From: rich Date: Sat, 8 Sep 2007 10:01:00 +0000 (+0000) Subject: Started writing. X-Git-Url: http://git.annexia.org/?a=commitdiff_plain;h=964d44d11ff6cca78e7cf90fe597efbfd51ea8f9;p=jonesforth.git Started writing. --- diff --git a/jonesforth.S b/jonesforth.S index 9a67b5d..dfa173c 100644 --- a/jonesforth.S +++ b/jonesforth.S @@ -1,16 +1,132 @@ -/* A sometimes minimal FORTH interpreter for Linux / i386 systems. -*- asm -*- - * By Richard W.M. Jones - * - * gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S - */ +/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- + By Richard W.M. Jones http://annexia.org/forth + + gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S + + 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. 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 + + Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html + + 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: + + <------------------------------------------------------------------------------------------------------------------------> + + 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 -o jonesforth jonesforth.S + ./jonesforth + + You will see lots of 'Warning: unterminated string; newline inserted' messages from the + assembler. That's just because the GNU assembler doesn't have a good syntax for multi-line + strings (or rather it used to, but the developers removed it!) so I've abused the syntax + slightly to make things readable. Ignore these warnings. + + 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". + + (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. + + INDIRECT THREADED CODE ---------------------------------------------------------------------- + + -#include -/* NOTES------------------------------------------------------------------------------------------------------------------- -Need to say something about $ before constants. -And about je/jne/ja/jb/jbe/etc @@ -398,6 +514,8 @@ var_\name : lea 4(%ebp),%ebp // pop return stack and throw away NEXT +#include + defcode "KEY",3,,KEY call _KEY push %eax // push return value on stack