/* 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 1.24 2007-09-23 20:06:00 rich Exp $
gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S
-
+*/
+ .set JONES_VERSION,24
+/*
INTRODUCTION ----------------------------------------------------------------------
FORTH is one of those alien languages which most working programmers regard in the same
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.
+
+ Also I used this document (http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design) which really
+ defies easy explanation.
+
+ 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.
+
ASSEMBLING ----------------------------------------------------------------------
If you want to actually run this FORTH, rather than just read it, you will need Linux on an
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:
+
+ ./jonesforth < myprog.f
+
+ If you want to load your own FORTH code and then continue reading user commands, you can do:
+
+ cat myfunctions.f - | ./jonesforth
+
ASSEMBLER ----------------------------------------------------------------------
(You can just skip to the next section -- you don't need to be able to read assembler to
Now the tutorial starts in earnest.
- INDIRECT THREADED CODE ----------------------------------------------------------------------
+ THE DICTIONARY ----------------------------------------------------------------------
+
+ In FORTH as you will know, functions are called "words", and just as in other languages they
+ have a name and a definition. Here are two FORTH words:
+
+ : DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +"
+ : QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE"
+
+ Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary
+ which is just a linked list of dictionary entries.
+
+ <--- DICTIONARY ENTRY (HEADER) ----------------------->
+ +------------------------+--------+---------- - - - - +----------- - - - -
+ | LINK POINTER | LENGTH/| NAME | DEFINITION
+ | | FLAGS | |
+ +--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - -
+
+ I'll come to the definition of the word later. For now just look at the header. The first
+ 4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for
+ the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte.
+ The length of the word can be up to 31 characters (5 bits used) and the top three bits are used
+ for various flags which I'll come to later. This is followed by the name itself, and in this
+ implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes.
+ That's just to ensure that the definition starts on a 32 bit boundary.
+
+ A FORTH variable called LATEST contains a pointer to the most recently defined word, in
+ other words, the head of this linked list.
+
+ DOUBLE and QUADRUPLE might look like this:
+
+ pointer to previous word
+ ^
+ |
+ +--|------+---+---+---+---+---+---+---+---+------------- - - - -
+ | LINK | 6 | D | O | U | B | L | E | 0 | (definition ...)
+ +---------+---+---+---+---+---+---+---+---+------------- - - - -
+ ^ len padding
+ |
+ +--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
+ | LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...)
+ +---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - -
+ ^ len padding
+ |
+ |
+ LATEST
+
+ You should be able to see from this how you might implement functions to find a word in
+ the dictionary (just walk along the dictionary entries starting at LATEST and matching
+ the names until you either find a match or hit the NULL pointer at the end of the dictionary);
+ and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set
+ LATEST to point to the new word). We'll see precisely these functions implemented in
+ assembly code later on.
+
+ One interesting consequence of using a linked list is that you can redefine words, and
+ a newer definition of a word overrides an older one. This is an important concept in
+ FORTH because it means that any word (even "built-in" or "standard" words) can be
+ overridden with a new definition, either to enhance it, to make it faster or even to
+ disable it. However because of the way that FORTH words get compiled, which you'll
+ understand below, words defined using the old definition of a word continue to use
+ the old definition. Only words defined after the new definition use the new definition.
+
+ DIRECT THREADED CODE ----------------------------------------------------------------------
+
+ Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea
+ or coffee and settle down. It's fair to say that if you don't understand this section, then you
+ won't "get" how FORTH works, and that would be a failure on my part for not explaining it well.
+ So if after reading this section a few times you don't understand it, please email me
+ (rich@annexia.org).
+
+ Let's talk first about what "threaded code" means. Imagine a peculiar version of C where
+ you are only allowed to call functions without arguments. (Don't worry for now that such a
+ language would be completely useless!) So in our peculiar C, code would look like this:
+
+ f ()
+ {
+ a ();
+ b ();
+ c ();
+ }
+
+ and so on. How would a function, say 'f' above, be compiled by a standard C compiler?
+ Probably into assembly code like this. On the right hand side I've written the actual
+ i386 machine code.
+
+ f:
+ CALL a E8 08 00 00 00
+ CALL b E8 1C 00 00 00
+ CALL c E8 2C 00 00 00
+ ; ignore the return from the function for now
+
+ "E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing
+ memory was hideously expensive and we might have worried about the wasted space being used
+ by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory)
+ by compressing this into just:
+
+ 08 00 00 00 Just the function addresses, without
+ 1C 00 00 00 the CALL prefix.
+ 2C 00 00 00
+
+ On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%.
+
+ [Historical note: If the execution model that FORTH uses looks strange from the following
+ paragraphs, then it was motivated entirely by the need to save memory on early computers.
+ This code compression isn't so important now when our machines have more memory in their L1
+ caches than those early computers had in total, but the execution model still has some
+ useful properties].
+
+ Of course this code won't run directly any more. Instead we need to write an interpreter
+ which takes each pair of bytes and calls it.
+
+ On an i386 machine it turns out that we can write this interpreter rather easily, in just
+ two assembly instructions which turn into just 3 bytes of machine code. Let's store the
+ pointer to the next word to execute in the %esi register:
+
+ 08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute.
+ %esi -> 1C 00 00 00
+ 2C 00 00 00
+
+ The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does
+ two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it
+ increments %esi by 4 bytes. So after LODSL, the situation now looks like this:
+
+ 08 00 00 00 <- We're still executing this one
+ 1C 00 00 00 <- %eax now contains this address (0x0000001C)
+ %esi -> 2C 00 00 00
+
+ Now we just need to jump to the address in %eax. This is again just a single x86 instruction
+ written JMP *(%eax). And after doing the jump, the situation looks like:
+
+ 08 00 00 00
+ 1C 00 00 00 <- Now we're executing this subroutine.
+ %esi -> 2C 00 00 00
+
+ To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)'
+ which literally make the jump to the next subroutine.
+
+ And that brings us to our first piece of actual code! Well, it's a macro.
+*/
-
+/* NEXT macro. */
+ .macro NEXT
+ lodsl
+ jmp *(%eax)
+ .endm
+/* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions.
+ Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like
+ a return.
+ The above describes what is known as direct threaded code.
+ To sum up: We compress our function calls down to a list of addresses and use a somewhat
+ magical macro to act as a "jump to next function in the list". We also use one register (%esi)
+ to act as a kind of instruction pointer, pointing to the next function in the list.
+ I'll just give you a hint of what is to come by saying that a FORTH definition such as:
+ : QUADRUPLE DOUBLE DOUBLE ;
+ actually compiles (almost, not precisely but we'll see why in a moment) to a list of
+ function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off.
-*/
+ At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!".
-/* NEXT macro. */
- .macro NEXT
- lodsl
- jmp *(%eax)
- .endm
+ I lied about JMP *(%eax).
+
+ INDIRECT THREADED CODE ----------------------------------------------------------------------
+
+ It turns out that direct threaded code is interesting but only if you want to just execute
+ a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE
+ was an assembly language function. In the direct threaded code, QUADRUPLE would look like:
+
+ +------------------+
+ | addr of DOUBLE --------------------> (assembly code to do the double)
+ +------------------+ NEXT
+ %esi -> | addr of DOUBLE |
+ +------------------+
+
+ We can add an extra indirection to allow us to run both words written in assembly language
+ (primitives written for speed) and words written in FORTH themselves as lists of addresses.
+
+ The extra indirection is the reason for the brackets in JMP *(%eax).
+
+ Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH:
+
+ : QUADRUPLE DOUBLE DOUBLE ;
+
+ +------------------+
+ | codeword | : DOUBLE DUP + ;
+ +------------------+
+ | addr of DOUBLE ---------------> +------------------+
+ +------------------+ | codeword |
+ | addr of DOUBLE | +------------------+
+ +------------------+ | addr of DUP --------------> +------------------+
+ | addr of EXIT | +------------------+ | codeword -------+
+ +------------------+ %esi -> | addr of + --------+ +------------------+ |
+ +------------------+ | | assembly to <-----+
+ | addr of EXIT | | | implement DUP |
+ +------------------+ | | .. |
+ | | .. |
+ | | NEXT |
+ | +------------------+
+ |
+ +-----> +------------------+
+ | codeword -------+
+ +------------------+ |
+ | assembly to <------+
+ | implement + |
+ | .. |
+ | .. |
+ | NEXT |
+ +------------------+
+
+ This is the part where you may need an extra cup of tea/coffee/favourite caffeinated
+ beverage. What has changed is that I've added an extra pointer to the beginning of
+ the definitions. In FORTH this is sometimes called the "codeword". The codeword is
+ a pointer to the interpreter to run the function. For primitives written in
+ assembly language, the "interpreter" just points to the actual assembly code itself.
+ They don't need interpreting, they just run.
+
+ In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter
+ function.
+
+ I'll show you the interpreter function shortly, but let's recall our indirect
+ JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE
+ as shown, and DUP has been called. Note that %esi is pointing to the address of +
+
+ The assembly code for DUP eventually does a NEXT. That:
+
+ (1) reads the address of + into %eax %eax points to the codeword of +
+ (2) increments %esi by 4
+ (3) jumps to the indirect %eax jumps to the address in the codeword of +,
+ ie. the assembly code to implement +
+
+ +------------------+
+ | codeword |
+ +------------------+
+ | addr of DOUBLE ---------------> +------------------+
+ +------------------+ | codeword |
+ | addr of DOUBLE | +------------------+
+ +------------------+ | addr of DUP --------------> +------------------+
+ | addr of EXIT | +------------------+ | codeword -------+
+ +------------------+ | addr of + --------+ +------------------+ |
+ +------------------+ | | assembly to <-----+
+ %esi -> | addr of EXIT | | | implement DUP |
+ +------------------+ | | .. |
+ | | .. |
+ | | NEXT |
+ | +------------------+
+ |
+ +-----> +------------------+
+ | codeword -------+
+ +------------------+ |
+ now we're | assembly to <-----+
+ executing | implement + |
+ this | .. |
+ function | .. |
+ | NEXT |
+ +------------------+
+
+ So I hope that I've convinced you that NEXT does roughly what you'd expect. This is
+ indirect threaded code.
+
+ I've glossed over four things. I wonder if you can guess without reading on what they are?
+
+ .
+ .
+ .
+
+ My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do
+ you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but
+ then point at part of DOUBLE. (3) What goes in the codeword for the words which are written
+ in FORTH? (4) How do you compile a function which does anything except call other functions
+ ie. a function which contains a number like : DOUBLE 2 * ; ?
+
+ THE INTERPRETER AND RETURN STACK ------------------------------------------------------------
+
+ Going at these in no particular order, let's talk about issues (3) and (2), the interpreter
+ and the return stack.
+
+ Words which are defined in FORTH need a codeword which points to a little bit of code to
+ give them a "helping hand" in life. They don't need much, but they do need what is known
+ as an "interpreter", although it doesn't really "interpret" in the same way that, say,
+ Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few
+ machine registers so that the word can then execute at full speed using the indirect
+ threaded model above.
+
+ One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old
+ %esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE.
+ Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like
+ a function call), we will need a stack to store these "return addresses" (old values of %esi).
+
+ As you will have 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.
+
+ 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
.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 COLD. COLD stands for cold-start. In ISO
+ FORTH (but not in this FORTH), COLD can be called at any time to completely reset
+ the state of FORTH, and there is another word called WARM which does a partial reset.
+*/
+
/* ELF entry point. */
.text
.globl _start
cold_start: // High-level code without a codeword.
.int COLD
-/* 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
+/*
+ We also allocate some space for the return stack and some space to store user
+ definitions. These are static memory allocations using fixed-size buffers, but it
+ wouldn't be a great deal of work to make them dynamic.
+*/
-/*----------------------------------------------------------------------
- * Fixed sized buffers for everything.
- */
.bss
-
/* FORTH return stack. */
-#define RETURN_STACK_SIZE 8192
+ .set RETURN_STACK_SIZE,8192
.align 4096
.space RETURN_STACK_SIZE
-return_stack:
+return_stack: // Initial top of return stack.
-/* Space for user-defined words. */
-#define USER_DEFS_SIZE 16384
+/* The user definitions area: space for user-defined words and general memory allocations. */
+ .set USER_DEFS_SIZE,16384
.align 4096
user_defs_start:
.space USER_DEFS_SIZE
+/*
+ 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
.align 4
.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.
+*/
+
+ .macro defcode name, namelen, flags=0, label
.section .rodata
.align 4
.globl name_\label
.align 4
.globl \label
\label :
- .int DOCOL // codeword - the interpreter
- // list of word pointers follow
- .endm
-
- .macro defvar name, namelen, flags=0, label, initial=0
- defcode \name,\namelen,\flags,\label
- push $var_\name
- NEXT
- .data
+ .int code_\label // codeword
+ .text
.align 4
-var_\name :
- .int \initial
+ .globl code_\label
+code_\label : // assembler code follows
.endm
- // Some easy ones, written in assembly for speed
- defcode "DROP",4,,DROP
- pop %eax // drop top of stack
- NEXT
+/*
+ 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 "DUP",3,,DUP
pop %eax // duplicate top of stack
push %eax
NEXT
+ defcode "DROP",4,,DROP
+ pop %eax // drop top of stack
+ NEXT
+
defcode "SWAP",4,,SWAP
pop %eax // swap top of stack
pop %ebx
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
1: pushl $0
NEXT
+ defcode "<",1,,LT
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ jl 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode ">",1,,GT
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ jg 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode "<=",2,,LE
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ jle 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode ">=",2,,GE
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ jge 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
defcode "0=",2,,ZEQU // top of stack equals 0?
pop %eax
test %eax,%eax
1: pushl $1
NEXT
+ defcode "0<>",3,,ZNEQU // top of stack not 0?
+ pop %eax
+ test %eax,%eax
+ jnz 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode "0<",2,,ZLT
+ pop %eax
+ test %eax,%eax
+ jl 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode "0>",2,,ZGT
+ pop %eax
+ test %eax,%eax
+ jg 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode "0<=",3,,ZLE
+ pop %eax
+ test %eax,%eax
+ jle 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
+ defcode "0>=",3,,ZGE
+ pop %eax
+ test %eax,%eax
+ jge 1f
+ pushl $0
+ NEXT
+1: pushl $1
+ NEXT
+
defcode "AND",3,,AND
pop %eax
andl %eax,(%esp)
orl %eax,(%esp)
NEXT
- defcode "INVERT",6,,INVERT
+ defcode "XOR",3,,XOR
+ pop %eax
+ xorl %eax,(%esp)
+ NEXT
+
+ defcode "INVERT",6,,INVERT // this is the FORTH "NOT" function
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 literal 2), grabs it, pushes it on the stack, then
+ manipulates %esi in order to skip the literal 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
push %eax // push value onto stack
NEXT
- // The STATE variable is 0 for execute mode, != 0 for compile mode
- defvar "STATE",5,,STATE
+/*
+ BUILT-IN VARIABLES ----------------------------------------------------------------------
- // This points to where compiled words go.
- defvar "HERE",4,,HERE,user_defs_start
+ 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 is the last definition in the dictionary.
- defvar "LATEST",6,,LATEST,name_SYSEXIT // SYSEXIT must be last in built-in dictionary
+ 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
- // _X, _Y and _Z are scratch variables used by standard words.
+/*
+ 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.
+ _X These are three scratch variables, used by some standard dictionary words.
+ _Y
+ _Z
+ S0 Stores the address of the top of the parameter stack.
+
+*/
+ defvar "STATE",5,,STATE
+ defvar "HERE",4,,HERE,user_defs_start
+ defvar "LATEST",6,,LATEST,name_SYSEXIT // SYSEXIT must be last in built-in dictionary
defvar "_X",2,,TX
defvar "_Y",2,,TY
defvar "_Z",2,,TZ
-
- // This stores the top of the data stack.
defvar "S0",2,,SZ
- // This stores the top of the return stack.
- defvar "R0",2,,RZ,return_stack
+/*
+ BUILT-IN CONSTANTS ----------------------------------------------------------------------
- defcode "DSP@",4,,DSPFETCH
- mov %esp,%eax
- push %eax
- NEXT
+ It's also useful to expose a few constants to FORTH. When the word is executed it pushes a
+ constant value on the stack.
- defcode "DSP!",4,,DSPSTORE
- pop %esp
+ 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.
+*/
+
+ .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
+ 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
+
+/*
+ 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
lea 4(%ebp),%ebp // pop return stack and throw away
NEXT
-#include <asm-i386/unistd.h>
+/*
+ PARAMETER (DATA) STACK ----------------------------------------------------------------------
- defcode "KEY",3,,KEY
- call _KEY
- push %eax // push return value on 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
-_KEY:
+
+ 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 the
+ program). 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.
+*/
+
+#include <asm-i386/unistd.h>
+
+ defcode "KEY",3,,KEY
+ call _KEY
+ push %eax // push return value on stack
+ NEXT
+_KEY:
mov (currkey),%ebx
cmp (bufftop),%ebx
jge 1f
mov $__NR_exit,%eax // syscall: exit
int $0x80
+/*
+ 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
.bss
2: .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 address at the top).
+
+ 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, so
+ in some crazy-world you could define FORTH words containing ASCII NULs, although why
+ you'd want to is a bit beyond me.
+
+ 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
// overwrite this buffer. Maximum word length is 32 chars.
5: .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
-
- mov $__NR_write,%eax // write syscall
- int $0x80
+/*
+ . (also called DOT) prints the top of the stack as an integer. In real FORTH implementations
+ it should print it in the current base, but this assembler version is simpler and can only
+ print in base 10.
- NEXT
+ Remember that you can override even built-in FORTH words easily, so if you want to write a
+ more advanced DOT then you can do so easily at a later point, and probably in FORTH.
+*/
defcode ".",1,,DOT
pop %eax // Get the number to print into %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.
+/*
+ Almost the opposite of DOT (but not quite), SNUMBER parses a numeric string such as one returned
+ by WORD and pushes the number on the parameter stack.
+
+ This function does absolutely no error checking, and in particular the length of the string
+ must be >= 1 bytes, and should contain only digits 0-9. If it doesn't you'll get random results.
+
+ This function is only used when reading literal numbers in code, and shouldn't really be used
+ in user code at all.
+*/
defcode "SNUMBER",7,,SNUMBER
pop %edi
pop %ecx
jnz 1b
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
// 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
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 |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+
+ 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.
+
+ 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 |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+
+ (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 INTERPRETER 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.
- defcode ":",1,,COLON
+ 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).
- // Get the word and create a dictionary entry header for it.
+ 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 word.
call _WORD // Returns %ecx = length, %edi = pointer to word.
mov %edi,%ebx // %ebx = address of the word
+ // 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
+ data area 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 CREATE // CREATE the dictionary entry / header
+ .int LIT, DOCOL, COMMA // Append DOCOL (the codeword).
+ .int 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 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
+
+/*
+ HIDDEN toggles the other flag, F_HIDDEN, of the latest word. Note that words flagged
+ as hidden are defined but cannot be called, so this is only used when you are trying to
+ hide the word as it is being defined.
+*/
- 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
+ defcode "HIDDEN",6,,HIDDEN
+ movl var_LATEST,%edi // LATEST word.
+ 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. */
+/*
+ ' (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
+
+/*
+ PRINTING STRINGS ----------------------------------------------------------------------
+
+ LITSTRING and EMITSTRING are primitives used to implement the ." operator (which is
+ written in FORTH). See the definition of that operator below.
+*/
+
+ 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 "EMITSTRING",10,,EMITSTRING
+ mov $1,%ebx // 1st param: stdout
+ pop %ecx // 2nd param: address of string
+ pop %edx // 3rd param: length of string
+ mov $__NR_write,%eax // write syscall
+ int $0x80
+ NEXT
+
+/*
+ COLD START AND INTERPRETER ----------------------------------------------------------------------
+
+ COLD is the first FORTH function called, almost immediately after the FORTH system "boots".
+
+ INTERPRETER is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate
+ description -- see: http://en.wikipedia.org/wiki/REPL).
+*/
+
+ // COLD must not return (ie. must not call EXIT).
+ defword "COLD",4,,COLD
+ .int INTERPRETER // call the interpreter loop (never returns)
+ .int LIT,1,SYSEXIT // hmmm, but in case it does, exit(1).
+
/* This interpreter is pretty simple, but remember that in FORTH you can always override
* it later with a more powerful one!
*/
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.
+
+ SYSEXIT exits the process using Linux exit syscall.
+
+ In this FORTH, SYSEXIT 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 SYSEXIT, 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
+
// NB: SYSEXIT must be the last entry in the built-in dictionary.
defcode SYSEXIT,7,,SYSEXIT
pop %ebx
mov $__NR_exit,%eax
int $0x80
-/*----------------------------------------------------------------------
- * Input buffer & initial input.
- */
+/*
+ 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.
+
+ As a kind of trick, I prefill the input buffer with the initial FORTH code. Once this code
+ has run (when we get to the "OK" prompt), this input buffer is reused for reading any further
+ user input.
+
+ Some notes about the code:
+
+ \ (backslash) is the FORTH way to start a comment which goes up to the next newline. However
+ because this is a C-style string, I have to escape the backslash, which is why they appear as
+ \\ comment.
+
+ Similarly, any backslashes in the code are doubled, and " becomes \" (eg. the definition of ."
+ is written as : .\" ... ;)
+
+ 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!
+
+ Enjoy!
+*/
+
.data
.align 4096
buffer:
- // XXX gives 'Warning: unterminated string; newline inserted' messages which you can ignore
+ // Multi-line constant 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 @ ;
: 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
- ;
-
-: ]
- 1 STATE ! \\ go back to compile mode
- ;
+\\ The primitive . (DOT) function doesn't follow with a blank, so redefine it to behave like FORTH.
+\\ Notice how we can trivially redefine existing words. Word definitions are not recursive by
+\\ default, but see below for the RECURSE word.
+: .
+ . SPACE \\ call built-in DOT, then print a space.
+;
\\ LITERAL takes whatever is on the stack and compiles LIT <foo>
: LITERAL IMMEDIATE
, \\ compile the literal itself (from the stack)
;
+\\ Now we can use [ and ] to insert literals which are calculated at compile time.
+\\ 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 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 ;
+
+\\ 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'
+\\ -- 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
+\\ -- 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
;
\\ BEGIN loop-part condition UNTIL
-\\ compiles to:
-\\ loop-part condition 0BRANCH OFFSET
-\\ where OFFSET points back to the loop-part
+\\ -- 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
;
\\ BEGIN loop-part AGAIN
-\\ compiles to:
-\\ loop-part BRANCH OFFSET
-\\ where OFFSET points back to the loop-part
+\\ -- 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
;
\\ 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
+\\ -- 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
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
+\\ 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
- SPACE \\ print a space
- 1- \\ until we count down to 0
- DUP 0=
- UNTIL
+ 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
;
-\\ .S prints the contents of the stack. Very useful for debugging.
-: .S
- DSP@ \\ get current stack pointer
+(
+ From now on we can use ( ... ) for comments.
+
+ 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
+)
+
+( With the looping constructs, we can now write SPACES, which writes n spaces to stdout. )
+: SPACES ( n -- )
BEGIN
- DUP @ . \\ print the stack element
- 4+ \\ move up
- DUP S0 @ 4- = \\ stop when we get to the top
- UNTIL
+ DUP 0> ( while n > 0 )
+ WHILE
+ SPACE ( print a space )
+ 1- ( until we count down to 0 )
+ REPEAT
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
+( .S prints the contents of the stack. Very useful for debugging. )
+: .S ( -- )
+ DSP@ ( get current stack pointer )
+ BEGIN
+ DUP S0 @ <
+ WHILE
+ DUP @ . ( print the stack element )
+ 4+ ( move up )
+ REPEAT
+ DROP
+;
+
+( DEPTH returns the depth of the stack. )
+: DEPTH ( -- n )
+ S0 @ DSP@ -
+ 4- ( adjust because S0 was on the stack when we pushed DSP )
+;
+
+(
+ [NB. The following may be a bit confusing because of the need to use backslash before
+ each double quote character. The backslashes are there to keep the assembler happy.
+ They are NOT part of the final output. So here we are defining a function called
+ 'dot double-quote' (not 'dot backslash double-quote').]
+
+ .\" 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 @ 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
+ 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
+ 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
+ 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
+ ' 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!
+ ( 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
+ DUP '\"' = IF
+ DROP ( drop the double quote character )
+ EXIT ( return from this function )
+ 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
+(
+ 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 by 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
+
+ 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 CREATE TEN 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 ;
+)
+: CONSTANT
+ CREATE ( make the dictionary entry (the name follows CONSTANT) )
+ 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 )
;
-\\ 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
+(
+ VARIABLE is a little bit harder because we need somewhere to put the variable. There is
+ nothing particularly special about the 'user definitions area' (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 |
+ +---------+---------+---+---+---+---+------------+------------+---|--------+------------+
+ | <var> | LINK | 3 | V | A | R | DOCOL | LIT | <addr var> | EXIT |
+ +---------+---------+---+---+---+---+------------+------------+------------+------------+
+ len codeword
+
+ where <var> is the place to store the variable, and <addr var> 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 definitions area.
+
+ 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 n 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 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 )
+ 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 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 directly on the stack
+ 30 TO VAL updates VAL, setting it to 30
+
+ 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
+ particular 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 <addr> !
+ and calculating <addr> (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 | <value> | EXIT |
+ +---------+---+---+---+---+------------+------------+------------+------------+
+ len codeword
+
+ where <value> 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 <addr>? Why of course a pointer to that <value>:
+
+ code compiled - - - - --+------------+------------+------------+-- - - - -
+ by TO VAL | LIT | <addr> | ! |
+ - - - - --+------------+-----|------+------------+-- - - - -
+ |
+ V
+ +---------+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 3 | V | A | L | DOCOL | LIT | <value> | 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 -- )
+ 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
+;
+
+(
+ 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 @b ( 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 @b ( 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
+ DROP DROP ( len addr -- )
+;
+
+(
+ WORDS prints all the words defined in the dictionary, starting with the word defined most recently.
+
+ The implementation simply iterates backwards from LATEST using the link pointers.
+)
+: WORDS
+ LATEST @ ( start at LATEST dictionary entry )
+ BEGIN
+ DUP 0<> ( while link pointer is not null )
+ WHILE
+ DUP ID. ( print the word )
+ SPACE
+ @ ( dereference the link pointer - go to previous word )
+ REPEAT
+ DROP
+ CR
+;
+
+(
+ 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 )
;
-\\ ALLOT is used to allocate (static) memory when compiling. It increases HERE by
-\\ the amount given on the stack.
-: ALLOT HERE +! ;
+( 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.
-\\ Finally print the welcome prompt.
+ 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 @ >CFA ( LATEST points to the word being compiled at the moment )
+ , ( compile it )
+;
+
+( Finally print the welcome prompt. )
+.\" JONESFORTH VERSION \" VERSION . CR
.\" OK \"
"
.int buffer
bufftop:
.int _initbufftop
+
+/* END OF jonesforth.S */
git.annexia.org / jonesforth.git / blobdiff