/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- By Richard W.M. Jones http://annexia.org/forth This is PUBLIC DOMAIN (see public domain release statement below). $Id: jonesforth.S,v 1.47 2009-09-11 08:33:13 rich Exp $ gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,–build-id=none -o jonesforth jonesforth.S */ .set JONES_VERSION,47 /* 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 .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 be 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 %ebx push %eax push %ecx NEXT defcode “-ROT”,4,,NROT pop %eax pop %ebx pop %ecx push %eax push %ecx push %ebx 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 ———————————————————————- An 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 ) */ //#include // you might need this instead #include .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