-\ -*- forth -*-
+\ -*- text -*-
\ A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
\ By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth
\ This is PUBLIC DOMAIN (see public domain release statement below).
-\ $Id: jonesforth.f,v 1.1 2007-09-24 00:18:19 rich Exp $
+\ $Id: jonesforth.f,v 1.2 2007-09-24 00:37:01 rich Exp $
\
\ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth
\
: ')' [ CHAR ) ] LITERAL ;
: '"' [ CHAR " ] LITERAL ;
+\ While compiling, '[COMPILE] word' compiles 'word' if it would otherwise be IMMEDIATE.
+: [COMPILE] IMMEDIATE
+ WORD \ get the next word
+ FIND \ find it in the dictionary
+ >CFA \ get its codeword
+ , \ and compile that
+;
+
\ So far we have defined only very simple definitions. Before we can go further, we really need to
\ make some control structures, like IF ... THEN and loops. Luckily we can define arbitrary control
\ structures directly in FORTH.
;
(
- [NB. The following may be a bit confusing because of the need to use backslash before
- each double quote character. The backslashes are there to keep the assembler happy.
- They are NOT part of the final output. So here we are defining a function called
- 'S double-quote' (not 'S backslash double-quote').]
-
S" string" is used in FORTH to define strings. It leaves the address of the string and
- its length on the stac,k with the address at the top. The space following S" is the normal
+ its length on the stack, with the address at the top. The space following S" is the normal
space between FORTH words and is not a part of the string.
+ This is tricky to define because it has to do different things depending on whether
+ we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can
+ detect this and do different things).
+
In compile mode we append
LITSTRING <string length> <string rounded up 4 bytes>
to the current word. The primitive LITSTRING does the right thing when the current
OVER !b ( save next character )
1+ ( increment address )
REPEAT
+ DROP ( drop the final " character )
HERE @ - ( calculate the length )
HERE @ ( push the start address )
THEN
(
." is the print string operator in FORTH. Example: ." Something to print"
- The space after the operator is the ordinary space required between words.
-
- This is tricky to define because it has to do different things depending on whether
- we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can
- detect this and do different things).
+ The space after the operator is the ordinary space required between words and is not
+ a part of what is printed.
In immediate mode we just keep reading characters and printing them until we get to
the next double quote.
- In compile mode we have the problem of where we're going to store the string (remember
- that the input buffer where the string comes from may be overwritten by the time we
- come round to running the function). We store the string in the compiled function
- like this:
- ..., LITSTRING, string length, string rounded up to 4 bytes, EMITSTRING, ...
+ In compile mode we use S" to store the string, then add EMITSTRING afterwards:
+ LITSTRING <string length> <string rounded up to 4 bytes> EMITSTRING
+
+ It may be interesting to note the use of [COMPILE] to turn the call to the immediate
+ word S" into compilation of that word. It compiles it into the definition of .",
+ not into the definition of the word being compiled when this is running (complicated
+ enough for you?)
)
: ." IMMEDIATE ( -- )
STATE @ IF ( compiling? )
- ' LITSTRING , ( compile LITSTRING )
- HERE @ ( save the address of the length word on the stack )
- 0 , ( dummy length - we don't know what it is yet )
- BEGIN
- KEY ( get next character of the string )
- DUP '"' <>
- WHILE
- HERE @ !b ( store the character in the compiled image )
- 1 HERE +! ( increment HERE pointer by 1 byte )
- REPEAT
- DROP ( drop the double quote character at the end )
- DUP ( get the saved address of the length word )
- HERE @ SWAP - ( calculate the length )
- 4- ( subtract 4 (because we measured from the start of the length word) )
- SWAP ! ( and back-fill the length location )
- HERE @ ( round up to next multiple of 4 bytes for the remaining code )
- 3 +
- 3 INVERT AND
- HERE !
+ [COMPILE] S" ( read the string, and compile LITSTRING, etc. )
' EMITSTRING , ( compile the final EMITSTRING )
ELSE
( In immediate mode, just read characters and print them until we get
- to the ending double quote. Much simpler than the above code! )
+ to the ending double quote. )
BEGIN
KEY
DUP '"' = IF
;
(
- While compiling, '[COMPILE] word' compiles 'word' if it would otherwise be IMMEDIATE.
-)
-: [COMPILE] IMMEDIATE
- WORD ( get the next word )
- FIND ( find it in the dictionary )
- >CFA ( get its codeword )
- , ( and compile that )
-;
-
-(
RECURSE makes a recursive call to the current word that is being compiled.
Normally while a word is being compiled, it is marked HIDDEN so that references to the
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