From 03b912a7db63d213cbfd9a1cb35535759632642b Mon Sep 17 00:00:00 2001 From: Dimitri Sokolyuk Date: Fri, 26 May 2017 01:59:36 +0200 Subject: jonesforth --- jonesforth/Dockerfile | 6 + jonesforth/jonesforth.S | 2313 ++++++++++++++++++++++++++++++++++++++++++++++ jonesforth/jonesforth.fs | 1790 +++++++++++++++++++++++++++++++++++ jonesforth/run.sh | 3 + 4 files changed, 4112 insertions(+) create mode 100644 jonesforth/Dockerfile create mode 100644 jonesforth/jonesforth.S create mode 100644 jonesforth/jonesforth.fs create mode 100755 jonesforth/run.sh (limited to 'jonesforth') diff --git a/jonesforth/Dockerfile b/jonesforth/Dockerfile new file mode 100644 index 0000000..14c1c0a --- /dev/null +++ b/jonesforth/Dockerfile @@ -0,0 +1,6 @@ +FROM debian +RUN apt-get update && apt-get install -y libc6-dev-i386 +COPY jonesforth.* / +RUN gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S +CMD cat jonesforth.fs - | ./jonesforth +# NOTE requires SYS_RAWIO diff --git a/jonesforth/jonesforth.S b/jonesforth/jonesforth.S new file mode 100644 index 0000000..8d13286 --- /dev/null +++ b/jonesforth/jonesforth.S @@ -0,0 +1,2313 @@ +/* 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 1.45 2007/10/22 18:53:13 rich Exp $ + + gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S +*/ + .set JONES_VERSION,45 +/* + INTRODUCTION ---------------------------------------------------------------------- + + FORTH is one of those alien languages which most working programmers regard in the same + way as Haskell, LISP, and so on. Something so strange that they'd rather any thoughts + of it just go away so they can get on with writing this paying code. But that's wrong + and if you care at all about programming then you should at least understand all these + languages, even if you will never use them. + + LISP is the ultimate high-level language, and features from LISP are being added every + decade to the more common languages. But FORTH is in some ways the ultimate in low level + programming. Out of the box it lacks features like dynamic memory management and even + strings. In fact, at its primitive level it lacks even basic concepts like IF-statements + and loops. + + Why then would you want to learn FORTH? There are several very good reasons. First + and foremost, FORTH is minimal. You really can write a complete FORTH in, say, 2000 + lines of code. I don't just mean a FORTH program, I mean a complete FORTH operating + system, environment and language. You could boot such a FORTH on a bare PC and it would + come up with a prompt where you could start doing useful work. The FORTH you have here + isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making + it a good tutorial). It's possible to completely understand the system. Who can say they + completely understand how Linux works, or gcc? + + Secondly FORTH has a peculiar bootstrapping property. By that I mean that after writing + a little bit of assembly to talk to the hardware and implement a few primitives, all the + rest of the language and compiler is written in FORTH itself. Remember I said before + that FORTH lacked IF-statements and loops? Well of course it doesn't really because + such a lanuage would be useless, but my point was rather that IF-statements and loops are + written in FORTH itself. + + Now of course this is common in other languages as well, and in those languages we call + them 'libraries'. For example in C, 'printf' is a library function written in C. But + in FORTH this goes way beyond mere libraries. Can you imagine writing C's 'if' in C? + And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict + yourself to the usual if/while/for/switch constructs? You want a construct that iterates + over every other element in a list of numbers? You can add it to the language. What + about an operator which pulls in variables directly from a configuration file and makes + them available as FORTH variables? Or how about adding Makefile-like dependencies to + the language? No problem in FORTH. How about modifying the FORTH compiler to allow + complex inlining strategies -- simple. This concept isn't common in programming languages, + but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not + the lame C preprocessor) and "domain specific languages" (DSLs). + + This tutorial isn't about learning FORTH as the language. I'll point you to some references + you should read if you're not familiar with using FORTH. This tutorial is about how to + write FORTH. In fact, until you understand how FORTH is written, you'll have only a very + superficial understanding of how to use it. + + So if you're not familiar with FORTH or want to refresh your memory here are some online + references to read: + + http://en.wikipedia.org/wiki/Forth_%28programming_language%29 + + http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm + + http://wiki.laptop.org/go/Forth_Lessons + + http://www.albany.net/~hello/simple.htm + + Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html + + Discussion and criticism of this FORTH here: http://lambda-the-ultimate.org/node/2452 + + ACKNOWLEDGEMENTS ---------------------------------------------------------------------- + + This code draws heavily on the design of LINA FORTH (http://home.hccnet.nl/a.w.m.van.der.horst/lina.html) + by Albert van der Horst. Any similarities in the code are probably not accidental. + + Some parts of this FORTH are also based on this IOCCC entry from 1992: + http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design. + I was very proud when Sean Barrett, the original author of the IOCCC entry, commented in the LtU thread + http://lambda-the-ultimate.org/node/2452#comment-36818 about this FORTH. + + And finally I'd like to acknowledge the (possibly forgotten?) authors of ARTIC FORTH because their + original program which I still have on original cassette tape kept nagging away at me all these years. + http://en.wikipedia.org/wiki/Artic_Software + + PUBLIC DOMAIN ---------------------------------------------------------------------- + + I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. + + In case this is not legally possible, I grant any entity the right to use this work for any purpose, + without any conditions, unless such conditions are required by law. + + SETTING UP ---------------------------------------------------------------------- + + Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of + ASCII-art diagrams to explain concepts, the best way to look at this is using a window which + uses a fixed width font and is at least this wide: + + <------------------------------------------------------------------------------------------------------------------------> + + Secondly make sure TABS are set to 8 characters. The following should be a vertical + line. If not, sort out your tabs. + + | + | + | + + Thirdly I assume that your screen is at least 50 characters high. + + ASSEMBLING ---------------------------------------------------------------------- + + If you want to actually run this FORTH, rather than just read it, you will need Linux on an + i386. Linux because instead of programming directly to the hardware on a bare PC which I + could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux + process with a few basic system calls (read, write and exit and that's about all). i386 + is needed because I had to write the assembly for a processor, and i386 is by far the most + common. (Of course when I say 'i386', any 32- or 64-bit x86 processor will do. I'm compiling + this on a 64 bit AMD Opteron). + + Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to + assemble and run the code (save this file as 'jonesforth.S') are: + + gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S + cat jonesforth.f - | ./jonesforth + + If you want to run your own FORTH programs you can do: + + cat jonesforth.f myprog.f | ./jonesforth + + If you want to load your own FORTH code and then continue reading user commands, you can do: + + cat jonesforth.f myfunctions.f - | ./jonesforth + + ASSEMBLER ---------------------------------------------------------------------- + + (You can just skip to the next section -- you don't need to be able to read assembler to + follow this tutorial). + + However if you do want to read the assembly code here are a few notes about gas (the GNU assembler): + + (1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator. The registers + available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them + have special purposes. + + (2) Add, mov, etc. take arguments in the form SRC,DEST. So mov %eax,%ecx moves %eax -> %ecx + + (3) Constants are prefixed with '$', and you mustn't forget it! If you forget it then it + causes a read from memory instead, so: + mov $2,%eax moves number 2 into %eax + mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake) + + (4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards" + and '1b' (etc.) means label '1:' "backwards". Notice that these labels might be mistaken + for hex numbers (eg. you might confuse 1b with $0x1b). + + (5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc. + + (6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and + less repetitive. + + For more help reading the assembler, do "info gas" at the Linux prompt. + + Now the tutorial starts in earnest. + + THE DICTIONARY ---------------------------------------------------------------------- + + In FORTH as you will know, functions are called "words", and just as in other languages they + have a name and a definition. Here are two FORTH words: + + : DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +" + : QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE" + + Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary + which is just a linked list of dictionary entries. + + <--- DICTIONARY ENTRY (HEADER) -----------------------> + +------------------------+--------+---------- - - - - +----------- - - - - + | LINK POINTER | LENGTH/| NAME | DEFINITION + | | FLAGS | | + +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - - + + I'll come to the definition of the word later. For now just look at the header. The first + 4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for + the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte. + The length of the word can be up to 31 characters (5 bits used) and the top three bits are used + for various flags which I'll come to later. This is followed by the name itself, and in this + implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes. + That's just to ensure that the definition starts on a 32 bit boundary. + + A FORTH variable called LATEST contains a pointer to the most recently defined word, in + other words, the head of this linked list. + + DOUBLE and QUADRUPLE might look like this: + + pointer to previous word + ^ + | + +--|------+---+---+---+---+---+---+---+---+------------- - - - - + | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...) + +---------+---+---+---+---+---+---+---+---+------------- - - - - + ^ len padding + | + +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - + | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...) + +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - + ^ len padding + | + | + LATEST + + You should be able to see from this how you might implement functions to find a word in + the dictionary (just walk along the dictionary entries starting at LATEST and matching + the names until you either find a match or hit the NULL pointer at the end of the dictionary); + and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set + LATEST to point to the new word). We'll see precisely these functions implemented in + assembly code later on. + + One interesting consequence of using a linked list is that you can redefine words, and + a newer definition of a word overrides an older one. This is an important concept in + FORTH because it means that any word (even "built-in" or "standard" words) can be + overridden with a new definition, either to enhance it, to make it faster or even to + disable it. However because of the way that FORTH words get compiled, which you'll + understand below, words defined using the old definition of a word continue to use + the old definition. Only words defined after the new definition use the new definition. + + DIRECT THREADED CODE ---------------------------------------------------------------------- + + Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea + or coffee and settle down. It's fair to say that if you don't understand this section, then you + won't "get" how FORTH works, and that would be a failure on my part for not explaining it well. + So if after reading this section a few times you don't understand it, please email me + (rich@annexia.org). + + Let's talk first about what "threaded code" means. Imagine a peculiar version of C where + you are only allowed to call functions without arguments. (Don't worry for now that such a + language would be completely useless!) So in our peculiar C, code would look like this: + + f () + { + a (); + b (); + c (); + } + + and so on. How would a function, say 'f' above, be compiled by a standard C compiler? + Probably into assembly code like this. On the right hand side I've written the actual + i386 machine code. + + f: + CALL a E8 08 00 00 00 + CALL b E8 1C 00 00 00 + CALL c E8 2C 00 00 00 + ; ignore the return from the function for now + + "E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing + memory was hideously expensive and we might have worried about the wasted space being used + by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory) + by compressing this into just: + + 08 00 00 00 Just the function addresses, without + 1C 00 00 00 the CALL prefix. + 2C 00 00 00 + + On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%. + + [Historical note: If the execution model that FORTH uses looks strange from the following + paragraphs, then it was motivated entirely by the need to save memory on early computers. + This code compression isn't so important now when our machines have more memory in their L1 + caches than those early computers had in total, but the execution model still has some + useful properties]. + + Of course this code won't run directly on the CPU any more. Instead we need to write an + interpreter which takes each set of bytes and calls it. + + On an i386 machine it turns out that we can write this interpreter rather easily, in just + two assembly instructions which turn into just 3 bytes of machine code. Let's store the + pointer to the next word to execute in the %esi register: + + 08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute. + %esi -> 1C 00 00 00 + 2C 00 00 00 + + The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does + two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it + increments %esi by 4 bytes. So after LODSL, the situation now looks like this: + + 08 00 00 00 <- We're still executing this one + 1C 00 00 00 <- %eax now contains this address (0x0000001C) + %esi -> 2C 00 00 00 + + Now we just need to jump to the address in %eax. This is again just a single x86 instruction + written JMP *(%eax). And after doing the jump, the situation looks like: + + 08 00 00 00 + 1C 00 00 00 <- Now we're executing this subroutine. + %esi -> 2C 00 00 00 + + To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)' + which literally make the jump to the next subroutine. + + And that brings us to our first piece of actual code! Well, it's a macro. +*/ + +/* NEXT macro. */ + .macro NEXT + lodsl + jmp *(%eax) + .endm + +/* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions. + + Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like + a return. + + The above describes what is known as direct threaded code. + + To sum up: We compress our function calls down to a list of addresses and use a somewhat + magical macro to act as a "jump to next function in the list". We also use one register (%esi) + to act as a kind of instruction pointer, pointing to the next function in the list. + + I'll just give you a hint of what is to come by saying that a FORTH definition such as: + + : QUADRUPLE DOUBLE DOUBLE ; + + actually compiles (almost, not precisely but we'll see why in a moment) to a list of + function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off. + + At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!". + + I lied about JMP *(%eax). + + INDIRECT THREADED CODE ---------------------------------------------------------------------- + + It turns out that direct threaded code is interesting but only if you want to just execute + a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE + was an assembly language function. In the direct threaded code, QUADRUPLE would look like: + + +------------------+ + | addr of DOUBLE --------------------> (assembly code to do the double) + +------------------+ NEXT + %esi -> | addr of DOUBLE | + +------------------+ + + We can add an extra indirection to allow us to run both words written in assembly language + (primitives written for speed) and words written in FORTH themselves as lists of addresses. + + The extra indirection is the reason for the brackets in JMP *(%eax). + + Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH: + + : QUADRUPLE DOUBLE DOUBLE ; + + +------------------+ + | codeword | : DOUBLE DUP + ; + +------------------+ + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP --------------> +------------------+ + | addr of EXIT | +------------------+ | codeword -------+ + +------------------+ %esi -> | addr of + --------+ +------------------+ | + +------------------+ | | assembly to <-----+ + | addr of EXIT | | | implement DUP | + +------------------+ | | .. | + | | .. | + | | NEXT | + | +------------------+ + | + +-----> +------------------+ + | codeword -------+ + +------------------+ | + | assembly to <------+ + | implement + | + | .. | + | .. | + | NEXT | + +------------------+ + + This is the part where you may need an extra cup of tea/coffee/favourite caffeinated + beverage. What has changed is that I've added an extra pointer to the beginning of + the definitions. In FORTH this is sometimes called the "codeword". The codeword is + a pointer to the interpreter to run the function. For primitives written in + assembly language, the "interpreter" just points to the actual assembly code itself. + They don't need interpreting, they just run. + + In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter + function. + + I'll show you the interpreter function shortly, but let's recall our indirect + JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE + as shown, and DUP has been called. Note that %esi is pointing to the address of + + + The assembly code for DUP eventually does a NEXT. That: + + (1) reads the address of + into %eax %eax points to the codeword of + + (2) increments %esi by 4 + (3) jumps to the indirect %eax jumps to the address in the codeword of +, + ie. the assembly code to implement + + + +------------------+ + | codeword | + +------------------+ + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP --------------> +------------------+ + | addr of EXIT | +------------------+ | codeword -------+ + +------------------+ | addr of + --------+ +------------------+ | + +------------------+ | | assembly to <-----+ + %esi -> | addr of EXIT | | | implement DUP | + +------------------+ | | .. | + | | .. | + | | NEXT | + | +------------------+ + | + +-----> +------------------+ + | codeword -------+ + +------------------+ | + now we're | assembly to <-----+ + executing | implement + | + this | .. | + function | .. | + | NEXT | + +------------------+ + + So I hope that I've convinced you that NEXT does roughly what you'd expect. This is + indirect threaded code. + + I've glossed over four things. I wonder if you can guess without reading on what they are? + + . + . + . + + My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do + you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but + then point at part of DOUBLE. (3) What goes in the codeword for the words which are written + in FORTH? (4) How do you compile a function which does anything except call other functions + ie. a function which contains a number like : DOUBLE 2 * ; ? + + THE INTERPRETER AND RETURN STACK ------------------------------------------------------------ + + Going at these in no particular order, let's talk about issues (3) and (2), the interpreter + and the return stack. + + Words which are defined in FORTH need a codeword which points to a little bit of code to + give them a "helping hand" in life. They don't need much, but they do need what is known + as an "interpreter", although it doesn't really "interpret" in the same way that, say, + Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few + machine registers so that the word can then execute at full speed using the indirect + threaded model above. + + One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old + %esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE. + Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like + a function call), we will need a stack to store these "return addresses" (old values of %esi). + + As you will have seen in the background documentation, FORTH has two stacks, an ordinary + stack for parameters, and a return stack which is a bit more mysterious. But our return + stack is just the stack I talked about in the previous paragraph, used to save %esi when + calling from a FORTH word into another FORTH word. + + In this FORTH, we are using the normal stack pointer (%esp) for the parameter stack. + We will use the i386's "other" stack pointer (%ebp, usually called the "frame pointer") + for our return stack. + + I've got two macros which just wrap up the details of using %ebp for the return stack. + You use them as for example "PUSHRSP %eax" (push %eax on the return stack) or "POPRSP %ebx" + (pop top of return stack into %ebx). +*/ + +/* Macros to deal with the return stack. */ + .macro PUSHRSP reg + lea -4(%ebp),%ebp // push reg on to return stack + movl \reg,(%ebp) + .endm + + .macro POPRSP reg + mov (%ebp),\reg // pop top of return stack to reg + lea 4(%ebp),%ebp + .endm + +/* + And with that we can now talk about the interpreter. + + In FORTH the interpreter function is often called DOCOL (I think it means "DO COLON" because + all FORTH definitions start with a colon, as in : DOUBLE DUP + ; + + The "interpreter" (it's not really "interpreting") just needs to push the old %esi on the + stack and set %esi to the first word in the definition. Remember that we jumped to the + function using JMP *(%eax)? Well a consequence of that is that conveniently %eax contains + the address of this codeword, so just by adding 4 to it we get the address of the first + data word. Finally after setting up %esi, it just does NEXT which causes that first word + to run. +*/ + +/* DOCOL - the interpreter! */ + .text + .align 4 +DOCOL: + PUSHRSP %esi // push %esi on to the return stack + addl $4,%eax // %eax points to codeword, so make + movl %eax,%esi // %esi point to first data word + NEXT + +/* + Just to make this absolutely clear, let's see how DOCOL works when jumping from QUADRUPLE + into DOUBLE: + + QUADRUPLE: + +------------------+ + | codeword | + +------------------+ DOUBLE: + | addr of DOUBLE ---------------> +------------------+ + +------------------+ %eax -> | addr of DOCOL | + %esi -> | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | etc. | + + First, the call to DOUBLE calls DOCOL (the codeword of DOUBLE). DOCOL does this: It + pushes the old %esi on the return stack. %eax points to the codeword of DOUBLE, so we + just add 4 on to it to get our new %esi: + + QUADRUPLE: + +------------------+ + | codeword | + +------------------+ DOUBLE: + | addr of DOUBLE ---------------> +------------------+ +top of return +------------------+ %eax -> | addr of DOCOL | +stack points -> | addr of DOUBLE | + 4 = +------------------+ + +------------------+ %esi -> | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | etc. | + + Then we do NEXT, and because of the magic of threaded code that increments %esi again + and calls DUP. + + Well, it seems to work. + + One minor point here. Because DOCOL is the first bit of assembly actually to be defined + in this file (the others were just macros), and because I usually compile this code with the + text segment starting at address 0, DOCOL has address 0. So if you are disassembling the + code and see a word with a codeword of 0, you will immediately know that the word is + written in FORTH (it's not an assembler primitive) and so uses DOCOL as the interpreter. + + STARTING UP ---------------------------------------------------------------------- + + Now let's get down to nuts and bolts. When we start the program we need to set up + a few things like the return stack. But as soon as we can, we want to jump into FORTH + code (albeit much of the "early" FORTH code will still need to be written as + assembly language primitives). + + This is what the set up code does. Does a tiny bit of house-keeping, sets up the + separate return stack (NB: Linux gives us the ordinary parameter stack already), then + immediately jumps to a FORTH word called QUIT. Despite its name, QUIT doesn't quit + anything. It resets some internal state and starts reading and interpreting commands. + (The reason it is called QUIT is because you can call QUIT from your own FORTH code + to "quit" your program and go back to interpreting). +*/ + +/* Assembler entry point. */ + .text + .globl _start +_start: + cld + mov %esp,var_S0 // Save the initial data stack pointer in FORTH variable S0. + mov $return_stack_top,%ebp // Initialise the return stack. + call set_up_data_segment + + mov $cold_start,%esi // Initialise interpreter. + NEXT // Run interpreter! + + .section .rodata +cold_start: // High-level code without a codeword. + .int QUIT + +/* + BUILT-IN WORDS ---------------------------------------------------------------------- + + Remember our dictionary entries (headers)? Let's bring those together with the codeword + and data words to see how : DOUBLE DUP + ; really looks in memory. + + pointer to previous word + ^ + | + +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ + ^ len pad codeword | + | V + LINK in next word points to codeword of DUP + + Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we + don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc. + So instead we will have to define built-in words using the GNU assembler data constructors + (like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are + unsure of them). + + The long way would be: + + .int + .byte 6 // len + .ascii "DOUBLE" // string + .byte 0 // padding +DOUBLE: .int DOCOL // codeword + .int DUP // pointer to codeword of DUP + .int PLUS // pointer to codeword of + + .int EXIT // pointer to codeword of EXIT + + That's going to get quite tedious rather quickly, so here I define an assembler macro + so that I can just write: + + defword "DOUBLE",6,,DOUBLE + .int DUP,PLUS,EXIT + + and I'll get exactly the same effect. + + Don't worry too much about the exact implementation details of this macro - it's complicated! +*/ + +/* Flags - these are discussed later. */ + .set F_IMMED,0x80 + .set F_HIDDEN,0x20 + .set F_LENMASK,0x1f // length mask + + // Store the chain of links. + .set link,0 + + .macro defword name, namelen, flags=0, label + .section .rodata + .align 4 + .globl name_\label +name_\label : + .int link // link + .set link,name_\label + .byte \flags+\namelen // flags + length byte + .ascii "\name" // the name + .align 4 // padding to next 4 byte boundary + .globl \label +\label : + .int DOCOL // codeword - the interpreter + // list of word pointers follow + .endm + +/* + Similarly I want a way to write words written in assembly language. There will quite a few + of these to start with because, well, everything has to start in assembly before there's + enough "infrastructure" to be able to start writing FORTH words, but also I want to define + some common FORTH words in assembly language for speed, even though I could write them in FORTH. + + This is what DUP looks like in memory: + + pointer to previous word + ^ + | + +--|------+---+---+---+---+------------+ + | LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly + +---------+---+---+---+---+------------+ code used to write DUP, + ^ len codeword which ends with NEXT. + | + LINK in next word + + Again, for brevity in writing the header I'm going to write an assembler macro called defcode. + As with defword above, don't worry about the complicated details of the macro. +*/ + + .macro defcode name, namelen, flags=0, label + .section .rodata + .align 4 + .globl name_\label +name_\label : + .int link // link + .set link,name_\label + .byte \flags+\namelen // flags + length byte + .ascii "\name" // the name + .align 4 // padding to next 4 byte boundary + .globl \label +\label : + .int code_\label // codeword + .text + //.align 4 + .globl code_\label +code_\label : // assembler code follows + .endm + +/* + Now some easy FORTH primitives. These are written in assembly for speed. If you understand + i386 assembly language then it is worth reading these. However if you don't understand assembly + you can skip the details. +*/ + + defcode "DROP",4,,DROP + pop %eax // drop top of stack + NEXT + + defcode "SWAP",4,,SWAP + pop %eax // swap top two elements on stack + pop %ebx + push %eax + push %ebx + NEXT + + defcode "DUP",3,,DUP + mov (%esp),%eax // duplicate top of stack + push %eax + NEXT + + defcode "OVER",4,,OVER + mov 4(%esp),%eax // get the second element of stack + push %eax // and push it on top + NEXT + + defcode "ROT",3,,ROT + pop %eax + pop %ebx + pop %ecx + push %eax + push %ecx + push %ebx + NEXT + + defcode "-ROT",4,,NROT + pop %eax + pop %ebx + pop %ecx + push %ebx + push %eax + push %ecx + NEXT + + defcode "2DROP",5,,TWODROP // drop top two elements of stack + pop %eax + pop %eax + NEXT + + defcode "2DUP",4,,TWODUP // duplicate top two elements of stack + mov (%esp),%eax + mov 4(%esp),%ebx + push %ebx + push %eax + NEXT + + defcode "2SWAP",5,,TWOSWAP // swap top two pairs of elements of stack + pop %eax + pop %ebx + pop %ecx + pop %edx + push %ebx + push %eax + push %edx + push %ecx + NEXT + + defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero + movl (%esp),%eax + test %eax,%eax + jz 1f + push %eax +1: NEXT + + defcode "1+",2,,INCR + incl (%esp) // increment top of stack + NEXT + + defcode "1-",2,,DECR + decl (%esp) // decrement top of stack + NEXT + + defcode "4+",2,,INCR4 + addl $4,(%esp) // add 4 to top of stack + NEXT + + defcode "4-",2,,DECR4 + subl $4,(%esp) // subtract 4 from top of stack + NEXT + + defcode "+",1,,ADD + pop %eax // get top of stack + addl %eax,(%esp) // and add it to next word on stack + NEXT + + defcode "-",1,,SUB + pop %eax // get top of stack + subl %eax,(%esp) // and subtract it from next word on stack + NEXT + + defcode "*",1,,MUL + pop %eax + pop %ebx + imull %ebx,%eax + push %eax // ignore overflow + NEXT + +/* + In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in + terms of the primitive /MOD. The design of the i386 assembly instruction idiv which + leaves both quotient and remainder makes this the obvious choice. +*/ + + defcode "/MOD",4,,DIVMOD + xor %edx,%edx + pop %ebx + pop %eax + idivl %ebx + push %edx // push remainder + push %eax // push quotient + NEXT + +/* + Lots of comparison operations like =, <, >, etc.. + + ANS FORTH says that the comparison words should return all (binary) 1's for + TRUE and all 0's for FALSE. However this is a bit of a strange convention + so this FORTH breaks it and returns the more normal (for C programmers ...) + 1 meaning TRUE and 0 meaning FALSE. +*/ + + defcode "=",1,,EQU // top two words are equal? + pop %eax + pop %ebx + cmp %ebx,%eax + sete %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "<>",2,,NEQU // top two words are not equal? + pop %eax + pop %ebx + cmp %ebx,%eax + setne %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "<",1,,LT + pop %eax + pop %ebx + cmp %eax,%ebx + setl %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode ">",1,,GT + pop %eax + pop %ebx + cmp %eax,%ebx + setg %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "<=",2,,LE + pop %eax + pop %ebx + cmp %eax,%ebx + setle %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode ">=",2,,GE + pop %eax + pop %ebx + cmp %eax,%ebx + setge %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0=",2,,ZEQU // top of stack equals 0? + pop %eax + test %eax,%eax + setz %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0<>",3,,ZNEQU // top of stack not 0? + pop %eax + test %eax,%eax + setnz %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0<",2,,ZLT // comparisons with 0 + pop %eax + test %eax,%eax + setl %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0>",2,,ZGT + pop %eax + test %eax,%eax + setg %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0<=",3,,ZLE + pop %eax + test %eax,%eax + setle %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "0>=",3,,ZGE + pop %eax + test %eax,%eax + setge %al + movzbl %al,%eax + pushl %eax + NEXT + + defcode "AND",3,,AND // bitwise AND + pop %eax + andl %eax,(%esp) + NEXT + + defcode "OR",2,,OR // bitwise OR + pop %eax + orl %eax,(%esp) + NEXT + + defcode "XOR",3,,XOR // bitwise XOR + pop %eax + xorl %eax,(%esp) + NEXT + + defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE and NOT) + notl (%esp) + NEXT + +/* + RETURNING FROM FORTH WORDS ---------------------------------------------------------------------- + + Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called + DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing): + + QUADRUPLE + +------------------+ + | codeword | + +------------------+ DOUBLE + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | addr of + | + +------------------+ + %esi -> | addr of EXIT | + +------------------+ + + What happens when the + function does NEXT? Well, the following code is executed. +*/ + + defcode "EXIT",4,,EXIT + POPRSP %esi // pop return stack into %esi + NEXT + +/* + EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi. + So after this (but just before NEXT) we get: + + QUADRUPLE + +------------------+ + | codeword | + +------------------+ DOUBLE + | addr of DOUBLE ---------------> +------------------+ + +------------------+ | codeword | + %esi -> | addr of DOUBLE | +------------------+ + +------------------+ | addr of DUP | + | addr of EXIT | +------------------+ + +------------------+ | addr of + | + +------------------+ + | addr of EXIT | + +------------------+ + + And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-) + + LITERALS ---------------------------------------------------------------------- + + The final point I "glossed over" before was how to deal with functions that do anything + apart from calling other functions. For example, suppose that DOUBLE was defined like this: + + : DOUBLE 2 * ; + + It does the same thing, but how do we compile it since it contains the literal 2? One way + would be to have a function called "2" (which you'd have to write in assembler), but you'd need + a function for every single literal that you wanted to use. + + FORTH solves this by compiling the function using a special word called LIT: + + +---------------------------+-------+-------+-------+-------+-------+ + | (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT | + +---------------------------+-------+-------+-------+-------+-------+ + + LIT is executed in the normal way, but what it does next is definitely not normal. It + looks at %esi (which now points to the number 2), grabs it, pushes it on the stack, then + manipulates %esi in order to skip the number as if it had never been there. + + What's neat is that the whole grab/manipulate can be done using a single byte single + i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams, + see if you can find out how LIT works: +*/ + + defcode "LIT",3,,LIT + // %esi points to the next command, but in this case it points to the next + // literal 32 bit integer. Get that literal into %eax and increment %esi. + // On x86, it's a convenient single byte instruction! (cf. NEXT macro) + lodsl + push %eax // push the literal number on to stack + NEXT + +/* + MEMORY ---------------------------------------------------------------------- + + As important point about FORTH is that it gives you direct access to the lowest levels + of the machine. Manipulating memory directly is done frequently in FORTH, and these are + the primitive words for doing it. +*/ + + defcode "!",1,,STORE + pop %ebx // address to store at + pop %eax // data to store there + mov %eax,(%ebx) // store it + NEXT + + defcode "@",1,,FETCH + pop %ebx // address to fetch + mov (%ebx),%eax // fetch it + push %eax // push value onto stack + NEXT + + defcode "+!",2,,ADDSTORE + pop %ebx // address + pop %eax // the amount to add + addl %eax,(%ebx) // add it + NEXT + + defcode "-!",2,,SUBSTORE + pop %ebx // address + pop %eax // the amount to subtract + subl %eax,(%ebx) // add it + NEXT + +/* + ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes + so we also define standard words C@ and C!. + + Byte-oriented operations only work on architectures which permit them (i386 is one of those). + */ + + defcode "C!",2,,STOREBYTE + pop %ebx // address to store at + pop %eax // data to store there + movb %al,(%ebx) // store it + NEXT + + defcode "C@",2,,FETCHBYTE + pop %ebx // address to fetch + xor %eax,%eax + movb (%ebx),%al // fetch it + push %eax // push value onto stack + NEXT + +/* C@C! is a useful byte copy primitive. */ + defcode "C@C!",4,,CCOPY + movl 4(%esp),%ebx // source address + movb (%ebx),%al // get source character + pop %edi // destination address + stosb // copy to destination + push %edi // increment destination address + incl 4(%esp) // increment source address + NEXT + +/* and CMOVE is a block copy operation. */ + defcode "CMOVE",5,,CMOVE + mov %esi,%edx // preserve %esi + pop %ecx // length + pop %edi // destination address + pop %esi // source address + rep movsb // copy source to destination + mov %edx,%esi // restore %esi + NEXT + +/* + BUILT-IN VARIABLES ---------------------------------------------------------------------- + + These are some built-in variables and related standard FORTH words. Of these, the only one that we + have discussed so far was LATEST, which points to the last (most recently defined) word in the + FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable) + on to the stack, so you can read or write it using @ and ! operators. For example, to print + the current value of LATEST (and this can apply to any FORTH variable) you would do: + + LATEST @ . CR + + To make defining variables shorter, I'm using a macro called defvar, similar to defword and + defcode above. (In fact the defvar macro uses defcode to do the dictionary header). +*/ + + .macro defvar name, namelen, flags=0, label, initial=0 + defcode \name,\namelen,\flags,\label + push $var_\name + NEXT + .data + .align 4 +var_\name : + .int \initial + .endm + +/* + The built-in variables are: + + STATE Is the interpreter executing code (0) or compiling a word (non-zero)? + LATEST Points to the latest (most recently defined) word in the dictionary. + HERE Points to the next free byte of memory. When compiling, compiled words go here. + S0 Stores the address of the top of the parameter stack. + BASE The current base for printing and reading numbers. + +*/ + defvar "STATE",5,,STATE + defvar "HERE",4,,HERE + defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary + defvar "S0",2,,SZ + defvar "BASE",4,,BASE,10 + +/* + BUILT-IN CONSTANTS ---------------------------------------------------------------------- + + It's also useful to expose a few constants to FORTH. When the word is executed it pushes a + constant value on the stack. + + The built-in constants are: + + VERSION Is the current version of this FORTH. + R0 The address of the top of the return stack. + DOCOL Pointer to DOCOL. + F_IMMED The IMMEDIATE flag's actual value. + F_HIDDEN The HIDDEN flag's actual value. + F_LENMASK The length mask in the flags/len byte. + + SYS_* and the numeric codes of various Linux syscalls (from ) +*/ + +//#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),%ebx + cmp (bufftop),%ebx + jge 1f // exhausted the input buffer? + xor %eax,%eax + mov (%ebx),%al // get next key from input buffer + inc %ebx + mov %ebx,(currkey) // increment currkey + ret + +1: // Out of input; use read(2) to fetch more input from stdin. + xor %ebx,%ebx // 1st param: stdin + mov $buffer,%ecx // 2nd param: buffer + mov %ecx,currkey + mov $BUFFER_SIZE,%edx // 3rd param: max length + mov $__NR_read,%eax // syscall: read + int $0x80 + test %eax,%eax // If %eax <= 0, then exit. + jbe 2f + addl %eax,%ecx // buffer+%eax = bufftop + mov %ecx,bufftop + jmp _KEY + +2: // Error or end of input: exit the program. + xor %ebx,%ebx + mov $__NR_exit,%eax // syscall: exit + int $0x80 + + .data + .align 4 +currkey: + .int buffer // Current place in input buffer (next character to read). +bufftop: + .int buffer // Last valid data in input buffer + 1. + +/* + By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout. + This implementation just uses the write system call. No attempt is made to buffer output, but + it would be a good exercise to add it. +*/ + + defcode "EMIT",4,,EMIT + pop %eax + call _EMIT + NEXT +_EMIT: + mov $1,%ebx // 1st param: stdout + + // write needs the address of the byte to write + mov %al,emit_scratch + mov $emit_scratch,%ecx // 2nd param: address + + mov $1,%edx // 3rd param: nbytes = 1 + + mov $__NR_write,%eax // write syscall + int $0x80 + ret + + .data // NB: easier to fit in the .data section +emit_scratch: + .space 1 // scratch used by EMIT + +/* + Back to input, WORD is a FORTH word which reads the next full word of input. + + What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on). + Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it + calculates the length of the word it read and returns the address and the length as + two words on the stack (with the length at the top of stack). + + Notice that WORD has a single internal buffer which it overwrites each time (rather like + a static C string). Also notice that WORD's internal buffer is just 32 bytes long and + there is NO checking for overflow. 31 bytes happens to be the maximum length of a + FORTH word that we support, and that is what WORD is used for: to read FORTH words when + we are compiling and executing code. The returned strings are not NUL-terminated. + + Start address+length is the normal way to represent strings in FORTH (not ending in an + ASCII NUL character as in C), and so FORTH strings can contain any character including NULs + and can be any length. + + WORD is not suitable for just reading strings (eg. user input) because of all the above + peculiarities and limitations. + + Note that when executing, you'll see: + WORD FOO + which puts "FOO" and length 3 on the stack, but when compiling: + : BAR WORD FOO ; + is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling + and immediate mode, and you'll understand why. +*/ + + defcode "WORD",4,,WORD + call _WORD + push %edi // push base address + push %ecx // push length + NEXT + +_WORD: + /* Search for first non-blank character. Also skip \ comments. */ +1: + call _KEY // get next key, returned in %eax + cmpb $'\\',%al // start of a comment? + je 3f // if so, skip the comment + cmpb $' ',%al + jbe 1b // if so, keep looking + + /* Search for the end of the word, storing chars as we go. */ + mov $word_buffer,%edi // pointer to return buffer +2: + stosb // add character to return buffer + call _KEY // get next key, returned in %al + cmpb $' ',%al // is blank? + ja 2b // if not, keep looping + + /* Return the word (well, the static buffer) and length. */ + sub $word_buffer,%edi + mov %edi,%ecx // return length of the word + mov $word_buffer,%edi // return address of the word + ret + + /* Code to skip \ comments to end of the current line. */ +3: + call _KEY + cmpb $'\n',%al // end of line yet? + jne 3b + jmp 1b + + .data // NB: easier to fit in the .data section + // A static buffer where WORD returns. Subsequent calls + // overwrite this buffer. Maximum word length is 32 chars. +word_buffer: + .space 32 + +/* + As well as reading in words we'll need to read in numbers and for that we are using a function + called NUMBER. This parses a numeric string such as one returned by WORD and pushes the + number on the parameter stack. + + The function uses the variable BASE as the base (radix) for conversion, so for example if + BASE is 2 then we expect a binary number. Normally BASE is 10. + + If the word starts with a '-' character then the returned value is negative. + + If the string can't be parsed as a number (or contains characters outside the current BASE) + then we need to return an error indication. So NUMBER actually returns two items on the stack. + At the top of stack we return the number of unconverted characters (ie. if 0 then all characters + were converted, so there is no error). Second from top of stack is the parsed number or a + partial value if there was an error. +*/ + defcode "NUMBER",6,,NUMBER + pop %ecx // length of string + pop %edi // start address of string + call _NUMBER + push %eax // parsed number + push %ecx // number of unparsed characters (0 = no error) + NEXT + +_NUMBER: + xor %eax,%eax + xor %ebx,%ebx + + test %ecx,%ecx // trying to parse a zero-length string is an error, but will return 0. + jz 5f + + movl var_BASE,%edx // get BASE (in %dl) + + // Check if first character is '-'. + movb (%edi),%bl // %bl = first character in string + inc %edi + push %eax // push 0 on stack + cmpb $'-',%bl // negative number? + jnz 2f + pop %eax + push %ebx // push <> 0 on stack, indicating negative + dec %ecx + jnz 1f + pop %ebx // error: string is only '-'. + movl $1,%ecx + ret + + // Loop reading digits. +1: imull %edx,%eax // %eax *= BASE + movb (%edi),%bl // %bl = next character in string + inc %edi + + // Convert 0-9, A-Z to a number 0-35. +2: subb $'0',%bl // < '0'? + jb 4f + cmp $10,%bl // <= '9'? + jb 3f + subb $17,%bl // < 'A'? (17 is 'A'-'0') + jb 4f + addb $10,%bl + +3: cmp %dl,%bl // >= BASE? + jge 4f + + // OK, so add it to %eax and loop. + add %ebx,%eax + dec %ecx + jnz 1b + + // Negate the result if first character was '-' (saved on the stack). +4: pop %ebx + test %ebx,%ebx + jz 5f + neg %eax + +5: ret + +/* + DICTIONARY LOOK UPS ---------------------------------------------------------------------- + + We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure. + + The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the + dictionary. What it actually returns is the address of the dictionary header, if it finds it, + or 0 if it didn't. + + So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer: + + pointer to this + | + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + + See also >CFA and >DFA. + + FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why. +*/ + + defcode "FIND",4,,FIND + pop %ecx // %ecx = length + pop %edi // %edi = address + call _FIND + push %eax // %eax = address of dictionary entry (or NULL) + NEXT + +_FIND: + push %esi // Save %esi so we can use it in string comparison. + + // Now we start searching backwards through the dictionary for this word. + mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary +1: test %edx,%edx // NULL pointer? (end of the linked list) + je 4f + + // Compare the length expected and the length of the word. + // Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery + // this won't pick the word (the length will appear to be wrong). + xor %eax,%eax + movb 4(%edx),%al // %al = flags+length field + andb $(F_HIDDEN|F_LENMASK),%al // %al = name length + cmpb %cl,%al // Length is the same? + jne 2f + + // Compare the strings in detail. + push %ecx // Save the length + push %edi // Save the address (repe cmpsb will move this pointer) + lea 5(%edx),%esi // Dictionary string we are checking against. + repe cmpsb // Compare the strings. + pop %edi + pop %ecx + jne 2f // Not the same. + + // The strings are the same - return the header pointer in %eax + pop %esi + mov %edx,%eax + ret + +2: mov (%edx),%edx // Move back through the link field to the previous word + jmp 1b // .. and loop. + +4: // Not found. + pop %esi + xor %eax,%eax // Return zero to indicate not found. + ret + +/* + FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall + that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH + word >CFA turns a dictionary pointer into a codeword pointer. + + The example below shows the result of: + + WORD DOUBLE FIND >CFA + + FIND returns a pointer to this + | >CFA converts it to a pointer to this + | | + V V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + codeword + + Notes: + + Because names vary in length, this isn't just a simple increment. + + In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but + that is not true in most FORTH implementations where they store a back pointer in the definition + (with an obvious memory/complexity cost). The reason they do this is that it is useful to be + able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions + quickly. + + What does CFA stand for? My best guess is "Code Field Address". +*/ + + defcode ">CFA",4,,TCFA + pop %edi + call _TCFA + push %edi + NEXT +_TCFA: + xor %eax,%eax + add $4,%edi // Skip link pointer. + movb (%edi),%al // Load flags+len into %al. + inc %edi // Skip flags+len byte. + andb $F_LENMASK,%al // Just the length, not the flags. + add %eax,%edi // Skip the name. + addl $3,%edi // The codeword is 4-byte aligned. + andl $~3,%edi + ret + +/* + Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and + returns a pointer to the first data field. + + FIND returns a pointer to this + | >CFA converts it to a pointer to this + | | + | | >DFA converts it to a pointer to this + | | | + V V V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + codeword + + (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is + different from theirs, because they have an extra indirection). + + You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA. +*/ + + defword ">DFA",4,,TDFA + .int TCFA // >CFA (get code field address) + .int INCR4 // 4+ (add 4 to it to get to next word) + .int EXIT // EXIT (return from FORTH word) + +/* + COMPILING ---------------------------------------------------------------------- + + Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this: + + : DOUBLE DUP + ; + + and we have to turn this into: + + pointer to previous word + ^ + | + +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ + ^ len pad codeword | + | V + LATEST points here points to codeword of DUP + + There are several problems to solve. Where to put the new word? How do we read words? How + do we define the words : (COLON) and ; (SEMICOLON)? + + FORTH solves this rather elegantly and as you might expect in a very low-level way which + allows you to change how the compiler works on your own code. + + FORTH has an INTERPRET function (a true interpreter this time, not DOCOL) which runs in a + loop, reading words (using WORD), looking them up (using FIND), turning them into codeword + pointers (using >CFA) and deciding what to do with them. + + What it does depends on the mode of the interpreter (in variable STATE). + + When STATE is zero, the interpreter just runs each word as it looks them up. This is known as + immediate mode. + + The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the + interpreter appends the codeword pointer to user memory (the HERE variable points to the next + free byte of user memory -- see DATA SEGMENT section below). + + So you may be able to see how we could define : (COLON). The general plan is: + + (1) Use WORD to read the name of the function being defined. + + (2) Construct the dictionary entry -- just the header part -- in user memory: + + pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where + ^ | the interpreter will start appending + | V codewords. + +--|------+---+---+---+---+---+---+---+---+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | + +---------+---+---+---+---+---+---+---+---+------------+ + len pad codeword + + (3) Set LATEST to point to the newly defined word, ... + + (4) .. and most importantly leave HERE pointing just after the new codeword. This is where + the interpreter will append codewords. + + (5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to + our partially-formed header. + + After : has run, our input is here: + + : DOUBLE DUP + ; + ^ + | + Next byte returned by KEY will be the 'D' character of DUP + + so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP", + looks it up in the dictionary, gets its codeword pointer, and appends it: + + +-- HERE updated to point here. + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + +---------+---+---+---+---+---+---+---+---+------------+------------+ + len pad codeword + + Next we read +, get the codeword pointer, and append it: + + +-- HERE updated to point here. + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+ + len pad codeword + + The issue is what happens next. Obviously what we _don't_ want to happen is that we + read ";" and compile it and go on compiling everything afterwards. + + At this point, FORTH uses a trick. Remember the length byte in the dictionary definition + isn't just a plain length byte, but can also contain flags. One flag is called the + IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as + IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_. + + This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE. + + And all it does is append the codeword for EXIT on to the current definition and switch + back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition + of ; and we'll see that it's really a very simple definition, declared IMMEDIATE. + + After the interpreter reads ; and executes it 'immediately', we get this: + + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + len pad codeword ^ + | + HERE + STATE is set to 0. + + And that's it, job done, our new definition is compiled, and we're back in immediate mode + just reading and executing words, perhaps including a call to test our new word DOUBLE. + + The only last wrinkle in this is that while our word was being compiled, it was in a + half-finished state. We certainly wouldn't want DOUBLE to be called somehow during + this time. There are several ways to stop this from happening, but in FORTH what we + do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is + being compiled. This prevents FIND from finding it, and thus in theory stops any + chance of it being called. + + The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm + going to define them. The : (COLON) function can be made a little bit more general by writing + it in two parts. The first part, called CREATE, makes just the header: + + +-- Afterwards, HERE points here. + | + V + +---------+---+---+---+---+---+---+---+---+ + | LINK | 6 | D | O | U | B | L | E | 0 | + +---------+---+---+---+---+---+---+---+---+ + len pad + + and the second part, the actual definition of : (COLON), calls CREATE and appends the + DOCOL codeword, so leaving: + + +-- Afterwards, HERE points here. + | + V + +---------+---+---+---+---+---+---+---+---+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | + +---------+---+---+---+---+---+---+---+---+------------+ + len pad codeword + + CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to + create other types of words (not just ones which contain code, but words which contain variables, + constants and other data). +*/ + + defcode "CREATE",6,,CREATE + + // Get the name length and address. + pop %ecx // %ecx = length + pop %ebx // %ebx = address of name + + // Link pointer. + movl var_HERE,%edi // %edi is the address of the header + movl var_LATEST,%eax // Get link pointer + stosl // and store it in the header. + + // Length byte and the word itself. + mov %cl,%al // Get the length. + stosb // Store the length/flags byte. + push %esi + mov %ebx,%esi // %esi = word + rep movsb // Copy the word + pop %esi + addl $3,%edi // Align to next 4 byte boundary. + andl $~3,%edi + + // Update LATEST and HERE. + movl var_HERE,%eax + movl %eax,var_LATEST + movl %edi,var_HERE + NEXT + +/* + Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words + to use. + + The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user + memory pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is: + + previous value of HERE + | + V + +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | | | + +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ + 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 + NEXT +_COMMA: + movl var_HERE,%edi // HERE + stosl // Store it. + movl %edi,var_HERE // Update HERE (incremented) + ret + +/* + Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode. + + Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this + variable we can switch between the two modes. + + For various reasons which may become apparent later, FORTH defines two standard words called + [ and ] (LBRAC and RBRAC) which switch between modes: + + Word Assembler Action Effect + [ LBRAC STATE := 0 Switch to immediate mode. + ] RBRAC STATE := 1 Switch to compile mode. + + [ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the + interpreter saw [ then it would compile it rather than running it. We would never be able to + switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode + the word runs immediately, switching us back to immediate mode. +*/ + + defcode "[",1,F_IMMED,LBRAC + xor %eax,%eax + movl %eax,var_STATE // Set STATE to 0. + NEXT + + defcode "]",1,,RBRAC + movl $1,var_STATE // Set STATE to 1. + NEXT + +/* + Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets + the word HIDDEN and goes into compile mode. +*/ + + defword ":",1,,COLON + .int WORD // Get the name of the new word + .int CREATE // CREATE the dictionary entry / header + .int LIT, DOCOL, COMMA // Append DOCOL (the codeword). + .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition). + .int RBRAC // Go into compile mode. + .int EXIT // Return from the function. + +/* + ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag. +*/ + + defword ";",1,F_IMMED,SEMICOLON + .int LIT, EXIT, COMMA // Append EXIT (so the word will return). + .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition). + .int LBRAC // Go back to IMMEDIATE mode. + .int EXIT // Return from the function. + +/* + EXTENDING THE COMPILER ---------------------------------------------------------------------- + + Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define + your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because + it allows you in effect to extend the compiler itself. Does gcc let you do that? + + Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic + compiler, and are all IMMEDIATE words. + + The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word, + or on the current word if you call it in the middle of a definition. + + Typical usage is: + + : MYIMMEDWORD IMMEDIATE + ...definition... + ; + + but some FORTH programmers write this instead: + + : MYIMMEDWORD + ...definition... + ; IMMEDIATE + + The two usages are equivalent, to a first approximation. +*/ + + defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE + movl var_LATEST,%edi // LATEST word. + addl $4,%edi // Point to name/flags byte. + xorb $F_IMMED,(%edi) // Toggle the IMMED bit. + NEXT + +/* + 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the + most recently defined word (used above in : and ; definitions) you would do: + + LATEST @ HIDDEN + + 'HIDE word' toggles the flag on a named 'word'. + + Setting this flag stops the word from being found by FIND, and so can be used to make 'private' + words. For example, to break up a large word into smaller parts you might do: + + : SUB1 ... subword ... ; + : SUB2 ... subword ... ; + : SUB3 ... subword ... ; + : MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ; + HIDE SUB1 + HIDE SUB2 + HIDE SUB3 + + After this, only MAIN is 'exported' or seen by the rest of the program. +*/ + + defcode "HIDDEN",6,,HIDDEN + pop %edi // Dictionary entry. + addl $4,%edi // Point to name/flags byte. + xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit. + NEXT + + defword "HIDE",4,,HIDE + .int WORD // Get the word (after HIDE). + .int FIND // Look up in the dictionary. + .int HIDDEN // Set F_HIDDEN flag. + .int EXIT // Return. + +/* + ' (TICK) is a standard FORTH word which returns the codeword pointer of the next word. + + The common usage is: + + ' FOO , + + which appends the codeword of FOO to the current word we are defining (this only works in compiled code). + + You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define + a literal 2 might be: + + : LIT2 IMMEDIATE + ' LIT , \ Appends LIT to the currently-being-defined word + 2 , \ Appends the number 2 to the currently-being-defined word + ; + + So you could do: + + : DOUBLE LIT2 * ; + + (If you don't understand how LIT2 works, then you should review the material about compiling words + and immediate mode). + + This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in + compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in + immediate mode too. +*/ + defcode "'",1,,TICK + lodsl // Get the address of the next word and skip it. + pushl %eax // Push it on the stack. + NEXT + +/* + BRANCHING ---------------------------------------------------------------------- + + It turns out that all you need in order to define looping constructs, IF-statements, etc. + are two primitives. + + BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the + top of stack is zero). + + The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes, + %esi starts by pointing to the offset field (compare to LIT above): + + +---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+ + | (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word | + +---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+ + ^ | ^ + | | | + | +-----------------------+ + %esi added to offset + + The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution + continues at the branch target. Negative offsets work as expected. + + 0BRANCH is the same except the branch happens conditionally. + + Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely + in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH + into the word currently being compiled. + + As an example, code written like this: + + condition-code IF true-part THEN rest-code + + compiles to: + + condition-code 0BRANCH OFFSET true-part rest-code + | ^ + | | + +-------------+ +*/ + + defcode "BRANCH",6,,BRANCH + add (%esi),%esi // add the offset to the instruction pointer + NEXT + + defcode "0BRANCH",7,,ZBRANCH + pop %eax + test %eax,%eax // top of stack is zero? + jz code_BRANCH // if so, jump back to the branch function above + lodsl // otherwise we need to skip the offset + NEXT + +/* + LITERAL STRINGS ---------------------------------------------------------------------- + + LITSTRING is a primitive used to implement the ." and S" operators (which are written in + FORTH). See the definition of those operators later. + + TELL just prints a string. It's more efficient to define this in assembly because we + can make it a single Linux syscall. +*/ + + defcode "LITSTRING",9,,LITSTRING + lodsl // get the length of the string + push %esi // push the address of the start of the string + push %eax // push it on the stack + addl %eax,%esi // skip past the string + addl $3,%esi // but round up to next 4 byte boundary + andl $~3,%esi + NEXT + + defcode "TELL",4,,TELL + mov $1,%ebx // 1st param: stdout + pop %edx // 3rd param: length of string + pop %ecx // 2nd param: address of string + mov $__NR_write,%eax // write syscall + int $0x80 + NEXT + +/* + QUIT AND INTERPRET ---------------------------------------------------------------------- + + QUIT is the first FORTH function called, almost immediately after the FORTH system "boots". + As explained before, QUIT doesn't "quit" anything. It does some initialisation (in particular + it clears the return stack) and it calls INTERPRET in a loop to interpret commands. The + reason it is called QUIT is because you can call it from your own FORTH words in order to + "quit" your program and start again at the user prompt. + + INTERPRET is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate + description -- see: http://en.wikipedia.org/wiki/REPL). +*/ + + // QUIT must not return (ie. must not call EXIT). + defword "QUIT",4,,QUIT + .int RZ,RSPSTORE // R0 RSP!, clear the return stack + .int INTERPRET // interpret the next word + .int BRANCH,-8 // and loop (indefinitely) + +/* + This interpreter is pretty simple, but remember that in FORTH you can always override + it later with a more powerful one! + */ + defcode "INTERPRET",9,,INTERPRET + call _WORD // Returns %ecx = length, %edi = pointer to word. + + // Is it in the dictionary? + xor %eax,%eax + movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...) + call _FIND // Returns %eax = pointer to header or 0 if not found. + test %eax,%eax // Found? + jz 1f + + // In the dictionary. Is it an IMMEDIATE codeword? + mov %eax,%edi // %edi = dictionary entry + movb 4(%edi),%al // Get name+flags. + push %ax // Just save it for now. + call _TCFA // Convert dictionary entry (in %edi) to codeword pointer. + pop %ax + andb $F_IMMED,%al // Is IMMED flag set? + mov %edi,%eax + jnz 4f // If IMMED, jump straight to executing. + + jmp 2f + +1: // Not in the dictionary (not a word) so assume it's a literal number. + incl interpret_is_lit + call _NUMBER // Returns the parsed number in %eax, %ecx > 0 if error + test %ecx,%ecx + jnz 6f + mov %eax,%ebx + mov $LIT,%eax // The word is LIT + +2: // Are we compiling or executing? + movl var_STATE,%edx + test %edx,%edx + jz 4f // Jump if executing. + + // Compiling - just append the word to the current dictionary definition. + call _COMMA + mov interpret_is_lit,%ecx // Was it a literal? + test %ecx,%ecx + jz 3f + mov %ebx,%eax // Yes, so LIT is followed by a number. + call _COMMA +3: NEXT + +4: // Executing - run it! + mov interpret_is_lit,%ecx // Literal? + test %ecx,%ecx // Literal? + jnz 5f + + // Not a literal, execute it now. This never returns, but the codeword will + // eventually call NEXT which will reenter the loop in QUIT. + jmp *(%eax) + +5: // Executing a literal, which means push it on the stack. + push %ebx + NEXT + +6: // Parse error (not a known word or a number in the current BASE). + // Print an error message followed by up to 40 characters of context. + mov $2,%ebx // 1st param: stderr + mov $errmsg,%ecx // 2nd param: error message + mov $errmsgend-errmsg,%edx // 3rd param: length of string + mov $__NR_write,%eax // write syscall + int $0x80 + + mov (currkey),%ecx // the error occurred just before currkey position + mov %ecx,%edx + sub $buffer,%edx // %edx = currkey - buffer (length in buffer before currkey) + cmp $40,%edx // if > 40, then print only 40 characters + jle 7f + mov $40,%edx +7: sub %edx,%ecx // %ecx = start of area to print, %edx = length + mov $__NR_write,%eax // write syscall + int $0x80 + + mov $errmsgnl,%ecx // newline + mov $1,%edx + mov $__NR_write,%eax // write syscall + int $0x80 + + NEXT + + .section .rodata +errmsg: .ascii "PARSE ERROR: " +errmsgend: +errmsgnl: .ascii "\n" + + .data // NB: easier to fit in the .data section + .align 4 +interpret_is_lit: + .int 0 // Flag used to record if reading a literal + +/* + ODDS AND ENDS ---------------------------------------------------------------------- + + CHAR puts the ASCII code of the first character of the following word on the stack. For example + CHAR A puts 65 on the stack. + + EXECUTE is used to run execution tokens. See the discussion of execution tokens in the + FORTH code for more details. + + SYSCALL0, SYSCALL1, SYSCALL2, SYSCALL3 make a standard Linux system call. (See + 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 + + 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: + .space BUFFER_SIZE + +/* + START OF FORTH CODE ---------------------------------------------------------------------- + + We've now reached the stage where the FORTH system is running and self-hosting. All further + words can be written as FORTH itself, including words like IF, THEN, .", etc which in most + languages would be considered rather fundamental. + + I used to append this here in the assembly file, but I got sick of fighting against gas's + crack-smoking (lack of) multiline string syntax. So now that is in a separate file called + jonesforth.f + + If you don't already have that file, download it from http://annexia.org/forth in order + to continue the tutorial. +*/ + +/* END OF jonesforth.S */ diff --git a/jonesforth/jonesforth.fs b/jonesforth/jonesforth.fs new file mode 100644 index 0000000..5a998cc --- /dev/null +++ b/jonesforth/jonesforth.fs @@ -0,0 +1,1790 @@ +\ -*- text -*- +\ 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.f,v 1.17 2007/10/12 20:07:44 rich Exp $ +\ +\ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth +\ +\ 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. +\ +\ START OF FORTH CODE ---------------------------------------------------------------------- +\ +\ We've now reached the stage where the FORTH system is running and self-hosting. All further +\ words can be written as FORTH itself, including words like IF, THEN, .", etc which in most +\ languages would be considered rather fundamental. +\ +\ Some notes about the code: +\ +\ I use indenting to show structure. The amount of whitespace has no meaning to FORTH however +\ except that you must use at least one whitespace character between words, and words themselves +\ cannot contain whitespace. +\ +\ FORTH is case-sensitive. Use capslock! + +\ The primitive word /MOD (DIVMOD) leaves both the quotient and the remainder on the stack. (On +\ i386, the idivl instruction gives both anyway). Now we can define the / and MOD in terms of /MOD +\ and a few other primitives. +: / /MOD SWAP DROP ; +: MOD /MOD DROP ; + +\ Define some character constants +: '\n' 10 ; +: BL 32 ; \ BL (BLank) is a standard FORTH word for space. + +\ CR prints a carriage return +: CR '\n' EMIT ; + +\ SPACE prints a space +: SPACE BL EMIT ; + +\ NEGATE leaves the negative of a number on the stack. +: NEGATE 0 SWAP - ; + +\ Standard words for booleans. +: TRUE 1 ; +: FALSE 0 ; +: NOT 0= ; + +\ LITERAL takes whatever is on the stack and compiles LIT +: LITERAL IMMEDIATE + ' LIT , \ compile LIT + , \ compile the literal itself (from the stack) + ; + +\ Now we can use [ and ] to insert literals which are calculated at compile time. (Recall that +\ [ and ] are the FORTH words which switch into and out of immediate mode.) +\ Within definitions, use [ ... ] LITERAL anywhere that '...' is a constant expression which you +\ would rather only compute once (at compile time, rather than calculating it each time your word runs). +: ':' + [ \ go into immediate mode (temporarily) + CHAR : \ push the number 58 (ASCII code of colon) on the parameter stack + ] \ go back to compile mode + LITERAL \ compile LIT 58 as the definition of ':' word +; + +\ A few more character constants defined the same way as above. +: ';' [ CHAR ; ] LITERAL ; +: '(' [ CHAR ( ] LITERAL ; +: ')' [ CHAR ) ] LITERAL ; +: '"' [ CHAR " ] LITERAL ; +: 'A' [ CHAR A ] LITERAL ; +: '0' [ CHAR 0 ] LITERAL ; +: '-' [ CHAR - ] LITERAL ; +: '.' [ CHAR . ] LITERAL ; + +\ 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. However we still have +\ access to the word which we are currently compiling through the LATEST pointer so we +\ can use that to compile a recursive call. +: RECURSE IMMEDIATE + LATEST @ \ LATEST points to the word being compiled at the moment + >CFA \ get the codeword + , \ compile it +; + +\ CONTROL STRUCTURES ---------------------------------------------------------------------- +\ +\ So far we have defined only very simple definitions. Before we can go further, we really need to +\ make some control structures, like IF ... THEN and loops. Luckily we can define arbitrary control +\ structures directly in FORTH. +\ +\ Please note that the control structures as I have defined them here will only work inside compiled +\ words. If you try to type in expressions using IF, etc. in immediate mode, then they won't work. +\ Making these work in immediate mode is left as an exercise for the reader. + +\ 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 +; + +\ UNLESS is the same as IF but the test is reversed. +\ +\ Note the use of [COMPILE]: Since IF is IMMEDIATE we don't want it to be executed while UNLESS +\ is compiling, but while UNLESS is running (which happens to be when whatever word using UNLESS is +\ being compiled -- whew!). So we use [COMPILE] to reverse the effect of marking IF as immediate. +\ This trick is generally used when we want to write our own control words without having to +\ implement them all in terms of the primitives 0BRANCH and BRANCH, but instead reusing simpler +\ control words like (in this instance) IF. +: UNLESS IMMEDIATE + ' NOT , \ compile NOT (to reverse the test) + [COMPILE] IF \ continue by calling the normal IF +; + +\ COMMENTS ---------------------------------------------------------------------- +\ +\ FORTH allows ( ... ) as comments within function definitions. This works by having an IMMEDIATE +\ word called ( which just drops input characters until it hits the corresponding ). +: ( IMMEDIATE + 1 \ allowed nested parens by keeping track of depth + BEGIN + KEY \ read next character + DUP '(' = IF \ open paren? + DROP \ drop the open paren + 1+ \ depth increases + ELSE + ')' = IF \ close paren? + 1- \ depth decreases + THEN + THEN + DUP 0= UNTIL \ continue until we reach matching close paren, depth 0 + DROP \ drop the depth counter +; + +( + From now on we can use ( ... ) for comments. + + STACK NOTATION ---------------------------------------------------------------------- + + In FORTH style we can also use ( ... -- ... ) to show the effects that a word has on the + parameter stack. For example: + + ( n -- ) means that the word consumes an integer (n) from the parameter stack. + ( b a -- c ) means that the word uses two integers (a and b, where a is at the top of stack) + and returns a single integer (c). + ( -- ) means the word has no effect on the stack +) + +( Some more complicated stack examples, showing the stack notation. ) +: NIP ( x y -- y ) SWAP DROP ; +: TUCK ( x y -- y x y ) DUP ROT ; +: PICK ( x_u ... x_1 x_0 u -- x_u ... x_1 x_0 x_u ) + 1+ ( add one because of 'u' on the stack ) + 4 * ( multiply by the word size ) + DSP@ + ( add to the stack pointer ) + @ ( and fetch ) +; + +( With the looping constructs, we can now write SPACES, which writes n spaces to stdout. ) +: SPACES ( n -- ) + BEGIN + DUP 0> ( while n > 0 ) + WHILE + SPACE ( print a space ) + 1- ( until we count down to 0 ) + REPEAT + DROP +; + +( Standard words for manipulating BASE. ) +: DECIMAL ( -- ) 10 BASE ! ; +: HEX ( -- ) 16 BASE ! ; + +( + PRINTING NUMBERS ---------------------------------------------------------------------- + + The standard FORTH word . (DOT) is very important. It takes the number at the top + of the stack and prints it out. However first I'm going to implement some lower-level + FORTH words: + + U.R ( u width -- ) which prints an unsigned number, padded to a certain width + U. ( u -- ) which prints an unsigned number + .R ( n width -- ) which prints a signed number, padded to a certain width. + + For example: + -123 6 .R + will print out these characters: + - 1 2 3 + + In other words, the number padded left to a certain number of characters. + + The full number is printed even if it is wider than width, and this is what allows us to + define the ordinary functions U. and . (we just set width to zero knowing that the full + number will be printed anyway). + + Another wrinkle of . and friends is that they obey the current base in the variable BASE. + BASE can be anything in the range 2 to 36. + + While we're defining . &c we can also define .S which is a useful debugging tool. This + word prints the current stack (non-destructively) from top to bottom. +) + +( This is the underlying recursive definition of U. ) +: U. ( u -- ) + BASE @ /MOD ( width rem quot ) + ?DUP IF ( if quotient <> 0 then ) + RECURSE ( print the quotient ) + THEN + + ( print the remainder ) + DUP 10 < IF + '0' ( decimal digits 0..9 ) + ELSE + 10 - ( hex and beyond digits A..Z ) + 'A' + THEN + + + EMIT +; + +( + FORTH word .S prints the contents of the stack. It doesn't alter the stack. + Very useful for debugging. +) +: .S ( -- ) + DSP@ ( get current stack pointer ) + BEGIN + DUP S0 @ < + WHILE + DUP @ U. ( print the stack element ) + SPACE + 4+ ( move up ) + REPEAT + DROP +; + +( This word returns the width (in characters) of an unsigned number in the current base ) +: UWIDTH ( u -- width ) + BASE @ / ( rem quot ) + ?DUP IF ( if quotient <> 0 then ) + RECURSE 1+ ( return 1+recursive call ) + ELSE + 1 ( return 1 ) + THEN +; + +: U.R ( u width -- ) + SWAP ( width u ) + DUP ( width u u ) + UWIDTH ( width u uwidth ) + -ROT ( u uwidth width ) + SWAP - ( u width-uwidth ) + ( At this point if the requested width is narrower, we'll have a negative number on the stack. + Otherwise the number on the stack is the number of spaces to print. But SPACES won't print + a negative number of spaces anyway, so it's now safe to call SPACES ... ) + SPACES + ( ... and then call the underlying implementation of U. ) + U. +; + +( + .R prints a signed number, padded to a certain width. We can't just print the sign + and call U.R because we want the sign to be next to the number ('-123' instead of '- 123'). +) +: .R ( n width -- ) + SWAP ( width n ) + DUP 0< IF + NEGATE ( width u ) + 1 ( save a flag to remember that it was negative | width n 1 ) + ROT ( 1 width u ) + SWAP ( 1 u width ) + 1- ( 1 u width-1 ) + ELSE + 0 ( width u 0 ) + ROT ( 0 width u ) + SWAP ( 0 u width ) + THEN + SWAP ( flag width u ) + DUP ( flag width u u ) + UWIDTH ( flag width u uwidth ) + -ROT ( flag u uwidth width ) + SWAP - ( flag u width-uwidth ) + + SPACES ( flag u ) + SWAP ( u flag ) + + IF ( was it negative? print the - character ) + '-' EMIT + THEN + + U. +; + +( Finally we can define word . in terms of .R, with a trailing space. ) +: . 0 .R SPACE ; + +( The real U., note the trailing space. ) +: U. U. SPACE ; + +( ? fetches the integer at an address and prints it. ) +: ? ( addr -- ) @ . ; + +( c a b WITHIN returns true if a <= c and c < b ) +: WITHIN + ROT ( b c a ) + OVER ( b c a c ) + <= IF + > IF ( b c -- ) + TRUE + ELSE + FALSE + THEN + ELSE + 2DROP ( b c -- ) + FALSE + THEN +; + +( DEPTH returns the depth of the stack. ) +: DEPTH ( -- n ) + S0 @ DSP@ - + 4- ( adjust because S0 was on the stack when we pushed DSP ) +; + +( + ALIGNED takes an address and rounds it up (aligns it) to the next 4 byte boundary. +) +: ALIGNED ( addr -- addr ) + 3 + 3 INVERT AND ( (addr+3) & ~3 ) +; + +( + ALIGN aligns the HERE pointer, so the next word appended will be aligned properly. +) +: ALIGN HERE @ ALIGNED HERE ! ; + +( + STRINGS ---------------------------------------------------------------------- + + S" string" is used in FORTH to define strings. It leaves the address of the string and + its length on the stack, (length at the top of stack). The space following S" is the normal + space between FORTH words and is not a part of the string. + + 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 compile mode we append + LITSTRING + to the current word. The primitive LITSTRING does the right thing when the current + word is executed. + + In immediate mode there isn't a particularly good place to put the string, but in this + case we put the string at HERE (but we _don't_ change HERE). This is meant as a temporary + location, likely to be overwritten soon after. +) +( C, appends a byte to the current compiled word. ) +: C, + HERE @ C! ( store the character in the compiled image ) + 1 HERE +! ( increment HERE pointer by 1 byte ) +; + +: S" IMMEDIATE ( -- addr len ) + STATE @ IF ( compiling? ) + ' 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 + C, ( copy character ) + 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 ) + ALIGN ( round up to next multiple of 4 bytes for the remaining code ) + ELSE ( immediate mode ) + HERE @ ( get the start address of the temporary space ) + BEGIN + KEY + DUP '"' <> + WHILE + OVER C! ( save next character ) + 1+ ( increment address ) + REPEAT + DROP ( drop the final " character ) + HERE @ - ( calculate the length ) + HERE @ ( push the start address ) + SWAP ( addr len ) + THEN +; + +( + ." is the print string operator in FORTH. Example: ." Something to print" + The space after the operator is the ordinary space required between words and is not + a part of what is printed. + + In immediate mode we just keep reading characters and printing them until we get to + the next double quote. + + In compile mode we use S" to store the string, then add TELL afterwards: + LITSTRING TELL + + It may be interesting to note the use of [COMPILE] to turn the call to the immediate + word S" into compilation of that word. It compiles it into the definition of .", + not into the definition of the word being compiled when this is running (complicated + enough for you?) +) +: ." IMMEDIATE ( -- ) + STATE @ IF ( compiling? ) + [COMPILE] S" ( read the string, and compile LITSTRING, etc. ) + ' TELL , ( compile the final TELL ) + ELSE + ( In immediate mode, just read characters and print them until we get + to the ending double quote. ) + BEGIN + KEY + DUP '"' = IF + DROP ( drop the double quote character ) + EXIT ( return from this function ) + THEN + EMIT + AGAIN + THEN +; + +( + CONSTANTS AND VARIABLES ---------------------------------------------------------------------- + + In FORTH, global constants and variables are defined like this: + + 10 CONSTANT TEN when TEN is executed, it leaves the integer 10 on the stack + VARIABLE VAR when VAR is executed, it leaves the address of VAR on the stack + + Constants can be read but not written, eg: + + TEN . CR prints 10 + + You can read a variable (in this example called VAR) by doing: + + VAR @ leaves the value of VAR on the stack + VAR @ . CR prints the value of VAR + VAR ? CR same as above, since ? is the same as @ . + + and update the variable by doing: + + 20 VAR ! sets VAR to 20 + + Note that variables are uninitialised (but see VALUE later on which provides initialised + variables with a slightly simpler syntax). + + How can we define the words CONSTANT and VARIABLE? + + The trick is to define a new word for the variable itself (eg. if the variable was called + 'VAR' then we would define a new word called VAR). This is easy to do because we exposed + dictionary entry creation through the CREATE word (part of the definition of : above). + A call to WORD [TEN] CREATE (where [TEN] means that "TEN" is the next word in the input) + leaves the dictionary entry: + + +--- HERE + | + V + +---------+---+---+---+---+ + | LINK | 3 | T | E | N | + +---------+---+---+---+---+ + len + + For CONSTANT we can continue by appending DOCOL (the codeword), then LIT followed by + the constant itself and then EXIT, forming a little word definition that returns the + constant: + + +---------+---+---+---+---+------------+------------+------------+------------+ + | LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT | + +---------+---+---+---+---+------------+------------+------------+------------+ + len codeword + + Notice that this word definition is exactly the same as you would have got if you had + written : TEN 10 ; + + Note for people reading the code below: DOCOL is a constant word which we defined in the + assembler part which returns the value of the assembler symbol of the same name. +) +: CONSTANT + WORD ( get the name (the name follows CONSTANT) ) + CREATE ( make the dictionary entry ) + DOCOL , ( append DOCOL (the codeword field of this word) ) + ' LIT , ( append the codeword LIT ) + , ( append the value on the top of the stack ) + ' EXIT , ( append the codeword EXIT ) +; + +( + VARIABLE is a little bit harder because we need somewhere to put the variable. There is + nothing particularly special about the user memory (the area of memory pointed to by HERE + where we have previously just stored new word definitions). We can slice off bits of this + memory area to store anything we want, so one possible definition of VARIABLE might create + this: + + +--------------------------------------------------------------+ + | | + V | + +---------+---------+---+---+---+---+------------+------------+---|--------+------------+ + | | LINK | 3 | V | A | R | DOCOL | LIT | | EXIT | + +---------+---------+---+---+---+---+------------+------------+------------+------------+ + len codeword + + where is the place to store the variable, and points back to it. + + To make this more general let's define a couple of words which we can use to allocate + arbitrary memory from the user memory. + + First ALLOT, where n ALLOT allocates n bytes of memory. (Note when calling this that + it's a very good idea to make sure that n is a multiple of 4, or at least that next time + a word is compiled that HERE has been left as a multiple of 4). +) +: ALLOT ( n -- addr ) + HERE @ SWAP ( here n ) + HERE +! ( adds n to HERE, after this the old value of HERE is still on the stack ) +; + +( + Second, CELLS. In FORTH the phrase 'n CELLS ALLOT' means allocate n integers of whatever size + is the natural size for integers on this machine architecture. On this 32 bit machine therefore + CELLS just multiplies the top of stack by 4. +) +: CELLS ( n -- n ) 4 * ; + +( + So now we can define VARIABLE easily in much the same way as CONSTANT above. Refer to the + diagram above to see what the word that this creates will look like. +) +: VARIABLE + 1 CELLS ALLOT ( allocate 1 cell of memory, push the pointer to this memory ) + WORD CREATE ( make the dictionary entry (the name follows VARIABLE) ) + DOCOL , ( append DOCOL (the codeword field of this word) ) + ' LIT , ( append the codeword LIT ) + , ( append the pointer to the new memory ) + ' EXIT , ( append the codeword EXIT ) +; + +( + VALUES ---------------------------------------------------------------------- + + VALUEs are like VARIABLEs but with a simpler syntax. You would generally use them when you + want a variable which is read often, and written infrequently. + + 20 VALUE VAL creates VAL with initial value 20 + VAL pushes the value (20) directly on the stack + 30 TO VAL updates VAL, setting it to 30 + VAL pushes the value (30) directly on the stack + + Notice that 'VAL' on its own doesn't return the address of the value, but the value itself, + making values simpler and more obvious to use than variables (no indirection through '@'). + The price is a more complicated implementation, although despite the complexity there is no + performance penalty at runtime. + + A naive implementation of 'TO' would be quite slow, involving a dictionary search each time. + But because this is FORTH we have complete control of the compiler so we can compile TO more + efficiently, turning: + TO VAL + into: + LIT ! + and calculating (the address of the value) at compile time. + + Now this is the clever bit. We'll compile our value like this: + + +---------+---+---+---+---+------------+------------+------------+------------+ + | LINK | 3 | V | A | L | DOCOL | LIT | | EXIT | + +---------+---+---+---+---+------------+------------+------------+------------+ + len codeword + + where is the actual value itself. Note that when VAL executes, it will push the + value on the stack, which is what we want. + + But what will TO use for the address ? Why of course a pointer to that : + + code compiled - - - - --+------------+------------+------------+-- - - - - + by TO VAL | LIT | | ! | + - - - - --+------------+-----|------+------------+-- - - - - + | + V + +---------+---+---+---+---+------------+------------+------------+------------+ + | LINK | 3 | V | A | L | DOCOL | LIT | | EXIT | + +---------+---+---+---+---+------------+------------+------------+------------+ + len codeword + + In other words, this is a kind of self-modifying code. + + (Note to the people who want to modify this FORTH to add inlining: values defined this + way cannot be inlined). +) +: VALUE ( n -- ) + WORD CREATE ( make the dictionary entry (the name follows VALUE) ) + DOCOL , ( append DOCOL ) + ' LIT , ( append the codeword LIT ) + , ( append the initial value ) + ' EXIT , ( append the codeword EXIT ) +; + +: TO IMMEDIATE ( n -- ) + WORD ( get the name of the value ) + FIND ( look it up in the dictionary ) + >DFA ( get a pointer to the first data field (the 'LIT') ) + 4+ ( increment to point at the value ) + STATE @ IF ( compiling? ) + ' LIT , ( compile LIT ) + , ( compile the address of the value ) + ' ! , ( compile ! ) + ELSE ( immediate mode ) + ! ( update it straightaway ) + THEN +; + +( x +TO VAL adds x to VAL ) +: +TO IMMEDIATE + WORD ( get the name of the value ) + FIND ( look it up in the dictionary ) + >DFA ( get a pointer to the first data field (the 'LIT') ) + 4+ ( increment to point at the value ) + STATE @ IF ( compiling? ) + ' LIT , ( compile LIT ) + , ( compile the address of the value ) + ' +! , ( compile +! ) + ELSE ( immediate mode ) + +! ( update it straightaway ) + THEN +; + +( + PRINTING THE DICTIONARY ---------------------------------------------------------------------- + + ID. takes an address of a dictionary entry and prints the word's name. + + For example: LATEST @ ID. would print the name of the last word that was defined. +) +: ID. + 4+ ( skip over the link pointer ) + DUP C@ ( get the flags/length byte ) + F_LENMASK AND ( mask out the flags - just want the length ) + + BEGIN + DUP 0> ( length > 0? ) + WHILE + SWAP 1+ ( addr len -- len addr+1 ) + DUP C@ ( len addr -- len addr char | get the next character) + EMIT ( len addr char -- len addr | and print it) + SWAP 1- ( len addr -- addr len-1 | subtract one from length ) + REPEAT + 2DROP ( len addr -- ) +; + +( + 'WORD word FIND ?HIDDEN' returns true if 'word' is flagged as hidden. + + 'WORD word FIND ?IMMEDIATE' returns true if 'word' is flagged as immediate. +) +: ?HIDDEN + 4+ ( skip over the link pointer ) + C@ ( get the flags/length byte ) + F_HIDDEN AND ( mask the F_HIDDEN flag and return it (as a truth value) ) +; +: ?IMMEDIATE + 4+ ( skip over the link pointer ) + C@ ( get the flags/length byte ) + F_IMMED AND ( mask the F_IMMED flag and return it (as a truth value) ) +; + +( + WORDS prints all the words defined in the dictionary, starting with the word defined most recently. + However it doesn't print hidden words. + + The implementation simply iterates backwards from LATEST using the link pointers. +) +: WORDS + LATEST @ ( start at LATEST dictionary entry ) + BEGIN + ?DUP ( while link pointer is not null ) + WHILE + DUP ?HIDDEN NOT IF ( ignore hidden words ) + DUP ID. ( but if not hidden, print the word ) + SPACE + THEN + @ ( dereference the link pointer - go to previous word ) + REPEAT + CR +; + +( + FORGET ---------------------------------------------------------------------- + + So far we have only allocated words and memory. FORTH provides a rather primitive method + to deallocate. + + 'FORGET word' deletes the definition of 'word' from the dictionary and everything defined + after it, including any variables and other memory allocated after. + + The implementation is very simple - we look up the word (which returns the dictionary entry + address). Then we set HERE to point to that address, so in effect all future allocations + and definitions will overwrite memory starting at the word. We also need to set LATEST to + point to the previous word. + + Note that you cannot FORGET built-in words (well, you can try but it will probably cause + a segfault). + + XXX: Because we wrote VARIABLE to store the variable in memory allocated before the word, + in the current implementation VARIABLE FOO FORGET FOO will leak 1 cell of memory. +) +: FORGET + WORD FIND ( find the word, gets the dictionary entry address ) + DUP @ LATEST ! ( set LATEST to point to the previous word ) + HERE ! ( and store HERE with the dictionary address ) +; + +( + DUMP ---------------------------------------------------------------------- + + DUMP is used to dump out the contents of memory, in the 'traditional' hexdump format. + + Notice that the parameters to DUMP (address, length) are compatible with string words + such as WORD and S". + + You can dump out the raw code for the last word you defined by doing something like: + + LATEST @ 128 DUMP +) +: DUMP ( addr len -- ) + BASE @ ROT ( save the current BASE at the bottom of the stack ) + HEX ( and switch to hexadecimal mode ) + + BEGIN + ?DUP ( while len > 0 ) + WHILE + OVER 8 U.R ( print the address ) + SPACE + + ( print up to 16 words on this line ) + 2DUP ( addr len addr len ) + 1- 15 AND 1+ ( addr len addr linelen ) + BEGIN + ?DUP ( while linelen > 0 ) + WHILE + SWAP ( addr len linelen addr ) + DUP C@ ( addr len linelen addr byte ) + 2 .R SPACE ( print the byte ) + 1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 ) + REPEAT + DROP ( addr len ) + + ( print the ASCII equivalents ) + 2DUP 1- 15 AND 1+ ( addr len addr linelen ) + BEGIN + ?DUP ( while linelen > 0) + WHILE + SWAP ( addr len linelen addr ) + DUP C@ ( addr len linelen addr byte ) + DUP 32 128 WITHIN IF ( 32 <= c < 128? ) + EMIT + ELSE + DROP '.' EMIT + THEN + 1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 ) + REPEAT + DROP ( addr len ) + CR + + DUP 1- 15 AND 1+ ( addr len linelen ) + DUP ( addr len linelen linelen ) + ROT ( addr linelen len linelen ) + - ( addr linelen len-linelen ) + ROT ( len-linelen addr linelen ) + + ( len-linelen addr+linelen ) + SWAP ( addr-linelen len-linelen ) + REPEAT + + DROP ( restore stack ) + BASE ! ( restore saved BASE ) +; + +( + CASE ---------------------------------------------------------------------- + + CASE...ENDCASE is how we do switch statements in FORTH. There is no generally + agreed syntax for this, so I've gone for the syntax mandated by the ISO standard + FORTH (ANS-FORTH). + + ( some value on the stack ) + CASE + test1 OF ... ENDOF + test2 OF ... ENDOF + testn OF ... ENDOF + ... ( default case ) + ENDCASE + + The CASE statement tests the value on the stack by comparing it for equality with + test1, test2, ..., testn and executes the matching piece of code within OF ... ENDOF. + If none of the test values match then the default case is executed. Inside the ... of + the default case, the value is still at the top of stack (it is implicitly DROP-ed + by ENDCASE). When ENDOF is executed it jumps after ENDCASE (ie. there is no "fall-through" + and no need for a break statement like in C). + + The default case may be omitted. In fact the tests may also be omitted so that you + just have a default case, although this is probably not very useful. + + An example (assuming that 'q', etc. are words which push the ASCII value of the letter + on the stack): + + 0 VALUE QUIT + 0 VALUE SLEEP + KEY CASE + 'q' OF 1 TO QUIT ENDOF + 's' OF 1 TO SLEEP ENDOF + ( default case: ) + ." Sorry, I didn't understand key <" DUP EMIT ." >, try again." CR + ENDCASE + + (In some versions of FORTH, more advanced tests are supported, such as ranges, etc. + Other versions of FORTH need you to write OTHERWISE to indicate the default case. + As I said above, this FORTH tries to follow the ANS FORTH standard). + + The implementation of CASE...ENDCASE is somewhat non-trivial. I'm following the + implementations from here: + http://www.uni-giessen.de/faq/archiv/forthfaq.case_endcase/msg00000.html + + The general plan is to compile the code as a series of IF statements: + + CASE (push 0 on the immediate-mode parameter stack) + test1 OF ... ENDOF test1 OVER = IF DROP ... ELSE + test2 OF ... ENDOF test2 OVER = IF DROP ... ELSE + testn OF ... ENDOF testn OVER = IF DROP ... ELSE + ... ( default case ) ... + ENDCASE DROP THEN [THEN [THEN ...]] + + The CASE statement pushes 0 on the immediate-mode parameter stack, and that number + is used to count how many THEN statements we need when we get to ENDCASE so that each + IF has a matching THEN. The counting is done implicitly. If you recall from the + implementation above of IF, each IF pushes a code address on the immediate-mode stack, + and these addresses are non-zero, so by the time we get to ENDCASE the stack contains + some number of non-zeroes, followed by a zero. The number of non-zeroes is how many + times IF has been called, so how many times we need to match it with THEN. + + This code uses [COMPILE] so that we compile calls to IF, ELSE, THEN instead of + actually calling them while we're compiling the words below. + + As is the case with all of our control structures, they only work within word + definitions, not in immediate mode. +) +: CASE IMMEDIATE + 0 ( push 0 to mark the bottom of the stack ) +; + +: OF IMMEDIATE + ' OVER , ( compile OVER ) + ' = , ( compile = ) + [COMPILE] IF ( compile IF ) + ' DROP , ( compile DROP ) +; + +: ENDOF IMMEDIATE + [COMPILE] ELSE ( ENDOF is the same as ELSE ) +; + +: ENDCASE IMMEDIATE + ' DROP , ( compile DROP ) + + ( keep compiling THEN until we get to our zero marker ) + BEGIN + ?DUP + WHILE + [COMPILE] THEN + REPEAT +; + +( + DECOMPILER ---------------------------------------------------------------------- + + CFA> is the opposite of >CFA. It takes a codeword and tries to find the matching + dictionary definition. (In truth, it works with any pointer into a word, not just + the codeword pointer, and this is needed to do stack traces). + + In this FORTH this is not so easy. In fact we have to search through the dictionary + because we don't have a convenient back-pointer (as is often the case in other versions + of FORTH). Because of this search, CFA> should not be used when performance is critical, + so it is only used for debugging tools such as the decompiler and printing stack + traces. + + This word returns 0 if it doesn't find a match. +) +: CFA> + LATEST @ ( start at LATEST dictionary entry ) + BEGIN + ?DUP ( while link pointer is not null ) + WHILE + 2DUP SWAP ( cfa curr curr cfa ) + < IF ( current dictionary entry < cfa? ) + NIP ( leave curr dictionary entry on the stack ) + EXIT + THEN + @ ( follow link pointer back ) + REPEAT + DROP ( restore stack ) + 0 ( sorry, nothing found ) +; + +( + SEE decompiles a FORTH word. + + We search for the dictionary entry of the word, then search again for the next + word (effectively, the end of the compiled word). This results in two pointers: + + +---------+---+---+---+---+------------+------------+------------+------------+ + | LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT | + +---------+---+---+---+---+------------+------------+------------+------------+ + ^ ^ + | | + Start of word End of word + + With this information we can have a go at decompiling the word. We need to + recognise "meta-words" like LIT, LITSTRING, BRANCH, etc. and treat those separately. +) +: SEE + WORD FIND ( find the dictionary entry to decompile ) + + ( Now we search again, looking for the next word in the dictionary. This gives us + the length of the word that we will be decompiling. (Well, mostly it does). ) + HERE @ ( address of the end of the last compiled word ) + LATEST @ ( word last curr ) + BEGIN + 2 PICK ( word last curr word ) + OVER ( word last curr word curr ) + <> ( word last curr word<>curr? ) + WHILE ( word last curr ) + NIP ( word curr ) + DUP @ ( word curr prev (which becomes: word last curr) ) + REPEAT + + DROP ( at this point, the stack is: start-of-word end-of-word ) + SWAP ( end-of-word start-of-word ) + + ( begin the definition with : NAME [IMMEDIATE] ) + ':' EMIT SPACE DUP ID. SPACE + DUP ?IMMEDIATE IF ." IMMEDIATE " THEN + + >DFA ( get the data address, ie. points after DOCOL | end-of-word start-of-data ) + + ( now we start decompiling until we hit the end of the word ) + BEGIN ( end start ) + 2DUP > + WHILE + DUP @ ( end start codeword ) + + CASE + ' LIT OF ( is it LIT ? ) + 4 + DUP @ ( get next word which is the integer constant ) + . ( and print it ) + ENDOF + ' LITSTRING OF ( is it LITSTRING ? ) + [ CHAR S ] LITERAL EMIT '"' EMIT SPACE ( print S" ) + 4 + DUP @ ( get the length word ) + SWAP 4 + SWAP ( end start+4 length ) + 2DUP TELL ( print the string ) + '"' EMIT SPACE ( finish the string with a final quote ) + + ALIGNED ( end start+4+len, aligned ) + 4 - ( because we're about to add 4 below ) + ENDOF + ' 0BRANCH OF ( is it 0BRANCH ? ) + ." 0BRANCH ( " + 4 + DUP @ ( print the offset ) + . + ." ) " + ENDOF + ' BRANCH OF ( is it BRANCH ? ) + ." BRANCH ( " + 4 + DUP @ ( print the offset ) + . + ." ) " + ENDOF + ' ' OF ( is it ' (TICK) ? ) + [ CHAR ' ] LITERAL EMIT SPACE + 4 + DUP @ ( get the next codeword ) + CFA> ( and force it to be printed as a dictionary entry ) + ID. SPACE + ENDOF + ' EXIT OF ( is it EXIT? ) + ( We expect the last word to be EXIT, and if it is then we don't print it + because EXIT is normally implied by ;. EXIT can also appear in the middle + of words, and then it needs to be printed. ) + 2DUP ( end start end start ) + 4 + ( end start end start+4 ) + <> IF ( end start | we're not at the end ) + ." EXIT " + THEN + ENDOF + ( default case: ) + DUP ( in the default case we always need to DUP before using ) + CFA> ( look up the codeword to get the dictionary entry ) + ID. SPACE ( and print it ) + ENDCASE + + 4 + ( end start+4 ) + REPEAT + + ';' EMIT CR + + 2DROP ( restore stack ) +; + +( + EXECUTION TOKENS ---------------------------------------------------------------------- + + Standard FORTH defines a concept called an 'execution token' (or 'xt') which is very + similar to a function pointer in C. We map the execution token to a codeword address. + + execution token of DOUBLE is the address of this codeword + | + V + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | + +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ + len pad codeword ^ + + There is one assembler primitive for execution tokens, EXECUTE ( xt -- ), which runs them. + + You can make an execution token for an existing word the long way using >CFA, + ie: WORD [foo] FIND >CFA will push the xt for foo onto the stack where foo is the + next word in input. So a very slow way to run DOUBLE might be: + + : DOUBLE DUP + ; + : SLOW WORD FIND >CFA EXECUTE ; + 5 SLOW DOUBLE . CR \ prints 10 + + We also offer a simpler and faster way to get the execution token of any word FOO: + + ['] FOO + + (Exercises for readers: (1) What is the difference between ['] FOO and ' FOO? + (2) What is the relationship between ', ['] and LIT?) + + More useful is to define anonymous words and/or to assign xt's to variables. + + To define an anonymous word (and push its xt on the stack) use :NONAME ... ; as in this + example: + + :NONAME ." anon word was called" CR ; \ pushes xt on the stack + DUP EXECUTE EXECUTE \ executes the anon word twice + + Stack parameters work as expected: + + :NONAME ." called with parameter " . CR ; + DUP + 10 SWAP EXECUTE \ prints 'called with parameter 10' + 20 SWAP EXECUTE \ prints 'called with parameter 20' + + Notice that the above code has a memory leak: the anonymous word is still compiled + into the data segment, so even if you lose track of the xt, the word continues to + occupy memory. A good way to keep track of the xt and thus avoid the memory leak is + to assign it to a CONSTANT, VARIABLE or VALUE: + + 0 VALUE ANON + :NONAME ." anon word was called" CR ; TO ANON + ANON EXECUTE + ANON EXECUTE + + Another use of :NONAME is to create an array of functions which can be called quickly + (think: fast switch statement). This example is adapted from the ANS FORTH standard: + + 10 CELLS ALLOT CONSTANT CMD-TABLE + : SET-CMD CELLS CMD-TABLE + ! ; + : CALL-CMD CELLS CMD-TABLE + @ EXECUTE ; + + :NONAME ." alternate 0 was called" CR ; 0 SET-CMD + :NONAME ." alternate 1 was called" CR ; 1 SET-CMD + \ etc... + :NONAME ." alternate 9 was called" CR ; 9 SET-CMD + + 0 CALL-CMD + 1 CALL-CMD +) + +: :NONAME + 0 0 CREATE ( create a word with no name - we need a dictionary header because ; expects it ) + HERE @ ( current HERE value is the address of the codeword, ie. the xt ) + DOCOL , ( compile DOCOL (the codeword) ) + ] ( go into compile mode ) +; + +: ['] IMMEDIATE + ' LIT , ( compile LIT ) +; + +( + EXCEPTIONS ---------------------------------------------------------------------- + + Amazingly enough, exceptions can be implemented directly in FORTH, in fact rather easily. + + The general usage is as follows: + + : FOO ( n -- ) THROW ; + + : TEST-EXCEPTIONS + 25 ['] FOO CATCH \ execute 25 FOO, catching any exception + ?DUP IF + ." called FOO and it threw exception number: " + . CR + DROP \ we have to drop the argument of FOO (25) + THEN + ; + \ prints: called FOO and it threw exception number: 25 + + CATCH runs an execution token and detects whether it throws any exception or not. The + stack signature of CATCH is rather complicated: + + ( a_n-1 ... a_1 a_0 xt -- r_m-1 ... r_1 r_0 0 ) if xt did NOT throw an exception + ( a_n-1 ... a_1 a_0 xt -- ?_n-1 ... ?_1 ?_0 e ) if xt DID throw exception 'e' + + where a_i and r_i are the (arbitrary number of) argument and return stack contents + before and after xt is EXECUTEd. Notice in particular the case where an exception + is thrown, the stack pointer is restored so that there are n of _something_ on the + stack in the positions where the arguments a_i used to be. We don't really guarantee + what is on the stack -- perhaps the original arguments, and perhaps other nonsense -- + it largely depends on the implementation of the word that was executed. + + THROW, ABORT and a few others throw exceptions. + + Exception numbers are non-zero integers. By convention the positive numbers can be used + for app-specific exceptions and the negative numbers have certain meanings defined in + the ANS FORTH standard. (For example, -1 is the exception thrown by ABORT). + + 0 THROW does nothing. This is the stack signature of THROW: + + ( 0 -- ) + ( * e -- ?_n-1 ... ?_1 ?_0 e ) the stack is restored to the state from the corresponding CATCH + + The implementation hangs on the definitions of CATCH and THROW and the state shared + between them. + + Up to this point, the return stack has consisted merely of a list of return addresses, + with the top of the return stack being the return address where we will resume executing + when the current word EXITs. However CATCH will push a more complicated 'exception stack + frame' on the return stack. The exception stack frame records some things about the + state of execution at the time that CATCH was called. + + When called, THROW walks up the return stack (the process is called 'unwinding') until + it finds the exception stack frame. It then uses the data in the exception stack frame + to restore the state allowing execution to continue after the matching CATCH. (If it + unwinds the stack and doesn't find the exception stack frame then it prints a message + and drops back to the prompt, which is also normal behaviour for so-called 'uncaught + exceptions'). + + This is what the exception stack frame looks like. (As is conventional, the return stack + is shown growing downwards from higher to lower memory addresses). + + +------------------------------+ + | return address from CATCH | Notice this is already on the + | | return stack when CATCH is called. + +------------------------------+ + | original parameter stack | + | pointer | + +------------------------------+ ^ + | exception stack marker | | + | (EXCEPTION-MARKER) | | Direction of stack + +------------------------------+ | unwinding by THROW. + | + | + + The EXCEPTION-MARKER marks the entry as being an exception stack frame rather than an + ordinary return address, and it is this which THROW "notices" as it is unwinding the + stack. (If you want to implement more advanced exceptions such as TRY...WITH then + you'll need to use a different value of marker if you want the old and new exception stack + frame layouts to coexist). + + What happens if the executed word doesn't throw an exception? It will eventually + return and call EXCEPTION-MARKER, so EXCEPTION-MARKER had better do something sensible + without us needing to modify EXIT. This nicely gives us a suitable definition of + EXCEPTION-MARKER, namely a function that just drops the stack frame and itself + returns (thus "returning" from the original CATCH). + + One thing to take from this is that exceptions are a relatively lightweight mechanism + in FORTH. +) + +: EXCEPTION-MARKER + RDROP ( drop the original parameter stack pointer ) + 0 ( there was no exception, this is the normal return path ) +; + +: CATCH ( xt -- exn? ) + DSP@ 4+ >R ( save parameter stack pointer (+4 because of xt) on the return stack ) + ' EXCEPTION-MARKER 4+ ( push the address of the RDROP inside EXCEPTION-MARKER ... ) + >R ( ... on to the return stack so it acts like a return address ) + EXECUTE ( execute the nested function ) +; + +: THROW ( n -- ) + ?DUP IF ( only act if the exception code <> 0 ) + RSP@ ( get return stack pointer ) + BEGIN + DUP R0 4- < ( RSP < R0 ) + WHILE + DUP @ ( get the return stack entry ) + ' EXCEPTION-MARKER 4+ = IF ( found the EXCEPTION-MARKER on the return stack ) + 4+ ( skip the EXCEPTION-MARKER on the return stack ) + RSP! ( restore the return stack pointer ) + + ( Restore the parameter stack. ) + DUP DUP DUP ( reserve some working space so the stack for this word + doesn't coincide with the part of the stack being restored ) + R> ( get the saved parameter stack pointer | n dsp ) + 4- ( reserve space on the stack to store n ) + SWAP OVER ( dsp n dsp ) + ! ( write n on the stack ) + DSP! EXIT ( restore the parameter stack pointer, immediately exit ) + THEN + 4+ + REPEAT + + ( No matching catch - print a message and restart the INTERPRETer. ) + DROP + + CASE + 0 1- OF ( ABORT ) + ." ABORTED" CR + ENDOF + ( default case ) + ." UNCAUGHT THROW " + DUP . CR + ENDCASE + QUIT + THEN +; + +: ABORT ( -- ) + 0 1- THROW +; + +( Print a stack trace by walking up the return stack. ) +: PRINT-STACK-TRACE + RSP@ ( start at caller of this function ) + BEGIN + DUP R0 4- < ( RSP < R0 ) + WHILE + DUP @ ( get the return stack entry ) + CASE + ' EXCEPTION-MARKER 4+ OF ( is it the exception stack frame? ) + ." CATCH ( DSP=" + 4+ DUP @ U. ( print saved stack pointer ) + ." ) " + ENDOF + ( default case ) + DUP + CFA> ( look up the codeword to get the dictionary entry ) + ?DUP IF ( and print it ) + 2DUP ( dea addr dea ) + ID. ( print word from dictionary entry ) + [ CHAR + ] LITERAL EMIT + SWAP >DFA 4+ - . ( print offset ) + THEN + ENDCASE + 4+ ( move up the stack ) + REPEAT + DROP + CR +; + +( + C STRINGS ---------------------------------------------------------------------- + + FORTH strings are represented by a start address and length kept on the stack or in memory. + + Most FORTHs don't handle C strings, but we need them in order to access the process arguments + and environment left on the stack by the Linux kernel, and to make some system calls. + + Operation Input Output FORTH word Notes + ---------------------------------------------------------------------- + + Create FORTH string addr len S" ..." + + Create C string c-addr Z" ..." + + C -> FORTH c-addr addr len DUP STRLEN + + FORTH -> C addr len c-addr CSTRING Allocated in a temporary buffer, so + should be consumed / copied immediately. + FORTH string should not contain NULs. + + For example, DUP STRLEN TELL prints a C string. +) + +( + Z" .." is like S" ..." except that the string is terminated by an ASCII NUL character. + + To make it more like a C string, at runtime Z" just leaves the address of the string + on the stack (not address & length as with S"). To implement this we need to add the + extra NUL to the string and also a DROP instruction afterwards. Apart from that the + implementation just a modified S". +) +: Z" IMMEDIATE + STATE @ IF ( compiling? ) + ' 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 @ C! ( store the character in the compiled image ) + 1 HERE +! ( increment HERE pointer by 1 byte ) + REPEAT + 0 HERE @ C! ( add the ASCII NUL byte ) + 1 HERE +! + 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 ) + ALIGN ( round up to next multiple of 4 bytes for the remaining code ) + ' DROP , ( compile DROP (to drop the length) ) + ELSE ( immediate mode ) + HERE @ ( get the start address of the temporary space ) + BEGIN + KEY + DUP '"' <> + WHILE + OVER C! ( save next character ) + 1+ ( increment address ) + REPEAT + DROP ( drop the final " character ) + 0 SWAP C! ( store final ASCII NUL ) + HERE @ ( push the start address ) + THEN +; + +: STRLEN ( str -- len ) + DUP ( save start address ) + BEGIN + DUP C@ 0<> ( zero byte found? ) + WHILE + 1+ + REPEAT + + SWAP - ( calculate the length ) +; + +: CSTRING ( addr len -- c-addr ) + SWAP OVER ( len saddr len ) + HERE @ SWAP ( len saddr daddr len ) + CMOVE ( len ) + + HERE @ + ( daddr+len ) + 0 SWAP C! ( store terminating NUL char ) + + HERE @ ( push start address ) +; + +( + THE ENVIRONMENT ---------------------------------------------------------------------- + + Linux makes the process arguments and environment available to us on the stack. + + The top of stack pointer is saved by the early assembler code when we start up in the FORTH + variable S0, and starting at this pointer we can read out the command line arguments and the + environment. + + Starting at S0, S0 itself points to argc (the number of command line arguments). + + S0+4 points to argv[0], S0+8 points to argv[1] etc up to argv[argc-1]. + + argv[argc] is a NULL pointer. + + After that the stack contains environment variables, a set of pointers to strings of the + form NAME=VALUE and on until we get to another NULL pointer. + + The first word that we define, ARGC, pushes the number of command line arguments (note that + as with C argc, this includes the name of the command). +) +: ARGC + S0 @ @ +; + +( + n ARGV gets the nth command line argument. + + For example to print the command name you would do: + 0 ARGV TELL CR +) +: ARGV ( n -- str u ) + 1+ CELLS S0 @ + ( get the address of argv[n] entry ) + @ ( get the address of the string ) + DUP STRLEN ( and get its length / turn it into a FORTH string ) +; + +( + ENVIRON returns the address of the first environment string. The list of strings ends + with a NULL pointer. + + For example to print the first string in the environment you could do: + ENVIRON @ DUP STRLEN TELL +) +: ENVIRON ( -- addr ) + ARGC ( number of command line parameters on the stack to skip ) + 2 + ( skip command line count and NULL pointer after the command line args ) + CELLS ( convert to an offset ) + S0 @ + ( add to base stack address ) +; + +( + SYSTEM CALLS AND FILES ---------------------------------------------------------------------- + + Miscellaneous words related to system calls, and standard access to files. +) + +( BYE exits by calling the Linux exit(2) syscall. ) +: BYE ( -- ) + 0 ( return code (0) ) + SYS_EXIT ( system call number ) + SYSCALL1 +; + +( + UNUSED returns the number of cells remaining in the user memory (data segment). + + For our implementation we will use Linux brk(2) system call to find out the end + of the data segment and subtract HERE from it. +) +: GET-BRK ( -- brkpoint ) + 0 SYS_BRK SYSCALL1 ( call brk(0) ) +; + +: UNUSED ( -- n ) + GET-BRK ( get end of data segment according to the kernel ) + HERE @ ( get current position in data segment ) + - + 4 / ( returns number of cells ) +; + +( + MORECORE increases the data segment by the specified number of (4 byte) cells. + + NB. The number of cells requested should normally be a multiple of 1024. The + reason is that Linux can't extend the data segment by less than a single page + (4096 bytes or 1024 cells). + + This FORTH doesn't automatically increase the size of the data segment "on demand" + (ie. when , (COMMA), ALLOT, CREATE, and so on are used). Instead the programmer + needs to be aware of how much space a large allocation will take, check UNUSED, and + call MORECORE if necessary. A simple programming exercise is to change the + implementation of the data segment so that MORECORE is called automatically if + the program needs more memory. +) +: BRK ( brkpoint -- ) + SYS_BRK SYSCALL1 +; + +: MORECORE ( cells -- ) + CELLS GET-BRK + BRK +; + +( + Standard FORTH provides some simple file access primitives which we model on + top of Linux syscalls. + + The main complication is converting FORTH strings (address & length) into C + strings for the Linux kernel. + + Notice there is no buffering in this implementation. +) + +: R/O ( -- fam ) O_RDONLY ; +: R/W ( -- fam ) O_RDWR ; + +: OPEN-FILE ( addr u fam -- fd 0 (if successful) | c-addr u fam -- fd errno (if there was an error) ) + ROT ( fam addr u ) + CSTRING ( fam cstring ) + SYS_OPEN SYSCALL2 ( open (filename, flags) ) + DUP ( fd fd ) + DUP 0< IF ( errno? ) + NEGATE ( fd errno ) + ELSE + DROP 0 ( fd 0 ) + THEN +; + +: CREATE-FILE ( addr u fam -- fd 0 (if successful) | c-addr u fam -- fd errno (if there was an error) ) + O_CREAT OR + O_TRUNC OR + ROT ( fam addr u ) + CSTRING ( fam cstring ) + 420 ROT ( 0644 fam cstring ) + SYS_OPEN SYSCALL3 ( open (filename, flags|O_TRUNC|O_CREAT, 0644) ) + DUP ( fd fd ) + DUP 0< IF ( errno? ) + NEGATE ( fd errno ) + ELSE + DROP 0 ( fd 0 ) + THEN +; + +: CLOSE-FILE ( fd -- 0 (if successful) | fd -- errno (if there was an error) ) + SYS_CLOSE SYSCALL1 + NEGATE +; + +: READ-FILE ( addr u fd -- u2 0 (if successful) | addr u fd -- 0 0 (if EOF) | addr u fd -- u2 errno (if error) ) + ROT SWAP -ROT ( u addr fd ) + SYS_READ SYSCALL3 + + DUP ( u2 u2 ) + DUP 0< IF ( errno? ) + NEGATE ( u2 errno ) + ELSE + DROP 0 ( u2 0 ) + THEN +; + +( + PERROR prints a message for an errno, similar to C's perror(3) but we don't have the extensive + list of strerror strings available, so all we can do is print the errno. +) +: PERROR ( errno addr u -- ) + TELL + ':' EMIT SPACE + ." ERRNO=" + . CR +; + +( + ASSEMBLER CODE ---------------------------------------------------------------------- + + This is just the outline of a simple assembler, allowing you to write FORTH primitives + in assembly language. + + Assembly primitives begin ': NAME' in the normal way, but are ended with ;CODE. ;CODE + updates the header so that the codeword isn't DOCOL, but points instead to the assembled + code (in the DFA part of the word). + + We provide a convenience macro NEXT (you guessed what it does). However you don't need to + use it because ;CODE will put a NEXT at the end of your word. + + The rest consists of some immediate words which expand into machine code appended to the + definition of the word. Only a very tiny part of the i386 assembly space is covered, just + enough to write a few assembler primitives below. +) + +HEX + +( Equivalent to the NEXT macro ) +: NEXT IMMEDIATE AD C, FF C, 20 C, ; + +: ;CODE IMMEDIATE + [COMPILE] NEXT ( end the word with NEXT macro ) + ALIGN ( machine code is assembled in bytes so isn't necessarily aligned at the end ) + LATEST @ DUP + HIDDEN ( unhide the word ) + DUP >DFA SWAP >CFA ! ( change the codeword to point to the data area ) + [COMPILE] [ ( go back to immediate mode ) +; + +( The i386 registers ) +: EAX IMMEDIATE 0 ; +: ECX IMMEDIATE 1 ; +: EDX IMMEDIATE 2 ; +: EBX IMMEDIATE 3 ; +: ESP IMMEDIATE 4 ; +: EBP IMMEDIATE 5 ; +: ESI IMMEDIATE 6 ; +: EDI IMMEDIATE 7 ; + +( i386 stack instructions ) +: PUSH IMMEDIATE 50 + C, ; +: POP IMMEDIATE 58 + C, ; + +( RDTSC instruction ) +: RDTSC IMMEDIATE 0F C, 31 C, ; + +DECIMAL + +( + RDTSC is an assembler primitive which reads the Pentium timestamp counter (a very fine- + grained counter which counts processor clock cycles). Because the TSC is 64 bits wide + we have to push it onto the stack in two slots. +) +: RDTSC ( -- lsb msb ) + RDTSC ( writes the result in %edx:%eax ) + EAX PUSH ( push lsb ) + EDX PUSH ( push msb ) +;CODE + +( + INLINE can be used to inline an assembler primitive into the current (assembler) + word. + + For example: + + : 2DROP INLINE DROP INLINE DROP ;CODE + + will build an efficient assembler word 2DROP which contains the inline assembly code + for DROP followed by DROP (eg. two 'pop %eax' instructions in this case). + + Another example. Consider this ordinary FORTH definition: + + : C@++ ( addr -- addr+1 byte ) DUP 1+ SWAP C@ ; + + (it is equivalent to the C operation '*p++' where p is a pointer to char). If we + notice that all of the words used to define C@++ are in fact assembler primitives, + then we can write a faster (but equivalent) definition like this: + + : C@++ INLINE DUP INLINE 1+ INLINE SWAP INLINE C@ ;CODE + + One interesting point to note is that this "concatenative" style of programming + allows you to write assembler words portably. The above definition would work + for any CPU architecture. + + There are several conditions that must be met for INLINE to be used successfully: + + (1) You must be currently defining an assembler word (ie. : ... ;CODE). + + (2) The word that you are inlining must be known to be an assembler word. If you try + to inline a FORTH word, you'll get an error message. + + (3) The assembler primitive must be position-independent code and must end with a + single NEXT macro. + + Exercises for the reader: (a) Generalise INLINE so that it can inline FORTH words when + building FORTH words. (b) Further generalise INLINE so that it does something sensible + when you try to inline FORTH into assembler and vice versa. + + The implementation of INLINE is pretty simple. We find the word in the dictionary, + check it's an assembler word, then copy it into the current definition, byte by byte, + until we reach the NEXT macro (which is not copied). +) +HEX +: =NEXT ( addr -- next? ) + DUP C@ AD <> IF DROP FALSE EXIT THEN + 1+ DUP C@ FF <> IF DROP FALSE EXIT THEN + 1+ C@ 20 <> IF FALSE EXIT THEN + TRUE +; +DECIMAL + +( (INLINE) is the lowlevel inline function. ) +: (INLINE) ( cfa -- ) + @ ( remember codeword points to the code ) + BEGIN ( copy bytes until we hit NEXT macro ) + DUP =NEXT NOT + WHILE + DUP C@ C, + 1+ + REPEAT + DROP +; + +: INLINE IMMEDIATE + WORD FIND ( find the word in the dictionary ) + >CFA ( codeword ) + + DUP @ DOCOL = IF ( check codeword <> DOCOL (ie. not a FORTH word) ) + ." Cannot INLINE FORTH words" CR ABORT + THEN + + (INLINE) +; + +HIDE =NEXT + +( + NOTES ---------------------------------------------------------------------- + + DOES> isn't possible to implement with this FORTH because we don't have a separate + data pointer. +) + +( + WELCOME MESSAGE ---------------------------------------------------------------------- + + Print the version and OK prompt. +) + +: WELCOME + S" TEST-MODE" FIND NOT IF + ." JONESFORTH VERSION " VERSION . CR + UNUSED . ." CELLS REMAINING" CR + ." OK " + THEN +; + +WELCOME +HIDE WELCOME diff --git a/jonesforth/run.sh b/jonesforth/run.sh new file mode 100755 index 0000000..8ba83b3 --- /dev/null +++ b/jonesforth/run.sh @@ -0,0 +1,3 @@ +#!/bin/sh +docker build -t jonesforth . +docker run --cap-add=SYS_RAWIO -ti --rm jonesforth -- cgit v1.2.3