2 \ A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
3 \ By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth
4 \ This is PUBLIC DOMAIN (see public domain release statement below).
5 \ $Id: jonesforth.f,v 1.4 2007-09-25 21:48:20 rich Exp $
7 \ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth
9 \ PUBLIC DOMAIN ----------------------------------------------------------------------
11 \ I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide.
13 \ In case this is not legally possible, I grant any entity the right to use this work for any purpose,
14 \ without any conditions, unless such conditions are required by law.
16 \ SETTING UP ----------------------------------------------------------------------
18 \ Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of
19 \ ASCII-art diagrams to explain concepts, the best way to look at this is using a window which
20 \ uses a fixed width font and is at least this wide:
22 \<------------------------------------------------------------------------------------------------------------------------>
24 \ Secondly make sure TABS are set to 8 characters. The following should be a vertical
25 \ line. If not, sort out your tabs.
31 \ Thirdly I assume that your screen is at least 50 characters high.
33 \ START OF FORTH CODE ----------------------------------------------------------------------
35 \ We've now reached the stage where the FORTH system is running and self-hosting. All further
36 \ words can be written as FORTH itself, including words like IF, THEN, .", etc which in most
37 \ languages would be considered rather fundamental.
39 \ Some notes about the code:
41 \ I use indenting to show structure. The amount of whitespace has no meaning to FORTH however
42 \ except that you must use at least one whitespace character between words, and words themselves
43 \ cannot contain whitespace.
45 \ FORTH is case-sensitive. Use capslock!
47 \ The primitive word /MOD (DIVMOD) leaves both the quotient and the remainder on the stack. (On
48 \ i386, the idivl instruction gives both anyway). Now we can define the / and MOD in terms of /MOD
49 \ and a few other primitives.
53 \ Define some character constants
57 \ CR prints a carriage return
60 \ SPACE prints a space
61 : SPACE 'SPACE' EMIT ;
63 \ DUP, DROP are defined in assembly for speed, but this is how you might define them
64 \ in FORTH. Notice use of the scratch variables _X and _Y.
65 \ : DUP _X ! _X @ _X @ ;
68 \ The 2... versions of the standard operators work on pairs of stack entries. They're not used
69 \ very commonly so not really worth writing in assembler. Here is how they are defined in FORTH.
73 \ More standard FORTH words.
77 \ NEGATE leaves the negative of a number on the stack.
80 \ Standard words for booleans.
85 \ LITERAL takes whatever is on the stack and compiles LIT <foo>
88 , \ compile the literal itself (from the stack)
91 \ Now we can use [ and ] to insert literals which are calculated at compile time. (Recall that
92 \ [ and ] are the FORTH words which switch into and out of immediate mode.)
93 \ Within definitions, use [ ... ] LITERAL anywhere that '...' is a constant expression which you
94 \ would rather only compute once (at compile time, rather than calculating it each time your word runs).
96 [ \ go into immediate mode (temporarily)
97 CHAR : \ push the number 58 (ASCII code of colon) on the parameter stack
98 ] \ go back to compile mode
99 LITERAL \ compile LIT 58 as the definition of ':' word
102 \ A few more character constants defined the same way as above.
103 : '(' [ CHAR ( ] LITERAL ;
104 : ')' [ CHAR ) ] LITERAL ;
105 : '"' [ CHAR " ] LITERAL ;
106 : 'A' [ CHAR A ] LITERAL ;
107 : '0' [ CHAR 0 ] LITERAL ;
108 : '-' [ CHAR - ] LITERAL ;
110 \ While compiling, '[COMPILE] word' compiles 'word' if it would otherwise be IMMEDIATE.
111 : [COMPILE] IMMEDIATE
112 WORD \ get the next word
113 FIND \ find it in the dictionary
114 >CFA \ get its codeword
118 \ RECURSE makes a recursive call to the current word that is being compiled.
120 \ Normally while a word is being compiled, it is marked HIDDEN so that references to the
121 \ same word within are calls to the previous definition of the word. However we still have
122 \ access to the word which we are currently compiling through the LATEST pointer so we
123 \ can use that to compile a recursive call.
125 LATEST @ \ LATEST points to the word being compiled at the moment
126 >CFA \ get the codeword
130 \ So far we have defined only very simple definitions. Before we can go further, we really need to
131 \ make some control structures, like IF ... THEN and loops. Luckily we can define arbitrary control
132 \ structures directly in FORTH.
134 \ Please note that the control structures as I have defined them here will only work inside compiled
135 \ words. If you try to type in expressions using IF, etc. in immediate mode, then they won't work.
136 \ Making these work in immediate mode is left as an exercise for the reader.
138 \ condition IF true-part THEN rest
139 \ -- compiles to: --> condition 0BRANCH OFFSET true-part rest
140 \ where OFFSET is the offset of 'rest'
141 \ condition IF true-part ELSE false-part THEN
142 \ -- compiles to: --> condition 0BRANCH OFFSET true-part BRANCH OFFSET2 false-part rest
143 \ where OFFSET if the offset of false-part and OFFSET2 is the offset of rest
145 \ IF is an IMMEDIATE word which compiles 0BRANCH followed by a dummy offset, and places
146 \ the address of the 0BRANCH on the stack. Later when we see THEN, we pop that address
147 \ off the stack, calculate the offset, and back-fill the offset.
149 ' 0BRANCH , \ compile 0BRANCH
150 HERE @ \ save location of the offset on the stack
151 0 , \ compile a dummy offset
156 HERE @ SWAP - \ calculate the offset from the address saved on the stack
157 SWAP ! \ store the offset in the back-filled location
161 ' BRANCH , \ definite branch to just over the false-part
162 HERE @ \ save location of the offset on the stack
163 0 , \ compile a dummy offset
164 SWAP \ now back-fill the original (IF) offset
165 DUP \ same as for THEN word above
170 \ BEGIN loop-part condition UNTIL
171 \ -- compiles to: --> loop-part condition 0BRANCH OFFSET
172 \ where OFFSET points back to the loop-part
173 \ This is like do { loop-part } while (condition) in the C language
175 HERE @ \ save location on the stack
179 ' 0BRANCH , \ compile 0BRANCH
180 HERE @ - \ calculate the offset from the address saved on the stack
181 , \ compile the offset here
184 \ BEGIN loop-part AGAIN
185 \ -- compiles to: --> loop-part BRANCH OFFSET
186 \ where OFFSET points back to the loop-part
187 \ In other words, an infinite loop which can only be returned from with EXIT
189 ' BRANCH , \ compile BRANCH
190 HERE @ - \ calculate the offset back
191 , \ compile the offset here
194 \ BEGIN condition WHILE loop-part REPEAT
195 \ -- compiles to: --> condition 0BRANCH OFFSET2 loop-part BRANCH OFFSET
196 \ where OFFSET points back to condition (the beginning) and OFFSET2 points to after the whole piece of code
197 \ So this is like a while (condition) { loop-part } loop in the C language
199 ' 0BRANCH , \ compile 0BRANCH
200 HERE @ \ save location of the offset2 on the stack
201 0 , \ compile a dummy offset2
205 ' BRANCH , \ compile BRANCH
206 SWAP \ get the original offset (from BEGIN)
207 HERE @ - , \ and compile it after BRANCH
209 HERE @ SWAP - \ calculate the offset2
210 SWAP ! \ and back-fill it in the original location
213 \ FORTH allows ( ... ) as comments within function definitions. This works by having an IMMEDIATE
214 \ word called ( which just drops input characters until it hits the corresponding ).
216 1 \ allowed nested parens by keeping track of depth
218 KEY \ read next character
219 DUP '(' = IF \ open paren?
220 DROP \ drop the open paren
223 ')' = IF \ close paren?
227 DUP 0= UNTIL \ continue until we reach matching close paren, depth 0
228 DROP \ drop the depth counter
232 From now on we can use ( ... ) for comments.
234 In FORTH style we can also use ( ... -- ... ) to show the effects that a word has on the
235 parameter stack. For example:
237 ( n -- ) means that the word consumes an integer (n) from the parameter stack.
238 ( b a -- c ) means that the word uses two integers (a and b, where a is at the top of stack)
239 and returns a single integer (c).
240 ( -- ) means the word has no effect on the stack
243 ( Standard words for manipulating BASE. )
244 : DECIMAL ( -- ) 10 BASE ! ;
245 : HEX ( -- ) 16 BASE ! ;
248 The standard FORTH word . (DOT) is very important. It takes the number at the top
249 of the stack and prints it out. However first I'm going to implement some lower-level
252 U.R ( u width -- ) which prints an unsigned number, padded to a certain width
253 U. ( u -- ) which prints an unsigned number
254 .R ( n width -- ) which prints a signed number, padded to a certain width.
258 will print out these characters:
259 <space> <space> - 1 2 3
261 In other words, the number padded left to a certain number of characters.
263 The full number is printed even if it is wider than width, and this is what allows us to
264 define the ordinary functions U. and . (we just set width to zero knowing that the full
265 number will be printed anyway).
267 Another wrinkle of . and friends is that they obey the current base in the variable BASE.
268 BASE can be anything in the range 2 to 36.
272 BASE @ /MOD ( width rem quot )
273 DUP 0<> IF ( if quotient <> 0 then )
274 RECURSE ( print the quotient )
276 DROP ( drop the zero quotient )
279 ( print the remainder )
281 '0' ( decimal digits 0..9 )
283 10 - ( hex and beyond digits A..Z )
290 ( U. is easy to define in terms of U.R Note the trailing space. )
293 ( .R is easy, we just need to print the sign and then call U.R )
297 '-' EMIT ( print the sign )
298 NEGATE ( negate the number so we can use U.R )
299 SWAP 1- ( n width-1 )
307 ( Finally we can define word . in terms of .R, with a trailing space. )
310 ( ? fetches the integer at an address and prints it. )
313 ( With the looping constructs, we can now write SPACES, which writes n spaces to stdout. )
316 DUP 0> ( while n > 0 )
318 SPACE ( print a space )
319 1- ( until we count down to 0 )
324 ( c a b WITHIN returns true if a <= c and c < b )
340 ( .S prints the contents of the stack. Very useful for debugging. )
342 DSP@ ( get current stack pointer )
346 DUP @ . ( print the stack element )
352 ( DEPTH returns the depth of the stack. )
355 4- ( adjust because S0 was on the stack when we pushed DSP )
359 ALIGNED takes an address and rounds it up (aligns it) to the next 4 byte boundary.
361 : ALIGNED ( addr -- addr )
362 3 + 3 INVERT AND ( (addr+3) & ~3 )
366 ALIGN aligns the HERE pointer, so the next word appended will be aligned properly.
368 : ALIGN HERE @ ALIGNED HERE ! ;
371 S" string" is used in FORTH to define strings. It leaves the address of the string and
372 its length on the stack, with the address at the top. The space following S" is the normal
373 space between FORTH words and is not a part of the string.
375 This is tricky to define because it has to do different things depending on whether
376 we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can
377 detect this and do different things).
379 In compile mode we append
380 LITSTRING <string length> <string rounded up 4 bytes>
381 to the current word. The primitive LITSTRING does the right thing when the current
384 In immediate mode there isn't a particularly good place to put the string, but in this
385 case we put the string at HERE (but we _don't_ change HERE). This is meant as a temporary
386 location, likely to be overwritten soon after.
388 : S" IMMEDIATE ( -- len addr )
389 STATE @ IF ( compiling? )
390 ' LITSTRING , ( compile LITSTRING )
391 HERE @ ( save the address of the length word on the stack )
392 0 , ( dummy length - we don't know what it is yet )
394 KEY ( get next character of the string )
397 HERE @ C! ( store the character in the compiled image )
398 1 HERE +! ( increment HERE pointer by 1 byte )
400 DROP ( drop the double quote character at the end )
401 DUP ( get the saved address of the length word )
402 HERE @ SWAP - ( calculate the length )
403 4- ( subtract 4 (because we measured from the start of the length word) )
404 SWAP ! ( and back-fill the length location )
405 ALIGN ( round up to next multiple of 4 bytes for the remaining code )
406 ELSE ( immediate mode )
407 HERE @ ( get the start address of the temporary space )
412 OVER C! ( save next character )
413 1+ ( increment address )
415 DROP ( drop the final " character )
416 HERE @ - ( calculate the length )
417 HERE @ ( push the start address )
422 ." is the print string operator in FORTH. Example: ." Something to print"
423 The space after the operator is the ordinary space required between words and is not
424 a part of what is printed.
426 In immediate mode we just keep reading characters and printing them until we get to
427 the next double quote.
429 In compile mode we use S" to store the string, then add EMITSTRING afterwards:
430 LITSTRING <string length> <string rounded up to 4 bytes> EMITSTRING
432 It may be interesting to note the use of [COMPILE] to turn the call to the immediate
433 word S" into compilation of that word. It compiles it into the definition of .",
434 not into the definition of the word being compiled when this is running (complicated
437 : ." IMMEDIATE ( -- )
438 STATE @ IF ( compiling? )
439 [COMPILE] S" ( read the string, and compile LITSTRING, etc. )
440 ' EMITSTRING , ( compile the final EMITSTRING )
442 ( In immediate mode, just read characters and print them until we get
443 to the ending double quote. )
447 DROP ( drop the double quote character )
448 EXIT ( return from this function )
456 In FORTH, global constants and variables are defined like this:
458 10 CONSTANT TEN when TEN is executed, it leaves the integer 10 on the stack
459 VARIABLE VAR when VAR is executed, it leaves the address of VAR on the stack
461 Constants can be read but not written, eg:
465 You can read a variable (in this example called VAR) by doing:
467 VAR @ leaves the value of VAR on the stack
468 VAR @ . CR prints the value of VAR
469 VAR ? CR same as above, since ? is the same as @ .
471 and update the variable by doing:
473 20 VAR ! sets VAR to 20
475 Note that variables are uninitialised (but see VALUE later on which provides initialised
476 variables with a slightly simpler syntax).
478 How can we define the words CONSTANT and VARIABLE?
480 The trick is to define a new word for the variable itself (eg. if the variable was called
481 'VAR' then we would define a new word called VAR). This is easy to do because we exposed
482 dictionary entry creation through the CREATE word (part of the definition of : above).
483 A call to CREATE TEN leaves the dictionary entry:
488 +---------+---+---+---+---+
489 | LINK | 3 | T | E | N |
490 +---------+---+---+---+---+
493 For CONSTANT we can continue by appending DOCOL (the codeword), then LIT followed by
494 the constant itself and then EXIT, forming a little word definition that returns the
497 +---------+---+---+---+---+------------+------------+------------+------------+
498 | LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT |
499 +---------+---+---+---+---+------------+------------+------------+------------+
502 Notice that this word definition is exactly the same as you would have got if you had
505 Note for people reading the code below: DOCOL is a constant word which we defined in the
506 assembler part which returns the value of the assembler symbol of the same name.
509 CREATE ( make the dictionary entry (the name follows CONSTANT) )
510 DOCOL , ( append DOCOL (the codeword field of this word) )
511 ' LIT , ( append the codeword LIT )
512 , ( append the value on the top of the stack )
513 ' EXIT , ( append the codeword EXIT )
517 VARIABLE is a little bit harder because we need somewhere to put the variable. There is
518 nothing particularly special about the 'user definitions area' (the area of memory pointed
519 to by HERE where we have previously just stored new word definitions). We can slice off
520 bits of this memory area to store anything we want, so one possible definition of
521 VARIABLE might create this:
523 +--------------------------------------------------------------+
526 +---------+---------+---+---+---+---+------------+------------+---|--------+------------+
527 | <var> | LINK | 3 | V | A | R | DOCOL | LIT | <addr var> | EXIT |
528 +---------+---------+---+---+---+---+------------+------------+------------+------------+
531 where <var> is the place to store the variable, and <addr var> points back to it.
533 To make this more general let's define a couple of words which we can use to allocate
534 arbitrary memory from the user definitions area.
536 First ALLOT, where n ALLOT allocates n bytes of memory. (Note when calling this that
537 it's a very good idea to make sure that n is a multiple of 4, or at least that next time
538 a word is compiled that HERE has been left as a multiple of 4).
540 : ALLOT ( n -- addr )
541 HERE @ SWAP ( here n )
542 HERE +! ( adds n to HERE, after this the old value of HERE is still on the stack )
546 Second, CELLS. In FORTH the phrase 'n CELLS ALLOT' means allocate n integers of whatever size
547 is the natural size for integers on this machine architecture. On this 32 bit machine therefore
548 CELLS just multiplies the top of stack by 4.
550 : CELLS ( n -- n ) 4 * ;
553 So now we can define VARIABLE easily in much the same way as CONSTANT above. Refer to the
554 diagram above to see what the word that this creates will look like.
557 1 CELLS ALLOT ( allocate 1 cell of memory, push the pointer to this memory )
558 CREATE ( make the dictionary entry (the name follows VARIABLE) )
559 DOCOL , ( append DOCOL (the codeword field of this word) )
560 ' LIT , ( append the codeword LIT )
561 , ( append the pointer to the new memory )
562 ' EXIT , ( append the codeword EXIT )
566 VALUEs are like VARIABLEs but with a simpler syntax. You would generally use them when you
567 want a variable which is read often, and written infrequently.
569 20 VALUE VAL creates VAL with initial value 20
570 VAL pushes the value directly on the stack
571 30 TO VAL updates VAL, setting it to 30
573 Notice that 'VAL' on its own doesn't return the address of the value, but the value itself,
574 making values simpler and more obvious to use than variables (no indirection through '@').
575 The price is a more complicated implementation, although despite the complexity there is no
576 performance penalty at runtime.
578 A naive implementation of 'TO' would be quite slow, involving a dictionary search each time.
579 But because this is FORTH we have complete control of the compiler so we can compile TO more
580 efficiently, turning:
584 and calculating <addr> (the address of the value) at compile time.
586 Now this is the clever bit. We'll compile our value like this:
588 +---------+---+---+---+---+------------+------------+------------+------------+
589 | LINK | 3 | V | A | L | DOCOL | LIT | <value> | EXIT |
590 +---------+---+---+---+---+------------+------------+------------+------------+
593 where <value> is the actual value itself. Note that when VAL executes, it will push the
594 value on the stack, which is what we want.
596 But what will TO use for the address <addr>? Why of course a pointer to that <value>:
598 code compiled - - - - --+------------+------------+------------+-- - - - -
599 by TO VAL | LIT | <addr> | ! |
600 - - - - --+------------+-----|------+------------+-- - - - -
603 +---------+---+---+---+---+------------+------------+------------+------------+
604 | LINK | 3 | V | A | L | DOCOL | LIT | <value> | EXIT |
605 +---------+---+---+---+---+------------+------------+------------+------------+
608 In other words, this is a kind of self-modifying code.
610 (Note to the people who want to modify this FORTH to add inlining: values defined this
611 way cannot be inlined).
614 CREATE ( make the dictionary entry (the name follows VALUE) )
615 DOCOL , ( append DOCOL )
616 ' LIT , ( append the codeword LIT )
617 , ( append the initial value )
618 ' EXIT , ( append the codeword EXIT )
621 : TO IMMEDIATE ( n -- )
622 WORD ( get the name of the value )
623 FIND ( look it up in the dictionary )
624 >DFA ( get a pointer to the first data field (the 'LIT') )
625 4+ ( increment to point at the value )
626 STATE @ IF ( compiling? )
627 ' LIT , ( compile LIT )
628 , ( compile the address of the value )
630 ELSE ( immediate mode )
631 ! ( update it straightaway )
635 ( x +TO VAL adds x to VAL )
637 WORD ( get the name of the value )
638 FIND ( look it up in the dictionary )
639 >DFA ( get a pointer to the first data field (the 'LIT') )
640 4+ ( increment to point at the value )
641 STATE @ IF ( compiling? )
642 ' LIT , ( compile LIT )
643 , ( compile the address of the value )
644 ' +! , ( compile +! )
645 ELSE ( immediate mode )
646 +! ( update it straightaway )
651 ID. takes an address of a dictionary entry and prints the word's name.
653 For example: LATEST @ ID. would print the name of the last word that was defined.
656 4+ ( skip over the link pointer )
657 DUP C@ ( get the flags/length byte )
658 F_LENMASK AND ( mask out the flags - just want the length )
661 DUP 0> ( length > 0? )
663 SWAP 1+ ( addr len -- len addr+1 )
664 DUP C@ ( len addr -- len addr char | get the next character)
665 EMIT ( len addr char -- len addr | and print it)
666 SWAP 1- ( len addr -- addr len-1 | subtract one from length )
668 2DROP ( len addr -- )
672 'WORD word FIND ?HIDDEN' returns true if 'word' is flagged as hidden.
674 'WORD word FIND ?IMMEDIATE' returns true if 'word' is flagged as immediate.
677 4+ ( skip over the link pointer )
678 C@ ( get the flags/length byte )
679 F_HIDDEN AND ( mask the F_HIDDEN flag and return it (as a truth value) )
682 4+ ( skip over the link pointer )
683 C@ ( get the flags/length byte )
684 F_IMMED AND ( mask the F_IMMED flag and return it (as a truth value) )
688 WORDS prints all the words defined in the dictionary, starting with the word defined most recently.
689 However it doesn't print hidden words.
691 The implementation simply iterates backwards from LATEST using the link pointers.
694 LATEST @ ( start at LATEST dictionary entry )
696 DUP 0<> ( while link pointer is not null )
698 DUP ?HIDDEN NOT IF ( ignore hidden words )
699 DUP ID. ( but if not hidden, print the word )
702 @ ( dereference the link pointer - go to previous word )
709 So far we have only allocated words and memory. FORTH provides a rather primitive method
712 'FORGET word' deletes the definition of 'word' from the dictionary and everything defined
713 after it, including any variables and other memory allocated after.
715 The implementation is very simple - we look up the word (which returns the dictionary entry
716 address). Then we set HERE to point to that address, so in effect all future allocations
717 and definitions will overwrite memory starting at the word. We also need to set LATEST to
718 point to the previous word.
720 Note that you cannot FORGET built-in words (well, you can try but it will probably cause
723 XXX: Because we wrote VARIABLE to store the variable in memory allocated before the word,
724 in the current implementation VARIABLE FOO FORGET FOO will leak 1 cell of memory.
727 WORD FIND ( find the word, gets the dictionary entry address )
728 DUP @ LATEST ! ( set LATEST to point to the previous word )
729 HERE ! ( and store HERE with the dictionary address )
733 DUMP is used to dump out the contents of memory, in the 'traditional' hexdump format.
735 : DUMP ( addr len -- )
736 BASE @ ROT ( save the current BASE at the bottom of the stack )
737 HEX ( and switch the hexadecimal mode )
740 DUP 0> ( while len > 0 )
742 OVER 8 .R ( print the address )
745 ( print up to 16 words on this line )
746 2DUP ( addr len addr len )
747 1- 15 AND 1+ ( addr len addr linelen )
749 DUP 0> ( while linelen > 0 )
751 SWAP ( addr len linelen addr )
752 DUP C@ ( addr len linelen addr byte )
753 2 .R SPACE ( print the byte )
754 1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 )
758 ( print the ASCII equivalents )
759 2DUP 1- 15 AND 1+ ( addr len addr linelen )
761 DUP 0> ( while linelen > 0)
763 SWAP ( addr len linelen addr )
764 DUP C@ ( addr len linelen addr byte )
765 DUP 32 128 WITHIN IF ( 32 <= c < 128? )
768 DROP [ CHAR ? ] LITERAL EMIT
770 1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 )
775 DUP 1- 15 AND 1+ ( addr len linelen )
776 DUP ( addr len linelen linelen )
777 ROT ( addr linelen len linelen )
778 - ( addr linelen len-linelen )
779 ROT ( len-linelen addr linelen )
780 + ( len-linelen addr+linelen )
781 SWAP ( addr-linelen len-linelen )
784 2DROP ( restore stack )
785 BASE ! ( restore saved BASE )
788 ( Finally print the welcome prompt. )
789 ." JONESFORTH VERSION " VERSION . CR
git.annexia.org / jonesforth.git / blob