2 3 v/ v* v. cr
This is equivalent to:
PRINT 0.7071 * 0.6666
Since a fraction is still a single-cell number, you can
manipulate the stack in the usual way. In some cases you can even
mix cells and fractions, as the following table will show you:
[float Table:
+-----------+-----------+-------+----------+
| 2OS | TOS | Word | Result |
+-----------+-----------+-------+----------+
+-----------+-----------+-------+----------+
| fraction | fraction | V* | fraction |
+-----------+-----------+-------+----------+
| fraction | cell | V* | cell |
+-----------+-----------+-------+----------+
| cell | fraction | V* | cell |
+-----------+-----------+-------+----------+
| fraction | fraction | V/ | fraction |
+-----------+-----------+-------+----------+
| cell | cell | V/ | fraction |
+-----------+-----------+-------+----------+
| cell | fraction | V/ | cell |
+-----------+-----------+-------+----------+
| fraction | fraction | + | fraction |
+-----------+-----------+-------+----------+
| fraction | fraction | - | fraction |
+-----------+-----------+-------+----------+
[Senseless!!!
Fraction words
]
]
With the word 'V>S' you can convert a fraction back to a '10K'
scaled fixed point number. You can print a fraction using the
word 'V.'.
Another example is SQRT from ”math.4th”. If you enter a number of
which the root is an integer, you will get a correct answer. You
don't even need a special formatting routine. If you enter any
other number, it will return only the integer part. You can fix
this by scaling the number.
However, scaling it by 10 will get you nowhere, since "3" is the
square root of "9", but "30" is not the square root of "90". In
that case, we have to square the scale itself; we need 100,
10,000 or even 1,000,000 to get a correct answer. In order to
retrieve the next digit of the square root of "650", we have to
multiply it by 100, which is the square of 10:
[needs lib/math.4th]
: .fp <# # [char] . hold #S #> type cr ;
650 100 * sqrt .fp
Which will print:
25.4
To acquire greater precision we have to scale it up even further,
like 10,000. This will show us, that "25.49" brings us even
closer to the correct answer. If you want to use the ”
fraction.4th” library, you will have to scale it to the square of
'10K', which means you're restricted to numbers upto ”21.475”:
include lib/math.4th
include lib/fraction.4th
10K 10K [*] constant 100M
21 100M * sqrt s>v v. cr
All these words and more are included in the library file ”
math.4th”. You might not find the word you actually need, because
we are not much of a mathematician. When we encounter new
routines they will be added to 4tH. We would appreciate your
input!
11.27 Double numbers
C'mon, indulge me, run this program:
max-n . cr
You will probably see some fairly large number displayed on your
screen. What is it? Well, it is the largest number that can fit
in a cell. Larger numbers and 4tH will start to behave
erratically:
max-n 1+ . cr
Still, it is large enough to do the accounting for a reasonably
sized enterprise. But it hasn't been always like that. Early
Forths could barely handle the accounting of an average
schoolboy. In order to get some real work done they had to expand
the range somehow. And if one cell isn't enough you simply take
two cells. That is what double numbers are all about: they are
numbers that are composed of two cells.
The problem is that Forths operators aren't overloaded. If you
try to add up two double numbers with '+' you will end up with
one double number and the addition of the two parts of the first
double number. So in order to add up two double numbers, a
separate word had to be defined. If you need a special word for
addition, you will also need one for multiplication, subtraction,
division and negation.
It is no secret that Charles Moore, the inventor of Forth,
thought that double numbers had become superfluous after the
introduction of modern processors and modern Forth compilers.
That is one of the reasons that 4tH doesn't have a native double
word implementation. But should you need this vastly expanded
range for one reason or another 4tH allows you to enter the murky
world of double, unsigned and mixed numbers.
So, how does it work. First of all, if you need a full
implementation you have to include these three libraries:
include lib/anscore.4th \ double storage words
include lib/todbl.4th \ double number input
include lib/dbldot.4th \ double number output
Then you probably need some variables. But hey, if double numbers
take up two cells, you can't use ordinary variables. That's true.
You will need small arrays:
2 array dVar1 \ double variable one
2 array dVar2 \ double variable two
And here comes the next problem. How do you enter double numbers?
Depending on the size of the number, you can use two approaches.
The easiest one is to convert a single number to a double number
with ”U>D”. The only catch is this only works for positive
numbers. If you want to enter a negative number, you have to
negate it afterwards:
500 u>d 60000 u>d d+ \ add 500 and 60000
2dup d. cr \ print the double number
dVar1 2! \ store it in variable one
Yes, every single operation has a double counterpart:
[float Table:
+---------+---------+
| Single | Double |
+---------+---------+
+---------+---------+
| + | d+ |
+---------+---------+
| negate | dnegate |
+---------+---------+
| . | d. |
+---------+---------+
| 2/ | d2/ |
+---------+---------+
| max | dmax |
+---------+---------+
| min | dmin |
+---------+---------+
| dup | 2dup |
+---------+---------+
| @ | 2@ |
+---------+---------+
| ! | 2! |
+---------+---------+
[Senseless!!!
Examples of single and double number counterparts
]
]
But what if you want to enter a very large number right away? In
that case you will have to convert a string to a double number
with ”S>DOUBLE[footnote:
Note it will bomb out when the string isn't a double number. If
you want to play safe, use ”>NUMBER”. There is also a single
number ”>NUMBER” word. In that case you can select the double
number version by using its alias ”>DOUBLE”.
]”.
s" 5000000000" s>double \ convert a string to double
2drop 2dup d. cr \ print the double number
dVar1 2@ dmax dVar2 2! \ save the largest in variable two
Finally, when you have done all you needed to do and you're left
with a number that is small enough you can convert it back to a
single number with ”D>U”. Note this will only work for positive
numbers:
dVar2 2@ dVar1 2@ d- \ subtract both double variables
2dup d. cr \ print the double number
d2/ d2/ d>u . cr \ divide by 4 and convert to
unsigned
Rule of the thumb is: stay away from double numbers if you can.
It is slow, cumbersome and error-prone. If you can't, goodnight
and good luck!
11.28 Floating point numbers (basic)
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning!
This is really complex stuff, I cannot guarantee that it
functions flawlessly. You may lose accuracy or get the wrong
result. Don't use any of this for any real life applications. |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
Ok, if you really, really, really want it, 4tH also provides
floating point number support. If you only need the basic
operations and are willing to settle for limited accuracy and
error checking, you should try zenfloat.4th. It is small and very
easy to use. Just include it[footnote:
You always have to include zenfloat.4th manually before you
include any other floating point library member.
] and go right ahead:
include lib/zenfloat.4th
314159265 -8 f. cr
That will print the first eight decimals of ”pi”. Just read it
like ”314159265e-8” or in laymans terms ”314159265” with eight
places after the decimal point. Of course you can use positive
exponents as well. Because ZEN floating point numbers are stored
as two numbers[footnote:
The exponent on the TOS, the mantissa on the 20S.
] on the datastack[footnote:
Consequently, this implementation uses what ANS Forth calls a ”
shared floating point stack”.
], you can use '2DUP', '2DROP' and '2SWAP' to manipulate them.
This is the way to calculate the surface of a circle with a
radius of 10.55:
include lib/zenfloat.4th
314159265 -8 1055 -2
2dup f* f* f. cr
Which will happily print:
349.667115
You can also convert a number to a float and back:
1960 s>f 2dup f. f>s . cr
If you try to convert a floating point number that is bigger than
'MAX-N' to a single number it doesn't work of course. You can
store a floating point number in a variable if you want:
include lib/zenfloat.4th
include lib/anscore.4th
2 array pi
314159265 -8 pi 2!
If you want to write ANS Forth compatible code, you can. Just
include zenans.4th just after zenfloat.4th. It will allow you to
write code like this:
[DEFINED] 4TH# [IF] \ if this is 4tH
include lib/zenfloat.4th \ include the ZEN fp library
include lib/zenans.4th \ make Zen ANS compatible
include lib/zenfsin.4th \ include the SIN library
[THEN]
[UNDEFINED] 4TH# [IF] \ if this is ANS Forth
s" easy.4th" included \ load the compatibility layer
[THEN]
FLOAT array fVar \ a floating point variable
FLOAT array Pi \ a FP variable holding PI
\ store PI in the variable
cr s" 314159265e-8" s>float Pi f!
Pi f@ 4 s>f f/ \ get PI and calculate sine
fsin fdup f. cr fVar f! \ print the result and save it
fVar f@ f. cr \ print the variable
And this is the way it is run:
habe@linux-471m:~/Forth> gforth
Gforth 0.6.2, Copyright (C) 1995-2003 Free Software Foundation,
Inc.
Gforth comes with ABSOLUTELY NO WARRANTY; for details type
`license'
Type `bye' to exit
s" fpdemo.4th" included
( Lots of messages you can ignore )
0.707106780551956
0.707106780551956
ok
bye
habe@linux-471m:~/Forth> 4th cxq fpdemo.4th
0.707106778
0.707106778
habe@linux-471m:~/Forth>
Apart from some rounding errors they are identical. Note that
this ”S>FLOAT” implementation is very basic. It only supports the
”e” notation with no decimal points. There is also a full ANS
Forth implementation available called zentoflt.4th. If you want
to use that instead, you will have to include if before including
zenans.4th.
If you decide to use zenans.4th, most of the constructions in
section [sec:Floating-point-numbers] will work with zenfloat.4th
as well. The error handling of zenfloat.4th is very basic. If you
try to get the square root of a negative number, it will just
stop with an appropriate message. There are lots of floating
point routines included[footnote:
All compatible library members are prefixed with 'zen*'. See also
chapter [cha:Library-dependencies].
], e.g. LN, EXP, SQRT, SIN, ASIN, SINH, ASINH, etc. Just check
the glossary for more details.
11.29 <sec:Floating-point-numbers>Floating point numbers (full)
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning!
This is really complex stuff, I cannot guarantee that it
functions flawlessly. You may lose accuracy or get the wrong
result. Don't use any of this for any real life applications. |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
If you want something more sophisticated than zenfloat.4th you
should try ansfloat.4th. This library is a full ANS-Forth
implementation. In ansfloat.4th mantissas are double-cell
unsigned. Single-cell exponents contain the mantissa's sign and a
signed exponent. The format used is non-standard for simplicity.
Exponents are almost a whole cell wide, leading to a wider
dynamic range than most IEEE formats. Range and digits of
precision versus cell width are listed below.
[float Table:
+-------------+----------------------+---------------------------+
| Cell width | Digits of precision | Range = 10^{-x} to 10^{x} |
+-------------+----------------------+---------------------------+
+-------------+----------------------+---------------------------+
| 32 | 19 | x=323196269 |
+-------------+----------------------+---------------------------+
| n | INT(n*0.602) | x=0.301*(2^{n-1}-2n) |
+-------------+----------------------+---------------------------+
[Senseless!!!
Range and digits of precision
]
]
Floating point arithmetic is easy to use, but be careful to watch
where your inaccuracies are coming from. Floating point numbers
are approximations. You can lose up to half a bit of precision in
each operation. Differences between large numbers can be trouble
spots. If you need transcendental functions, they can often be
done in integer arithmetic since you don't need floating point's
run-time auto-scaling. Floating point arithmetic uses a
dedicated, shallow stack[footnote:
ANS Forth defines a six item stack. A larger FP stack is an
environmental dependency.
]. There is no depth checking, so underflows and overflows may
occur unless error checking is added. Since floating point
support is implemented in high level 4tH it is also rather slow
and big.
So, how does it work. First of all, if you want to use floating
point, you always need to include this library before any other
floating point library:
include lib/ansfloat.4th \ floating point words
This will also create the floating point stack. If you want a
larger stack, just create this constant accordingly before
including the library:
32 CONSTANT FLOATING-STACK \ size of float stack
include lib/ansfloat.4th \ floating point words
This will create a stack of 32 floating point items. You probably
want to do some I/O, so let's take care of that one too:
32 CONSTANT FLOATING-STACK \ size of float stack
include lib/ansfloat.4th \ floating point words
include lib/ansfpio.4th \ floating point I/O words
Then you probably need some variables. But hey, if floating point
numbers take up more than a cell, you can't use ordinary
variables. That's true. You will need small arrays:
FLOAT array fVar1 \ double variable one
FLOAT array fVar2 \ double variable two
Finally, you have to initialize the library and specify the
precision[footnote:
The number of significant digits used by F.
]:
fclear \ initialize library
8 set-precision \ set precision to eight
And here comes the next problem. How do you enter floating point
numbers? Depending on the size of the number, you can use two
approaches. The easiest one is to convert a single number to a
floating point number with ”S>F”.
500 s>f 60000 s>f f+ \ add 500 and 60000
fdup f. cr \ print the floating point number
fVar1 f! \ store it in variable one
Note the double numbers library is loaded by default as well, so ”
D>F” is available too. Every single operation has a floating
point counterpart:
[float Table:
+---------+---------+
| Single | Float |
+---------+---------+
+---------+---------+
| + | f+ |
+---------+---------+
| / | f/ |
+---------+---------+
| * | f* |
+---------+---------+
| negate | fnegate |
+---------+---------+
| . | f. |
+---------+---------+
| max | fmax |
+---------+---------+
| min | fmin |
+---------+---------+
| dup | fdup |
+---------+---------+
| @ | f@ |
+---------+---------+
| ! | f! |
+---------+---------+
[Senseless!!!
Examples of single and floating point number counterparts
]
]
But what if you want to enter a floating point number right away?
In that case you will have to convert a string to a double number
with ”>FLOAT”.
s" 5000.575" >float \ convert a string to a float
drop fdup f. cr \ drop the flag and print the
number
fVar1 f@ fmax fVar2 f! \ save the largest in variable two
Most ANS-Forth floating point words are available, although these
words usually have their own library file. For technical reasons
they will not automatically include the floating point number
library for you, but abort instead if it is not loaded. So if you
want to calculate the sine of 45 degrees, you have to do this:
include lib/ansfloat.4th \ include the fp library
include lib/ansfpio.4th \ include the fp i/o library
include lib/fsinfcos.4th \ include the library
fclear \ clear the fp stack
8 set-precision \ set precision to eight
pi 4 s>f f/ fsin f. cr \ calculate the sine
Since a full circle (360 degrees) requires 2 times PI, we have to
divide it by 4 to get the equivalent in radians[footnote:
”FSIN” takes its parameter in radians.
]. But what if you make an error? What if a conversion overflows,
you divide by zero or try to get the square root of a negative
number? There is where ”FERROR” comes in. It is a variable that
holds the last floating point error that occurred. Here you got
an example:
include lib/ansfloat.4th \ include the library
fclear \ clear the fp stack
8 set-precision \ set precision to eight
1 s>f 0 s>f f/ \ divide by zero
ferror ? cr \ examine FERROR
In this case, ”FERROR” returns two. But what does that mean?
Well, here you got a handy table of all IEEE 754 exceptions. In
4tH, these are predefined constants.
[float Table:
+---------------+--------------------+
| Code | Meaning |
+---------------+--------------------+
+---------------+--------------------+
| FE.NOERRORS | No error |
+---------------+--------------------+
| FE.OVERFLOW | FP overflow |
+---------------+--------------------+
| FE.UNDERFLOW | FP underflow |
+---------------+--------------------+
| FE.DIVBYZERO | Division by zero |
+---------------+--------------------+
| FE.INEXACT | Inexact result |
+---------------+--------------------+
| FE.INVALID | Invalid operation |
+---------------+--------------------+
[Senseless!!!
IEEE 754 FP math errors
]
]
You can simply clear any errors by invoking ”FCLEAR” again. Note
that this clears your floating point stack too, so you have to
start all over again. And no, 'THROW' and 'CATCH' do not restore
your floating point stack. No easy recovery here!
If you enter the realm of full fledged floating point numbers,
you may find it very hard to resolve all dependencies. Well, it
is not so hard as you think as long as you apply the following
rule of the tumb. Before including any other floating point
library file include these first in this order:
include lib/ansfloat.4th
include lib/ansfpio.4th
\ now you may include other library files
If you don't mind the size or the speed but do require full ANS
Forth compliance, begin your program like this:
include lib/ansfloat.4th
include lib/fpin.4th
include lib/fpout.4th
\ now you may include other library files
You will see that this resolves most of your dependency problems[footnote:
If not, take a look at chapter [cha:Library-dependencies].
]. Bottom line: if you don't need floating point numbers, avoid
them and apply other techniques. But sometimes it cannot be
avoided and I guess you'll agree with me that it's good it's
there.
11.30 Forth Scientific Library
4tH includes serveral library members of the Forth Scientific
Library. The FSL contains several very complex and highly
specialized mathematical words. Some of these words have been
adapted to work with 4tH. If a floating point word does not
require fsl-util.4th, it will usually be prefixed by an 'f'. If
it does, all you have to do is to include fsl-util.4th after
you've included ansfloat.4th, ansfpio.4th and preferably also
fpconst.4th. It won't work with zenfloat.4th.
include lib/ansfloat.4th
include lib/ansfpio.4th
include lib/fsl-util.4th
Note that like ansfloat.4th, fsl-util.4th isn't included
automatically by other library members, so you have to include it
explicitly. If not, the compiler will simply abort. Since the FSL
uses special datatypes, you have to do some work in order to get
them to work with 4tH. If you want to declare an fsl-array of
e.g. ten floats:
10 FLOAT MARRAY MyFSL
You have to declare it like this:
10 FLOAT [*] 1 [+] ARRAY MyFSL ( allocation)
FLOAT MyFSL FSL-ARRAY ( initialization)
:THIS MyFSL DOES> (FSL-ARRAY) ; ( runtime behavior)
If you want to declare an fsl-matrix of e.g. 16 by 8 floats:
16 8 FLOAT MMATRIX MyFSL
You have to declare it like this:
16 8 [*] FLOAT [*] 2 [+] ARRAY MyFSL ( allocation)
16 8 FLOAT MyFSL FSL-MATRIX ( initialization)
:THIS MyFSL DOES> (FSL-MATRIX) ; ( runtime behavior)
Surely, this is a bit awkward, but it is required to be as
compatible as possible with sources that use the FSL. Note that
all restrictions and reservations concerning floating point
support also apply to the FSL words.
11.31 Complex control structures
Sometimes, the normal control structures of 4tH are not enough.
Take this implementation of '-TRAILING':
: -trailing ( a n1 -- a n2)
begin
dup \ quit if length is zero
while
2dup 1- chars + c@ bl <> \ is it still a space?
if exit else 1- then \ if not, quit
repeat \ if so, decrement length
;
No one will tell you that this is elegant. You have to perform a
test and quit the word. And this is still palatable. Imagine you
have to test several conditions like this! It will become
horrible pretty soon! Therefore, 4tH supports extended control
structures. We've seen the basic control structures in sections [sec:WHILE-DO]
, [sec:REPEAT-UNTIL] and [sec:Infinite-loops]. Now we're
expanding those into:
BEGIN .. WHILE .. WHILE .. AGAIN | REPEAT
BEGIN .. WHILE .. WHILE .. UNTIL
Yes, that's right: 'REPEAT' and 'AGAIN' are actually aliases. But
what can we do with them? Well, take a look at our modified
'-TRAILING' word:
: -trailing ( a n1 -- a n2)
begin
dup
while \ quit if length is zero
2dup 1- chars + c@ bl =
while \ quit if it is not a space
1- \ decrement length
repeat
;
You have to admit that the latter version is much more elegant
and readable.
11.32 Sorting
Yes, 4tH can do that too. You just have to include 'qsort.4th' to
make it all possible. It works pretty much like the sort routines
you've seen in C, which means you have to devise a word to
compare two values. Note that 'qsort.4th' can only sort integer
arrays. Setting it up is pretty simple. First you have to include
it:
include lib/qsort.4th
Then you have to create a word that returns a true flag when the
second value on the stack is smaller than the top of the stack.
In this example we will just compare two integers, so that is
pretty easy:
: MyPrecedes < ;
'qsort.4th' creates a deferred word[footnote:
See section [Vectors].
] called ”PRECEDES”. Now we have to assign our word to ”PRECEDES”
, so that it is executed when ”PRECEDES” is called:
' MyPrecedes is Precedes
That's it! We're ready to rock 'n roll now. Let's set up a simple
testing environment:
10 constant #elements
#elements array elements
: InitElements #elements 0 do random elements i th ! loop ;
: ShowElements #elements 0 do elements i th @ . loop cr ;
This creates an array of ten elements, which is filled with
random values by ”InitElements”. ”ShowElements” will show on
screen what is stored there. The actual sort is straightforward:
tell ”SORT” which array and how many elements there are to sort
and you're done:
: SortElements elements #elements sort ;
Now let's put it all together:
InitElements
ShowElements
SortElements
ShowElements
It will initialize the array, show its contents, sort it and show
it again. It will output something like this:
12717 6028 1389 31870 14234 15884 31062 14788 18186 149
149 1389 6028 12717 14234 14788 15884 18186 31062 31870
And what if these were string addresses? Well, ”SORT” would have
sorted them too, from the lowest addresses up to the highest
addresses, but that's probably not what you meant. You wanted to
sort the actual strings, not just their addresses. Can 4tH do
that too? Sure, you just got to create another ”PRECEDES” word.
Something like this:
: SPrecedes >R COUNT R> COUNT COMPARE 0< ;
This will take the two values and treat them as strings. Now the
actual strings are sorted, not just the addresses itself. Note
that the strings themselves will not move in memory. The pointers
move, the strings themselves don't.
11.33 Tokenizing strings
Sometimes you want to split up a string in several different
parts. This is called ”tokenizing”. Doing it with 4tH is (as
usual ;-) quite easy. Just include 'tokenize.4th'. Now you got
several words to get what you want. 'tokenize.4th' creates a
deferred word[footnote:
See section [Vectors].
] called ”IS-TYPE”. It decides whether a character is of a
certain type. In this example, we just want to know whether it is
a lowercase 'a':
include lib/tokenize.4th
:noname [char] a = ; is is-type
Now it's time to play ball:
s" 01234aBcDe01234" scan type cr
”SCAN” will now skip all characters unless it is an 'a'. When it
is found, it stops and returns the remainder of the string:
aBcDe01234
Yes, ”SCAN” starts at the beginning of the string. But there is
also a word that starts at the end of the string:
s" 01234aBcDe01234" -scan type cr
It returns a different result too:
01234a
And what about the rest of the string? Well, that is discarded.
But if you need it, there is also a word that just splits up the
string:
s" 01234aBcDe01234" split type cr type cr
So, ”SPLIT” returns two strings:
01234
aBcDe01234
And of course, he's got a little brother that works the other way
around:
s" 01234aBcDe01234" -split type cr type cr
”-SPLIT” returns two strings too:
01234a
BcDe01234
That was quite easy. But what if you want to find the first
non-digit? That's what we got ”SKIP” for! ”SKIP” skips all
characters of a certain kind. We've already seen how we can
distinguish characters in section [sec:Distinguishing-characters]
. So in this case we just got to include 'istype.4th' and assign ”
IS-DIGIT” to ”IS-TYPE”:
include lib/tokenize.4th
include lib/istype.4th
' is-digit is is-type
s" 01234aBcDe01234" skip type cr
”SKIP” returns a single string:
aBcDe01234
And he has a counterpart too:
include lib/tokenize.4th
include lib/istype.4th
' is-digit is is-type
s" 01234aBcDe01234" -skip type cr
May be this result will surprise you, although it is completely
correct:
01234aBcDe
What exactly did we ask for? We wanted the first non-digit,
starting from the end. 'e' is the first non-digit, so ”-SKIP” is
completely correct.
11.34 Regular expressions
Well, we don't offer full regular expressions (yet), but you can
use wildcards for basic pattern matching. First you have to
include it:
include lib/wildcard.4th
You've probably used wildcards before. It is very easy. A ”*”
stands for zero or more characters and a ”?” stands for a single
character. E.g. if you're looking for a line that begins with a
date in the 21st century, then the word ”INVOICE” and finally
ends with a name, you could try this:
s" 20??-??-?? == INVOICE == *" mystring count wild-match
”WILD-MATCH” returns true if the string matches and false if the
string doesn't match, which is different from ”COMPARE”. Both
strings are consumed, so save their address/count pair if you
need them later on. Note that ”WILD-MATCH” is case sensitive, so
if you need a case insensitive comparison you will have to
convert them first.
Finally, ”WILD-MATCH” is faster than regular expressions, but
also less precise. E.g. ”gr?y” will not only return ”grey” and ”
gray”, but also ”groy”; ”reg*exp*” will not only return ”regular
expressions”, but also ”registered express mail”.
11.35 <sec:Escape-characters>Escape characters
Sometimes you need a string with embedded control characters. You
could try to patch them into your sourcecode, but that is usually
not a good idea. You could also patch them into a variable, but
that is cumbersome. 4tH offers a word that will allow you to use
the control characters listed in table [tab:Supported-control-characters]
.
[float Table:
+-------------------+-----------------+
| Escape character | Meaning |
+-------------------+-----------------+
+-------------------+-----------------+
| \a | Bell |
+-------------------+-----------------+
| \b | Backspace |
+-------------------+-----------------+
| \e | Escape |
+-------------------+-----------------+
| \f | Formfeed |
+-------------------+-----------------+
| \l | Linefeed |
+-------------------+-----------------+
| \n | Linefeed |
+-------------------+-----------------+
| \q | Quote (”) |
+-------------------+-----------------+
| \r | Carriage return |
+-------------------+-----------------+
| \t | Tab |
+-------------------+-----------------+
| \v | Vertical tab |
+-------------------+-----------------+
| \z | Null character |
+-------------------+-----------------+
[Senseless!!!
<tab:Supported-control-characters>Supported control characters
]
]
Using this facility is quite simple:
include lib/escape.4th
s" \tThis is the \qC:\\4tH\q directory” s>escape type cr
Which will print:
This is the "C:\4tH" directory
Note that other escaped characters are printed ”as is”, like the
backslash in this example. If you need even more flexibility, we
have another card up our sleeve. The only catch is you'll have to
know the ASCII code of the characters you want to insert, either
in binary, octal, decimal or hex:
include lib/embed.4th
s" %1001This is the&42C:\4tH#34$20directory" s>embed type cr
Which will also print:
This is the "C:\4tH" directory
• Every number prefixed with a '%' character is interpreted as a
binary ASCII code;
• Every number prefixed with '&' character is interpreted as an
octal ASCII code;
• Every number prefixed with a '#' character is interpreted as a
decimal ASCII code;
• Every number prefixed with a '$' character is interpreted as a
hexadecimal ASCII code.
How much flexibility do you need? Of course, all this flexibility
comes at a price. The strings are not expanded at compile-time
but at run-time, so it will cost you a little more space and a
little more time compared to other string literals.
11.36 Running 4tH programs from the Unix shell
If you're using Unix (which we highly recommend), you can run 4tH
programs right from the Unix shell. All you have to do is to add
one single line at the top:
#! /usr/lbin/4th cxq
." Hello world!" cr
It indicates the way you normally compile and run a 4tH program,
but without the filename, e.g.:
/usr/lbin/4th cxq hello.4th
In this case, you're using the classic 4tH compiler, which is
located in the /usr/lbin directory. Note that you can add options
if you want. The 'cxq' options tell the compiler to silently
compile and execute a program.
Note this trick only works with 4tH sources, not compiled
programs. You also have to flag the 4tH source as 'executable'.
You can do that by issuing this command:
chmod 555 hello.4th
Now you can simply enter:
hello.4th
at the Unix prompt and your program will be compiled and
executed. Don't worry about compromising the portability of your
program. It will still compile and run happily under other
Operating Systems, since '#!' is an alias for '\'. It only has a
special meaning to the Unix shell.
11.37 Embedding 4tH programs in a batch file
If you're running a Microsoft Operating System like Windows or
DOS[footnote:
DOS version 3.3 or higher.
], you can embed 4tH source code in an ordinary batch file[footnote:
This method was taken from CSL, the ”C Scripting Language”. You
can learn more about CSL at ”http://csl.sourceforge.net”.
]. All you have to do is to make the shell ignore the 4tH code,
e.g.:
@goto exec
." Hello world!" cr
(
:exec
@4th cxq %0.bat %1 %2 %3 %4 %5 %6 %7 %8 %9
@rem )
Now save your file as ”EXAMPLE.BAT” in the current working
directory[footnote:
If you want to store it permanently in another directory, you may
have to add additional path information.
] and run it:
example
Don't add the ”.BAT” extension or the whole thing won't work. 4tH
will now automatically pick up the batch file and execute it.
Well, how does it work?
It's simple: the shell silently jumps to the ”EXEC” label and
executes 4tH. 4tH will compile the batch file. It ignores the
line that starts with '@GOTO', since '@GOTO' is an alias for '\'.
It compiles anything up to the opening parenthesis, since that is
the start of a multiline comment. The shell on its turn ignores
the closing parenthesis, since that has been commented out by ”
@REM”.
11.38 This is the end
This is the end of it. If you mastered all we have written about
4tH, you may be just as proficient as we are. Or even better. In
the meanwhile you may even have acquired a taste for this
strange, but elegant language. If you do, it may be time to step
up to Forth, since 4tH does have it limitations. This is in no
way an obligation. If you feel comfortable with 4tH, please do
stick with it!
If you need any help, you can contact us by sending an email to:
[email protected]
Note that we do appreciate any input, so if you've written a
state of the art application in 4tH, used 4tH in some special way
or do have any comments or suggestions on 4tH, we'd like to hear
from you! We do also have a web-site:
http://www.xs4all.nl/~thebeez/4tH
You will find there lots of documentation and news on 4tH. We'd
like to thank you for putting so much effort in 4tH. We tried to
be of assistance and we hope we did it well!
Reference guide
Glossary
This glossary contains all of the word definitions used in
version 3.5d of 4tH. The definitions are presented in order of
their ASCII sort. Availability of the word in the appropriate
ANS-Forth wordset is listed. This does not mean any conformance
to the ANS-Forth definition.
Pronunciation: Natural-language pronunciation if it differs from
English.
Include: Following library file provides this word.
Compiles to: Describes the transformation of the word to tokens
through all passes. Compiler directives will lack this section.
Syntax: Describes definition characteristics if non-conformance
should lead to a compilation error.
+------------+-------------------------------------------------+
| <char> | Character |
+------------+-------------------------------------------------+
| <string> | String constant, delimited by spaces |
+------------+-------------------------------------------------+
| <literal> | Expression which compiles to LITERAL (n) |
+------------+-------------------------------------------------+
| <name> | String of characters, stored in the symboltable |
+------------+-------------------------------------------------+
| <space> | Space character |
+------------+-------------------------------------------------+
| <word> | Any valid 4tH word. |
+------------+-------------------------------------------------+
Compiler: Describes special actions the compiler takes when
compiling this word.
Stack effects: Describes the action of the tokens on the
parameter stack at runtime. The symbols indicate the order in
which input parameters have been placed on the stack. Two dashes
indicate the execution point. Any parameters left on the stack
are listed. In this notation, the top of the stack is to the
right.
+-------+--------------------------+-------------------+
| n | 32 bits signed number | |
+-------+--------------------------+-------------------+
| c | 8 bits character | |
+-------+--------------------------+-------------------+
| f | boolean flag | |
+-------+--------------------------+-------------------+
| fam | file access method | |
+-------+--------------------------+-------------------+
| h | file handle (stream) | |
+-------+--------------------------+-------------------+
| d | double number (2 cells) | |
+-------+--------------------------+-------------------+
| sp | stack pointer | Stack Area |
+-------+--------------------------+-------------------+
| x | address of a cell | Variable Area |
+-------+--------------------------+-------------------+
| addr | address of a character | Character Segment |
+-------+--------------------------+-------------------+
| xt | execution token | Code Segment |
+-------+--------------------------+-------------------+
Floating: Describes the action of floating point words on the
floating point stack at runtime. The symbols[footnote:
'r' stands for real number.
] indicate the order in which input parameters have been placed
on the stack. Two dashes indicate the execution point. Any
parameters left on the floating point stack are listed. In this
notation, the top of the stack is to the right.
Forth: Describes the deviation of 4tH from ANS-Forth and gives
suggestions for porting Forth programs.
! CORE
Pronunciation: store
Compiles to: ! (0)
Stack effects: n x ---
Stores n in the variable at address x.
# CORE
Pronunciation: number-sign
Compiles to: # (0)
Stack effects: n1 --- n2
Forth: In Forth a double number is required.
Generate from n1 the next ASCII character which is placed in an
output string, stored in PAD. Result n2 is the quotient after the
division by BASE, and is remained for further processing. Used
between <# and #>.
#! 4TH
Syntax: #!<space><string>
The remainder of the line is discarded. This word is used to
start a 4tH source program from a Unix type shell. An alias for
\.
#> CORE
Pronunciation: number-sign-greater
Compiles to: #> (0)
Stack effects: n1 --- addr n2
Forth: In Forth a double number is required.
Terminates numeric output conversion by dropping n1, leaving the
address in PAD and character count n2 suitable for TYPE.
#S CORE
Pronunciation: number-sign-s
Compiles to: #S (0)
Stack effects: n1 --- n2
Forth: In Forth a double number is required.
Generates ASCII text in PAD by the use of # until a zero number
n2 results. Used between <# and #>.
#TIB CORE EXT
Pronunciation: number-t-i-b
Include: obsolete.4th
Stack effects: --- x
X is the address of a cell containing the number of characters in
the terminal input buffer (see /TIB).
' CORE
Pronunciation: tick
Compiles to: LITERAL (<argument of symbol>)
Syntax: '<space><name>
Stack effects: --- x | xt | n
Forth: In Forth you can determine the address of variables,
constants, etc. In 4tH the contents of the symboltable entry is
returned. Of course the token addresses of built-in primitives
cannot be determined either. E.g. use
: _+ + ; ' _+
instead of
' +
Compile the value contents of the symboltable entry identified as
symbol <name> as a literal.
( CORE FILE
Pronunciation: paren
Syntax: (<space><string>)
Ignore a comment that will be delimited by a right parenthesis.
May occur inside or outside a colon-definition. A blank after the
leading parenthesis is required.
(ERROR) 4TH
Compiles to: LITERAL (<largest negative integer>)
Stack effects: --- n
Returns 4tHs internal error-flag. This number cannot be printed.
Usually -2^31.
) 4TH
Compiles to: EQ0 (0)
0BRANCH (<address of THROW>)
LITERAL (<M4ASSERT>)
THROW (0)
Stack effects: f ---
Forth: Similar constructions are available in GForth and
Win32Forth.
If flag f is FALSE, the program will terminate with an error. Its
compilation is dependant on the presence of [ASSERT] (see:
[ASSERT] and ASSERT().
* CORE
Pronunciation: star
Compiles to: * (0)
Stack effects: n1 n2 --- n3
Leave the product n3 of two numbers n1 and n2.
*/ CORE
Pronunciation: star-slash
Compiles to: */ (0)
Stack effects: n1 n2 n3 --- n4
Leave the ratio n4 = n1*n2/n3.
*/MOD CORE
Pronunciation: star-slash-mod
Compiles to: >R (0)
* (0)
R> (0)
/MOD (0)
Stack effects: n1 n2 n3 --- n4 n5
Leave the quotient n5 and remainder n4 of the operation n1*n2/n3.
+ CORE
Pronunciation: plus
Compiles to: + (0)
Stack effects: n1 n2 --- n3
Leave the sum n3 of n1+n2.
+! CORE
Pronunciation: plus-store
Compiles to: +! (0)
Stack effects: n x ---
Add n to the value in variable at address x.
+CONSTANT 4TH
Syntax: <literal><space>+CONSTANT<space><name>
Compiler: The previously compiled literal is taken as an argument
for +CONSTANT. The instruction pointer is decremented, actually
deleting the literal.
A defining word used to create word <name>. When <name> is later
executed, it will add the value of <literal> on the top of the
stack.
+FIELD FACILITY EXT
Syntax:
STRUCT<space><literal><space>+FIELD<space><name><space>END-STRUCT<space><name>
Compiler: Take two previous compiled literals. The last literal
is added to the first and recompiled. The first literal is the
value of a named +CONSTANT. The instruction pointer does not
change.
Create a field for STRUCTURE implementations. The created
fieldname is an +CONSTANT that memorizes the current offset (see:
+FIELD, STRUCT, END-STRUCT).
+LOOP CORE
Pronunciation: plus-loop
Compiles to: +LOOP (<address of matching DO token>)
Syntax: DO<space>..<space>+LOOP
Stack effects: n ---
Used in the form DO .. n1 +LOOP. At runtime, +LOOP selectively
controls branching back to the corresponding DO based on n1, the
loop index and the loop limit. The increment n1 is added to the
index and the total compared to the limit. The branch back to DO
occurs until the new index is equal to or greater than the limit
(n > 0), or until the new index is less than the limit (n < 0).
Upon exiting the loop, the parameters are discarded and execution
continues ahead.
+PLACE COMUS
Compiles to: COUNT (0)
+ (0)
PLACE (0)
Stack effects: addr1 n addr2 ---
Copies the string at address addr1 with count n to address addr2.
, CORE
Pronunciation: comma
Compiles to: , (<literal>)
Syntax: <literal><space>,
Compiler: The previously compiled literal is changed into a NOOP
instruction. The instruction pointer is not incremented.
Forth: Forth pops a value from the stack. This is not possible in
4tH. Instead the previously compiled literal has its codefield
changed to NOOP.
Store the literal into the next available location.
," COMUS
Compiles to: ," (<address of string constant>)
Syntax: ,"<space><string>"
Forth: Compilation characteristics are quite different. 4tH
compiles only the address, Forth compiles the entire string.
Compile the string, delimited by " in the String Segment and
leave the offset as the address of a string constant (see: @C).
,| 4TH
Compiles to: ," (<address of string constant>)
Syntax: ,|<space><string>|
Compile the string, delimited by | in the String Segment and
leave the offset as the address of a string constant (see: @C).
- CORE
Pronunciation: minus
Compiles to: - (0)
Stack effects: n1 n2 --- n3
Leave the difference of n1 - n2 in n3.
-> 4TH
Compiler: The instruction pointer is not incremented. In fact, ->
is a dummy.
Syntax: <name><space>-><space><name>
Separation between a structure and its member.
-ROT COMUS
Compiles to: ROT (0)
ROT (0)
Stack effects: n1 n2 n3 --- n3 n1 n2
Rotate top stack item below the next two items.
-TRAILING STRING
Pronunciation: dash-trailing
Compiles to: -TRAILING (0)
Stack effects: addr n1 --- addr n2
Adjusts the character count n1 of a string beginning address to
suppress the output of trailing blanks, i.e. the characters from
addr+n1 to addr+n2 are blanks.
. CORE
Pronunciation: dot
Compiles to: . (0)
Stack effects: n ---
Print a number to the current output device, converted according
to the numeric BASE. A trailing blank follows.
." CORE
Pronunciation: dot-quote
Compiles to: ." (<address of string constant>)
Syntax: ."<space><string>"
Compiles string in the String Segment with an execution procedure
to transmit the string to the selected output device.
.( CORE EXT
Pronunciation: dot-paren
Compiles to: ." (<address of string constant>)
Syntax: .(<space><string>)
Compiles string in the String Segment with an execution procedure
to transmit the string to the selected output device. An alias
for .".
.R CORE EXT
Pronunciation: dot-r
Compiles to: .R (0)
Stack effects: n1 n2 ---
Print the number n1 right aligned in a field whose width is n2 to
the current output device. No following blank is printed.
.S TOOLS
Pronunciation: dot-s
Include: anstools.4th
Stack effects: ---
Copy and display the values currently on the data stack.
.| 4TH
Compiles to: ." (<address of string constant>)
Syntax: .|<space><string>|
Compiles string in the String Segment with an execution procedure
to transmit the string to the selected output device.
/ CORE
Pronunciation: slash
Compiles to: / (0)
Stack effects: n1 n2 --- n3
Leaves the quotient n3 of n1/n2.
/CELL COMUS
Compiles to: LITERAL (<size of a cell>)
Stack effects: --- n
Returns the size of a cell in address units.
/CHAR COMUS
Compiles to: LITERAL (<size of char>)
Stack effects: --- n
Returns the size of a character in address units.
/MOD CORE
Pronunciation: slash-mod
Compiles to: /MOD (0)
Stack effects: n1 n2 --- n3 n4
Leave the remainder n3 and quotient n4 of n1/n2.
/PAD 4TH
Compiles to: LITERAL (<size of PAD>)
Stack effects: --- n
Forth: Equivalent to:
: /PAD S" /PAD" ENVIRONMENT? DROP ;
Returns the size of PAD.
/STRING STRING
Pronunciation: slash-string
Compiles to: SWAP (0)
OVER (0)
- (0)
>R (0)
+ (0)
R> (0)
Stack effects: addr1 n1 n2 --- addr2 n3
Adjust the character string at addr1 by n2 characters. The
resulting character string, specified by addr2 n3 , begins at
addr1 plus n2 characters and is n1 minus n characters long.
/TIB 4TH
Compiles to: LITERAL (<size of TIB>)
Stack effects: --- n
Returns the size of the terminal input buffer.
0< CORE
Pronunciation: zero-less
Compiles to: 0< (0)
Stack effects: n --- f
Leave a TRUE flag if number n is less than zero (negative),
otherwise leave a FALSE flag in f.
0<> CORE EXT
Pronunciation: zero-not-equals
Compiles to: 0<> (0)
Stack effects: n --- f
Leave a TRUE flag if number n is not equal to zero, otherwise
leave a FALSE flag in f.
0= CORE
Pronunciation: zero-equals
Compiles to: 0= (0)
Stack effects: n --- f
Leave a TRUE flag if number n is equal to zero, otherwise leave a
FALSE flag in f.
0> CORE EXT
Pronunciation: zero-greater
Compiles to: 0> (0)
Stack effects: n --- f
Leave a TRUE flag if number n is greater than zero (positive),
otherwise leave a FALSE flag in f.
1+ CORE
Pronunciation: one-plus
Compiles to: 1+ (0)
Stack effects: n --- n+1
Increment n by 1.
1- CORE
Pronunciation: one-minus
Compiles to: 1+ (0)
Stack effects: n --- n+1
Decrement n by 1.
2! CORE
Pronunciation: two-store
Include: anscore.4th
Stack effects: n1 n2 x ---
Store the cell pair n1 n2 at x, with n2 at x and n2 at the next
consecutive cell.
2* CORE
Pronunciation: two-star
Compiles to: 2* (0)
Stack effects: n --- n*2
Multiply n by 2. Performs a left shift.
2/ CORE
Pronunciation: two-slash
Compiles to: 2/ (0)
Stack effects: n --- n/2
Divide n by 2. Performs a right shift.
2>R CORE EXT
Pronunciation: two-to-r
Compiles to: >R (0)
>R (0)
Stack effects: n1 n2 ---
Forth: Forth swaps both values before transfering them to the
return stack.
Transfer cell pair n1 n2 to the return stack.
2@ CORE
Pronunciation: two-fetch
Include: anscore.4th
Stack effects: x --- n1 n2
Fetch the cell pair n1 n2 stored at x. n2 is stored at x and n1
at the next consecutive cell.
2DROP CORE
Pronunciation: two-drop
Compiles to: DROP (0)
DROP (0)
Stack effects: n1 n2 ---
Drop cell pair n1 n2 from the stack.
2DUP CORE
Pronunciation: two-dupe
Compiles to: OVER (0)
OVER (0)
Stack effects: n1 n2 --- n1 n2 n1 n2
Duplicate cell pair n1 n2.
2OVER CORE
Pronunciation: two-over
Include: 2rotover.4th
Stack effects: n1 n2 n3 n4 --- n1 n2 n3 n4 n1 n2
Copy cell pair n1 n2 to the top of the stack.
2R> CORE EXT
Pronunciation: two-r-from
Compiles to: R> (0)
R> (0)
Stack effects: --- n1 n2
Forth: Forth swaps both values after transfering them from the
return stack.
Transfer cell pair n1 n2 from the return stack.
2R@ CORE EXT
Pronunciation: two-r-fetch
Compiles to: R> (0)
I (0)
OVER (0)
>R (0)
Stack effects: --- n1 n2
Forth: Forth swaps both values after transfering them from the
return stack.
Copy cell pair n1 n2 from the return stack.
2ROT DOUBLE EXT
Pronunciation: two-rote
Include: 2rotover.4th
Stack effects: n1 n2 n3 n4 n5 n6 --- n3 n4 n5 n6 n1 n2
Rotate the top three cell pairs on the stack bringing cell pair
n1 n2 to the top of the stack.
2SWAP CORE
Pronunciation: two-swap
Compiles to: ROT (0)
>R (0)
ROT (0)
R> (0)
Stack effects: n1 n2 n3 n4 --- n3 n4 n1 n2
Exchange the top two cell pairs.
4TH# 4TH
Compiles to: LITERAL (<4tH version in hexadecimal>)
Stack effects: --- n
Constant containing the 4tH version in hexadecimal.
: CORE
Pronunciation: colon
Compiles to: BRANCH (<address of matching ; token>)
Syntax: :<space><name>..<space>;
Creates a subroutine defining <name> as equivalent to the
following sequence of 4tH word definitions until the next ;.
:NONAME CORE EXT
Pronunciation: colon-no-name
Compiles to: LITERAL (<address of next BRANCH>)
BRANCH (<address of matching ; token>)
Syntax: :NONAME<space>..<space>;
Stack effects: --- xt
Create an execution token xt and compile the current definition.
The execution semantics of xt will be determined by the words
compiled into the body of the definition. This definition can be
executed later by using xt EXECUTE.
:THIS 4TH
Compiles to: BRANCH (<address of matching ; token>)
LITERAL (<original value>) | VARIABLE (<original value>)
Syntax: :THIS<space><name><space>DOES><space>..<space>;
Create an subroutine <name> that first pushes the original value
of <name> on the stack. The words after DOES> determine what the
actual execution behaviour will be (see: DOES>).
; CORE
Pronunciation: semi-colon
Compiles to: EXIT (0)
Syntax: See :
Terminate a colon definition. At runtime, return to the calling
word by popping a token-address from the return stack.
< CORE
Pronunciation: less-than
Compiles to: < (0)
Stack effects: n1 n2 --- f
Leave a TRUE flag if n1 is less than n2; otherwise leave a FALSE
flag in f.
<# CORE
Pronunciation: less-number-sign
Compiles to: <# (0)
Forth: In Forth a double number is required.
Setup for pictured numeric output formatting in PAD using the
words <#, #, #S, SIGN, HOLD, #>.
<> CORE EXT
Pronunciation: not-equals
Compiles to: <> (0)
Stack effects: n1 n2 --- f
Leave a TRUE flag if n1 does not equal n2; otherwise leave a
FALSE flag in f.
<= 4TH
Compiles to: > (0)
0= (0)
Stack effects: n1 n2 --- f
Leave a TRUE flag if n1 is less or equal than n2; otherwise leave
a FALSE flag in f.
= CORE
Pronunciation: equals
Compiles to: = (0)
Stack effects: n1 n2 --- f
Leave a TRUE flag if n1 equals n2; otherwise leave a FALSE flag
in f.
> CORE
Pronunciation: greater-than
Compiles to: > (0)
Stack effects: n1 n2 --- f
Leave a TRUE flag if n1 is greater than n2; otherwise leave a
FALSE flag in f.
>= 4TH
Compiles to: < (0)
0= (0)
Stack effects: n1 n2 --- f
Leave a TRUE flag if n1 is greater or equal than n2; otherwise
leave a FALSE flag in f.
>BODY CORE
Pronunciation: to-body
Compiles to: ENVIRON (<address of FIRST>)
+ (0)
Stack effects: n --- x
Forth: In Forth, >BODY works with every CREATEd datatype.
n is the ticked value of a VARIABLE, VALUE, DEFER or FILE. >BODY
returns its address in the Variable Area.
>FLOAT FLOATING
Pronunciation: to-float
Include: ansfpio.4th
fpin.4th
zenans.4th
zentoflt.4th
Stack effects: addr n --- f
Floating: --- r
An attempt is made to convert the string specified by addr and n
to internal floating-point representation. If the string
represents a valid floating-point number in the syntax ”mantissa
(with optional exponent)”, its value r and true are returned. If
the string does not represent a valid floating-point number only
false is returned.
>IN CORE
Pronunciation: to-in
Compiles to: LITERAL (<address of >IN>)
Stack effects: --- x
A variable containing the address within the Character Segment
from which the next text will be parsed. PARSE uses and moves the
value of >IN.
>NUMBER CORE
Pronunciation: to-number
Include: tonumber.4th
Stack effects: n1 a1 n2 --- n3 a2 n4
n3 is the unsigned result of converting the characters within the
string specified by a1 n2 into digits, using the number in BASE,
and adding each into n1 after multiplying n1 by the number in
BASE. Conversion continues left-to-right until a character that
is not convertible, including any + or -, is encountered or the
string is entirely converted. a2 is the location of the first
unconverted character or the first character past the end of the
string if the string was entirely converted. n4 is the number of
unconverted characters in the string. An ambiguous condition
exists if n3 overflows during the conversion.
>NUMBER CORE
Pronunciation: to-number
Include: todbl.4th
Stack effects: d1 a1 n1 --- d2 a2 n2
d2 is the unsigned result of converting the characters within the
string specified by a1 n2 into digits, using the number in BASE,
and adding each into d1 after multiplying d1 by the number in
BASE. Conversion continues left-to-right until a character that
is not convertible, including any + or -, is encountered or the
string is entirely converted. a2 is the location of the first
unconverted character or the first character past the end of the
string if the string was entirely converted. n2 is the number of
unconverted characters in the string. An ambiguous condition
exists if d2 overflows during the conversion.
>R CORE
Pronunciation: to-r
Compiles to: >R (0)
Stack effects: n ---
Remove n from the stack and place it on the return stack. Use
should be balanced with R> in the same definition.
? TOOLS
Pronunciation: question
Compiles to: @ (0)
. (0)
Stack effects: x ---
Print the value contained in the variable at address x in free
format according to the current BASE.
?DO CORE EXT
Pronunciation: question-do
Compiles to: ?DO (0)
Syntax: ?DO<space>..<space>+LOOP
?DO<space>..<space>LOOP
Stack effects: n1 n2 ---
If n1 is equal to n2, continue execution at LOOP or +LOOP.
Otherwise set up loop control parameters with index n2 and limit
n1 and continue executing immediately following ?DO. Anything
already on the return stack becomes unavailable until the loop
control parameters are discarded.
?DUP CORE
Pronunciation: question-dupe
Include: anscore.4th
Stack effects: n --- 0 | n n
Duplicate n if it is non-zero.
@ CORE
Pronunciation: fetch
Compiles to: @ (0)
Stack effects: x --- n
Leave the contents n of the variable at address x on the stack.
@C CROSS EXT
Compiles to: @C (0)
Stack effects: xt --- n | addr
Forth: In Forth the word @ can also be used to fetch values from
the dictionary. Due to 4tHs internal structure this is not
possible.
Leave the contents n of the parameter field of token address xt
on the stack. If n contains an string constant compiled by ,” it
is copied to the PAD. Its address is returned as addr.
@GOTO 4TH
Syntax: @GOTO<space><string>
The remainder of the line is discarded. This word is used to
start a 4tH source program from a MS type shell. An alias for \.
ABORT CORE
Forth: In Forth the behaviour of ABORT is different from QUIT (-1
THROW). In 4tH it doesn't really matter which one you use.
An alias for QUIT.
ABORT” CORE
Pronunciation: abort-quote
Compiles to: 0BRANCH (<address of QUIT>)
LITERAL (stdout)
USE (0)
.” (<address of string constant>)
CR (0)
QUIT (0)
Syntax: ABORT”<space><string>”
Stack effects: n ---
Forth: In Forth the behaviour of ABORT” is different from QUIT
(-2 THROW).
Remove n from the stack. If any bit of n is not zero, display the
string and set the program counter to the end of the program.
Effectively quits execution.
ABS CORE
Pronunciation: abs
Compiles to: ABS (0)
Stack effects: n1 --- n2
Leave the absolute value of n1 as n2.
ACCEPT CORE
Compiles to: ACCEPT (0)
Stack effects: addr n1 --- n2
Forth: In Forth no null character is appended.
Read n1 characters from the current input device to address addr.
If input is read from the terminal CR will terminate the input
stream. All other devices will terminate reading when an EOF
occurs. In all cases input will end when n1 characters have been
read. A null character is added to the end of the input when
reading from the keyboard. The number n2 represents the number of
characters actually read.
AGAIN CORE EXT
Compiles to: BRANCH (<address of the token following BEGIN>)
Syntax: BEGIN<space>..<space>AGAIN
At runtime, AGAIN forces execution to return to the corresponding
BEGIN. Execution cannot leave this loop. AGAIN is an alias for
REPEAT.
AKA 4TH
Syntax: AKA<space><word name><space><name>
Create a word <name> with the same compilation and execution
semantics as the existing word <word name>. The word <word name>
has to be user defined.
ALIAS 4TH
Compiles to: TO (<variable address>)
Stack effects: xt ---
Syntax: ALIAS<space><name>
Store xt in the value identified by name. ALIAS is an alias for
IS, but does not require a previously defined DEFER.
ALIGN CORE
Compiler: The instruction pointer is not incremented. In fact,
ALIGN is a dummy.
If the dataspace pointer is not aligned, reserve enough space to
align it.
ALIGNED CORE
Compiler: The instruction pointer is not incremented. In fact,
ALIGNED is a dummy.
Stack effects: n --- n
n is the first aligned address greater than or equal to n.
ALLOCATE MEMORY
Include: ansmem.4th
Stack effects: n --- addr f
Allocate n address units of contiguous data space. The initial
content of the allocated space is undefined. If the allocation
succeeds, addr is the aligned starting address of the allocated
space and f is false. If the operation fails, addr does not
represent a valid address and f is true.
AND CORE
Compiles to: AND (0)
Stack effects: n1 n2 --- n3
Leave the bitwise logical AND of n1 AND n2 as n3.
APP 4TH
Compiles to: LITERAL (<application variable>)
Stack effects: --- x
This word returns the variable address x in the Variable Area to
an array of application specific variables. If APP equals FIRST
no application specific variables have been defined.
APPEND 4TH
Compiles to: LITERAL (<fam>)
Stack effects: --- fam
This will leave a file access method modifier on the stack,
signalling that output will be appended. Must be added to another
file access modifier. Used in combination with OUTPUT.
ARGN 4TH
Compiles to: ARGN (0)
Stack effects: --- n
Returns the number of arguments that have been passed to 4tH
(see: ARGS).
ARGS 4TH
Compiles to: ARGS (0)
Stack effects: n1 --- addr n2
Copies argument n1 to the PAD and leaves address addr and length
n2 on the stack (see: ARGN).
ARRAY 4TH
Syntax: <literal><space>ARRAY<space><name>
Compiler: The previously compiled literal is taken as an argument
for ARRAY. The instruction pointer is decremented, actually
deleting the literal.
Forth: Roughly equivalent to:
: ARRAY CREATE CELLS ALLOT ;
Allocate <literal> cells of contiguous data space beginning at
<name> in the Integer Segment. The initial content of the
allocated space is undefined.
ASSERT( 4TH
Syntax: ASSERT(<space><word>..<word><space>)
Forth: Similar constructions are available in GForth and
Win32For.
Mark the beginning of an assertion. If assertions are disabled
all words following upto ) are commented out (see: [ASSERT] and )
).
BASE CORE
Compiles to: LITERAL (<address of BASE>)
Stack effects: --- x
A variable containing the current number BASE used for input and
output.
BEGIN CORE
Syntax: BEGIN<space>..<space>AGAIN
BEGIN<space>..<space>WHILE<space>..<space>UNTIL
BEGIN<space>..<space>WHILE<space>..<space>REPEAT
Forth: Within a BEGIN .. REPEAT construct, multiple WHILEs may be
used as well, but additional words are necessary to complete the
construct.
At runtime begin marks the start of a sequence that may be
repetitively executed. It serves as a return point from the
corresponding UNTIL, AGAIN or REPEAT. When executing UNTIL, a
return to BEGIN will occur if the top of the stack is false; for
AGAIN and REPEAT a return to BEGIN always occurs. Multiple WHILEs
may be used.
BIN FILE
Include: ansfile.4th
Stack effects: fam1 --- fam2
Modify file access method fam1 to additionally select a binary,
i.e., not line oriented, file access method, giving access method
fam2. Since 4tH does this automatically, BIN is a dummy.
BL CORE
Pronunciation: b-l
Compiles to: LITERAL (<ASCII value of space>)
Stack effects: --- c
A constant that leaves the ASCII value for "blank".
BLANK STRING
Compiles to: LITERAL (<ASCII value of space)
FILL (0)
Stack effects: n addr ---
If n is greater than zero, store the character value for space in
n consecutive character positions beginning at addr.
BLK BLOCK
Pronunciation: b-l-k
Include: ansblock.4th
Stack effects: --- x
Forth: In Forth, a block cannot have the number zero. BLK
contains the number of the block being interpreted.
x is the address of a cell containing the number of the
mass-storage block currently cached. Altering the contents of BLK
will have no lasting effects.
BLOCK BLOCK
Include: ansblock.4th
Stack effects: n --- addr
Addr is the address of the first character of the block buffer
assigned to mass-storage block n. An ambiguous condition exists
if u is not an available block number. If block n is already in a
block buffer, addr is the address of that block buffer. If block
n is not already in memory, unassign the block buffer. If the
block in that buffer has been UPDATEd, transfer the block to mass
storage and transfer block n from mass storage into that buffer.
a-addr is the address of that block buffer. At the conclusion of
the operation, the block buffer pointed to by addr is the current
block buffer and is assigned to n.
BOUNDS COMUS
Compiles to: OVER (0)
+ (0)
SWAP (0)
Stack effects: addr n --- addr addr+n
Convert a starting value and count into the form required for a
DO or ?DO loop.
BUFFER BLOCK
Include: ansblock.4th
Stack effects: n --- addr
Addr is the address of the first character of the block buffer
assigned to mass-storage block n. An ambiguous condition exists
if n is not an available block number. If block n is already in a
block buffer, addr is the address of that block buffer. If block
n is not already in memory, unassign the block buffer. If the
block in that buffer has been UPDATEd, transfer the block to mass
storage and transfer block n from mass storage into that buffer.
a-addr is the address of that block buffer. At the conclusion of
the operation, the block buffer pointed to by addr is the current
block buffer and is assigned to n.
BUFFER: CORE EXT
Syntax: <literal><space>BUFFER:<space><name>
Compiler: The previously compiled literal is taken as an argument
for BUFFER:. The instruction pointer is decremented, actually
deleting the literal.
Allocate <literal> address units of contiguous data space
beginning at <name> in the Character Segment. The initial content
of the allocated space is undefined.
C! CORE
Pronunciation: c-store
Compiles to: C! (0)
Stack effects: c addr ---
Store 8 bits of c at address addr in the Character Segment.
C, CORE
Pronunciation: c-comma
Syntax: <literal><space>C,
Compiler: The previously compiled literal is added as a character
to the String Segment. The instruction pointer is decremented,
actually deleting the literal.
Forth: Forth pops a value from the stack. This is not possible in
4tH.
Reserve space for one character in the String Segment and store
char in the space.
C@ CORE
Pronunciation: c-fetch
Compiles to: C@ (0)
Stack effects: addr --- c
Leave the 8 bits contents of Character Segment address addr as c.
CATCH EXCEPTION
Compiles to: CATCH (0)
(CATCH) (0)
Stack effects: xt --- n
Push an exception frame on the return stack and execute the
execution token xt in such a way that control can be transferred
to a point just after CATCH if THROW is executed during the
execution of xt (see: THROW).
CELL+ CORE
Pronunciation: cell-plus
Compiles to: 1+ (0)
Stack effects: x1 --- x2
Add the the size of a cell in cells to x1 giving x2.
CELL- COMUS
Compiles to: 1- (0)
Stack effects: x1 --- x2
Subtract the the size of a cell in cells to x1 giving x2.
CELLS CORE
Compiler: The instruction pointer is not incremented. In fact,
CELLS is a dummy.
Stack effects: n --- n
n is the size in cells of n cells.
CHAR CORE
Pronunciation: char
Compiles to: LITERAL (<ASCII-value of character>)
Syntax: CHAR<space><char>
Stack effects: --- c
Compiles the ASCII-value of <char> as a literal. At runtime the
value is thrown on the stack.
CHAR+ CORE
Pronunciation: char-plus
Compiles to: 1+ (0)
Stack effects: addr1 --- addr2
Add the the size of a character in characters to addr1 giving
addr2.
CHAR- 4TH
Compiles to: 1- (0)
Stack effects: addr1 --- addr2
Subtract the the size of a character in characters to addr1
giving addr2.
CHARS CORE
Pronunciation: chars
Compiler: The instruction pointer is not incremented. In fact,
CHARS is a dummy.
Stack effects: n --- n
Forth: In 4tH CHARS is a dummy, but it can be used to make a
program ANS-compatible.
n is the size in characters of n characters.
CHOP 4TH
Compiles to: 1- (0)
SWAP (0)
1+ (0)
SWAP (0)
Stack effects: a n --- a+1 n-1
Deletes the first character from the string defined by address a
and length n.
CIN 4TH
Compiles to: ENVIRON (<address of CIN>)
Stack effects: --- n
Identifies the input source.
CLOSE 4TH
Compiles to: CLOSE (0)
Stack effects: h ---
CLOSE will close a file or pipe, previously opened by OPEN and
release the stream. Depending on the file access method, the
terminal will be made the current input-device, otherwise the
screen will be made the current output-device.
CLOSE-FILE FILE
Include: ansfile.4th
Stack effects: h --- f
Close the file identified by handle h. Flag f is the
implementation-defined I/O result code.
CMOVE STRING
Pronunciation: c-move
Compiles to: CMOVE (0)
Stack effects: addr1 addr2 n ---
Forth: In Forth there are two words for this operation, CMOVE and
CMOVE>. Usage depends on the direction of the move. In 4tH CMOVE
is smart, like MOVE.
Move the specified quantity of bytes (n) beginning at address
addr1 to addr2.
CMOVE> STRING
Pronunciation: c-move-up
Compiles to: CMOVE (0)
Stack effects: addr1 addr2 n ---
An alias for CMOVE (see: CMOVE).
COMPARE STRING
Include: compare.4th
Stack effects: addr1 n1 addr2 n2 --- n3
Compare the string specified by addr1 n1 to the string specified
by addr2 n2 . The strings are compared, beginning at the given
addresses, character by character, up to the length of the
shorter string or until a difference is found. If the two strings
are identical, n3 is zero. If the two strings are identical up to
the length of the shorter string, n3 is -1 if n1 is less than n2
and 1 otherwise. If the two strings are not identical up to the
length of the shorter string, n3 is -1 if the first non-matching
character in the string specified by addr1 n1 has a lesser
numeric value than the corresponding character in the string
specified by addr2 n2 and 1 otherwise.
CONSTANT CORE
Syntax: <literal><space>CONSTANT<space><name>
Compiler: The previously compiled literal is taken as an argument
for CONSTANT. The instruction pointer is decremented, actually
deleting the literal.
Forth: In Forth, the literal value is popped from the stack. This
cannot be done in 4tH.
A defining word used to create word <name>. When <name> is later
executed, it will push the value of <literal> on the stack.
COUNT CORE
Compiles to: COUNT (0)
Stack effects: addr1 --- addr2 n
Forth: Programs assuming that the string is a so-called counted
string will not work. Well-written programs only assume the
correct input- and output-parameters.
Leave the Character Segment address addr2 and count n of an
ASCIIZ string beginning at Character Segment address addr1.
Typically COUNT is followed by TYPE.
COUT 4TH
Compiles to: ENVIRON (<address of COUT>)
Stack effects: --- n
Identifies the output source.
CR CORE
Pronunciation: c-r
Compiles to: CR (0)
Transmit a carriage return to the selected output-device. The
actual sequence sent is OS- and stream-dependant.
CREATE CORE
Syntax: CREATE<space><name>
Forth: In Forth this will create a dictionary header.
Leaves <name> in the symboltable and replace further occurences
with LITERAL <xt>. <xt> represents the address in the Code
Segment where CREATE was compiled.
CREATE-FILE FILE
Include: ansfile.4th
Stack effects: addr n fam --- h f
Create the file named in the character string specified by addr
and n, and open it with file access method fam. The meaning of
values of fam is implementation defined. If a file with the same
name already exists, recreate it as an empty file. If the file
was successfully created and opened, f is zero, handle h is its
identifier, and the file has been positioned to the start of the
file. Otherwise, f is the implementation-defined I/O result code
and h is undefined.
D+ DOUBLE
Pronunciation: d-plus
Include: ansdbl.4th
Stack effects: d1 d2 --- d3
Add d1 to d2, giving the sum d3.
D- DOUBLE
Pronunciation: d-minus
Include: ansdbl.4th
Stack effects: d1 d2 --- d3
Subtract d2 from d1, giving the difference d3.
D. DOUBLE
Pronunciation: d-dot
Include: dbldot.4th
Stack effects: d ---
Display d in free field format.
D.R DOUBLE
Pronunciation: d-dot-r
Include: dbldot.4th
Stack effects: d n ---
Display d right aligned in a field n characters wide. If the
number of characters required to display d is greater than n, all
digits are displayed with no leading spaces in a field as wide as
necessary.
D0< DOUBLE
Pronunciation: d-zero-less
Include: ansdbl.4th
Stack effects: d --- f
Flag f is true if and only if d is less than zero.
D0= DOUBLE
Pronunciation: d-zero-equals
Include: ansdbl.4th
Stack effects: d --- f
Flag f is true if and only if d is equal to zero.
D2* DOUBLE
Pronunciation: d-two-star
Include: ansdbl.4th
Stack effects: d1 --- d2
D2 is the result of shifting d1 one bit toward the
most-significant bit, filling the vacated least-significant bit
with zero.
D2/ DOUBLE
Pronunciation: d-two-slash
Include: ansdbl.4th
Stack effects: d1 --- d2
D2 is the result of shifting d1 one bit toward the
least-significant bit, leaving the most-significant bit
unchanged.
D< DOUBLE
Pronunciation: d-less-than
Include: ansdbl.4th
Stack effects: d1 d2 --- f
Flag f is true if and only if d1 is less than d2.
D= DOUBLE
Pronunciation: d-equals
Include: ansdbl.4th
Stack effects: d1 d2 --- f
Flag f is true if and only if d1 is equal to d2.
D>F FLOATING
Pronunciation: d-to-f
Include: ansfloat.4th
Stack effects: d ---
Floating: --- r
r is the floating-point equivalent of d. An ambiguous condition
exists if d cannot be precisely represented as a floating-point
value.
D>S DOUBLE
Pronunciation: d-to-s
Compiler: The instruction pointer is not incremented. In fact,
D>S is a dummy.
Stack effects: n --- n
Convert the number n to number n with the same numerical value.
DABS DOUBLE
Pronunciation: d-abs
Include: ansdbl.4th
Stack effects: d1 --- d2
D2 is the absolute value of d1.
DMAX DOUBLE
Pronunciation: d-max
Include: ansdbl.4th
Stack effects: d1 d2 --- d3
D3 is the greater of d1 and d2.
DMIN DOUBLE
Pronunciation: d-min
Include: ansdbl.4th
Stack effects: d1 d2 --- d3
D3 is the lesser of d1 and d2.
DNEGATE DOUBLE
Pronunciation: d-negate
Include: ansdbl.4th
Stack effects: d1 --- d2
D2 is the negation of d1.
DECIMAL CORE
Compiles to: RADIX (10)
Forth: See HEX.
Set the numeric conversion BASE for decimal output at runtime.
DEFER CORE EXT
Compiles to: LITERAL ((ERROR))
TO (<variable address>)
Syntax: DEFER<space><name>
Stack effects: ---
Create a value name which will hold an execution token for a word
whose behavior will be determined later and may be varied. The
initial value will trigger an error if used before proper
assignment.
DEFER! CORE EXT
Compiles to: ENVIRON (<address of FIRST>)
+ (0)
! (0)
Stack effects: xt x ---
Set the vector x to execute xt.
DEFER@ CORE EXT
Compiles to: ENVIRON (<address of FIRST>)
+ (0)
@ (0)
Stack effects: x --- xt
xt is the xt associated with the deferred word corresponding to
x.
DEPTH CORE
Compiles to: SP@ (0)
Stack effects: --- n
Returns the number of items on the stack in n, before DEPTH was
executed. An alias for SP@.
DO CORE
Compiles to: DO (0)
Syntax: DO<space>..<space>+LOOP
DO<space>..<space>LOOP
Stack effects: n1 n2 ---
At runtime DO begins a sequence with repetitive execution
controlled by a loop limit n1 and an index with initial value n2.
DO removes these from the stack. Upon reaching LOOP or +LOOP the
index is altered. Until the new index equals or exceeds the
limit, execution loops back to just after DO; otherwise the loop
parameters are discarded and execution continues ahead. Both n1
and n2 are determined at runtime and may be the result of other
operations. Within a loop I will copy the current value of the
index on the stack.
DOES> CORE
Pronunciation: does
Compiler: The instruction pointer is not incremented. In fact,
DOES> is a dummy.
Syntax: :THIS<space><name><space>DOES><space>..<space>;
Forth: In Forth, DOES> is usually combined with CREATE, changing
the behaviour of an entire datatype. In 4tH it is used with
:THIS, changing only the referenced definition.
Append the run-time semantics to the referenced definition.
Execute the portion of the definition that begins with the
initiation semantics appended by the DOES> which modified name.
DROP CORE
Compiles to: DROP (0)
Stack effects: n ---
Drop the number from the stack.
DU< DOUBLE EXT
Pronunciation: d-u-less
Include: ansdbl.4th
Stack effects: d1 d2 --- f
Flag is true if and only if usigned double d1 is less than
unsigned double d2.
DUMP TOOLS
Include: dump.4th
Stack effects: addr n ---
Display the contents of n consecutive addresses starting at addr.
DUP CORE
Pronunciation: dupe
Compiles to: DUP (0)
Stack effects: n --- n n
Duplicate the value on the stack.
ELSE CORE
Compiles to: BRANCH (<address of matching THEN token>)
Syntax: IF<space>..<space>ELSE<space>..<space>THEN
At runtime ELSE executes after the true following IF. ELSE forces
execution to skip over the following false part and resumes
execution after the THEN.
EMIT CORE
Compiles to: EMIT (0)
Stack effects: c ---
Transmit the ASCII character with code n to the selected output
device.
EMPTY-BUFFERS BLOCK EXT
Include: ansblock.4th
Stack effects: ---
Unassign all block buffers. Do not transfer the contents of any
UPDATEd block buffer to mass storage.
END-STRUCT 4TH
Syntax:
STRUCT<space><literal><space>+FIELD<space><name><space>END-STRUCT<space><name>
Compiler: The previously compiled literal is taken as an argument
for END-STRUCT, creating a constant that holds the length of the
STRUCT. The instruction pointer is decremented, actually deleting
the literal.
Forth: Similar constructions are available in gForth. +FIELD is
part of the Forth 200x draft.
Terminate the definition of a STRUCT. The created structure is an
constant that memorizes the size of the structure (see: +FIELD,
STRUCT).
ENUM 4TH
Syntax: <literal><space>ENUM<space><name>
Compiler: The previously compiled literal is taken as an argument
for ENUM and incremented afterwards. The instruction pointer is
left unchanged.
Forth: This word is available in some Forths.
A defining word used to create word <name>. When <name> is later
executed, it will push the value of <literal> on the stack.
ENVIRONMENT? CORE
Pronunciation: environment-query
Include: environ.4th
Stack effects: addr n --- -f
addr is the address of a character string and n is the string's
character count. The character string should contain a keyword
from ANS-Forth environmental queries or the optional word sets to
be checked for correspondence with an attribute of the present
environment. The system treats the attribute as unknown, the
returned flag is false.
ERASE CORE EXT
Compiles to: LITERAL (0)
FILL (0)
Stack effects: addr n ---
If n is greater than zero, clear all bits in each of n
consecutive address units of memory beginning at addr.
ERROR? 4TH
Compiles to: LITERAL (<largest negative integer>)
OVER (0)
= (0)
Stack effects: n --- n f
If n equals (ERROR), leave a true flag, otherwise leave a false
flag. Determines whether n indicates an error condition. The
resulting stack diagram is ANS-Forth compliant.
EVALUATE CORE
Include: evaluate.4th
Stack effects: addr n ---
Forth: In Forth, the entire dictionary is available. In 4tH, the
only words available are explicitly defined by the program.
Make the string described by addr and n the input buffer and
interpret. Other stack effects are due to the words EVALUATEd.
EXECUTE CORE
Compiles to: EXECUTE (0)
Stack effects: xt ---
Execute the colon definition whose token-address xt is on the
stack. The current token-address is pushed on the returnstack.
EXIT CORE
Compiles to: EXIT (0)
When compiled within a colon-definition, terminates execution of
that definition at that point. At runtime functionally equivalent
to ;.
EXPECT CORE EXT
Include: obsolete.4th
Stack effects: addr n ---
Receive a string of at most n-1 characters. The editing
functions, if any, that the system performs in order to construct
the string of characters are implementation-defined. Input
terminates when an implementation-defined line terminator is
received or when the string is n-1 characters long. When input
terminates the display is maintained in an implementation-defined
way. Store the string at addr and its length in SPAN (see SPAN).
F! FLOATING
Pronunciation: f-store
Include: ansfloat.4th
zenans.4th
Stack effects: x ---
Floating: r ---
Store r at address x.
F* FLOATING
Pronunciation: f-star
Include: ansfloat.4th
zenfloat.4th
Stack effects: ---
Floating: r1 r2 --- r3
Multiply r1 by r2 giving r3.
F** FLOATING EXT
Pronunciation: f-star-star
Include: falog.4th
zenfalog.4th
Stack effects: ---
Floating: r1 r2 --- r3
Raise r1 to the power r2, giving the product r3.
F+ FLOATING
Pronunciation: f-plus
Include: ansfloat.4th
zenfloat.4th
Stack effects: ---
Floating: r1 r2 --- r3
Add r1 to r2 giving the sum r3.
F- FLOATING
Pronunciation: f-minus
Include: ansfloat.4th
zenfloat.4th
Stack effects: ---
Floating: r1 r2 --- r3
Subtract r2 from r1 giving r3.
F. FLOATING EXT
Pronunciation: f-dot
Include: ansfloat.4th
fpout.4th
zenfloat.4th
Stack effects: ---
Floating: r ---
Display, with a trailing space, the top number on the
floating-point stack using fixedpoint notation. An ambiguous
condition exists if the value of BASE is not (decimal) ten or if
the character string representation exceeds the size of the
pictured numeric output string buffer.
F/ FLOATING
Pronunciation: f-slash
Include: ansfloat.4th
zenfloat.4th
Stack effects: ---
Floating: r1 r2 --- r3
Divide r1 by r2, giving the quotient r3. An ambiguous condition
exists if r2 is zero, or the quotient lies outside of the range
of a floating-point number.
F0< FLOATING
Pronunciation: f-zero-less-than
Include: ansfloat.4th
zenfloat.4th
Stack effects: --- f
Floating: r ---
Flag f is true if and only if r is less than zero.
F0= FLOATING
Pronunciation: f-zero-equals
Include: ansfloat.4th
zenfloat.4th
Stack effects: --- f
Floating: r ---
Flag f is true if and only if r is equal to zero.
F< FLOATING
Pronunciation: f-less-than
Include: ansfloat.4th
zenfloat.4th
Stack effects: --- f
Floating: r1 r2 ---
Flag f is true if and only if r1 is less than r2.
F>D FLOATING
Pronunciation: f-to-d
Include: ansfloat.4th
Stack effects: --- d
Floating: r ---
Double number d is the double-cell signed-integer equivalent of
the integer portion of r. The fractional portion of r is
discarded. An ambiguous condition exists if the integer portion
of r cannot be precisely represented as a double-cell signed
integer.
F@ FLOATING
Pronunciation: f-fetch
Include: ansfloat.4th
zenans.4th
Stack effects: x ---
Floating: --- r
Float r is the value stored at address x.
FABS FLOATING EXT
Pronunciation: f-abs
Include: ansfloat.4th
zenfloat.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the absolute value of r1.
FACOS FLOATING EXT
Pronunciation: f-a-cos
Include: asinacos.4th
zenfasin.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the principal radian angle whose cosine is r1. An
ambiguous condition exists if | r1 | is greater than one.
FACOSH FLOATING EXT
Pronunciation: f-a-cosh
Include: fatanh.4th
zenatanh.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the floating-point value whose hyperbolic cosine is
r1. An ambiguous condition exists if r1 is less than one.
FALIGN FLOATING
Pronunciation: f-align
Include: ansfloat.4th
Stack effects: ---
Floating: ---
If the data-space pointer is not float aligned, reserve enough
data space to make it so. In 4tH, it is a dummy.
FALIGNED FLOATING
Pronunciation: f-aligned
Include: ansfloat.4th
Stack effects: x --- x
Floating: ---
Address x is the first float-aligned address greater than or
equal to address x. In 4tH, it is a dummy.
FALOG FLOATING EXT
Pronunciation: f-a-log
Include: falog.4th
zenfalog.4th
Stack effects: ---
Floating: r1 --- r2
Raise ten to the power r1, giving r2.
FALSE CORE EXT
Compiles to: LITERAL (<false>)
Stack effects: --- -f
Returns a FALSE flag on the stack.
FASIN FLOATING EXT
Pronunciation: f-a-sine
Include: asinacos.4th
zenfasin.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the principal radian angle whose sine is r1. An
ambiguous condition exists if | r1 | is greater than one.
FASINH FLOATING EXT
Pronunciation: f-a-cinch
Include: fatanh.4th
zenatanh.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the floating-point value whose hyperbolic sine is r1.
An ambiguous condition exists if r1 is less than zero.
FATAN FLOATING EXT
Pronunciation: f-a-tan
Include: asinacos.4th
zenfasin.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the principal radian angle whose tangent is r1.
FATAN2 FLOATING EXT
Pronunciation: f-a-tan-two
Include: fatan2.4th
zenatan2.4th
Stack effects: ---
Floating: r1 r2 --- r3
Float r3 is the radian angle whose tangent is r1/r2. An ambiguous
condition exists if r1 and r2 are zero.
FATANH FLOATING EXT
Pronunciation: f-a-tan-h
Include: fatanh.4th
zenatanh.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the floating-point value whose hyperbolic tangent is
r1. An ambiguous condition exists if r1 is outside the range of
-1 to 1.
FCOS FLOATING EXT
Pronunciation: f-cos
Include: fsinfcos.4th
zenfsin.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the cosine of the radian angle r1.
FCOSH FLOATING EXT
Pronunciation: f-cosh
Include: sinhcosh.4th
zenfsinh.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the hyperbolic cosine of r1.
FDEPTH FLOATING
Pronunciation: f-depth
Include: ansfloat.4th
Stack effects: --- n
Floating: ---
N is the number of values contained on the default separate
floating-point stack.
FDROP FLOATING
Pronunciation: f-drop
Include: ansfloat.4th
zenans.4th
Stack effects: ---
Floating: r ---
Remove r from the floating-point stack.
FDUP FLOATING
Pronunciation: f-dupe
Include: ansfloat.4th
zenans.4th
Stack effects: ---
Floating: r --- r r
Duplicate r.
FE. FLOATING EXT
Pronunciation: f-e-dot
Include: ansfpio.4th
fpout.4th
Stack effects: ---
Floating: r ---
Display, with a trailing space, the top number on the
floating-point stack using engineering notation, where the
significand is greater than or equal to 1.0 and less than 1000.0
and the decimal exponent is a multiple of three. An ambiguous
condition exists if the value of BASE is not (decimal) ten or if
the character string representation exceeds the size of the
pictured numeric output string buffer.
FEXP FLOATING EXT
Pronunciation: f-cos
Include: fexp.4th
zenfexp.4th
Stack effects: ---
Floating: r1 --- r2
Raise e to the power r1, giving r2.
FILE 4TH
Compiles to: LITERAL ((ERROR))
TO (<variable address>)
Syntax: FILE<space><name>
Stack effects: ---
Create a value name which will hold a filehandle. The initial
value will trigger an error if used before proper assignment.
FILE-POSITION FILE
Include: ansfile.4th
Stack effects: h --- n f
n is the current file position for the file identified by handle
h. Flag f is the implementation-defined I/O result code. n is
undefined if f is non-zero.
FILE-SIZE FILE
Include: ansfile.4th
Stack effects: h --- n f
n is the size, in characters, of the file identified by handle h.
Flag f is the implementation-defined I/O result code. This
operation does not affect the value returned by FILE-POSITION. n
is undefined if f is true.
FILE-STATUS FILE EXT
Include: ansfile.4th
Stack effects: addr n1 --- n2 f
Return the status of the file identified by the character string
addr n1. If the file exists, flag f is zero; otherwise flag f is
the implementation-defined I/O result code. n2 contains
implementation defined information about the file.
FILES 4TH
Compiles to: LITERAL (<number of open files>)
Stack_effects: --- n
Returns the maximum number of open streams 4tH can handle. Two of
these streams are predefined, STDIN and STDOUT.
FILL CORE
Compiles to: FILL (0)
Stack effects: addr n c ---
Fills n bytes in the Character Segment, beginning at address
addr, with character c.
FIRST 4TH
Compiles to: ENVIRON (<address of FIRST>)
Stack effects: --- x
Leaves the variable address x of the first user-variable. If
FIRST is greater than LAST, no user-variables have been defined.
FLN FLOATING EXT
Pronunciation: f-l-n
Include: flnflog.4th
zenfln.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the natural logarithm of r1. An ambiguous condition
exists if r1 is less than or equal to zero.
FLOAT+ FLOATING
Pronunciation: float-plus
Include: ansfloat.4th
zenans.4th
Stack effects: x1 --- x2
Floating: ---
Add the size in address units of a floating-point number to
address x1, giving address x2.
FLOATS FLOATING
Include: ansfloat.4th
zenans.4th
Stack effects: n1 --- n2
Floating: ---
Forth: In Forth, this word also works at compile time.
Number n2 is the size in address units of n1 floating-point
numbers.
FLOG FLOATING EXT
Pronunciation: f-log
Include: flnflog.4th
zenfln.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the base-ten logarithm of r1. An ambiguous condition
exists if r1 is less than or equal to zero.
FLOOR FLOATING
Include: ansfloat.4th
zenfloor.4th
Stack effects: ---
Floating: r1 --- r2
Round r1 to an integral value using the ”round toward negative
infinity” rule, giving r2.
FLUSH BLOCK
Include: ansblock.4th
Stack effects: ---
Perform the function of SAVE-BUFFERS, then unassign the block
buffer.
FLUSH-FILE FILE EXT
Include: ansfile.4th
Stack effects: h --- f
Attempt to force any buffered information written to the file
referred to by handle h to be written to mass storage, and the
size information for the file to be recorded in the storage
directory if changed. If the operation is successful, f is zero.
Otherwise, it is an implementation-defined I/O result code.
FM/MOD CORE
Pronunciation: f-m-slash-mod
Include: mixed.4th
Stack effects: d1 n1 --- n2 n3
Divide d1 by n1, giving the floored quotient n3 and the remainder
n2. Input and output stack arguments are signed. An ambiguous
condition exists if n1 is zero or if the quotient lies outside
the range of a single-cell signed integer.
FMAX FLOATING
Pronunciation: f-max
Include: ansfloat.4th
zenfmin.4th
Stack effects: ---
Floating: r1 r2 --- r3
Float r3 is the greater of r1 and r2.
FMIN FLOATING
Pronunciation: f-min
Include: ansfloat.4th
zenfmin.4th
Stack effects: ---
Floating: r1 r2 --- r3
Float r3 is the lesser of r1 and r2.
FNEGATE FLOATING
Pronunciation: f-negate
Include: ansfloat.4th
zenfloat.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the negation of r1.
FOVER FLOATING
Pronunciation: f-over
Include: ansfloat.4th
zenans.4th
Stack effects: ---
Floating: r1 r2 --- r1 r2 r1
Place a copy of r1 on top of the floating-point stack.
FREE MEMORY
Include: ansmem.4th
Stack effects: addr --- f
Return the contiguous region of data space indicated by addr to
the system for later allocation. addr shall indicate a region of
data space that was previously obtained by ALLOCATE or RESIZE. If
the operation succeeds, f is false. If the operation fails, f is
true.
FROT FLOATING
Pronunciation: f-rot
Include: ansfloat.4th
zenans.4th
Stack effects: ---
Floating: r1 r2 r3 --- r2 r3 r1
Rotate the top three floating-point stack entries.
FROUND FLOATING
Pronunciation: f-round
Include: ansfloat.4th
zenround.4th
Stack effects: ---
Floating: r1 --- r2
Round r1 to an integral value using the ”round to nearest” rule,
giving r2.
FS. FLOATING EXT
Pronunciation: f-s-dot
Include: ansfpio.4th
fpout.4th
Stack effects: ---
Floating: r ---
Display, with a trailing space, the top number on the
floating-point stack in scientific notation. An ambiguous
condition exists if the value of BASE is not (decimal) ten or if
the character string representation exceeds the size of the
pictured numeric output string buffer.
FSIN FLOATING EXT
Pronunciation: f-sine
Include: fsinfcos.4th
zenfsin.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the sine of the radian angle r1.
FSINCOS FLOATING EXT
Pronunciation: f-sine-cos
Include: fsinfcos.4th
zenfsin.4th
Stack effects: ---
Floating: r1 --- r2 r3
Float r2 is the sine of the radian angle r1. Float r3 is the
cosine of the radian angle r1.
FSINH FLOATING EXT
Pronunciation: f-cinch
Include: sinhcosh.4th
zenfsinh.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the hyperbolic sine of r1.
FSQRT FLOATING EXT
Pronunciation: f-square-root
Include: ansfloat.4th
zenfsqrt.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the square root of r1. An ambiguous condition exists
if r1 is less than zero.
FSWAP FLOATING
Pronunciation: f-swap
Include: ansfloat.4th
zenans.4th
Stack effects: ---
Floating: r1 r2 --- r2 r1
Exchange the top two floating-point stack items.
FTAN FLOATING EXT
Pronunciation: f-tan
Include: fsinfcos.4th
zenfsin.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the tangent of the radian angle r1. An ambiguous
condition exists if cos (r1) is zero.
FTANH FLOATING EXT
Pronunciation: f-tan-h
Include: sinhcosh.4th
zenfsinh.4th
Stack effects: ---
Floating: r1 --- r2
Float r2 is the hyperbolic tangent of r1.
F~ FLOATING EXT
Pronunciation: f-proximate
Include: ansfloat.4th
Stack effects: --- f
Floating: r1 r2 r3 ---
If r3 is positive, flag is true if the absolute value of (r1
minus r2) is less than r3. If r3 is zero, flag is true if the
implementation-dependent encoding of r1 and r2 are exactly
identical. If r3 is negative, flag is true if the absolute value
of (r1 minus r2) is less than the absolute value of r3 times the
sum of the absolute values of r1 and r2.
HERE CORE
Compiles to: LITERAL (<token address>)
Stack effects: --- xt
Forth: Leaves the address of the next available dictionary
location. Since 4tH doesn't have a dictionary location, its use
is very different.
At runtime, HERE leaves the address xt in the Code Segment where
it was compiled.
HEX CORE EXT
Compiles to: RADIX (16)
Forth: In Forth this construction
HEX : SOMETIN 16 ;
will compile 16 as a hexadecimal number. In 4tH it will simply
be compiled and 16 will be compiled as a decimal number. To
emulate this construction use
[HEX] : SOMETIN 16 ;
instead.
Set the numeric conversion BASE for hexadecimal output at
runtime.
HI 4TH
Compiles to: ENVIRON (<address of HI>)
Stack effects: --- addr
Leaves the address of the last character in the Character
Segment.
HIDE 4TH
Syntax: HIDE<space><name>
Find <name>, then delete name from the symbol table. Used to
create private definitions.
HOLD CORE
Compiles to: HOLD (0)
Stack effects: c ---
Used between <# and #> to insert an ASCII character into a
pictured numeric output string, e.g. [HEX] 2E HOLD will place a
decimal point.
I CORE
Compiles to: I (0)
Stack effects: --- n
Used with a DO .. LOOP to copy the loop index to the stack. An
alias for R.
IF CORE
Compiles to: 0BRANCH (<address of matching ELSE|THEN token>)
Stack effects: f ---
Syntax: See ELSE, THEN
At runtime, IF selects execution based on f. If f is non-zero,
execution continues ahead through the true part. If f is zero
execution skips till just after ELSE to execute the false part.
After each part, execution resumes after THEN.
IMMEDIATE CORE
Compiler: The instruction pointer is not incremented. In fact,
IMMEDIATE is a dummy.
Stack effects: ---
Make the most recent definition an immediate word.
INCLUDE COMUS
Syntax: INCLUDE<space><string><space>
Compiler: The contents of the file are inserted at this position.
An alias for [NEEDS (see: [NEEDS).
INPUT 4TH
Compiles to: LITERAL (<fam>)
Stack effects: --- fam
This will leave a file access method on the stack, signalling an
operation on an input-device.
INVERT CORE
Compiles to: INVERT (0)
Stack effects: n1 --- n2
Leave n1's binary complement as n2. This word is not equivalent
to 0=.
IS CORE EXT
Compiles to: TO (<variable address>)
Stack effects: xt ---
Syntax: IS<space><name>
Store xt in the value identified by name, previously defined by
DEFER (see: DEFER).
J CORE
Compiles to: J (0)
Stack effects: --- n
Used with an embedded DO .. +LOOP to copy the outer loop index to
the stack. Copies in fact the third item of the returnstack.
LAST 4TH
Compiles to: ENVIRON (<address of LAST>)
Stack effects: --- x
Leaves the variable address x of the last variable in the
Variable Area.
LEAVE CORE
Compiles to: LEAVE (0)
Force termination of a DO .. +LOOP at the next opportunity by
setting the loop index equal to the loop limit. The limit itself
remains unchanged, and execution proceeds normally until +LOOP is
encountered.
LIST BLOCK EXT
Include: ansblock.4th
Stack effects: n ---
Display block n in an implementation-defined format. Store n in
SCR.
LO 4TH
Compiles to: LITERAL (<TIB+PAD>)
Stack effects: --- addr
Leaves the offset of the first character of the Allocation Area
in the Character Segment. If LO is greater than HI, no memory has
been allocated.
LOAD BLOCK
Include: ansblock.4th
Before: evaluate.4th
Stack effects: n ---
Forth: In Forth, the entire dictionary is available. In 4tH, the
only words available are explicitly defined by the program.
Save the current input-source specification. Store n in BLK (thus
making block n the input source and setting the input buffer to
encompass its contents), set >IN to zero, and execute EVALUATE.
When the parse area is exhausted, restore the prior input source
specification.
LOOP CORE
Compiles to: LOOP (<address of matching DO token>)
Syntax: DO<space>..<space>+LOOP
Used in the form DO .. LOOP. At runtime, LOOP selectively
controls branching back to the corresponding DO based on the loop
index and the loop limit. The index is incrementex and compared
to the limit. The branch back to DO occurs until the new index is
equal to the limit. Upon exiting the loop, the parameters are
discarded and execution continues ahead.
LSHIFT CORE
Pronunciation: l-shift
Compiles to: SHIFT (0)
Stack effects: n1 n2 --- n3
Performs a logical bit shift on n1. Specifically, SHIFT shifts a
number a number of bits, specified in n2, using a logical
register shift. An alias for SHIFT.
M* CORE
Pronunciation: m-star
Include: mixed.4th
Stack effects: n1 n2 --- d
d is the signed product of n1 times n2.
M*/ DOUBLE
Pronunciation: m-star
Include: mixed.4th
Stack effects: d1 n1 n2 --- d2
Multiply d1 by n1 producing the triple-cell intermediate result
t. Divide t by n2 giving the double-cell quotient d2. An
ambiguous condition exists if n2 is zero or negative, or the
quotient lies outside of the range of a double-precision signed
integer.
M+ DOUBLE
Pronunciation: m-star
Include: mixed.4th
Stack effects: d1 n1 --- d2
Add n1 to d1, giving the sum d2.
MAX CORE
Compiles to: MAX (0)
Stack effects: n1 n2 --- n3
Leave n3 as the greater of the two numbers n1 and n2.
MAX-N COMUS
Compiles to: LITERAL (<largest positive integer>)
Stack effects: --- n
Forth: Equivalent to:
: MAX-N S" MAX-N" ENVIRONMENT? DROP ;
Returns the largest positive integer that 4tH can handle. Usually
2^31.
MAX-RAND 4TH
Compiles to: LITERAL (<largest integer returned by RANDOM>)
Stack effect: --- n
Returns the largest positive integer that RANDOM can return.
MIN CORE
Compiles to: MIN (0)
Stack effects: n1 n2 --- n3
Leave n3 as the smaller of the two numbers n1 and n2.
MOD CORE
Compiles to: MOD (0)
Stack effects: n1 n2 --- n3
Leave the remainder of n1/n2 with the same sign as n1 in n3.
MOVE CORE
Compiles to: CMOVE (0)
Stack effects: addr1 addr2 n ---
Move the specified quantity of bytes (n) beginning at address
addr1 to addr2 in the Character Segment.
MS FACILITY EXT
Include: ansfacil.4th
Stack effects: n ---
Forth: In Forth, the resolution is significantly higher than
between +0 and +1999 ms.
Wait at least u milliseconds.
NEGATE CORE
Compiles to: NEGATE (0)
Stack effects: n1 --- -n1
Leave n1 negated (two's complement).
NIP CORE EXT
Compiles to: SWAP (0)
DROP (0)
Stack effects: n1 n2 --- n2
Drop the first item below the top of stack.
NOT COMUS
Compiles to: 0= (0)
Stack effects: n --- f
An alias for 0= (see: 0=).
NUMBER 4TH
Compiles to: NUMBER (0)
Stack effects: addr n1 --- n2
Forth: Some Forths support this word too, but issue a message on
error.
Convert an string at offset addr with length n1 in the Character
Segment to number n2. If numeric conversion is not possible
(ERROR) is left on the stack.
OCTAL 4TH
Compiles to: RADIX (8)
Forth: See HEX.
Set the numeric conversion BASE for octal output at runtime.
OFFSET 4TH
Syntax: OFFSET<space><name>
Forth: Equivalent to:
: OFFSET CREATE DOES> SWAP CHARS + C@ ;
Leaves <name> in the symboltable and replaces further occurences
of <name> with an execution procedure which takes an index from
the stack and leaves the character concerned on the stack.
OMIT 4TH
Compiles to: OMIT (0)
Stack effects: c ---
Skips all leading delimiters in the Character Segment, using
character c as a delimiter.
OPEN 4TH
Compiles to: OPEN (0)
Stack effects: addr n fam --- h
OPEN will open the file, which name has been specified by an
ASCIIZ string, starting at offset addr in the Character Segment
and having length n. Depending on the file access method, the
file or pipe will be opened for reading, otherwise for writing.
If the file or pipe was succesfully opened it will be connected
to a stream and a valid filehandle will be left on the stack. If
not, (ERROR) will be left on the stack. Note that OPEN does not
connect a stream to a channel (see: USE).
OPEN-FILE FILE
Include: ansfile.4th
Stack effects: addr n fam --- h f
Open the file named in the character string specified by addr n,
with file access method indicated by fam. The meaning of values
of fam is implementation defined. If the file is successfully
opened, flag f is zero, handle h is its identifier, and the file
has been positioned to the start of the file. Otherwise, f is the
implementation-defined I/O result code and h is undefined.
OR CORE
Compiles to: OR (0)
Stack effects: n1 n2 --- n3
Leave the bitwise logical OR in n3 of the numbers n1 and n2.
OUT COMUS
Compiles to: LITERAL (<address of OUT>)
Stack effects: --- x
A variable containing the the value that will be returned to the
host program.
OUTPUT 4TH
Compiles to: LITERAL (<fam>)
Stack effects: --- fam
This will leave a file access method on the stack, signalling an
operation on an output-device.
OVER CORE
Compiles to: OVER (0)
Stack effects: n1 n2 --- n1 n2 n1
Copy the second stack value to the top of the stack.
PAD CORE EXT
Compiles to: LITERAL (<address of PAD>)
Stack effects: --- addr
Leave the address of the text output buffer.
PARSE CORE EXT
Compiles to: PARSE (0)
Stack effects: c --- addr n
Reads a string from the Character Segment, using character c as a
delimiter. Leaves the addr/count pair addr n. The resulting
string is not zero-terminated. If the parse area was empty, the
resulting string has a zero length.
PARSE-WORD 4TH
Compiles to: DUP (0)
OMIT (0)
PARSE (0)
Stack effects: c --- addr n
Reads a string from the Character Segment, using character c as a
delimiter and skipping all leading delimiters. Leaves the
addr/count pair addr n. The resulting string is not
zero-terminated.
PAUSE 4TH
Compiles to: PAUSE (0)
Stack_effects: ---
Saves a stackframe, closes all files and quits execution. Leaves
the virtual machine in a state where it can resume execution.
PICK CORE EXT
Include: anscore.4th
Stack effects: nu .. n1 n2 u --- nu .. n1 n2 nu
Remove u. Copy the nu to the top of the stack.
PIPE 4TH
Compiles to: LITERAL (<fam>)
Stack effects: --- fam
This will leave a file access method modifier on the stack,
signalling an operation on a pipe. Must be added to another file
access modifier. Used in combination with INPUT and OUTPUT. If an
OS does not support pipes, opening a pipe will always fail.
PLACE COMUS
Compiles to: PLACE (0)
Stack effects: addr1 n addr2 ---
Copies the string at address addr1 with count n to address addr2.
PRECISION FLOATING EXT
Include: ansfloat.4th
Stack effects: --- n
Floating: ---
Return the number of significant digits currently used by F, FE,
or FS as n.
QUERY CORE EXT
Include: obsolete.4th
Stack effects: ---
Make the user input device the input source. Receive input into
the terminal input buffer, replacing any previous contents. Make
the result, whose address is returned by TIB, the input buffer.
Set >IN to zero.
QUIT CORE
Compiles to: QUIT (0)
Forth: This word has quite another meaning in Forth.
Sets the program counter to the end of the program. Effectively
quits execution.
R> CORE
Pronunciation: r-from
Compiles to: R> (0)
Stack effects: --- n
Remove the top value from the return stack and leave it on the
stack.
R'@ TOOLBELT
Compiles to: R> (0)
I (0)
SWAP (0)
>R (0)
Stack effects: --- n
Copy the second return stack item to the stack.
R/O FILE
Pronunciation: r-o
Include: ansfile.4th
Stack effects: --- fam
fam is the implementation-defined value for selecting the read
only file access method.
R/W FILE
Pronunciation: r-w
Include: ansfile.4th
Stack effects: --- fam
fam is the implementation-defined value for selecting the read
write file access method.
R@ CORE
Pronunciation: r-fetch
Compiles to: I (0)
Stack effects: --- n
Copy the top of the return stack to the stack.
RANDOM COMUS
Compiles to: RANDOM (0)
Stack effects: --- n
Returns a pseudo-random number in n, between 0 and MAX-RAND. Seed
is automatically set.
READ-FILE FILE
Include: ansfile.4th
Stack effects: addr n1 h --- n2 f
Read n1 consecutive characters to addr from the current position
of the file identified by handle h. If n1 characters are read
without an exception, flag f is zero and n2 is equal to n1. If
the end of the file is reached before n1 characters are read,
flag f is zero and n2 is the number of characters actually read.
At the conclusion of the operation, FILE-POSITION returns the
next file position after the last character read.
READ-LINE FILE
Include: ansfile.4th
Stack effects: addr n1 h --- n2 f1 f2
Read the next line from the file specified by handle h into
memory at the address addr. At most n1 characters are read. Up to
two implementation-defined line terminating characters may be
read into memory at the end of the line, but are not included in
the count n2. The line buffer provided by addr should be at least
n1+2 characters long. If the operation succeeded, flag f1 is true
and flag f2 is zero. If a line terminator was received before n1
characters were read, then n2 is the number of characters, not
including the line terminator, actually read (0 <= n2 <= n1).
When n1 = n2 the line terminator has yet to be reached. If the
operation is initiated when the value returned by FILE-POSITION
is equal to the value returned by FILE-SIZE for the file
identified by handle h, flag f1 is false, flag f2 is zero, and n2
is zero. If flag f2 is non-zero, an exception occurred during the
operation and f2 is the implementation-defined I/O result code.
At the conclusion of the operation, FILE-POSITION returns the
next file position after the last character read.
RECURSE CORE
Compiles to: CALL (<last defined word>)
Compile a call to the current colon-definition inside the current
colon-definition. If this word is used outside a colon definition
it is undefined.
REFILL CORE EXT FILE EXT
Compiles to: REFILL (0)
Stack effects: --- f
Attempt to fill the input buffer from the input source, returning
a true flag if successful. When the input source is the user
input device, attempt to receive input into the terminal input
buffer. When the input source is a text file, attempt to read the
next line from the text-input file. If successful, make the
result the input buffer, set >IN to zero, and return true.
Receipt of a line containing no characters is considered
successful. If there is no input available from the current input
source, return false.
REPEAT CORE
Compiles to: BRANCH (<address of matching BEGIN>)
Syntax: BEGIN<space>..<space>WHILE<space>..<space>REPEAT
Forth: Within a BEGIN .. REPEAT construct, multiple WHILEs may be
used as well, but additional words are necessary to complete the
construct.
At runtime, REPEAT forces an unconditional branch back to just
after the corresponding BEGIN. Multiple WHILEs may be used.
REPOSITION-FILE FILE
Include: ansfile.4th
Stack effects: n h --- f
Reposition the file identified by handle h to n. Flag f is the
implementation-defined I/O result code. An ambiguous condition
exists if the file is positioned outside the file boundaries. At
the conclusion of the operation, FILE-POSITION returns the value
n.
REPRESENT FLOATING
Include: represnt.4th
Stack effects: addr n1 --- n2 f1 f2
Floating: r ---
At addr, place the character-string external representation of
the significand of the floating-point number r. The size of the
buffer identified by addr must be greater than or equal to
MAXDIGITS and will not be terminated by REPRESENT. Return the
decimal-base exponent as n2, the sign as f1 and ”valid result” as
f2. The character string shall consist of the n1 most significant
digits of the significand represented as a decimal fraction with
the implied decimal point to the left of the first digit, and the
first digit zero only if all digits are zero. The significand is
rounded to n1 digits following the ”round to nearest” rule; n2 is
adjusted, if necessary, to correspond to the rounded magnitude of
the significand. If f2 is true then r was in the
implementation-defined range of floating-point numbers. If f1 is
true then r is negative. An ambiguous condition exists if the
value of BASE is not decimal ten.
RESIZE MEMORY
Include: ansmem.4th
Stack effects: addr1 n --- addr2 f
Change the allocation of the contiguous data space starting at
the address addr1, previously allocated by ALLOCATE or RESIZE, to
n address units. n may be either larger or smaller than the
current size of the region. If the operation succeeds, addr2 is
the aligned starting address of n address units of allocated
memory and f is false. The values contained in the region at
addr1 are copied to addr2, up to the minimum size of either of
the two regions. If they are the same, the values contained in
the region are preserved to the minimum of n or the original
size. If addr2 is not the same as addr1, the region of memory at
addr1 is returned to the system according to the operation of
FREE. If the operation fails, addr2 equals addr1, the region of
memory at addr1 is unaffected, and f is true.
RESTORE-INPUT CORE EXT
Include: evaluate.4th
Stack effects: n1 n2 a1 n3 h n4 --- f
Attempt to restore the input source specification to the state
described by n1 through h. Flag is true if the input source
specification cannot be so restored.
ROLL CORE EXT
Include: anscore.4th
Stack effects: nu n1 .. n2 u --- n1 .. n2 nu
Remove u. Rotate u+1 items on the top of the stack.
ROT CORE
Pronunciation: rote
Compiles to: ROT (0)
Stack effects: n1 n2 n3 --- n2 n3 n1
Rotate the top three values on the stack, bringing the third to
the top.
RP@ 4TH
Compiles to: RP@ (0)
Stack effects: --- sp
Return the address sp of the stack position of the top of the
return stack as it was before RP@ was executed.
RSHIFT CORE
Pronunciation: r-shift
Compiles to: NEGATE (0)
SHIFT (0)
Stack effects: n1 n2 --- n3 )
Perform a logical right shift of n2 bit-places on n1, giving n2.
Put zeroes into the most significant bits vacated by the shift
(depends on implementation[footnote:
Some C compilers do an arithmetic shift, leaving the most
significant bit set.
]).
S" CORE FILE
Pronunciation: s-quote
Compiles to: S" (<address of string constant>)
Syntax: S"<space><string>"
Stack effects: --- addr n
Compiles string delimited by " in the String Segment with an
execution procedure to move the string to PAD. Leaves the address
and the length of the string on the stack.
S>D CORE
Pronunciation: s-to-d
Compiler: The instruction pointer is not incremented. In fact,
S>D is a dummy.
Stack effects: n --- n
Convert the number n to double number n with the same numerical
value.
S| 4TH
Compiles to: S" (<address of string constant>)
Syntax: S|<space><string>|
Stack effects: --- addr n
Compiles string delimited by | in the String Segment with an
execution procedure to move the string to PAD. Leaves the address
and the length of the string on the stack.
SAVE-BUFFERS BLOCK
Include: ansblock.4th
Stack effects: ---
Transfer the contents of each UPDATEd block buffer to mass
storage. Mark the buffer as unmodified.
SAVE-INPUT CORE EXT
Include: evaluate.4th
Stack effects: --- n1 n2 a1 n3 h n4
n1 through h describe the current state of the input source
specification for later use by RESTORE-INPUT.
SCONSTANT 4TH
Syntax: S"<space><string>"<space>SCONSTANT<space><name>
Compiler: The previously compiled string address is taken as an
argument for SCONSTANT. The instruction pointer is decremented,
actually deleting the string address.
A defining word used to create word <name>. When <name> is later
executed, it will push the current address and the length of the
string constant on the stack.
SCR BLOCK EXT
Pronunciation: s-c-r
Include: ansblock.4th
Stack effects: --- x
x is the address of a cell containing the block number of the
block most recently LISTed.
SEARCH STRING
Include: search.4th
Stack effects: addr1 n1 addr2 n2 --- addr3 n3 f
Search the string specified by addr1 n1 for the string specified
by addr2 n2 . If flag is true, a match was found at addr3 with n3
characters remaining. If flag is false there was no match and
addr3 is addr1 and n3 is n1.
SEEK 4TH
Compiles to: SEEK (0)
Stack effects: n h --- f
Reposition the file identified by handle h to n. If n is
positive, TELL returns the value n. If n is negative, the file is
repositioned relative to the end of the file. If the operation is
successful, FALSE is returned, otherwise TRUE.
SET-PRECISION FLOATING EXT
Include: ansfloat.4th
Stack effects: n ---
Floating: ---
Set the number of significant digits currently used by F, FE, or
FS to n.
SIGN CORE
Compiles to: SIGN (0)
Stack effects: n1 n2 --- n2
Stores an ASCII '-' sign just before the converted numeric output
string in PAD when n1 is negative. n1 is discarded, but n2 is
maintained. Must be used between <# and #>.
SM/REM CORE
Pronunciation: s-m-slash-rem
Include: mixed.4th
Stack effects: d n1 --- n2 n3
Divide d by n1, giving the symmetric quotient n3 and the
remainder n2. Input and output stack arguments are signed. An
ambiguous condition exists if n1 is zero or if the quotient lies
outside the range of a single-cell signed integer.
SOURCE CORE
Compiles to: LITERAL (<address of TIB variable>)
@ (0)
LITERAL (<address of TIB-size variable>)
@ (0)
Stack effects: --- addr n
addr is the address of, and n is the number of characters in, the
currently used TIB.
SOURCE! SOURCEFORGE
Compiles to: LITERAL (<address of TIB-size variable>)
! (0)
LITERAL (<address of TIB variable>)
! (0)
Stack effects: addr n ---
Forth: In Forth, >IN is set to zero. In 4tH this is left up to
the application programmer.
Make the string described by c-addr and u the current input
buffer. A program is allowed to refill the input buffer without
restoring the original input source; upon a refill, the system
shall accept the new portion of text to the current refill buffer
and make it the input buffer.
SOURCE-ID CORE EXT FILE
Pronunciation: source-i-d
Compiles to: ENVIRON (<address of CIN>)
Stack effects: --- n
Identifies the input source.
SP@ 4TH
Compiles to: SP@ (0)
Stack effects: --- sp
Return the address sp of the stack position of the top of the
stack as it was before SP@ was executed.
SPACE CORE
Compiles to: LITERAL (<ASCII value of space>)
EMIT (0)
Transmit an ASCII blank to the current output device.
SPACES CORE
Compiles to: SPACES (0)
Stack effects: n ---
Transmit n ASCII blanks to the current output device.
SPAN CORE EXT
Include: obsolete.4th
Stack effects: --- x
X is the address of a cell containing the count of characters
stored by the last execution of EXPECT (see EXPECT).
STACK-CELLS 4TH
Compiles to: LITERAL (<number of integers>)
Stack effects: --- n
Forth: Equivalent to:
: STACK-CELLS S" STACK-CELLS" ENVIRONMENT? DROP ;
Returns the number of integers that the Stack Area can contain.
Both stacks share the Stack Area.
STDIN 4TH
Compiles to: LITERAL (<address of stream>)
Stack effects: --- h
Leaves a filehandle on the stack associated with the standard
keyboard input device. This stream cannot be closed.
STDOUT 4TH
Compiles to: LITERAL (<address of stream>)
Stack effects: --- h
Leaves a filehandle on the stack associated with the standard
screen output device. This stream cannot be closed.
STRING 4TH
Syntax: <literal><space>STRING<space><name>
Compiler: The previously compiled literal is taken as an argument
for STRING. The instruction pointer is decremented, actually
deleting the literal.
Forth: This word is 4tH specific. Roughly equivalent to:
: STRING CREATE CHARS ALLOT ;
Allocate <literal> characters of contiguous data space beginning
at <name> in the Character Segment. The initial content of the
allocated space is undefined.
STRUCT 4TH
Compiles to: LITERAL (0)
Syntax:
STRUCT<space><literal><space>+FIELD<space><name><space>END-STRUCT<space><name>
Stack effects: --- n
Forth: Similar constructions are available in GForth. +FIELD is
part of the Forth 200x draft.
A constant, which initiates a STRUCT definition (see: +FIELD,
END-STRUCT).
SWAP CORE
Compiles to: SWAP (0)
Stack effects: n1 n2 --- n2 n1
Exchange the top two values on the stack.
SYNC 4TH
Compiles to: SYNC (0)
Stack effects: ---
Attempt to force any buffered information written to the device
referred to by the output channel to be written.
TABLE 4TH
Syntax: TABLE<space><name>
Forth: Available in some Forths.
Leaves <name> in the symboltable and replace further occurences
with LITERAL <xt>. <xt> represents the address in the Code
Segment where TABLE was compiled. An alias for CREATE.
TELL 4TH
Compiles to: TELL (0)
Stack effects: h --- n
n is the current file position for the file identified by handle
h.
TH COMUS
Compiles to: + (0)
Stack effects: x1 n --- x2
Forth: This word is not part of ANS-Forth or Forth-79, but can be
found in other Forths. It can be very handy when porting 4tH
programs. Just define TH as:
: TH CELLS + ;
When you're using a construction like:
VAR 2 TH
In both 4tH and Forth the third element will be referenced. The
use of TH to reference an element of a string in the Character
Segment is allowed in 4tH, but the resulting source cannot be
ported to Forth.
Used to reference an element in an array of integers. Will return
the address of the n-th element in array x1 as x2. An alias for
+.
THEN CORE
Syntax: IF<space>..<space>ELSE<space>..<space>THEN
At runtime THEN serves only as the destination of a forward
branch from IF or ELSE. It marks the conclusion of the
conditional structure.
THROW EXCEPTION
Compiles to: THROW (0)
Stack effects: n ---
Forth: The values of THROW are not conforming the ANS-Forth
standard.
If n is non-zero, pop the topmost exception frame from the return
stack, along with everything beyond that frame. Then adjust the
return- and datastacks so they are the same as the depths saved
in the exception frame, put n on top of the data stack, and
transfer control to a point just after the CATCH that pushed that
exception frame (see: CATCH).
TIB CORE EXT
Pronunciation: t-i-b
Compiles to: LITERAL (<address of Terminal Input Buffer>)
Stack effects: --- addr
Forth: In Forth this is a variable. However, it is unlikely
you'll ever find a program which assigns another value to it.
A constant which leaves the address of the Terminal Input Buffer
on the stack.
TIME 4TH
Compiles to: TIME (0)
Stack effects: --- n
Returns the number of seconds since January 1st, 1970.
TIME&DATE FACILITY
Pronunciation: time-and-date
Include: ansfacil.4th
Stack effects: --- n1 n2 n3 n4 n5 n6
Return the current time and date. n1 is the second {0...59}, n2
is the minute {0...59}, n3 is the hour {0...23}, n4 is the day
{1...31}, n5 is the month {1...12}, and n6 is the year (e.g.,
1991).
TO CORE EXT
Compiles to: TO (<variable address>)
Stack effects: n ---
Syntax: TO<space><name>
Store n in the value identified by name.
TRUE CORE EXT
Compiles to: LITERAL (<flag>)
Stack effects: --- f
Forth: In ANS-Forth TRUE is represented by a -1 value.
Returns a true flag on the stack.
TUCK CORE EXT
Compiles to: SWAP (0)
OVER (0)
Stack effects: n1 n2 --- n2 n1 n2 )
Copy the first (top) stack item below the second stack item.
TYPE CORE
Compiles to: TYPE (0)
Stack effects: addr n ---
Transmit n characters from addr to the selected output device.
U. CORE
Pronunciation: u-dot
Include: dbldot.4th
Stack effects: n ---
Display n in free field format as an unsigned number.
U.R CORE EXT
Pronunciation: u-dot-r
Include: dbldot.4th
Stack effects: n1 n2 ---
Display unsigned number n1 right aligned in a field n2 characters
wide. If the number of characters required to display n1 is
greater than n2, all digits are displayed with no leading spaces
in a field as wide as necessary.
U< CORE
Pronunciation: u-less-than
Include: ansdbl.4th
Stack effects: n1 n2 --- f
Flag f is true if and only if unsigned number n1 is less than
unsigned number n2.
U> CORE EXT
Pronunciation: u-greater-than
Include: ansdbl.4th
Stack effects: n1 n2 --- f
Flag f is true if and only if unsigned number n1 is greater than
unsigned number n2.
UM* CORE
Pronunciation: u-m-star
Include: mixed.4th
Stack effects: n1 n2 --- d
Multiply n1 by n2, giving the unsigned double-cell product d. All
values and arithmetic are unsigned.
UM/MOD CORE
Pronunciation: u-m-slash-mod
Include: mixed.4th
Stack effects: d n1 --- n2 n3
Divide d by n1, giving the quotient n3 and the remainder n2. All
values and arithmetic are unsigned. An ambiguous condition exists
if n1 is zero or if the quotient lies outside the range of a
single-cell unsigned integer.
UNLOOP CORE
Compiles to: R> (0)
R> (0)
DROP (0)
DROP (0)
Stack effects: ---
Discard the loop-control parameters for the current nesting
level. An UNLOOP is required for each nesting level before the
definition may be EXITed.
UNTIL CORE
Compiles to: 0BRANCH (<address of matching BEGIN>)
Stack effects: f ---
Syntax: BEGIN<space>..<space>WHILE<space>..<space>UNTIL
Forth: The optional WHILE word is not supported.
At runtime UNTIL controls the conditional branch back to the
corresponding BEGIN. If f is FALSE execution returns to just
after BEGIN; if f is TRUE execution continues ahead.
UPDATE BLOCK
Include: ansblock.4th
Stack effects: ---
Mark the current block buffer as modified. An ambiguous condition
exists if there is no current block buffer. UPDATE does not
immediately cause I/O.
USE 4TH
Compiles to: USE (0)
Stack effects: h ---
USE will associate the stream identified by filehandle h with the
appropriate input- or output-channel, depending on the file
access method used when opening the stream (see: OPEN). No
streams are closed.
VALUE CORE EXT
Compiles to: TO (<variable address>)
Stack effects: n ---
Syntax: <literal><space>VALUE<space><name>
Create a symboltable entry for the value name with an initial
value n. At runtime, n will be placed on the stack. In 4tH it is
an alias for TO.
VARIABLE CORE
Syntax: VARIABLE<space><name>
A defining word used to create variable <name>. When <name> is
later executed, it will push the address <var> on the stack, so
that a fetch or store may access this location.
VARS 4TH
Compiles to: VARS (0)
Stack effects: --- x
This word returns the begin of the variables area.
W/O FILE
Pronunciation: w-o
Include: ansfile.4th
Stack effects: --- fam
fam is the implementation-defined value for selecting the write
only file access method.
WHILE CORE
Compiles to: 0BRANCH (<address of matching REPEAT token>)
Stack effects: f ---
Syntax: BEGIN<space>..<space>WHILE<space>..<space>REPEAT
BEGIN<space>..<space>WHILE<space>..<space>UNTIL
Forth: Within a BEGIN .. REPEAT construct, multiple WHILEs may be
used as well, but additional words are necessary to complete the
construct.
At runtime, WHILE selects conditional execution based on number
n. If f is TRUE, WHILE continues execution of the code thru to
REPEAT, which branches back to BEGIN. If f is FALSE, execution
skips to just after REPEAT, exiting the structure. Multiple
WHILEs may be used.
WIDTH 4TH
Compiles to: LITERAL (<number of characters>)
Stack effects: --- n
A constant which leaves the maximum number of characters, allowed
in a <name> label.
WITHIN CORE EXT
Include: range.4th
Stack effects: n1 n2 n3 --- f
Perform a comparison of a test value n1 with a lower limit n2 and
an upper limit n3 , returning true if either (n2 < n3 and (n2 <=
n1 and n1 < n3)) or (n2 > n3 and (n2 <= n1 or n1 < n3)) is true,
returning false otherwise.
WORD CORE
Include: word.4th
Stack effects: c --- addr
Skip leading delimiters and parse characters delimited by c. Addr
is the address of the parsed word. If the parse area was empty or
contained no characters other than the delimiter, the resulting
string has a zero length.
WRITE-FILE FILE
Include: ansfile.4th
Stack effects: addr n h --- f
Write n characters from addr to the file identified by handle h
starting at its current position. Flag f is the
implementation-defined I/O result code. At the conclusion of the
operation, FILE-POSITION returns the next file position after the
last character written to the file, and FILE-SIZE returns a value
greater than or equal to the value returned by FILE-POSITION.
WRITE-LINE FILE
Include: ansfile.4th
Stack effects: addr n h --- f
Write n characters from addr followed by the
implementation-dependent line terminator to the file identified
by handle h starting at its current position. Flag f is the
implementation-defined I/O result code. At the conclusion of the
operation, FILE-POSITION returns the next file position after the
last character written to the file, and FILE-SIZE returns a value
greater than or equal to the value returned by FILE-POSITION.
XOR CORE
Pronunciation: x-or
Compiles to: XOR (0)
Stack effects: n1 n2 --- n3
Leave the bitwise logical XOR of n1 XOR n2 as n3.
['] CORE
Pronunciation: bracket-tick
Compiles to: LITERAL (<tok>)
Syntax: [']<space><name>
Stack effects: --- n | x | xt
Forth: See '.
Compile the value contents of the symboltable entry identified as
symbol <name> as a literal. An alias for '.
[*] 4TH
Compiles to: LITERAL (<product>)
Syntax: <literal><space><literal><space>[*]
Compiler: Two previously compiled literals are taken as
arguments, multiplied and their product recompiled. The
instruction pointer is decremented, actually deleting the
literals.
Stack effects: --- n
Forth: Equivalent to *.
[+] 4TH
Compiles to: LITERAL (<sum>)
Syntax: <literal><space><literal><space>[+]
Compiler: Two previously compiled literals are taken as
arguments, added and their sum recompiled. The instruction
pointer is decremented, actually deleting the literals.
Stack effects: --- n
Forth: Equivalent to +.
[/] 4TH
Compiles to: LITERAL (<quotient>)
Syntax: <literal><space><literal><space>[/]
Compiler: Two previously compiled literals are taken as
arguments, divided and their quotient recompiled. The instruction
pointer is decremented, actually deleting the literals.
Stack effects: --- n
Forth: Equivalent to /.
[=] 4TH
Compiles to: LITERAL (<flag>)
Syntax: <literal><space><literal><space>[=]
Compiler: Two previously compiled literals are taken as arguments
and a true flag is recompiled when they are equal. The
instruction pointer is decremented, actually deleting the
literals.
Stack effects: --- f
Forth: Equivalent to =.
[ABORT] 4TH
Forth: Roughly equivalent to:
[ ABORT ]
Compilation is aborted immediately.
[ASSERT] 4TH
Toggles assertions. Assertions are disabled by default (see:
ASSERT( and ) ).
[BINARY] 4TH
Forth: Roughly equivalent to:
[ 2 BASE ! ]
When encountered during compilation it will set the radix to
binary. All subsequent literals will be interpreted as binary
numbers. Runtime behaviour will be controlled by HEX, OCTAL and
DECIMAL.
[CHAR] CORE
Pronunciation: bracket-char
Compiles to: LITERAL (<ASCII-value of character>)
Syntax: [CHAR]<space><char>
Stack effects: --- c
Compiles the ASCII-value of <char> as a literal. At runtime the
value is thrown on the stack. An alias for CHAR.
[DECIMAL] 4TH
Forth: Roughly equivalent to:
[ DECIMAL ]
When encountered during compilation it will set the radix to
decimal. All subsequent literals will be interpreted as decimal
numbers. Runtime behaviour will be controlled by HEX, OCTAL and
DECIMAL.
[DEFINED] SEARCH EXT
Compiles to: LITERAL (<flag>)
Stack effects: --- f
Syntax: [DEFINED]<space><name>
If the name is defined, return TRUE, else return FALSE.
[HEX] 4TH
Forth: Roughly equivalent to:
[ HEX ]
When encountered during compilation it will set the radix to
hexadecimal. All subsequent literals will be interpreted as
hexadecimal numbers. Runtime behaviour will be controlled by HEX,
OCTAL and DECIMAL.
[IF] TOOLS EXT
Pronunciation: bracket-if
Syntax: <literal><space>[IF]<space><word>..<word><space>[THEN]
Compiler: The previously compiled literal is taken as an argument
for [IF]. The instruction pointer is decremented, actually
deleting the literal.
Forth: Forth pops a value from the stack. This is not possible in
4tH.
If the literal is nonzero, do nothing. Otherwise, skipping
leading spaces, parse and discard space-delimited words from the
source, including nested occurences of [IF] .. [THEN], until the
word [THEN] has been parsed and discarded.
[NEEDS 4TH
Syntax: [NEEDS<space><string>]
Compiler: The contents of the file are inserted at this position.
Open the file specified by <string> and include its contents at
the current position. When the end of the file is reached, close
the file and continue compilation. An error condition exists if
the named file can not be opened, if an I/O exception occurs
reading the file, or if an I/O exception occurs while closing the
file.
[NEGATE] 4TH
Compiles to: LITERAL (<negation>)
Syntax: <literal><space>[NEGATE]
Compiler: A previously compiled literal is taken as an argument,
negated and recompiled. The instruction pointer is decremented,
actually deleting the literal.
Stack effects: --- -n
Forth: Equivalent to NEGATE.
[NOT] 4TH
Compiles to: LITERAL (<flag>)
Syntax: <literal><space>[NOT]
Compiler: A previously compiled literal is taken as an argument
and a true flag is recompiled when it is equal to zero. The
instruction pointer is decremented, actually deleting the
literal.
Stack effects: --- f
Forth: Equivalent to 0=.
[OCTAL] 4TH
Forth: Roughly equivalent to:
[ 8 BASE ! ]
When encountered during compilation it will set the radix to
octal. All subsequent literals will be interpreted as octal
numbers. Runtime behaviour will be controlled by HEX, OCTAL and
DECIMAL.
[SIGN] 4TH
Compiles to: LITERAL (<sign>)
Syntax: <literal><space>[SIGN]
Compiler: A previously compiled literal is taken as an argument,
and its sign recompiled. The instruction pointer is decremented,
actually deleting the literal.
Stack effects: --- -1|0|1
[THEN] TOOLS EXT
Pronunciation: bracket-then
Does nothing. Acts as a marker for [IF] (see: [IF]).
[UNDEFINED] SEARCH EXT
Compiles to: LITERAL (<flag>)
Stack effects: --- f
Syntax: [UNDEFINED]<space><name>
If the name is defined, return FALSE, else return TRUE.
\ CORE EXT
Pronunciation: backslash
Syntax: \<space><string>
The remainder of the line is discarded. Used for comment.
Editor manual
13.1 Introduction
Forth organises its mass storage into "screens" of 1024
characters. Forth may have one screen in memory at a time for
storing text. The screens are numbered, starting with screen 0.
Each screen is organised as 16 lines with 64 characters. The
Forth screens are merely an arrangement of virtual memory and do
not correspond to the screen format of the target machine. Due to
this format, the use of the comment word '\' is not allowed. Use
'(' instead.
13.2 Selecting a screen and input of text
After you've started an editing session, you need to select a
screen to edit. The screen is given a number and selected by
using:
n CLEAR (clear screen n and select for editing).
To input new text to screen after CLEAR, the P (put) command is
used. Example:
0 P THIS IS HOW
1 P TO INPUT TEXT
2 P TO LINES 0, 1, 2 OF SELECTED SCREEN.
13.3 Line editing
During this description of the editor, reference is made to PAD.
This is a text buffer which may hold a line of text to be found
or deleted by a string editing command. Do not confuse this PAD
with 4tHs PAD. It is only called that way by convention.
13.4 Line editing commands
n D Delete line n but hold it in PAD. Line 15 becomes free as all
statements move up 1 line.
n E Erase line n with blanks.
n I Insert the text from PAD at line n, moving the old line n and
following lines down. Line 15 is lost.
n H Hold line n at PAD (used by system more often than by user).
n R Replace line n with the text in PAD.
n S Spread at line n. Line n and following lines move down 1
line. Line n becomes blank. Line 15 is lost.
n T Display line n and copy it to PAD.
n P text Put 'text' at line n, overwriting its previous contents.
13.5 Screen editing commands
n LIST List screen n and select it for editing: if screen n is
not the current screen, it will request to load from memory.
n CLEAR Clear screen n with blanks and select it for editing.
n INSERT Insert screen n. The current screen n and all screens
following it are moved down. The last screen is lost.
n m COPY Copy the contents of screen n to screen m. The original
contents of screen m are lost.
FLUSH Used at the end of an editing session to save the current
screen to memory.
UNDO Used to reload the current screen again, thus undoing all
changes since the last flush (triggered by CLEAR, FLUSH or LIST).
L List the current screen. The cursor line is relisted after the
screen listing to show the cursor position.
13.6 Cursor control and string editing
The screen of text being edited resides in a buffer area of
storage. The editing cursor is a variable holding an offset into
this buffer area. Commands are provided from the user to position
the cursor either directly or by searching for a string of buffer
text, and to insert or delete text at the cursor position.
13.7 Commands to position the cursor
n M Move the cursor by n characters and the cursor line. The
position of the cursor on its line is shown by a ^ (caret).
n W Wipe n characters to the left of the cursor.
TOP Position the cursor at the start of the screen.
13.8 String editing commands
B Used after F to back up the cursor by the length of the most
recent text.
C text Copy in text to the cursor line at the cursor position.
F text Search forward from the current cursor position until
string 'text' is found. The cursor is left at the end of the
string and the cursor line printed. If the string is not found an
error message is given and the cursor repositioned to the top of
the screen.
N Find the next occurrence of the string found by an F command
TlLL text Delete on the cursor line from the cursor till the end
of string text.
X text Find and delete the next occurrence of the string 'text'.
13.9 Saving and exiting
WRITE Saves the current contents of all screens to the
block-file. No flushing is done.
WQ Flushes the current screen and saves the current contents of
all screens to the block-file.
Q Quits the editor without saving.
EXPORT name Saves the current contents of all screens to the
text-file with the name 'name'. No flushing is done.
13.10 Calculator mode
The calculator mode is a simulation of what is known as the
"Forth calculator mode". You can use it to try out a host of 4tH
words in interactive mode. It also serves nicely as a
deskcalculator. You can freely mix editor and calculator
commands.
We tried to include as many 4tH words as possible, although we
had to modify some due to the limitations imposed by the system.
There are eight pre-defined user-variables called "A." though
"H.". You can use these variables like any other user-variable.
You cannot declare new variables or make any colon-definitions in
interactive mode. If you are unclear how to use the built-in
calculator please refer to the Primer and the Glossary. By
convention, calculator mode uses "OK" as the prompt. The
following table shows you which commands are available:[float Table:
+--------+-----------------++---------------+----------------+
| Editor | 4tH equivalent || Editor | 4tH equivalent |
+--------+-----------------++---------------+----------------+
+--------+-----------------++---------------+----------------+
| + | + || @ | @ |
+--------+-----------------++---------------+----------------+
| th | th || ? | ? |
+--------+-----------------++---------------+----------------+
| - | - || base! | base ! |
+--------+-----------------++---------------+----------------+
| * | * || decimal | decimal |
+--------+-----------------++---------------+----------------+
| / | / || octal | octal |
+--------+-----------------++---------------+----------------+
| q | quit || binary | 2 base ! |
+--------+-----------------++---------------+----------------+
| quit | quit || .( <string>) | .( <string>) |
+--------+-----------------++---------------+----------------+
| bye | quit || mod | mod |
+--------+-----------------++---------------+----------------+
| . | . || abs | abs |
+--------+-----------------++---------------+----------------+
| .r | .r || negate | negate |
+--------+-----------------++---------------+----------------+
| drop | drop || invert | invert |
+--------+-----------------++---------------+----------------+
| dup | dup || min | min |
+--------+-----------------++---------------+----------------+
| rot | rot || max | max |
+--------+-----------------++---------------+----------------+
| swap | swap || or | or |
+--------+-----------------++---------------+----------------+
| over | over || and | and |
+--------+-----------------++---------------+----------------+
| A. | variable a. a. || xor | xor |
+--------+-----------------++---------------+----------------+
| B. | variable b. b. || lshift | lshift |
+--------+-----------------++---------------+----------------+
| C. | variable c. c. || rshift | rshift |
+--------+-----------------++---------------+----------------+
| D. | variable d. d. || depth | depth |
+--------+-----------------++---------------+----------------+
| E. | variable e. e. || cells | cells |
+--------+-----------------++---------------+----------------+
| F. | variable f. f. || 1+ | 1+ |
+--------+-----------------++---------------+----------------+
| G. | variable g. g. || cell+ | cell+ |
+--------+-----------------++---------------+----------------+
| H. | variable h. h. || 1- | 1- |
+--------+-----------------++---------------+----------------+
| ! | ! || cell- | cell- |
+--------+-----------------++---------------+----------------+
| +! | +! || space | space |
+--------+-----------------++---------------+----------------+
| random | random || spaces | spaces |
+--------+-----------------++---------------+----------------+
| wait | wait || 2* | 2* |
+--------+-----------------++---------------+----------------+
| time | time || 2/ | 2/ |
+--------+-----------------++---------------+----------------+
| char | char || /mod | /mod |
+--------+-----------------++---------------+----------------+
| [char] | [char] || */ | */ |
+--------+-----------------++---------------+----------------+
| emit | emit || */mod | */mod |
+--------+-----------------++---------------+----------------+
| cr | cr || ( <string>) | ( <string>) |
+--------+-----------------++---------------+----------------+
[Senseless!!!
DC commands
]
]
Shell manual
14.1 Introduction
The 4tsh shell is a multitasking environment for 4tH. 4tH
features cooperative multitasking, which means programs have to
relinquish control to the shell using 'PAUSE', otherwise the
program will keep in control. The best place to add 'PAUSE' is
usually somewhere in a loop. 4tH comes with several example
multitasking programs for you to try out. 4tsh can be used as a
command line replacement for 4th, since you can enable
multitasking in the editor.
4tsh is scriptable. Scripts are stored in blockfiles, because
block I/O is completed within a single context. If you prefer to
use your own editor, you need to convert your script to a
blockfile. 4tH comes with a conversion program, called
txt2blk.4th. Every twelve lines are converted to a block, leaving
four additional lines for future modifications. Your lines should
be limited to 63 characters or less.
When a script it loaded, the first block is executed
automatically. By convention, the first block is block 0. When
the execution of a block has completed, the script stops. You can
call other blocks by using ”LOAD”. When the execution of a called
block has completed, the execution of the previous block will
resume at the point where execution was transferred to the called
block. It is recommended to use the first block as an application
load screen[footnote:
See ”Thinking Forth”, chapter 5.
], e.g.
( 4tsh application load screen)
1 load ( initialization)
2 load ( checking conditions)
3 load ( error handling)
An application load screen is simply a block that consecutively
loads all the blocks that make up your script. You can run an
arbitrary number of scripts at startup by issuing them on the
command line, e.g.
4tsh boot.scr startup.scr tasks.scr
When all scripts have finished execution, control will
automatically be transferred to the monitor.
14.2 Loading and saving
load” s” Loads HX file s from disk and leaves the task number on
the stack.
compile” s” Loads and compiles source file s and leaves the task
number on the stack.
n save” s” Saves task number n to HX file s.
n write” s” Generates C source file s from task number n.
n1 n2 see Decompiles task number n1 from opcode n2 on.
14.3 Task management
task Leaves the task number of the current monitor on the stack.
boot Starts a new monitor and runs the boot scripts.
pause Deactivates the monitor for one cycle.
n pauses Deactivates the monitor for n cycles.
n run Awakes and switches to task number n.
n awake Awakes task number n.
n sleep Deactivates task number n, but leaves it in memory.
n kill Deactivates task number n and removes it from memory.
tasks Lists all tasks.
halt Kills all tasks and shuts down 4tsh.
14.4 Scripting
script” s" Run script s. Does only work in interactive mode.
n load Load and interpret block n. Does not work in interactive
mode.
:: s Define label s.
goto s Goto label s. Works in interactive mode, but only if s
resides on the same line.
n if Execute the words between if and the corresponding then, but
only if n is non-zero. Works in interactive mode. If you fail to
provide a corresponding then you will be prompted to provide it
manually.
then Marker for if.
n not Leaves a non-zero value on the stack if n is zero,
otherwise zero.
n status Leaves the status of task number n on the stack.
done Constant holding the termination status returned by status.
running Constant holding the active status returned by status.
sleeping Constant holding the inactive status returned by status.
14.5 Stack, I/O and arithmetic
[float Table:
+-------+----------------++-------+----------------+
| 4tsh | 4tH equivalent || 4tsh | 4tH equivalent |
+-------+----------------++-------+----------------+
+-------+----------------++-------+----------------+
| + | + || . | . |
+-------+----------------++-------+----------------+
| - | - || dup | dup |
+-------+----------------++-------+----------------+
| * | * || rot | rot |
+-------+----------------++-------+----------------+
| / | / || over | over |
+-------+----------------++-------+----------------+
| cr | cr || swap | swap |
+-------+----------------++-------+----------------+
| .( | .( || drop | drop |
+-------+----------------++-------+----------------+
| ( | ( || = | = |
+-------+----------------++-------+----------------+
[Senseless!!!
4tsh commands
]
]
<cha:Preprocessor-manual>Preprocessor manual
15.1 Introduction
The preprocessor is a tool written in 4tH that has the following
features:
• It expands special macro definitions. These macro definitions
can contain anything you want;
• It strips whitespace and comments;
• It collects all include files and inserts them into the source;
• It expands CASE..ENDCASE, [CHAR], CHAR, ACTION-OF, 2VARIABLE,
2CONSTANT, FVARIABLE and FCONSTANT constructs.
• It simplifies the entry of double or floating point numbers.
This tool can help you to solve several problems:
• Your 4tH implementation has serious memory restrictions, so
certain sources cannot be compiled;
• You have to port Forth programs that use CASE..ENDCASE and
ACTION-OF statements, double or floating point numbers, complex
jump constructs or specialized data definitions like FVARIABLE,
FCONSTANT, 2VARIABLE or 2CONSTANT;
• You have to write Forth compatible programs without the use of
easy.4th;
• You can simplify development by writing more complex programs
more easily;
• It may serve in some situations as a debugging tool.
Note that the preprocessor is just a tool to automate certain
sourcecode manipulations. It doesn't compile anything nor does it
check the syntax. Succesfully processed source can be rejected by
the compiler for any number of reasons. Every valid 4tH program
is automatically valid 4tH preprocessor source. In addition the
preprocessor supports special preprocessor words, which are
listed below. Note that the preprocessor is not suited yet for
processing block files.
15.2 Macros
A macro starts with the word :MACRO and is delimited by a
semi-colon like a normal definition. A macro may span several
lines and may contain anything you want, including conditional
and loop statements. It may not contain any include file
directives or another macro.
This ANS Forth definition is impossible to define in 4tH:
: STEP 8 POSTPONE LITERAL POSTPONE +LOOP ;
In the preprocessor you can define it like this:
:MACRO STEP 8 +LOOP ;
Macros are expanded by the preprocessor which means that every
time it finds ”STEP” it will be replaced by ”8 +LOOP”.
15.3 Invocation
Since the preprocessor is written in 4tH, you can invoke it in
the usual way:
4th cxq pp4th.4th mysource.4pp mysource.4th
You may opt to create an executable or script suited for your
operating system. In that case, you can invoke it like any other
program:
pp4th mysource.4pp mysource.4th
You can also use make, see section [sec:Using-the-library]. Note
the preprocessor uses the DIR4TH environment variable to locate
include files.
15.4 Preprocessor commands
\ s The remainder of the line s is discarded. Used for comment.
( s) Discard comment s that is delimited by a right parenthesis.
A blank after the leading parenthesis is required.
char c Replaces character c as with its ASCII value.
[char] c Replaces character c as with its ASCII value.
d% s The string s represents a double number. D% is replaced with
an expression that leaves a double number on the stack.
f% s The string s represents a floating point number. F% is
replaced with an expression that leaves a floating point number
on the stack.
n 2constant s Store double number n and put it on the stack when
s is executed.
2variable s Reserve enough space to allocate a double number.
Leave its address on the stack when s is executed.
n fconstant s Store floating point number n and put it on the
stack when s is executed.
fvariable s Reserve enough space to allocate a floating point
number. Leave its address on the stack when s is executed.
include s The contents of file s, delimited by whitespace, are
inserted at this position.
[needs s] The contents of file s, delimited by a right bracket,
are inserted at this position.
:macro s ; Create a macro with the name s, delimited by a
semi-colon. When s is encountered it is replaced by the contents
of the macro. A macro may contain any sequence of valid 4tH
words, including conditional and loop statements, but no macros
or include files.
action-of s Leave the execution token on the stack that is
associated with name s.
case Mark the start of the CASE..OF..ENDOF..ENDCASE structure.
n of If n does not equal the 2OS, discard n and continue
execution at the location following the next ENDOF. Otherwise,
discard both values and continue execution in line.
endof Mark the end of the OF..ENDOF part of the CASE structure.
Jump to ENDCASE.
endcase Mark the end of the CASE..OF..ENDOF..ENDCASE structure.
15.5 Error messages
Usage: pp4th infile outfile Issue a preprocessor file and a
source file on the commandline
Macro space exhausted The combined size of all macro definitions
is too big
Macro not allowed here You may not use a macro within a macro
Unexpected macro The preprocessor found a :MACRO word within a
previously started macro definition
Too many macros There are too many macro definitions in the
currently processed file
Unexpected end of line A number, character, string, name or other
expression was expected on the same line.
Nesting too deep Too many CASE..ENDCASE constructs within other
CASE..ENDCASE constructs
Missing CASE The preprocessor found an ENDCASE word without a
matching CASE word
Unmatched CASE The preprocessor found a CASE word without a
matching ENDCASE word
Seek failed The preprocessor was unable to restore the file
status after processing an include file
Cannot open <file> Could not find a preprocessor file
Include file not allowed here You may not call an include file
within a macro
Include file nested too deep Too many include files included
within other include files
Cannot open include file Could not find an include file
Cannot read include file Could not read the include file
ANS Forth statement
Forth, like BASIC, has always suffered from a lack - or may be an
abundance of standards. Both languages had many dialects, which
were highly incompatible. However, although there was never a
generally accepted BASIC standard, a simple BASIC program can be
easily converted to almost any existing implementation of the
language.
The Forth community had a different approach to the problem. They
kept changing the core every few years, so even now it's very
hard to find a program which can run on any Forth with little
modification. Calling those very different versions a standard
didn't really help.
So when the ANSI-standard committee began its work they had a few
very though nuts to crack. In our view the ANS-Forth standard is
big step forward, but not perfect. It has not fully regained the
simplicity we found in the Forth-79 and still has some serious
flaws, although most are an inheritance from Forth-83.
We do feel the need for a real Forth standard, so we tried to
make 4tH as ANS-Forth compatible as possible without sacrificing
the ease of use that we had in mind when we designed it. About
95% of the CORE wordset is supported.
4tH was built according to the ANS-Forth standard, but with a
tiny Forth-79 flavor. Full compliance to the ANS-Forth standard
was never an objective. According to the ANS-Forth standard 4tH
cannot be an "ANS-Forth System", since the standard does not
cover this kind of implementation.
16.1 ANS-Forth Label
According to the ANS-Forth standard, section 5.2.2, this system
is capable of compiling:
ANS Forth Programs
Requiring:
• the Double number Extensions word set
• the Exception word set
• the Memory word set
Requiring selected words from:
• the Core Extensions word set
• the Block word set
• the Block Extensions word set
• the Double number word set
• the Facility Extensions word set
• the File-Access word set
• the File-Access Extensions word set
• the Floating-Point word set
• the Floating-Point Extensions word set
• the Programming-Tools word set
• the Programming-Tools Extensions word set
• the String word set.
End of label. Although the ANS-Forth standard (section 4.1)
requires documentation to be presented in a prescribed format,
4tH does not comply for the simple reason that due to its
architecture it is not considered to be a "ANS Forth System"
(sections 3.3, 3.4, 5.1).
Note that due to this special architecture some words are missing
from the CORE wordset or behave slightly different, so some "ANS
Forth Programs" with the requirements mentioned above may not
compile or compile only with modifications.
16.2 Unsupported CORE words
These words are not available in 4tH. Some CORE words are only
available in source (ANS-Forth, section 3). You can find them in
the 4tH glossary. The behaviour of some 4tH words may differ from
the ANS-Forth definition.
ALLOT
FIND
KEY
LITERAL
POSTPONE
STATE
[
]
16.3 Supported ANS Forth word sets
The words in the following sections are supported by 4tH;
external words are in italics. Please note that due to 4tHs
special architecture some words may behave slightly different, so
some "ANS Forth Programs" using these words may need
modifications in order to run properly. More words are available
in source and can be loaded when required.
16.3.1 Core Extensions word set
#TIB
.(
.R
0<>
0>
2>R
2R>
2R@
:NONAME
<>
?DO
AGAIN
ERASE
EXPECT
FALSE
HEX
NIP
PAD
PARSE
PICK
QUERY
REFILL
RESTORE-INPUT
ROLL
SAVE-INPUT
SOURCE-ID
TIB
TO
TRUE
TUCK
U.R
U>
VALUE
WITHIN
\
16.3.2 Block word set
BLK
BLOCK
BUFFER
FLUSH
LOAD
SAVE-BUFFERS
UPDATE
16.3.3 Block Extensions word set
EMPTY-BUFFERS
LIST
SCR
16.3.4 Double number word set
D+
D-
D.
D.R
D0<
D0=
D2*
D2/
D<
D=
D>S
DABS
DMAX
DMIN
DNEGATE
M+
M*/
16.3.5 Double number Extensions word set
2ROT
DU<
16.3.6 Facility Extensions word set
+FIELD
MS
TIME&DATE
16.3.7 File-Access word set
(
BIN
CLOSE-FILE
CREATE-FILE
FILE-POSITION
FILE-SIZE
OPEN-FILE
R/O
R/W
READ-FILE
READ-LINE
REPOSITION-FILE
S"
SOURCE-ID
W/O
WRITE-FILE
WRITE-LINE
16.3.8 File-Access Extensions word set
FILE-STATUS
FLUSH-FILE
REFILL
16.3.9 Floating-Point word set
>FLOAT
D>F
F!
F*
F+
F-
F0<
F0=
F<
F>D
F@
FALIGN
FALIGNED
FDEPTH
FDROP
FDUP
FLOAT+
FLOATS
FLOOR
FMAX
FMIN
FNEGATE
FOVER
FROT
FROUND
FSWAP
REPRESENT
16.3.10 Floating-Point Extensions word set
F**
F.
FABS
FACOS
FACOSH
FALOG
FASIN
FASINH
FATAN
FATAN2
FATANH
FCOS
FCOSH
FE.
FEXP
FLN
FLOG
FS.
FSIN
FSINCOS
FSINH
FSQRT
FTAN
FTANH
F~
PRECISION
SET-PRECISION
16.3.11 Programming-Tools word set
.S
?
DUMP
16.3.12 Programming-Tools Extensions word set
[IF]
[THEN]
16.3.13 String word set
-TRAILING
/STRING
BLANK
CMOVE
CMOVE>
COMPARE
SEARCH
Errors guide
17.1 How to use this manual
This manual contains all the error messages 4tH can possibly
issue. It is organized like this:
Message: This features the message from "errs_4th.c", the
error-code returned in ErrNo and the C-mnemonic.
Words: Words that can trigger this error.
Example: This features a 4tH one-liner that will trigger the
error.
Cause: This lists all possible causes of the error.
Hints: This will give you some directions on how to fix the
error.
17.2 Interpreter (exec_4th)
When exiting this function ErrLine will contain the address of
the word in the Code Segment where the error occured.
Message: No errors (#0 M4NOERRS)
Words: Not applicable
Example: Not applicable
Cause: A program was succesfully executed.
Hints: Make an error ;)
Message: Out of memory (#1 M4NOMEM)
Words: Not applicable
Example: Not applicable
Cause: There was not enough free memory to allocate the Character
Segment or the Integer Segment.
Hints:
1. Reduce the amount of memory your program allocates and
recompile.
2. Add more physical memory or increase swap space.
3. Recompile 4tH under another operating system (flat memory
space) or another memory model.
Message: Bad object (#2 M4BADOBJ)
Words: Not applicable
Example: Not applicable
Cause: An unknown token was encountered in the H-code.
Hints: Contact us, this should never happen.
Message: Stack overflow (#3 M4SOVFLW)
Words: Any word that pushes items on the Data Stack.
Example: STACK 1+ 0 DO I LOOP
Cause: The Data Stack collided with the Return Stack.
Hints:
1. Don't push too many elements on the Data Stack.
2. Merge colon-definitions. Reduce the number of nested
DO..LOOPs.
3. If you are using recursion, try if you can achieve the same
result with a loop.
4. Make sure that your stacks are still balanced when returning
from a colon-definition. Don't leave any unused data on the
Data Stack. Flow-control words can have unexpected stack
effects!
Message: Stack empty (#4 M4SEMPTY)
Words: Any word that pops items from the Data Stack.
Example: 0 SWAP
Cause: The Data Stack did not contain the required number of
items to complete the operation.
Hints:
1. Make sure that your stack is still balanced when returning
from a colon-definition.
2. Make sure that the required number of items are on the stack
when performing the operation.
3. If the problem occurs within an interpreter driven
application, make sure that you check the number of elements
are on the stack before allowing the operation.
Message: Return stack overflow (#5 M4ROVFLW)
Words: Any word that pushes items on the Return Stack; calling a
user defined word
Example: : DUMMY DUMMY ; DUMMY
Cause: The Return Stack collided with the Data Stack.
Hints:
1. Don't push too many elements on the Data Stack.
2. Merge colon-definitions. Reduce the number of nested
DO..LOOPs.
3. If you are using recursion, try if you can achieve the same
result with a loop.
4. Make sure that your stacks are still balanced when returning
from a colon-definition. Don't leave any unused data on the
Data Stack. Flow-control words can have unexpected stack
effects!
Message: Return stack empty (#6 M4REMPTY)
Words: Any word that pops items from the Return Stack; returning
from a user defined word
Example: R>
Cause: The Return Stack did not contain the required number of
items to complete the operation.
Hints:
1. Balance R> and >R inside your colon-definition. Flow-control
words can have unexpected stack effects!
2. Be careful when using R> and >R inside a DO..LOOP.
Message: Bad string (#7 M4BADSTR)
Words: ARGS OFFSET
Example: -1 ARGS
Cause: There was either no argument on the command line or no
binary string constant with this index.
Hints:
1. Use a valid index for ARGS.
2. Use a valid index for the offset.
Message: Bad variable (#8 M4BADVAR)
Words: ! @ +! ?
Example: 6 ARRAY NAME NAME -5 TH @
Cause: You tried to access a variable or array element, but its
address in the Integer Segment is invalid.
Hints:
1. Be sure that all stack-items are in the right order when
address calculations, fetches or stores are made.
2. Use a valid array index or address.
Message: Bad address (#9 M4BADADR)
Words: All string handling words
Example: 10 STRING BUFFER TIB CHAR- BUFFER /TIB CMOVE
Cause: You tried to access a character, but its address in the
Character Segment is invalid.
Hints:
1. Be sure that all stack-items are in the right order when
address calculations, fetches or stores are made.
2. Make sure that the number of elements is correct when you use
words like CMOVE, COUNT, FILL.
3. Terminate strings.
4. You exceeded the maximum length of PAD when you defined a
string constant using S".
5. You exceeded the maximum length of PAD when you fetched a
commandline argument using ARGS.
Message: Divide by zero (#10 M4DIVBY0)
Words: / MOD /MOD */ */MOD
Example: 1 0 / . CR
Cause: You tried to divide by zero.
Hints: Check the divisor before you use it.
Message: Bad token (#11 M4BADTOK)
Words: @C EXECUTE EXIT CATCH
Example: : DUMMY ; ' DUMMY 5 - DUP @C SWAP EXECUTE
Cause: You tried to jump to a token or access the argument of a
token, but its address in the Code Segment is invalid.
Hints:
1. Be sure that all stack-items are in the right order when
address calculations, fetches or jumps are made.
2. Make sure the address you're using is within the Code Segment.
3. Be sure that the name after ' is that of a colon-definition.
Message: Bad radix (#12 M4BADRDX)
Words: .R . # NUMBER
Example: 1 BASE ! 5 . CR
Cause: The 4tH variable BASE contained a value outside the 2 to
36 range during a conversion.
Hints: Take care that BASE stays within the 2 to 36 range.
Message: Bad pointer (#13 M4BADPTR)
Words: CATCH THROW PAUSE
Example: : ME R> R> R> DROP -5 >R >R >R 1 THROW ; ' ME CATCH
Cause: The stack pointer THROW or PAUSE tried to use was invalid.
Hints:
1. Be careful when you manipulate the Return Stack.
2. Contact us, this should never happen.
Message: I/O error (#14 M4IOERR)
Words: All words performing I/O
Example: OUTPUT FILE 5 . CR
Cause:
1. You tried to read from or write to an unopened file.
2. You tried to USE, SEEK or TELL an unused stream.
3. There was an I/O error when you tried to read from or write to
a file.
4. There was an error when you tried to close an open file with
CLOSE.
5. There was an error when 4tH tried to close a file after the
program terminated.
Hints:
1. Open a file before you try to read or write to it. Check the
value OPEN returns.
2. Make sure the values on the stack are correct when you perform
I/O.
3. Make sure the values on the stack are correct when addressing
streams.
4. Make sure that there is enough space left on the device you
try to write to. Make sure it functions correctly.
Message: Assertion failed (#15 M4ASSERT)
Words: )
Example: [ASSERT] ASSERT( FALSE )
Cause: The top of the stack was FALSE when ) executed.
Hints: Correct the condition ) acted upon.
Message: Unhandled exception (#16 M4THROW)
Words: THROW
Example: 1 THROW
Cause: A THROW was encountered without a previous call from
CATCH. The top of stack contained an error number outside the
range of system errors.
Hints: Make sure that a THROW can only be reached from a previous
CATCH.
Message: Bad stream (#17 M4BADDEV)
Words: USE SEEK TELL CLOSE
Example: -1 CLOSE
Cause:
1. The filehandle you tried to use was out of range.
2. You may not SEEK, TELL or CLOSE the streams STDIN and STDOUT.
3. You may not SEEK or TELL a pipe.
Hints:
1. Make sure you use a proper stream when using USE, SEEK, TELL
or CLOSE.
2. Check stack manipulations or use a variable or value.
17.3 Compiler (comp_4th)
When exiting this function ErrLine will contain the address in
the Code Segment where the next word would have been compiled if
the error hadn't occured. This is logical, since 4tH always
reports where the error occured. And all previous words have been
succesfully compiled.
Message: No errors (#0 M4NOERRS)
Words: Not applicable
Example: Not applicable
Cause: A source was succesfully compiled.
Hints: Make an error ;)
Message: Out of memory (#1 M4NOMEM)
Words: Not applicable
Example: Not applicable
Cause: There was not enough free memory to allocate the H-code
header, the Code Segment, the symbol-table or the control-stack.
Hints:
1. Compact your source by removing all comment and whitespace or
use the preprocessor[footnote:
See chapter [cha:Preprocessor-manual].
].
2. Add more physical memory or increase swap space.
3. Recompile 4tH under another compiler (flat memory space) or
another memory model.
Message: Bad object (#2 M4BADOBJ)
Words: All defining words
Example: CR CR 20 STRING
Cause:
1. A word could not be compiled due to lack of space in the Code
Segment.
2. A definition could not be compiled due to lack of space in the
symbol-table.
Hints:
1. Trying to make words private by using conditional compilation
may trigger this error. Remove the offending HIDE.
2. In certain circumstances, incomplete data declarations may
trigger this error. Complete the declaration.
3. Contact us, this should never happen with normal source-code.
Message: I/O error (#14 M4IOERR)
Words: [NEEDS INCLUDE
Example: [NEEDS nosuchfile.4th]
Cause:
1. The source file you tried to read doesn't exist.
2. There was an error reading the source file.
3. There was an error when 4tH tried to close the source file.
Hints:
1. Make sure the file you try to open exists and is in the path.
Change your working directory if necessary. Check the DIR4TH
environment variable.
2. Make sure the device functions correctly.
Message: Bad literal (#18 M4BADLIT)
Words: All words requiring a literal expression
Example: 10 5 * ARRAY NAME
Cause: The expression, which was compiled right before the word
which caused the error, did not compile to a literal.
Hints: Use a literal expression.
Message: Undefined name (#19 M4NONAME)
Words: <name> ' ['] RECURSE :THIS AKA HIDE
Example: ' HELLO ( "hello" is not defined)
Cause:
1. The name which caused the error is not present in the
symbol-table.
2. The name is not defined at all.
3. You tried to create a :THIS definition for an invalid
datatype.
4. It is not a valid number in the current radix.
5. RECURSE is used outside a colon definition.
6. The name you used is longer than WIDTH characters.
Hints:
1. Note that the words above only work with names defined inside
the program and not with built-in names.
2. Usually a typo; correct spelling.
3. Use a proper datatype when creating a :THIS definition.
4. Set the appropriate radix by using [BINARY], [OCTAL],
[DECIMAL] or [HEX].
5. Remove the offending RECURSE.
6. Use a shorter name.
Message: Nesting too deep (#20 M4NONEST)
Words: All flow control words and colon definitions
Example: 10 0 DO 10 0 DO <more flow-control structures> LOOP LOOP
Cause: The control-stack, that holds all references to addresses
of flow-control structures in the Code Segment, overflowed.
Hints: Make separate colon-definitions of the flow-control
structures that caused the error.
Message: No program (#21 M4NOPROG)
Words: All words that do not compile any tokens
Example: 10 ARRAY NAME ( Won't compile)
Cause:
1. The source didn't contain any compilable words.
2. The source was corrupt.
3. A runaway comment or conditional compilation clause.
4. In rare cases use of reserved words as names.
Hints:
1. Make a program that does something.
2. Make sure that the source actually contains 4tH source- code.
3. Terminate your comments and conditional compilations properly.
4. Don't use any reserved words as names.
Message: Incomplete declaration (#22 M4NODECL)
Words: All defining words and compiler directives
Example: 10 CONSTANT CONSTANT NAME
Cause:
1. Syntax errors; usually a missing name or a literal expression.
2. Incomplete compiler directives or expressions, like a leading
comma or a trailing CHAR.
3. An assertion, beginning with ASSERT(, is missing a right
parenthesis. Assertions are not enabled at that point.
4. An [IF] is not balanced by a [THEN].
Hints:
1. Use an appropriate expression or name.
2. Complete compiler directives and expressions.
3. Add a right parenthesis at the end of the expression.
4. Add a [THEN] for each [IF] statement.
Message: Unmatched conditional (#23 M4NOJUMP)
Words: All flow control words and colon definitions
Example: : WRONG IF DROP BEGIN FALSE LOOP ;
Cause: The flow-control word that caused the error didn't match
with the previous flow-control word (BEGIN after IF) or was
missing.
Hints: Use the appropriate flow-control word to terminate a
flow-control structure.
Message: Unterminated string (#24 M4NOSTR)
Words: ." \ ( .( ," S" ABORT" S| ,| [CHAR] CHAR @GOTO [NEEDS
INCLUDE [DEFINED] [UNDEFINED]
Example: ." Hello world
Cause:
1. A required delimiter is missing at the end of a string.
2. An internal error occured at the very end of the source.
Hints:
1. Add the required delimiter at the end of the string.
2. Contact us, this should never happen.
Message: Null string (#25 M4NULSTR)
Words: See error #24
Example: ." "
Cause:
1. The string between the word and its delimiter did not contain
any characters.
2. There was more than one whitespace character between a
[DEFINED], [UNDEFINED], [CHAR], CHAR or INCLUDE and the string
following it.
Hints:
1. Use a string that contains at least one single character.
2. Delete all superfluous whitespace characters between
[DEFINED], [UNDEFINED], [CHAR], CHAR or INCLUDE and the string
following it.
Message: Duplicate name (#26 M4DUPNAM)
Words: All defining words
Example: : TH CELLS + ;
Cause: The name you used for a definition is already in use by
4tH or your own program.
Hints: Use a different name.
Message: Compilation aborted (#27 M4CABORT)
Words: [ABORT]
Example: [ABORT]
Cause: An [ABORT] directive was encountered during compilation.
Hints: The original programmer must have had a reason to abort
compilation in this particular circumstance. See the program for
additional information.
17.4 Loader (load_4th)
Since the loader works with complete segments, the words don't
have to do much with fixing an error. Therefore, it reports that
nothing has been loaded (word 0) or everything has been loaded
(the last word).
Message: No errors (#0 M4NOERRS)
Words: Not applicable
Example: Not applicable
Cause: A program was succesfully loaded.
Hints: Keep up the good work. ;)
Message: Out of memory (#1 M4NOMEM)
Words: Not applicable
Example: Not applicable
Cause: There was not enough free memory to allocate the header,
the Code Segment or the String Segment.
Hints:
1. Reduce the amount of memory your program allocates and
recompile.
2. Add more physical memory or increase swap space.
3. Recompile 4tH under another compiler (flat memory space) or
another memory model.
Message: Bad object (#2 M4BADOBJ)
Words: Not applicable
Example: Not applicable
Cause:
1. You tried to load a file, that was not an HX-file.
2. You tried to load an HX-file from a previous version of 4tH.
3. You tried to load an HX-file for a different application.
4. You tried to load an inconsistent HX-file.
Hints:
1. Use a proper HX-file.
2. Recompile the source, using the current 4tH compiler.
3. If the source is compatible, you might recompile the source,
using your own 4tH compiler.
4. Recompile the source, using your own 4tH compiler.
Message: I/O error (#14 M4IOERR)
Words: Not applicable
Example: Not applicable
Cause:
1. The file could not be opened.
2. There was an I/O error while the file was read.
3. The file could not be closed.
Hints:
1. Use a valid filename.
2. Make sure the device functions correctly.
3. Make sure the device functions correctly.
17.5 Saver (save_4th)
Since the saver works with complete segments, the words don't
have to do much with fixing an error. Therefore, it reports that
nothing has been saved (word 0) or everything has been saved (the
last word).
Message: No errors (#0 M4NOERRS)
Words: Not applicable
Example: Not applicable
Cause: A program was succesfully saved.
Hints: Keep up the good work. ;)
Message: I/O error (#14 M4IOERR)
Words: Not applicable
Example: Not applicable
Cause:
1. The file could not be opened.
2. There was an I/O error while the file was written.
3. The file could not be closed.
Hints:
1. Make sure you got enough inodes or directory-entries left on
the device you want to write to. Use a valid filename.
2. Make sure that there is enough space left on the device you
try to write to. Make sure it functions correctly.
3. Make sure the device functions correctly.
Library dependencies<cha:Library-dependencies>
Here all mixed, double and floating point library files and their
dependencies are listed. If only one of the listed files is
needed resolve a dependency, they are printed in italics.
+---------------+----------+---------------+
| Library | Family | Depends on |
+---------------+----------+---------------+
+---------------+----------+---------------+
| ansdbl.4th | | 2rotover.4th |
+---------------+----------+---------------+
| ansfloat.4th | ANS | anscore.4th |
| | | mixed.4th |
+---------------+----------+---------------+
| ansfpio.4th | ANS | ansfloat.4th |
| | | dblsharp.4th |
+---------------+----------+---------------+
| asinacos.4th | ANS | fpconst.4th |
| | | taylor.4th |
+---------------+----------+---------------+
| dbldot.4th | | dblsharp.4th |
+---------------+----------+---------------+
| dblsharp.4th | | mixed.4th |
+---------------+----------+---------------+
| ellipint.4th | ANS | flnflog.4th |
| | | horner.4th |
+---------------+----------+---------------+
| falog.4th | ANS | fexp.4th |
| | | flnflog.4th |
+---------------+----------+---------------+
| fatan2.4th | ANS | asinacos.4th |
+---------------+----------+---------------+
| fatanh.4th | ANS | flnflog.4th |
+---------------+----------+---------------+
| fcbrt.4th | ANS | ansfloat.4th |
+---------------+----------+---------------+
| felip.4th | ANS | ansfloat.4th |
| | | fpconst.4th |
+---------------+----------+---------------+
| fequals.4th | ANS/Zen | ansfloat.4th |
| | | zenans.4th |
+---------------+----------+---------------+
| ferf.4th | ANS | fpconst.4th |
| | | taylor.4th |
+---------------+----------+---------------+
| fexp.4th | ANS | taylor.4th |
+---------------+----------+---------------+
| fexpint.4th | ANS | fexp.4th |
| | | flnflog.4th |
+---------------+----------+---------------+
| flnflog.4th | ANS | ansfloat.4th |
| | | fpconst.4th |
+---------------+----------+---------------+
| flogist.4th | ANS | fexp.4th |
+---------------+----------+---------------+
| fpconst.4th | ANS/Zen | ansfpio.4th |
| | | fpin.4th |
| | | zentoflt.4th |
+---------------+----------+---------------+
| fpin.4th | ANS | ansfloat.4th |
| | | tonumber.4th |
+---------------+----------+---------------+
| fpout.4th | ANS | range.4th |
| | | represnt.4th |
+---------------+----------+---------------+
| fsinfcos.4th | ANS | fpconst.4th |
| | | taylor.4th |
+---------------+----------+---------------+
| fsl-util.4th | ANS | ansfpio.4th |
| | | fpout.4th |
+---------------+----------+---------------+
| ftrunc.4th | ANS | ansfloat.4th |
+---------------+----------+---------------+
| gamma.4th | ANS | fequals.4th |
| | | fexp.4th |
| | | flnflog.4th |
| | | fsinfcos.4th |
| | | horner.4th |
+---------------+----------+---------------+
| gauss.4th | ANS | horner.4th |
+---------------+----------+---------------+
| horner.4th | ANS | fsl-util.4th |
+---------------+----------+---------------+
| mixed.4th | | ansdbl.4th |
| | | constant.4th |
+---------------+----------+---------------+
| pcylfun.4th | ANS | falog.4th |
| | | gamma.4th |
+---------------+----------+---------------+
| permcomb.4th | | mixed.4th |
+---------------+----------+---------------+
| represnt.4th | ANS | ansfloat.4th |
| | | dbldot.4th |
+---------------+----------+---------------+
| sinhcosh.4th | ANS | fexp.4th |
+---------------+----------+---------------+
| taylor.4th | ANS | ansfloat.4th |
+---------------+----------+---------------+
| todbl.4th | | digit.4th |
| | | mixed.4th |
+---------------+----------+---------------+
| tonumber.4th | | digit.4th |
+---------------+----------+---------------+
| zenans.4th | Zen | anscore.4th |
| | | zenfloat.4th |
+---------------+----------+---------------+
| zenatan2.4th | Zen | zenfasin.4th |
+---------------+----------+---------------+
| zenatanh.4th | Zen | zenfln.4th |
| | | zenfsqrt.4th |
+---------------+----------+---------------+
| zenfalog.4th | Zen | zenfexp.4th |
| | | zenfln.4th |
+---------------+----------+---------------+
| zenfasin.4th | Zen | zenfsqrt.4th |
| | | zentaylr.4th |
+---------------+----------+---------------+
| zenferf.4th | Zen | zentaylr.4th |
+---------------+----------+---------------+
| zenfexp.4th | Zen | zentaylr.4th |
| | | zentrunc.4th |
+---------------+----------+---------------+
| zenfln.4th | Zen | anscore.4th |
| | | zenfloat.4th |
+---------------+----------+---------------+
| zenfloat.4th | Zen | mixed.4th |
+---------------+----------+---------------+
| zenfloor.4th | Zen | zenfloat.4th |
+---------------+----------+---------------+
| zenfmin.4th | Zen | zenfloat.4th |
+---------------+----------+---------------+
| zenfsin.4th | Zen | zenfloor.4th |
| | | zentaylr.4th |
+---------------+----------+---------------+
| zenfsinh.4th | Zen | zenfexp.4th |
+---------------+----------+---------------+
| zenfsqrt.4th | Zen | zenfloat.4th |
+---------------+----------+---------------+
| zenround.4th | Zen | zentrunc.4th |
+---------------+----------+---------------+
| zentaylr.4th | Zen | 2rotover.4th |
| | | zenfloat.4th |
+---------------+----------+---------------+
| zentoflt.4th | Zen | anscore.4th |
| | | tonumber.4th |
| | | zenfloat.4th |
+---------------+----------+---------------+
| zentrunc.4th | Zen | zenfloor.4th |
+---------------+----------+---------------+
[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/deps.dot.pdf>
[Senseless!!!
Double, mixed and floating point word dependencies
]
]
• The grey ovals depict library file dependencies that are not
automatically resolved.
• Dotted lines indicate that only one of the pictured
dependencies needs to be resolved.
Porting guide
19.1 Introduction
4tH is ANS-Forth compatible. That means that 4tH and ANS-Forth
share a common wordset, so you can write programs that run on
both systems. This guide will show you how you can write portable
programs or convert eligible ANS-Forth programs to 4tH with as
little effort as possible.
19.2 General guidelines
We have already stated that 4tH and ANS-Forth have much in
common, but it is unlikely that you can write a non-trivial
program that runs unmodified on both platforms without resorting
to conditional compilation, which allows you to "hide"
implementation specific code. The word '4TH#' not only holds
4tH's version number, but is also an effective way to
differentiate between 4tH and other compilers:
[DEFINED] 4TH# [IF]
variable span
: expect 1- accept span ! ;
[THEN]
Of course, the opposite works too:
[UNDEFINED] 4TH# [IF]
s" easy.4th" included
[THEN]
If you have an interactive program you might want to disable the
4tH autostart:
[DEFINED] 4TH# [IF] start-program [THEN]
Otherwise 'REFILL' will try to get its input from the file
instead of the keyboard.
19.3 Differences between 4tH and ANS-Forth
Like any software, 4tH is a compromise. We have to address the
requirements of both newbies and power users, which means we have
to make choices[footnote:
You may or may not agree with the choices we made, but you can
rest assured we have given them considerable thought.
] concerning ANS-Forth compliancy. There are several reasons why
4tH is not completely ANS-Forth compliant:
1. 4tH uses a different architecture which makes it impossible to
be ANS-Forth compliant, so some constructions are simply not
feasible;
2. Some constructions in ANS-Forth are considered to be
illogical, unelegant, bloated, not intuitive, error prone,
inefficient or otherwise not acceptable;
3. 4tH maintains a close relationship with C, so it is more
logical and efficient to use C-conventions instead of ANS-Forth
conventions.
Where possible, we try to minimize the consequences for our users
by hiding the differences behind abstractions or other
transparent solutions. But sometimes, we simply can't. In this
section we will show you which differences there are between 4tH
and ANS-Forth and how you can either avoid or resolve them.
19.3.1 Strings
In 4tH, strings are stored in an ASCIIZ format. ANS-Forth uses
counted strings. In 4tH there is no such thing as a countbyte,
since it uses a terminator. If you limit the use of 'COUNT' only
to string variables and constants, and exclusively use 'PLACE' or
'+PLACE' you should be fine, since the address/count convention
of ANS-Forth is fully supported. Should you resort to low level
operations which require a terminator, you might have to define
an equivalent word in ANS-Forth to make your program portable.
'S"' does have interpretation semantics, but the string stored at
the address 'S"' returns might have a very short lifespan,
depending on your ANS-Forth compiler. 4tH has a transparent,
circular buffer that protects the string from overwriting, but
when you port your program you might not be that lucky. Note that
ANS-Forth does not require compilers to provide these facilities.
19.3.2 Double numbers
4tH uses only signed 32 bit cells, but some words in ANS-Forth,
like '<#', '#>', 'FILE-SIZE', 'FILE-POSITION' and
'REPOSITION-FILE' require the use of double numbers. You can
easily fix this by adding 'S>D', which converts a number to a
double number. Its counterpart, 'D>S', is available too. In 4tH
these words have no effect.
19.3.3 Booleans
Another nice topic for a flame war is the value of truth. In
ANS-Forth the 'TRUE' has the value "-1", which means all bits are
set. Which is very clever. You can 'XOR', 'OR', 'AND' and
'INVERT' it with any other value and it will behave as logical
value. But "the all bits set" flag has its drawbacks too. Let's
see what the ANS-Forth standard says about flags:
"A FALSE flag is a single-cell datum with all bits unset, and a
TRUE flag is a single-cell datum with all bits set. While Forth
words which test flags accept any non-null bit pattern as true,
there exists the concept of the well-formed flag. If an operation
whose result is to be used as a flag may produce any bit-mask
other than TRUE or FALSE, the recommended discipline is to
convert the result to a well-formed flag by means of the Forth
word 0<> so that the result of any subsequent logical operations
on the flag will be predictable. In addition to the words which
move, fetch and store single-cell items, the following words are
valid for operations on one or more flag data residing on the
data stack: AND OR XOR INVERT"
We highly recommend the discipline of converting a non-zero value
to a well-formed flag. But we don't understand why 'INVERT' is a
valid way to manipulate a flag. We'll try to explain you why.
Forth traditionally has no specific logical operators. Instead,
binary operators were used. This put 'INVERT' (or 'NOT' as it was
called in Forth-79) in a difficult position. 'INVERT'ing any
non-zero value will result in a non-zero value, except when all
bits are set.
That is why '0=' was introduced, a full-fledged logical operator.
So why use 'INVERT' when you want to perform a logical operation?
Another quote:
"Since a "char" can store small positive numbers and since the
character data type is a sub-range of the unsigned integer data
type, C! must store the n least-significant bits of a cell (8 <=
n <= bits/cell). Given the enumeration of allowed number
representations and their known encodings, "TRUE xx C! xx C@"
must leave a stack item with some number of bits set, which will
thus will be accepted as non-zero by IF."
This is another problem of using "all bits set" as a true flag:
you store a well formed flag in an address unit that should
easily be able to handle it and you'll never get it back. A flag
is a boolean and can have two values: either true or false. The
smallest unit that can hold a boolean is a bit. ANS-Forth
programmers are denied that privilege.
But why are some Forth programmers so keen on their “all bits set”
flag? Well, you can do neat things with it.
: >CHAR DUP 9 > 7 AND + ASCII 0 + ;
This will convert a digit to its ASCII representation. True, it
is a clever piece of programming, but in our opinion it is bad
style. Why? Because you are using a flag as a bitmask, which is a
completely different datatype. Although there is no such thing as
“data typing” in Forth, this way of programming makes it
difficult to understand and maintain a program, which the
ANS-Forth standard acknowledges:
"The discipline of circumscribing meaning which a program may
assign to various combinations of bit patterns is sometimes
called data typing. Many computer languages impose explicit data
typing and have compilers that prevent ill-defined operations.
Forth rarely explicitly imposes data-type restrictions. Still,
data types implicitly do exist, and discipline is required,
particularly if portability of programs is a goal. In Forth, it
is incumbent upon the programmer (rather than the compiler) to
determine that data are accurately typed."
That is why 4tH uses "1" as a true flag. Usually, it won't make
much difference. Except when you use 'INVERT' to invert a flag or
intend to make obfuscated programs. If you use '0=' instead, you
won't run in any trouble, not even when you port your program to
ANS-Forth. Clarity may introduce a little overhead, but in this
age of multi-gigaherz machines, who is counting? E.g. you could
program “>CHAR” like this:
\ convert a flag to a
bit mask
: >MASK 0 SWAP IF INVERT THEN ; ( f -- mask)
\ convert a digit to
ASCII
: >CHAR DUP 9 > >MASK 7 AND + ASCII 0 + ; ( n -- c)
If you still want to change the true flag, you can by simply
changing a #define in "cmds_4th.h":
#define F_T ~(0L)
But we doubt whether it will be a great benefit to your
programming style.
19.3.4 CREATE..DOES><sub:CREATE..DOES>>
In both 4tH and ANS-Forth it is possible to change the runtime
behaviour of variables. E.g. in ANS-Forth, 'CONSTANT' is usually
defined as:
: CONSTANT CREATE , DOES> @ ;
10 CONSTANT MY_CONST
MY_CONST . CR
Of course there is a predefined word in 4tH that does this, but
if you wanted to mimic this behaviour you would have to define it
like this:
CREATE MY_CONST 10 , \ CREATE part
:THIS MY_CONST DOES> @C ; \ DOES> part
MY_CONST . CR \ Works the same way
The point is that the ANS-Forth "CREATE DOES>" construct cannot
be ported to 4tH, although all words seem to be supported. A rule
of the thumb is that defining words cannot be used to define new
defining words, like in ANS-Forth. Most errors will be trapped by
4tH's compiler, though.
Just remember that a ':THIS' definition can easily be ported to
ANS-Forth. If you want to write a portable program, ':THIS' is
the way to go.
19.3.5 HERE
Be careful with 'HERE'. 'HERE' looks and acts a lot like the
ANS-Forth 'HERE', but since the architecture is different it
serves quite another function. When 'HERE' is used for address
arithmetic with definitions or arrays of constants, it works
right out of the box. If not, it usually doesn't.
19.3.6 Interpretation and compilation mode
There are several words, which act differently in interpretation
and compilation mode. In Forth-79, some of them were
"state-smart", which means they adjusted their behaviour
depending on the mode the system was in. In Forth-83 and
subsequently ANS-Forth, they became "dumb" words and counterparts
were designed for each mode. Other words lacked interpretation
semantics all together.
4tH has got neither a true interpretation mode nor a state. But
if you want to port 4tH code to ANS-Forth, this has to be dealt
with. In 4tH this porting issue is resolved by several aliases.
Some words have an alias since they do not have interpretation
semantics in the ANS-Forth standard, but are often used outside
colon-definitons in 4tH. This will enable you to make a word that
mimics these interpretation semantics.
This table lists all "dumb" words with their counterparts.
"Interpretation" means it has to be used outside
colon-definitions. "Compilation" means it has to be used inside
colon-definitions.
[float Table:
+-----------------+-------------+
| Interpretation | Compilation |
+-----------------+-------------+
| ' | ['] |
+-----------------+-------------+
| .( | ." |
+-----------------+-------------+
| CHAR | [CHAR] |
+-----------------+-------------+
[Senseless!!!
Dumb words
]
]
Finally, in ANS-Forth all flowcontrol words (like IF, THEN,
BEGIN, WHILE, DO, LOOP) may only be used inside
colon-definitions.
19.3.7 BEGIN..WHILE..REPEAT
4tH allows you to use multiple WHILE's in a BEGIN..WHILE..REPEAT
construct. ANS-Forth allows that too, but requires an extra
'THEN' for each additional 'WHILE'. In short, this is the 4tH
version:
0 begin dup 10 < while 1+ dup 5 mod while 1+ repeat
And this is the ANS-Forth version:
0 begin dup 10 < while 1+ dup 5 mod while 1+ repeat then
To make this work, we have to resort to conditional compilation:
0 begin dup 10 < while 1+ dup 5 mod while 1+ repeat
[undefined] 4th# [if] then [then]
It's not beautiful, but it works. The same applies when 'UNTIL'
is used instead of 'REPEAT'. BEGIN..WHILE..AGAIN constructs are
not supported by ANS-Forth, so be careful when considering
'AGAIN' too much of an alias of 'REPEAT'.
19.3.8 CASE..OF..ENDOF..ENDCASE
Many users wonder why this ”essential” construct is missing in
4tH. The explanation is pretty simple: it is horrible! First,
this construct puts a heavy burden on the controlstack, which is
pretty shallow in 4tH. Why? Because it is essentially a nested
IF..ELSE..THEN construct as you will see later on. Second, these
kinds of problems are better handled by a lookup table. 4tH has
excellent support for lookup tables, much better than other
Forths.
Of course, sometimes you just want to convert a program and don't
feel like redesigning it. Fortunately, converting
CASE..OF..ENDOF..ENDCASE constructs is pretty straightforward. It
only requires four simple steps. Let's examine this simple
example:
: .WEEKDAY ( daynum --- )
CASE
1 OF ." Sunday" ENDOF
2 OF ." Monday" ENDOF
3 OF ." Tuesday" ENDOF
4 OF ." Wednesday" ENDOF
5 OF ." Thursday" ENDOF
6 OF ." Friday" ENDOF
7 OF ." Saturday" ENDOF
ENDCASE ;
Step 1: Replace ENDOF with ELSE and eliminate CASE
: .WEEKDAY ( daynum --- )
1 OF ." Sunday" ELSE
2 OF ." Monday" ELSE
3 OF ." Tuesday" ELSE
4 OF ." Wednesday" ELSE
5 OF ." Thursday" ELSE
6 OF ." Friday" ELSE
7 OF ." Saturday" ELSE
ENDCASE ;
Step 2: Replace OF with OVER = IF DROP
: .WEEKDAY ( daynum --- )
1 OVER = IF DROP ." Sunday" ELSE
2 OVER = IF DROP ." Monday" ELSE
3 OVER = IF DROP ." Tuesday" ELSE
4 OVER = IF DROP ." Wednesday" ELSE
5 OVER = IF DROP ." Thursday" ELSE
6 OVER = IF DROP ." Friday" ELSE
7 OVER = IF DROP ." Saturday" ELSE
ENDCASE ;
Step 3: Replace ENDCASE with DROP
: .WEEKDAY ( daynum --- )
1 OVER = IF DROP ." Sunday" ELSE
2 OVER = IF DROP ." Monday" ELSE
3 OVER = IF DROP ." Tuesday" ELSE
4 OVER = IF DROP ." Wednesday" ELSE
5 OVER = IF DROP ." Thursday" ELSE
6 OVER = IF DROP ." Friday" ELSE
7 OVER = IF DROP ." Saturday" ELSE
DROP ;
Step 4: Add as many THENs as there are ELSEs
: .WEEKDAY ( daynum --- )
1 OVER = IF DROP ." Sunday" ELSE
2 OVER = IF DROP ." Monday" ELSE
3 OVER = IF DROP ." Tuesday" ELSE
4 OVER = IF DROP ." Wednesday" ELSE
5 OVER = IF DROP ." Thursday" ELSE
6 OVER = IF DROP ." Friday" ELSE
7 OVER = IF DROP ." Saturday" ELSE
DROP THEN THEN THEN THEN THEN THEN THEN ;
Done! Ugly? Yes, but that's what a CASE..OF..ENDOF..ENDCASE
construct internally looks like! Wrapping it in some slick
keywords doesn't change that. If 4tH issues an error message
saying that you're nesting too deep, you understand why. If you
happen to convert a program that contains many
CASE..OF..ENDOF..ENDCASE constructs it may be a bit tiresome, but
remember you only have to do it once. If you don't feel like
editing your source manually, you can use the script case24th.th
to do the job for you.
19.3.9 DO..LOOP
It is well-known in the Forth community that DO..LOOP is flawed.
There have been several attempts to correct this, but they never
got it right. On many occasions it even got worse. But why is
DO..LOOP flawed?
'DO' puts the limit and the index on the Return Stack, but it
doesn't decide whether the loop is actually entered. So, every
loop is executed at least once. After each iteration 'LOOP'
decides whether it iterates once more.
In our opinion it would have been better when 'DO' had made that
decision (like any other language), but we can still live with
that. The real trouble came with DO..+LOOP.
'+LOOP' is a logical extension. Every single language allows you
to change the step. But contrary to what one might expect,
'+LOOP' doesn't terminate when the loop limit is reached or
exceeded, but when the loop index crosses "the boundary between
the loop limit minus one and the loop limit".
What does that mean? Well, consider these three loops and try to
predict what will be printed. Note: every loop is executed at
least once:
( 1) 5 0 do i . 1 +loop cr
( 2) 5 0 do i . -1 +loop cr
( 3) -5 0 do i . -1 +loop cr
You would probably expect to see:
0 1 2 3 4
0
0 -1 -2 -3 -4
And that is what you get when you use 4tH. But this is not what
you will get with ANS-Forth:
0 1 2 3 4
0 -1 -2 -3 .. 6 5
0 -1 -2 -3 -4 -5
The behaviour of the second loop is caused, because '+LOOP'
doesn't take into account that it is counting down. So it
iterates until the loop index reaches the loop limit by
wrap-around arithmetic.
The behaviour of the third loop is caused by the ANS-Forth
definition: the loop index must "cross the boundary between the
loop limit minus one and the loop limit". In this case, the
boundary is between -5 and -6.
DO..+LOOP didn't behave like this since the beginning of Forth:
it was introduced in Forth-83. We preserved the Forth-79
definition as closely as possible, because it is much more
intuitive.
Some claim that ?DO..+LOOP will save it. As a matter of fact, it
does. But only when the loop index and the loop limit are the
same:
0 0 ?do i . loop
In that case the loop won't be entered. But it still won't save
us for loops like this:
5 0 do i . -1 +loop cr
The authors of gForth claim that a whole host of new DO..LOOP
words are the solution. We don't think so:
100 -100 do i . i 1+ 2/ negate +loop cr
The bottomline is: you can't let two words make the same
decision. 4tHs '+LOOP' checks which direction it is going (up or
down) and evaluates the loop arguments accordingly. We feel it is
the best we can do for you.
Is there no way we can circumvent these problems? Yes, there is.
It may not be too elegant or even fast, but it solves the
problem. We just emulate C's for():
12 >r begin \ set up loop index
r@ 10 < \ check loop limit
while
r@ . \ access loop index
r> 2+ >r \ increment loop index
repeat \ next iteration
r> drop cr \ drop loop index
Which is "equivalent" to:
10 12 ?do i . 2 +loop cr
Except that it works as expected. And as an extra bonus it is
portable to ANS-Forth. Are these differences between the
ANS-Forth and 4tH implementation of DO..LOOP really that
important? Not in practice. Nobody really wants a loop that
depends on wrap-around arithmetic, and you'll hardly ever see a
'+LOOP' with a negative subscript. Everybody wants their programs
to be understandable and maintainable, so the DO..LOOPs you'll
encounter will usually be well-behaved.
19.3.10 I/O
It is trivial to define the ANS-Forth FILE wordset in 4tH, but
almost impossible to do the opposite. So if you want to make a
portable program use the ANS-Forth FILE wordset by including the
'ansfile.4th' library file. The reason why 4tH uses a different
I/O subsystem is twofold:
1. 4tH's I/O subsystem is far more powerful and elegant. Instead
of defining a whole new wordset, 4tH reuses most of the
available I/O words, like 'TYPE', 'EMIT', 'ACCEPT' and 'REFILL'[footnote:
As a matter of fact, a new set of words have been proposed that
allow redirection of these words. 4tH already has that
functionality.
], which is very Forth-like.
2. 4tH's I/O was initially quite primitive and this was the only
way to extend the system without breaking too much code.
This example is taken from gForth, but runs identically on both
4tH and gForth:
[defined] 4th# [if] \ if this is 4tH, include
include lib/ansfile.4th \ ANS Forth FILE wordset
include lib/compare.4th \ and the word COMPARE
[then]
[undefined] 4th# [if] \ if this is not 4tH,
s" lib/easy.4th" included \ include 4tH
compatibility
[then]
0 Value fd-in \ input file handle
0 Value fd-out \ output file handle
: open-input ( addr u -- ) r/o open-file throw to fd-in ;
: open-output ( addr u -- ) w/o create-file throw to fd-out ;
s" foo.in" open-input \ open input file
s" foo.out" open-output \ open output file
: show 2dup type cr ; \ show the line
256 Constant max-line \ size of basic buffer
max-line 2 [+] string line-buffer \ extend by two bytes:
\ ANS Forth requirement!
: scan-file ( addr u -- )
begin \ read a line
line-buffer max-line fd-in read-line throw
while \ is it identical?
>r 2dup line-buffer r> show compare dup
while \ if so, exit loop
drop \ clean up
repeat \ ANS requires an extra
[undefined] 4th# [if] then [then] \ then after each WHILE
drop 2drop
;
\ now scan the file
s" The text I search is here" scan-file
fd-in close-file throw \ close input file
fd-out close-file throw \ close output file
Since section 11.3.12 of the ANS-Forth standard clearly states
that an I/O exception shall not cause a 'THROW', we have to
mention that 'FILE-SIZE', 'FILE-STATUS' and 'REPOSITION-FILE' are
not entirely ANS-Forth compliant.
19.4 Easy 4tH
4tH programs won't run on ANS-Forth all by itself. You'll usually
need several definitions to make them work. In collaboration with
Wil Baden we have developed an interface between ANS-Forth and
4tH. It consists of two files, ”easy.4th” and ”ezneeds.4th”.
These library files enable you to run most 4tH programs under
ANS-Forth. In order to succesfully compile and run a 4tH program
under ANS-Forth it must have been written with ANS-Forth in mind.
The rest is simple: just add a couple of lines at the beginning
of your 4tH program:
[UNDEFINED] 4TH# [IF]
s" easy4th.4th" included
[THEN]
That's all! Most of the 4tH words are now known to your very own
ANS-Forth compiler. If your compiler already supports 'INCLUDE',
you might be tempted to use:
[UNDEFINED] 4TH# [IF]
include easy4th.4th
[THEN]
This will actually work but since 4tH recognizes and acts on the
'INCLUDE' directive, it will load the interface. A slight memory
and CPU penalty is the result.
19.4.1 Disabling DOES>
Easy 4tH will effectively disable 'DOES>', since 'DOES>' is
nothing more than some syntactic sugar in 4tH. If you want to use
the standard Forth 'DOES>', you have to define '(_KEEP_DOES_)'
somewhere, e.g.
[UNDEFINED] 4TH# [IF]
0 constant (_keep_does_)
s" easy4th.4th" included
[THEN]
Easy 4tH will now leave 'DOES>' alone. Note that you have to
refrain from using 'DOES>' in your ':THIS' definitions (see
section [sub:CREATE..DOES>]).
19.4.2 Enabling the String Space
Optionally, you can define a 'CONSTANT' before including Easy
4tH, which enables support for arrays of string constants:
<size> constant /STRING-SPACE
The parameter "SIZE" represents the size of the String Segment.
When you decompile a 4tH program, it will show you exactly how
much space is allocated to the String Segment. In order to port a
single 4tH program, this is all the information you need!
4tH message : No errors at word 1105
Object size : 1106 words
String size : 2539 chars
Variables : 19 cells
Strings : 262 chars
Reliable : Yes
In this case the "/STRING-SPACE" must be at least 2539 bytes. So,
give or take a few changes, let's say 3072 bytes. We advise you
to allocate a little more memory than is strictly necessary. You
can also use Easy 4tH to make ANS-Forth understand 4tH. Just
type:
16384 constant /STRING-SPACE
s" easy4th.4th" included
Now you can play around with ANS-Forth using the 4tH language. If
you use a lot of string constants you might run out of space, but
your ANS-Forth compiler will give you a message when that
happens.
Note that - depending on the ANS-Forth compiler you're using -
Easy 4tH may redefine some words, although it will try to
minimize these redefinitions as much as possible.
19.4.3 The structure of Easy 4tH
Easy 4tH may look like a large program, but it isn't. It
basically tries to figure out what your compiler supports and
what is still left to define. It always prefers the native
definition to its own. E.g. if your compiler already supports
'PLACE', Easy 4tH will leave that definition intact and assume it
has been defined correctly.
• Easy 4tH will start by defining several defining words like
'STRING', 'STRUCT' and 'ARRAY'. Since there is no standard
definition for these words, it will overwrite any existing
definition.
• After that, Easy 4tH will query the environment and define a
'CONSTANT' when successful. When not, a warning is issued.
• Several 4tH specific compiling words are defined.
• Easy 4tH checks for the presence of several ANS-Forth and COMUS
words. If they are not there, they are defined. Warnings are
issued where applicable.
• In the next stage the parsing, conversion, time and random
number subsystems of 4tH are defined. Warnings are issued where
applicable.
• All 4tH words that cannot be defined in ANS-Forth are marked as
unsupported. When used, an error message is issued and
compilation aborted.
• The standard ANS-Forth 'DOES>' is disabled (unless you override
it).
• ”ezneeds.4th” is loaded and '[NEEDS' and 'INCLUDE' are defined
if needed.
Note that your own compiler may issue error messages or warnings
too, e.g. about redefinitions.
19.5 Converting ANS-Forth programs to 4tH
4tH is a subset of ANS-Forth, so it might be difficult to find a
program that will run on 4tH without at least some rewriting. And
there is no guarantee that it will work, because most ANS-Forth
programs weren't written with 4tH in mind. We'll list the major
pitfalls:
• Programs requiring unsupported words or most words from the
FACILITY, FACILITY EXT, SEARCH and SEARCH EXT wordsets are
generally impossible to port.
• Definitions manipulating the dictionary or the stacks. But 4tH
has no dictionary and does not allow direct access to the
stacks.
• Definitions that switch between interpretation and compilation
mode. 4tH either interprets or compiles; you cannot switch
between the two on the fly. User-defined 'IMMEDIATE' words
generally don't work.
• Definitions using 'CREATE' and 'DOES>' can be difficult to
port. The only way is to do the 'CREATE' part manually and wrap
the 'DOES>' part into a ':THIS' definition.
• Definitions requiring the LOCAL and LOCAL EXT wordsets are
difficult to port. You'll need to rewrite them extensively by
using the 'locals.4th' library file.
• Definitions using ANS-Forth enhanced flow control require some
rewriting and conditional compilation.
• Programs that assume they may store cells and characters in the
same dataspace require some rewriting. Use the 'ncoding.4th'
library file.
Development guide
Compiling the source
20.1 Introduction
4tH is primarily designed as a powerful and easy to use toolkit
for developers. You can use it "as is" and you've got a very
flexible calculation-engine. You can tailor it to a specific
application and push it even further. Or you just might want to
use one of the safest and easiest Forth-alike environments ever
created. These are all valid reasons and I will try to address
them all.
First, I will show you how to compile the example applications.
They are far from useless. In fact, you will have created a
complete programming environment. Second, I will show you how to
create the 4tH library and how to use it. Third, I will explain
to you how the compiler works and how you can make simple
additions to 4tH.
If you find any errors in this document please contact me by
sending email to "[email protected]". You would be helping a
lot of future 4tH users.
20.2 Recommended and preferred compilers
4tH is written in ANSI-C and K&R C and should be portable to any
platform that supports such a compiler. All memory-models are
supported, although the usual restrictions apply. Of course, it
is impossible to test every single compiler on the market, but
there are a number of compilers that are known to work. Preferred
compilers are Open Source and are available for a number of
platforms. When properly installed, the entire compilation can be
performed in two simple steps:
make
Then login as root and enter:
make install
That's all. Any documentation, library files or example programs
must be installed manually. Recommended compilers are free (as in
beer) and are known to work. It's up to you to figure out the
correct installation procedure. Unlisted compilers may or may not
work.
[float Table:
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| Compiler | URL | Platform | Label |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| GCC 2.95 msvcrt |
http://downloads.activestate.com/pub/staff/gsar/gcc-2.95.2-msvcrt.zip | Win32 | Preferred |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| Cygwin | http://www.cygwin.com/ | Win32 | Preferred |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| MinGW | http://sourceforge.net/projects/mingw/ | Win32 | Preferred |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| Pelles C | http://www.smorgasbordet.com/pellesc/ | Win32 | Recommended |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| LCC | http://www.cs.virginia.edu/~lcc-win32/ | Win32 | Recommended |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| TCC | http://fabrice.bellard.free.fr/tcc/ | Win32 | Recommended |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| Turbo C V2.01 | http://bdn.borland.com/article/images/20841/tc201.zip | DOS | Recommended |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
| DJGPP | http://www.delorie.com/djgpp/ | DOS | Preferred |
+------------------+-------------------------------------------------------------------------+-----------+-------------+
[Senseless!!!
<tab:List-of-compilers>List of compilers
]
]
There is no such list for OS/X or Linux. Those platforms usually
already come with an Open Source GCC compiler.
20.3 Compiling 4th
First copy all files to any directory you like. Now make the
latter directory your current directory. The following commands
are applicable to Linux, OS/X and other unices.
If you aren't using an ANSI-C compiler add the "-DARCHAIC"
switch. If you are working on a Unix platform add the "-DUNIX"
switch. If you add the include-files to your own /usr/include
directory, use the "-DUSRLIB4TH" switch. Note that this isn't a
recommended practice.
Finally, if your compiler features a stricmp() function you want
to use instead of 4tHs builtin MatchName(), use the "-DSTRICMP"
switch. These optional switches will be referred to in the
following examples as "$(CFLAGS)".
Unfortunately, not all C-compilers are created equal, so you have
to check the documentation that came with your compiler. You have
to check for three things:
1. First, the 4tH-toolkit assumes that all chars are signed. I
know there are a few compilers out there, that assume that
chars are unsigned (like the RS/6000 and GNU compilers). In
most cases switches are available to correct that. Some K&R
compilers do not support the type "void". If so, you might have
to add "#define void" to 4th.h.
2. Second, compilers on non-Unix platforms might use different or
additional switches, like those that determine the
memory-model.
3. Third, this documentation assumes you call your compiler with
"cc". Borland compilers are called with "bcc" or "tcc". Watcom
compilers are called with "wcc". GNU compilers are called with
"gcc". Microsoft compilers are called with "cl" for some
obscure reason (C Language?).
The 4th program is a do-all compiler. It compiles the source,
executes it and if you made a programming error it will decompile
whatever it could compile. You can also load existing objects or
save new objects. It takes at least two arguments, which are the
command string and the 4tH-program. In Linux, OS/X or other
unices, entering:
make
su
make install
should do the trick. Most MS-DOS compilers contain a version of
`make`, but you probably have to recreate the makefile.
Otherwise, compile it with:
cc $(CFLAGS) -O -o 4th 4th.c open_4th.c comp_4th.c exec_4th.c
dump_4th.c load_4th.c save_4th.c errs_4th.c name_4th.c free_4th.c
cgen_4th.c
If your compiler does not support a command line interface, you
have to include these files:
free_4th.c
errs_4th.c
name_4th.c
dump_4th.c
exec_4th.c
load_4th.c
save_4th.c
comp_4th.c
open_4th.c
cgen_4th.c
4th.c
20.4 Compiling the library
If you want to add the 4tH compiler to your own programs, I
strongly advise you to create a library. As a matter of fact, I
will assume that you have created the library later on. The
library uses only a handful of functions, so it is feasible to
create the library manually. However, we advise you to use the
makefile. Carefully check all macros. In Linux, OS/X or other
unices, entering:
make
su
make install
should do the trick. Most MS-DOS compilers contain a version of
`make`, but you probably have to recreate the makefile. The
library consists of 10 functions:
comp_4th()
exec_4th()
dump_4th()
free_4th()
save_4th()
load_4th()
errs_4th[]
name_4th[]
open_4th()
cgen_4th()
You can compile each function manually by issuing the command:
cc $(CFLAGS) -O -c <function>.c
You'll end up with 11 objectfiles. You can add these functions to
a library by issuing:
ar r lib4th.a <function>.o
The resulting library must be moved to the /usr/lib directory.
When using MS-DOS we strongly advise you to create a library for
each memory-model, like "4ths.lib" for a small memory-model
library, "4thl.lib" for a large memory-model library, etc. When
you compile a function for a particular memory-model you'll have
to add the memorymodel switch. Then create a library by issuing
this command for each function:
lib 4th<model>.lib + <function>.obj
Whether you use a makefile or create the library manually, you
should end up with a working 4tH library. If you are unable to
make a working makefile, write a script- or batchfile. If you
have to recreate the library, it will save you a lot of work.
20.5 <sec:Using-the-library>Using the library
Before we dive into the depths of the API, we will tell you how
to compile a program that uses the 4tH library. If you are using
an advanced MS-DOS or MS-Windows compiler this may or may not
apply to you. In that case we advise you to check your
documentation. If you are using a plain vanilla compiler this
will usually work. Compile the program with:
cc -c <program>.c
When you use a non-Unix system, link the objectfile with the
startup code and libraries to create an executable program. The
<model>.obj is the startup code for C-programs and <model>.lib is
the runtime-library.
link <model>.obj <program.obj>, <program>,, 4th<model>.lib
<model>.lib
Unix developers just give the command:
cc <program>.c -o <program> -l4th
If you happen to use the GNU Compiler Collection (gcc), just
issue:
gcc -s -Wall -fsigned-char <program>.c -o <program> -l4th
After you've built the library you can also issue these commands
to (re)compile 4th. If you happen to use a GNU based platform,
you can configure make to work with 4tH. You just have to create
a small Makefile in your current working directory[footnote:
That is the directory where you are when you call 4tH and is
usually pointed to by the environment variable DIR4TH.
]:
# GNU Make - implicit rules for 4tH
# Copyright 2006, 2009 Hans Bezemer
%.c : %.4th
4th cgq $< $@
%.c : %.hx
4th lgq $< $@
%.4th : %.4pp
pp4th $< $@
CC=gcc
CFLAGS=-fsigned-char -Wall -O3 -s
LDLIBS=-l4th
LDFLAGS=-s
Note this is only an example, so you may have to change it for
your system. Now copy 4th.h to the directory where the 4tH source
is located and you're done:
$ make examples/eliza
4th cgq examples/eliza.4th examples/eliza.c
gcc -fsigned-char -Wall -O3 -s -c examples/eliza.c -o
examples/eliza.o
gcc -s examples/eliza.o -l4th -o examples/eliza
rm examples/eliza.c examples/eliza.o
$ examples/eliza
HI! I'M ELIZA. WHAT'S YOUR PROBLEM?
>
Isn't that easy? You can also use this procedure to turn 4tH
utilities like the preprocessor into full fledged programs. In
fact, the Makefile takes the preprocessor into account if
required:
$ make startrek
pp4th startrek.4pp startrek.4th
4th cgq startrek.4th startrek.c
gcc -fsigned-char -Wall -O3 -s -c -o startrek.o startrek.c
gcc -s startrek.o -l4th -o startrek
rm startrek.4th startrek.c startrek.o
20.6 Shared library
If you're working with Linux, you can create shared libraries
very easily. Just build the libraries with this switch:
make SHARED=1
Then become root and continue the installation as follows:
su
make SHARED=1 install
This will install the shared library and all executables. Now
start 4tH. If you get a message, you have to run ldconfig:
su
ldconfig
Now try again. It should work. If you want to compile against the
shared library, you don't have to a thing. Just type:
gcc -s -Wall -fsigned-char <program>.c -o <program> -l4th
This procedure might work on other GNU platforms too, but this
has not been tested.
20.7 64-bit platforms
Although 4tH will work perfectly well on a 64-bit platform there
are some disadvantages:
• HX files generated by this compiler are not portable to 32-bit
platforms
• Some library files will not work properly without some
modifications
• You'll have to regenerate the editor.h file
A quick fix is to change the size of a cell to a four byte
datatype. The following procedure will usually work. Open 4th.h
and change these lines:
#define CELL_MIN LONG_MIN
#define CELL_MAX LONG_MAX
#define strtoc(a,b,c) strtol((a),(b),(c))
typedef long cell;
To this:
#define CELL_MIN INT_MIN
#define CELL_MAX INT_MAX
#define strtoc(a,b,c) (int)strtol((a),(b),(c))
typedef int cell;
Save 4th.h and compile as described in the previous sections.
20.8 Generating the editor
If you compile the source on a 64-bit or otherwise incompatible
platform, you will find the editor won't work. That is because
the source was generated for a 32-bit platform. You can generate
compatible source for your platform yourself. Just proceed as
follows.
First, generate 4tH as usual but do not install it. The editor
still won't work, but that is not important right now. Rename the
original editor.h to editor.org. Next, copy the editor.4th file
from the examples to your development directory. Change the first
few lines to look like this:
true constant Embedded? \ compile for embedded
version?
false constant Multitask? \ compile for multitasking
version?
large constant #scr \ number of screens (see
above)
64 constant c/l \ characters per line
16 constant l/scr \ lines per editing screen
If you have very little memory available you can change #scr to
'medium'. If you happen to work on a tiny screen, change c/l to
32 and l/scr to 8. Leave Multitask? alone. Now compile the
source:
./4th cvsq editor.4th editor.hx
Use the 4tH program bin2h.4th from the examples to convert the
Hcode executable to C. It is easier when you copy it to your
development directory. Then execute the following command:
./4th cxq bin2h.4th EmbeddedHX editor.hx editor.h
Now compile 4tH again:
make mostlyclean
make
Run 4tH and check if the editor loads properly:
./4th
You should see something like this:
4tH library V3.5d - Copyright 1994,2009 J.L. Bezemer
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>e
Cannot open file OK
q
(S)creen file: new.scr
(O)bject file: out
(E)dit (C)ompile (R)un (A)rguments
(Q)uit (G)enerate (B)uild (D)ecompile
>q
If it doesn't, try another C compiler. Yours may not be
compatible[footnote:
See table [tab:List-of-compilers] for a list of compatible
compilers.
]. If it does, you can safely proceed with the installation:
su
make install
You can use the same procedure when adding a modified or even a
completely different editor.
20.9 Optimizations
It really depends on your compiler. Some compilers allow
optimizations up to level 3 (-O3), others won't even produce a
usable compilant with any optimization enabled. It can even
depend on the platform you're compiling for. It is hard to give a
general recommendation, but try compiling 4tH without any
optimization first, then crank optimization gradually up until
the compilant doesn't work properly anymore or optimization
doesn't give you any more speed or size advantages. You may have
to do some benchmarking to find this out[footnote:
4tH comes with a wide selection of benchmarking programs.
]. There is also a program that allows you to test the virtual
machine. All listed preferred compilers allow the highest
optimization level. The optimization level can be set manually or
can be adjusted in the Makefile. Consult your compiler
documentation for details.
Using the 4tH API
21.1 Introduction
One of the design requirements of the 4tH library was that it had
to be very easy to use. We've seen many APIs that were impossible
to use and put most of the burden on the developer.
4tH takes a different direction. We've designed an API that
almost exactly matches the tasks you want to perform. Want to
compile? Compile. Want to decompile? Decompile. Want to save?
Save. Just like that. No difficult to understand datatypes, no
initialization, no garbage collection, no checks.
The only error, you, the developer can make is fill up memory.
Virtually all other errors are caught by the API. E.g. 4tH will
refuse to save or execute a 4tH-program when compilation failed.
Of course, if you manipulate 4tHs datastructures directly you can
still bring it to its knees, but I assume that is not what you
want.
21.2 A sample program
There are ten API-functions.
[float Table:
+--------------+------------------------------------------------------+
| API | Function |
+--------------+------------------------------------------------------+
+--------------+------------------------------------------------------+
| comp_4th() | Loads and compiles a 4tH source to an H-code object |
+--------------+------------------------------------------------------+
| exec_4th() | Executes an H-code object |
+--------------+------------------------------------------------------+
| dump_4th() | Decompiles an H-code object |
+--------------+------------------------------------------------------+
| free_4th() | Frees an H-code object from memory |
+--------------+------------------------------------------------------+
| save_4th() | Saves an H-code object to an HX-file on disk |
+--------------+------------------------------------------------------+
| store_4th() | Saves an H-code object to an HX-file in memory |
+--------------+------------------------------------------------------+
| load_4th() | Loads an HX-file from disk and installs H-code |
+--------------+------------------------------------------------------+
| fetch_4th() | Loads an HX-file from memory and installs H-code |
+--------------+------------------------------------------------------+
| open_4th() | Creates a small 4tH program that loads a 4tH source |
+--------------+------------------------------------------------------+
| cgen_4th() | Generates a standalone C source with embedded H-code |
+--------------+------------------------------------------------------+
[Senseless!!!
API functions
]
]
That's really all! So if you want to compile a 4tH source, save
it to disk, execute it and finally discard it, you've virtually
written the program.
H-code is nothing but a pointer to a structure. Even if you
thought you never worked with structures before, it's as easy as
working with files. We'll show you.
When you want to use files you first have to include "stdio.h",
like:
#include <stdio.h>
If you want to use a single file for output, you've got to
declare a file-pointer like:
FILE* Outfile;
Before you can use a file, you have to open it:
Outfile = fopen ("filename.ext", "w");
If the file cannot be opened, fopen() returns a NULL-pointer. You
have to check that before you can safely use the file:
if (Outfile == NULL) printf ("Unable to open file");
else fprintf (Outfile, "This is written to disk");
Finally, when you're done, you have to close the file:
fclose (Outfile);
Working with the 4tH library is very similar. When you want to
use the 4tH library you write:
#include "4th.h"
When you use an H-code object (which is a compiled 4tH program),
you declare a H-code pointer:
Hcode* Program;
Before you can use the H-code pointer, we first got to compile a
4tH source. 4tH source is simply a malloc()ed ASCIIZ string. That
means that anything you can convert to a string stored in dynamic
memory can be used as 4tH source. That includes constant strings,
environment variables, lines read from a file, etc.
In this example we use the strdup() function to convert a
constant string. We know that not all runtime-libraries contain
such a function, so if you want give it a try, here is the
source:
char *strdup (char* str)
{
char *p;
p = calloc (strlen (str) + 1, sizeof (char));
if (p) strcpy (p, str);
return (p);
}
Now we can all create a 4tH source and compile it. This one will
create the famous "Hello world" program:
Program = comp_4th (strdup (".\" Hello world\" cr"));
Without any checking you can try to execute it:
exec_4th (Program, 0, NULL, 0);
The "0" means we do not want to transfer any variables or
constants to the execution environment, but we'll get to that
later on. You could make a second call to exec_4th() and see the
program execute twice. The H-code is still in memory. In order to
free it from memory, we have to end our program with:
free_4th (Program);
We are finished now. The full program looks like this:
#include "4th.h"
#include <stdlib.h>
int main(int argc, char** argv)
{
Hcode* Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
exec_4th (Program, 0, NULL, 0);
free_4th (Program);
return (EXIT_SUCCESS);
}
Now compile the C-program and execute it. You should see that it
prints "Hello world" on the screen. Now, that wasn't too hard,
was it?
21.3 A first look at open_4th()
You probably don't want to compile constant strings. Most
certainly you want to create a source-file and compile it. If you
come to think of it, that could be the hardest part when you want
to make your own 4tH-compiler.
Don't worry. We've created a function that handles just that:
open_4th(). The function creates a tiny 4tH program that tells
comp_4th() which file to load. open_4th() just wants to know
which file to load.
If an error occurs (which is very rare), open_4th() returns a
NULL pointer. If open_4th() is successful it returns a
char-pointer to the program in memory. You can feed the
return-value of open_4th() directly to comp_4th(). So, we've got
to declare a char-pointer for the return-value of open_4th() and
call the function:
char* source;
source = open_4th ("venture.4th");
No need to open or close files, comp_4th() will take care of
that. You're one step closer to the creation of your own
compiler.
21.4 A closer look at H-code
H-code is not just a simple pointer to some simple structure. In
fact, it is more complex than a file-pointer. It is comprised of
several parts.[float Figure:
<Graphics file: /home/habe/Lyx/4tHdocs/hcode.eps>
[Senseless!!!
Hcode structure
]
]
1. First, the header. The header contains all information about
the actual program, e.g. the number of variables, the size of
allocated space, its status. The mere existence of an H-code
pointer doesn't mean you actually got a program you can
execute.
2. Second, the Code Segment. This contains the actual program. If
you don't have a Code Segment, there is nothing to execute
since you have no program. The Code Segment is an array of
words. A word consists of a token and an argument. Every token
matches a piece of compiled C in the interpreter. We'll get to
that later on.
3. Third, the String Segment. This segment only contains constant
strings, defined by e.g. S" , ," , .( and .".
4. Fourth, the Integer Segment. This segment contains the stacks
and all writable integer data. It is only present when a
program is sleeping (or hibernating, if you prefer).
5. Fifth, the Character Segment. This segment contains all
writable character data. It is only present when a program is
sleeping.
The first three parts are read-only to the 4tH-programmer. If you
are smart, you consider them to be read-only too. There is no
need whatsoever to change anything here. The API knows best.
21.5 A closer look at HX-code
First of all, you have to understand how numbers are stored in
HX-code. There are four different kinds of numbers in HX-code:
1. Constants
2. Tiny numbers
3. Short numbers
4. Long numbers
All numbers are preceded by a type-byte, which depicts the kind
of number that is stored. The type-byte encoding is listed in
table [tab:HX-type-byte-encoding].
[float Table:
+------+-----------+--------+-----------------+
| Bit | Mnemonic | Value | Significance |
+------+-----------+--------+-----------------+
+------+-----------+--------+-----------------+
| 0 | HCSIGN | 1 | Negative number |
+------+-----------+--------+-----------------+
| 1 | HCBYTE | 2 | Tiny number |
+------+-----------+--------+-----------------+
| 2 | HCSHRT | 4 | Short number |
+------+-----------+--------+-----------------+
| 3 | HCZERO | 8 | 0 or CELL_MIN |
+------+-----------+--------+-----------------+
| 4 | HCONE | 16 | 1 or -1 |
+------+-----------+--------+-----------------+
[Senseless!!!
<tab:HX-type-byte-encoding>HX type-byte encoding
]
]
When HCZERO or HCONE are set, there won't be any additional
bytes. If the type-byte contains 17 (which is 16 + 1) we're
actually looking at -1. If the type-byte contains 8, we're
looking at zero. The first three bytes of any HX-file depict the
length in bytes of a tiny number, a short number and a long
number. Typically they contain 1, 2 and 4. That means if bit 1 is
set, the type-byte contains a 2, which means only a single byte
will follow. If bit 2 is set, two bytes will follow and finally,
if no bit is set four bytes will follow.
The bytes following it will always depict a small-endian,
positive number. Bit 0 is valid in any combination and signals a
negative number. Since CELL_MIN can differ depending on the
platform it is a constant, depicted by both bit 0 and bit 3 set,
which is 1 + 8 (9). HX-code contains several sections:
• The header
• The Code Segment
• The String Segment
• The Character Segment (optionally)
• The Integer Segment (optionally)
• The checksum
First, we have the header. The header contains certain
information so 4tH can establish its compatibility and create the
required environment. The header contains the following elements:
• The length of a tiny number (in bytes)
• The length of a short number (in bytes)
• The length of a long number (in bytes)
• The largest positive number a cell can contain (CELL_MAX)
• The version of 4tH (in hexadecimal)
• The application byte (usually 0)
• The size of the Code Segment (in elements)
• The size of the String Segment (in characters)
• The offset of the Variable Area (>0 for sleeping VMs)
• The size of the Variable Area (in cells)
• The size of the Character Area (in characters)
The numbers are stored in the format, we described above. The
Code Segment requires some explanation. If a 4tH token does not
require any arguments (e.g. 'CR') no arguments are stored. If it
does (e.g. 'LITERAL') the actual value is stored right after it
in the number format we have described above.
The Integer Segment and Character Segment are only stored when
the HX-code contains a sleeping VM. When saved, the Integer
Segment is completely contained in the HX-code, stack and all.
All numbers are stored in the now famil1ar format. HX-code
contains a sleeping VM when the offset in the header is greater
than 0. The offset is computed by adding the number of 4tH system
variables (both writable and non-writable) to the number of
C-variables that have been passed to exec_4th().
The checksum is the way 4tH checks the integrity of HX-code.
Every time 4tH writes a byte of HX-code, it is XORed with the
previous byte. The byte resulting from the sum of these
operations is written last[footnote:
The same algorithm was used for the Sinclair ZX Spectrum tape
handling routines.
]. When 4tH reads back the HX-code the same operation is
performed, hence the values should be the same or some kind of
corruption has crept in. If that happens, 4tH will reject the
HX-code read.
Let's take a look at a real life example:
: hello ." Hello world!" cr ;
hello
When decompiled, it looks like this:
4tH message: No errors at word 5
Object size: 5 words
String size: 13 chars
Variables : 0 cells
Strings : 0 chars
Reliable : Yes
[ 0] branch (3)
[ 1] ." (0) Hello world!
[ 2] cr (0)
[ 3] exit (0)
[ 4] call (0)
And finally, this is the resulting HX-code:
01 Size of a tiny number
02 Size of a short number
03 Size of a long number
00 Start of a long number
FF 1st byte long number
FF 2nd byte long number
FF 3rd byte long number
7F Long number: 7FFFFFFFh
04 Start of short number
5D 1st byte of short number
03 Short number: 035Dh
08 Constant: 00h
02 Start of tiny number
05 Tiny number: 05h
02 Start of tiny number
0D Tiny number: 0Dh
08 Constant: 00h
08 Constant: 00h
08 Constant: 00h
1D BRANCH
02 Start of tiny number
03 Tiny number: 03h
05 .”
08 Constant: 00h
02 CR
45 EXIT
4C CALL
08 Constant: 00h
48 H
65 e
6C l
6C l
6F o
20 <space>
77 w
6F o
72 r
6C l
64 d
21 !
00 <null>
E6 Checksum
Why this elaborate scheme? Well, for two reasons. First, it
allows HX-code to be portable across a wide range of platforms[footnote:
As long as the 7-bit ASCII characterset is supported. EBCDIC
HX-files will not be portable for obvious reasons.
]. Second, it is very compact. If we would write out all numbers
in full, most space would be occupied by zeros. That would be a
waste, especially when embedding HX-files. The speed penalty is
negligible.
21.6 A first look at comp_4th()
The function comp_4th() is one of the most complex and most
important functions of the API. It takes a source and compiles
that to H-code. If you want to save the source, copy it or reload
it, since comp_4th() consumes it entirely. That is why source has
to be allocated in dynamic memory. On the other hand, it explains
why compilation needs very little memory.
If comp_4th() can't compile anything, it returns just the header,
containing all the information on what went wrong and where. If
there is not enough memory to allocate even the header it returns
a NULL-pointer. If it could compile part of the code it returns
what it could compile. But that might not be everything and you
need it all to get a program you can execute.
Lucky for you, but the API can determine whether comp_4th()
returned an executable H-code or not. If exec_4th() doesn't
execute the program, it just couldn't.
The comp_4th() function is just as smart. It can survive the NULL
pointer you feed it. When you read on, you'll find out that the
comp_4th() function is a toolkit by itself with lots of tools to
create your own 4tH words.
If we extend the example we started with open_4th(), we could
continue like this. In order to make an executable H-code we feed
source to comp_4th() and get H-code in return. We just need a
pointer to store its address:
Hcode *Program;
So the entire 4tH-compiler now looks like:
char *source;
Hcode *Program;
source = open_4th ("venture.4th");
Program = comp_4th (source);
We assume you want to execute the program you've just compiled,
so we'll continue with the interpreter-function called
exec_4th().
21.7 A first look at exec_4th()
The exec_4th() function is essentially very simple. It contains
small pieces of C that can be matched with the tokens in the Code
Segment. The exec_4th() function executes these small pieces of C
until there are no more words left to execute or an error occurs.
Additionally, it creates two new segments, which are discarded
when exec_4th() terminates without hibernation. Each segment is
essentially an array of a specific datatype. The sizes of these
segments are specified in the header of the H-code.
The Character Segment contains characters. First, the Terminal
Input Buffer, abbreviated to TIB. When you execute REFILL, this
is the place where the string you typed is stored. Second, the
PAD. This is the place where exec_4th() stores temporary strings.
Finally the Allocation Area where strings, defined by STRING are
allocated.
The Integer Segment contains signed 32 bit integers. First, the
Stack Area. It contains both the return stack and the data stack.
The return stack grows downward and the data stack grows upward.
Second, the System Variable Area. The System Variable Area
contains three variables, HANDLER, HERE and HLD. They can only be
accessed by 4tH itself. Third, the Variable Area. The Variable
Area contains four basic types of variables.
First, the environment variables. In standard 4tH there are five
environment variables, HI, FIRST, LAST, CIN and COUT. They are
all read-only. Second, the predefined variables. In standard 4tH
there are five predefined variables, BASE, >IN, OUT and the
variable pair SOURCE. Later on, we'll teach you how to add your
own. Third, the application variables. These are copies of
C-variables or constants. Fourth, the user-defined variables.
These are defined by the application-programmer with VARIABLE,
VALUE, DEFER, FILE or ARRAY.
The exec_4th() function takes an H-code pointer and returns the
current value of the variable OUT when it exits. When an error
occurs, exec_4th() returns always the largest negative 32 bit
integer, which is also the default value of OUT. In order to give
you maximum control, you can also transfer string-arrays and
C-variables or constants to exec_4th().
C-variables have to be of type 'cell'. This type is predefined in
the '4th.h' headerfile. So you can just write:
#include "4th.h"
cell february = 29;
You can mix constants or variables as you like. Just add the
appropriate cast. You also have to declare the number of
variables or constants you transfer (in this case 12). Let's say
you transfer the number of days of each month to exec_4th():
#include "4th.h"
#include <stdlib.h>
int main (int argc, char** argv)
{
cell february = 29;
cell Result;
Hcode *Program;
char *source;
source = open_4th ("months.4th");
Program = comp_4th (source);
Result = exec_4th (Program, 0, NULL, 12, (cell) 31,
february, (cell) 31, (cell) 30, (cell) 31, (cell) 30,
(cell)
31, (cell) 31, (cell) 30, (cell) 31, (cell) 30, (cell) 31);
return (EXIT_SUCCESS);
}
The application-programmer can use the 'APP' variable to access
all months:
12 0 do app i th ." days: " ? cr loop
Later, we'll learn you how to assign names to those
application-dependant variables. Note that although 'Result'
contains the return-value of the 'months.4th' program, the
C-program does absolutely nothing with it. In the next example
we'll show you how you can use this value.
Take this C-program:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
cell Result;
Hcode *Program;
char *source;
source = open_4th ("calc.4th");
Program = comp_4th (source);
Result = exec_4th (Program, 0, NULL, 2, (cell) 5, (cell)
7);
printf ("Result: %ld\n", (long) Result);
return (EXIT_SUCCESS);
}
This program compiles 'calc.4th' and transfers '5' and '7' to
exec_4th(). What is returned in 'Result' depends on what
'calc.4th' does. Let's take a look at 'calc.4th':
app 0 th @ app 1 th @ + out !
It fetches both variables, adds them and assigns the sum to OUT.
Thus, when exec_4th() terminates it returns the value of OUT.
This value is assigned to 'Result'. Then, 'Result' is cast to
'long' and displayed:
Result: 12
We haven't used the other two arguments of exec_4th() yet. These
are used to pass string constants to your 4tH application. Most
of you will use it to pass commandline arguments to 4tH.
Let's say we want to pass these arguments to 4tH in C-style. So
"0 ARGS" is the name of our 4tH-program. That means we have to
skip the name of the C-program itself. The name of our
4tH-program in in argv [1]. That also means we have to decrement
argc, before passing it to exec_4th():
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
Hcode *Program;
char *source;
if (argc > 0) {
source = open_4th (argv [1]);
Program = comp_4th (source);
(void) exec_4th (Program, argc - 1, argv + 1, 0);
return (EXIT_SUCCESS);
}
return (EXIT_FAILURE);
}
Note that argv [1] is not the same as argv + 1! When we call our
C-program with:
main args.4th hello here I am
argc will be 6. That means exec_4th() will get a stringarray with
five strings (since we discarded one). We can access these
strings by:
argn 0> if argn 0 do i args count type cr loop then
Which would print:
args.4th
hello
here
I
am
You do not have to pass argc and argv; other string arrays of the
same type ('**char' or '*char[]') are okay too. Just don't forget
to pass the correct number of elements in the string array.
So now you know how your H-code program can communicate with your
C-program. You can transfer any number of variables to the
4tH-environment and retrieve the result. You can even pass
commandline arguments to the 4tH-environment and use them inside
your application. In the next section you will be introduced to
some other interesting properties of the 4tH-environment.
21.8 A first look at free_4th()
Let's take a look at this program:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
cell Result;
Hcode *Program;
char *source;
source = open_4th ("calc.4th");
Program = comp_4th (source);
Result = exec_4th (Program, 0, NULL, 2, (cell) 5, (cell)
7);
printf ("Result: %ld\n", (long) Result);
Result = exec_4th (Program, 0, NULL, 2, (cell) 12, (cell)
9);
printf ("Result: %ld\n", (long) Result);
return (EXIT_SUCCESS);
}
Essentially, it is the very same program we've seen in the
previous section. But here the 'calc.4th' program is executed
twice with different arguments. Can that be done without
recompiling the program? Yes! The pointer 'Program' is still
valid and points to the very same 4tH program in memory. You can
execute it any number of times without recompiling. You can go
even further:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
cell Result;
char *source;
Hcode *Multiply;
Hcode *Subtract;
source = open_4th ("multiply.4th");
Multiply = comp_4th (source);
source = open_4th ("subtract.4th");
Subtract = comp_4th (source);
Result = exec_4th (Multiply, 0, NULL, 2, (cell) 7, (cell)
5);
printf ("Result: %ld\n", (long) Result);
Result = exec_4th (Subtract, 0, NULL, 2, (cell) 7, (cell)
5);
printf ("Result: %ld\n", (long) Result);
return (EXIT_SUCCESS);
}
The file 'multiply.4th' contains:
app 0 th @ app 1 th @ * out !
The file 'subtract.4th' contains:
app 0 th @ app 1 th @ - out !
So executing this C-program will give the following result:
Result: 35
Result: 2
And yes, both programs can be re-executed any number of times.
But what if some 4tH program has served his purpose? Does it have
to remain in memory all the time? No. Since it is located in
dynamic memory it can be freed. Not with free(), since H-code is
too complex to be served with a simple function like free(). But
a special function is included in the API, which serves the same
purpose:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
cell Result;
char *source;
Hcode *Multiply;
Hcode *Subtract;
source = open_4th ("multiply.4th");
Multiply = comp_4th (source);
source = open_4th ("subtract.4th");
Subtract = comp_4th (source);
Result = exec_4th (Multiply, 0, NULL, 2, (cell) 7, (cell)
5);
printf ("Result: %ld\n", (long) Result);
free_4th (Multiply);
Result = exec_4th (Subtract, 0, NULL, 2, (cell) 7, (cell)
5);
printf ("Result: %ld\n", (long) Result);
free_4th (Subtract);
return (EXIT_SUCCESS);
}
The function free_4th() takes an Hcode-pointer and frees all
resources. There is really nothing more to it. Remember that
free_4th() doesn't alter the pointer itself. It may still contain
a value, but of course using that value is asking for trouble.
The API checks quite a few things by itself, but that doesn't
mean you can start to write sloppy programs!
21.9 A first look at save_4th()
We've already seen that we can compile a 4tH-program and keep it
in memory for as long as we want. We can also discard it if we
don't need it anymore. But what if we want to reuse the compilant
later? Or if we want to distribute 4tH programs without revealing
our source-code?
You can do that easily. 4tH uses another main format which not
only enables you to load compiled programs, but also run them on
a multitude of platforms. It is called the 'Hcode eXecutable'
(HX-file) and it is fully portable across all platforms 4tH
supports.
Saving a program is very easy too. You don't even have to open or
close files. Here is a very simple compiler:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
char *source;
Hcode *Program;
if (argc == 3) {
source = open_4th (argv [1]);
Program = comp_4th (source);
save_4th (Program, argv [2]);
free_4th (Program);
return (EXIT_SUCCESS);
}
return (EXIT_FAILURE);
}
You just declare the input and the output file on the commandline
and when no errors occur an HX-file is saved to disk. The
save_4th() function takes the Hcode pointer and the filename you
want to save it to. Note save_4th() supports hibernation too;
just feed it a sleeping virtual machine. That's all!
21.10 A first look at load_4th()
But you don't just save compiled programs. You want to be able to
reuse them too. There is a special function that reads an HX-file
and restores the H-code to its original form. This API-function
is easy to use too. Just feed it the name of the file and it
returns a pointer to the H-code. This is the listing of a simple
HX-execute:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
Hcode *Program;
cell Result;
if (argc == 2) {
Program = load_4th (argv [1]);
Result = exec_4th (Program, 0, NULL, 0);
printf ("Result: %ld\n", (long) Result);
free_4th (Program);
return (EXIT_SUCCESS);
}
return (EXIT_FAILURE);
}
You just declare the HX-file on the commandline and when no
errors occur it is executed. Finally, it displays the result of
the program.
21.11 A first look at error-trapping
If you are a professional programmer you might appreciate the
ease of use of the 4tH toolkit, but you have the feeling you
don't have any control. If that were the case, we would feel the
same way. In fact, you have all the control you'll ever need. In
the background, 4tH keeps track of everything that is happening.
We've already discussed the header of the H-code. All
status-information is stored here. And it's all available. May be
you'll find it a little more complex and intimidating, but you
can easily master it. Let's take a look at this piece of code:
Hcode *Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
This piece of code tries to compile the classic "Hello world"
program. But did it compile? If comp_4th() returned a
NULL-pointer, you know there was not enough memory. But like any
other compiler, there are a million other things that can go
wrong. Although other API functions will refuse unreliable
H-code, sometimes we want to check it ourselves and take
alternative action if necessary.
All information regarding the status is saved in the header. But
if comp_4th() returns a NULL-pointer there is no header. So we
have to check that first:
Hcode *Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
if (Program == NULL) printf ("Not enough memory\n");
If the program enters the 'else' clause we know that a header
exists. Now we need to check the status. Did an error occur?
There are two members in the header we can check. First, 'ErrNo'
which contains an error-code. If 'ErrNo' contains '0', there were
no errors:
Hcode *Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
if (Program == NULL)
printf ("Not enough memory\n");
else {
if (Program->ErrNo == 0)
(void) exec_4th (Program, 0, NULL, 0);
else
printf ("There were errors\n");
}
Note that the member 'ErrNo' is closely linked to the H-code.
That is hardly surprising since it is part of the H-code! But we
still don't know which error occurred.
Fortunately, there is a predefined array of error-messages we can
use. It is called errs_4th[] and you can use it without declaring
it explicitly, since '4th.h' takes care of that. If you have
correctly built the library it will automatically be linked in:
Hcode *Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
if (Program == NULL)
printf ("Not enough memory\n");
else {
if (Program->ErrNo == 0)
(void) exec_4th (Program, 0, NULL, 0);
else
printf ("Error: %s\n", errs_4th [Program->ErrNo]);
}
Of course, checking error-codes by the number is not ideal from a
maintenance point of view. In '4th.h' you'll find a lot of
#define-s describing these errors. The mnemonic for 'no errors'
is 'M4NOERRS', so we can slightly alter our program to:
Hcode *Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
if (Program == NULL)
printf ("Not enough memory\n");
else {
if (Program->ErrNo == M4NOERRS)
(void) exec_4th (Program, 0, NULL, 0);
else
printf ("Error: %s\n", errs_4th [Program->ErrNo]);
}
In the next section we'll show how you can help the
4tH-programmer to pinpoint his errors even more precisely.
21.12 A first look at dump_4th()
Although 4tH can tell you what error you made and where you made
it, you may find it pretty hard to locate it anyway. That is
because 4tH makes a reference to the compilant instead of the
source.
That is because 4tH preprocesses the source and never looks
further ahead than one single word, so a reference to the source
wouldn't help you much anyway. That is the bad news.
The good news is that the instructions 4tH uses internally are
virtually identical to the ones you used in your source. If you
decompile the program you should still be able to recognize your
source. The function dump_4th() is essentially a decompiler. Let
us show you a small part of a program by Leo Brodie we converted
to 4tH:
VARIABLE SPAN
: EXPECT ACCEPT SPAN ! ;
16 CONSTANT #NAME
8 CONSTANT #EYES
16 CONSTANT #ME ( length of fields )
#NAME STRING NAME
#EYES STRING EYES
#ME STRING ME ( calculate values )
...
If you decompile the entire program you will get a listing, which
consists of two parts. First the header:
4tH message : No errors at word 80
Object size : 81 words
String size : 208 chars
Variables : 1 cells
Strings : 40 chars
Reliable : Yes
First it will present the current status of this Hcode program.
The words are numbered and we begin counting at zero. This means
this program is okay, since word 80 is the very last word. We can
derive that information from the second field that lists that
there are 81 words, numbered from 0 to 80. The third field tells
us there are 208 characters stored in the String Segment.
The next two fields tell us something about the
runtime-environment. The total number of strings we defined take
up 40 bytes and we defined one single variable. Finally, 4tH
tells us this piece of Hcode is reliable. That means it can be
saved to disk or executed. If it had told us the Hcode was not
reliable, we could still have decompiled it. Otherwise it could
get very hard to pinpoint an error. Next is the decompiled
program itself:
[ 0] branch (4)
[ 1] accept (0)
[ 2] variable (0)
[ 3] ! (0)
[ 4] exit (0)
...
As you will see, you can still tell what this program is all
about. Since 4tH has no dictionary, but uses a symbol-table, all
lexical references are gone. There is no indication that the
first word was ever called 'EXPECT' or that variable #0 was named
'SPAN'. In fact, if you would name them differently, it would
still compile to the very same Hcode.
The bracketed numbers are the 'addresses' of the words. Then it
prints the name of the compiled token. Finally the argument part
of the word is printed within parentheses.
Not all tokens have arguments. 'ACCEPT' and 'EXIT' don't need
one. They either take their arguments from the stack or don't
have any. But 'LITERAL' and 'BRANCH' do need them. 'LITERAL'
needs the value of the number it has to throw on the stack and
'BRANCH' needs the address it has to branch to (in fact, it
branches to the next token after the indicated address). The
interpreter "knows" which token has a valid argument and which
ones it can ignore.
But you surely want to know how you can integrate this decompiler
into your own programs. Like all other functions, it needs an
Hcode-pointer. It also needs a device where you can send the
report to.
To give you maximum flexibility we used an open stream, so you
can use the screen, a printer or a file. A disadvantage is you
have to open and close the file yourself when applicable.
We also gave you the opportunity to do a partial listing. You can
tell dump_4th() what range you want to decompile. These
parameters are protected too. If you feed dump_4th() an invalid
range it will try to figure out what range is most applicable.
This allows you to do a full listing with minimum effort by
issuing a range from word 0 to word -1.
A sample application may look like this:
FILE *ErrFile;
Hcode *Program;
/* other code */
if ((ErrFile = fopen ("error.lst", "w")) == NULL)
printf ("Cannot open file\n");
else {
dump_4th (Program, ErrFile, 0, -1);
if (fclose (ErrFile)
printf ("Error closing file\n");
}
If you want to print it to screen, you can use either 'stdout' or
'stderr'. Note that 'stderr' cannot be redirected easily under
MS-DOS, so we'll use 'stdout' here:
Hcode *Program;
/* other code */
dump_4th (Program, stdout, 0, -1);
You can always provide a report when compiling or you can use
error-checking to decide whether you execute or save Hcode or
print a report. This is the listing of a complete compiler:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
char *source;
Hcode *Program;
if (argc == 2) {
source = open_4th (argv [1]);
Program = comp_4th (source);
if (Program == NULL)
printf ("Not enough memory\n");
else {
if (Program->ErrNo == M4NOERRS)
(void) exec_4th (Program, argc - 1, argv +
1, 0);
else dump_4th (Program, stdout, 0, -1);
free_4th (Program);
return (EXIT_SUCCESS);
}
}
return (EXIT_FAILURE);
}
This program loads a source, compiles it and executes the
resulting Hcode if no errors occurred. It tells you when there is
not enough memory and provides a decompiler-listing on screen
when a programming error was made. Pretty neat, huh?
21.13 A first look at cgen_4th()
cgen_4th() allows you to create native standalone programs with
minimal effort. It is a lot like save_4th(). All you need is a
Hcode object in memory and an open stream. The stream will allow
you to send the C program cgen_4th() generates to screen, file or
a printer. A sample application could look like this:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
Hcode *Program;
cell Result;
FILE *CFile;
if (argc == 2) {
if ((CFile = fopen ("myfile.c", "w")) != NULL) {
Program = load_4th (argv [1]);
cgen_4th (Program, CFile);
free_4th (Program);
fclose (CFile);
return (EXIT_SUCCESS);
}
}
return (EXIT_FAILURE);
}
This program will load an HX file and send a complete C source to
"myfile.c". You can compile it by issuing[footnote:
See section [sec:Using-the-library].
]:
cc myfile.c exec_4th.c errs_4th.c -o myfile
or, if you have the 4tH library installed by:
cc myfile.c -o myfile -l4th
Just like any other 4tH related C program. You will have a native
executable for your platform and nobody will ever know it's
actually a 4tH program. But there are even more ways to use
embedded 4tH as we will see.
21.14 Converting HX-files
With the 4tH program bin2h.4th you can convert HX-files to
portable C-source. This opens a whole new range of applications.
cgen_4th() is a quick way to create standalone programs, but
bin2h.4th allows you to embed highly compacted HX code into your
C program.
This is particularly useful when memory is tight, because you can
load the HX code when it is actually needed and discard it
afterwards. Furthermore, you can have several HX code snippets
inside your C program, which is not possible when using
cgen_4th().
On the downside, HX code is a little more difficult to handle.
bin2h.4th just creates the embedded code, not an entire program.
You will have to write that yourself. Furthermore, in order to
use bin2h.4th you need to create an HX file first. Still, using
bin2h.4th is dead easy:
4th cxq bin2h.4th HelloWorld myprog.hx myprog.h
You might have noticed that the only thing you have to provide is
the name of the HX file (myprog.hx), the name of the include file
(myprog.h) and the name of the variable in which the embedded HX
code is stored (HelloWorld).
21.15 A first look at fetch_4th()
A typical bin2h.4th generated includefile looks like this:
static unit HelloWorld [] = {
'\x01', '\x02', '\x04', '\x00', '\xff', '\xff', '\xff', '\x7f',
'\x04',
'\x5c', '\x03', '\x08', '\x02', '\x02', '\x02', '\x0d', '\x08',
'\x08',
'\x08', '\x05', '\x08', '\x02', '\x48', '\x65', '\x6c', '\x6c',
'\x6f',
'\x20', '\x77', '\x6f', '\x72', '\x6c', '\x64', '\x21', '\x00',
'\xfd',
};
Since it contains compiled code there is no need for functions
like comp_4th(). However, HX code can not be fed to exec_4th()
directly. It has to be loaded first. The function load_4th() does
this automatically. There is a function in the 4tH API to load
bytecode from memory, called fetch_4th(). Just pass the HX code
pointer to it:
Hcode* Program;
Program = fetch_4th (HelloWorld);
Now the bytecode is installed and can be executed by exec_4th()
in the usual way:
(void) exec_4th (Program, 0, NULL, 0);
In this case, it will simply print "Hello world!" to your screen.
21.16 A first look at store_4th()
And what if you don't want to store bytecode on disk, but in
memory? Well, you can do that too. 4tH provides the function
store_4th(), which takes a Hcode pointer, a pointer to a buffer
and the size of that buffer. It is very easy to use. Just create
a sufficiently large buffer, either dynamic or static, and load
the Hcode. No need to worry about buffer overflow, when properly
used store_4th() will prevent such mishaps. It will even return
the amount of bytes it has written:
#include "4th.h"
#include <stdlib.h>
int main (int argc, char **argv)
{
Hcode *Object; /* Hcode object */
unit Buffer [1024]; /* memory allocated to
bytecode */
char *Program; /* 4tH sourcecode */
size_t MySize; /* number of bytes written
to memory */
Program = open_4th ("hello.4th"); /* create sourcecode */
Object = comp_4th (Program); /* compile and save the
sourcecode */
MySize = store_4th (Object, Buffer, sizeof (Buffer));
printf ("%s, %ld bytes used\n", errs_4th [Object->ErrNo],
MySize);
free_4th (Object); /* destroy the Hcode
object */
Object = fetch_4th (Buffer); /* read the bytecode from
'Buffer' */
exec_4th (Object, 0, NULL, 0); /* execute the Hcode
object */
free_4th (Object); /* destroy the Hcode
object again */
return (EXIT_SUCCESS); /* signal 'everything ok'
*/
}
This program compiles some 4tH source, saves the HX code in a
buffer and then discards the original Hcode. Finally, the HX code
is reloaded, run and discarded again. Note that store_4th()
supports hibernation, like its close brother save_4th(). When
this program is run it will display:
No errors, 36 bytes used
Hello world!
Needless to say that you can do very neat things with this
function, like paging programs in and out very quickly using very
little memory, storing multiple programs in a single buffer, etc.
Use your imagination!
21.17 Examples of embedded HX code
The include-files bin2h.4th generates contain global variables.
You can either integrate them in your sourcecode or include them,
e.g:
#include <stdlib.h>
#include "4th.h"
#include "hello.h"
or:
#include <stdlib.h>
#include "4th.h"
static unit HelloWorld [] = {
'\x01', '\x02', '\x04', '\x00', '\xff', '\xff', '\xff', '\x7f',
'\x04',
'\x5d', '\x03', '\x08', '\x02', '\x02', '\x02', '\x0d', '\x08',
'\x08',
'\x08', '\x05', '\x08', '\x02', '\x48', '\x65', '\x6c', '\x6c',
'\x6f',
'\x20', '\x77', '\x6f', '\x72', '\x6c', '\x64', '\x21', '\x00',
'\xfc'
};
It's really up to you. You can install and uninstall HX code as
often as you want. You can also have multiple instances of the HX
code in memory if you need to. E.g. this is perfectly valid:
#include <stdlib.h>
#include "4th.h"
static unit HelloWorld [] = {
'\x01', '\x02', '\x04', '\x00', '\xff', '\xff', '\xff', '\x7f',
'\x04',
'\x5d', '\x03', '\x08', '\x02', '\x02', '\x02', '\x0d', '\x08',
'\x08',
'\x08', '\x05', '\x08', '\x02', '\x48', '\x65', '\x6c', '\x6c',
'\x6f',
'\x20', '\x77', '\x6f', '\x72', '\x6c', '\x64', '\x21', '\x00',
'\xfc'
};
int main (int argc, char** argv)
{
Hcode* Instance1;
Hcode* Instance2;
/* load two instances of HX code */
Instance1 = fetch_4th (HelloWorld);
Instance2 = fetch_4th (HelloWorld);
/* execute both instances */
(void) exec_4th (Instance1, 0, NULL, 0);
(void) exec_4th (Instance2, 0, NULL, 0);
/* free first instance */
free_4th (Instance1);
/* execute and free second instance */
(void) exec_4th (Instance2, 0, NULL, 0);
free_4th (Instance2);
/* reinstall first instance and execute */
Instance1 = fetch_4th (HelloWorld);
(void) exec_4th (Instance1, 0, NULL, 0);
free_4th (Instance1);
return (EXIT_SUCCESS);
}
The combination of different pieces of HX code is possible too.
This code contains two pieces of HX code. The first one adds up
two numbers, the second one divides two numbers. Both return the
result of the calculation to the variable "Result":
#include <stdlib.h>
#include <stdio.h>
#include "4th.h"
/* app dup @ swap cell+ @ + out ! */
static unit Addition [] = {
'\x01', '\x02', '\x04', '\x00', '\xff', '\xff', '\xff', '\x7f',
'\x04',
'\x5d', '\x03', '\x08', '\x02', '\x09', '\x08', '\x08', '\x08',
'\x08',
'\x39', '\x02', '\x0a', '\x11', '\x07', '\x10', '\x33', '\x07',
'\x0b',
'\x39', '\x02', '\x07', '\x08', '\xe2'
};
/* app dup @ swap cell+ @ / out ! */
static unit Division [] = {
'\x01', '\x02', '\x04', '\x00', '\xff', '\xff', '\xff', '\x7f',
'\x04',
'\x5d', '\x03', '\x08', '\x02', '\x09', '\x08', '\x08', '\x08',
'\x08',
'\x39', '\x02', '\x0a', '\x11', '\x07', '\x10', '\x33', '\x07',
'\x0e',
'\x39', '\x02', '\x07', '\x08', '\xe7'
};
int main (int argc, char** argv)
{
Hcode* Instance;
cell Result;
/* load addition HX code */
Instance = fetch_4th (Addition);
/* execute: add 5 to 7 */
Result = exec_4th (Instance, 0, NULL, 2, (cell) 5, (cell)
7);
/* free instance */
free_4th (Instance);
/* load division HX code */
Instance = fetch_4th (Division);
/* execute: div Result by 6 */
Result = exec_4th (Instance, 0, NULL, 2, Result, (cell) 6);
/* free instance */
free_4th (Instance);
/* print Result and exit */
printf ("Result: %ld\n", (long) Result);
return (EXIT_SUCCESS);
}
There are no restrictions whatsoever to the use of the rest of
the 4tH API, since fetch_4th() returns an ordinary Hcode pointer.
For instance, you can still use load_4th() to load additional
HX-files. Happy embedding!
21.18 Suspended execution
People are wondering how they can enable hibernation[footnote:
Also referred to as 'hibernation', 'sleeping VMs' or 'dormant
VMs'.
]. Well, you can't. Only a 4tH programmer can do that by using
the word 'PAUSE'. Normally, 4tH closes all files, releases the
runtime environment and exits. When 'PAUSE' is encountered, 4tH
creates a stackframe, closes all files and exits. All API
functions recognize a dormant VM and act accordingly, so there is
not much you can do. You can recognize a dormant VM by examining
the ”Offset” member of the Hcode structure. If is non-zero, you
got a dormant VM at your hands:
Object->Offset
Still, there is a lot you can do with a dormant VM as long as you
have created special provisions in your 4tH program. Take this
very simple interpreter:
include lib/interprt.4th
\ The words supported by the interpreter
: bye ." ZZzzzz.." cr pause ." Waky, waky!" cr ;
: test ." Test successfully executed!" cr ;
: _+ + ;
: _- - ;
: _* * ;
: _/ / ;
: _.( [char] ) parse type ;
: _cr cr ;
: _. . ;
\ The dictionary of the interpreter
create wordlist
," bye" ' bye ,
," test" ' test ,
," +" ' _+ ,
," -" ' _- ,
," *" ' _* ,
," /" ' _/ ,
," .(" ' _.( ,
," cr" ' _cr ,
," ." ' _. ,
NULL ,
wordlist to dictionary
\ The interpreter itself
: go ['] interpret catch if ." Oops!" cr then ;
: prompt ." OK" cr refill drop go ;
: script 1 args tib place 0 >in ! go bye ;
: run begin argn 2 = if script else prompt then again ;
run
Note you can only suspend this program. When you provide a
commandline argument, it will interpret it as a script and
execute it. That is neat! Now take a look at this C program:
#include "4th.h"
#include <stdio.h>
#include <stdlib.h>
#define HX "tiny.hx"
/*
This function starts an interactive session
*/
void Prompt (Hcode *Program)
{
puts ("Keyboard control enabled..\n");
(void) exec_4th (Program, 0, NULL, 0);
puts ("\nHost control enabled..");
}
/*
This function builds an argument list and starts a script
*/
void Script (Hcode *Program, char *script)
{
char *(Args [3]); /* mimics argv[][] */
Args [0] = HX; /* the program name */
Args [1] = script; /* the script itself */
Args [2] = NULL; /* the list terminator */
puts ("Script control enabled..\n");
(void) exec_4th (Program, 2, Args, 0);
puts ("\nHost control enabled..");
}
/*
Host program, which calls the shots
*/
int main (int argc, char** argv)
{
Hcode *Program;
Program = load_4th (HX);
if (Program) /* if loading was
successful */
{
Prompt (Program); /* interactive session */
Script (Program, ".( This is a test) cr test test test");
Prompt (Program); /* interactive session */
Script (Program, ".( Leaving an item on the stack) cr 23
45 +");
Prompt (Program); /* interactive session */
puts ("Host shutting down..");
free_4th (Program); /* free resources */
}
return (EXIT_SUCCESS);
}
There is a function that supplies arguments and executes a script
and a function that does not supply arguments and enters
interactive mode. In main() we call these functions alternately.
It is a pretty mean program! You can allow a user to do what he
wants and when he relinguishes control, you can execute whatever
you want from your C program. It is so mean that even when the
user enters something like:
11 13 * 4 + bye .( I still gotta do this!) cr
The part after 'BYE' will still be executed before the
interpreter starts executing a script. Whatever is on the stack
stays on the stack, no matter if it was left there by a script or
an interactive session. Now that is powerful! But there are more
neat things you can do with suspended execution. You can also use
it to read or write 4tH data. That may seem a bit tricky at
first, but as a matter of fact it is very easy. Take a look at
this example:
10 constant NUMCELLS \ array size
NUMCELLS array Xarray \ array to be exported
32 string Xstring \ string to be exported
: export out ! pause ; \ save literal and sleep
: run
Xarray export \ export Xarray
Xarray 10 bounds do i ? loop cr \ show array contents
s" This comes straight from 4tH!" Xstring place
Xstring export \ export Xstring
." Famous last words.." cr \ final message
;
run
You probably remember that the contents of OUT are returned by
exec_4th(), so what 'EXPORT' actually does is saving an address
of a variable before returning control to the C program. 'EXPORT'
is called almost immediately in this example. Obviously something
has been done, since the contents of 'XARRAY' are dumped. Then a
string variable is initialized and 'EXPORT' is called again.
Finally a message is printed. Doesn't that make you wonder what
the C program does?
#include "4th.h"
#include "cmds_4th.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MK_CP(a,b) ((a)->CellSeg + STACKSIZ + SYS4TH + (b))
#define MK_UP(a,b) ((a)->UnitSeg + (b))
#define NUMCELLS 10
#define HX "bulk.hx"
int main (int argc, char** argv)
{
cell Pointer;
cell Carray [] = { 0L, 10L, 20L, 30L, 40L, 50L, 60L, 70L,
80L, 90L };
Hcode *Program;
cell *p;
int x;
Program = load_4th (HX);
if (Program) /* if loading was
successful */
{ /* fill the 4tH array */
Pointer = exec_4th (Program, 0, NULL, 0);
p = MK_CP (Program, Pointer);
for (x = 0; x < NUMCELLS; x++, p++) *p = Carray [x];
/* show array and setup
string */
Pointer = exec_4th (Program, 0, NULL, 0);
puts (MK_UP (Program, Pointer));
/* show famous last words
*/
Pointer = exec_4th (Program, 0, NULL, 0);
free_4th (Program); /* free hcode */
}
return (EXIT_SUCCESS);
}
The first thing you notice are the two macros, MK_CP() and
MK_UP(). They have been defined to create pointers to the Integer
Segment and the Character Segment. It is very easy: just call 4tH
from C and export your variable of choice with 'EXPORT'. In this
program, the value is stored in 'Pointer'. Note that it is
essential that you know exactly what has been returned: a Cell or
a Unit.
In this example, exec_4th() first returns a cell, so we have to
call MK_CP() to convert it to a pointer to a cell. After that we
can transfer the contents of a C array to 4tH. The second time
exec_4th() returns a unit, so we'll have to call MK_UP() to
convert it to a pointer to an unit. After that we can use the
pointer to print the string. Then exec_4tH() is called for the
last time and a string is printed. Note that you don't have to
let 4tH finish. You can call free_4th() any moment you want to.
That is still not enough for you? You'd like to see something
even fancier? Well, what do you think about this tiny cooperative
multitasker:
#include "4th.h"
#include <stdlib.h>
#define MAX_TASK 16
Hcode *Processes [MAX_TASK]; /* process slots */
/*
This routine adds a task to the process space. It returns the PID
if successful, CELL_MIN if not.
*/
cell task_4th (Hcode **Process, Hcode *Task)
{
cell x;
if (Task) /* if there is a task */
for (x = 0; x < MAX_TASK; x++) /* search all process
slots */
if (Process [x] == NULL) /* if empty slot found */
{
Process [x] = Task; /* add the task */
return (x); /* and return success */
}
return (CELL_MIN); /* if not, return failure
*/
}
/*
This routine searches the process space for a given task. If
found, it is
executed and the return value returned to the calling process. If
not found,
it signals termination.
*/
cell wake_4th (Hcode **Process, cell task)
{
cell x, y;
for (x = task; x < MAX_TASK; x++) /* search slots beginning
with task */
if (Process [x]) /* if it is an active
process */
{ /* execute it */
y = exec_4th (Process [x], 0, NULL, 1, x);
/* if the process has
terminated */
if (Process [x]->Offset == 0)
{ /* free the process */
free_4th (Process [x]); /* and set the pointer to
NULL */
Process [x] = NULL;
}
/* return next process
number */
return (y == CELL_MIN ? ++x : y);
}
/* signal no active
processes found */
return (task == 0 ? CELL_MIN : MAX_TASK);
}
/*
This is the true multitasker. It keeps on looping through the
processes until
it receives a kill signal from wake_4th().
*/
void multi_4th (Hcode **Process)
{
cell pid = 0L; /* process id */
while (pid >= 0L) /* seach all process slots
*/
{
pid = wake_4th (Process, pid); /* now wake this process
*/
if (pid >= MAX_TASK) pid = 0; /* and loop around */
}
}
/*
Host program, which calls the shots
*/
int main (int argc, char** argv)
{
cell x;
/* set all slots to NULL */
for (x = 0; x < MAX_TASK; x++) Processes [x] = NULL;
/* now load two processes
*/
printf ("Process %ld installed\n", task_4th (Processes,
load_4th ("1.hx")));
printf ("Process %ld installed\n", task_4th (Processes,
load_4th ("2.hx")));
multi_4th (Processes); /* start the multitasker */
return (EXIT_SUCCESS); /* return success */
}
Since load_4th() returns a NULL pointer when it doesn't succeed,
it is safe to pass it to task_4th(). Even if programs aren't
suited to do any multitasking, you can use this program: they
will just be executed consecutively. task_4th() will add a
program to the process list. wake_4th() will try to wake up a
program beginning with the PID which is passed to it. Note that
the PID is passed to the 4tH program, so it can be queried. If
'OUT' contains a valid PID, it will be the next process that the
program will try to awaken. When a process terminates, it is
taken from the process list. When all processes have terminated,
wake_4th() returns CELL_MIN. Note that a program can terminate
the multitasker by returning a non-zero value. That's as fancy as
it gets, folks!
The String Segment and the Code Segment are read-only for a
reason. Although you can access them, we advise you not to
attempt it. The same goes for the Stack Area and the System Area.
The interfaces we've provided here are a relatively safe way to
exchange information between 4tH and C, but if you make any
errors your program might crash. Therefore, it is a good idea to
add exception handlers to critical sections of your 4tH program.
21.19 Useful variables
We've already seen that dump_4th() can provide you with a lot of
information about Hcode. If you need this information, you don't
have to call dump_4th(). The dump_4th() function simply uses the
information that is already available. This small program shows
you how to obtain it:
Hcode *Program;
Program = comp_4th (strdup (".\" Hello world\" cr"));
if (Program == NULL)
printf ("Not enough memory\n");
else {
printf ("Error# : %u\n", Program->ErrNo);
printf ("Error at word: %d\n", Program->ErrLine);
printf ("Object size : %d\n", Program->CodeSiz);
printf ("String size : %u\n", Program->StringSiz);
printf ("Var. offset : %u\n", Program->Offset);
printf ("Variables : %u\n", Program->Variables);
printf ("Strings : %u\n", Program->Strings);
printf ("Reliable : %s\n", Program->Reliable ? "Yes" :
"No");
}
The labels are kept the same as in dump_4th(), so if you need
more information, read that section again.
Modifying 4tH
22.1 Introduction
A good scripting language must be easy extendible. We will cover
the most common extensions. You will acquire indepth knowledge of
the inner workings of 4tH. All of 4tHs functions are a toolkit in
itself and can be put to your own use (especially comp_4th(),
which is 4tHs most complex function).
As we proceed, there will be more files to edit and the
modifications will get more complex. Be sure you mastered the
previous extensions, before you get on with the more elaborate
ones. A good knowledge of C is required for most operations.
22.2 <sec:A-closer-look>A closer look at comp_4th()
As we already know, comp_4th() compiles source to H-code. First
of all, we need to have a source. This is a simple character
array, which is pointed to by "Source". Then an H-code header is
created.
The header is initialized by InitObject(), which calls
ParseText() to get the initial size of the Code Segment. The
initial size of the Code Segment is stored in the header-member
"CodeSiz".
ParseText() calls two other functions: ParseDirectives(), which
picks out all directives and calls ParseStrings() if need be,
which parses the source for string-arguments.
If ParseStrings() encounters a '[NEEDS' or 'INCLUDE' directive,
it will call DoNeeds(), which will create enough room for the
file to be included and read the actual file. If DoNeeds() fails,
it will set the ”ErrNo” member of the header accordingly and
exit. MakeRoom() just moves the last part of the source to the
end of the reallocated space. Since all variables are adjusted
accordingly, ParseText() will pick up parsing after the '[NEEDS'
or 'INCLUDE' directive. It never knows the difference.
After parsing ParseText() returns the initial size of the Code
Segment (the number of words). The function ParseText() sets
three important variables:
[float Table:
+----------------+------------------------------+
| Variable | Content |
+----------------+------------------------------+
+----------------+------------------------------+
| SourceStrings | The number of source-words |
+----------------+------------------------------+
| SourceWords | The size of the Code Segment |
+----------------+------------------------------+
| SourceSymbols | The size of the symbol-table |
+----------------+------------------------------+
[Senseless!!!
comp_4th() variables
]
]
It is imperative to know these numbers since the 4tH-environment
has to size its resources. No resizing is required until all
compilation is done. All allocated resources should be large
enough to contain the resulting compilant, so extending these
resources should never be necessary. In the end, they are only
shrunk to their actual sizes. Now we can start to allocate the
compiler-resources:
• Code Segment
• Symboltable
• Controlstack
This is done by simply calling AllocResource(). Note that it is
not necessary to allocate the String Segment. Strings remain in
memory already allocated to the source and are just shifted to
the front.
Then compilation can begin. First, the variable "Cursor" is set
to the beginning of the source. Then every call to GetNextWord()
sets the variable "CurrentWord" to point to the next source-word.
If there are no more source-words, "CurrentWord" is set to NULL
and compilation is terminated.
Now compilation really gets off. It is important to know that not
all words are created equal. There are five kinds of words:
• Immediate words
• Words
• Constants
• Symbols
• Numbers
So, there are five distinct functions which handle these words:
• Immediate words are compiled by GetImmediate()
• Words are compiled by GetWord()
• Constants are compiled by GetConstant()
• Symbols are compiled by GetSymbol()
• Numbers are compiled by comp_4th()
If the first four functions fail there is one more chance that
this is a valid source-word: it might be a number. So, the
source-word is converted to a number in the current radix. If
this works, the number is compiled. If it doesn't, it isn't a
valid source-word and the member "ErrNo" is set.
Compiling is done by a single function called CompileWord(). You
just provide the token and its argument and CompileWord() takes
care of the rest.
When all compiling is done, we can discard the symboltable and
the controlstack. This is done by calling the function
FreeResource(). It is called by ReallocSegs(), which shrinks the
Code Segment and the String Segment to their actual sizes, and
AbortCompile(), which shuts the compiler down in case of an
error.
If an error occurred before any compiling took place
AbortCompile() also discards the Code Segment and the String
Segment, thus returning a bare H-code header. As you already
know, the error-code is stored in the member "ErrNo" of the
H-code header.
If the error occurred after words have been compiled, this
partial compilant is not discarded. Instead the member "Reliable"
is set to FALSE, indicating that the compilant cannot be run or
saved. It can be decompiled, thus enabling the user to track the
error.
In the next sections we will take a closer look at the three main
tables in comp_4th(), which contain all of 4tHs built-in words.
22.3 Adding a constant
Adding a constant is very easy. You only have to update a single
table in comp_4th(). The table with constants, which is embedded
in GetConstant() has four members:
1. Length byte
2. Name
3. Type
4. Value
The length-byte is used to quickly scan the table. All words are
skipped until it reaches the constants with the same length. Then
it starts to compare the names.
What happens then depends on the argument "mode". When it equals
W_EXEC, the constant is compiled (a literal with the value as
argument). Otherwise, only the index in the table is returned,
pointing to the constant we just found. This enables mere
searches in the table. When a name isn't found at all, it returns
MISSING.
Say, we want to add a constant called "TWENTY". At least, we know
its name: "TWENTY". The name "TWENTY" is six characters long. And
of course, we want to compile the number "20" each time it is
referenced in the source. To make comp_4th() compile a number, we
need the token LITERAL. The four members are:
1. Length byte = 6
2. Name = "TWENTY"
3. Type = LITERAL
4. Value = 20
Since every constant is a signed 32 bits number, we add the
modifier 'L' to the "20". So the complete line we have to add to
the table reads:
{ 6, "TWENTY", LITERAL, 20L },
Now we have to insert this line into the table. Note that
constants with the same length have to be grouped together:
{ 5, "INPUT", LITERAL, F4_READ },
{ 6, "OUTPUT", LITERAL, F4_WRITE },
{ 6, "APPEND", LITERAL, F4_APPND },
{ 7, "(ERROR)", LITERAL, CELL_MIN },
We decide to put our constant behind "APPEND". Of course, we
could have put it behind "INPUT" or "OUTPUT" as well:
{ 5, "INPUT", LITERAL, F4_READ },
{ 6, "OUTPUT", LITERAL, F4_WRITE },
{ 6, "APPEND", LITERAL, F4_APPND },
{ 6, "TWENTY", LITERAL, 20L },
{ 7, "(ERROR)", LITERAL, CELL_MIN },
Now recompile 4tH and run this simple program:
twenty . cr
It should compile without errors and print "20". That's all there
is to it! Not to difficult to begin with, huh?
22.4 Adding a word<sec:Adding-a-word>
Now for something a little more difficult. Let's say we want to
implement 'NIP'. Of course, 'NIP' is already available, but if
you compile this program:
1 2 nip
You will see that it actually compiles to:
[0] literal (1)
[1] literal (2)
[2] swap (0)
[3] drop (0)
So, 'NIP' is actually expanded to 'SWAP' and 'DROP'. That is
because 'NIP' can be defined as:
: nip swap drop ; ( n1 n2 -- n2)
It removes the number under the top of stack. We call this an
inline macro, which we will discuss later on. If you really want
to try out 'NIP' in the following example, you have to remove a
line from ImmedList[]:
{ 3, 0, 1, "NIP", "", DoNip },
Your compiler might complain about unused functions, but it will
work.
Inline macros are not the only way to add 'NIP' to 4tH. We can
also implement 'NIP' as a word. Most words can be found in the
function GetWord(). There is a single table, which is laid out
like the table of constants we encountered in the previous
section, so:
1. Length byte = 3
2. Name = "NIP"
But instead of the value of the constant, we have to add
something else. That something is called the token. The tokens
are defined in "cmds_4th.h". Let's have a look:
#define PAUSE 100
#define VECTOR 101
#define ENVIRON 102
#define PLITERAL 103
#define FSEEK 104
#define FTELL 105
/* ranges */
#define LastWord4th FTELL
#define LastMsg4th M4CABORT
Well, in fact you can place a new token anywhere, but then you
have to renumber all other tokens. The easiest way is to place it
after the last token you defined, which in this case is "FTELL".
The token "FTELL" has the number "105". Well, the next number is
"106" and that is the number "NIP" is going to get. So we add:
#define PAUSE 100
#define VECTOR 101
#define ENVIRON 102
#define PLITERAL 103
#define FSEEK 104
#define FTELL 105
#define NIP 106
/* ranges */
#define LastWord4th FTELL
#define LastMsg4th M4CABORT
What part don't you understand? But we're not ready yet. If you
look at the #define below the entry we just made, you will see
that it says that the last word in 4tH is "FTELL". That is
incorrect now. We've just added "NIP". Fixing it is very easy.
Just change the line to:
#define LastWord4tH NIP
We're done now with "cmds_4th.h". Now we have change
"comp_4th.c", so that the compiler can recognize and compile
"NIP". Now we can complete the entry for GetWord():
1. Length byte = 3
2. Name = "NIP"
3. Token = NIP
So the complete entry reads:
{ 3, "NIP", NIP },
And like we did with our constant "TWENTY", we have to give it a
place inside the table between all other 3-letter words. We
decided that pasting "NIP" between "USE" and "SEEK" would be a
good idea (but there are plenty of other places too):
{ 3, "HEX", HEX },
{ 3, "USE", USE },
{ 3, "NIP", NIP },
{ 4, "SEEK", FSEEK },
{ 4, "TELL", FTELL },
{ 3, "HEX", HEX },
Are we done now? No. The compiler will recognize and compile
"NIP", but what does it do? That behavior will have to be defined
in "exec_4th.c", but we'll discuss that in the next section.
22.5 A closer look at exec_4th()
Since 4tH has a segmented structure, there are special words for
each segment, e.g. "C!" for the Character Segment and "!" for the
Variable Area. But when one wants a Virtual Machine that checks
every access, the parameters of these words need to be checked.
There are very few words that access the Variable Area, so these
are checked within the code for the token itself. Others, like
the ones accessing the data stack, are used more often. So,
special functions were created that allow or check access to
those areas. There are thirteen functions you should know about.
They form the basic API.
[float Table:
+-------------------------------+-------------------------------------------------+
| Function/Macro | Description |
+-------------------------------+-------------------------------------------------+
+-------------------------------+-------------------------------------------------+
| DPOP | Gets an item from the data stack |
+-------------------------------+-------------------------------------------------+
| DPUSH (cell) | Puts an item on the data stack |
+-------------------------------+-------------------------------------------------+
| DFREE (cell) | Checks amount of free space on the data stack |
+-------------------------------+-------------------------------------------------+
| DSIZE (cell) | Checks the number of items on the data stack |
+-------------------------------+-------------------------------------------------+
| DS (cell) | Random acces item on data stack |
+-------------------------------+-------------------------------------------------+
| RPOP | Gets an item from the return stack |
+-------------------------------+-------------------------------------------------+
| RPUSH (cell) | Puts an item on the return stack |
+-------------------------------+-------------------------------------------------+
| RFREE (cell) | Checks amount of free space on the return stack |
+-------------------------------+-------------------------------------------------+
| RSIZE (cell) | Checks the number of items on the return stack |
+-------------------------------+-------------------------------------------------+
| RS (cell) | Random acces item on return stack |
+-------------------------------+-------------------------------------------------+
| unit fetch (cell) | Gets a character from the Character Segment |
+-------------------------------+-------------------------------------------------+
| void store (cell, unit) | Puts a character in the Character Segment |
+-------------------------------+-------------------------------------------------+
| cell toPAD (char*) | Puts a string in the PAD |
+-------------------------------+-------------------------------------------------+
| char* toCstring (cell, cell) | Puts a addr/count string in the PAD |
+-------------------------------+-------------------------------------------------+
[Senseless!!!
exec_4th() basic API
]
]
We strongly recommend you use these functions when accessing any
of these segments. But you'll probably need more than functions
to create your code. What about variables?
Well, of course there is a host of variables you can use, but
there are three variables that are used more frequently. They are
called "a", "b" and "c" and are of type cell. Now, what did NIP
do?
NIP ( n1 n2 -- n2)
That means the first cell is taken from the data stack and saved,
then the second cell is taken from the data stack and dropped and
finally the first cell is replaced on the data stack. Note there
need to be at least two items on the stack to make it work. Since
there will be one item less on the stack after the operation, we
don't need to check whether there is enough space. So this will
do the trick:
DSIZE (2);
a = DPOP;
DDROP;
DPUSH (a);
There is even a faster way to do it. This implementation uses the
DS() macro which allows direct access to the datastack:
DSIZE (2);
DS (2) = DS (1);
DDROP;
DS(1) equals ”Top of stack” and DS(2) equals ”Second of stack”.
Note that the DS() macro doesn't adjust the stack pointer. That
is where DDROP comes in. It discards the superfluous item on the
stack. Now we add a label and a "break" (which is necessary in a
switch()):
case (NIP): DSIZE (2);
DS (2) = DS (1);
DDROP;
break;
Although this is slightly more difficult than the previous API,
this implementation is much, much faster. So, now we're finally
done! We can compile the whole thing and test our new command:
2 3 NIP . cr
Can we? No, there is one more thing we have to do. Your source
will compile OK and execute OK, but it is nowhere to be found
when we decompile it. Does that mean we have to edit dump_4th()?
Wrong again, but we will see that in the following section.
Before we leave, we'll take a look at another word, OVER. This
once differs from NIP, since it leaves more items than it
consumes. If we wouldn't take any precautions, we might run into
serious trouble. So, what does OVER do?
OVER ( n1 n2 -- n1 n2 n1)
'OVER' requires two items and leaves three items on the stack.
That means we need 3 - 2 = 1 extra item on the stack:
DSIZE (2);
DFREE (1);
a = DS (2);
DPUSH (a);
Note we must use DPUSH() to adjust the stack pointer. Of course,
there are macros that manipulate the return stack in a similar
way. One tip: be careful with nesting macros, since you might not
get what you want.
Strings are quite another ballgame. In 4tH these are usually
address/count strings and consequently incompatible with C, since
they are not terminated. Strings in the String Segment are not
accessible by a 4tH programmer at all. In order to solve these
problems, the PAD was created. The PAD is essentially a circular
string buffer where temporary strings are stored. The address and
count are usually returned to the 4tH programmer enabling him to
manipulate these strings. At the same time you can be assured
that they are terminated and C-compatible.
There are two API functions which allow you to access the PAD,
toPAD() and toCstring(). The first one allows you to copy a C
string to the PAD. It puts the address on the stack and returns
the count. The second one allows you to copy an address/count
string to the PAD. It returns a pointer to a C string. In
exec_4th() there are two C string pointers which you can use, p
and q. A little example: suppose there is an address/count string
on the stack which you need to access:
DSIZE (2);
a = DPOP;
b = DPOP;
p = toCstring (b, a);
Note that the count resides on the top of the stack and is popped
first. The address comes after that. You can use toPAD() to copy
any C string to the PAD, e.g.
DFREE (2);
DPUSH (toPAD ("Hello world!"));
Note that toPAD() already left the address on the stack, so you
only need to push the count it returns.
22.6 A first look at name_4th()<sec:A-first-look>
May be "name_4th.c" looks like a function, but it definitely
isn't. It is a global array that is as global as globals can get.
You can refer to it in any program that uses the 4tH library
without defining it first. It contains the names of all the
tokens, so when we decompile, the tokens get a readable name:
#include "4th.h"
#include "cmds_4th.h"
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char** argv)
{
puts (name_4th [SWAP]);
return (EXIT_SUCCESS);
}
This program will print "swap". Yes, you can use the token-code
as an index to this array. We can't make it any easier than that!
Now you understand that we can't do the same thing for "NIP",
since we didn't add that one. In fact, any program trying it will
either print garbage or crash. Let's take a look at the last few
lines of name_4th[]:
"environ", "+literal", "seek", "tell"
};
Yeah, we've seen that before. All entries are ordered in the same
way as in "cmds_4th.h". Since "NIP" was added at the end of
"cmds_4th.h", we have to do the very same thing here. Don't
forget the comma after "TELL"
"environ", "+literal", "seek", "tell", "nip"
};
Now we are finally done. We can recompile the library (and the
compiler) and we can use "NIP" like any other 4tH word.
22.7 <sec:Extending-the-compiler>Extending the compiler
So far we've only added words that are directly compiled into a
token/argument pair. If that was the way all 4tH words worked, we
would never have branches, variables or other things that make up
a language. In fact, you would never construct this complex
architecture, since there are easier ways to achieve the same
functionality.
The secret lies in the last major function of comp_4th() we have
discussed, which is called GetImmediate(). When you look at it
for the first time, it looks quite like GetConstant() and
GetWord(). The associated table ImmedList[] has a length-byte and
a name but instead of a token or a value the last field is a
pointer to a function.
You'll find these functions above GetImmediate() and they all
start with Do..(). In fact, they are the icing on the cake. They
make 4tH a compiler, since they allow non-linear compiling. That
includes:
• Branching
• Comments
• Allocation of variables
• String handling
• Assertions
• Constants
If there is something you want to do that you cannot define in a
single token/argument, this is the place where you have to be.
But before you are starting to make new functions, note there are
many functions that you can use.
[float Table:
+-------------------------------------------------------------------------+
| Branching |
+------------------+------------------------------------------------------+
| MarkLink() | Adds a link to the controlstack |
+------------------+------------------------------------------------------+
| MakeLink() | Makes a back-link |
+------------------+------------------------------------------------------+
| PairLink() | Retrieves a link from the controlstack |
+------------------+------------------------------------------------------+
| CompileMark() | Compiles a token and adds a link to the controlstack |
+------------------+------------------------------------------------------+
| Symboltable |
+-------------------------------------------------------------------------+
| AddSymbol() | Adds a symbol to the symboltable |
+------------------+------------------------------------------------------+
| MakeSymbol() | Adds the current word as a symbol to the symboltable |
+------------------+------------------------------------------------------+
| GetSymbol() | Retrieves an entry from the symboltable |
+------------------+------------------------------------------------------+
| Parsing |
+-------------------------------------------------------------------------+
| GetNextWord() | Gets next word in the source |
+------------------+------------------------------------------------------+
| DecodeSymbol() | Decodes a symbol from source |
+------------------+------------------------------------------------------+
| DecodeLiteral() | Gets a compiled literal expression |
+------------------+------------------------------------------------------+
| DecodeOperand() | Gets a compiled expression |
+------------------+------------------------------------------------------+
| DecodeWord() | Gets a name from source |
+------------------+------------------------------------------------------+
| DecodeName() | Gets a previously delared name from source |
+------------------+------------------------------------------------------+
| SkipSource() | Discards all source between two labels |
+------------------+------------------------------------------------------+
| Compiling |
+-------------------------------------------------------------------------+
| CompileWord() | Compiles a token and its argument |
+------------------+------------------------------------------------------+
| InlineWords() | Compiles a sequence of tokens without arguments |
+------------------+------------------------------------------------------+
| Strings |
+-------------------------------------------------------------------------+
| MoveString() | Moves a string inside the String Segment |
+------------------+------------------------------------------------------+
| CompileString() | Compiles a token and its associated string |
+------------------+------------------------------------------------------+
[Senseless!!!
comp_4th() basic API
]
]
E.g. when you want to compile a word, you don't have to bother
yourself with error-checking or other 4tH-internals. Just make a
call to CompileWord():
CompileWord (NIP, 0);
Extending the compiler is quite easy. We will illustrate that by
using a very simple example. We have some compiler-words that
handle the radix at compile time. They are:
[BINARY]
[OCTAL]
[DECIMAL]
[HEX]
Now we want to add a new one called "[SEXTAL]", which sets the
radix to six. The radix at compile-time is handled by a single
variable called "Base". First we have to make a function, which
sets "Base". We call it DoSextal(). Note that this function
cannot receive or return any values, like all Do..() functions:
#ifndef ARCHAIC
static void DoSextal (void)
#else
static void DoSextal ()
#endif
{
Base = 6;
}
Just place it anywhere before ImmedList[] and you're safe. Now we
have to make it work. Like the other Get..() functions, there is
a table called ImmedList[] which drives this behaviour, so let's
get started:
1. Length byte = 8
2. Symboltable entries = 0[footnote:
See section [sec:Using-the-symbol]
]
3. Additional tokens = -1[footnote:
See section [sec:Sizing-the-Code]
]
4. Delimiter = ""[footnote:
See section [sec:Adding-string-words]
]
5. Name = "[SEXTAL]"
6. Function = DoSextal
Now let's update that table:
{ 7, 0, -1, "ALIGNED", "", DoDummy },
{ 7, 0, 0, "RECURSE", "", DoRecurse },
{ 8, 0, -1, "[SEXTAL]", "", DoSextal },
{ 8, 1, -3, "CONSTANT", "", DoConstant },
{ 8, 1, -2, "VARIABLE", "", DoVariable },
You're done now! Recompile the program and "[SEXTAL]" has become
a part of 4tH. Note that not all words within ImmedList[] can be
defined that easily. If special tokens are required, you might
have to edit other files as well.
22.8 Making aliases
Sometimes you need two different names that do the same thing.
Well-known examples are "CHAR" and "[CHAR]" or "I" and "R@". How
can this be done?
In fact, it is very simple. Think about it. 4tHs vocabulary is
stored in tables. These tables link a name with some kind of
behavior. So we have to make two different names that are linked
to the same thing. Take a look at this excerpt of ImmedList[]:
{ 5, 0, -1, "ALIGN", "", DoDummy },
{ 5, 0, -1, "CELLS", "", DoDummy },
{ 5, 0, -1, "DOES>", "", DoDummy },
{ 5, 0, -1, "CHARS", "", DoDummy },
{ 5, 1, -2, "TABLE", "", DoCreate },
{ 6, 1, -2, "CREATE", "", DoCreate },
{ 6, 0, -1, "[THEN]", "", DoDummy },
You might have noticed a few aliases.
[float Table:
+-------------------------------------+-------------+
| Word | Compiled by |
+-------------------------------------+-------------+
+-------------------------------------+-------------+
| [THEN], ALIGN, CELLS, DOES>, CHARS | DoDummy() |
+-------------------------------------+-------------+
| TABLE, CREATE | DoCreate() |
+-------------------------------------+-------------+
[Senseless!!!
Examples of aliases
]
]
That means that if you write your 4tH program you can choose
between "TABLE" and "CREATE". It doesn't matter, it will compile
to the same thing. But we have to add to that some aliases are
created because in Forth they do have different meanings, like
"[CHAR]" and "CHAR". Read the glossary for details.
We will show you another example. This one comes from GetWord():
{ 5, "CHAR+", INC },
{ 5, "CHAR-", DEC },
{ 5, "CELL+", INC },
{ 5, "CELL-", DEC },
You see that "CHAR+" and "CELL+" do two very different things. At
least in Forth. Within 4tH the smallest addressunit is always an
element of that particular segment, thus one. So in 4tH these
words are aliases and will compile to the very same code.
22.9 Giving a name to an application variable
We already learned that you can transfer variables to 4tH:
Result = exec_4th (Program, 0, NULL, 12, (cell) 31, february,
(cell) 31, (cell) 30, (cell) 31, (cell) 30, (cell) 31, (cell) 31,
(cell) 30, (cell) 31, (cell) 30, (cell) 31);
These kind of variables are called "application variables". Of
course, you don't have to use the same variables every time you
call exec_4th(), but if you do it may be a good idea to give them
a significant name. That makes it a lot easier for a 4tH
application programmer to reference your variables. Like
everything in 4tH, that is very easy too.
If we take a look at "cmds_4th.h" you will see a C-constant named
"VAR4TH". This constant has two functions. First, it shows how
many internal 4tH variables there are. Second, it is an index to
the first application variable, so 'APP' is defined as "VAR4TH".
That means that:
app 0 th
Is the very first application variable and:
app 1 th
Is the second application variable. You can do the same. Let's
say you have three application variables, which contain the
document-number, the page-number and the line-number. You'd like
to call them "&DOC", "&PAGE" and "&LINE". The ampersands are not
really necessary, but we add them in order to identify the
application specific words. To make it work you have to call
exec_4th() by:
Result = exec_4th (Program, 0, NULL, 3, (cell) Doc, (cell) Page,
(cell) Line);
Now these are the mappings.
[float Table:
+-------------+----------------+
| C variable | 4tH expression |
+-------------+----------------+
+-------------+----------------+
| Doc | app 0 th |
+-------------+----------------+
| Page | app 1 th |
+-------------+----------------+
| Line | app 2 th |
+-------------+----------------+
[Senseless!!!
Mapping between 4tH and C variables
]
]
Now all we have to do is add constants that are equivalent to
these addresses. As we've seen before, we can do that by
modifying comp_4th(). That is GetConstant() to be exact:
{ 4, "&DOC", LITERAL, VAR4TH+0 },
{ 5, "&PAGE", LITERAL, VAR4TH+1 },
{ 5, "&LINE", LITERAL, VAR4TH+2 },
That's all! You can now refer to these variables with their
proper names.
[float Table:
+-------------+-----------------+--------------+
| C variable | 4tH expression | 4tH variable |
+-------------+-----------------+--------------+
+-------------+-----------------+--------------+
| Doc | app 0 th | &doc |
+-------------+-----------------+--------------+
| Page | app 1 th | &page |
+-------------+-----------------+--------------+
| Line | app 2 th | &line |
+-------------+-----------------+--------------+
[Senseless!!!
Mapping between 4tH and C variable names
]
]
Note that if you use this technique you are bound to calling
exec_4th() with these arguments in this order! Failure to do so
may cause unpredictable results (but no crashes of course).
22.10 Adding new variables
In standard 4tH there are five environment variables, HI, FIRST,
LAST, CIN and COUT. There are also five predefined variables,
'>IN', 'BASE', 'OUT and the variable pair 'SOURCE'. These
variables are initialized by exec_4th(), so their initial value
should be known by then.
Adding new variables is not difficult. We're going to make a
variable that contains 4tHs release number, called "VERSION".
First take a look at "cmds_4th.h". It contains a #define called
"VAR4TH":
/* variables and environs */
#define SYS4TH 3
#define VAR4TH 10
#define ENV4TH 5
Now remember that number behind "VAR4TH". You will need it later.
Then increment it:
/* variables and environs */
#define SYS4TH 3
#define VAR4TH 11
#define ENV4TH 5
If you would have preferred to make 'VERSION' an environment, you
should also have incremented "ENV4TH". But we assume you'll allow
the variable to be overwritten. Now add a symbolic value for the
variable. Just append it to the list and increment the number:
#define VBASE 5
#define VIN 6
#define VOUT 7
#define VTIB 8
#define VTIBS 9
#define VVERS 10
Or if you prefer it to make an environment variable, add it to
the environment variable list:
#define VHI 0
#define VFIRST 1
#define VLAST 2
#define VCIN 3
#define VCOUT 4
#define VVERS 5
#define VBASE 6
#define VIN 7
#define VOUT 8
#define VTIB 9
#define VTIBS 10
That's all. Now save "cmds_4th.h" and load comp_4th() in your
editor. This stage is very much like adding a name to an
application variable. We simply define a constant that contains
the address of our new internal variable. You will remember how
we add a constant. Right, we add an entry to the GetConstant()
table:
{ 7, "VERSION", LITERAL, VVERS },
Making it an environment variable is very easy too: just replace
the 'LITERAL' token by an 'ENVIRON' token:
{ 7, "VERSION", ENVIRON, VVERS },
All we need to do now is to initialize the variable in
exec_4th(). Since it is a variable, it resides in the Variable
Area of the Integer Segment. The Integer Segment is just a large
array of unsigned longs.
The pointer "Stack" points to the beginning of the Integer
Segment, which is also the beginning of the Stack Area. The
pointer "Vars" points to the area that is assigned to 4tHs
variables. Our constant "VERSION" is an index to that array, so
the expression "Vars [VVERS]" is a valid reference to our
"VERSION" variable.
However, this indexed way of referencing is slower than a
pointer. Therefore, we have created pointers that reference these
frequently used variables:
cell *In; /* equivalent of forth >IN */
cell *Result; /* return value for apps */
Base = &(Vars [VBASE]); /* assign pointer to BASE */
In = &(Vars [VIN]); /* assign pointer to >IN */
You might have noticed the absence of "Base". Well, since it is
referenced elsewhere as well, this is a global variable. But
don't worry, there is no need to reference "VERSION" globally.
So, we need to define a pointer to a cell, assign it to "Vars
[3]" and initialize it:
Vars [VVERS] = Version4th; /* initialize it */
That is all! Any questions? Where does "Version4th" come from? It
is defined in "cmds_4th.h". Anybody else? Next subject, please.
22.11 Resizing the 4tH environment
You might come up with a situation that the stack isn't big
enough. Or that you want to give your programmers deeper nesting.
Or that 512 characters isn't just good enough for temporary
storage.
Relax! All these things can be changed with very little effort.
And after that, you just need to recompile 4tH like we've done
before.
There is a single file you need to edit, "cmds_4th.h". You will
find several easy to change #defines there.
/* compiler */
#define LINKSIZ 64
#define SYMLEN 16
/* interpreter */
#define STACKSIZ 512
#define TIBSIZ 256
#define PADSIZ 512
#define DOTSIZ 64
#define RNDMASK 32767
#define MAXDEVS 8
#define PIPEWAIT 102400L
You already know "VAR4TH", since we discussed that one earlier in
this document. Right, it determines the number of internal
variables! "LINKSIZ" determines the nesting depth. Nesting depth
has to do with the number of nested branches, e.g.
IF
IF
IF
THEN
THEN
THEN
Each 'IF' puts its address and a reference (I'm an IF) on the
flow control stack. Each 'THEN' takes an entry off the flow
control stack and takes the appropriate action. So, in the
current version of 4tH you can nest upto 64 consecutive
conditionals, before you get an error. You may increase or
decrease that number.
"SYMLEN" is the maximum length of any name you define, e.g. a
colon-definition, a constant, a variable. The default is 16,
which is enough to define a name like "multiplications". You can
define a longer name, but only the first fifteen characters will
be significant. You can increase the maximum number of
significant characters, but beware: this can take up a lot of
memory!
"STACKSIZ" is the combined size of both data and return stack.
This size will do for most applications, since it allows you a
combination of high usage of the data stack and low usage of the
return stack or vice versa. You might encounter a situation where
recursion forces you to resize the Stack Area. Decreasing is
possible too, of course, but at your own risk.
"TIBSIZ" is the size of the Terminal Input Buffer used by
'REFILL'. If you need 'REFILL' to accept longer lines than 256
characters and you don't want to allocate your own buffer, resize
it.
"PADSIZ" is the size of the scratch PAD, used to store temporary
strings. A part of the PAD is reserved to numbers. The size of
this area is determined by "DOTSIZ". The rest of PAD (PADSIZ -
DOTSIZ) is a circular string buffer. A bigger PAD will allow you
to store longer temporary strings that survive longer before
getting overwritten.
"RNDMASK" is used to truncate the value returned by rand(). Some
compilers return a 32 bit number and others a 16 bit number. In
order to maintain maximal compatibility across all platforms, 4tH
always returns a 16 bit value. You can fiddle around with it, but
you will compromise the portability of your 4tH programs.
"MAXDEVS" is the maximum number of I/O devices that 4tH can
manage. Note that two of them (STDIN, STDOUT) are already in use,
so you can open up to six additional devices concurrently.
Finally, " PIPEWAIT" is discussed in detail in the next section.
You will find there are other defines here too. Please, do not
change them. That just doesn't work. In fact, 4tH just won't work
properly anymore.
22.12 Tuning pipe failure detection
Pipes in 4tH are opened by the popen() function. This has one big
disadvantage. Although popen() is able to detect a failed fork(),
it is unable to detect whether the program was successfully
started or not. E.g. if the program cannot be found in the path
the pipe fails, although popen() has already reported it was
successful. In some cases this can have serious consequences.
After careful study we decided to monitor the process for a while
and then report success or failure. The default value works very
well on most modern systems, but with some systems it may be
neccesary to adjust it. This is the case when you experience one
of the following symptoms:
• Opening a pipe is slow; there is a long delay before 4tH
reports the pipe is successfully opened.
• 4tH reports that the pipe was successfully opened, but most of
the time this was not the case.
In that case, you have to adjust a #define in ”cmds_4th.h”. That
is a lot easier than you might think. We've developed a small
program to do that. It should be portable across most Unixes:
#include <stdio.h>
#include <limits.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
long TimeBadPipe (void)
{
FILE *p; /* filepointer to pipe */
long x; /* simple counter */
int s = 0; /* status of child */
p = popen ("nosuchprogram", "r"); /* perform a normal
popen() */
for (x = 0; x < INT_MAX && s == 0; x++) waitpid (-1, &s,
WNOHANG);
pclose (p); /* close the pipe */
return (x); /* return the count */
}
int main (int argc, char **argv)
{
int x; /* simple counter */
int now; /* return of TimeBadPipe()
*/
int sofar = 0; /* highest count */
int total = 0; /* total of all counts */
/* warn the user */
puts ("Doing 1000 iterations, wait..");
puts ("(This is going to be messy..)");
sleep (5); /* allow him to read the
message */
for (x = 0; x < 1000; x++) {
now = TimeBadPipe (); /* time a bad pipe */
if (now > sofar) sofar = now; /* adjust sofar */
total += now; /* add to total */
}
/* show the results */
printf ("\nAverage : %d\n", total / 1000);
printf ("Maximum : %d\n", sofar);
return (EXIT_SUCCESS);
}
If you run it, it will print something like this:
Doing 1000 iterations, wait..
(This is going to be messy..)
sh: nosuchprogram: command not found
sh: nosuchprogram: command not found
...
sh: nosuchprogram: command not found
sh: nosuchprogram: command not found
Average : 8685
Maximum : 41235
This means that on average the process had to be checked 8685
times, but at no occasion a process died after it had been
checked 41235 times. Run it several times, so you will get a good
impression of how your particular system behaves. Ignore
extremely high and extremely low values. Then take the highest
value that pops up several times and change ”cmds_4th.h”
appropriately:
#define PIPEWAIT 49152L
Note we rounded the value a little (we're a binary kind of guy)
and it doesn't have to be exact. Now you can safely use pipes on
your system and the result returned will assure you that the pipe
was actually successful opened and ready for use.
If this still doesn't work, you may have to adjust this #define
manually: decrease it if opening a pipe is slow, increase it if
4tH incorrectly reports a successfully opened pipe. If you use
MS-DOS, just forget all this. We don't provide any pipes there.
22.13 Adding new error messages
Adding new messages is quite simple. It requires not much more
than adding a #define and adding a string. You might have noticed
that every 4tH message has a mnemonic. Although this is not
required, it makes it much easier to read and thus maintain your
code.
This mnemonic is no longer than eight characters, all uppercase
and begins with "M4" (which stands for Message 4tH). Let's say
you've added the ANS-Forth floating point wordset and you want to
add the error message "Floating point exception". We'll do it the
easy way and just append the message at the end of the table.
First we have to come up with a mnemonic. We decide to use
"M4FLOATE". Now we start up our favorite editor and load "4th.h".
Then we look for the table with error mnemonics:
#define M4NOSTR 24
#define M4NULSTR 25
#define M4DUPNAM 26
#define M4CABORT 27
Now we simply add "M4FLOATE" to the end of the table. Since the
last message had code 27, we give our message code 28:
#define M4NOSTR 24
#define M4NULSTR 25
#define M4DUPNAM 26
#define M4CABORT 27
#define M4FLOATE 28
We can now save "4th.h". Now we have to add the message itself.
That is done by adding it to "errs_4th.c". That file just
contains an array of messages. Note that the messages are listed
in order of their codes:
"Unterminated string",
"Null string",
"Duplicate name",
"Compilation aborted"
};
If you change that order, your compiler might display the right
errorcode, but the wrong error message. Since our mnemonic comes
last, our message comes last:
"Unterminated string",
"Null string",
"Duplicate name",
"Compilation aborted"
"Floating point exception"
};
Don't forget adding a comma after the last message! If you don't
your compiler will certainly complain about that. Are we done
now. No, not quite yet. The 'THROW' routine wants to know which
codes are exceptions generated by the system and which one are
generated by the user. Why? Because user exceptions do not have
messages attached to them! We can change that in "cmds_4th.h".
/* ranges */
#define LastWord4th FTELL
#define LastMsg4th M4CABORT
Now it still points to the "duplicate name" error. We simply
change "LastMsg4th" to our mnemonic:
/* ranges */
#define LastWord4th FTELL
#define LastMsg4th M4FLOATE
We're done now! Note that this final step is not necessary when
you insert messages. Instead, you will have to renumber the table
in "4th.h". No two mnemonics may ever share the same error code,
remember that! If you don't keep the mnemonics, the errorcodes
and the messages properly synchronized you may get some pretty
strange error messages. Which is less than helpful.
22.14 Sizing the Code Segment<sec:Sizing-the-Code>
By default 4tH assumes a 1:1 relationship between a word in
source and a compiled word (in the Code Segment). When
ParseText() is called it will count the number of words in the
source. This number is later used to size the initial Code
Segment. This 1:1 relationship is not so strange as it may seem
at first, e.g.:
BL DROP
Will compile to:
[0] literal (32)
[1] drop (0)
Two words in source, two compiled words. But there are exceptions
too,
e.g.:
BL ,
Will compile to:
[0] , (32)
That is because ',' does not compile to anything, but changes the
previously compiled literal to an constant array element. Note
that ',' is an immediate word. In fact, all exceptions to this
1:1 relationship rule are immediate words! The vast majority of
4tH words obey this 'one on one' rule:
• All numbers and constants compile to literals
• All ordinary words compile to a word without argument
• All symbols compile to a word with argument
In a previous chapter we've created an immediate word called
"[SEXTAL]". When you take a closer look, you will see that it
just changes the base; it doesn't compile to anything. Still,
without the proper argument 4tH assumes it will compile a token
and reserves space in the Code Segment.
Can you prevent this? Yes, you can. There is a member in the
table of ImmedList[] which allows you to signal 4tH that it
shouldn't reserve space in the Code Segment for "[SEXTAL]". The
first field indicates the length of the keyword, the third
indicates the correction 4tH should make to the sizing of the
Code Segment when this keyword is encountered, the fourth is the
keyword itself and the last one is the C function that compiles
the word. The second and the fifth field will be discussed later.
{ 8, 1, -2, "VARIABLE", "", DoVariable },
{ 8, 0, -1, "[ASSERT]", "", DoAssert },
{ 8, 0, -1, "[BINARY]", "", DoBinary },
{ 9, 0, -1, "[DECIMAL]", "", DoDecimal },
Now, the correction we want to make is that 4tH should allocate
one word less in the Code Segment, since "[SEXTAL]" does not
compile to anything. One less means "-1". We can now change the
table accordingly:
{ 8, 1, -2, "VARIABLE", "", DoVariable },
{ 8, 0, -1, "[ASSERT]", "", DoAssert },
{ 8, 0, -1, "[BINARY]", "", DoBinary },
{ 8, 0, -1, "[SEXTAL]", "", DoSextal },
{ 9, 0, -1, "[DECIMAL]", "", DoDecimal },
That's all. We'll give to a few more examples.
E.g. 'VARIABLE' does not compile to anything either; it just
reserves space in the Variable Area. But 'VARIABLE' always comes
with a name, which doesn't compile to anything either. So we
should decrease the the number of words in the Code Segment by
two!
'CONSTANT' not only requires a name, but consumes a previously
compiled literal as well. Initially this literal allocates space
in the Code Segment, but it is gone after 'CONSTANT' has been
compiled. So we decrease the number of words in the Code Segment
by three!
'VALUE' is even more complicated. You can write something like:
10 value ten
But this will compile to:
[0] literal (10)
[0] to (0)
'VALUE' does not consume the previously compiled literal! But the
name does not compile to anything. 'VALUE' takes a value from the
Data Stack at run time, while 'CONSTANT', 'STRING' and 'ARRAY'
take a previously compiled literal at compile time. If you don't
believe us, check the glossary.
'VALUE' itself compiles to something, the literal is undisturbed,
only the name vanishes. That means only one word less, thus "-1".
22.15 Adding inline macros
We've already seen how we can add new words to 4tH. We add a
token and write the runtime. But this approach has a few
disadvantages. First, the number of tokens is limited. You can
use them, but once you run out of them, that's it. Second,
writing a runtime is a little complex if you only have limited
knowledge of C.
You can add as many inline macros as you want. From a user point
of view there is not much difference between an ordinary 4tH word
and an inline macro. The word is recognized by the compiler and
works as expected.
Inline macros are simply sequences of existing tokens. As we've
seen before, 'NIP' is implemented as an inline macro, so this
source:
1 2 nip drop
Will compile to:
[0] literal (1)
[1] literal (2)
[2] swap (0)
[3] drop (0)
[4] drop (0)
There are disadvantages to inline macros as well. Every time you
use 'NIP' it will expand to two words, so your Hcode will become
a little bigger. We recommend to limit inline macros to three
words. Second, an implementation using inline macros will make
the compiler less compact compared to an implementation using
tokens.
On the other hand, you only need to change the compiler when you
use an inline macro. No changes to the interpreter or the
decompiler will be necessary. Existing HX files will still run,
although some 4tH sources will need modification.
Let's go to business. How can we implement an inline macro. Let's
take 'NIP'. First we have to make an entry in the ImmedList[]
table.
{ 2, 1, -1, "TO", "", DoValue },
{ 2, 1, -1, "IS", "", DoIs },
{ 2, 1, -1, "AS", "", DoValue },
{ 2, 0, 0, "IF", "", DoIf },
{ 2, 0, 0, "DO", "", DoDo },
{ 2, 0, 0, "->", "", DoDummy },
{ 3, 0, 1, "2R>", "", DoTwoRGet },
{ 3, 0, 1, "2>R", "", DoTwoRPut },
{ 3, 0, 3, "2R@", "", DoTwoRCopy },
Since 'NIP' is defined by:
: nip swap drop ;
It will compile to:
swap (0)
drop (0)
Which is one token more than the parser would expect. So the
value of the fourth field is "1". But this will only reserve
space for 'NIP'. The word won't be recognized by the compiler
yet. In order to do that we have to make a word that compiles the
tokens for 'NIP'. You can only do that with an "immediate" word:
#ifndef ARCHAIC
static void DoNip (void)
#else
static void DoNip ()
#endif
{
CompileWord (SWAP, 0L);
CompileWord (DROP, 0L);
}
This will compile 'SWAP' and 'DROP' into the compilant. Now we
have to make an entry in ImmedList[] to link this function to the
name "NIP":
{ 2, 1, -1, "TO", "", DoValue },
{ 2, 1, -1, "IS", "", DoIs },
{ 2, 1, -1, "AS", "", DoValue },
{ 2, 0, 0, "IF", "", DoIf },
{ 2, 0, 0, "DO", "", DoDo },
{ 2, 0, 0, "->", "", DoDummy },
{ 3, 0, 1, "2R>", "", DoTwoRGet },
{ 3, 0, 1, "2>R", "", DoTwoRPut },
{ 3, 0, 3, "2R@", "", DoTwoRCopy },
{ 3, 0, 1, "NIP", "", DoNip },
That's all! Since 'NIP' uses existing tokens, the compiler can
handle it all by itself. There is no need to write runtime code
or define tokens. All the burden is put on the compiler.
22.16 Adding string words<sec:Adding-string-words>
We've already seen that some words in 4tH have a name attached to
them, like 'STRING', 'CREATE' or 'VARIABLE'. Since all these
names are delimited by whitespace (like any other 4tH word),
there is no need for special code.
Some words have special strings attached to them, like '."', '('
or '\'. These string are not delimited by whitespace, so they
need special treatment. It's even more complex: each word has a
different delimiter. '."' is delimited by '"', '(' is delimited
by ')', and '\' is delimited by an end-of-line marker.
In this section we're going to explain how we added '."', since
it's the most complex string word. Other string words like '('
are handled by the compiler only.
The first step to adding a string word is letting the compiler
know, what delimiter is used. We do that by modifying
ImmedList[]:
{ 2, 0, -2, "#!", EOL, DoComment },
{ 2, 0, -1, ",\"", "\"", DoCommaQuote },
{ 2, 0, -1, ",|", "|", DoCommaQuote },
{ 2, 0, -1, ".\"", "\"", DoDotQuote },
{ 2, 0, -1, ".(", ")", DoDotQuote },
{ 2, 0, 1, ">=", "", DoGreaterEqual },
{ 2, 0, 1, "<=", "", DoLessEqual },
{ 2, 0, -1, "S\"", "\"", DoSQuote },
{ 2, 0, -1, "S|", "|", DoSQuote },
The fifth field tells the parser whether this word requires a
special delimiter. Yes, '."' is delimited by '"', so we enter a
quote in the fifth field. If you enter an empty string, the
parser assumes the word isn't a string word at all.
Now, what will the parser do when it encounters '."'? It will
find an entry in ImmedList[] for '."'. It will see that this is a
string word. Then it will make a call to ParseString() to find
the delimiter and flag everything in between as a word. Which
means that if you call GetNextWord(), you will get the entire
string and not just the next word, e.g.:
: hello ." Hello world" cr ;
Will be parsed as:
GetNextWord (): :
GetNextWord (): hello
GetNextWord (): ."
GetNextWord (): Hello world
GetNextWord (): cr
GetNextWord (): ;
You will notice that there are some words that are delimited by
whitespace, e.g. 'CHAR'. Why is that? Isn't every word delimited
by whitespace? Yes, it is. But note that every word, which is
parsed by ParseText() is also checked by ParseDirectives(). This
expression would cause problems:
CHAR (
After 'CHAR' is parsed by ParseText(), '(' follows and is
recognized by ParseDirectives() to be the start of a comment. To
prevent this, we let the string following 'CHAR' be parsed by
ParseString(). By the time ParseText() regains control, the
character following 'CHAR' is already parsed and can cause no
more problems. In short, if an expression like:
word (
or
word \
is valid, let it be parsed by ParseString() by making an entry in
the delimiter field of the ImmedList[] table. If not, don't.
Next, we have to develop a word that compiles '."'. Now, how can
we compile '."'? First, we have to get the string and move it to
the String Segment. We can do that by calling GetNextWord()
manually, but then we have to check for NULL-pointers. It is much
easier to call DecodeWord() which sets the ErrNo member
automatically when an error occurs.
DecodeWord() takes one argument, which is the error code it
should set the ErrNo member to. It returns TRUE if GetNextWord()
was called successfully. "CurrentWord" now points to the string
after '."'.
Then we have to move the string to the String Segment.
MoveString() does just that. It expects "CurrentWord" to point to
the string that has to be moved. It returns a number. We'll need
that when we design the runtime code.
There is no token or combination of tokens for printing strings.
So this one will need a token of its own. We'll call it "PRINT"
for the time being. Now, we got all components.
• The string can be parsed
• We can move it to the String Segment
• We can compile a token and an argument
This is the code for DoDotQuote():
#ifndef ARCHAIC
static void DoDotQuote (void)
#else
static void DoDotQuote ()
#endif
{
CompileString (PRINT);
}
We aren't done yet. We still have to link the string '."' to this
routine by adding an entry to ImmedList[]:
{ 2, 0, -1, ",\"", "\"", DoCommaQuote },
{ 2, 0, -1, ",|", "|", DoCommaQuote },
{ 2, 0, -1, ".\"", "\"", DoDotQuote },
{ 2, 0, -1, ".(", ")", DoDotQuote },
Now we can save comp_4th() and get on with the next file.
Remember, we still got to add a token. As you will probably know,
we do that in "cmds_4th.h":
#define NOOP 0
#define CELLD 0
#define EXECUTE 1
#define CR 2
#define SPACES 3
#define EMIT 4
#define PRINT 5
#define DOT 6
#define FETCH 7
And of course, we have to add a name to "name_4th.c", so it can
be decompiled properly:
char *name_4th [] = {
",", "execute", "cr", "spaces", "emit",
".\"",
Are we done yet? Not by a long shot. We have created a word with
an argument, which is the offset of the string in the String
Segment. That requires some special techniques. But we'll go into
that in the next section.
22.17 <sec:Adding-words-with>Adding words with arguments
The very first thing you have to do is to make sure that your
code can be saved and loaded again. Words that only consist of a
token are saved without the argument. That reduces the size of
the HX file. If you want to save the argument you have to add a
line to both load_4th() and save_4th():
case (LITERAL):
case (PRINT):
case (BRANCH):
case (BRANCH0):
Now we have to add code to exec_4th() in order to execute '."'.
The first problem we encounter is: how do we access the argument?
Accessing an argument is quite an expression:
Object->CodeSeg [Object->ErrLine].Value
In which:
Object = Hcode pointer
CodeSeg = Member of Hcode, pointing to the Code Segment
ErrLine = Member of Hcode, pointing to the current word
Value = Member of word, holding the argument
In plain English it means: give me the argument of the currently
executed word in the Code Segment. But we can also make our lives
a lot easier by using this macro:
OPERAND
But this is only half the problem. How can we access the String
Segment where the stringconstant is stored? We made a pretty
table on that subject.
[float Table:
+------------+---------------------------------+-------+
| Datatype | Expression | Type |
+------------+---------------------------------+-------+
+------------+---------------------------------+-------+
| String | Object->StringSeg [{cell}] | char |
+------------+---------------------------------+-------+
| String | Object->StringSeg + {cell} | char* |
+------------+---------------------------------+-------+
| Character | Object->UnitSeg [{cell}] | char |
+------------+---------------------------------+-------+
| Character | Object->UnitSeg + {cell} | char* |
+------------+---------------------------------+-------+
| Variable | Vars [Object->Offset + {cell}] | cell |
+------------+---------------------------------+-------+
| Code | Object->CodeSeg [{cell}] | dict |
+------------+---------------------------------+-------+
[Senseless!!!
Accessing 4tH data from C
]
]
Most of the time it is more convenient to use functions to access
those segments instead of addressing them directly:
[float Table:
+--------------------+-------------------+------------------------+
| Area | Fetch | Store |
+--------------------+-------------------+------------------------+
+--------------------+-------------------+------------------------+
| Data Stack | DPOP | DPUSH (value) |
+--------------------+-------------------+------------------------+
| Return Stack | RPOP | RPUSH (value) |
+--------------------+-------------------+------------------------+
| Character Segment | fetch (location) | store (location, char) |
+--------------------+-------------------+------------------------+
[Senseless!!!
exec_4th() data access API
]
]
You might consider using other functions too if certain datatypes
are accessed more frequently.
Back to '."'. We have to access the String Segment for this one.
Since all output is channelled through emit(), we have to convert
the string to "units", which are unsigned characters. We could
use the expression "Object->StringSeg [{arg}]", but that would be
slower than pointer access on most systems. We decide to use "p",
which is a temporary stringpointer:
case (PRINT): for (p = Object->StringSeg + (unsigned) OPERAND;
*p; p++)
emit ((unit) *p);
break;
We assign "p" to a pointer to the string (Object->StringSeg +
{cell}). We check for null-characters (*p). If it is not a
null-character, we 'EMIT' it (emit (*p)) and advance the pointer
(p++) before entering the loop again.
Now we are done. Let's do something more complicated now, like
adding conditionals.
22.18 Adding conditionals
Basically, there are ten branch-instructions in 4tH:
1. BRANCH, which unconditionally branches to an address in the
Code Segment.
2. 0BRANCH, which branches to an address if the Code Segment if
the top of the Data Stack is zero.
3. CALL, which unconditionally branches to an address in the Code
Segment, throwing its origin on the Return Stack.
4. EXIT, which unconditionally branches to an address in the Code
Segment, which is taken from the top of the Return Stack.
5. VECTOR, which unconditionally branches to an address in the
Code Segment, which is taken from the contents of a variable,
throwing its origin on the Return Stack.
6. EXECUTE, which unconditionally branches to an address in the
Code Segment, which is taken from the top of the Data Stack,
throwing its origin on the Return Stack.
7. CATCH, which unconditionally branches to an address in the
Code Segment, which is taken from the top of the Data Stack,
throwing the data stack pointer, the previous handler and its
origin on the Return Stack.
8. LOOP, which branches to an address in the Code Segment if the
top of the Return Stack is less than value below it.
9. +LOOP, which branches to an address in the Code Segment if the
top of the Return Stack plus the top of the Data Stack is not
equal to the value below the top of the Return Stack.
10. ?DO, which branches to an address in the Code Segment if the
top of the Return Stack plus the top of the Data Stack is equal
to the value below the top of the Return Stack.
The first four are the most common and the most useful ones.
Together, they control your entire program. But how do they know
where to branch to? There is no instruction like 'BRANCH'. And
where is 'DO'?
Well, 'DO' doesn't do any branching. It just puts the loop
parameters on the Return stack. And as for 'BRANCH', this is how
it works:
IF something ELSE other thing THEN
This is an expression we are very familiar with. We pronounce it
as:
"If TOS is non-zero then something is executed."
This is not entirely true. In fact, it is:
"If TOS is zero then branch after 'ELSE'"
Which in effect results in the execution of "something". But when
"something" has executed, it has to branch after the "other
thing". Unconditionally, that is. 'THEN' does nothing, except
serve as a marker for the branch. It doesn't have to compile to
anything.
So this little piece to code will compile to:
[0] 0branch (2)
[1] ...
[2] branch (3)
[3] ...
[4] ...
You see that 'IF' compiles to a '0BRANCH' instruction, 'ELSE' to
a 'BRANCH' instruction and 'THEN' to nothing! If you have a
closer look you might assume that 4tH will branch to instruction
[2] and then branch directly to instruction [3]. This is not
quite what was intended.
What a 'BRANCH' instruction actually does is setting the
instruction counter to a specific value. Then, like after every
instruction, the instruction counter is incremented. Why make
exceptions? That only slows the interpreter down. Let's take a
look at this piece of code:
10 dup if 1+ else . then cr
This will compile into:
[0] literal (10)
[1] dup (0)
[2] 0branch (4)
[3] 1+ (0)
[4] branch (5)
[5] . (0)
[6] cr (0)
Now how does this execute. We will show you by giving the value
of the instruction pointer before execution, after execution and
after the automatic increment.
[float Table:
+---------------+--------------+---------+--------+-----------+
| Instruction# | Instruction | Before | After | Increment |
+---------------+--------------+---------+--------+-----------+
+---------------+--------------+---------+--------+-----------+
| [0] | literal | [0] | [0] | [1] |
+---------------+--------------+---------+--------+-----------+
| [1] | dup | [1] | [1] | [2] |
+---------------+--------------+---------+--------+-----------+
| [2] | 0branch | [2] | [2] | [3] |
+---------------+--------------+---------+--------+-----------+
| [3] | 1+ | [3] | [3] | [4] |
+---------------+--------------+---------+--------+-----------+
| [4] | branch | [4] | [5] | [6] |
+---------------+--------------+---------+--------+-----------+
| [6] | cr | [6] | [6] | [7] |
+---------------+--------------+---------+--------+-----------+
[Senseless!!!
Example execution plan
]
]
You see that '0BRANCH' has no effect when the top of the Data
Stack is non-zero. And while 'BRANCH' sets the instruction
pointer to "5", it will resume execution at location "6".
If you compile this little piece of code by hand and start
compiling from the beginning, you will also see that you can't
fill in the destination until it has been compiled. So how does
4tH do that?
4tH has a small stack (Control Stack) where it stores these
addresses. So when it encounters an 'IF' or 'ELSE' or 'THEN'
instruction, it stores its current address there. What would have
happened during the compile of the previous program:
• 'IF' is encountered. It compiles a '0BRANCH' instruction and
stores '2' on the Control Stack.
• 'ELSE' is encountered. It compiles a 'BRANCH' instruction,
takes '2' from the Control Stack and changes the argument of
the '0BRANCH' word to its own address, which is '4'. It stores
'4' on the Control Stack again.
• 'THEN' is encountered. It takes '4' from the Control Stack and
changes the argument of the 'BRANCH' word to address of the
last compiled word, which is '5'.
But this example was correct. What would have happened if we had
written something like:
5 0 : test begin 1 dup if ; while . then dup dup repeat
To prevent the compiler from accepting these kind of
constructions, a reference is added. This reference tells the
compiler what conditional was put on the stack. If the reference
isn't correct, the compiler will throw an exception. There are
five predefined references, but you may add your own.
There are three functions which handle conditionals:
[float Table:
+-----------+--------------------------------+
| Function | Description |
+-----------+--------------------------------+
+-----------+--------------------------------+
| MarkLink | Throws an address on the stack |
+-----------+--------------------------------+
| PairLink | Gets an address from the stack |
+-----------+--------------------------------+
| MakeLink | Makes a "backlink" |
+-----------+--------------------------------+
[Senseless!!!
Branch resolving API
]
]
They all take a reference as argument. All address calculation
and errorchecking is done by these functions. Let's get to
business and retrace our steps when we added 'BEGIN.. WHILE..
REPEAT'.
All conditionals are "immediate" words. So they have to be added
to ImmedList[]. That also means, that each word has its own
function. Let's design the one for 'BEGIN'. 'BEGIN' is just a
marker, which means we have little more to do than to save the
address on the stack:
#ifndef ARCHAIC
static void DoBegin (void)
#else
static void DoBegin ()
#endif
{
MarkLink (R_BEGIN);
}
Yes, that's all. Just call MakeLink() with the proper reference!
Just make sure, you've compiled everything you wanted to compile.
Jumps resolved by MarkLink() will in effect always continue from
the word that will be compiled next, although it seems they jump
to the word compiled last.
Now we have to make 'WHILE'. 'WHILE' executes a piece of code
when the top of the DataStack is non-zero. Which means it jumps
to 'REPEAT' when the top of the DataStack is zero. Sounds like a
'0BRANCH' instruction to us. Note, that 'BEGIN' doesn't play any
part whatsoever here!
Because the address it has to jump to isn't known yet (we haven't
encountered 'REPEAT', we can only compile the '0BRANCH'
instruction with an arbitrary address. But 'REPEAT' will have to
know the address in order to make a backlink, so we have to throw
it on the stack:
#ifndef ARCHAIC
static void DoWhile (void)
#else
static void DoWhile ()
#endif
{
CompileMark (BRANCH0, R_WHILE);
}
Note that CompileMark() is equivalent to:
CompileWord (BRANCH0, 0L);
MarkLink (R_WHILE);
Now we come to 'REPEAT'. It has a lot of things to do. First, it
has to compile a 'BRANCH' instruction in order to get back to
'BEGIN'. Second, it will have to resolve the backlink from
'WHILE'.
In a way, 'REPEAT' has an advantage over 'WHILE'. It doesn't have
to compile an arbitrary address, since it is already on the
control stack. It has been provided by 'BEGIN'. It can retrieve
that address by calling PairLink() with the proper reference:
CompileWord (BRANCH, PairLink (R_BEGIN));
But there is a problem. MakeLink() should always make a link to
the last compiled word. And we can't compile the 'BRANCH'
instruction first, because of the "WHILE" reference on the top of
the controlstack!
So we have to resolve the 'WHILE' backlink first. For that
purpose, MakeLink() has an extra argument. Usually, MakeLink() is
called with "LASTW", which means it will jump to the word
compiled last. Then the instruction counter will be incremented
and the interpreter will continue from there.
In order to compile 'REPEAT', we have to make a backlink that
points to the word compiled next. So, this statement is inserted
before CompileWord():
MakeLink (R_WHILE, NEXTW);
CompileWord (BRANCH, PairLink (R_BEGIN));
And what if we had made a "BEGIN..AGAIN" or a "BEGIN..UNTIL"
loop? Well, in any case we would have to branch back to 'BEGIN'.
'UNTIL' conditionally and 'AGAIN' unconditionally. The address of
'BEGIN' would have already been on the control stack, so a single
statement could have taken care of it:
CompileWord (0BRANCH, PairLink (R_BEGIN));
CompileWord (BRANCH, PairLink (R_BEGIN));
Actually, since 4tH supports multiple 'WHILE's the problem is a
little more complex. 'REPEAT' must resolve all 'WHILE's on the
control stack before it can even think of compiling a 'BRANCH'
instruction. We've already seen that the only difference between
an 'AGAIN' and an 'UNTIL' is the branch instruction which is
compiled. So in 4tH 'REPEAT', 'UNTIL' and 'AGAIN' are handled in
a similar way:
#ifndef ARCHAIC
static void CompileAgain (unit AgainToken)
#else
static void CompileAgain (AgainToken) unit AgainToken;
#endif
{
while ((ToCS > 0) && (ControlStack [ToCS - 1].Mark == R_WHILE))
MakeLink (R_WHILE, NEXTW);
CompileWord (AgainToken, PairLink (R_BEGIN));
}
As long as the control stack is not empty and there is a 'WHILE'
reference on top of the control stack, backlinks are made.
Finally, the branch instruction is compiled, which jumps back to
'BEGIN'. 'REPEAT' can now be reduced to:
CompileAgain (BRANCH);
Note that a colon definition also uses the control stack. This
reference is resolved by ';', which compiles 'EXIT' and creates a
backlink. The 'BRANCH' instruction will prevent the interpreter
from entering the definition. At the same time, ':' creates a
symbol. We'll go into that in the next section.
If you want to create your own branch instructions, you'll have
to define their behaviour in exec_4th(). If the argument of the
token contains the address you want to jump to in the end, you'll
have to define it like this:
JUMP (OPERAND);
That is pretty easy. This macro changes the Program Counter,
which is part of the Hcode header:
Object->ErrLine
Of course, we've defined a macro for that:
PROGCOUNT
If the address you want to jump to is issued by the user, you
probably want to check whether it is a valid execution token.
Just use the macro XT():
DSIZE (1);
a = DPOP;
XT (a);
JUMP (a);
Consequently, leaving the current Program Counter value on the
return stack is pretty easy too:
RFREE (1);
RPUSH (PROGCOUNT);
I think that covers it all, don't you?
22.19 Extending the I/O subsystem
The 4tH I/O system is entirely built upon the buffered C streams[footnote:
See: http://www.aquaphoenix.com/ref/gnu_c_library/libc_118.html
] concept. That means every device that can be assigned to a
FILE* and accessed through fgetc() and fputc() can be integrated
into the 4tH I/O system. If can open a device with fopen() and
close it with fclose(), you're done, it's already supported. If
not, you have to design two functions that take the same
parameters and return the same values as fopen() and fclose(). If
you can't you can probably still use those devices within 4tH,
but you can't integrate them into the I/O system.
If you have defined these functions, you'll have to make changes
to OpenStream() in exec_4th(). 'Mode' is the sum of all file
access methods, e.g.
s" ls" input pipe + open
This definition contains two file access methods, INPUT and PIPE.
You can find these values in cmds_4th.h. INPUT equals 1 and PIPE
equals 8. That makes 9 and that is what ends up in the 'Mode'
parameter of OpenStream(). However, if one would allow all
possible combinations of all file access methods some would
surely make little sense. That is why 'Mode' is filtered by
Mapping[]. You will find that element 9 of Mapping[][footnote:
We start counting at 0.
] contains the value 5. That number corresponds to element 5 in
Modelist[], which lists the correct file access method (which is
still 9, of course) and the mode parameter for fopen().
OpenStream() continues by initializing the members of the
Stream[] structure.
[float Table:
+-------------+--------------------------------------------+
| Member | Function |
+-------------+--------------------------------------------+
+-------------+--------------------------------------------+
| Mode | Uniform file access method |
+-------------+--------------------------------------------+
| Device | FILE pointer to opened device |
+-------------+--------------------------------------------+
| Connect | Function pointer to fopen() like function |
+-------------+--------------------------------------------+
| Disconnect | Function pointer to fclose() like function |
+-------------+--------------------------------------------+
[Senseless!!!
Members of Stream[] structure
]
]
When this is done, it uses the Connect() member to open the
device. After that, everything is completely transparent to the
programmer.
If you want to add a new device, you probably want to signal
which type of device you're using. In order to do that, you must
first add a #define to the 'file modes' section in cmds_4th.h.
Each new file access method has exactly twice the value of the
previous one, which means that the first one you define would
have to be 16. Then you have to figure out which modes are
actually supported. Can your device be opened in read-write mode?
Does appending make sense? You add those 'ideal' states to
ModeList[]. Then map all possible combinations of all file access
methods to the 'ideal' states in ModeList[] using Mapping[]
conversion table. Every additional file access method doubles the
size of Mapping[], so beware!
[float Table:
+----------+----------------------------------------+----------+
| Macro | Function | THROW |
+----------+----------------------------------------+----------+
+----------+----------------------------------------+----------+
| DEV(n) | Aborts if n is not a device | M4BADDEV |
+----------+----------------------------------------+----------+
| UDEV(n) | Aborts if n is not opened by the user | M4BADDEV |
+----------+----------------------------------------+----------+
| ODEV(n) | Aborts if n is not opened | M4IOERR |
+----------+----------------------------------------+----------+
| SDEV(n) | Aborts if n is a pipe | M4BADDEV |
+----------+----------------------------------------+----------+
[Senseless!!!
Device status macros
]
]
If you want to check a device in exec_4th(), there are several
macros you can use. They all take the value returned by
OpenStream() as parameter.
22.20 Using the symbol table<sec:Using-the-symbol>
The symboltable is a way to dynamically add words to the
vocabulary of 4tH. All other words are hard-coded into the
compiler. If you want to add any, you have change the entire
compiler and make a new executable. We've seen that before.
Without the symboltable there wouldn't be any strings, tables,
variables or even colon-definitions. May be you think that such a
powerful feature must be hard to work with. No, it isn't!
The only thing you have to tell the symboltable is "hey, if that
word comes along, compile this token and this argument into the
object". That's all. There are three functions that control the
symbol-table:
[float Table:
+---------------------+------------------------------------+
| Function | Description |
+---------------------+------------------------------------+
+---------------------+------------------------------------+
| AddSymbol() | Adds a symbol to the symboltable |
+---------------------+------------------------------------+
| MakeSymbol() | Makes a symbol of the current word |
+---------------------+------------------------------------+
| GetSymbol() | Searches the symboltable |
+---------------------+------------------------------------+
| SearchDictionary() | Searches the entire dictionary |
+---------------------+------------------------------------+
[Senseless!!!
Symboltable API
]
]
You can forget about the GetSymbol(). You will hardly ever need
it. Let's see how it works. We'll continue with ':':
if (DecodeWord (M4NODECL))
{
AddSymbol (CALL, Object->ErrLine, CurrentWord);
CompileMark (BRANCH, R_COLON);
}
First it uses DecodeWord() to set "CurrentWord" to the next word
in the source, which is the name of the definition. Then it adds
a symbol to the symboltable. Hey, shouldn't we compile a word
first?
No. The member "ErrLine" of an Hcode header always points to the
next available word in the Code Segment. That is the place where
we will compile our 'BRANCH' instruction. When the instruction
pointer is set to that location, it will be automatically
incremented and get inside the definition. So that is okay.
The token we'll use to branch inside that definition is not
'BRANCH' or '0BRANCH', but 'CALL'. 'CALL' throws the address of
its own location on the Return Stack. When the definition is
done, 'EXIT' takes that address off the Return Stack and jumps
backs.
AddSymbol() adds an entry to the symboltable. No, you don't need
to check the symboltable when you add a symbol. AddSymbol() does
that for you and sets the member "ErrNo" when needed.
Finally, a 'BRANCH' token is compiled with a dummy argument. It
will be solved later with a backlink, marked by MarkLink(). Now,
how does it actually work? We'll give you an example. Take this
small program:
: hello's 0 do ." Hello " loop cr ;
20 hello's 10 hello's
When the ':' is reached by the compiler, it hasn't compiled a
thing, so "ErrLine" still points to the first word in the Code
Segment (0). DecodeWord() is called, so "CurrentWord" points to
"HELLO'S". Then a symbol is added to the symboltable by calling
AddSymbol(). The entry looks like this:
HELLO'S -> CALL (0)
That means that every time the name "HELLO'S" is found in the
source, the word "CALL (0)" will be compiled. See for yourself:
[0] branch (6)
[1] literal (0)
[2] do (0)
[3] ." (0)
[4] loop (2)
[5] cr (0)
[6] exit (0)
[7] literal (20)
[8] call (0)
[9] literal (10)
[10] call (0)
Basically, that is all you need to know about the symboltable.
Yes, you can search it yourself, but why should you? It is done
automatically for you. But if you really want to know: you do it
by calling GetSymbol().
All you need is the name of the symbol you're looking for and
what you want the compiler to do when i finds it. E.g. if you
were looking for "HELLO'S" and didn't want to compile it, you'd
have to write:
int x = GetSymbol ("HELLO\'S", W_SEARCH);
GetSymbol() returns the index of "HELLO'S" in the symboltable.
You can use this index to access the symboltable, called
"SymTable":
printf ("%d, %ld, %s\n", (int) SymTable [x].Token,
(long) SymTable [x].Value, SymTable [x].Name);
Which prints the token, the argument and the name of the symbol.
If the name isn't listed GetSymbol() returns "MISSING":
if ((x = GetSymbol ("HELLO\'S", W_SEARCH)) == MISSING)
printf ("Not found\n");
If you search the symboltable in order to compile a word, you
only have to tell:
int x = GetSymbol ("HELLO\'S", W_EXEC);
This will not only return the index, but compile the word as
well. Note that this function can only search the symboltable. It
cannot look for other words. These words have their own function,
but basically work the same:
[float Table:
+------------------+----------------+
| Class | Function |
+------------------+----------------+
+------------------+----------------+
| Immediate words | GetImmediate() |
+------------------+----------------+
| Simple words | GetWord() |
+------------------+----------------+
| Constants | GetConstant() |
+------------------+----------------+
[Senseless!!!
Table search API
]
]
They will return an index as well, but that will be of little use
since the tables they search are private and cannot be accessed
outside the function. SearchDictionary() combines all these
functions (including GetSymbol()) but will only return a boolean
to indicate that that the word was found. It is the most common
way to access these lower level functions.
If you decide to add your own words that use the symboltable, you
have to make an entry in ImmedList[]. Let's say you want to add a
word, which defines a floating point number, e.g.:
float fp_number
Now we have to let 4tH know that for each "FLOAT" a symboltable
entry has to be reserved:
{ 5, 1, -2, "FLOAT", "", DoFloat },
Yes, that is where that famous second field is for! It tells 4tH
how many symboltable entries it has to reserve for a specific
immediate word.
22.21 Using variables and datatypes
We're slowly entering the area where extensions are becoming
projects on its own. You should be able to make the most common
extensions yourself now. What we have ahead is just for the
interested reader of someone who want to add a completely new
wordset.
We're going to explain you how 4tH handles strings and other
datatypes. Variables (any variable!) are not created during
compilation. That means no space is reserved. 4tH only monitors
how much space has been allocated to each datatype. This
information is saved in the header.
At the moment there are only two basic datatypes: characters
(Character Segment) and 32 bit signed integers (Integer Segment).
You'll find the size of these segments in the Hcode members
"Variables" and "Strings". The Character Segment and Integer
Segment are created when a Hcode program is executed and
discarded when the Hcode program is terminated.
So the only thing the compiler has to do is keep track of the
sizes of the segments and assign pointers to variables. This is
quite easy. When an Hcode header is initialized by InitObject(),
both "Variables" and "Strings" are set to zero. Then it parses
this declaration:
variable one
As a consequence, DoVariable() is called:
if (DecodeWord (M4NODECL))
AddSymbol (VARIABLE, Object->Variables++, CurrentWord);
It calls DecodeWord(), so "CurrentWord" now points to "ONE".
"Variables" is still zero. If DecodeWord() was called
successfully, it just adds a symbol by calling AddSymbol(), which
looks like this:
ONE -> VARIABLE (0)
After that "Variables" is incremented, so it now holds the value
"1". It doesn't matter, what comes next: "ONE" will always
compile to "VARIABLE(0)". The next variable will compile to
"VARIABLE(1)". Unless it is an array:
10 array list
The "10" is compiled as a literal. Then the compiler encounters
"ARRAY", so DoArray() is called:
cell val = DecodeSymbol ();
if (! Object->ErrNo) {
AddSymbol (VARIABLE, (cell) Object->Variables,
CurrentWord);
Object->Variables += (unsigned) val;
}
First it calls DecodeSymbol(), which does two things:
1. It calls DecodeWord(), so "CurrentWord" now points to "LIST".
2. It removes the previously compiled literal (by decrementing
the member "ErrLine") and returns it.
Now "val" holds the value "10". If no error occurred, DoArray()
will call AddSymbol(). There an entry is created that looks like
this:
LIST -> VARIABLE (1)
So every time the name "LIST" is encountered a 'VARIABLE' token
will be compiled with argument "1". Finally, the number of
variables is incremented by "val", so the member "Variables" now
holds "11". This means that 10 variables have been added, which
is correct.
It works about the same for 'STRING', only we compile a literal
value here. Why? Because the system areas in the Character
Segment are fixed. In the Variable Area there are also the
application variables and 4tH cannot know at compile time how
many there will be at runtime.
We could have placed the variables right after the system
variables, but that would have made it much more difficult to add
names to your application variables. But now we have to resolve
what 'VARIABLE' has to do at runtime. So we have to edit
exec_4th().
Well, the only thing it has to do is calculate its address in the
Variable Area. There is a member in the header that holds the
offset of the user variables inside the Variable Area. The only
thing we have to do is to add the operand to it and push the
result:
DPUSH (Object->Offset + OPERAND);
Since the next word takes the address of the Data Stack there is
no real difference with a literal. The changes you want to use
the address of a variable as a literal expression are quite
remote.
There are two macros that check the status of a variable. VAR(n)
checks whether n is a variable at all. UVAR (n) checks whether n
is a writable variable. When any of these macros fail, M4BADVAR
will be thrown. n is the value that VARIABLE leaves on the stack.
Of course, if you want to add an entirely new datatype, you have
lots of work to do, but you can use the same tools as we have
used. We have to stress that you use a separate segment for each
datatype. That keeps 4tH simple and it won't take more memory
than other implementations.
Note that if you want to create constants for a certain datatype
you have to work out a scheme to load and save them. If this
scheme depends on a certain, non-portable encoding, you won't be
able to use the resulting .HX files on different platforms.
22.22 Other tools
We have known assertions since version 3.1c and conditional
compilation since version 3.1d. Both conditionally skip source
between to markers. And they can be nested. That sounds like
quite a challenge, but it isn't. In fact, there is only one
simple routine that handles it.
If we encounter a situation where source has to be skipped, we
just call SkipSource(). In case of conditional compilation, the
source that we have to skip is between the markers '[IF]' and
'[THEN]'.
First, we call DecodeLiteral(). This function gets the argument
part of the previously compiled literal and removes that literal
from the compilant (actually, the member "ErrLine" is
decremented, so it will be overwritten):
cell val = DecodeLiteral ();
if (! Object->ErrNo) if (! val) SkipSource ("[IF]", "[THEN]");
Then SkipSource() is called with the argument "[IF]" and
"[THEN]". It will handle everything, including any nested
markers. Note that "CurrentWord" still has to point to the "[IF]"
that triggered the action.
22.23 Patching 4tH
We're getting at the end of the story here. There is one topic
left we want to discuss with you.
It is a drag when you have made some nice extensions to 4tH and
you have to reapply them each time a new version of 4tH is
released. However, there is a solution. 4tH comes with a program
called patch4th.4th which can help you. The only thing you have
to do is to create a 4tH patch file. It consists of six parts,
which all have to appear in the order presented to you here. If a
section is not applicable, leave it blank.
22.23.1 Tokens
The first part are the tokens or the instructions of the virtual
machine, if you prefer. Every entry consists of three fields,
delimited by a tilde[footnote:
A tilde is rarely used by 4tH, so that seemed a good choice. If
you prefer another delimiter, you have to change the source of
patch4th.4th.
]:
1. The first field is the C constant of the token, as it appears
in cmds_4th.h (see [sec:Adding-a-word]);
2. The second field indicates whether the token needs a parameter
(see [sec:Adding-words-with]);
3. The third field is the mnemonic, as used in name_4th.c (see [sec:A-first-look]
).
So a sample entry might look like:
[tokens]
NIP~no~"nip"
To terminate this section, add an empty line.
22.23.2 Words
The second part are the words you actually use in a 4tH program.
As you will know by now, a word can compile to zero or more
tokens. Every entry consists of eight fields, delimited by a
tilde:
1. The first field is the name of the word as you will use it in
a 4tH program;
2. The second field is the token it will compile to;
3. The third field contains the type of word, constant, immediate
or word;
4. The fourth field is the fixed parameter of a constant;
5. The fifth field is the number of symbol entries it will need;
6. The sixth field is the source correction that will be applied;
7. The seventh field is the delimiter it uses;
8. The eighth field is the C function which handles the immediate
word.
A simple word requires fields 1, 2 and 3. A constant requires
fields 1, 2, 3 and 4. An immediate word requires 1, 3, 5, 6, 7
and 8. If a field is not applicable for a certain type it will
not matter what you enter there. See section [sec:A-closer-look]
for more information. A sample entry might look like:
[words]
BIRTHDAY~LITERAL~constant~19600902L~0~0~~
BINARY~RADIX~constant~2L~0~0~~
NIP~NIP~word~~0~0~~
[SEXTAL]~~immediate~~0~-1~""~DoSextal
To terminate this section, add an empty line.
22.23.3 The virtual machine
These sections are copied verbatim - including indententation -
into exec_4th.c. The first section are the additional #include
directives you might need. These are located in the [vm.include]
section:
[vm.include]
#include <sys/stat.h>
You terminate this section by directly continuing with the next
section, [vm.globals]. This contains any global variables you use
in the virtual machine, e.g. Sleeping. They will appear right
before the prototype of the throw() function:
[vm.include]
#include <sys/stat.h>
[vm.globals]
static unsigned MyGlobal;
You terminate this section by directly continuing with the next
section, [vm.io]. This contains the I/O functions emit() and
Accept() if you want to replace the ones that exec_4th()
provides. If you want to keep the standard I/O functions, leave
it completely blank, not even an empty line, e.g.:
[vm.include]
#include <sys/stat.h>
[vm.globals]
static unsigned MyGlobal;
[vm.io]
[vm.support]
You terminate this section by directly continuing with the next
section, [vm.support]. This contains any support functions for
the virtual machine, e.g. OpenStream(). They will appear right
before the main exec_4th() function:
[vm.include]
#include <sys/stat.h>
[vm.globals]
static unsigned MyGlobal;
[vm.io]
[vm.support]
/*
Custom support functions
*/
You terminate this section by directly continuing with the next
section, [vm.vars]. This contains any additional local variables
of the exec_4th() function, e.g. VarMax, which will be added to
the local variable list:
[vm.include]
#include <sys/stat.h>
[vm.globals]
static unsigned MyGlobal;
[vm.io]
[vm.support]
/*
Custom support functions
*/
[vm.vars]
time_t MyLocal;
You terminate this section by directly continuing with the next
section, [vm.extension]. This contains the actual C code which
will be copied into the main loop of exec_4th():
[vm.include]
#include <sys/stat.h>
[vm.globals]
static unsigned MyGlobal;
[vm.io]
[vm.support]
/*
Custom support functions
*/
[vm.vars]
time_t MyLocal;
[vm.extension]
case (NIP): DSIZE (2);
DS (2) = DS (1);
DDROP;
break;
You terminate this section by directly continuing with the next
section.
22.23.4 Immediate words
This section contains the C functions that are executed when
immediate words are compiled (see [sec:A-closer-look] and [sec:Extending-the-compiler]
). They will be inserted verbatim just before ImmedList[]:
[vm.include]
#include <sys/stat.h>
[vm.globals]
static unsigned MyGlobal;
[vm.io]
[vm.support]
/*
Custom support functions
*/
[vm.vars]
time_t MyLocal;
[vm.extension]
case (NIP): DSIZE (2);
DS (2) = DS (1);
DDROP;
break;
[immediate.words]
#ifndef ARCHAIC
static void DoSextal (void)
#else
static void DoSextal ()
#endif
{
Base = 6;
}
You don't have to explicitly terminate this section.
22.23.5 Applying the patches
Make a subdirectory, copy the original cmds_4th.h, comp_4th.c,
exec_4th.c, name_4th.c, save_4th.c and load_4th.c sources into it
and rename them to .txt. They will serve as templates for your
custom 4tH sources. In this example we will assume your custom
patchfile and the compiled patch4th.4th are also located there,
but that is not required. When you make a directory listing you
will see the following files:
cmds_4th.txt
comp_4th.txt
exec_4th.txt
name_4th.txt
save_4th.txt
load_4th.txt
mypatch.txt
patch4th.hx
Now run it:
4th lxq patch4th.hx mypatch.txt
When everything is alright, you will see the following messages:
Opening mypatch.txt
.. 1 tokens read
.. 4 words read
Processing cmds_4th.txt
.. done
Processing save_4th.txt
.. done
Processing load_4th.txt
.. done
Processing name_4th.txt
.. done
Processing exec_4th.txt
.. done
Processing comp_4th.txt
.. done
Closing mypatch.txt
.. done
When you list the directory again, you will see that new
cmds_4th.h, comp_4th.c, exec_4th.c, name_4th.c, save_4th.c and
load_4th.c sources have been created.
22.23.6 Error messages
Usage: patch4th patch-file Issue a patchfile on the commandline
Bad boolean ”yes” or ”no” was expected in this field
Bad datatype ”word”, ”constant” or ”immediate” was expected in
this field
Bad number A number was expected in this field
Cannot find [tokens] A [tokens] section was expected in the
patchfile
Cannot find [words] A [words] section was expected in the
patchfile
Cannot find [vm.include] A [vm.include] section was expected in
the patchfile
Cannot find [vm.support] A [vm.support] section was expected in
the patchfile
Too many tokens Too many tokens were defined in the patchfile
Cannot find /* ranges */ Corrupted cmds_4th.txt file
Cannot find NOOP token Corrupted cmds_4th.txt file
Cannot find LastWord4th Corrupted cmds_4th.txt file
Cannot open <file>.h|c Could not create a source file
Cannot open <file>.txt Could not find a template file
Document ends here
Copyright 1997, 2009 J.L. Bezemer
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