/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth
+ This is PUBLIC DOMAIN (see public domain release statement below).
+ $Id: jonesforth.S,v 1.47 2009-09-11 08:33:13 rich Exp $
- gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S
-
+ gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S
+*/
+ .set JONES_VERSION,47
+/*
INTRODUCTION ----------------------------------------------------------------------
FORTH is one of those alien languages which most working programmers regard in the same
over every other element in a list of numbers? You can add it to the language. What
about an operator which pulls in variables directly from a configuration file and makes
them available as FORTH variables? Or how about adding Makefile-like dependencies to
- the language? No problem in FORTH. This concept isn't common in programming languages,
+ the language? No problem in FORTH. How about modifying the FORTH compiler to allow
+ complex inlining strategies -- simple. This concept isn't common in programming languages,
but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not
the lame C preprocessor) and "domain specific languages" (DSLs).
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
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.
Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to
assemble and run the code (save this file as 'jonesforth.S') are:
- gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S
- ./jonesforth
+ gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S
+ cat jonesforth.f - | ./jonesforth
- You will see lots of 'Warning: unterminated string; newline inserted' messages from the
- assembler. That's just because the GNU assembler doesn't have a good syntax for multi-line
- strings (or rather it used to, but the developers removed it!) so I've abused the syntax
- slightly to make things readable. Ignore these warnings.
+ If you want to 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 ----------------------------------------------------------------------
mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake)
(4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards"
- and '1b' (etc.) means label '1:' "backwards".
+ and '1b' (etc.) means label '1:' "backwards". Notice that these labels might be mistaken
+ for hex numbers (eg. you might confuse 1b with $0x1b).
(5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc.
THE DICTIONARY ----------------------------------------------------------------------
- In FORTH as you will know, functions are called "words", as just as in other languages they
+ 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 +"
|
LATEST
- You shoud be able to see from this how you might implement functions to find a word in
+ 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),
+ 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.
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
- 16 bit machine code.
+ i386 machine code.
f:
CALL a E8 08 00 00 00
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 any more. Instead we need to write an interpreter
- which takes each pair of bytes and calls it.
+ 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
%esi -> 1C 00 00 00
2C 00 00 00
- The all-important x86 instruction is called LODSL (or in Intel manuals, LODSW). It does
+ 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:
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 +.
+ 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:
+-----> +------------------+
| codeword -------+
+------------------+ |
- now we're | assembly to <------+
+ now we're | assembly to <-----+
executing | implement + |
this | .. |
function | .. |
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 read, when reading 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.
+ 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:
+ 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. */
| addr of DOUBLE ---------------> +------------------+
+------------------+ %eax -> | addr of DOCOL |
%esi -> | addr of DOUBLE | +------------------+
- +------------------+ | addr of DUP -------------->
+ +------------------+ | addr of DUP |
| addr of EXIT | +------------------+
+------------------+ | etc. |
- First, the call to DOUBLE causes DOCOL (the codeword of DOUBLE). DOCOL does this: It
+ 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:
| codeword |
+------------------+ DOUBLE:
| addr of DOUBLE ---------------> +------------------+
- +------------------+ | addr of DOCOL |
- | addr of DOUBLE | +------------------+
- +------------------+ %esi -> | addr of DUP -------------->
+top of return +------------------+ %eax -> | addr of DOCOL |
+stack points -> | addr of DOUBLE | + 4 = +------------------+
+ +------------------+ %esi -> | addr of DUP |
| addr of EXIT | +------------------+
+------------------+ | etc. |
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).
+*/
-/* ELF entry point. */
+/* Assembler entry point. */
.text
.globl _start
_start:
cld
- mov %esp,var_S0 // Store the initial data stack pointer.
- mov $return_stack,%ebp // Initialise the return stack.
+ 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 COLD
-
-/*----------------------------------------------------------------------
- * Fixed sized buffers for everything.
- */
- .bss
-
-/* FORTH return stack. */
-#define RETURN_STACK_SIZE 8192
- .align 4096
- .space RETURN_STACK_SIZE
-return_stack:
-
-/* Space for user-defined words. */
-#define USER_DEFS_SIZE 16384
- .align 4096
-user_defs_start:
- .space USER_DEFS_SIZE
-
-
+ .int QUIT
+/*
+ BUILT-IN WORDS ----------------------------------------------------------------------
+ Remember our dictionary entries (headers)? Let's bring those together with the codeword
+ and data words to see how : DOUBLE DUP + ; really looks in memory.
+ pointer to previous word
+ ^
+ |
+ +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
+ ^ len pad codeword |
+ | V
+ LINK in next word points to codeword of DUP
+
+ Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we
+ don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc.
+ So instead we will have to define built-in words using the GNU assembler data constructors
+ (like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are
+ unsure of them).
+
+ The long way would be:
+
+ .int <link to previous word>
+ .byte 6 // len
+ .ascii "DOUBLE" // string
+ .byte 0 // padding
+DOUBLE: .int DOCOL // codeword
+ .int DUP // pointer to codeword of DUP
+ .int PLUS // pointer to codeword of +
+ .int EXIT // pointer to codeword of EXIT
+
+ That's going to get quite tedious rather quickly, so here I define an assembler macro
+ so that I can just write:
+
+ defword "DOUBLE",6,,DOUBLE
+ .int DUP,PLUS,EXIT
+
+ and I'll get exactly the same effect.
+
+ Don't worry too much about the exact implementation details of this macro - it's complicated!
+*/
-/*----------------------------------------------------------------------
- * Built-in words defined the long way.
- */
-#define F_IMMED 0x80
-#define F_HIDDEN 0x20
+/* Flags - these are discussed later. */
+ .set F_IMMED,0x80
+ .set F_HIDDEN,0x20
+ .set F_LENMASK,0x1f // length mask
// Store the chain of links.
.set link,0
- .macro defcode name, namelen, flags=0, label
+ .macro defword name, namelen, flags=0, label
.section .rodata
.align 4
.globl name_\label
.set link,name_\label
.byte \flags+\namelen // flags + length byte
.ascii "\name" // the name
- .align 4
+ .align 4 // padding to next 4 byte boundary
.globl \label
\label :
- .int code_\label // codeword
- .text
- .align 4
- .globl code_\label
-code_\label : // assembler code follows
+ .int DOCOL // codeword - the interpreter
+ // list of word pointers follow
.endm
- .macro defword name, namelen, flags=0, label
+/*
+ Similarly I want a way to write words written in assembly language. There will quite a few
+ of these to start with because, well, everything has to start in assembly before there's
+ enough "infrastructure" to be able to start writing FORTH words, but also I want to define
+ some common FORTH words in assembly language for speed, even though I could write them in FORTH.
+
+ This is what DUP looks like in memory:
+
+ pointer to previous word
+ ^
+ |
+ +--|------+---+---+---+---+------------+
+ | LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly
+ +---------+---+---+---+---+------------+ code used to write DUP,
+ ^ len codeword which ends with NEXT.
+ |
+ LINK in next word
+
+ Again, for brevity in writing the header I'm going to write an assembler macro called defcode.
+ As with defword above, don't worry about the complicated details of the macro.
+*/
+
+ .macro defcode name, namelen, flags=0, label
.section .rodata
.align 4
.globl name_\label
.set link,name_\label
.byte \flags+\namelen // flags + length byte
.ascii "\name" // the name
- .align 4
+ .align 4 // padding to next 4 byte boundary
.globl \label
\label :
- .int DOCOL // codeword - the interpreter
- // list of word pointers follow
+ .int code_\label // codeword
+ .text
+ //.align 4
+ .globl code_\label
+code_\label : // assembler code follows
.endm
- .macro defvar name, namelen, flags=0, label, initial=0
- defcode \name,\namelen,\flags,\label
- push $var_\name
- NEXT
- .data
- .align 4
-var_\name :
- .int \initial
- .endm
+/*
+ Now some easy FORTH primitives. These are written in assembly for speed. If you understand
+ i386 assembly language then it is worth reading these. However if you don't understand assembly
+ you can skip the details.
+*/
- // Some easy ones, written in assembly for speed
defcode "DROP",4,,DROP
pop %eax // drop top of stack
NEXT
- defcode "DUP",3,,DUP
- pop %eax // duplicate top of stack
- push %eax
- push %eax
- NEXT
-
defcode "SWAP",4,,SWAP
- pop %eax // swap top of stack
+ pop %eax // swap top two elements on stack
pop %ebx
push %eax
push %ebx
NEXT
+ defcode "DUP",3,,DUP
+ mov (%esp),%eax // duplicate top of stack
+ push %eax
+ NEXT
+
defcode "OVER",4,,OVER
mov 4(%esp),%eax // get the second element of stack
push %eax // and push it on top
pop %eax
pop %ebx
pop %ecx
+ push %ebx
push %eax
push %ecx
- push %ebx
NEXT
defcode "-ROT",4,,NROT
pop %eax
pop %ebx
pop %ecx
+ push %eax
+ push %ecx
+ push %ebx
+ NEXT
+
+ defcode "2DROP",5,,TWODROP // drop top two elements of stack
+ pop %eax
+ pop %eax
+ NEXT
+
+ defcode "2DUP",4,,TWODUP // duplicate top two elements of stack
+ mov (%esp),%eax
+ mov 4(%esp),%ebx
+ push %ebx
+ push %eax
+ NEXT
+
+ defcode "2SWAP",5,,TWOSWAP // swap top two pairs of elements of stack
+ pop %eax
+ pop %ebx
+ pop %ecx
+ pop %edx
push %ebx
push %eax
+ push %edx
push %ecx
NEXT
+ defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero
+ movl (%esp),%eax
+ test %eax,%eax
+ jz 1f
+ push %eax
+1: NEXT
+
defcode "1+",2,,INCR
incl (%esp) // increment top of stack
NEXT
NEXT
defcode "4+",2,,INCR4
- addl $4,(%esp) // increment top of stack
+ addl $4,(%esp) // add 4 to top of stack
NEXT
defcode "4-",2,,DECR4
- subl $4,(%esp) // decrement top of stack
+ subl $4,(%esp) // subtract 4 from top of stack
NEXT
defcode "+",1,,ADD
- pop %eax
- addl %eax,(%esp)
+ pop %eax // get top of stack
+ addl %eax,(%esp) // and add it to next word on stack
NEXT
defcode "-",1,,SUB
- pop %eax
- subl %eax,(%esp)
+ pop %eax // get top of stack
+ subl %eax,(%esp) // and subtract it from next word on stack
NEXT
defcode "*",1,,MUL
push %eax // ignore overflow
NEXT
- defcode "/",1,,DIV
- xor %edx,%edx
- pop %ebx
- pop %eax
- idivl %ebx
- push %eax // push quotient
- NEXT
+/*
+ In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in
+ terms of the primitive /MOD. The design of the i386 assembly instruction idiv which
+ leaves both quotient and remainder makes this the obvious choice.
+*/
- defcode "MOD",3,,MOD
+ defcode "/MOD",4,,DIVMOD
xor %edx,%edx
pop %ebx
pop %eax
idivl %ebx
push %edx // push remainder
+ push %eax // push quotient
NEXT
+/*
+ Lots of comparison operations like =, <, >, etc..
+
+ ANS FORTH says that the comparison words should return all (binary) 1's for
+ TRUE and all 0's for FALSE. However this is a bit of a strange convention
+ so this FORTH breaks it and returns the more normal (for C programmers ...)
+ 1 meaning TRUE and 0 meaning FALSE.
+*/
+
defcode "=",1,,EQU // top two words are equal?
pop %eax
pop %ebx
cmp %ebx,%eax
- je 1f
- pushl $0
- NEXT
-1: pushl $1
+ sete %al
+ movzbl %al,%eax
+ pushl %eax
NEXT
defcode "<>",2,,NEQU // top two words are not equal?
pop %eax
pop %ebx
cmp %ebx,%eax
- je 1f
- pushl $1
+ setne %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "<",1,,LT
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ setl %al
+ movzbl %al,%eax
+ pushl %eax
NEXT
-1: pushl $0
+
+ 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
- jz 1f
- pushl $0
+ setz %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "0<>",3,,ZNEQU // top of stack not 0?
+ pop %eax
+ test %eax,%eax
+ setnz %al
+ movzbl %al,%eax
+ pushl %eax
NEXT
-1: pushl $1
+
+ 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
+ defcode "AND",3,,AND // bitwise AND
pop %eax
andl %eax,(%esp)
NEXT
- defcode "OR",2,,OR
+ defcode "OR",2,,OR // bitwise OR
pop %eax
orl %eax,(%esp)
NEXT
- defcode "INVERT",6,,INVERT
+ defcode "XOR",3,,XOR // bitwise XOR
+ pop %eax
+ xorl %eax,(%esp)
+ NEXT
+
+ defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE and NOT)
notl (%esp)
NEXT
- // COLD must not return (ie. must not call EXIT).
- defword "COLD",4,,COLD
- // XXX reinitialisation of the interpreter
- .int INTERPRETER // call the interpreter loop (never returns)
- .int LIT,1,SYSEXIT // hmmm, but in case it does, exit(1).
+/*
+ RETURNING FROM FORTH WORDS ----------------------------------------------------------------------
+
+ Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called
+ DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing):
+
+ QUADRUPLE
+ +------------------+
+ | codeword |
+ +------------------+ DOUBLE
+ | addr of DOUBLE ---------------> +------------------+
+ +------------------+ | codeword |
+ | addr of DOUBLE | +------------------+
+ +------------------+ | addr of DUP |
+ | addr of EXIT | +------------------+
+ +------------------+ | addr of + |
+ +------------------+
+ %esi -> | addr of EXIT |
+ +------------------+
+
+ What happens when the + function does NEXT? Well, the following code is executed.
+*/
defcode "EXIT",4,,EXIT
POPRSP %esi // pop return stack into %esi
NEXT
+/*
+ EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi.
+ So after this (but just before NEXT) we get:
+
+ QUADRUPLE
+ +------------------+
+ | codeword |
+ +------------------+ DOUBLE
+ | addr of DOUBLE ---------------> +------------------+
+ +------------------+ | codeword |
+ %esi -> | addr of DOUBLE | +------------------+
+ +------------------+ | addr of DUP |
+ | addr of EXIT | +------------------+
+ +------------------+ | addr of + |
+ +------------------+
+ | addr of EXIT |
+ +------------------+
+
+ And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-)
+
+ LITERALS ----------------------------------------------------------------------
+
+ The final point I "glossed over" before was how to deal with functions that do anything
+ apart from calling other functions. For example, suppose that DOUBLE was defined like this:
+
+ : DOUBLE 2 * ;
+
+ It does the same thing, but how do we compile it since it contains the literal 2? One way
+ would be to have a function called "2" (which you'd have to write in assembler), but you'd need
+ a function for every single literal that you wanted to use.
+
+ FORTH solves this by compiling the function using a special word called LIT:
+
+ +---------------------------+-------+-------+-------+-------+-------+
+ | (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT |
+ +---------------------------+-------+-------+-------+-------+-------+
+
+ LIT is executed in the normal way, but what it does next is definitely not normal. It
+ looks at %esi (which now points to the number 2), grabs it, pushes it on the stack, then
+ manipulates %esi in order to skip the number as if it had never been there.
+
+ What's neat is that the whole grab/manipulate can be done using a single byte single
+ i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams,
+ see if you can find out how LIT works:
+*/
+
defcode "LIT",3,,LIT
// %esi points to the next command, but in this case it points to the next
// literal 32 bit integer. Get that literal into %eax and increment %esi.
push %eax // push the literal number on to stack
NEXT
- defcode "LITSTRING",9,,LITSTRING
- lodsl // get the length of the string
- push %eax // push it on the stack
- push %esi // push the address of the start of the string
- addl %eax,%esi // skip past the string
- addl $3,%esi // but round up to next 4 byte boundary
- andl $~3,%esi
- NEXT
-
- defcode "BRANCH",6,,BRANCH
- add (%esi),%esi // add the offset to the instruction pointer
- NEXT
+/*
+ MEMORY ----------------------------------------------------------------------
- defcode "0BRANCH",7,,ZBRANCH
- pop %eax
- test %eax,%eax // top of stack is zero?
- jz code_BRANCH // if so, jump back to the branch function above
- lodsl // otherwise we need to skip the offset
- NEXT
+ As important point about FORTH is that it gives you direct access to the lowest levels
+ of the machine. Manipulating memory directly is done frequently in FORTH, and these are
+ the primitive words for doing it.
+*/
defcode "!",1,,STORE
pop %ebx // address to store at
subl %eax,(%ebx) // add it
NEXT
-/* ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes.
- * I don't know whether FORTH has these words, so I invented my own, called !b and @b.
- * Byte-oriented operations only work on architectures which permit them (i386 is one of those).
- * UPDATE: writing a byte to the dictionary pointer is called C, in FORTH.
+/*
+ ! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes
+ so we also define standard words C@ and C!.
+
+ Byte-oriented operations only work on architectures which permit them (i386 is one of those).
*/
- defcode "!b",2,,STOREBYTE
+
+ defcode "C!",2,,STOREBYTE
pop %ebx // address to store at
pop %eax // data to store there
movb %al,(%ebx) // store it
NEXT
- defcode "@b",2,,FETCHBYTE
+ defcode "C@",2,,FETCHBYTE
pop %ebx // address to fetch
xor %eax,%eax
movb (%ebx),%al // fetch it
push %eax // push value onto stack
NEXT
- // The STATE variable is 0 for execute mode, != 0 for compile mode
- defvar "STATE",5,,STATE
+/* C@C! is a useful byte copy primitive. */
+ defcode "C@C!",4,,CCOPY
+ movl 4(%esp),%ebx // source address
+ movb (%ebx),%al // get source character
+ pop %edi // destination address
+ stosb // copy to destination
+ push %edi // increment destination address
+ incl 4(%esp) // increment source address
+ NEXT
- // This points to where compiled words go.
- defvar "HERE",4,,HERE,user_defs_start
+/* and CMOVE is a block copy operation. */
+ defcode "CMOVE",5,,CMOVE
+ mov %esi,%edx // preserve %esi
+ pop %ecx // length
+ pop %edi // destination address
+ pop %esi // source address
+ rep movsb // copy source to destination
+ mov %edx,%esi // restore %esi
+ NEXT
- // This is the last definition in the dictionary.
- defvar "LATEST",6,,LATEST,name_SYSEXIT // SYSEXIT must be last in built-in dictionary
+/*
+ BUILT-IN VARIABLES ----------------------------------------------------------------------
- // _X, _Y and _Z are scratch variables used by standard words.
- defvar "_X",2,,TX
- defvar "_Y",2,,TY
- defvar "_Z",2,,TZ
+ These are some built-in variables and related standard FORTH words. Of these, the only one that we
+ have discussed so far was LATEST, which points to the last (most recently defined) word in the
+ FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable)
+ on to the stack, so you can read or write it using @ and ! operators. For example, to print
+ the current value of LATEST (and this can apply to any FORTH variable) you would do:
- // This stores the top of the data stack.
- defvar "S0",2,,SZ
+ LATEST @ . CR
- // This stores the top of the return stack.
- defvar "R0",2,,RZ,return_stack
+ To make defining variables shorter, I'm using a macro called defvar, similar to defword and
+ defcode above. (In fact the defvar macro uses defcode to do the dictionary header).
+*/
- defcode "DSP@",4,,DSPFETCH
- mov %esp,%eax
- push %eax
+ .macro defvar name, namelen, flags=0, label, initial=0
+ defcode \name,\namelen,\flags,\label
+ push $var_\name
NEXT
+ .data
+ .align 4
+var_\name :
+ .int \initial
+ .endm
- defcode "DSP!",4,,DSPSTORE
- pop %esp
+/*
+ The built-in variables are:
+
+ STATE Is the interpreter executing code (0) or compiling a word (non-zero)?
+ LATEST Points to the latest (most recently defined) word in the dictionary.
+ HERE Points to the next free byte of memory. When compiling, compiled words go here.
+ S0 Stores the address of the top of the parameter stack.
+ BASE The current base for printing and reading numbers.
+
+*/
+ defvar "STATE",5,,STATE
+ defvar "HERE",4,,HERE
+ defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary
+ defvar "S0",2,,SZ
+ defvar "BASE",4,,BASE,10
+
+/*
+ BUILT-IN CONSTANTS ----------------------------------------------------------------------
+
+ It's also useful to expose a few constants to FORTH. When the word is executed it pushes a
+ constant value on the stack.
+
+ The built-in constants are:
+
+ VERSION Is the current version of this FORTH.
+ R0 The address of the top of the return stack.
+ DOCOL Pointer to DOCOL.
+ F_IMMED The IMMEDIATE flag's actual value.
+ F_HIDDEN The HIDDEN flag's actual value.
+ F_LENMASK The length mask in the flags/len byte.
+
+ SYS_* and the numeric codes of various Linux syscalls (from <asm/unistd.h>)
+*/
+
+//#include <asm-i386/unistd.h> // you might need this instead
+#include <asm/unistd.h>
+
+ .macro defconst name, namelen, flags=0, label, value
+ defcode \name,\namelen,\flags,\label
+ push $\value
NEXT
+ .endm
+
+ defconst "VERSION",7,,VERSION,JONES_VERSION
+ defconst "R0",2,,RZ,return_stack_top
+ defconst "DOCOL",5,,__DOCOL,DOCOL
+ defconst "F_IMMED",7,,__F_IMMED,F_IMMED
+ defconst "F_HIDDEN",8,,__F_HIDDEN,F_HIDDEN
+ defconst "F_LENMASK",9,,__F_LENMASK,F_LENMASK
+
+ defconst "SYS_EXIT",8,,SYS_EXIT,__NR_exit
+ defconst "SYS_OPEN",8,,SYS_OPEN,__NR_open
+ defconst "SYS_CLOSE",9,,SYS_CLOSE,__NR_close
+ defconst "SYS_READ",8,,SYS_READ,__NR_read
+ defconst "SYS_WRITE",9,,SYS_WRITE,__NR_write
+ defconst "SYS_CREAT",9,,SYS_CREAT,__NR_creat
+ defconst "SYS_BRK",7,,SYS_BRK,__NR_brk
+
+ defconst "O_RDONLY",8,,__O_RDONLY,0
+ defconst "O_WRONLY",8,,__O_WRONLY,1
+ defconst "O_RDWR",6,,__O_RDWR,2
+ defconst "O_CREAT",7,,__O_CREAT,0100
+ defconst "O_EXCL",6,,__O_EXCL,0200
+ defconst "O_TRUNC",7,,__O_TRUNC,01000
+ defconst "O_APPEND",8,,__O_APPEND,02000
+ defconst "O_NONBLOCK",10,,__O_NONBLOCK,04000
+
+/*
+ RETURN STACK ----------------------------------------------------------------------
+
+ These words allow you to access the return stack. Recall that the register %ebp always points to
+ the top of the return stack.
+*/
defcode ">R",2,,TOR
pop %eax // pop parameter stack into %eax
NEXT
defcode "RDROP",5,,RDROP
- lea 4(%ebp),%ebp // pop return stack and throw away
+ addl $4,%ebp // pop return stack and throw away
NEXT
-#include <asm-i386/unistd.h>
+/*
+ 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
_KEY:
mov (currkey),%ebx
cmp (bufftop),%ebx
- jge 1f
+ jge 1f // exhausted the input buffer?
xor %eax,%eax
- mov (%ebx),%al
+ mov (%ebx),%al // get next key from input buffer
inc %ebx
- mov %ebx,(currkey)
+ mov %ebx,(currkey) // increment currkey
ret
-1: // out of input; use read(2) to fetch more input from stdin
+1: // Out of input; use read(2) to fetch more input from stdin.
xor %ebx,%ebx // 1st param: stdin
mov $buffer,%ecx // 2nd param: buffer
mov %ecx,currkey
- mov $buffend-buffer,%edx // 3rd param: max length
+ mov $BUFFER_SIZE,%edx // 3rd param: max length
mov $__NR_read,%eax // syscall: read
int $0x80
test %eax,%eax // If %eax <= 0, then exit.
mov %ecx,bufftop
jmp _KEY
-2: // error or out of input: exit
+2: // Error or end of input: exit the program.
xor %ebx,%ebx
mov $__NR_exit,%eax // syscall: exit
int $0x80
+ .data
+ .align 4
+currkey:
+ .int buffer // Current place in input buffer (next character to read).
+bufftop:
+ .int buffer // Last valid data in input buffer + 1.
+
+/*
+ By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout.
+ This implementation just uses the write system call. No attempt is made to buffer output, but
+ it would be a good exercise to add it.
+*/
+
defcode "EMIT",4,,EMIT
pop %eax
call _EMIT
mov $1,%ebx // 1st param: stdout
// write needs the address of the byte to write
- mov %al,(2f)
- mov $2f,%ecx // 2nd param: address
+ mov %al,emit_scratch
+ mov $emit_scratch,%ecx // 2nd param: address
mov $1,%edx // 3rd param: nbytes = 1
int $0x80
ret
- .bss
-2: .space 1 // scratch used by EMIT
+ .data // NB: easier to fit in the .data section
+emit_scratch:
+ .space 1 // scratch used by EMIT
+
+/*
+ Back to input, WORD is a FORTH word which reads the next full word of input.
+
+ What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on).
+ Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it
+ calculates the length of the word it read and returns the address and the length as
+ two words on the stack (with the length at the top of stack).
+
+ Notice that WORD has a single internal buffer which it overwrites each time (rather like
+ a static C string). Also notice that WORD's internal buffer is just 32 bytes long and
+ there is NO checking for overflow. 31 bytes happens to be the maximum length of a
+ FORTH word that we support, and that is what WORD is used for: to read FORTH words when
+ we are compiling and executing code. The returned strings are not NUL-terminated.
+
+ Start address+length is the normal way to represent strings in FORTH (not ending in an
+ ASCII NUL character as in C), and so FORTH strings can contain any character including NULs
+ and can be any length.
+
+ WORD is not suitable for just reading strings (eg. user input) because of all the above
+ peculiarities and limitations.
+
+ Note that when executing, you'll see:
+ WORD FOO
+ which puts "FOO" and length 3 on the stack, but when compiling:
+ : BAR WORD FOO ;
+ is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling
+ and immediate mode, and you'll understand why.
+*/
defcode "WORD",4,,WORD
call _WORD
- push %ecx // push length
push %edi // push base address
+ push %ecx // push length
NEXT
_WORD:
jbe 1b // if so, keep looking
/* Search for the end of the word, storing chars as we go. */
- mov $5f,%edi // pointer to return buffer
+ mov $word_buffer,%edi // pointer to return buffer
2:
stosb // add character to return buffer
call _KEY // get next key, returned in %al
ja 2b // if not, keep looping
/* Return the word (well, the static buffer) and length. */
- sub $5f,%edi
+ sub $word_buffer,%edi
mov %edi,%ecx // return length of the word
- mov $5f,%edi // return address of the word
+ mov $word_buffer,%edi // return address of the word
ret
/* Code to skip \ comments to end of the current line. */
jne 3b
jmp 1b
- .bss
+ .data // NB: easier to fit in the .data section
// A static buffer where WORD returns. Subsequent calls
// overwrite this buffer. Maximum word length is 32 chars.
-5: .space 32
+word_buffer:
+ .space 32
- defcode "EMITSTRING",10,,EMITSTRING
- mov $1,%ebx // 1st param: stdout
- pop %ecx // 2nd param: address of string
- pop %edx // 3rd param: length of string
+/*
+ As well as reading in words we'll need to read in numbers and for that we are using a function
+ called NUMBER. This parses a numeric string such as one returned by WORD and pushes the
+ number on the parameter stack.
- mov $__NR_write,%eax // write syscall
- int $0x80
+ The function uses the variable BASE as the base (radix) for conversion, so for example if
+ BASE is 2 then we expect a binary number. Normally BASE is 10.
- NEXT
+ If the word starts with a '-' character then the returned value is negative.
- defcode ".",1,,DOT
- pop %eax // Get the number to print into %eax
- call _DOT // Easier to do this recursively ...
+ If the string can't be parsed as a number (or contains characters outside the current BASE)
+ then we need to return an error indication. So NUMBER actually returns two items on the stack.
+ At the top of stack we return the number of unconverted characters (ie. if 0 then all characters
+ were converted, so there is no error). Second from top of stack is the parsed number or a
+ partial value if there was an error.
+*/
+ defcode "NUMBER",6,,NUMBER
+ pop %ecx // length of string
+ pop %edi // start address of string
+ call _NUMBER
+ push %eax // parsed number
+ push %ecx // number of unparsed characters (0 = no error)
NEXT
-_DOT:
- mov $10,%ecx // Base 10
-1:
- cmp %ecx,%eax
- jb 2f
- xor %edx,%edx // %edx:%eax / %ecx -> quotient %eax, remainder %edx
- idivl %ecx
- pushl %edx
- call _DOT
- popl %eax
- jmp 1b
-2:
- xor %ah,%ah
- aam $10
- cwde
- addl $'0',%eax
- call _EMIT
- ret
- // Parse a number from a string on the stack -- almost the opposite of . (DOT)
- // Note that there is absolutely no error checking. In particular the length of the
- // string must be >= 1 bytes.
- defcode "SNUMBER",7,,SNUMBER
- pop %edi
- pop %ecx
- call _SNUMBER
- push %eax
- NEXT
-_SNUMBER:
+_NUMBER:
xor %eax,%eax
xor %ebx,%ebx
-1:
- imull $10,%eax // %eax *= 10
- movb (%edi),%bl
+
+ test %ecx,%ecx // trying to parse a zero-length string is an error, but will return 0.
+ jz 5f
+
+ movl var_BASE,%edx // get BASE (in %dl)
+
+ // Check if first character is '-'.
+ movb (%edi),%bl // %bl = first character in string
+ inc %edi
+ push %eax // push 0 on stack
+ cmpb $'-',%bl // negative number?
+ jnz 2f
+ pop %eax
+ push %ebx // push <> 0 on stack, indicating negative
+ dec %ecx
+ jnz 1f
+ pop %ebx // error: string is only '-'.
+ movl $1,%ecx
+ ret
+
+ // Loop reading digits.
+1: imull %edx,%eax // %eax *= BASE
+ movb (%edi),%bl // %bl = next character in string
inc %edi
- subb $'0',%bl // ASCII -> digit
+
+ // Convert 0-9, A-Z to a number 0-35.
+2: subb $'0',%bl // < '0'?
+ jb 4f
+ cmp $10,%bl // <= '9'?
+ jb 3f
+ subb $17,%bl // < 'A'? (17 is 'A'-'0')
+ jb 4f
+ addb $10,%bl
+
+3: cmp %dl,%bl // >= BASE?
+ jge 4f
+
+ // OK, so add it to %eax and loop.
add %ebx,%eax
dec %ecx
jnz 1b
- ret
+
+ // Negate the result if first character was '-' (saved on the stack).
+4: pop %ebx
+ test %ebx,%ebx
+ jz 5f
+ neg %eax
+
+5: ret
+
+/*
+ DICTIONARY LOOK UPS ----------------------------------------------------------------------
+
+ We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure.
+
+ The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the
+ dictionary. What it actually returns is the address of the dictionary header, if it finds it,
+ or 0 if it didn't.
+
+ So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer:
+
+ pointer to this
+ |
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+
+ See also >CFA and >DFA.
+
+ FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why.
+*/
defcode "FIND",4,,FIND
- pop %edi // %edi = address
pop %ecx // %ecx = length
+ pop %edi // %edi = address
call _FIND
- push %eax
+ push %eax // %eax = address of dictionary entry (or NULL)
NEXT
_FIND:
// Now we start searching backwards through the dictionary for this word.
mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary
-1:
- test %edx,%edx // NULL pointer? (end of the linked list)
+1: test %edx,%edx // NULL pointer? (end of the linked list)
je 4f
// Compare the length expected and the length of the word.
// this won't pick the word (the length will appear to be wrong).
xor %eax,%eax
movb 4(%edx),%al // %al = flags+length field
- andb $(F_HIDDEN|0x1f),%al // %al = name length
+ andb $(F_HIDDEN|F_LENMASK),%al // %al = name length
cmpb %cl,%al // Length is the same?
jne 2f
mov %edx,%eax
ret
-2:
- mov (%edx),%edx // Move back through the link field to the previous word
+2: mov (%edx),%edx // Move back through the link field to the previous word
jmp 1b // .. and loop.
4: // Not found.
xor %eax,%eax // Return zero to indicate not found.
ret
- defcode ">CFA",4,,TCFA // DEA -> Codeword address
+/*
+ FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall
+ that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH
+ word >CFA turns a dictionary pointer into a codeword pointer.
+
+ The example below shows the result of:
+
+ WORD DOUBLE FIND >CFA
+
+ FIND returns a pointer to this
+ | >CFA converts it to a pointer to this
+ | |
+ V V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ codeword
+
+ Notes:
+
+ Because names vary in length, this isn't just a simple increment.
+
+ In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but
+ that is not true in most FORTH implementations where they store a back pointer in the definition
+ (with an obvious memory/complexity cost). The reason they do this is that it is useful to be
+ able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions
+ quickly.
+
+ What does CFA stand for? My best guess is "Code Field Address".
+*/
+
+ defcode ">CFA",4,,TCFA
pop %edi
call _TCFA
push %edi
add $4,%edi // Skip link pointer.
movb (%edi),%al // Load flags+len into %al.
inc %edi // Skip flags+len byte.
- andb $0x1f,%al // Just the length, not the flags.
+ andb $F_LENMASK,%al // Just the length, not the flags.
add %eax,%edi // Skip the name.
addl $3,%edi // The codeword is 4-byte aligned.
andl $~3,%edi
ret
- defcode "CHAR",4,,CHAR
- call _WORD // Returns %ecx = length, %edi = pointer to word.
- xor %eax,%eax
- movb (%edi),%al // Get the first character of the word.
- push %eax // Push it onto the stack.
- NEXT
+/*
+ Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and
+ returns a pointer to the first data field.
+
+ FIND returns a pointer to this
+ | >CFA converts it to a pointer to this
+ | |
+ | | >DFA converts it to a pointer to this
+ | | |
+ V V V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ codeword
+
+ (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is
+ different from theirs, because they have an extra indirection).
+
+ You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA.
+*/
- defcode ":",1,,COLON
+ defword ">DFA",4,,TDFA
+ .int TCFA // >CFA (get code field address)
+ .int INCR4 // 4+ (add 4 to it to get to next word)
+ .int EXIT // EXIT (return from FORTH word)
- // Get the word and create a dictionary entry header for it.
- call _WORD // Returns %ecx = length, %edi = pointer to word.
- mov %edi,%ebx // %ebx = address of the word
+/*
+ COMPILING ----------------------------------------------------------------------
+
+ Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this:
+ : DOUBLE DUP + ;
+
+ and we have to turn this into:
+
+ pointer to previous word
+ ^
+ |
+ +--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+
+ ^ len pad codeword |
+ | V
+ LATEST points here points to codeword of DUP
+
+ There are several problems to solve. Where to put the new word? How do we read words? How
+ do we define the words : (COLON) and ; (SEMICOLON)?
+
+ FORTH solves this rather elegantly and as you might expect in a very low-level way which
+ allows you to change how the compiler works on your own code.
+
+ FORTH has an INTERPRET function (a true interpreter this time, not DOCOL) which runs in a
+ loop, reading words (using WORD), looking them up (using FIND), turning them into codeword
+ pointers (using >CFA) and deciding what to do with them.
+
+ What it does depends on the mode of the interpreter (in variable STATE).
+
+ When STATE is zero, the interpreter just runs each word as it looks them up. This is known as
+ immediate mode.
+
+ The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the
+ interpreter appends the codeword pointer to user memory (the HERE variable points to the next
+ free byte of user memory -- see DATA SEGMENT section below).
+
+ So you may be able to see how we could define : (COLON). The general plan is:
+
+ (1) Use WORD to read the name of the function being defined.
+
+ (2) Construct the dictionary entry -- just the header part -- in user memory:
+
+ pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where
+ ^ | the interpreter will start appending
+ | V codewords.
+ +--|------+---+---+---+---+---+---+---+---+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
+ +---------+---+---+---+---+---+---+---+---+------------+
+ len pad codeword
+
+ (3) Set LATEST to point to the newly defined word, ...
+
+ (4) .. and most importantly leave HERE pointing just after the new codeword. This is where
+ the interpreter will append codewords.
+
+ (5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to
+ our partially-formed header.
+
+ After : has run, our input is here:
+
+ : DOUBLE DUP + ;
+ ^
+ |
+ Next byte returned by KEY will be the 'D' character of DUP
+
+ so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP",
+ looks it up in the dictionary, gets its codeword pointer, and appends it:
+
+ +-- HERE updated to point here.
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+
+ len pad codeword
+
+ Next we read +, get the codeword pointer, and append it:
+
+ +-- HERE updated to point here.
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+
+ len pad codeword
+
+ The issue is what happens next. Obviously what we _don't_ want to happen is that we
+ read ";" and compile it and go on compiling everything afterwards.
+
+ At this point, FORTH uses a trick. Remember the length byte in the dictionary definition
+ isn't just a plain length byte, but can also contain flags. One flag is called the
+ IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as
+ IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_.
+
+ This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE.
+
+ And all it does is append the codeword for EXIT on to the current definition and switch
+ back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition
+ of ; and we'll see that it's really a very simple definition, declared IMMEDIATE.
+
+ After the interpreter reads ; and executes it 'immediately', we get this:
+
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ len pad codeword ^
+ |
+ HERE
+ STATE is set to 0.
+
+ And that's it, job done, our new definition is compiled, and we're back in immediate mode
+ just reading and executing words, perhaps including a call to test our new word DOUBLE.
+
+ The only last wrinkle in this is that while our word was being compiled, it was in a
+ half-finished state. We certainly wouldn't want DOUBLE to be called somehow during
+ this time. There are several ways to stop this from happening, but in FORTH what we
+ do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is
+ being compiled. This prevents FIND from finding it, and thus in theory stops any
+ chance of it being called.
+
+ The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm
+ going to define them. The : (COLON) function can be made a little bit more general by writing
+ it in two parts. The first part, called CREATE, makes just the header:
+
+ +-- Afterwards, HERE points here.
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+
+ | LINK | 6 | D | O | U | B | L | E | 0 |
+ +---------+---+---+---+---+---+---+---+---+
+ len pad
+
+ and the second part, the actual definition of : (COLON), calls CREATE and appends the
+ DOCOL codeword, so leaving:
+
+ +-- Afterwards, HERE points here.
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL |
+ +---------+---+---+---+---+---+---+---+---+------------+
+ len pad codeword
+
+ CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to
+ create other types of words (not just ones which contain code, but words which contain variables,
+ constants and other data).
+*/
+
+ defcode "CREATE",6,,CREATE
+
+ // Get the name length and address.
+ pop %ecx // %ecx = length
+ pop %ebx // %ebx = address of name
+
+ // Link pointer.
movl var_HERE,%edi // %edi is the address of the header
movl var_LATEST,%eax // Get link pointer
stosl // and store it in the header.
+ // Length byte and the word itself.
mov %cl,%al // Get the length.
- orb $F_HIDDEN,%al // Set the HIDDEN flag on this entry.
stosb // Store the length/flags byte.
push %esi
mov %ebx,%esi // %esi = word
addl $3,%edi // Align to next 4 byte boundary.
andl $~3,%edi
- movl $DOCOL,%eax // The codeword for user-created words is always DOCOL (the interpreter)
- stosl
-
- // Header built, so now update LATEST and HERE.
- // We'll be compiling words and putting them HERE.
+ // Update LATEST and HERE.
movl var_HERE,%eax
movl %eax,var_LATEST
movl %edi,var_HERE
-
- // And go into compile mode by setting STATE to 1.
- movl $1,var_STATE
NEXT
+/*
+ Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words
+ to use.
+
+ The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user
+ memory pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is:
+
+ previous value of HERE
+ |
+ V
+ +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | | <data> |
+ +---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+
+ len pad ^
+ |
+ new value of HERE
+
+ and <data> is whatever 32 bit integer was at the top of the stack.
+
+ , (COMMA) is quite a fundamental operation when compiling. It is used to append codewords
+ to the current word that is being compiled.
+*/
+
defcode ",",1,,COMMA
pop %eax // Code pointer to store.
call _COMMA
movl %edi,var_HERE // Update HERE (incremented)
ret
- defcode "HIDDEN",6,,HIDDEN
- call _HIDDEN
+/*
+ Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode.
+
+ Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this
+ variable we can switch between the two modes.
+
+ For various reasons which may become apparent later, FORTH defines two standard words called
+ [ and ] (LBRAC and RBRAC) which switch between modes:
+
+ Word Assembler Action Effect
+ [ LBRAC STATE := 0 Switch to immediate mode.
+ ] RBRAC STATE := 1 Switch to compile mode.
+
+ [ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the
+ interpreter saw [ then it would compile it rather than running it. We would never be able to
+ switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode
+ the word runs immediately, switching us back to immediate mode.
+*/
+
+ defcode "[",1,F_IMMED,LBRAC
+ xor %eax,%eax
+ movl %eax,var_STATE // Set STATE to 0.
NEXT
-_HIDDEN:
- movl var_LATEST,%edi // LATEST word.
- addl $4,%edi // Point to name/flags byte.
- xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit.
- ret
- defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE
- call _IMMEDIATE
+ defcode "]",1,,RBRAC
+ movl $1,var_STATE // Set STATE to 1.
NEXT
-_IMMEDIATE:
+
+/*
+ Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets
+ the word HIDDEN and goes into compile mode.
+*/
+
+ defword ":",1,,COLON
+ .int WORD // Get the name of the new word
+ .int CREATE // CREATE the dictionary entry / header
+ .int LIT, DOCOL, COMMA // Append DOCOL (the codeword).
+ .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition).
+ .int RBRAC // Go into compile mode.
+ .int EXIT // Return from the function.
+
+/*
+ ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag.
+*/
+
+ defword ";",1,F_IMMED,SEMICOLON
+ .int LIT, EXIT, COMMA // Append EXIT (so the word will return).
+ .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition).
+ .int LBRAC // Go back to IMMEDIATE mode.
+ .int EXIT // Return from the function.
+
+/*
+ EXTENDING THE COMPILER ----------------------------------------------------------------------
+
+ Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define
+ your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because
+ it allows you in effect to extend the compiler itself. Does gcc let you do that?
+
+ Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic
+ compiler, and are all IMMEDIATE words.
+
+ The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word,
+ or on the current word if you call it in the middle of a definition.
+
+ Typical usage is:
+
+ : MYIMMEDWORD IMMEDIATE
+ ...definition...
+ ;
+
+ but some FORTH programmers write this instead:
+
+ : MYIMMEDWORD
+ ...definition...
+ ; IMMEDIATE
+
+ The two usages are equivalent, to a first approximation.
+*/
+
+ defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE
movl var_LATEST,%edi // LATEST word.
addl $4,%edi // Point to name/flags byte.
xorb $F_IMMED,(%edi) // Toggle the IMMED bit.
- ret
+ NEXT
+
+/*
+ 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the
+ most recently defined word (used above in : and ; definitions) you would do:
+
+ 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
- defcode ";",1,F_IMMED,SEMICOLON
- movl $EXIT,%eax // EXIT is the final codeword in compiled words.
- call _COMMA // Store it.
- call _HIDDEN // Toggle the HIDDEN flag (unhides the new word).
- xor %eax,%eax // Set STATE to 0 (back to execute mode).
- movl %eax,var_STATE
+ After this, only MAIN is 'exported' or seen by the rest of the program.
+*/
+
+ defcode "HIDDEN",6,,HIDDEN
+ pop %edi // Dictionary entry.
+ addl $4,%edi // Point to name/flags byte.
+ xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit.
NEXT
-/* This definiton of ' (TICK) is strictly cheating - it also only works in compiled code. */
+ defword "HIDE",4,,HIDE
+ .int WORD // Get the word (after HIDE).
+ .int FIND // Look up in the dictionary.
+ .int HIDDEN // Set F_HIDDEN flag.
+ .int EXIT // Return.
+
+/*
+ ' (TICK) is a standard FORTH word which returns the codeword pointer of the next word.
+
+ The common usage is:
+
+ ' FOO ,
+
+ which appends the codeword of FOO to the current word we are defining (this only works in compiled code).
+
+ You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define
+ a literal 2 might be:
+
+ : LIT2 IMMEDIATE
+ ' LIT , \ Appends LIT to the currently-being-defined word
+ 2 , \ Appends the number 2 to the currently-being-defined word
+ ;
+
+ So you could do:
+
+ : DOUBLE LIT2 * ;
+
+ (If you don't understand how LIT2 works, then you should review the material about compiling words
+ and immediate mode).
+
+ This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in
+ compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in
+ immediate mode too.
+*/
defcode "'",1,,TICK
lodsl // Get the address of the next word and skip it.
pushl %eax // Push it on the stack.
NEXT
-/* This interpreter is pretty simple, but remember that in FORTH you can always override
- * it later with a more powerful one!
- */
- defword "INTERPRETER",11,,INTERPRETER
- .int INTERPRET,RDROP,INTERPRETER
+/*
+ BRANCHING ----------------------------------------------------------------------
+
+ It turns out that all you need in order to define looping constructs, IF-statements, etc.
+ are two primitives.
+
+ BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the
+ top of stack is zero).
+
+ The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes,
+ %esi starts by pointing to the offset field (compare to LIT above):
+
+ +---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+
+ | (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word |
+ +---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+
+ ^ | ^
+ | | |
+ | +-----------------------+
+ %esi added to offset
+
+ The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution
+ continues at the branch target. Negative offsets work as expected.
+
+ 0BRANCH is the same except the branch happens conditionally.
+
+ Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely
+ in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH
+ into the word currently being compiled.
+
+ As an example, code written like this:
+
+ condition-code IF true-part THEN rest-code
+
+ compiles to:
+
+ condition-code 0BRANCH OFFSET true-part rest-code
+ | ^
+ | |
+ +-------------+
+*/
+
+ defcode "BRANCH",6,,BRANCH
+ add (%esi),%esi // add the offset to the instruction pointer
+ NEXT
+
+ defcode "0BRANCH",7,,ZBRANCH
+ pop %eax
+ test %eax,%eax // top of stack is zero?
+ jz code_BRANCH // if so, jump back to the branch function above
+ lodsl // otherwise we need to skip the offset
+ NEXT
+
+/*
+ LITERAL STRINGS ----------------------------------------------------------------------
+
+ LITSTRING is a primitive used to implement the ." and S" operators (which are written in
+ FORTH). See the definition of those operators later.
+
+ TELL just prints a string. It's more efficient to define this in assembly because we
+ can make it a single Linux syscall.
+*/
+
+ defcode "LITSTRING",9,,LITSTRING
+ lodsl // get the length of the string
+ push %esi // push the address of the start of the string
+ push %eax // push it on the stack
+ addl %eax,%esi // skip past the string
+ addl $3,%esi // but round up to next 4 byte boundary
+ andl $~3,%esi
+ NEXT
+
+ defcode "TELL",4,,TELL
+ mov $1,%ebx // 1st param: stdout
+ pop %edx // 3rd param: length of string
+ pop %ecx // 2nd param: address of string
+ mov $__NR_write,%eax // write syscall
+ int $0x80
+ NEXT
+
+/*
+ QUIT AND INTERPRET ----------------------------------------------------------------------
+
+ QUIT is the first FORTH function called, almost immediately after the FORTH system "boots".
+ As explained before, QUIT doesn't "quit" anything. It does some initialisation (in particular
+ it clears the return stack) and it calls INTERPRET in a loop to interpret commands. The
+ reason it is called QUIT is because you can call it from your own FORTH words in order to
+ "quit" your program and start again at the user prompt.
+
+ INTERPRET is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate
+ description -- see: http://en.wikipedia.org/wiki/REPL).
+*/
+
+ // QUIT must not return (ie. must not call EXIT).
+ defword "QUIT",4,,QUIT
+ .int RZ,RSPSTORE // R0 RSP!, clear the return stack
+ .int INTERPRET // interpret the next word
+ .int BRANCH,-8 // and loop (indefinitely)
+/*
+ This interpreter is pretty simple, but remember that in FORTH you can always override
+ it later with a more powerful one!
+ */
defcode "INTERPRET",9,,INTERPRET
call _WORD // Returns %ecx = length, %edi = pointer to word.
1: // Not in the dictionary (not a word) so assume it's a literal number.
incl interpret_is_lit
- call _SNUMBER // Returns the parsed number in %eax
+ call _NUMBER // Returns the parsed number in %eax, %ecx > 0 if error
+ test %ecx,%ecx
+ jnz 6f
mov %eax,%ebx
mov $LIT,%eax // The word is LIT
jnz 5f
// Not a literal, execute it now. This never returns, but the codeword will
- // eventually call NEXT which will reenter the loop in INTERPRETER.
+ // eventually call NEXT which will reenter the loop in QUIT.
jmp *(%eax)
5: // Executing a literal, which means push it on the stack.
push %ebx
NEXT
- .data
+6: // Parse error (not a known word or a number in the current BASE).
+ // Print an error message followed by up to 40 characters of context.
+ mov $2,%ebx // 1st param: stderr
+ mov $errmsg,%ecx // 2nd param: error message
+ mov $errmsgend-errmsg,%edx // 3rd param: length of string
+ mov $__NR_write,%eax // write syscall
+ int $0x80
+
+ mov (currkey),%ecx // the error occurred just before currkey position
+ mov %ecx,%edx
+ sub $buffer,%edx // %edx = currkey - buffer (length in buffer before currkey)
+ cmp $40,%edx // if > 40, then print only 40 characters
+ jle 7f
+ mov $40,%edx
+7: sub %edx,%ecx // %ecx = start of area to print, %edx = length
+ mov $__NR_write,%eax // write syscall
+ int $0x80
+
+ mov $errmsgnl,%ecx // newline
+ mov $1,%edx
+ mov $__NR_write,%eax // write syscall
+ int $0x80
+
+ NEXT
+
+ .section .rodata
+errmsg: .ascii "PARSE ERROR: "
+errmsgend:
+errmsgnl: .ascii "\n"
+
+ .data // NB: easier to fit in the .data section
.align 4
interpret_is_lit:
.int 0 // Flag used to record if reading a literal
- // NB: SYSEXIT must be the last entry in the built-in dictionary.
- defcode SYSEXIT,7,,SYSEXIT
- pop %ebx
- mov $__NR_exit,%eax
+/*
+ ODDS AND ENDS ----------------------------------------------------------------------
+
+ CHAR puts the ASCII code of the first character of the following word on the stack. For example
+ CHAR A puts 65 on the stack.
+
+ EXECUTE is used to run execution tokens. See the discussion of execution tokens in the
+ FORTH code for more details.
+
+ SYSCALL0, SYSCALL1, SYSCALL2, SYSCALL3 make a standard Linux system call. (See <asm/unistd.h>
+ for a list of system call numbers). As their name suggests these forms take between 0 and 3
+ syscall parameters, plus the system call number.
+
+ In this FORTH, SYSCALL0 must be the last word in the built-in (assembler) dictionary because we
+ initialise the LATEST variable to point to it. This means that if you want to extend the assembler
+ part, you must put new words before SYSCALL0, or else change how LATEST is initialised.
+*/
+
+ defcode "CHAR",4,,CHAR
+ call _WORD // Returns %ecx = length, %edi = pointer to word.
+ xor %eax,%eax
+ movb (%edi),%al // Get the first character of the word.
+ push %eax // Push it onto the stack.
+ NEXT
+
+ defcode "EXECUTE",7,,EXECUTE
+ pop %eax // Get xt into %eax
+ jmp *(%eax) // and jump to it.
+ // After xt runs its NEXT will continue executing the current word.
+
+ defcode "SYSCALL3",8,,SYSCALL3
+ pop %eax // System call number (see <asm/unistd.h>)
+ pop %ebx // First parameter.
+ pop %ecx // Second parameter
+ pop %edx // Third parameter
int $0x80
+ push %eax // Result (negative for -errno)
+ NEXT
-/*----------------------------------------------------------------------
- * Input buffer & initial input.
- */
- .data
+ defcode "SYSCALL2",8,,SYSCALL2
+ pop %eax // System call number (see <asm/unistd.h>)
+ pop %ebx // First parameter.
+ pop %ecx // Second parameter
+ int $0x80
+ push %eax // Result (negative for -errno)
+ NEXT
+
+ defcode "SYSCALL1",8,,SYSCALL1
+ pop %eax // System call number (see <asm/unistd.h>)
+ pop %ebx // First parameter.
+ int $0x80
+ push %eax // Result (negative for -errno)
+ NEXT
+
+ defcode "SYSCALL0",8,,SYSCALL0
+ pop %eax // System call number (see <asm/unistd.h>)
+ int $0x80
+ push %eax // Result (negative for -errno)
+ NEXT
+
+/*
+ DATA SEGMENT ----------------------------------------------------------------------
+
+ Here we set up the Linux data segment, used for user definitions and variously known as just
+ the 'data segment', 'user memory' or 'user definitions area'. It is an area of memory which
+ grows upwards and stores both newly-defined FORTH words and global variables of various
+ sorts.
+
+ It is completely analogous to the C heap, except there is no generalised 'malloc' and 'free'
+ (but as with everything in FORTH, writing such functions would just be a Simple Matter
+ Of Programming). Instead in normal use the data segment just grows upwards as new FORTH
+ words are defined/appended to it.
+
+ There are various "features" of the GNU toolchain which make setting up the data segment
+ more complicated than it really needs to be. One is the GNU linker which inserts a random
+ "build ID" segment. Another is Address Space Randomization which means we can't tell
+ where the kernel will choose to place the data segment (or the stack for that matter).
+
+ Therefore writing this set_up_data_segment assembler routine is a little more complicated
+ than it really needs to be. We ask the Linux kernel where it thinks the data segment starts
+ using the brk(2) system call, then ask it to reserve some initial space (also using brk(2)).
+
+ You don't need to worry about this code.
+*/
+ .text
+ .set INITIAL_DATA_SEGMENT_SIZE,65536
+set_up_data_segment:
+ xor %ebx,%ebx // Call brk(0)
+ movl $__NR_brk,%eax
+ int $0x80
+ movl %eax,var_HERE // Initialise HERE to point at beginning of data segment.
+ addl $INITIAL_DATA_SEGMENT_SIZE,%eax // Reserve nn bytes of memory for initial data segment.
+ movl %eax,%ebx // Call brk(HERE+INITIAL_DATA_SEGMENT_SIZE)
+ movl $__NR_brk,%eax
+ int $0x80
+ ret
+
+/*
+ We allocate static buffers for the return static and input buffer (used when
+ reading in files and text that the user types in).
+*/
+ .set RETURN_STACK_SIZE,8192
+ .set BUFFER_SIZE,4096
+
+ .bss
+/* FORTH return stack. */
+ .align 4096
+return_stack:
+ .space RETURN_STACK_SIZE
+return_stack_top: // Initial top of return stack.
+
+/* This is used as a temporary input buffer when reading from files or the terminal. */
.align 4096
buffer:
- // XXX gives 'Warning: unterminated string; newline inserted' messages which you can ignore
- .ascii "\
-\\ Define some character constants
-: '\\n' 10 ;
-: 'SPACE' 32 ;
-: '\"' 34 ;
-: ':' 58 ;
-
-\\ CR prints a carriage return
-: CR '\\n' EMIT ;
-
-\\ SPACE prints a space
-: SPACE 'SPACE' EMIT ;
-
-\\ Primitive . (DOT) function doesn't follow with a blank, so redefine it to behave like FORTH.
-\\ Notice how we can trivially redefine existing functions.
-: . . SPACE ;
-
-\\ DUP, DROP are defined in assembly for speed, but this is how you might define them
-\\ in FORTH. Notice use of the scratch variables _X and _Y.
-\\ : DUP _X ! _X @ _X @ ;
-\\ : DROP _X ! ;
-
-\\ The 2... versions of the standard operators work on pairs of stack entries. They're not used
-\\ very commonly so not really worth writing in assembler. Here is how they are defined in FORTH.
-: 2DUP OVER OVER ;
-: 2DROP DROP DROP ;
-
-\\ More standard FORTH words.
-: 2* 2 * ;
-: 2/ 2 / ;
-
-\\ [ and ] allow you to break into immediate mode while compiling a word.
-: [ IMMEDIATE \\ define [ as an immediate word
- 0 STATE ! \\ go into immediate mode
- ;
+ .space BUFFER_SIZE
-: ]
- 1 STATE ! \\ go back to compile mode
- ;
+/*
+ START OF FORTH CODE ----------------------------------------------------------------------
-\\ LITERAL takes whatever is on the stack and compiles LIT <foo>
-: LITERAL IMMEDIATE
- ' LIT , \\ compile LIT
- , \\ compile the literal itself (from the stack)
- ;
+ We've now reached the stage where the FORTH system is running and self-hosting. All further
+ words can be written as FORTH itself, including words like IF, THEN, .", etc which in most
+ languages would be considered rather fundamental.
-\\ condition IF true-part THEN rest
-\\ compiles to:
-\\ condition 0BRANCH OFFSET true-part rest
-\\ where OFFSET is the offset of 'rest'
-\\ condition IF true-part ELSE false-part THEN
-\\ compiles to:
-\\ condition 0BRANCH OFFSET true-part BRANCH OFFSET2 false-part rest
-\\ where OFFSET if the offset of false-part and OFFSET2 is the offset of rest
-
-\\ IF is an IMMEDIATE word which compiles 0BRANCH followed by a dummy offset, and places
-\\ the address of the 0BRANCH on the stack. Later when we see THEN, we pop that address
-\\ off the stack, calculate the offset, and back-fill the offset.
-: IF IMMEDIATE
- ' 0BRANCH , \\ compile 0BRANCH
- HERE @ \\ save location of the offset on the stack
- 0 , \\ compile a dummy offset
-;
-
-: THEN IMMEDIATE
- DUP
- HERE @ SWAP - \\ calculate the offset from the address saved on the stack
- SWAP ! \\ store the offset in the back-filled location
-;
-
-: ELSE IMMEDIATE
- ' BRANCH , \\ definite branch to just over the false-part
- HERE @ \\ save location of the offset on the stack
- 0 , \\ compile a dummy offset
- SWAP \\ now back-fill the original (IF) offset
- DUP \\ same as for THEN word above
- HERE @ SWAP -
- SWAP !
-;
-
-\\ BEGIN loop-part condition UNTIL
-\\ compiles to:
-\\ loop-part condition 0BRANCH OFFSET
-\\ where OFFSET points back to the loop-part
-\\ This is like do { loop-part } while (condition) in the C language
-: BEGIN IMMEDIATE
- HERE @ \\ save location on the stack
-;
-
-: UNTIL IMMEDIATE
- ' 0BRANCH , \\ compile 0BRANCH
- HERE @ - \\ calculate the offset from the address saved on the stack
- , \\ compile the offset here
-;
-
-\\ BEGIN loop-part AGAIN
-\\ compiles to:
-\\ loop-part BRANCH OFFSET
-\\ where OFFSET points back to the loop-part
-\\ In other words, an infinite loop which can only be returned from with EXIT
-: AGAIN IMMEDIATE
- ' BRANCH , \\ compile BRANCH
- HERE @ - \\ calculate the offset back
- , \\ compile the offset here
-;
-
-\\ BEGIN condition WHILE loop-part REPEAT
-\\ compiles to:
-\\ condition 0BRANCH OFFSET2 loop-part BRANCH OFFSET
-\\ where OFFSET points back to condition (the beginning) and OFFSET2 points to after the whole piece of code
-\\ So this is like a while (condition) { loop-part } loop in the C language
-: WHILE IMMEDIATE
- ' 0BRANCH , \\ compile 0BRANCH
- HERE @ \\ save location of the offset2 on the stack
- 0 , \\ compile a dummy offset2
-;
-
-: REPEAT IMMEDIATE
- ' BRANCH , \\ compile BRANCH
- SWAP \\ get the original offset (from BEGIN)
- HERE @ - , \\ and compile it after BRANCH
- DUP
- HERE @ SWAP - \\ calculate the offset2
- SWAP ! \\ and back-fill it in the original location
-;
-
-\\ With the looping constructs, we can now write SPACES, which writes n spaces to stdout.
-: SPACES
- BEGIN
- SPACE \\ print a space
- 1- \\ until we count down to 0
- DUP 0=
- UNTIL
-;
-
-\\ .S prints the contents of the stack. Very useful for debugging.
-: .S
- DSP@ \\ get current stack pointer
- BEGIN
- DUP @ . \\ print the stack element
- 4+ \\ move up
- DUP S0 @ 4- = \\ stop when we get to the top
- UNTIL
- DROP
-;
-
-\\ DEPTH returns the depth of the stack.
-: DEPTH S0 @ DSP@ - ;
-
-\\ .\" is the print string operator in FORTH. Example: .\" Something to print\"
-\\ The space after the operator is the ordinary space required between words.
-\\ This is tricky to define because it has to do different things depending on whether
-\\ we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can
-\\ detect this and do different things).
-\\ In immediate mode we just keep reading characters and printing them until we get to
-\\ the next double quote.
-\\ In compile mode we have the problem of where we're going to store the string (remember
-\\ that the input buffer where the string comes from may be overwritten by the time we
-\\ come round to running the function). We store the string in the compiled function
-\\ like this:
-\\ LITSTRING, string length, string rounded up to 4 bytes, EMITSTRING, ...
-: .\" IMMEDIATE
- STATE @ \\ compiling?
- IF
- ' LITSTRING , \\ compile LITSTRING
- HERE @ \\ save the address of the length word on the stack
- 0 , \\ dummy length - we don't know what it is yet
- BEGIN
- KEY \\ get next character of the string
- DUP '\"' <>
- WHILE
- HERE @ !b \\ store the character in the compiled image
- 1 HERE +! \\ increment HERE pointer by 1 byte
- REPEAT
- DROP \\ drop the double quote character at the end
- DUP \\ get the saved address of the length word
- HERE @ SWAP - \\ calculate the length
- 4- \\ subtract 4 (because we measured from the start of the length word)
- SWAP ! \\ and back-fill the length location
- HERE @ \\ round up to next multiple of 4 bytes for the remaining code
- 3 +
- 3 INVERT AND
- HERE !
- ' EMITSTRING , \\ compile the final EMITSTRING
- ELSE
- \\ In immediate mode, just read characters and print them until we get
- \\ to the ending double quote. Much simpler than the above code!
- BEGIN
- KEY
- DUP '\"' = IF EXIT THEN
- EMIT
- AGAIN
- THEN
-;
-
-\\ While compiling, [COMPILE] WORD compiles WORD if it would otherwise be IMMEDIATE.
-: [COMPILE] IMMEDIATE
- WORD \\ get the next word
- FIND \\ find it in the dictionary
- >CFA \\ get its codeword
- , \\ and compile that
-;
-
-\\ RECURSE makes a recursive call to the current word that is being compiled.
-\\ Normally while a word is being compiled, it is marked HIDDEN so that references to the
-\\ same word within are calls to the previous definition of the word.
-: RECURSE IMMEDIATE
- LATEST @ >CFA \\ LATEST points to the word being compiled at the moment
- , \\ compile it
-;
-
-\\ ALLOT is used to allocate (static) memory when compiling. It increases HERE by
-\\ the amount given on the stack.
-: ALLOT HERE +! ;
-
-
-\\ Finally print the welcome prompt.
-.\" OK \"
-"
-
-_initbufftop:
- .align 4096
-buffend:
+ I used to append this here in the assembly file, but I got sick of fighting against gas's
+ crack-smoking (lack of) multiline string syntax. So now that is in a separate file called
+ jonesforth.f
-currkey:
- .int buffer
-bufftop:
- .int _initbufftop
+ If you don't already have that file, download it from http://annexia.org/forth in order
+ to continue the tutorial.
+*/
+
+/* END OF jonesforth.S */
git.annexia.org / jonesforth.git / blobdiff