1 /* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
2 By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth
3 This is PUBLIC DOMAIN (see public domain release statement below).
4 $Id: jonesforth.S,v 1.41 2007-09-29 23:11:27 rich Exp $
6 gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S
10 INTRODUCTION ----------------------------------------------------------------------
12 FORTH is one of those alien languages which most working programmers regard in the same
13 way as Haskell, LISP, and so on. Something so strange that they'd rather any thoughts
14 of it just go away so they can get on with writing this paying code. But that's wrong
15 and if you care at all about programming then you should at least understand all these
16 languages, even if you will never use them.
18 LISP is the ultimate high-level language, and features from LISP are being added every
19 decade to the more common languages. But FORTH is in some ways the ultimate in low level
20 programming. Out of the box it lacks features like dynamic memory management and even
21 strings. In fact, at its primitive level it lacks even basic concepts like IF-statements
24 Why then would you want to learn FORTH? There are several very good reasons. First
25 and foremost, FORTH is minimal. You really can write a complete FORTH in, say, 2000
26 lines of code. I don't just mean a FORTH program, I mean a complete FORTH operating
27 system, environment and language. You could boot such a FORTH on a bare PC and it would
28 come up with a prompt where you could start doing useful work. The FORTH you have here
29 isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making
30 it a good tutorial). It's possible to completely understand the system. Who can say they
31 completely understand how Linux works, or gcc?
33 Secondly FORTH has a peculiar bootstrapping property. By that I mean that after writing
34 a little bit of assembly to talk to the hardware and implement a few primitives, all the
35 rest of the language and compiler is written in FORTH itself. Remember I said before
36 that FORTH lacked IF-statements and loops? Well of course it doesn't really because
37 such a lanuage would be useless, but my point was rather that IF-statements and loops are
38 written in FORTH itself.
40 Now of course this is common in other languages as well, and in those languages we call
41 them 'libraries'. For example in C, 'printf' is a library function written in C. But
42 in FORTH this goes way beyond mere libraries. Can you imagine writing C's 'if' in C?
43 And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict
44 yourself to the usual if/while/for/switch constructs? You want a construct that iterates
45 over every other element in a list of numbers? You can add it to the language. What
46 about an operator which pulls in variables directly from a configuration file and makes
47 them available as FORTH variables? Or how about adding Makefile-like dependencies to
48 the language? No problem in FORTH. How about modifying the FORTH compiler to allow
49 complex inlining strategies -- simple. This concept isn't common in programming languages,
50 but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not
51 the lame C preprocessor) and "domain specific languages" (DSLs).
53 This tutorial isn't about learning FORTH as the language. I'll point you to some references
54 you should read if you're not familiar with using FORTH. This tutorial is about how to
55 write FORTH. In fact, until you understand how FORTH is written, you'll have only a very
56 superficial understanding of how to use it.
58 So if you're not familiar with FORTH or want to refresh your memory here are some online
61 http://en.wikipedia.org/wiki/Forth_%28programming_language%29
63 http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm
65 http://wiki.laptop.org/go/Forth_Lessons
67 http://www.albany.net/~hello/simple.htm
69 Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html
71 Discussion and criticism of this FORTH here: http://lambda-the-ultimate.org/node/2452
73 ACKNOWLEDGEMENTS ----------------------------------------------------------------------
75 This code draws heavily on the design of LINA FORTH (http://home.hccnet.nl/a.w.m.van.der.horst/lina.html)
76 by Albert van der Horst. Any similarities in the code are probably not accidental.
78 Some parts of this FORTH are also based on this IOCCC entry from 1992:
79 http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design.
80 I was very proud when Sean Barrett, the original author of the IOCCC entry, commented in the LtU thread
81 http://lambda-the-ultimate.org/node/2452#comment-36818 about this FORTH.
83 And finally I'd like to acknowledge the (possibly forgotten?) authors of ARTIC FORTH because their
84 original program which I still have on original cassette tape kept nagging away at me all these years.
85 http://en.wikipedia.org/wiki/Artic_Software
87 PUBLIC DOMAIN ----------------------------------------------------------------------
89 I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide.
91 In case this is not legally possible, I grant any entity the right to use this work for any purpose,
92 without any conditions, unless such conditions are required by law.
94 SETTING UP ----------------------------------------------------------------------
96 Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of
97 ASCII-art diagrams to explain concepts, the best way to look at this is using a window which
98 uses a fixed width font and is at least this wide:
100 <------------------------------------------------------------------------------------------------------------------------>
102 Secondly make sure TABS are set to 8 characters. The following should be a vertical
103 line. If not, sort out your tabs.
109 Thirdly I assume that your screen is at least 50 characters high.
111 ASSEMBLING ----------------------------------------------------------------------
113 If you want to actually run this FORTH, rather than just read it, you will need Linux on an
114 i386. Linux because instead of programming directly to the hardware on a bare PC which I
115 could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux
116 process with a few basic system calls (read, write and exit and that's about all). i386
117 is needed because I had to write the assembly for a processor, and i386 is by far the most
118 common. (Of course when I say 'i386', any 32- or 64-bit x86 processor will do. I'm compiling
119 this on a 64 bit AMD Opteron).
121 Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to
122 assemble and run the code (save this file as 'jonesforth.S') are:
124 gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S
125 cat jonesforth.f - | ./jonesforth
127 If you want to run your own FORTH programs you can do:
129 cat jonesforth.f myprog.f | ./jonesforth
131 If you want to load your own FORTH code and then continue reading user commands, you can do:
133 cat jonesforth.f myfunctions.f - | ./jonesforth
135 ASSEMBLER ----------------------------------------------------------------------
137 (You can just skip to the next section -- you don't need to be able to read assembler to
138 follow this tutorial).
140 However if you do want to read the assembly code here are a few notes about gas (the GNU assembler):
142 (1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator. The registers
143 available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them
144 have special purposes.
146 (2) Add, mov, etc. take arguments in the form SRC,DEST. So mov %eax,%ecx moves %eax -> %ecx
148 (3) Constants are prefixed with '$', and you mustn't forget it! If you forget it then it
149 causes a read from memory instead, so:
150 mov $2,%eax moves number 2 into %eax
151 mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake)
153 (4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards"
154 and '1b' (etc.) means label '1:' "backwards".
156 (5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc.
158 (6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and
161 For more help reading the assembler, do "info gas" at the Linux prompt.
163 Now the tutorial starts in earnest.
165 THE DICTIONARY ----------------------------------------------------------------------
167 In FORTH as you will know, functions are called "words", and just as in other languages they
168 have a name and a definition. Here are two FORTH words:
170 : DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +"
171 : QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE"
173 Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary
174 which is just a linked list of dictionary entries.
176 <--- DICTIONARY ENTRY (HEADER) ----------------------->
177 +------------------------+--------+---------- - - - - +----------- - - - -
178 | LINK POINTER | LENGTH/| NAME | DEFINITION
180 +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - -
182 I'll come to the definition of the word later. For now just look at the header. The first
183 4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for
184 the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte.
185 The length of the word can be up to 31 characters (5 bits used) and the top three bits are used
186 for various flags which I'll come to later. This is followed by the name itself, and in this
187 implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes.
188 That's just to ensure that the definition starts on a 32 bit boundary.
190 A FORTH variable called LATEST contains a pointer to the most recently defined word, in
191 other words, the head of this linked list.
193 DOUBLE and QUADRUPLE might look like this:
195 pointer to previous word
198 +--|------+---+---+---+---+---+---+---+---+------------- - - - -
199 | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...)
200 +---------+---+---+---+---+---+---+---+---+------------- - - - -
203 +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
204 | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...)
205 +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
211 You should be able to see from this how you might implement functions to find a word in
212 the dictionary (just walk along the dictionary entries starting at LATEST and matching
213 the names until you either find a match or hit the NULL pointer at the end of the dictionary);
214 and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set
215 LATEST to point to the new word). We'll see precisely these functions implemented in
216 assembly code later on.
218 One interesting consequence of using a linked list is that you can redefine words, and
219 a newer definition of a word overrides an older one. This is an important concept in
220 FORTH because it means that any word (even "built-in" or "standard" words) can be
221 overridden with a new definition, either to enhance it, to make it faster or even to
222 disable it. However because of the way that FORTH words get compiled, which you'll
223 understand below, words defined using the old definition of a word continue to use
224 the old definition. Only words defined after the new definition use the new definition.
226 DIRECT THREADED CODE ----------------------------------------------------------------------
228 Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea
229 or coffee and settle down. It's fair to say that if you don't understand this section, then you
230 won't "get" how FORTH works, and that would be a failure on my part for not explaining it well.
231 So if after reading this section a few times you don't understand it, please email me
234 Let's talk first about what "threaded code" means. Imagine a peculiar version of C where
235 you are only allowed to call functions without arguments. (Don't worry for now that such a
236 language would be completely useless!) So in our peculiar C, code would look like this:
245 and so on. How would a function, say 'f' above, be compiled by a standard C compiler?
246 Probably into assembly code like this. On the right hand side I've written the actual
250 CALL a E8 08 00 00 00
251 CALL b E8 1C 00 00 00
252 CALL c E8 2C 00 00 00
253 ; ignore the return from the function for now
255 "E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing
256 memory was hideously expensive and we might have worried about the wasted space being used
257 by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory)
258 by compressing this into just:
260 08 00 00 00 Just the function addresses, without
261 1C 00 00 00 the CALL prefix.
264 On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%.
266 [Historical note: If the execution model that FORTH uses looks strange from the following
267 paragraphs, then it was motivated entirely by the need to save memory on early computers.
268 This code compression isn't so important now when our machines have more memory in their L1
269 caches than those early computers had in total, but the execution model still has some
272 Of course this code won't run directly any more. Instead we need to write an interpreter
273 which takes each pair of bytes and calls it.
275 On an i386 machine it turns out that we can write this interpreter rather easily, in just
276 two assembly instructions which turn into just 3 bytes of machine code. Let's store the
277 pointer to the next word to execute in the %esi register:
279 08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute.
283 The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does
284 two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it
285 increments %esi by 4 bytes. So after LODSL, the situation now looks like this:
287 08 00 00 00 <- We're still executing this one
288 1C 00 00 00 <- %eax now contains this address (0x0000001C)
291 Now we just need to jump to the address in %eax. This is again just a single x86 instruction
292 written JMP *(%eax). And after doing the jump, the situation looks like:
295 1C 00 00 00 <- Now we're executing this subroutine.
298 To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)'
299 which literally make the jump to the next subroutine.
301 And that brings us to our first piece of actual code! Well, it's a macro.
310 /* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions.
312 Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like
315 The above describes what is known as direct threaded code.
317 To sum up: We compress our function calls down to a list of addresses and use a somewhat
318 magical macro to act as a "jump to next function in the list". We also use one register (%esi)
319 to act as a kind of instruction pointer, pointing to the next function in the list.
321 I'll just give you a hint of what is to come by saying that a FORTH definition such as:
323 : QUADRUPLE DOUBLE DOUBLE ;
325 actually compiles (almost, not precisely but we'll see why in a moment) to a list of
326 function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off.
328 At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!".
330 I lied about JMP *(%eax).
332 INDIRECT THREADED CODE ----------------------------------------------------------------------
334 It turns out that direct threaded code is interesting but only if you want to just execute
335 a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE
336 was an assembly language function. In the direct threaded code, QUADRUPLE would look like:
339 | addr of DOUBLE --------------------> (assembly code to do the double)
340 +------------------+ NEXT
341 %esi -> | addr of DOUBLE |
344 We can add an extra indirection to allow us to run both words written in assembly language
345 (primitives written for speed) and words written in FORTH themselves as lists of addresses.
347 The extra indirection is the reason for the brackets in JMP *(%eax).
349 Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH:
351 : QUADRUPLE DOUBLE DOUBLE ;
354 | codeword | : DOUBLE DUP + ;
356 | addr of DOUBLE ---------------> +------------------+
357 +------------------+ | codeword |
358 | addr of DOUBLE | +------------------+
359 +------------------+ | addr of DUP --------------> +------------------+
360 | addr of EXIT | +------------------+ | codeword -------+
361 +------------------+ %esi -> | addr of + --------+ +------------------+ |
362 +------------------+ | | assembly to <-----+
363 | addr of EXIT | | | implement DUP |
364 +------------------+ | | .. |
367 | +------------------+
369 +-----> +------------------+
371 +------------------+ |
372 | assembly to <------+
379 This is the part where you may need an extra cup of tea/coffee/favourite caffeinated
380 beverage. What has changed is that I've added an extra pointer to the beginning of
381 the definitions. In FORTH this is sometimes called the "codeword". The codeword is
382 a pointer to the interpreter to run the function. For primitives written in
383 assembly language, the "interpreter" just points to the actual assembly code itself.
384 They don't need interpreting, they just run.
386 In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter
389 I'll show you the interpreter function shortly, but let's recall our indirect
390 JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE
391 as shown, and DUP has been called. Note that %esi is pointing to the address of +
393 The assembly code for DUP eventually does a NEXT. That:
395 (1) reads the address of + into %eax %eax points to the codeword of +
396 (2) increments %esi by 4
397 (3) jumps to the indirect %eax jumps to the address in the codeword of +,
398 ie. the assembly code to implement +
403 | addr of DOUBLE ---------------> +------------------+
404 +------------------+ | codeword |
405 | addr of DOUBLE | +------------------+
406 +------------------+ | addr of DUP --------------> +------------------+
407 | addr of EXIT | +------------------+ | codeword -------+
408 +------------------+ | addr of + --------+ +------------------+ |
409 +------------------+ | | assembly to <-----+
410 %esi -> | addr of EXIT | | | implement DUP |
411 +------------------+ | | .. |
414 | +------------------+
416 +-----> +------------------+
418 +------------------+ |
419 now we're | assembly to <-----+
420 executing | implement + |
426 So I hope that I've convinced you that NEXT does roughly what you'd expect. This is
427 indirect threaded code.
429 I've glossed over four things. I wonder if you can guess without reading on what they are?
435 My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do
436 you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but
437 then point at part of DOUBLE. (3) What goes in the codeword for the words which are written
438 in FORTH? (4) How do you compile a function which does anything except call other functions
439 ie. a function which contains a number like : DOUBLE 2 * ; ?
441 THE INTERPRETER AND RETURN STACK ------------------------------------------------------------
443 Going at these in no particular order, let's talk about issues (3) and (2), the interpreter
444 and the return stack.
446 Words which are defined in FORTH need a codeword which points to a little bit of code to
447 give them a "helping hand" in life. They don't need much, but they do need what is known
448 as an "interpreter", although it doesn't really "interpret" in the same way that, say,
449 Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few
450 machine registers so that the word can then execute at full speed using the indirect
451 threaded model above.
453 One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old
454 %esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE.
455 Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like
456 a function call), we will need a stack to store these "return addresses" (old values of %esi).
458 As you will have read, when reading the background documentation, FORTH has two stacks,
459 an ordinary stack for parameters, and a return stack which is a bit more mysterious. But
460 our return stack is just the stack I talked about in the previous paragraph, used to save
461 %esi when calling from a FORTH word into another FORTH word.
463 In this FORTH, we are using the normal stack pointer (%esp) for the parameter stack.
464 We will use the i386's "other" stack pointer (%ebp, usually called the "frame pointer")
465 for our return stack.
467 I've got two macros which just wrap up the details of using %ebp for the return stack.
468 You use them as for example "PUSHRSP %eax" (push %eax on the return stack) or "POPRSP %ebx"
469 (pop top of return stack into %ebx).
472 /* Macros to deal with the return stack. */
474 lea -4(%ebp),%ebp // push reg on to return stack
479 mov (%ebp),\reg // pop top of return stack to reg
484 And with that we can now talk about the interpreter.
486 In FORTH the interpreter function is often called DOCOL (I think it means "DO COLON" because
487 all FORTH definitions start with a colon, as in : DOUBLE DUP + ;
489 The "interpreter" (it's not really "interpreting") just needs to push the old %esi on the
490 stack and set %esi to the first word in the definition. Remember that we jumped to the
491 function using JMP *(%eax)? Well a consequence of that is that conveniently %eax contains
492 the address of this codeword, so just by adding 4 to it we get the address of the first
493 data word. Finally after setting up %esi, it just does NEXT which causes that first word
497 /* DOCOL - the interpreter! */
501 PUSHRSP %esi // push %esi on to the return stack
502 addl $4,%eax // %eax points to codeword, so make
503 movl %eax,%esi // %esi point to first data word
507 Just to make this absolutely clear, let's see how DOCOL works when jumping from QUADRUPLE
513 +------------------+ DOUBLE:
514 | addr of DOUBLE ---------------> +------------------+
515 +------------------+ %eax -> | addr of DOCOL |
516 %esi -> | addr of DOUBLE | +------------------+
517 +------------------+ | addr of DUP |
518 | addr of EXIT | +------------------+
519 +------------------+ | etc. |
521 First, the call to DOUBLE calls DOCOL (the codeword of DOUBLE). DOCOL does this: It
522 pushes the old %esi on the return stack. %eax points to the codeword of DOUBLE, so we
523 just add 4 on to it to get our new %esi:
528 +------------------+ DOUBLE:
529 | addr of DOUBLE ---------------> +------------------+
530 top of return +------------------+ %eax -> | addr of DOCOL |
531 stack points -> | addr of DOUBLE | + 4 = +------------------+
532 +------------------+ %esi -> | addr of DUP |
533 | addr of EXIT | +------------------+
534 +------------------+ | etc. |
536 Then we do NEXT, and because of the magic of threaded code that increments %esi again
539 Well, it seems to work.
541 One minor point here. Because DOCOL is the first bit of assembly actually to be defined
542 in this file (the others were just macros), and because I usually compile this code with the
543 text segment starting at address 0, DOCOL has address 0. So if you are disassembling the
544 code and see a word with a codeword of 0, you will immediately know that the word is
545 written in FORTH (it's not an assembler primitive) and so uses DOCOL as the interpreter.
547 STARTING UP ----------------------------------------------------------------------
549 Now let's get down to nuts and bolts. When we start the program we need to set up
550 a few things like the return stack. But as soon as we can, we want to jump into FORTH
551 code (albeit much of the "early" FORTH code will still need to be written as
552 assembly language primitives).
554 This is what the set up code does. Does a tiny bit of house-keeping, sets up the
555 separate return stack (NB: Linux gives us the ordinary parameter stack already), then
556 immediately jumps to a FORTH word called COLD. COLD stands for cold-start. In ISO
557 FORTH (but not in this FORTH), COLD can be called at any time to completely reset
558 the state of FORTH, and there is another word called WARM which does a partial reset.
561 /* ELF entry point. */
566 mov %esp,var_S0 // Store the initial data stack pointer.
567 mov $return_stack,%ebp // Initialise the return stack.
569 mov $cold_start,%esi // Initialise interpreter.
570 NEXT // Run interpreter!
573 cold_start: // High-level code without a codeword.
577 We also allocate some space for the return stack and some space to store user
578 definitions. These are static memory allocations using fixed-size buffers, but it
579 wouldn't be a great deal of work to make them dynamic.
583 /* FORTH return stack. */
584 .set RETURN_STACK_SIZE,8192
586 .space RETURN_STACK_SIZE
587 return_stack: // Initial top of return stack.
589 /* The user definitions area: space for user-defined words and general memory allocations. */
590 .set USER_DEFS_SIZE,65536
593 .space USER_DEFS_SIZE
595 /* This is used as a temporary input buffer when reading from files or the terminal. */
596 .set BUFFER_SIZE,4096
608 BUILT-IN WORDS ----------------------------------------------------------------------
610 Remember our dictionary entries (headers)? Let's bring those together with the codeword
611 and data words to see how : DOUBLE DUP + ; really looks in memory.
613 pointer to previous word
616 +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
617 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
618 +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
621 LINK in next word points to codeword of DUP
623 Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we
624 don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc.
625 So instead we will have to define built-in words using the GNU assembler data constructors
626 (like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are
629 The long way would be:
630 .int <link to previous word>
632 .ascii "DOUBLE" // string
634 DOUBLE: .int DOCOL // codeword
635 .int DUP // pointer to codeword of DUP
636 .int PLUS // pointer to codeword of +
637 .int EXIT // pointer to codeword of EXIT
639 That's going to get quite tedious rather quickly, so here I define an assembler macro
640 so that I can just write:
642 defword "DOUBLE",6,,DOUBLE
645 and I'll get exactly the same effect.
647 Don't worry too much about the exact implementation details of this macro - it's complicated!
650 /* Flags - these are discussed later. */
653 .set F_LENMASK,0x1f // length mask
655 // Store the chain of links.
658 .macro defword name, namelen, flags=0, label
664 .set link,name_\label
665 .byte \flags+\namelen // flags + length byte
666 .ascii "\name" // the name
670 .int DOCOL // codeword - the interpreter
671 // list of word pointers follow
675 Similarly I want a way to write words written in assembly language. There will quite a few
676 of these to start with because, well, everything has to start in assembly before there's
677 enough "infrastructure" to be able to start writing FORTH words, but also I want to define
678 some common FORTH words in assembly language for speed, even though I could write them in FORTH.
680 This is what DUP looks like in memory:
682 pointer to previous word
685 +--|------+---+---+---+---+------------+
686 | LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly
687 +---------+---+---+---+---+------------+ code used to write DUP,
688 ^ len codeword which ends with NEXT.
692 Again, for brevity in writing the header I'm going to write an assembler macro called defcode.
695 .macro defcode name, namelen, flags=0, label
701 .set link,name_\label
702 .byte \flags+\namelen // flags + length byte
703 .ascii "\name" // the name
707 .int code_\label // codeword
711 code_\label : // assembler code follows
715 Now some easy FORTH primitives. These are written in assembly for speed. If you understand
716 i386 assembly language then it is worth reading these. However if you don't understand assembly
717 you can skip the details.
721 pop %eax // duplicate top of stack
726 defcode "DROP",4,,DROP
727 pop %eax // drop top of stack
730 defcode "SWAP",4,,SWAP
731 pop %eax // swap top of stack
737 defcode "OVER",4,,OVER
738 mov 4(%esp),%eax // get the second element of stack
739 push %eax // and push it on top
751 defcode "-ROT",4,,NROT
760 defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero
769 incl (%esp) // increment top of stack
773 decl (%esp) // decrement top of stack
776 defcode "4+",2,,INCR4
777 addl $4,(%esp) // add 4 to top of stack
780 defcode "4-",2,,DECR4
781 subl $4,(%esp) // subtract 4 from top of stack
785 pop %eax // get top of stack
786 addl %eax,(%esp) // and add it to next word on stack
790 pop %eax // get top of stack
791 subl %eax,(%esp) // and subtract it from next word on stack
798 push %eax // ignore overflow
802 In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in
803 terms of the primitive /MOD. The design of the i386 assembly instruction idiv which
804 leaves both quotient and remainder makes this obvious choice.
807 defcode "/MOD",4,,DIVMOD
812 push %edx // push remainder
813 push %eax // push quotient
816 defcode "=",1,,EQU // top two words are equal?
826 defcode "<>",2,,NEQU // top two words are not equal?
876 defcode "0=",2,,ZEQU // top of stack equals 0?
885 defcode "0<>",3,,ZNEQU // top of stack not 0?
894 defcode "0<",2,,ZLT // comparisons with 0
930 defcode "AND",3,,AND // bitwise AND
935 defcode "OR",2,,OR // bitwise OR
940 defcode "XOR",3,,XOR // bitwise XOR
945 defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE)
950 RETURNING FROM FORTH WORDS ----------------------------------------------------------------------
952 Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called
953 DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing):
958 +------------------+ DOUBLE
959 | addr of DOUBLE ---------------> +------------------+
960 +------------------+ | codeword |
961 | addr of DOUBLE | +------------------+
962 +------------------+ | addr of DUP |
963 | addr of EXIT | +------------------+
964 +------------------+ | addr of + |
966 %esi -> | addr of EXIT |
969 What happens when the + function does NEXT? Well, the following code is executed.
972 defcode "EXIT",4,,EXIT
973 POPRSP %esi // pop return stack into %esi
977 EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi.
978 So after this (but just before NEXT) we get:
983 +------------------+ DOUBLE
984 | addr of DOUBLE ---------------> +------------------+
985 +------------------+ | codeword |
986 %esi -> | addr of DOUBLE | +------------------+
987 +------------------+ | addr of DUP |
988 | addr of EXIT | +------------------+
989 +------------------+ | addr of + |
994 And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-)
996 LITERALS ----------------------------------------------------------------------
998 The final point I "glossed over" before was how to deal with functions that do anything
999 apart from calling other functions. For example, suppose that DOUBLE was defined like this:
1003 It does the same thing, but how do we compile it since it contains the literal 2? One way
1004 would be to have a function called "2" (which you'd have to write in assembler), but you'd need
1005 a function for every single literal that you wanted to use.
1007 FORTH solves this by compiling the function using a special word called LIT:
1009 +---------------------------+-------+-------+-------+-------+-------+
1010 | (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT |
1011 +---------------------------+-------+-------+-------+-------+-------+
1013 LIT is executed in the normal way, but what it does next is definitely not normal. It
1014 looks at %esi (which now points to the literal 2), grabs it, pushes it on the stack, then
1015 manipulates %esi in order to skip the literal as if it had never been there.
1017 What's neat is that the whole grab/manipulate can be done using a single byte single
1018 i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams,
1019 see if you can find out how LIT works:
1022 defcode "LIT",3,,LIT
1023 // %esi points to the next command, but in this case it points to the next
1024 // literal 32 bit integer. Get that literal into %eax and increment %esi.
1025 // On x86, it's a convenient single byte instruction! (cf. NEXT macro)
1027 push %eax // push the literal number on to stack
1031 MEMORY ----------------------------------------------------------------------
1033 As important point about FORTH is that it gives you direct access to the lowest levels
1034 of the machine. Manipulating memory directly is done frequently in FORTH, and these are
1035 the primitive words for doing it.
1038 defcode "!",1,,STORE
1039 pop %ebx // address to store at
1040 pop %eax // data to store there
1041 mov %eax,(%ebx) // store it
1044 defcode "@",1,,FETCH
1045 pop %ebx // address to fetch
1046 mov (%ebx),%eax // fetch it
1047 push %eax // push value onto stack
1050 defcode "+!",2,,ADDSTORE
1052 pop %eax // the amount to add
1053 addl %eax,(%ebx) // add it
1056 defcode "-!",2,,SUBSTORE
1058 pop %eax // the amount to subtract
1059 subl %eax,(%ebx) // add it
1063 ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes
1064 so we also define standard words C@ and C!.
1066 Byte-oriented operations only work on architectures which permit them (i386 is one of those).
1069 defcode "C!",2,,STOREBYTE
1070 pop %ebx // address to store at
1071 pop %eax // data to store there
1072 movb %al,(%ebx) // store it
1075 defcode "C@",2,,FETCHBYTE
1076 pop %ebx // address to fetch
1078 movb (%ebx),%al // fetch it
1079 push %eax // push value onto stack
1083 BUILT-IN VARIABLES ----------------------------------------------------------------------
1085 These are some built-in variables and related standard FORTH words. Of these, the only one that we
1086 have discussed so far was LATEST, which points to the last (most recently defined) word in the
1087 FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable)
1088 on to the stack, so you can read or write it using @ and ! operators. For example, to print
1089 the current value of LATEST (and this can apply to any FORTH variable) you would do:
1093 To make defining variables shorter, I'm using a macro called defvar, similar to defword and
1094 defcode above. (In fact the defvar macro uses defcode to do the dictionary header).
1097 .macro defvar name, namelen, flags=0, label, initial=0
1098 defcode \name,\namelen,\flags,\label
1108 The built-in variables are:
1110 STATE Is the interpreter executing code (0) or compiling a word (non-zero)?
1111 LATEST Points to the latest (most recently defined) word in the dictionary.
1112 HERE Points to the next free byte of memory. When compiling, compiled words go here.
1113 _X These are three scratch variables, used by some standard dictionary words.
1116 S0 Stores the address of the top of the parameter stack.
1117 BASE The current base for printing and reading numbers.
1120 defvar "STATE",5,,STATE
1121 defvar "HERE",4,,HERE,user_defs_start
1122 defvar "LATEST",6,,LATEST,name_SYSCALL3 // SYSCALL3 must be last in built-in dictionary
1127 defvar "BASE",4,,BASE,10
1130 BUILT-IN CONSTANTS ----------------------------------------------------------------------
1132 It's also useful to expose a few constants to FORTH. When the word is executed it pushes a
1133 constant value on the stack.
1135 The built-in constants are:
1137 VERSION Is the current version of this FORTH.
1138 R0 The address of the top of the return stack.
1139 DOCOL Pointer to DOCOL.
1140 F_IMMED The IMMEDIATE flag's actual value.
1141 F_HIDDEN The HIDDEN flag's actual value.
1142 F_LENMASK The length mask in the flags/len byte.
1144 SYS_* and the numeric codes of various Linux syscalls (from <asm/unistd.h>)
1147 //#include <asm-i386/unistd.h> // you might need this instead
1148 #include <asm/unistd.h>
1150 .macro defconst name, namelen, flags=0, label, value
1151 defcode \name,\namelen,\flags,\label
1156 defconst "VERSION",7,,VERSION,JONES_VERSION
1157 defconst "R0",2,,RZ,return_stack
1158 defconst "DOCOL",5,,__DOCOL,DOCOL
1159 defconst "F_IMMED",7,,__F_IMMED,F_IMMED
1160 defconst "F_HIDDEN",8,,__F_HIDDEN,F_HIDDEN
1161 defconst "F_LENMASK",9,,__F_LENMASK,F_LENMASK
1163 defconst "SYS_EXIT",8,,SYS_EXIT,__NR_exit
1164 defconst "SYS_OPEN",8,,SYS_OPEN,__NR_open
1165 defconst "SYS_CLOSE",9,,SYS_CLOSE,__NR_close
1166 defconst "SYS_READ",8,,SYS_READ,__NR_read
1167 defconst "SYS_WRITE",9,,SYS_WRITE,__NR_write
1168 defconst "SYS_CREAT",9,,SYS_CREAT,__NR_creat
1170 defconst "O_RDONLY",8,,__O_RDONLY,0
1171 defconst "O_WRONLY",8,,__O_WRONLY,1
1172 defconst "O_RDWR",6,,__O_RDWR,2
1173 defconst "O_CREAT",7,,__O_CREAT,0100
1174 defconst "O_EXCL",6,,__O_EXCL,0200
1175 defconst "O_TRUNC",7,,__O_TRUNC,01000
1176 defconst "O_APPEND",8,,__O_APPEND,02000
1177 defconst "O_NONBLOCK",10,,__O_NONBLOCK,04000
1180 RETURN STACK ----------------------------------------------------------------------
1182 These words allow you to access the return stack. Recall that the register %ebp always points to
1183 the top of the return stack.
1187 pop %eax // pop parameter stack into %eax
1188 PUSHRSP %eax // push it on to the return stack
1191 defcode "R>",2,,FROMR
1192 POPRSP %eax // pop return stack on to %eax
1193 push %eax // and push on to parameter stack
1196 defcode "RSP@",4,,RSPFETCH
1200 defcode "RSP!",4,,RSPSTORE
1204 defcode "RDROP",5,,RDROP
1205 lea 4(%ebp),%ebp // pop return stack and throw away
1209 PARAMETER (DATA) STACK ----------------------------------------------------------------------
1211 These functions allow you to manipulate the parameter stack. Recall that Linux sets up the parameter
1212 stack for us, and it is accessed through %esp.
1215 defcode "DSP@",4,,DSPFETCH
1220 defcode "DSP!",4,,DSPSTORE
1225 INPUT AND OUTPUT ----------------------------------------------------------------------
1227 These are our first really meaty/complicated FORTH primitives. I have chosen to write them in
1228 assembler, but surprisingly in "real" FORTH implementations these are often written in terms
1229 of more fundamental FORTH primitives. I chose to avoid that because I think that just obscures
1230 the implementation. After all, you may not understand assembler but you can just think of it
1231 as an opaque block of code that does what it says.
1233 Let's discuss input first.
1235 The FORTH word KEY reads the next byte from stdin (and pushes it on the parameter stack).
1236 So if KEY is called and someone hits the space key, then the number 32 (ASCII code of space)
1237 is pushed on the stack.
1239 In FORTH there is no distinction between reading code and reading input. We might be reading
1240 and compiling code, we might be reading words to execute, we might be asking for the user
1241 to type their name -- ultimately it all comes in through KEY.
1243 The implementation of KEY uses an input buffer of a certain size (defined at the end of the
1244 program). It calls the Linux read(2) system call to fill this buffer and tracks its position
1245 in the buffer using a couple of variables, and if it runs out of input buffer then it refills
1246 it automatically. The other thing that KEY does is if it detects that stdin has closed, it
1247 exits the program, which is why when you hit ^D the FORTH system cleanly exits.
1250 defcode "KEY",3,,KEY
1252 push %eax // push return value on stack
1264 1: // out of input; use read(2) to fetch more input from stdin
1265 xor %ebx,%ebx // 1st param: stdin
1266 mov $buffer,%ecx // 2nd param: buffer
1268 mov $buffend-buffer,%edx // 3rd param: max length
1269 mov $__NR_read,%eax // syscall: read
1271 test %eax,%eax // If %eax <= 0, then exit.
1273 addl %eax,%ecx // buffer+%eax = bufftop
1277 2: // error or out of input: exit
1279 mov $__NR_exit,%eax // syscall: exit
1283 By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout.
1284 This implementation just uses the write system call. No attempt is made to buffer output, but
1285 it would be a good exercise to add it.
1288 defcode "EMIT",4,,EMIT
1293 mov $1,%ebx // 1st param: stdout
1295 // write needs the address of the byte to write
1297 mov $2f,%ecx // 2nd param: address
1299 mov $1,%edx // 3rd param: nbytes = 1
1301 mov $__NR_write,%eax // write syscall
1306 2: .space 1 // scratch used by EMIT
1309 Back to input, WORD is a FORTH word which reads the next full word of input.
1311 What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on).
1312 Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it
1313 calculates the length of the word it read and returns the address and the length as
1314 two words on the stack (with the length at the top of stack).
1316 Notice that WORD has a single internal buffer which it overwrites each time (rather like
1317 a static C string). Also notice that WORD's internal buffer is just 32 bytes long and
1318 there is NO checking for overflow. 31 bytes happens to be the maximum length of a
1319 FORTH word that we support, and that is what WORD is used for: to read FORTH words when
1320 we are compiling and executing code. The returned strings are not NUL-terminated, so
1321 in some crazy-world you could define FORTH words containing ASCII NULs, although why
1322 you'd want to is a bit beyond me.
1324 WORD is not suitable for just reading strings (eg. user input) because of all the above
1325 peculiarities and limitations.
1327 Note that when executing, you'll see:
1329 which puts "FOO" and length 3 on the stack, but when compiling:
1331 is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling
1332 and immediate mode, and you'll understand why.
1335 defcode "WORD",4,,WORD
1337 push %edi // push base address
1338 push %ecx // push length
1342 /* Search for first non-blank character. Also skip \ comments. */
1344 call _KEY // get next key, returned in %eax
1345 cmpb $'\\',%al // start of a comment?
1346 je 3f // if so, skip the comment
1348 jbe 1b // if so, keep looking
1350 /* Search for the end of the word, storing chars as we go. */
1351 mov $5f,%edi // pointer to return buffer
1353 stosb // add character to return buffer
1354 call _KEY // get next key, returned in %al
1355 cmpb $' ',%al // is blank?
1356 ja 2b // if not, keep looping
1358 /* Return the word (well, the static buffer) and length. */
1360 mov %edi,%ecx // return length of the word
1361 mov $5f,%edi // return address of the word
1364 /* Code to skip \ comments to end of the current line. */
1367 cmpb $'\n',%al // end of line yet?
1372 // A static buffer where WORD returns. Subsequent calls
1373 // overwrite this buffer. Maximum word length is 32 chars.
1377 As well as reading in words we'll need to read in numbers and for that we are using a function
1378 called SNUMBER. This parses a numeric string such as one returned by WORD and pushes the
1379 number on the parameter stack.
1381 This function does absolutely no error checking, and in particular the length of the string
1382 must be >= 1 bytes, and should contain only digits 0-9. If it doesn't you'll get random results.
1384 This function is only used when reading literal numbers in code, and shouldn't really be used
1385 in user code at all.
1387 defcode "SNUMBER",7,,SNUMBER
1397 imull $10,%eax // %eax *= 10
1400 subb $'0',%bl // ASCII -> digit
1407 DICTIONARY LOOK UPS ----------------------------------------------------------------------
1409 We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure.
1411 The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the
1412 dictionary. What it actually returns is the address of the dictionary header, if it finds it,
1415 So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer:
1421 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1422 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
1423 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1425 See also >CFA and >DFA.
1427 FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why.
1430 defcode "FIND",4,,FIND
1431 pop %ecx // %ecx = length
1432 pop %edi // %edi = address
1434 push %eax // %eax = address of dictionary entry (or NULL)
1438 push %esi // Save %esi so we can use it in string comparison.
1440 // Now we start searching backwards through the dictionary for this word.
1441 mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary
1443 test %edx,%edx // NULL pointer? (end of the linked list)
1446 // Compare the length expected and the length of the word.
1447 // Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery
1448 // this won't pick the word (the length will appear to be wrong).
1450 movb 4(%edx),%al // %al = flags+length field
1451 andb $(F_HIDDEN|F_LENMASK),%al // %al = name length
1452 cmpb %cl,%al // Length is the same?
1455 // Compare the strings in detail.
1456 push %ecx // Save the length
1457 push %edi // Save the address (repe cmpsb will move this pointer)
1458 lea 5(%edx),%esi // Dictionary string we are checking against.
1459 repe cmpsb // Compare the strings.
1462 jne 2f // Not the same.
1464 // The strings are the same - return the header pointer in %eax
1470 mov (%edx),%edx // Move back through the link field to the previous word
1471 jmp 1b // .. and loop.
1475 xor %eax,%eax // Return zero to indicate not found.
1479 FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall
1480 that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH
1481 word >CFA turns a dictionary pointer into a codeword pointer.
1483 The example below shows the result of:
1485 WORD DOUBLE FIND >CFA
1487 FIND returns a pointer to this
1488 | >CFA converts it to a pointer to this
1491 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1492 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
1493 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1497 Because names vary in length, this isn't just a simple increment.
1499 In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but
1500 that is not true in most FORTH implementations where they store a back pointer in the definition
1501 (with an obvious memory/complexity cost). The reason they do this is that it is useful to be
1502 able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions.
1504 What does CFA stand for? My best guess is "Code Field Address".
1507 defcode ">CFA",4,,TCFA
1514 add $4,%edi // Skip link pointer.
1515 movb (%edi),%al // Load flags+len into %al.
1516 inc %edi // Skip flags+len byte.
1517 andb $F_LENMASK,%al // Just the length, not the flags.
1518 add %eax,%edi // Skip the name.
1519 addl $3,%edi // The codeword is 4-byte aligned.
1524 Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and
1525 returns a pointer to the first data field.
1527 FIND returns a pointer to this
1528 | >CFA converts it to a pointer to this
1530 | | >DFA converts it to a pointer to this
1533 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1534 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
1535 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1537 (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is
1538 different from theirs, because they have an extra indirection).
1540 You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA.
1543 defword ">DFA",4,,TDFA
1544 .int TCFA // >CFA (get code field address)
1545 .int INCR4 // 4+ (add 4 to it to get to next word)
1546 .int EXIT // EXIT (return from FORTH word)
1549 COMPILING ----------------------------------------------------------------------
1551 Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this:
1555 and we have to turn this into:
1557 pointer to previous word
1560 +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1561 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
1562 +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
1563 ^ len pad codeword |
1565 LATEST points here points to codeword of DUP
1567 There are several problems to solve. Where to put the new word? How do we read words? How
1568 do we define the words : (COLON) and ; (SEMICOLON)?
1570 FORTH solves this rather elegantly and as you might expect in a very low-level way which
1571 allows you to change how the compiler works on your own code.
1573 FORTH has an INTERPRETER function (a true interpreter this time, not DOCOL) which runs in a
1574 loop, reading words (using WORD), looking them up (using FIND), turning them into codeword
1575 pointers (using >CFA) and deciding what to do with them.
1577 What it does depends on the mode of the interpreter (in variable STATE).
1579 When STATE is zero, the interpreter just runs each word as it looks them up. This is known as
1582 The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the
1583 interpreter appends the codeword pointer to user memory (the HERE variable points to the next
1584 free byte of user memory).
1586 So you may be able to see how we could define : (COLON). The general plan is:
1588 (1) Use WORD to read the name of the function being defined.
1590 (2) Construct the dictionary entry -- just the header part -- in user memory:
1592 pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where
1593 ^ | the interpreter will start appending
1595 +--|------+---+---+---+---+---+---+---+---+------------+
1596 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
1597 +---------+---+---+---+---+---+---+---+---+------------+
1600 (3) Set LATEST to point to the newly defined word, ...
1602 (4) .. and most importantly leave HERE pointing just after the new codeword. This is where
1603 the interpreter will append codewords.
1605 (5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to
1606 our partially-formed header.
1608 After : has run, our input is here:
1613 Next byte returned by KEY will be the 'D' character of DUP
1615 so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP",
1616 looks it up in the dictionary, gets its codeword pointer, and appends it:
1618 +-- HERE updated to point here.
1621 +---------+---+---+---+---+---+---+---+---+------------+------------+
1622 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP |
1623 +---------+---+---+---+---+---+---+---+---+------------+------------+
1626 Next we read +, get the codeword pointer, and append it:
1628 +-- HERE updated to point here.
1631 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
1632 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + |
1633 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
1636 The issue is what happens next. Obviously what we _don't_ want to happen is that we
1637 read ";" and compile it and go on compiling everything afterwards.
1639 At this point, FORTH uses a trick. Remember the length byte in the dictionary definition
1640 isn't just a plain length byte, but can also contain flags. One flag is called the
1641 IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as
1642 IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_.
1644 This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE.
1645 And all it does is append the codeword for EXIT on to the current definition and switch
1646 back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition
1647 of ; and we'll see that it's really a very simple definition, declared IMMEDIATE.
1649 After the interpreter reads ; and executes it 'immediately', we get this:
1651 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1652 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
1653 +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
1660 And that's it, job done, our new definition is compiled, and we're back in immediate mode
1661 just reading and executing words, perhaps including a call to test our new word DOUBLE.
1663 The only last wrinkle in this is that while our word was being compiled, it was in a
1664 half-finished state. We certainly wouldn't want DOUBLE to be called somehow during
1665 this time. There are several ways to stop this from happening, but in FORTH what we
1666 do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is
1667 being compiled. This prevents FIND from finding it, and thus in theory stops any
1668 chance of it being called.
1670 The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm
1671 going to define them. The : (COLON) function can be made a little bit more general by writing
1672 it in two parts. The first part, called CREATE, makes just the header:
1674 +-- Afterwards, HERE points here.
1677 +---------+---+---+---+---+---+---+---+---+
1678 | LINK | 6 | D | O | U | B | L | E | 0 |
1679 +---------+---+---+---+---+---+---+---+---+
1682 and the second part, the actual definition of : (COLON), calls CREATE and appends the
1683 DOCOL codeword, so leaving:
1685 +-- Afterwards, HERE points here.
1688 +---------+---+---+---+---+---+---+---+---+------------+
1689 | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
1690 +---------+---+---+---+---+---+---+---+---+------------+
1693 CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to
1694 create other types of words (not just ones which contain code, but words which contain variables,
1695 constants and other data).
1698 defcode "CREATE",6,,CREATE
1701 call _WORD // Returns %ecx = length, %edi = pointer to word.
1702 mov %edi,%ebx // %ebx = address of the word
1705 movl var_HERE,%edi // %edi is the address of the header
1706 movl var_LATEST,%eax // Get link pointer
1707 stosl // and store it in the header.
1709 // Length byte and the word itself.
1710 mov %cl,%al // Get the length.
1711 stosb // Store the length/flags byte.
1713 mov %ebx,%esi // %esi = word
1714 rep movsb // Copy the word
1716 addl $3,%edi // Align to next 4 byte boundary.
1719 // Update LATEST and HERE.
1721 movl %eax,var_LATEST
1726 Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words
1729 The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user
1730 data area pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is:
1732 previous value of HERE
1735 +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+
1736 | LINK | 6 | D | O | U | B | L | E | 0 | | <data> |
1737 +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+
1742 and <data> is whatever 32 bit integer was at the top of the stack.
1744 , (COMMA) is quite a fundamental operation when compiling. It is used to append codewords
1745 to the current word that is being compiled.
1748 defcode ",",1,,COMMA
1749 pop %eax // Code pointer to store.
1753 movl var_HERE,%edi // HERE
1755 movl %edi,var_HERE // Update HERE (incremented)
1759 Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode.
1761 Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this
1762 variable we can switch between the two modes.
1764 For various reasons which may become apparent later, FORTH defines two standard words called
1765 [ and ] (LBRAC and RBRAC) which switch between modes:
1767 Word Assembler Action Effect
1768 [ LBRAC STATE := 0 Switch to immediate mode.
1769 ] RBRAC STATE := 1 Switch to compile mode.
1771 [ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the
1772 interpreter saw [ then it would compile it rather than running it. We would never be able to
1773 switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode
1774 the word runs immediately, switching us back to immediate mode.
1777 defcode "[",1,F_IMMED,LBRAC
1779 movl %eax,var_STATE // Set STATE to 0.
1782 defcode "]",1,,RBRAC
1783 movl $1,var_STATE // Set STATE to 1.
1787 Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets
1788 the word HIDDEN and goes into compile mode.
1791 defword ":",1,,COLON
1792 .int CREATE // CREATE the dictionary entry / header
1793 .int LIT, DOCOL, COMMA // Append DOCOL (the codeword).
1794 .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition).
1795 .int RBRAC // Go into compile mode.
1796 .int EXIT // Return from the function.
1799 ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag.
1802 defword ";",1,F_IMMED,SEMICOLON
1803 .int LIT, EXIT, COMMA // Append EXIT (so the word will return).
1804 .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition).
1805 .int LBRAC // Go back to IMMEDIATE mode.
1806 .int EXIT // Return from the function.
1809 EXTENDING THE COMPILER ----------------------------------------------------------------------
1811 Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define
1812 your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because
1813 it allows you in effect to extend the compiler itself. Does gcc let you do that?
1815 Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic
1816 compiler, and are all IMMEDIATE words.
1818 The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word,
1819 or on the current word if you call it in the middle of a definition.
1823 : MYIMMEDWORD IMMEDIATE
1827 but some FORTH programmers write this instead:
1833 The two usages are equivalent, to a first approximation.
1836 defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE
1837 movl var_LATEST,%edi // LATEST word.
1838 addl $4,%edi // Point to name/flags byte.
1839 xorb $F_IMMED,(%edi) // Toggle the IMMED bit.
1843 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the
1844 most recently defined word (used above in : and ; definitions) you would do:
1848 Setting this flag stops the word from being found by FIND, and so can be used to make 'private'
1849 words. For example, to break up a large word into smaller parts you might do:
1851 : SUB1 ... subword ... ;
1852 : SUB2 ... subword ... ;
1853 : SUB3 ... subword ... ;
1854 : MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ;
1855 WORD SUB1 FIND HIDDEN \ Hide SUB1
1856 WORD SUB2 FIND HIDDEN \ Hide SUB2
1857 WORD SUB3 FIND HIDDEN \ Hide SUB3
1859 After this, only MAIN is 'exported' or seen by the rest of the program.
1862 defcode "HIDDEN",6,,HIDDEN
1863 pop %edi // Dictionary entry.
1864 addl $4,%edi // Point to name/flags byte.
1865 xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit.
1869 ' (TICK) is a standard FORTH word which returns the codeword pointer of the next word.
1871 The common usage is:
1875 which appends the codeword of FOO to the current word we are defining (this only works in compiled code).
1877 You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define
1878 a literal 2 might be:
1881 ' LIT , \ Appends LIT to the currently-being-defined word
1882 2 , \ Appends the number 2 to the currently-being-defined word
1889 (If you don't understand how LIT2 works, then you should review the material about compiling words
1890 and immediate mode).
1892 This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in
1893 compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in
1897 lodsl // Get the address of the next word and skip it.
1898 pushl %eax // Push it on the stack.
1902 BRANCHING ----------------------------------------------------------------------
1904 It turns out that all you need in order to define looping constructs, IF-statements, etc.
1907 BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the
1908 top of stack is zero).
1910 The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes,
1911 %esi starts by pointing to the offset field (compare to LIT above):
1913 +---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+
1914 | (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word |
1915 +---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+
1918 | +-----------------------+
1919 %esi added to offset
1921 The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution
1922 continues at the branch target. Negative offsets work as expected.
1924 0BRANCH is the same except the branch happens conditionally.
1926 Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely
1927 in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH
1928 into the word currently being compiled.
1930 As an example, code written like this:
1932 condition-code IF true-part THEN rest-code
1936 condition-code 0BRANCH OFFSET true-part rest-code
1942 defcode "BRANCH",6,,BRANCH
1943 add (%esi),%esi // add the offset to the instruction pointer
1946 defcode "0BRANCH",7,,ZBRANCH
1948 test %eax,%eax // top of stack is zero?
1949 jz code_BRANCH // if so, jump back to the branch function above
1950 lodsl // otherwise we need to skip the offset
1954 LITERAL STRINGS ----------------------------------------------------------------------
1956 LITSTRING is a primitive used to implement the ." and S" operators (which are written in
1957 FORTH). See the definition of those operators later.
1959 TELL just prints a string. It's more efficient to define this in assembly because we
1960 can make it a single Linux syscall.
1963 defcode "LITSTRING",9,,LITSTRING
1964 lodsl // get the length of the string
1965 push %esi // push the address of the start of the string
1966 push %eax // push it on the stack
1967 addl %eax,%esi // skip past the string
1968 addl $3,%esi // but round up to next 4 byte boundary
1972 defcode "TELL",4,,TELL
1973 mov $1,%ebx // 1st param: stdout
1974 pop %edx // 3rd param: length of string
1975 pop %ecx // 2nd param: address of string
1976 mov $__NR_write,%eax // write syscall
1981 COLD START AND INTERPRETER ----------------------------------------------------------------------
1983 COLD is the first FORTH function called, almost immediately after the FORTH system "boots".
1985 INTERPRETER is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate
1986 description -- see: http://en.wikipedia.org/wiki/REPL).
1989 // COLD must not return (ie. must not call EXIT).
1990 defword "COLD",4,,COLD
1991 .int INTERPRETER // call the interpreter loop (never returns)
1993 /* This interpreter is pretty simple, but remember that in FORTH you can always override
1994 * it later with a more powerful one!
1996 defword "INTERPRETER",11,,INTERPRETER
1997 .int INTERPRET,RDROP,INTERPRETER
1999 defcode "INTERPRET",9,,INTERPRET
2000 call _WORD // Returns %ecx = length, %edi = pointer to word.
2002 // Is it in the dictionary?
2004 movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...)
2005 call _FIND // Returns %eax = pointer to header or 0 if not found.
2006 test %eax,%eax // Found?
2009 // In the dictionary. Is it an IMMEDIATE codeword?
2010 mov %eax,%edi // %edi = dictionary entry
2011 movb 4(%edi),%al // Get name+flags.
2012 push %ax // Just save it for now.
2013 call _TCFA // Convert dictionary entry (in %edi) to codeword pointer.
2015 andb $F_IMMED,%al // Is IMMED flag set?
2017 jnz 4f // If IMMED, jump straight to executing.
2021 1: // Not in the dictionary (not a word) so assume it's a literal number.
2022 incl interpret_is_lit
2023 call _SNUMBER // Returns the parsed number in %eax
2025 mov $LIT,%eax // The word is LIT
2027 2: // Are we compiling or executing?
2030 jz 4f // Jump if executing.
2032 // Compiling - just append the word to the current dictionary definition.
2034 mov interpret_is_lit,%ecx // Was it a literal?
2037 mov %ebx,%eax // Yes, so LIT is followed by a number.
2041 4: // Executing - run it!
2042 mov interpret_is_lit,%ecx // Literal?
2043 test %ecx,%ecx // Literal?
2046 // Not a literal, execute it now. This never returns, but the codeword will
2047 // eventually call NEXT which will reenter the loop in INTERPRETER.
2050 5: // Executing a literal, which means push it on the stack.
2057 .int 0 // Flag used to record if reading a literal
2060 ODDS AND ENDS ----------------------------------------------------------------------
2062 CHAR puts the ASCII code of the first character of the following word on the stack. For example
2063 CHAR A puts 65 on the stack.
2065 SYSCALL3 makes a standard Linux system call. (See <asm/unistd.h> for a list of system call
2066 numbers). This is the form to use when the function takes up to three parameters.
2068 In this FORTH, SYSCALL3 must be the last word in the built-in (assembler) dictionary because we
2069 initialise the LATEST variable to point to it. This means that if you want to extend the assembler
2070 part, you must put new words before SYSCALL3, or else change how LATEST is initialised.
2073 defcode "CHAR",4,,CHAR
2074 call _WORD // Returns %ecx = length, %edi = pointer to word.
2076 movb (%edi),%al // Get the first character of the word.
2077 push %eax // Push it onto the stack.
2080 defcode "SYSCALL3",8,,SYSCALL3
2081 pop %eax // System call number (see <asm/unistd.h>)
2082 pop %ebx // First parameter.
2083 pop %ecx // Second parameter
2084 pop %edx // Third parameter
2086 push %eax // Result (negative for -errno)
2090 START OF FORTH CODE ----------------------------------------------------------------------
2092 We've now reached the stage where the FORTH system is running and self-hosting. All further
2093 words can be written as FORTH itself, including words like IF, THEN, .", etc which in most
2094 languages would be considered rather fundamental.
2096 I used to append this here in the assembly file, but I got sick of fighting against gas's
2097 stupid (lack of) multiline string syntax. So now that is in a separate file called jonesforth.f
2099 If you don't already have that file, download it from http://annexia.org/forth in order
2100 to continue the tutorial.
2103 /* END OF jonesforth.S */