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authorDimitri Sokolyuk <demon@dim13.org>2019-10-30 20:04:56 +0100
committerDimitri Sokolyuk <demon@dim13.org>2019-10-30 20:04:56 +0100
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tree56cf4177d5bc0e3ead781d1c60818c13b1df0f3c /jonesforth/jonesforth.S
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-/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
- By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth
- This is PUBLIC DOMAIN (see public domain release statement below).
- $Id: jonesforth.S,v 1.45 2007/10/22 18:53:13 rich Exp $
-
- gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S
-*/
- .set JONES_VERSION,45
-/*
- INTRODUCTION ----------------------------------------------------------------------
-
- FORTH is one of those alien languages which most working programmers regard in the same
- way as Haskell, LISP, and so on. Something so strange that they'd rather any thoughts
- of it just go away so they can get on with writing this paying code. But that's wrong
- and if you care at all about programming then you should at least understand all these
- languages, even if you will never use them.
-
- LISP is the ultimate high-level language, and features from LISP are being added every
- decade to the more common languages. But FORTH is in some ways the ultimate in low level
- programming. Out of the box it lacks features like dynamic memory management and even
- strings. In fact, at its primitive level it lacks even basic concepts like IF-statements
- and loops.
-
- Why then would you want to learn FORTH? There are several very good reasons. First
- and foremost, FORTH is minimal. You really can write a complete FORTH in, say, 2000
- lines of code. I don't just mean a FORTH program, I mean a complete FORTH operating
- system, environment and language. You could boot such a FORTH on a bare PC and it would
- come up with a prompt where you could start doing useful work. The FORTH you have here
- isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making
- it a good tutorial). It's possible to completely understand the system. Who can say they
- completely understand how Linux works, or gcc?
-
- Secondly FORTH has a peculiar bootstrapping property. By that I mean that after writing
- a little bit of assembly to talk to the hardware and implement a few primitives, all the
- rest of the language and compiler is written in FORTH itself. Remember I said before
- that FORTH lacked IF-statements and loops? Well of course it doesn't really because
- such a lanuage would be useless, but my point was rather that IF-statements and loops are
- written in FORTH itself.
-
- Now of course this is common in other languages as well, and in those languages we call
- them 'libraries'. For example in C, 'printf' is a library function written in C. But
- in FORTH this goes way beyond mere libraries. Can you imagine writing C's 'if' in C?
- And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict
- yourself to the usual if/while/for/switch constructs? You want a construct that iterates
- over every other element in a list of numbers? You can add it to the language. What
- about an operator which pulls in variables directly from a configuration file and makes
- them available as FORTH variables? Or how about adding Makefile-like dependencies to
- the language? No problem in FORTH. How about modifying the FORTH compiler to allow
- complex inlining strategies -- simple. This concept isn't common in programming languages,
- but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not
- the lame C preprocessor) and "domain specific languages" (DSLs).
-
- This tutorial isn't about learning FORTH as the language. I'll point you to some references
- you should read if you're not familiar with using FORTH. This tutorial is about how to
- write FORTH. In fact, until you understand how FORTH is written, you'll have only a very
- superficial understanding of how to use it.
-
- So if you're not familiar with FORTH or want to refresh your memory here are some online
- references to read:
-
- http://en.wikipedia.org/wiki/Forth_%28programming_language%29
-
- http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm
-
- http://wiki.laptop.org/go/Forth_Lessons
-
- http://www.albany.net/~hello/simple.htm
-
- Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html
-
- Discussion and criticism of this FORTH here: http://lambda-the-ultimate.org/node/2452
-
- ACKNOWLEDGEMENTS ----------------------------------------------------------------------
-
- This code draws heavily on the design of LINA FORTH (http://home.hccnet.nl/a.w.m.van.der.horst/lina.html)
- by Albert van der Horst. Any similarities in the code are probably not accidental.
-
- Some parts of this FORTH are also based on this IOCCC entry from 1992:
- http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design.
- I was very proud when Sean Barrett, the original author of the IOCCC entry, commented in the LtU thread
- http://lambda-the-ultimate.org/node/2452#comment-36818 about this FORTH.
-
- And finally I'd like to acknowledge the (possibly forgotten?) authors of ARTIC FORTH because their
- original program which I still have on original cassette tape kept nagging away at me all these years.
- http://en.wikipedia.org/wiki/Artic_Software
-
- PUBLIC DOMAIN ----------------------------------------------------------------------
-
- I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide.
-
- In case this is not legally possible, I grant any entity the right to use this work for any purpose,
- without any conditions, unless such conditions are required by law.
-
- SETTING UP ----------------------------------------------------------------------
-
- Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of
- ASCII-art diagrams to explain concepts, the best way to look at this is using a window which
- uses a fixed width font and is at least this wide:
-
- <------------------------------------------------------------------------------------------------------------------------>
-
- Secondly make sure TABS are set to 8 characters. The following should be a vertical
- line. If not, sort out your tabs.
-
- |
- |
- |
-
- Thirdly I assume that your screen is at least 50 characters high.
-
- ASSEMBLING ----------------------------------------------------------------------
-
- If you want to actually run this FORTH, rather than just read it, you will need Linux on an
- i386. Linux because instead of programming directly to the hardware on a bare PC which I
- could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux
- process with a few basic system calls (read, write and exit and that's about all). i386
- is needed because I had to write the assembly for a processor, and i386 is by far the most
- common. (Of course when I say 'i386', any 32- or 64-bit x86 processor will do. I'm compiling
- this on a 64 bit AMD Opteron).
-
- Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to
- assemble and run the code (save this file as 'jonesforth.S') are:
-
- gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S
- cat jonesforth.f - | ./jonesforth
-
- If you want to run your own FORTH programs you can do:
-
- cat jonesforth.f myprog.f | ./jonesforth
-
- If you want to load your own FORTH code and then continue reading user commands, you can do:
-
- cat jonesforth.f myfunctions.f - | ./jonesforth
-
- ASSEMBLER ----------------------------------------------------------------------
-
- (You can just skip to the next section -- you don't need to be able to read assembler to
- follow this tutorial).
-
- However if you do want to read the assembly code here are a few notes about gas (the GNU assembler):
-
- (1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator. The registers
- available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them
- have special purposes.
-
- (2) Add, mov, etc. take arguments in the form SRC,DEST. So mov %eax,%ecx moves %eax -> %ecx
-
- (3) Constants are prefixed with '$', and you mustn't forget it! If you forget it then it
- causes a read from memory instead, so:
- mov $2,%eax moves number 2 into %eax
- mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake)
-
- (4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards"
- and '1b' (etc.) means label '1:' "backwards". Notice that these labels might be mistaken
- for hex numbers (eg. you might confuse 1b with $0x1b).
-
- (5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc.
-
- (6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and
- less repetitive.
-
- For more help reading the assembler, do "info gas" at the Linux prompt.
-
- Now the tutorial starts in earnest.
-
- THE DICTIONARY ----------------------------------------------------------------------
-
- In FORTH as you will know, functions are called "words", and just as in other languages they
- have a name and a definition. Here are two FORTH words:
-
- : DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +"
- : QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE"
-
- Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary
- which is just a linked list of dictionary entries.
-
- <--- DICTIONARY ENTRY (HEADER) ----------------------->
- +------------------------+--------+---------- - - - - +----------- - - - -
- | LINK POINTER | LENGTH/| NAME | DEFINITION
- | | FLAGS | |
- +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - -
-
- I'll come to the definition of the word later. For now just look at the header. The first
- 4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for
- the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte.
- The length of the word can be up to 31 characters (5 bits used) and the top three bits are used
- for various flags which I'll come to later. This is followed by the name itself, and in this
- implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes.
- That's just to ensure that the definition starts on a 32 bit boundary.
-
- A FORTH variable called LATEST contains a pointer to the most recently defined word, in
- other words, the head of this linked list.
-
- DOUBLE and QUADRUPLE might look like this:
-
- pointer to previous word
- ^
- |
- +--|------+---+---+---+---+---+---+---+---+------------- - - - -
- | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...)
- +---------+---+---+---+---+---+---+---+---+------------- - - - -
- ^ len padding
- |
- +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
- | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...)
- +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
- ^ len padding
- |
- |
- LATEST
-
- You should be able to see from this how you might implement functions to find a word in
- the dictionary (just walk along the dictionary entries starting at LATEST and matching
- the names until you either find a match or hit the NULL pointer at the end of the dictionary);
- and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set
- LATEST to point to the new word). We'll see precisely these functions implemented in
- assembly code later on.
-
- One interesting consequence of using a linked list is that you can redefine words, and
- a newer definition of a word overrides an older one. This is an important concept in
- FORTH because it means that any word (even "built-in" or "standard" words) can be
- overridden with a new definition, either to enhance it, to make it faster or even to
- disable it. However because of the way that FORTH words get compiled, which you'll
- understand below, words defined using the old definition of a word continue to use
- the old definition. Only words defined after the new definition use the new definition.
-
- DIRECT THREADED CODE ----------------------------------------------------------------------
-
- Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea
- or coffee and settle down. It's fair to say that if you don't understand this section, then you
- won't "get" how FORTH works, and that would be a failure on my part for not explaining it well.
- So if after reading this section a few times you don't understand it, please email me
- (rich@annexia.org).
-
- Let's talk first about what "threaded code" means. Imagine a peculiar version of C where
- you are only allowed to call functions without arguments. (Don't worry for now that such a
- language would be completely useless!) So in our peculiar C, code would look like this:
-
- f ()
- {
- a ();
- b ();
- c ();
- }
-
- and so on. How would a function, say 'f' above, be compiled by a standard C compiler?
- Probably into assembly code like this. On the right hand side I've written the actual
- i386 machine code.
-
- f:
- CALL a E8 08 00 00 00
- CALL b E8 1C 00 00 00
- CALL c E8 2C 00 00 00
- ; ignore the return from the function for now
-
- "E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing
- memory was hideously expensive and we might have worried about the wasted space being used
- by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory)
- by compressing this into just:
-
- 08 00 00 00 Just the function addresses, without
- 1C 00 00 00 the CALL prefix.
- 2C 00 00 00
-
- On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%.
-
- [Historical note: If the execution model that FORTH uses looks strange from the following
- paragraphs, then it was motivated entirely by the need to save memory on early computers.
- This code compression isn't so important now when our machines have more memory in their L1
- caches than those early computers had in total, but the execution model still has some
- useful properties].
-
- Of course this code won't run directly on the CPU any more. Instead we need to write an
- interpreter which takes each set of bytes and calls it.
-
- On an i386 machine it turns out that we can write this interpreter rather easily, in just
- two assembly instructions which turn into just 3 bytes of machine code. Let's store the
- pointer to the next word to execute in the %esi register:
-
- 08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute.
- %esi -> 1C 00 00 00
- 2C 00 00 00
-
- The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does
- two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it
- increments %esi by 4 bytes. So after LODSL, the situation now looks like this:
-
- 08 00 00 00 <- We're still executing this one
- 1C 00 00 00 <- %eax now contains this address (0x0000001C)
- %esi -> 2C 00 00 00
-
- Now we just need to jump to the address in %eax. This is again just a single x86 instruction
- written JMP *(%eax). And after doing the jump, the situation looks like:
-
- 08 00 00 00
- 1C 00 00 00 <- Now we're executing this subroutine.
- %esi -> 2C 00 00 00
-
- To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)'
- which literally make the jump to the next subroutine.
-
- And that brings us to our first piece of actual code! Well, it's a macro.
-*/
-
-/* NEXT macro. */
- .macro NEXT
- lodsl
- jmp *(%eax)
- .endm
-
-/* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions.
-
- Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like
- a return.
-
- The above describes what is known as direct threaded code.
-
- To sum up: We compress our function calls down to a list of addresses and use a somewhat
- magical macro to act as a "jump to next function in the list". We also use one register (%esi)
- to act as a kind of instruction pointer, pointing to the next function in the list.
-
- I'll just give you a hint of what is to come by saying that a FORTH definition such as:
-
- : QUADRUPLE DOUBLE DOUBLE ;
-
- actually compiles (almost, not precisely but we'll see why in a moment) to a list of
- function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off.
-
- At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!".
-
- I lied about JMP *(%eax).
-
- INDIRECT THREADED CODE ----------------------------------------------------------------------
-
- It turns out that direct threaded code is interesting but only if you want to just execute
- a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE
- was an assembly language function. In the direct threaded code, QUADRUPLE would look like:
-
- +------------------+
- | addr of DOUBLE --------------------> (assembly code to do the double)
- +------------------+ NEXT
- %esi -> | addr of DOUBLE |
- +------------------+
-
- We can add an extra indirection to allow us to run both words written in assembly language
- (primitives written for speed) and words written in FORTH themselves as lists of addresses.
-
- The extra indirection is the reason for the brackets in JMP *(%eax).
-
- Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH:
-
- : QUADRUPLE DOUBLE DOUBLE ;
-
- +------------------+
- | codeword | : DOUBLE DUP + ;
- +------------------+
- | addr of DOUBLE ---------------> +------------------+
- +------------------+ | codeword |
- | addr of DOUBLE | +------------------+
- +------------------+ | addr of DUP --------------> +------------------+
- | addr of EXIT | +------------------+ | codeword -------+
- +------------------+ %esi -> | addr of + --------+ +------------------+ |
- +------------------+ | | assembly to <-----+
- | addr of EXIT | | | implement DUP |
- +------------------+ | | .. |
- | | .. |
- | | NEXT |
- | +------------------+
- |
- +-----> +------------------+
- | codeword -------+
- +------------------+ |
- | assembly to <------+
- | implement + |
- | .. |
- | .. |
- | NEXT |
- +------------------+
-
- This is the part where you may need an extra cup of tea/coffee/favourite caffeinated
- beverage. What has changed is that I've added an extra pointer to the beginning of
- the definitions. In FORTH this is sometimes called the "codeword". The codeword is
- a pointer to the interpreter to run the function. For primitives written in
- assembly language, the "interpreter" just points to the actual assembly code itself.
- They don't need interpreting, they just run.
-
- In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter
- function.
-
- I'll show you the interpreter function shortly, but let's recall our indirect
- JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE
- as shown, and DUP has been called. Note that %esi is pointing to the address of +
-
- The assembly code for DUP eventually does a NEXT. That:
-
- (1) reads the address of + into %eax %eax points to the codeword of +
- (2) increments %esi by 4
- (3) jumps to the indirect %eax jumps to the address in the codeword of +,
- ie. the assembly code to implement +
-
- +------------------+
- | codeword |
- +------------------+
- | addr of DOUBLE ---------------> +------------------+
- +------------------+ | codeword |
- | addr of DOUBLE | +------------------+
- +------------------+ | addr of DUP --------------> +------------------+
- | addr of EXIT | +------------------+ | codeword -------+
- +------------------+ | addr of + --------+ +------------------+ |
- +------------------+ | | assembly to <-----+
- %esi -> | addr of EXIT | | | implement DUP |
- +------------------+ | | .. |
- | | .. |
- | | NEXT |
- | +------------------+
- |
- +-----> +------------------+
- | codeword -------+
- +------------------+ |
- now we're | assembly to <-----+
- executing | implement + |
- this | .. |
- function | .. |
- | NEXT |
- +------------------+
-
- So I hope that I've convinced you that NEXT does roughly what you'd expect. This is
- indirect threaded code.
-
- I've glossed over four things. I wonder if you can guess without reading on what they are?
-
- .
- .
- .
-
- My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do
- you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but
- then point at part of DOUBLE. (3) What goes in the codeword for the words which are written
- in FORTH? (4) How do you compile a function which does anything except call other functions
- ie. a function which contains a number like : DOUBLE 2 * ; ?
-
- THE INTERPRETER AND RETURN STACK ------------------------------------------------------------
-
- Going at these in no particular order, let's talk about issues (3) and (2), the interpreter
- and the return stack.
-
- Words which are defined in FORTH need a codeword which points to a little bit of code to
- give them a "helping hand" in life. They don't need much, but they do need what is known
- as an "interpreter", although it doesn't really "interpret" in the same way that, say,
- Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few
- machine registers so that the word can then execute at full speed using the indirect
- threaded model above.
-
- One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old
- %esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE.
- Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like
- a function call), we will need a stack to store these "return addresses" (old values of %esi).
-
- As you will have seen in the background documentation, FORTH has two stacks, an ordinary
- stack for parameters, and a return stack which is a bit more mysterious. But our return
- stack is just the stack I talked about in the previous paragraph, used to save %esi when
- calling from a FORTH word into another FORTH word.
-
- In this FORTH, we are using the normal stack pointer (%esp) for the parameter stack.
- We will use the i386's "other" stack pointer (%ebp, usually called the "frame pointer")
- for our return stack.
-
- I've got two macros which just wrap up the details of using %ebp for the return stack.
- You use them as for example "PUSHRSP %eax" (push %eax on the return stack) or "POPRSP %ebx"
- (pop top of return stack into %ebx).
-*/
-
-/* Macros to deal with the return stack. */
- .macro PUSHRSP reg
- lea -4(%ebp),%ebp // push reg on to return stack
- movl \reg,(%ebp)
- .endm
-
- .macro POPRSP reg
- mov (%ebp),\reg // pop top of return stack to reg
- lea 4(%ebp),%ebp
- .endm
-
-/*
- And with that we can now talk about the interpreter.
-
- In FORTH the interpreter function is often called DOCOL (I think it means "DO COLON" because
- all FORTH definitions start with a colon, as in : DOUBLE DUP + ;
-
- The "interpreter" (it's not really "interpreting") just needs to push the old %esi on the
- stack and set %esi to the first word in the definition. Remember that we jumped to the
- function using JMP *(%eax)? Well a consequence of that is that conveniently %eax contains
- the address of this codeword, so just by adding 4 to it we get the address of the first
- data word. Finally after setting up %esi, it just does NEXT which causes that first word
- to run.
-*/
-
-/* DOCOL - the interpreter! */
- .text
- .align 4
-DOCOL:
- PUSHRSP %esi // push %esi on to the return stack
- addl $4,%eax // %eax points to codeword, so make
- movl %eax,%esi // %esi point to first data word
- NEXT
-
-/*
- Just to make this absolutely clear, let's see how DOCOL works when jumping from QUADRUPLE
- into DOUBLE:
-
- QUADRUPLE:
- +------------------+
- | codeword |
- +------------------+ DOUBLE:
- | addr of DOUBLE ---------------> +------------------+
- +------------------+ %eax -> | addr of DOCOL |
- %esi -> | addr of DOUBLE | +------------------+
- +------------------+ | addr of DUP |
- | addr of EXIT | +------------------+
- +------------------+ | etc. |
-
- First, the call to DOUBLE calls DOCOL (the codeword of DOUBLE). DOCOL does this: It
- pushes the old %esi on the return stack. %eax points to the codeword of DOUBLE, so we
- just add 4 on to it to get our new %esi:
-
- QUADRUPLE:
- +------------------+
- | codeword |
- +------------------+ DOUBLE:
- | addr of DOUBLE ---------------> +------------------+
-top of return +------------------+ %eax -> | addr of DOCOL |
-stack points -> | addr of DOUBLE | + 4 = +------------------+
- +------------------+ %esi -> | addr of DUP |
- | addr of EXIT | +------------------+
- +------------------+ | etc. |
-
- Then we do NEXT, and because of the magic of threaded code that increments %esi again
- and calls DUP.
-
- Well, it seems to work.
-
- One minor point here. Because DOCOL is the first bit of assembly actually to be defined
- in this file (the others were just macros), and because I usually compile this code with the
- text segment starting at address 0, DOCOL has address 0. So if you are disassembling the
- code and see a word with a codeword of 0, you will immediately know that the word is
- written in FORTH (it's not an assembler primitive) and so uses DOCOL as the interpreter.
-
- STARTING UP ----------------------------------------------------------------------
-
- Now let's get down to nuts and bolts. When we start the program we need to set up
- a few things like the return stack. But as soon as we can, we want to jump into FORTH
- code (albeit much of the "early" FORTH code will still need to be written as
- assembly language primitives).
-
- This is what the set up code does. Does a tiny bit of house-keeping, sets up the
- separate return stack (NB: Linux gives us the ordinary parameter stack already), then
- immediately jumps to a FORTH word called QUIT. Despite its name, QUIT doesn't quit
- anything. It resets some internal state and starts reading and interpreting commands.
- (The reason it is called QUIT is because you can call QUIT from your own FORTH code
- to "quit" your program and go back to interpreting).
-*/
-
-/* Assembler entry point. */
- .text
- .globl _start
-_start:
- cld
- mov %esp,var_S0 // Save the initial data stack pointer in FORTH variable S0.
- mov $return_stack_top,%ebp // Initialise the return stack.
- call set_up_data_segment
-
- mov $cold_start,%esi // Initialise interpreter.
- NEXT // Run interpreter!
-
- .section .rodata
-cold_start: // High-level code without a codeword.
- .int QUIT
-
-/*
- BUILT-IN WORDS ----------------------------------------------------------------------
-
- Remember our dictionary entries (headers)? Let's bring those together with the codeword
- and data words to see how : DOUBLE DUP + ; really looks in memory.
-
- pointer to previous word
- ^
- |
- +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
- +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
- ^ len pad codeword |
- | V
- LINK in next word points to codeword of DUP
-
- Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we
- don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc.
- So instead we will have to define built-in words using the GNU assembler data constructors
- (like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are
- unsure of them).
-
- The long way would be:
-
- .int <link to previous word>
- .byte 6 // len
- .ascii "DOUBLE" // string
- .byte 0 // padding
-DOUBLE: .int DOCOL // codeword
- .int DUP // pointer to codeword of DUP
- .int PLUS // pointer to codeword of +
- .int EXIT // pointer to codeword of EXIT
-
- That's going to get quite tedious rather quickly, so here I define an assembler macro
- so that I can just write:
-
- defword "DOUBLE",6,,DOUBLE
- .int DUP,PLUS,EXIT
-
- and I'll get exactly the same effect.
-
- Don't worry too much about the exact implementation details of this macro - it's complicated!
-*/
-
-/* Flags - these are discussed later. */
- .set F_IMMED,0x80
- .set F_HIDDEN,0x20
- .set F_LENMASK,0x1f // length mask
-
- // Store the chain of links.
- .set link,0
-
- .macro defword name, namelen, flags=0, label
- .section .rodata
- .align 4
- .globl name_\label
-name_\label :
- .int link // link
- .set link,name_\label
- .byte \flags+\namelen // flags + length byte
- .ascii "\name" // the name
- .align 4 // padding to next 4 byte boundary
- .globl \label
-\label :
- .int DOCOL // codeword - the interpreter
- // list of word pointers follow
- .endm
-
-/*
- Similarly I want a way to write words written in assembly language. There will quite a few
- of these to start with because, well, everything has to start in assembly before there's
- enough "infrastructure" to be able to start writing FORTH words, but also I want to define
- some common FORTH words in assembly language for speed, even though I could write them in FORTH.
-
- This is what DUP looks like in memory:
-
- pointer to previous word
- ^
- |
- +--|------+---+---+---+---+------------+
- | LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly
- +---------+---+---+---+---+------------+ code used to write DUP,
- ^ len codeword which ends with NEXT.
- |
- LINK in next word
-
- Again, for brevity in writing the header I'm going to write an assembler macro called defcode.
- As with defword above, don't worry about the complicated details of the macro.
-*/
-
- .macro defcode name, namelen, flags=0, label
- .section .rodata
- .align 4
- .globl name_\label
-name_\label :
- .int link // link
- .set link,name_\label
- .byte \flags+\namelen // flags + length byte
- .ascii "\name" // the name
- .align 4 // padding to next 4 byte boundary
- .globl \label
-\label :
- .int code_\label // codeword
- .text
- //.align 4
- .globl code_\label
-code_\label : // assembler code follows
- .endm
-
-/*
- Now some easy FORTH primitives. These are written in assembly for speed. If you understand
- i386 assembly language then it is worth reading these. However if you don't understand assembly
- you can skip the details.
-*/
-
- defcode "DROP",4,,DROP
- pop %eax // drop top of stack
- NEXT
-
- defcode "SWAP",4,,SWAP
- pop %eax // swap top two elements on stack
- pop %ebx
- push %eax
- push %ebx
- NEXT
-
- defcode "DUP",3,,DUP
- mov (%esp),%eax // duplicate top of stack
- push %eax
- NEXT
-
- defcode "OVER",4,,OVER
- mov 4(%esp),%eax // get the second element of stack
- push %eax // and push it on top
- NEXT
-
- defcode "ROT",3,,ROT
- pop %eax
- pop %ebx
- pop %ecx
- push %eax
- push %ecx
- push %ebx
- NEXT
-
- defcode "-ROT",4,,NROT
- pop %eax
- pop %ebx
- pop %ecx
- push %ebx
- push %eax
- push %ecx
- NEXT
-
- defcode "2DROP",5,,TWODROP // drop top two elements of stack
- pop %eax
- pop %eax
- NEXT
-
- defcode "2DUP",4,,TWODUP // duplicate top two elements of stack
- mov (%esp),%eax
- mov 4(%esp),%ebx
- push %ebx
- push %eax
- NEXT
-
- defcode "2SWAP",5,,TWOSWAP // swap top two pairs of elements of stack
- pop %eax
- pop %ebx
- pop %ecx
- pop %edx
- push %ebx
- push %eax
- push %edx
- push %ecx
- NEXT
-
- defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero
- movl (%esp),%eax
- test %eax,%eax
- jz 1f
- push %eax
-1: NEXT
-
- defcode "1+",2,,INCR
- incl (%esp) // increment top of stack
- NEXT
-
- defcode "1-",2,,DECR
- decl (%esp) // decrement top of stack
- NEXT
-
- defcode "4+",2,,INCR4
- addl $4,(%esp) // add 4 to top of stack
- NEXT
-
- defcode "4-",2,,DECR4
- subl $4,(%esp) // subtract 4 from top of stack
- NEXT
-
- defcode "+",1,,ADD
- pop %eax // get top of stack
- addl %eax,(%esp) // and add it to next word on stack
- NEXT
-
- defcode "-",1,,SUB
- pop %eax // get top of stack
- subl %eax,(%esp) // and subtract it from next word on stack
- NEXT
-
- defcode "*",1,,MUL
- pop %eax
- pop %ebx
- imull %ebx,%eax
- push %eax // ignore overflow
- NEXT
-
-/*
- In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in
- terms of the primitive /MOD. The design of the i386 assembly instruction idiv which
- leaves both quotient and remainder makes this the obvious choice.
-*/
-
- defcode "/MOD",4,,DIVMOD
- xor %edx,%edx
- pop %ebx
- pop %eax
- idivl %ebx
- push %edx // push remainder
- push %eax // push quotient
- NEXT
-
-/*
- Lots of comparison operations like =, <, >, etc..
-
- ANS FORTH says that the comparison words should return all (binary) 1's for
- TRUE and all 0's for FALSE. However this is a bit of a strange convention
- so this FORTH breaks it and returns the more normal (for C programmers ...)
- 1 meaning TRUE and 0 meaning FALSE.
-*/
-
- defcode "=",1,,EQU // top two words are equal?
- pop %eax
- pop %ebx
- cmp %ebx,%eax
- sete %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "<>",2,,NEQU // top two words are not equal?
- pop %eax
- pop %ebx
- cmp %ebx,%eax
- setne %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "<",1,,LT
- pop %eax
- pop %ebx
- cmp %eax,%ebx
- setl %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode ">",1,,GT
- pop %eax
- pop %ebx
- cmp %eax,%ebx
- setg %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "<=",2,,LE
- pop %eax
- pop %ebx
- cmp %eax,%ebx
- setle %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode ">=",2,,GE
- pop %eax
- pop %ebx
- cmp %eax,%ebx
- setge %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "0=",2,,ZEQU // top of stack equals 0?
- pop %eax
- test %eax,%eax
- setz %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "0<>",3,,ZNEQU // top of stack not 0?
- pop %eax
- test %eax,%eax
- setnz %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "0<",2,,ZLT // comparisons with 0
- pop %eax
- test %eax,%eax
- setl %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "0>",2,,ZGT
- pop %eax
- test %eax,%eax
- setg %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "0<=",3,,ZLE
- pop %eax
- test %eax,%eax
- setle %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "0>=",3,,ZGE
- pop %eax
- test %eax,%eax
- setge %al
- movzbl %al,%eax
- pushl %eax
- NEXT
-
- defcode "AND",3,,AND // bitwise AND
- pop %eax
- andl %eax,(%esp)
- NEXT
-
- defcode "OR",2,,OR // bitwise OR
- pop %eax
- orl %eax,(%esp)
- NEXT
-
- defcode "XOR",3,,XOR // bitwise XOR
- pop %eax
- xorl %eax,(%esp)
- NEXT
-
- defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE and NOT)
- notl (%esp)
- NEXT
-
-/*
- RETURNING FROM FORTH WORDS ----------------------------------------------------------------------
-
- Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called
- DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing):
-
- QUADRUPLE
- +------------------+
- | codeword |
- +------------------+ DOUBLE
- | addr of DOUBLE ---------------> +------------------+
- +------------------+ | codeword |
- | addr of DOUBLE | +------------------+
- +------------------+ | addr of DUP |
- | addr of EXIT | +------------------+
- +------------------+ | addr of + |
- +------------------+
- %esi -> | addr of EXIT |
- +------------------+
-
- What happens when the + function does NEXT? Well, the following code is executed.
-*/
-
- defcode "EXIT",4,,EXIT
- POPRSP %esi // pop return stack into %esi
- NEXT
-
-/*
- EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi.
- So after this (but just before NEXT) we get:
-
- QUADRUPLE
- +------------------+
- | codeword |
- +------------------+ DOUBLE
- | addr of DOUBLE ---------------> +------------------+
- +------------------+ | codeword |
- %esi -> | addr of DOUBLE | +------------------+
- +------------------+ | addr of DUP |
- | addr of EXIT | +------------------+
- +------------------+ | addr of + |
- +------------------+
- | addr of EXIT |
- +------------------+
-
- And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-)
-
- LITERALS ----------------------------------------------------------------------
-
- The final point I "glossed over" before was how to deal with functions that do anything
- apart from calling other functions. For example, suppose that DOUBLE was defined like this:
-
- : DOUBLE 2 * ;
-
- It does the same thing, but how do we compile it since it contains the literal 2? One way
- would be to have a function called "2" (which you'd have to write in assembler), but you'd need
- a function for every single literal that you wanted to use.
-
- FORTH solves this by compiling the function using a special word called LIT:
-
- +---------------------------+-------+-------+-------+-------+-------+
- | (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT |
- +---------------------------+-------+-------+-------+-------+-------+
-
- LIT is executed in the normal way, but what it does next is definitely not normal. It
- looks at %esi (which now points to the number 2), grabs it, pushes it on the stack, then
- manipulates %esi in order to skip the number as if it had never been there.
-
- What's neat is that the whole grab/manipulate can be done using a single byte single
- i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams,
- see if you can find out how LIT works:
-*/
-
- defcode "LIT",3,,LIT
- // %esi points to the next command, but in this case it points to the next
- // literal 32 bit integer. Get that literal into %eax and increment %esi.
- // On x86, it's a convenient single byte instruction! (cf. NEXT macro)
- lodsl
- push %eax // push the literal number on to stack
- NEXT
-
-/*
- MEMORY ----------------------------------------------------------------------
-
- As important point about FORTH is that it gives you direct access to the lowest levels
- of the machine. Manipulating memory directly is done frequently in FORTH, and these are
- the primitive words for doing it.
-*/
-
- defcode "!",1,,STORE
- pop %ebx // address to store at
- pop %eax // data to store there
- mov %eax,(%ebx) // store it
- NEXT
-
- defcode "@",1,,FETCH
- pop %ebx // address to fetch
- mov (%ebx),%eax // fetch it
- push %eax // push value onto stack
- NEXT
-
- defcode "+!",2,,ADDSTORE
- pop %ebx // address
- pop %eax // the amount to add
- addl %eax,(%ebx) // add it
- NEXT
-
- defcode "-!",2,,SUBSTORE
- pop %ebx // address
- pop %eax // the amount to subtract
- subl %eax,(%ebx) // add it
- NEXT
-
-/*
- ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes
- so we also define standard words C@ and C!.
-
- Byte-oriented operations only work on architectures which permit them (i386 is one of those).
- */
-
- defcode "C!",2,,STOREBYTE
- pop %ebx // address to store at
- pop %eax // data to store there
- movb %al,(%ebx) // store it
- NEXT
-
- defcode "C@",2,,FETCHBYTE
- pop %ebx // address to fetch
- xor %eax,%eax
- movb (%ebx),%al // fetch it
- push %eax // push value onto stack
- NEXT
-
-/* C@C! is a useful byte copy primitive. */
- defcode "C@C!",4,,CCOPY
- movl 4(%esp),%ebx // source address
- movb (%ebx),%al // get source character
- pop %edi // destination address
- stosb // copy to destination
- push %edi // increment destination address
- incl 4(%esp) // increment source address
- NEXT
-
-/* and CMOVE is a block copy operation. */
- defcode "CMOVE",5,,CMOVE
- mov %esi,%edx // preserve %esi
- pop %ecx // length
- pop %edi // destination address
- pop %esi // source address
- rep movsb // copy source to destination
- mov %edx,%esi // restore %esi
- NEXT
-
-/*
- BUILT-IN VARIABLES ----------------------------------------------------------------------
-
- These are some built-in variables and related standard FORTH words. Of these, the only one that we
- have discussed so far was LATEST, which points to the last (most recently defined) word in the
- FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable)
- on to the stack, so you can read or write it using @ and ! operators. For example, to print
- the current value of LATEST (and this can apply to any FORTH variable) you would do:
-
- LATEST @ . CR
-
- To make defining variables shorter, I'm using a macro called defvar, similar to defword and
- defcode above. (In fact the defvar macro uses defcode to do the dictionary header).
-*/
-
- .macro defvar name, namelen, flags=0, label, initial=0
- defcode \name,\namelen,\flags,\label
- push $var_\name
- NEXT
- .data
- .align 4
-var_\name :
- .int \initial
- .endm
-
-/*
- The built-in variables are:
-
- STATE Is the interpreter executing code (0) or compiling a word (non-zero)?
- LATEST Points to the latest (most recently defined) word in the dictionary.
- HERE Points to the next free byte of memory. When compiling, compiled words go here.
- S0 Stores the address of the top of the parameter stack.
- BASE The current base for printing and reading numbers.
-
-*/
- defvar "STATE",5,,STATE
- defvar "HERE",4,,HERE
- defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary
- defvar "S0",2,,SZ
- defvar "BASE",4,,BASE,10
-
-/*
- BUILT-IN CONSTANTS ----------------------------------------------------------------------
-
- It's also useful to expose a few constants to FORTH. When the word is executed it pushes a
- constant value on the stack.
-
- The built-in constants are:
-
- VERSION Is the current version of this FORTH.
- R0 The address of the top of the return stack.
- DOCOL Pointer to DOCOL.
- F_IMMED The IMMEDIATE flag's actual value.
- F_HIDDEN The HIDDEN flag's actual value.
- F_LENMASK The length mask in the flags/len byte.
-
- SYS_* and the numeric codes of various Linux syscalls (from <asm/unistd.h>)
-*/
-
-//#include <asm-i386/unistd.h> // you might need this instead
-#include <asm/unistd.h>
-
- .macro defconst name, namelen, flags=0, label, value
- defcode \name,\namelen,\flags,\label
- push $\value
- NEXT
- .endm
-
- defconst "VERSION",7,,VERSION,JONES_VERSION
- defconst "R0",2,,RZ,return_stack_top
- defconst "DOCOL",5,,__DOCOL,DOCOL
- defconst "F_IMMED",7,,__F_IMMED,F_IMMED
- defconst "F_HIDDEN",8,,__F_HIDDEN,F_HIDDEN
- defconst "F_LENMASK",9,,__F_LENMASK,F_LENMASK
-
- defconst "SYS_EXIT",8,,SYS_EXIT,__NR_exit
- defconst "SYS_OPEN",8,,SYS_OPEN,__NR_open
- defconst "SYS_CLOSE",9,,SYS_CLOSE,__NR_close
- defconst "SYS_READ",8,,SYS_READ,__NR_read
- defconst "SYS_WRITE",9,,SYS_WRITE,__NR_write
- defconst "SYS_CREAT",9,,SYS_CREAT,__NR_creat
- defconst "SYS_BRK",7,,SYS_BRK,__NR_brk
-
- defconst "O_RDONLY",8,,__O_RDONLY,0
- defconst "O_WRONLY",8,,__O_WRONLY,1
- defconst "O_RDWR",6,,__O_RDWR,2
- defconst "O_CREAT",7,,__O_CREAT,0100
- defconst "O_EXCL",6,,__O_EXCL,0200
- defconst "O_TRUNC",7,,__O_TRUNC,01000
- defconst "O_APPEND",8,,__O_APPEND,02000
- defconst "O_NONBLOCK",10,,__O_NONBLOCK,04000
-
-/*
- RETURN STACK ----------------------------------------------------------------------
-
- These words allow you to access the return stack. Recall that the register %ebp always points to
- the top of the return stack.
-*/
-
- defcode ">R",2,,TOR
- pop %eax // pop parameter stack into %eax
- PUSHRSP %eax // push it on to the return stack
- NEXT
-
- defcode "R>",2,,FROMR
- POPRSP %eax // pop return stack on to %eax
- push %eax // and push on to parameter stack
- NEXT
-
- defcode "RSP@",4,,RSPFETCH
- push %ebp
- NEXT
-
- defcode "RSP!",4,,RSPSTORE
- pop %ebp
- NEXT
-
- defcode "RDROP",5,,RDROP
- addl $4,%ebp // pop return stack and throw away
- NEXT
-
-/*
- PARAMETER (DATA) STACK ----------------------------------------------------------------------
-
- These functions allow you to manipulate the parameter stack. Recall that Linux sets up the parameter
- stack for us, and it is accessed through %esp.
-*/
-
- defcode "DSP@",4,,DSPFETCH
- mov %esp,%eax
- push %eax
- NEXT
-
- defcode "DSP!",4,,DSPSTORE
- pop %esp
- NEXT
-
-/*
- INPUT AND OUTPUT ----------------------------------------------------------------------
-
- These are our first really meaty/complicated FORTH primitives. I have chosen to write them in
- assembler, but surprisingly in "real" FORTH implementations these are often written in terms
- of more fundamental FORTH primitives. I chose to avoid that because I think that just obscures
- the implementation. After all, you may not understand assembler but you can just think of it
- as an opaque block of code that does what it says.
-
- Let's discuss input first.
-
- The FORTH word KEY reads the next byte from stdin (and pushes it on the parameter stack).
- So if KEY is called and someone hits the space key, then the number 32 (ASCII code of space)
- is pushed on the stack.
-
- In FORTH there is no distinction between reading code and reading input. We might be reading
- and compiling code, we might be reading words to execute, we might be asking for the user
- to type their name -- ultimately it all comes in through KEY.
-
- The implementation of KEY uses an input buffer of a certain size (defined at the end of this
- file). It calls the Linux read(2) system call to fill this buffer and tracks its position
- in the buffer using a couple of variables, and if it runs out of input buffer then it refills
- it automatically. The other thing that KEY does is if it detects that stdin has closed, it
- exits the program, which is why when you hit ^D the FORTH system cleanly exits.
-
- buffer bufftop
- | |
- V V
- +-------------------------------+--------------------------------------+
- | INPUT READ FROM STDIN ....... | unused part of the buffer |
- +-------------------------------+--------------------------------------+
- ^
- |
- currkey (next character to read)
-
- <---------------------- BUFFER_SIZE (4096 bytes) ---------------------->
-*/
-
- defcode "KEY",3,,KEY
- call _KEY
- push %eax // push return value on stack
- NEXT
-_KEY:
- mov (currkey),%ebx
- cmp (bufftop),%ebx
- jge 1f // exhausted the input buffer?
- xor %eax,%eax
- mov (%ebx),%al // get next key from input buffer
- inc %ebx
- mov %ebx,(currkey) // increment currkey
- ret
-
-1: // Out of input; use read(2) to fetch more input from stdin.
- xor %ebx,%ebx // 1st param: stdin
- mov $buffer,%ecx // 2nd param: buffer
- mov %ecx,currkey
- mov $BUFFER_SIZE,%edx // 3rd param: max length
- mov $__NR_read,%eax // syscall: read
- int $0x80
- test %eax,%eax // If %eax <= 0, then exit.
- jbe 2f
- addl %eax,%ecx // buffer+%eax = bufftop
- mov %ecx,bufftop
- jmp _KEY
-
-2: // Error or end of input: exit the program.
- xor %ebx,%ebx
- mov $__NR_exit,%eax // syscall: exit
- int $0x80
-
- .data
- .align 4
-currkey:
- .int buffer // Current place in input buffer (next character to read).
-bufftop:
- .int buffer // Last valid data in input buffer + 1.
-
-/*
- By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout.
- This implementation just uses the write system call. No attempt is made to buffer output, but
- it would be a good exercise to add it.
-*/
-
- defcode "EMIT",4,,EMIT
- pop %eax
- call _EMIT
- NEXT
-_EMIT:
- mov $1,%ebx // 1st param: stdout
-
- // write needs the address of the byte to write
- mov %al,emit_scratch
- mov $emit_scratch,%ecx // 2nd param: address
-
- mov $1,%edx // 3rd param: nbytes = 1
-
- mov $__NR_write,%eax // write syscall
- int $0x80
- ret
-
- .data // NB: easier to fit in the .data section
-emit_scratch:
- .space 1 // scratch used by EMIT
-
-/*
- Back to input, WORD is a FORTH word which reads the next full word of input.
-
- What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on).
- Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it
- calculates the length of the word it read and returns the address and the length as
- two words on the stack (with the length at the top of stack).
-
- Notice that WORD has a single internal buffer which it overwrites each time (rather like
- a static C string). Also notice that WORD's internal buffer is just 32 bytes long and
- there is NO checking for overflow. 31 bytes happens to be the maximum length of a
- FORTH word that we support, and that is what WORD is used for: to read FORTH words when
- we are compiling and executing code. The returned strings are not NUL-terminated.
-
- Start address+length is the normal way to represent strings in FORTH (not ending in an
- ASCII NUL character as in C), and so FORTH strings can contain any character including NULs
- and can be any length.
-
- WORD is not suitable for just reading strings (eg. user input) because of all the above
- peculiarities and limitations.
-
- Note that when executing, you'll see:
- WORD FOO
- which puts "FOO" and length 3 on the stack, but when compiling:
- : BAR WORD FOO ;
- is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling
- and immediate mode, and you'll understand why.
-*/
-
- defcode "WORD",4,,WORD
- call _WORD
- push %edi // push base address
- push %ecx // push length
- NEXT
-
-_WORD:
- /* Search for first non-blank character. Also skip \ comments. */
-1:
- call _KEY // get next key, returned in %eax
- cmpb $'\\',%al // start of a comment?
- je 3f // if so, skip the comment
- cmpb $' ',%al
- jbe 1b // if so, keep looking
-
- /* Search for the end of the word, storing chars as we go. */
- mov $word_buffer,%edi // pointer to return buffer
-2:
- stosb // add character to return buffer
- call _KEY // get next key, returned in %al
- cmpb $' ',%al // is blank?
- ja 2b // if not, keep looping
-
- /* Return the word (well, the static buffer) and length. */
- sub $word_buffer,%edi
- mov %edi,%ecx // return length of the word
- mov $word_buffer,%edi // return address of the word
- ret
-
- /* Code to skip \ comments to end of the current line. */
-3:
- call _KEY
- cmpb $'\n',%al // end of line yet?
- jne 3b
- jmp 1b
-
- .data // NB: easier to fit in the .data section
- // A static buffer where WORD returns. Subsequent calls
- // overwrite this buffer. Maximum word length is 32 chars.
-word_buffer:
- .space 32
-
-/*
- As well as reading in words we'll need to read in numbers and for that we are using a function
- called NUMBER. This parses a numeric string such as one returned by WORD and pushes the
- number on the parameter stack.
-
- The function uses the variable BASE as the base (radix) for conversion, so for example if
- BASE is 2 then we expect a binary number. Normally BASE is 10.
-
- If the word starts with a '-' character then the returned value is negative.
-
- If the string can't be parsed as a number (or contains characters outside the current BASE)
- then we need to return an error indication. So NUMBER actually returns two items on the stack.
- At the top of stack we return the number of unconverted characters (ie. if 0 then all characters
- were converted, so there is no error). Second from top of stack is the parsed number or a
- partial value if there was an error.
-*/
- defcode "NUMBER",6,,NUMBER
- pop %ecx // length of string
- pop %edi // start address of string
- call _NUMBER
- push %eax // parsed number
- push %ecx // number of unparsed characters (0 = no error)
- NEXT
-
-_NUMBER:
- xor %eax,%eax
- xor %ebx,%ebx
-
- test %ecx,%ecx // trying to parse a zero-length string is an error, but will return 0.
- jz 5f
-
- movl var_BASE,%edx // get BASE (in %dl)
-
- // Check if first character is '-'.
- movb (%edi),%bl // %bl = first character in string
- inc %edi
- push %eax // push 0 on stack
- cmpb $'-',%bl // negative number?
- jnz 2f
- pop %eax
- push %ebx // push <> 0 on stack, indicating negative
- dec %ecx
- jnz 1f
- pop %ebx // error: string is only '-'.
- movl $1,%ecx
- ret
-
- // Loop reading digits.
-1: imull %edx,%eax // %eax *= BASE
- movb (%edi),%bl // %bl = next character in string
- inc %edi
-
- // Convert 0-9, A-Z to a number 0-35.
-2: subb $'0',%bl // < '0'?
- jb 4f
- cmp $10,%bl // <= '9'?
- jb 3f
- subb $17,%bl // < 'A'? (17 is 'A'-'0')
- jb 4f
- addb $10,%bl
-
-3: cmp %dl,%bl // >= BASE?
- jge 4f
-
- // OK, so add it to %eax and loop.
- add %ebx,%eax
- dec %ecx
- jnz 1b
-
- // Negate the result if first character was '-' (saved on the stack).
-4: pop %ebx
- test %ebx,%ebx
- jz 5f
- neg %eax
-
-5: ret
-
-/*
- DICTIONARY LOOK UPS ----------------------------------------------------------------------
-
- We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure.
-
- The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the
- dictionary. What it actually returns is the address of the dictionary header, if it finds it,
- or 0 if it didn't.
-
- So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer:
-
- pointer to this
- |
- |
- V
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
-
- See also >CFA and >DFA.
-
- FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why.
-*/
-
- defcode "FIND",4,,FIND
- pop %ecx // %ecx = length
- pop %edi // %edi = address
- call _FIND
- push %eax // %eax = address of dictionary entry (or NULL)
- NEXT
-
-_FIND:
- push %esi // Save %esi so we can use it in string comparison.
-
- // Now we start searching backwards through the dictionary for this word.
- mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary
-1: test %edx,%edx // NULL pointer? (end of the linked list)
- je 4f
-
- // Compare the length expected and the length of the word.
- // Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery
- // this won't pick the word (the length will appear to be wrong).
- xor %eax,%eax
- movb 4(%edx),%al // %al = flags+length field
- andb $(F_HIDDEN|F_LENMASK),%al // %al = name length
- cmpb %cl,%al // Length is the same?
- jne 2f
-
- // Compare the strings in detail.
- push %ecx // Save the length
- push %edi // Save the address (repe cmpsb will move this pointer)
- lea 5(%edx),%esi // Dictionary string we are checking against.
- repe cmpsb // Compare the strings.
- pop %edi
- pop %ecx
- jne 2f // Not the same.
-
- // The strings are the same - return the header pointer in %eax
- pop %esi
- mov %edx,%eax
- ret
-
-2: mov (%edx),%edx // Move back through the link field to the previous word
- jmp 1b // .. and loop.
-
-4: // Not found.
- pop %esi
- xor %eax,%eax // Return zero to indicate not found.
- ret
-
-/*
- FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall
- that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH
- word >CFA turns a dictionary pointer into a codeword pointer.
-
- The example below shows the result of:
-
- WORD DOUBLE FIND >CFA
-
- FIND returns a pointer to this
- | >CFA converts it to a pointer to this
- | |
- V V
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- codeword
-
- Notes:
-
- Because names vary in length, this isn't just a simple increment.
-
- In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but
- that is not true in most FORTH implementations where they store a back pointer in the definition
- (with an obvious memory/complexity cost). The reason they do this is that it is useful to be
- able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions
- quickly.
-
- What does CFA stand for? My best guess is "Code Field Address".
-*/
-
- defcode ">CFA",4,,TCFA
- pop %edi
- call _TCFA
- push %edi
- NEXT
-_TCFA:
- xor %eax,%eax
- add $4,%edi // Skip link pointer.
- movb (%edi),%al // Load flags+len into %al.
- inc %edi // Skip flags+len byte.
- andb $F_LENMASK,%al // Just the length, not the flags.
- add %eax,%edi // Skip the name.
- addl $3,%edi // The codeword is 4-byte aligned.
- andl $~3,%edi
- ret
-
-/*
- Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and
- returns a pointer to the first data field.
-
- FIND returns a pointer to this
- | >CFA converts it to a pointer to this
- | |
- | | >DFA converts it to a pointer to this
- | | |
- V V V
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- codeword
-
- (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is
- different from theirs, because they have an extra indirection).
-
- You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA.
-*/
-
- defword ">DFA",4,,TDFA
- .int TCFA // >CFA (get code field address)
- .int INCR4 // 4+ (add 4 to it to get to next word)
- .int EXIT // EXIT (return from FORTH word)
-
-/*
- COMPILING ----------------------------------------------------------------------
-
- Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this:
-
- : DOUBLE DUP + ;
-
- and we have to turn this into:
-
- pointer to previous word
- ^
- |
- +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
- +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
- ^ len pad codeword |
- | V
- LATEST points here points to codeword of DUP
-
- There are several problems to solve. Where to put the new word? How do we read words? How
- do we define the words : (COLON) and ; (SEMICOLON)?
-
- FORTH solves this rather elegantly and as you might expect in a very low-level way which
- allows you to change how the compiler works on your own code.
-
- FORTH has an INTERPRET function (a true interpreter this time, not DOCOL) which runs in a
- loop, reading words (using WORD), looking them up (using FIND), turning them into codeword
- pointers (using >CFA) and deciding what to do with them.
-
- What it does depends on the mode of the interpreter (in variable STATE).
-
- When STATE is zero, the interpreter just runs each word as it looks them up. This is known as
- immediate mode.
-
- The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the
- interpreter appends the codeword pointer to user memory (the HERE variable points to the next
- free byte of user memory -- see DATA SEGMENT section below).
-
- So you may be able to see how we could define : (COLON). The general plan is:
-
- (1) Use WORD to read the name of the function being defined.
-
- (2) Construct the dictionary entry -- just the header part -- in user memory:
-
- pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where
- ^ | the interpreter will start appending
- | V codewords.
- +--|------+---+---+---+---+---+---+---+---+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
- +---------+---+---+---+---+---+---+---+---+------------+
- len pad codeword
-
- (3) Set LATEST to point to the newly defined word, ...
-
- (4) .. and most importantly leave HERE pointing just after the new codeword. This is where
- the interpreter will append codewords.
-
- (5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to
- our partially-formed header.
-
- After : has run, our input is here:
-
- : DOUBLE DUP + ;
- ^
- |
- Next byte returned by KEY will be the 'D' character of DUP
-
- so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP",
- looks it up in the dictionary, gets its codeword pointer, and appends it:
-
- +-- HERE updated to point here.
- |
- V
- +---------+---+---+---+---+---+---+---+---+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP |
- +---------+---+---+---+---+---+---+---+---+------------+------------+
- len pad codeword
-
- Next we read +, get the codeword pointer, and append it:
-
- +-- HERE updated to point here.
- |
- V
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + |
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
- len pad codeword
-
- The issue is what happens next. Obviously what we _don't_ want to happen is that we
- read ";" and compile it and go on compiling everything afterwards.
-
- At this point, FORTH uses a trick. Remember the length byte in the dictionary definition
- isn't just a plain length byte, but can also contain flags. One flag is called the
- IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as
- IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_.
-
- This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE.
-
- And all it does is append the codeword for EXIT on to the current definition and switch
- back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition
- of ; and we'll see that it's really a very simple definition, declared IMMEDIATE.
-
- After the interpreter reads ; and executes it 'immediately', we get this:
-
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
- +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- len pad codeword ^
- |
- HERE
- STATE is set to 0.
-
- And that's it, job done, our new definition is compiled, and we're back in immediate mode
- just reading and executing words, perhaps including a call to test our new word DOUBLE.
-
- The only last wrinkle in this is that while our word was being compiled, it was in a
- half-finished state. We certainly wouldn't want DOUBLE to be called somehow during
- this time. There are several ways to stop this from happening, but in FORTH what we
- do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is
- being compiled. This prevents FIND from finding it, and thus in theory stops any
- chance of it being called.
-
- The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm
- going to define them. The : (COLON) function can be made a little bit more general by writing
- it in two parts. The first part, called CREATE, makes just the header:
-
- +-- Afterwards, HERE points here.
- |
- V
- +---------+---+---+---+---+---+---+---+---+
- | LINK | 6 | D | O | U | B | L | E | 0 |
- +---------+---+---+---+---+---+---+---+---+
- len pad
-
- and the second part, the actual definition of : (COLON), calls CREATE and appends the
- DOCOL codeword, so leaving:
-
- +-- Afterwards, HERE points here.
- |
- V
- +---------+---+---+---+---+---+---+---+---+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
- +---------+---+---+---+---+---+---+---+---+------------+
- len pad codeword
-
- CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to
- create other types of words (not just ones which contain code, but words which contain variables,
- constants and other data).
-*/
-
- defcode "CREATE",6,,CREATE
-
- // Get the name length and address.
- pop %ecx // %ecx = length
- pop %ebx // %ebx = address of name
-
- // Link pointer.
- movl var_HERE,%edi // %edi is the address of the header
- movl var_LATEST,%eax // Get link pointer
- stosl // and store it in the header.
-
- // Length byte and the word itself.
- mov %cl,%al // Get the length.
- stosb // Store the length/flags byte.
- push %esi
- mov %ebx,%esi // %esi = word
- rep movsb // Copy the word
- pop %esi
- addl $3,%edi // Align to next 4 byte boundary.
- andl $~3,%edi
-
- // Update LATEST and HERE.
- movl var_HERE,%eax
- movl %eax,var_LATEST
- movl %edi,var_HERE
- NEXT
-
-/*
- Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words
- to use.
-
- The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user
- memory pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is:
-
- previous value of HERE
- |
- V
- +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+
- | LINK | 6 | D | O | U | B | L | E | 0 | | <data> |
- +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+
- len pad ^
- |
- new value of HERE
-
- and <data> is whatever 32 bit integer was at the top of the stack.
-
- , (COMMA) is quite a fundamental operation when compiling. It is used to append codewords
- to the current word that is being compiled.
-*/
-
- defcode ",",1,,COMMA
- pop %eax // Code pointer to store.
- call _COMMA
- NEXT
-_COMMA:
- movl var_HERE,%edi // HERE
- stosl // Store it.
- movl %edi,var_HERE // Update HERE (incremented)
- ret
-
-/*
- Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode.
-
- Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this
- variable we can switch between the two modes.
-
- For various reasons which may become apparent later, FORTH defines two standard words called
- [ and ] (LBRAC and RBRAC) which switch between modes:
-
- Word Assembler Action Effect
- [ LBRAC STATE := 0 Switch to immediate mode.
- ] RBRAC STATE := 1 Switch to compile mode.
-
- [ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the
- interpreter saw [ then it would compile it rather than running it. We would never be able to
- switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode
- the word runs immediately, switching us back to immediate mode.
-*/
-
- defcode "[",1,F_IMMED,LBRAC
- xor %eax,%eax
- movl %eax,var_STATE // Set STATE to 0.
- NEXT
-
- defcode "]",1,,RBRAC
- movl $1,var_STATE // Set STATE to 1.
- NEXT
-
-/*
- Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets
- the word HIDDEN and goes into compile mode.
-*/
-
- defword ":",1,,COLON
- .int WORD // Get the name of the new word
- .int CREATE // CREATE the dictionary entry / header
- .int LIT, DOCOL, COMMA // Append DOCOL (the codeword).
- .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition).
- .int RBRAC // Go into compile mode.
- .int EXIT // Return from the function.
-
-/*
- ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag.
-*/
-
- defword ";",1,F_IMMED,SEMICOLON
- .int LIT, EXIT, COMMA // Append EXIT (so the word will return).
- .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition).
- .int LBRAC // Go back to IMMEDIATE mode.
- .int EXIT // Return from the function.
-
-/*
- EXTENDING THE COMPILER ----------------------------------------------------------------------
-
- Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define
- your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because
- it allows you in effect to extend the compiler itself. Does gcc let you do that?
-
- Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic
- compiler, and are all IMMEDIATE words.
-
- The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word,
- or on the current word if you call it in the middle of a definition.
-
- Typical usage is:
-
- : MYIMMEDWORD IMMEDIATE
- ...definition...
- ;
-
- but some FORTH programmers write this instead:
-
- : MYIMMEDWORD
- ...definition...
- ; IMMEDIATE
-
- The two usages are equivalent, to a first approximation.
-*/
-
- defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE
- movl var_LATEST,%edi // LATEST word.
- addl $4,%edi // Point to name/flags byte.
- xorb $F_IMMED,(%edi) // Toggle the IMMED bit.
- NEXT
-
-/*
- 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the
- most recently defined word (used above in : and ; definitions) you would do:
-
- LATEST @ HIDDEN
-
- 'HIDE word' toggles the flag on a named 'word'.
-
- Setting this flag stops the word from being found by FIND, and so can be used to make 'private'
- words. For example, to break up a large word into smaller parts you might do:
-
- : SUB1 ... subword ... ;
- : SUB2 ... subword ... ;
- : SUB3 ... subword ... ;
- : MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ;
- HIDE SUB1
- HIDE SUB2
- HIDE SUB3
-
- After this, only MAIN is 'exported' or seen by the rest of the program.
-*/
-
- defcode "HIDDEN",6,,HIDDEN
- pop %edi // Dictionary entry.
- addl $4,%edi // Point to name/flags byte.
- xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit.
- NEXT
-
- defword "HIDE",4,,HIDE
- .int WORD // Get the word (after HIDE).
- .int FIND // Look up in the dictionary.
- .int HIDDEN // Set F_HIDDEN flag.
- .int EXIT // Return.
-
-/*
- ' (TICK) is a standard FORTH word which returns the codeword pointer of the next word.
-
- The common usage is:
-
- ' FOO ,
-
- which appends the codeword of FOO to the current word we are defining (this only works in compiled code).
-
- You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define
- a literal 2 might be:
-
- : LIT2 IMMEDIATE
- ' LIT , \ Appends LIT to the currently-being-defined word
- 2 , \ Appends the number 2 to the currently-being-defined word
- ;
-
- So you could do:
-
- : DOUBLE LIT2 * ;
-
- (If you don't understand how LIT2 works, then you should review the material about compiling words
- and immediate mode).
-
- This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in
- compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in
- immediate mode too.
-*/
- defcode "'",1,,TICK
- lodsl // Get the address of the next word and skip it.
- pushl %eax // Push it on the stack.
- NEXT
-
-/*
- BRANCHING ----------------------------------------------------------------------
-
- It turns out that all you need in order to define looping constructs, IF-statements, etc.
- are two primitives.
-
- BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the
- top of stack is zero).
-
- The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes,
- %esi starts by pointing to the offset field (compare to LIT above):
-
- +---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+
- | (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word |
- +---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+
- ^ | ^
- | | |
- | +-----------------------+
- %esi added to offset
-
- The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution
- continues at the branch target. Negative offsets work as expected.
-
- 0BRANCH is the same except the branch happens conditionally.
-
- Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely
- in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH
- into the word currently being compiled.
-
- As an example, code written like this:
-
- condition-code IF true-part THEN rest-code
-
- compiles to:
-
- condition-code 0BRANCH OFFSET true-part rest-code
- | ^
- | |
- +-------------+
-*/
-
- defcode "BRANCH",6,,BRANCH
- add (%esi),%esi // add the offset to the instruction pointer
- NEXT
-
- defcode "0BRANCH",7,,ZBRANCH
- pop %eax
- test %eax,%eax // top of stack is zero?
- jz code_BRANCH // if so, jump back to the branch function above
- lodsl // otherwise we need to skip the offset
- NEXT
-
-/*
- LITERAL STRINGS ----------------------------------------------------------------------
-
- LITSTRING is a primitive used to implement the ." and S" operators (which are written in
- FORTH). See the definition of those operators later.
-
- TELL just prints a string. It's more efficient to define this in assembly because we
- can make it a single Linux syscall.
-*/
-
- defcode "LITSTRING",9,,LITSTRING
- lodsl // get the length of the string
- push %esi // push the address of the start of the string
- push %eax // push it on the stack
- addl %eax,%esi // skip past the string
- addl $3,%esi // but round up to next 4 byte boundary
- andl $~3,%esi
- NEXT
-
- defcode "TELL",4,,TELL
- mov $1,%ebx // 1st param: stdout
- pop %edx // 3rd param: length of string
- pop %ecx // 2nd param: address of string
- mov $__NR_write,%eax // write syscall
- int $0x80
- NEXT
-
-/*
- QUIT AND INTERPRET ----------------------------------------------------------------------
-
- QUIT is the first FORTH function called, almost immediately after the FORTH system "boots".
- As explained before, QUIT doesn't "quit" anything. It does some initialisation (in particular
- it clears the return stack) and it calls INTERPRET in a loop to interpret commands. The
- reason it is called QUIT is because you can call it from your own FORTH words in order to
- "quit" your program and start again at the user prompt.
-
- INTERPRET is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate
- description -- see: http://en.wikipedia.org/wiki/REPL).
-*/
-
- // QUIT must not return (ie. must not call EXIT).
- defword "QUIT",4,,QUIT
- .int RZ,RSPSTORE // R0 RSP!, clear the return stack
- .int INTERPRET // interpret the next word
- .int BRANCH,-8 // and loop (indefinitely)
-
-/*
- This interpreter is pretty simple, but remember that in FORTH you can always override
- it later with a more powerful one!
- */
- defcode "INTERPRET",9,,INTERPRET
- call _WORD // Returns %ecx = length, %edi = pointer to word.
-
- // Is it in the dictionary?
- xor %eax,%eax
- movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...)
- call _FIND // Returns %eax = pointer to header or 0 if not found.
- test %eax,%eax // Found?
- jz 1f
-
- // In the dictionary. Is it an IMMEDIATE codeword?
- mov %eax,%edi // %edi = dictionary entry
- movb 4(%edi),%al // Get name+flags.
- push %ax // Just save it for now.
- call _TCFA // Convert dictionary entry (in %edi) to codeword pointer.
- pop %ax
- andb $F_IMMED,%al // Is IMMED flag set?
- mov %edi,%eax
- jnz 4f // If IMMED, jump straight to executing.
-
- jmp 2f
-
-1: // Not in the dictionary (not a word) so assume it's a literal number.
- incl interpret_is_lit
- call _NUMBER // Returns the parsed number in %eax, %ecx > 0 if error
- test %ecx,%ecx
- jnz 6f
- mov %eax,%ebx
- mov $LIT,%eax // The word is LIT
-
-2: // Are we compiling or executing?
- movl var_STATE,%edx
- test %edx,%edx
- jz 4f // Jump if executing.
-
- // Compiling - just append the word to the current dictionary definition.
- call _COMMA
- mov interpret_is_lit,%ecx // Was it a literal?
- test %ecx,%ecx
- jz 3f
- mov %ebx,%eax // Yes, so LIT is followed by a number.
- call _COMMA
-3: NEXT
-
-4: // Executing - run it!
- mov interpret_is_lit,%ecx // Literal?
- test %ecx,%ecx // Literal?
- jnz 5f
-
- // Not a literal, execute it now. This never returns, but the codeword will
- // eventually call NEXT which will reenter the loop in QUIT.
- jmp *(%eax)
-
-5: // Executing a literal, which means push it on the stack.
- push %ebx
- NEXT
-
-6: // Parse error (not a known word or a number in the current BASE).
- // Print an error message followed by up to 40 characters of context.
- mov $2,%ebx // 1st param: stderr
- mov $errmsg,%ecx // 2nd param: error message
- mov $errmsgend-errmsg,%edx // 3rd param: length of string
- mov $__NR_write,%eax // write syscall
- int $0x80
-
- mov (currkey),%ecx // the error occurred just before currkey position
- mov %ecx,%edx
- sub $buffer,%edx // %edx = currkey - buffer (length in buffer before currkey)
- cmp $40,%edx // if > 40, then print only 40 characters
- jle 7f
- mov $40,%edx
-7: sub %edx,%ecx // %ecx = start of area to print, %edx = length
- mov $__NR_write,%eax // write syscall
- int $0x80
-
- mov $errmsgnl,%ecx // newline
- mov $1,%edx
- mov $__NR_write,%eax // write syscall
- int $0x80
-
- NEXT
-
- .section .rodata
-errmsg: .ascii "PARSE ERROR: "
-errmsgend:
-errmsgnl: .ascii "\n"
-
- .data // NB: easier to fit in the .data section
- .align 4
-interpret_is_lit:
- .int 0 // Flag used to record if reading a literal
-
-/*
- ODDS AND ENDS ----------------------------------------------------------------------
-
- CHAR puts the ASCII code of the first character of the following word on the stack. For example
- CHAR A puts 65 on the stack.
-
- EXECUTE is used to run execution tokens. See the discussion of execution tokens in the
- FORTH code for more details.
-
- SYSCALL0, SYSCALL1, SYSCALL2, SYSCALL3 make a standard Linux system call. (See <asm/unistd.h>
- for a list of system call numbers). As their name suggests these forms take between 0 and 3
- syscall parameters, plus the system call number.
-
- In this FORTH, SYSCALL0 must be the last word in the built-in (assembler) dictionary because we
- initialise the LATEST variable to point to it. This means that if you want to extend the assembler
- part, you must put new words before SYSCALL0, or else change how LATEST is initialised.
-*/
-
- defcode "CHAR",4,,CHAR
- call _WORD // Returns %ecx = length, %edi = pointer to word.
- xor %eax,%eax
- movb (%edi),%al // Get the first character of the word.
- push %eax // Push it onto the stack.
- NEXT
-
- defcode "EXECUTE",7,,EXECUTE
- pop %eax // Get xt into %eax
- jmp *(%eax) // and jump to it.
- // After xt runs its NEXT will continue executing the current word.
-
- defcode "SYSCALL3",8,,SYSCALL3
- pop %eax // System call number (see <asm/unistd.h>)
- pop %ebx // First parameter.
- pop %ecx // Second parameter
- pop %edx // Third parameter
- int $0x80
- push %eax // Result (negative for -errno)
- NEXT
-
- defcode "SYSCALL2",8,,SYSCALL2
- pop %eax // System call number (see <asm/unistd.h>)
- pop %ebx // First parameter.
- pop %ecx // Second parameter
- int $0x80
- push %eax // Result (negative for -errno)
- NEXT
-
- defcode "SYSCALL1",8,,SYSCALL1
- pop %eax // System call number (see <asm/unistd.h>)
- pop %ebx // First parameter.
- int $0x80
- push %eax // Result (negative for -errno)
- NEXT
-
- defcode "SYSCALL0",8,,SYSCALL0
- pop %eax // System call number (see <asm/unistd.h>)
- int $0x80
- push %eax // Result (negative for -errno)
- NEXT
-
-/*
- DATA SEGMENT ----------------------------------------------------------------------
-
- Here we set up the Linux data segment, used for user definitions and variously known as just
- the 'data segment', 'user memory' or 'user definitions area'. It is an area of memory which
- grows upwards and stores both newly-defined FORTH words and global variables of various
- sorts.
-
- It is completely analogous to the C heap, except there is no generalised 'malloc' and 'free'
- (but as with everything in FORTH, writing such functions would just be a Simple Matter
- Of Programming). Instead in normal use the data segment just grows upwards as new FORTH
- words are defined/appended to it.
-
- There are various "features" of the GNU toolchain which make setting up the data segment
- more complicated than it really needs to be. One is the GNU linker which inserts a random
- "build ID" segment. Another is Address Space Randomization which means we can't tell
- where the kernel will choose to place the data segment (or the stack for that matter).
-
- Therefore writing this set_up_data_segment assembler routine is a little more complicated
- than it really needs to be. We ask the Linux kernel where it thinks the data segment starts
- using the brk(2) system call, then ask it to reserve some initial space (also using brk(2)).
-
- You don't need to worry about this code.
-*/
- .text
- .set INITIAL_DATA_SEGMENT_SIZE,65536
-set_up_data_segment:
- xor %ebx,%ebx // Call brk(0)
- movl $__NR_brk,%eax
- int $0x80
- movl %eax,var_HERE // Initialise HERE to point at beginning of data segment.
- addl $INITIAL_DATA_SEGMENT_SIZE,%eax // Reserve nn bytes of memory for initial data segment.
- movl %eax,%ebx // Call brk(HERE+INITIAL_DATA_SEGMENT_SIZE)
- movl $__NR_brk,%eax
- int $0x80
- ret
-
-/*
- We allocate static buffers for the return static and input buffer (used when
- reading in files and text that the user types in).
-*/
- .set RETURN_STACK_SIZE,8192
- .set BUFFER_SIZE,4096
-
- .bss
-/* FORTH return stack. */
- .align 4096
-return_stack:
- .space RETURN_STACK_SIZE
-return_stack_top: // Initial top of return stack.
-
-/* This is used as a temporary input buffer when reading from files or the terminal. */
- .align 4096
-buffer:
- .space BUFFER_SIZE
-
-/*
- START OF FORTH CODE ----------------------------------------------------------------------
-
- We've now reached the stage where the FORTH system is running and self-hosting. All further
- words can be written as FORTH itself, including words like IF, THEN, .", etc which in most
- languages would be considered rather fundamental.
-
- I used to append this here in the assembly file, but I got sick of fighting against gas's
- crack-smoking (lack of) multiline string syntax. So now that is in a separate file called
- jonesforth.f
-
- If you don't already have that file, download it from http://annexia.org/forth in order
- to continue the tutorial.
-*/
-
-/* END OF jonesforth.S */