X-Git-Url: http://git.annexia.org/?p=jonesforth.git;a=blobdiff_plain;f=jonesforth.S;h=ad932613969f820c01d75d06ffaf17429dfbdfd7;hp=dfa173ca71e3af883bc0fddc2867c1b908f843b2;hb=ae7508776fa28d069e0d890e6878f09a8e3f05ee;hpb=964d44d11ff6cca78e7cf90fe597efbfd51ea8f9 diff --git a/jonesforth.S b/jonesforth.S index dfa173c..ad93261 100644 --- a/jonesforth.S +++ b/jonesforth.S @@ -1,8 +1,12 @@ /* 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 1.44 2007-10-11 07:38:31 rich Exp $ - gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S - + gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S +*/ + .set JONES_VERSION,44 +/* INTRODUCTION ---------------------------------------------------------------------- FORTH is one of those alien languages which most working programmers regard in the same @@ -41,7 +45,8 @@ 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. This concept isn't common in programming languages, + 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). @@ -59,8 +64,33 @@ 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 @@ -69,6 +99,15 @@ <------------------------------------------------------------------------------------------------------------------------> + 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 @@ -82,13 +121,16 @@ 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 -o jonesforth jonesforth.S - ./jonesforth + 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 - You will see lots of 'Warning: unterminated string; newline inserted' messages from the - assembler. That's just because the GNU assembler doesn't have a good syntax for multi-line - strings (or rather it used to, but the developers removed it!) so I've abused the syntax - slightly to make things readable. Ignore these warnings. + 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 ---------------------------------------------------------------------- @@ -109,7 +151,8 @@ 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". + 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. @@ -120,24 +163,312 @@ Now the tutorial starts in earnest. - INDIRECT THREADED CODE ---------------------------------------------------------------------- + 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!". -/* NEXT macro. */ - .macro NEXT - lodsl - jmp *(%eax) - .endm + 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 @@ -150,20 +481,19 @@ lea 4(%ebp),%ebp .endm -/* ELF entry point. */ - .text - .globl _start -_start: - cld - mov %esp,var_S0 // Store the initial data stack pointer. - mov $return_stack,%ebp // Initialise the return stack. +/* + And with that we can now talk about the interpreter. - mov $cold_start,%esi // Initialise interpreter. - NEXT // Run 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 + ; - .section .rodata -cold_start: // High-level code without a codeword. - .int COLD + 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 @@ -174,38 +504,131 @@ DOCOL: movl %eax,%esi // %esi point to first data word NEXT -/*---------------------------------------------------------------------- - * Fixed sized buffers for everything. - */ - .bss +/* + 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). +*/ -/* FORTH return stack. */ -#define RETURN_STACK_SIZE 8192 - .align 4096 - .space RETURN_STACK_SIZE -return_stack: +/* 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 -/* Space for user-defined words. */ -#define USER_DEFS_SIZE 16384 - .align 4096 -user_defs_start: - .space USER_DEFS_SIZE + 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. -/*---------------------------------------------------------------------- - * Built-in words defined the long way. - */ -#define F_IMMED 0x80 -#define F_HIDDEN 0x20 + 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 defcode name, namelen, flags=0, label + .macro defword name, namelen, flags=0, label .section .rodata .align 4 .globl name_\label @@ -214,17 +637,36 @@ name_\label : .set link,name_\label .byte \flags+\namelen // flags + length byte .ascii "\name" // the name - .align 4 + .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 + .int DOCOL // codeword - the interpreter + // list of word pointers follow .endm - .macro defword name, namelen, flags=0, label +/* + 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 @@ -233,41 +675,38 @@ name_\label : .set link,name_\label .byte \flags+\namelen // flags + length byte .ascii "\name" // the name - .align 4 + .align 4 // padding to next 4 byte boundary .globl \label \label : - .int DOCOL // codeword - the interpreter - // list of word pointers follow + .int code_\label // codeword + .text + //.align 4 + .globl code_\label +code_\label : // assembler code follows .endm - .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 +/* + 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. +*/ - // Some easy ones, written in assembly for speed defcode "DROP",4,,DROP pop %eax // drop top of stack NEXT - defcode "DUP",3,,DUP - pop %eax // duplicate top of stack - push %eax - push %eax - NEXT - defcode "SWAP",4,,SWAP - pop %eax // swap top of stack + 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 @@ -291,6 +730,36 @@ var_\name : 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 @@ -300,21 +769,21 @@ var_\name : NEXT defcode "4+",2,,INCR4 - addl $4,(%esp) // increment top of stack + addl $4,(%esp) // add 4 to top of stack NEXT defcode "4-",2,,DECR4 - subl $4,(%esp) // decrement top of stack + subl $4,(%esp) // subtract 4 from top of stack NEXT defcode "+",1,,ADD - pop %eax - addl %eax,(%esp) + pop %eax // get top of stack + addl %eax,(%esp) // and add it to next word on stack NEXT defcode "-",1,,SUB - pop %eax - subl %eax,(%esp) + pop %eax // get top of stack + subl %eax,(%esp) // and subtract it from next word on stack NEXT defcode "*",1,,MUL @@ -324,75 +793,224 @@ var_\name : push %eax // ignore overflow NEXT - defcode "/",1,,DIV - xor %edx,%edx - pop %ebx - pop %eax - idivl %ebx - push %eax // push quotient - 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",3,,MOD + 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 - je 1f - pushl $0 - NEXT -1: pushl $1 + sete %al + movzbl %al,%eax + pushl %eax NEXT defcode "<>",2,,NEQU // top two words are not equal? pop %eax pop %ebx cmp %ebx,%eax - je 1f - pushl $1 + 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 -1: pushl $0 + + 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 - jz 1f - pushl $0 + 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 -1: pushl $1 + + defcode "0>=",3,,ZGE + pop %eax + test %eax,%eax + setge %al + movzbl %al,%eax + pushl %eax NEXT - defcode "AND",3,,AND + defcode "AND",3,,AND // bitwise AND pop %eax andl %eax,(%esp) NEXT - defcode "OR",2,,OR + defcode "OR",2,,OR // bitwise OR pop %eax orl %eax,(%esp) NEXT - defcode "INVERT",6,,INVERT + 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 - // COLD must not return (ie. must not call EXIT). - defword "COLD",4,,COLD - // XXX reinitialisation of the interpreter - .int INTERPRETER // call the interpreter loop (never returns) - .int LIT,1,SYSEXIT // hmmm, but in case it does, exit(1). +/* + 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. @@ -401,25 +1019,13 @@ var_\name : push %eax // push the literal number on to stack NEXT - defcode "LITSTRING",9,,LITSTRING - lodsl // get the length of the string - push %eax // push it on the stack - push %esi // push the address of the start of the string - addl %eax,%esi // skip past the string - addl $3,%esi // but round up to next 4 byte boundary - andl $~3,%esi - NEXT - - defcode "BRANCH",6,,BRANCH - add (%esi),%esi // add the offset to the instruction pointer - NEXT +/* + MEMORY ---------------------------------------------------------------------- - 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 + 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 @@ -445,52 +1051,144 @@ var_\name : 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. - * I don't know whether FORTH has these words, so I invented my own, called !b and @b. - * Byte-oriented operations only work on architectures which permit them (i386 is one of those). - * UPDATE: writing a byte to the dictionary pointer is called C, in FORTH. +/* + ! 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 "!b",2,,STOREBYTE + + defcode "C!",2,,STOREBYTE pop %ebx // address to store at pop %eax // data to store there movb %al,(%ebx) // store it NEXT - defcode "@b",2,,FETCHBYTE + defcode "C@",2,,FETCHBYTE pop %ebx // address to fetch xor %eax,%eax movb (%ebx),%al // fetch it push %eax // push value onto stack NEXT - // The STATE variable is 0 for execute mode, != 0 for compile mode - defvar "STATE",5,,STATE +/* 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 - // This points to where compiled words go. - defvar "HERE",4,,HERE,user_defs_start +/* 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 - // This is the last definition in the dictionary. - defvar "LATEST",6,,LATEST,name_SYSEXIT // SYSEXIT must be last in built-in dictionary +/* + BUILT-IN VARIABLES ---------------------------------------------------------------------- - // _X, _Y and _Z are scratch variables used by standard words. - defvar "_X",2,,TX - defvar "_Y",2,,TY - defvar "_Z",2,,TZ + 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: - // This stores the top of the data stack. - defvar "S0",2,,SZ + LATEST @ . CR - // This stores the top of the return stack. - defvar "R0",2,,RZ,return_stack + 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). +*/ - defcode "DSP@",4,,DSPFETCH - mov %esp,%eax - push %eax + .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 - defcode "DSP!",4,,DSPSTORE - pop %esp +/* + 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 @@ -511,30 +1209,82 @@ var_\name : NEXT defcode "RDROP",5,,RDROP - lea 4(%ebp),%ebp // pop return stack and throw away + addl $4,%ebp // pop return stack and throw away NEXT -#include +/* + PARAMETER (DATA) STACK ---------------------------------------------------------------------- - defcode "KEY",3,,KEY - call _KEY - push %eax // push return value on stack - NEXT -_KEY: - mov (currkey),%ebx - cmp (bufftop),%ebx - jge 1f - xor %eax,%eax - mov (%ebx),%al - inc %ebx - mov %ebx,(currkey) - ret + 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. +*/ -1: // out of input; use read(2) to fetch more input from stdin + 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 $buffend-buffer,%edx // 3rd param: max length + mov $BUFFER_SIZE,%edx // 3rd param: max length mov $__NR_read,%eax // syscall: read int $0x80 test %eax,%eax // If %eax <= 0, then exit. @@ -543,11 +1293,24 @@ _KEY: mov %ecx,bufftop jmp _KEY -2: // error or out of input: exit +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 @@ -556,8 +1319,8 @@ _EMIT: mov $1,%ebx // 1st param: stdout // write needs the address of the byte to write - mov %al,(2f) - mov $2f,%ecx // 2nd param: address + mov %al,emit_scratch + mov $emit_scratch,%ecx // 2nd param: address mov $1,%edx // 3rd param: nbytes = 1 @@ -565,13 +1328,43 @@ _EMIT: int $0x80 ret - .bss -2: .space 1 // scratch used by EMIT + .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 %ecx // push length push %edi // push base address + push %ecx // push length NEXT _WORD: @@ -584,7 +1377,7 @@ _WORD: jbe 1b // if so, keep looking /* Search for the end of the word, storing chars as we go. */ - mov $5f,%edi // pointer to return buffer + mov $word_buffer,%edi // pointer to return buffer 2: stosb // add character to return buffer call _KEY // get next key, returned in %al @@ -592,9 +1385,9 @@ _WORD: ja 2b // if not, keep looping /* Return the word (well, the static buffer) and length. */ - sub $5f,%edi + sub $word_buffer,%edi mov %edi,%ecx // return length of the word - mov $5f,%edi // return address of the word + mov $word_buffer,%edi // return address of the word ret /* Code to skip \ comments to end of the current line. */ @@ -604,71 +1397,118 @@ _WORD: jne 3b jmp 1b - .bss + .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. -5: .space 32 +word_buffer: + .space 32 - defcode "EMITSTRING",10,,EMITSTRING - mov $1,%ebx // 1st param: stdout - pop %ecx // 2nd param: address of string - pop %edx // 3rd param: length of string +/* + 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. - mov $__NR_write,%eax // write syscall - int $0x80 + 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. - NEXT + If the word starts with a '-' character then the returned value is negative. - defcode ".",1,,DOT - pop %eax // Get the number to print into %eax - call _DOT // Easier to do this recursively ... + 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 -_DOT: - mov $10,%ecx // Base 10 -1: - cmp %ecx,%eax - jb 2f - xor %edx,%edx // %edx:%eax / %ecx -> quotient %eax, remainder %edx - idivl %ecx - pushl %edx - call _DOT - popl %eax - jmp 1b -2: - xor %ah,%ah - aam $10 - cwde - addl $'0',%eax - call _EMIT - ret - // Parse a number from a string on the stack -- almost the opposite of . (DOT) - // Note that there is absolutely no error checking. In particular the length of the - // string must be >= 1 bytes. - defcode "SNUMBER",7,,SNUMBER - pop %edi - pop %ecx - call _SNUMBER - push %eax - NEXT -_SNUMBER: +_NUMBER: xor %eax,%eax xor %ebx,%ebx -1: - imull $10,%eax // %eax *= 10 - movb (%edi),%bl + + 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 - subb $'0',%bl // ASCII -> digit + + // 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 - ret + + // 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 %edi // %edi = address pop %ecx // %ecx = length + pop %edi // %edi = address call _FIND - push %eax + push %eax // %eax = address of dictionary entry (or NULL) NEXT _FIND: @@ -676,8 +1516,7 @@ _FIND: // 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) +1: test %edx,%edx // NULL pointer? (end of the linked list) je 4f // Compare the length expected and the length of the word. @@ -685,7 +1524,7 @@ _FIND: // 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|0x1f),%al // %al = name length + andb $(F_HIDDEN|F_LENMASK),%al // %al = name length cmpb %cl,%al // Length is the same? jne 2f @@ -703,8 +1542,7 @@ _FIND: mov %edx,%eax ret -2: - mov (%edx),%edx // Move back through the link field to the previous word +2: mov (%edx),%edx // Move back through the link field to the previous word jmp 1b // .. and loop. 4: // Not found. @@ -712,7 +1550,38 @@ _FIND: xor %eax,%eax // Return zero to indicate not found. ret - defcode ">CFA",4,,TCFA // DEA -> Codeword address +/* + 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 @@ -722,31 +1591,201 @@ _TCFA: add $4,%edi // Skip link pointer. movb (%edi),%al // Load flags+len into %al. inc %edi // Skip flags+len byte. - andb $0x1f,%al // Just the length, not the flags. + 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 - 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 +/* + 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. +*/ - defcode ":",1,,COLON + 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) - // Get the word and create a dictionary entry header for it. - call _WORD // Returns %ecx = length, %edi = pointer to word. - mov %edi,%ebx // %ebx = address of the 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. - orb $F_HIDDEN,%al // Set the HIDDEN flag on this entry. stosb // Store the length/flags byte. push %esi mov %ebx,%esi // %esi = word @@ -755,19 +1794,35 @@ _TCFA: addl $3,%edi // Align to next 4 byte boundary. andl $~3,%edi - movl $DOCOL,%eax // The codeword for user-created words is always DOCOL (the interpreter) - stosl - - // Header built, so now update LATEST and HERE. - // We'll be compiling words and putting them HERE. + // Update LATEST and HERE. movl var_HERE,%eax movl %eax,var_LATEST movl %edi,var_HERE - - // And go into compile mode by setting STATE to 1. - movl $1,var_STATE 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 | | | + +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ + len pad ^ + | + new value of HERE + + and 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 @@ -778,44 +1833,260 @@ _COMMA: movl %edi,var_HERE // Update HERE (incremented) ret - defcode "HIDDEN",6,,HIDDEN - call _HIDDEN +/* + 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 -_HIDDEN: - movl var_LATEST,%edi // LATEST word. - addl $4,%edi // Point to name/flags byte. - xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit. - ret - defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE - call _IMMEDIATE + defcode "]",1,,RBRAC + movl $1,var_STATE // Set STATE to 1. NEXT -_IMMEDIATE: + +/* + 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. - ret + 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: - defcode ";",1,F_IMMED,SEMICOLON - movl $EXIT,%eax // EXIT is the final codeword in compiled words. - call _COMMA // Store it. - call _HIDDEN // Toggle the HIDDEN flag (unhides the new word). - xor %eax,%eax // Set STATE to 0 (back to execute mode). - movl %eax,var_STATE + 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 -/* This definiton of ' (TICK) is strictly cheating - it also only works in compiled code. */ + 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 -/* This interpreter is pretty simple, but remember that in FORTH you can always override - * it later with a more powerful one! - */ - defword "INTERPRETER",11,,INTERPRETER - .int INTERPRET,RDROP,INTERPRETER +/* + 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. @@ -840,7 +2111,9 @@ _IMMEDIATE: 1: // Not in the dictionary (not a word) so assume it's a literal number. incl interpret_is_lit - call _SNUMBER // Returns the parsed number in %eax + 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 @@ -864,256 +2137,177 @@ _IMMEDIATE: jnz 5f // Not a literal, execute it now. This never returns, but the codeword will - // eventually call NEXT which will reenter the loop in INTERPRETER. + // 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 - .data +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 - // NB: SYSEXIT must be the last entry in the built-in dictionary. - defcode SYSEXIT,7,,SYSEXIT - pop %ebx - mov $__NR_exit,%eax +/* + 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 + 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 ) + pop %ebx // First parameter. + pop %ecx // Second parameter + pop %edx // Third parameter int $0x80 + push %eax // Result (negative for -errno) + NEXT -/*---------------------------------------------------------------------- - * Input buffer & initial input. - */ - .data + defcode "SYSCALL2",8,,SYSCALL2 + pop %eax // System call number (see ) + 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 ) + pop %ebx // First parameter. + int $0x80 + push %eax // Result (negative for -errno) + NEXT + + defcode "SYSCALL0",8,,SYSCALL0 + pop %eax // System call number (see ) + 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: - // XXX gives 'Warning: unterminated string; newline inserted' messages which you can ignore - .ascii "\ -\\ Define some character constants -: '\\n' 10 ; -: 'SPACE' 32 ; -: '\"' 34 ; -: ':' 58 ; - -\\ CR prints a carriage return -: CR '\\n' EMIT ; - -\\ SPACE prints a space -: SPACE 'SPACE' EMIT ; - -\\ Primitive . (DOT) function doesn't follow with a blank, so redefine it to behave like FORTH. -\\ Notice how we can trivially redefine existing functions. -: . . SPACE ; - -\\ DUP, DROP are defined in assembly for speed, but this is how you might define them -\\ in FORTH. Notice use of the scratch variables _X and _Y. -\\ : DUP _X ! _X @ _X @ ; -\\ : DROP _X ! ; - -\\ The 2... versions of the standard operators work on pairs of stack entries. They're not used -\\ very commonly so not really worth writing in assembler. Here is how they are defined in FORTH. -: 2DUP OVER OVER ; -: 2DROP DROP DROP ; - -\\ More standard FORTH words. -: 2* 2 * ; -: 2/ 2 / ; - -\\ [ and ] allow you to break into immediate mode while compiling a word. -: [ IMMEDIATE \\ define [ as an immediate word - 0 STATE ! \\ go into immediate mode - ; + .space BUFFER_SIZE -: ] - 1 STATE ! \\ go back to compile mode - ; +/* + START OF FORTH CODE ---------------------------------------------------------------------- -\\ LITERAL takes whatever is on the stack and compiles LIT -: LITERAL IMMEDIATE - ' LIT , \\ compile LIT - , \\ compile the literal itself (from the stack) - ; + 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. -\\ condition IF true-part THEN rest -\\ compiles to: -\\ condition 0BRANCH OFFSET true-part rest -\\ where OFFSET is the offset of 'rest' -\\ condition IF true-part ELSE false-part THEN -\\ compiles to: -\\ condition 0BRANCH OFFSET true-part BRANCH OFFSET2 false-part rest -\\ where OFFSET if the offset of false-part and OFFSET2 is the offset of rest - -\\ IF is an IMMEDIATE word which compiles 0BRANCH followed by a dummy offset, and places -\\ the address of the 0BRANCH on the stack. Later when we see THEN, we pop that address -\\ off the stack, calculate the offset, and back-fill the offset. -: IF IMMEDIATE - ' 0BRANCH , \\ compile 0BRANCH - HERE @ \\ save location of the offset on the stack - 0 , \\ compile a dummy offset -; - -: THEN IMMEDIATE - DUP - HERE @ SWAP - \\ calculate the offset from the address saved on the stack - SWAP ! \\ store the offset in the back-filled location -; - -: ELSE IMMEDIATE - ' BRANCH , \\ definite branch to just over the false-part - HERE @ \\ save location of the offset on the stack - 0 , \\ compile a dummy offset - SWAP \\ now back-fill the original (IF) offset - DUP \\ same as for THEN word above - HERE @ SWAP - - SWAP ! -; - -\\ BEGIN loop-part condition UNTIL -\\ compiles to: -\\ loop-part condition 0BRANCH OFFSET -\\ where OFFSET points back to the loop-part -\\ This is like do { loop-part } while (condition) in the C language -: BEGIN IMMEDIATE - HERE @ \\ save location on the stack -; - -: UNTIL IMMEDIATE - ' 0BRANCH , \\ compile 0BRANCH - HERE @ - \\ calculate the offset from the address saved on the stack - , \\ compile the offset here -; - -\\ BEGIN loop-part AGAIN -\\ compiles to: -\\ loop-part BRANCH OFFSET -\\ where OFFSET points back to the loop-part -\\ In other words, an infinite loop which can only be returned from with EXIT -: AGAIN IMMEDIATE - ' BRANCH , \\ compile BRANCH - HERE @ - \\ calculate the offset back - , \\ compile the offset here -; - -\\ BEGIN condition WHILE loop-part REPEAT -\\ compiles to: -\\ condition 0BRANCH OFFSET2 loop-part BRANCH OFFSET -\\ where OFFSET points back to condition (the beginning) and OFFSET2 points to after the whole piece of code -\\ So this is like a while (condition) { loop-part } loop in the C language -: WHILE IMMEDIATE - ' 0BRANCH , \\ compile 0BRANCH - HERE @ \\ save location of the offset2 on the stack - 0 , \\ compile a dummy offset2 -; - -: REPEAT IMMEDIATE - ' BRANCH , \\ compile BRANCH - SWAP \\ get the original offset (from BEGIN) - HERE @ - , \\ and compile it after BRANCH - DUP - HERE @ SWAP - \\ calculate the offset2 - SWAP ! \\ and back-fill it in the original location -; - -\\ With the looping constructs, we can now write SPACES, which writes n spaces to stdout. -: SPACES - BEGIN - SPACE \\ print a space - 1- \\ until we count down to 0 - DUP 0= - UNTIL -; - -\\ .S prints the contents of the stack. Very useful for debugging. -: .S - DSP@ \\ get current stack pointer - BEGIN - DUP @ . \\ print the stack element - 4+ \\ move up - DUP S0 @ 4- = \\ stop when we get to the top - UNTIL - DROP -; - -\\ DEPTH returns the depth of the stack. -: DEPTH S0 @ DSP@ - ; - -\\ .\" is the print string operator in FORTH. Example: .\" Something to print\" -\\ The space after the operator is the ordinary space required between words. -\\ This is tricky to define because it has to do different things depending on whether -\\ we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can -\\ detect this and do different things). -\\ In immediate mode we just keep reading characters and printing them until we get to -\\ the next double quote. -\\ In compile mode we have the problem of where we're going to store the string (remember -\\ that the input buffer where the string comes from may be overwritten by the time we -\\ come round to running the function). We store the string in the compiled function -\\ like this: -\\ LITSTRING, string length, string rounded up to 4 bytes, EMITSTRING, ... -: .\" IMMEDIATE - STATE @ \\ compiling? - IF - ' LITSTRING , \\ compile LITSTRING - HERE @ \\ save the address of the length word on the stack - 0 , \\ dummy length - we don't know what it is yet - BEGIN - KEY \\ get next character of the string - DUP '\"' <> - WHILE - HERE @ !b \\ store the character in the compiled image - 1 HERE +! \\ increment HERE pointer by 1 byte - REPEAT - DROP \\ drop the double quote character at the end - DUP \\ get the saved address of the length word - HERE @ SWAP - \\ calculate the length - 4- \\ subtract 4 (because we measured from the start of the length word) - SWAP ! \\ and back-fill the length location - HERE @ \\ round up to next multiple of 4 bytes for the remaining code - 3 + - 3 INVERT AND - HERE ! - ' EMITSTRING , \\ compile the final EMITSTRING - ELSE - \\ In immediate mode, just read characters and print them until we get - \\ to the ending double quote. Much simpler than the above code! - BEGIN - KEY - DUP '\"' = IF EXIT THEN - EMIT - AGAIN - THEN -; - -\\ While compiling, [COMPILE] WORD compiles WORD if it would otherwise be IMMEDIATE. -: [COMPILE] IMMEDIATE - WORD \\ get the next word - FIND \\ find it in the dictionary - >CFA \\ get its codeword - , \\ and compile that -; - -\\ RECURSE makes a recursive call to the current word that is being compiled. -\\ Normally while a word is being compiled, it is marked HIDDEN so that references to the -\\ same word within are calls to the previous definition of the word. -: RECURSE IMMEDIATE - LATEST @ >CFA \\ LATEST points to the word being compiled at the moment - , \\ compile it -; - -\\ ALLOT is used to allocate (static) memory when compiling. It increases HERE by -\\ the amount given on the stack. -: ALLOT HERE +! ; - - -\\ Finally print the welcome prompt. -.\" OK \" -" - -_initbufftop: - .align 4096 -buffend: + 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 -currkey: - .int buffer -bufftop: - .int _initbufftop + 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 */