/* 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 1.19 2007-09-08 22:51:28 rich Exp $
+ $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,19
+ .set JONES_VERSION,47
/*
INTRODUCTION ----------------------------------------------------------------------
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.
- Also I used this document (http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design) which really
- defies easy explanation.
+ 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 ----------------------------------------------------------------------
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
-
- 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.
+ gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S
+ cat jonesforth.f - | ./jonesforth
If you want to run your own FORTH programs you can do:
- ./jonesforth < myprog.f
+ 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 myfunctions.f - | ./jonesforth
+ 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.
|
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);
and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set
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
+-----> +------------------+
| 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")
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.
+ 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
-
-/*
- 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.
-*/
-
- .bss
-/* FORTH return stack. */
-#define RETURN_STACK_SIZE 8192
- .align 4096
- .space RETURN_STACK_SIZE
-return_stack: // Initial top of 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
+ 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
unsure of them).
The long way would be:
+
.int <link to previous word>
.byte 6 // len
.ascii "DOUBLE" // string
*/
/* Flags - these are discussed later. */
-#define F_IMMED 0x80
-#define F_HIDDEN 0x20
+ .set F_IMMED,0x80
+ .set F_HIDDEN,0x20
+ .set F_LENMASK,0x1f // length mask
// Store the chain of links.
.set link,0
.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
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
.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
+ //.align 4
.globl code_\label
code_\label : // assembler code follows
.endm
you can skip the details.
*/
- defcode "DUP",3,,DUP
- pop %eax // duplicate top of stack
- push %eax
- 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 %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
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
-1: pushl $0
+
+ defcode "<",1,,LT
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ setl %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode ">",1,,GT
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ setg %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "<=",2,,LE
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ setle %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode ">=",2,,GE
+ pop %eax
+ pop %ebx
+ cmp %eax,%ebx
+ setge %al
+ movzbl %al,%eax
+ pushl %eax
NEXT
defcode "0=",2,,ZEQU // top of stack equals 0?
pop %eax
test %eax,%eax
- jz 1f
- pushl $0
+ setz %al
+ movzbl %al,%eax
+ pushl %eax
NEXT
-1: pushl $1
+
+ defcode "0<>",3,,ZNEQU // top of stack not 0?
+ pop %eax
+ test %eax,%eax
+ setnz %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "0<",2,,ZLT // comparisons with 0
+ pop %eax
+ test %eax,%eax
+ setl %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "0>",2,,ZGT
+ pop %eax
+ test %eax,%eax
+ setg %al
+ movzbl %al,%eax
+ pushl %eax
NEXT
- defcode "AND",3,,AND
+ defcode "0<=",3,,ZLE
+ pop %eax
+ test %eax,%eax
+ setle %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "0>=",3,,ZGE
+ pop %eax
+ test %eax,%eax
+ setge %al
+ movzbl %al,%eax
+ pushl %eax
+ NEXT
+
+ defcode "AND",3,,AND // bitwise AND
pop %eax
andl %eax,(%esp)
NEXT
- defcode "OR",2,,OR
+ defcode "OR",2,,OR // bitwise OR
pop %eax
orl %eax,(%esp)
NEXT
- defcode "INVERT",6,,INVERT // this is the FORTH "NOT" function
+ 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
| addr of EXIT |
+------------------+
- And NEXT just completes the job by, well in this case just by calling DOUBLE again :-)
+ And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-)
LITERALS ----------------------------------------------------------------------
+---------------------------+-------+-------+-------+-------+-------+
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.
+ 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,
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
+/* C@C! is a useful byte copy primitive. */
+ defcode "C@C!",4,,CCOPY
+ movl 4(%esp),%ebx // source address
+ movb (%ebx),%al // get source character
+ pop %edi // destination address
+ stosb // copy to destination
+ push %edi // increment destination address
+ incl 4(%esp) // increment source address
+ NEXT
+
+/* and CMOVE is a block copy operation. */
+ defcode "CMOVE",5,,CMOVE
+ mov %esi,%edx // preserve %esi
+ pop %ecx // length
+ pop %edi // destination address
+ pop %esi // source address
+ rep movsb // copy source to destination
+ mov %edx,%esi // restore %esi
+ NEXT
+
/*
BUILT-IN VARIABLES ----------------------------------------------------------------------
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.
- R0 Stores the address of the top of the return stack.
- VERSION Is the current version of this FORTH.
+ BASE The current base for printing and reading numbers.
*/
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
+ defvar "HERE",4,,HERE
+ defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary
defvar "S0",2,,SZ
- defvar "R0",2,,RZ,return_stack
- defvar "VERSION",7,,VERSION,JONES_VERSION
+ 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 ----------------------------------------------------------------------
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
/*
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
+ 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.
-*/
-#include <asm-i386/unistd.h>
+ 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
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 address at the top).
+ 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, so
- in some crazy-world you could define FORTH words containing ASCII NULs, although why
- you'd want to is a bit beyond me.
+ 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.
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
/*
- . (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.
-
- 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 _DOT // Easier to do this recursively ...
- 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
+ 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.
-/*
- 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.
+ 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.
- 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.
+ If the word starts with a '-' character then the returned value is negative.
- This function is only used when reading literal numbers in code, and shouldn't really be used
- in user code at all.
+ 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 "SNUMBER",7,,SNUMBER
- pop %edi
- pop %ecx
- call _SNUMBER
- push %eax
+ 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
-_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
- subb $'0',%bl // ASCII -> digit
+ push %eax // push 0 on stack
+ cmpb $'-',%bl // negative number?
+ jnz 2f
+ pop %eax
+ push %ebx // push <> 0 on stack, indicating negative
+ dec %ecx
+ jnz 1f
+ pop %ebx // error: string is only '-'.
+ movl $1,%ecx
+ ret
+
+ // Loop reading digits.
+1: imull %edx,%eax // %eax *= BASE
+ movb (%edi),%bl // %bl = next character in string
+ inc %edi
+
+ // Convert 0-9, A-Z to a number 0-35.
+2: subb $'0',%bl // < '0'?
+ jb 4f
+ cmp $10,%bl // <= '9'?
+ jb 3f
+ subb $17,%bl // < 'A'? (17 is 'A'-'0')
+ jb 4f
+ addb $10,%bl
+
+3: cmp %dl,%bl // >= BASE?
+ jge 4f
+
+ // OK, so add it to %eax and loop.
add %ebx,%eax
dec %ecx
jnz 1b
- 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 ----------------------------------------------------------------------
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
- See also >CFA which takes a dictionary entry pointer and returns a pointer to the codeword.
+ 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.
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ codeword
Notes:
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.
+ 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".
*/
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
/*
+ Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and
+ returns a pointer to the first data field.
+
+ FIND returns a pointer to this
+ | >CFA converts it to a pointer to this
+ | |
+ | | >DFA converts it to a pointer to this
+ | | |
+ V V V
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ | LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT |
+ +---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+
+ codeword
+
+ (Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is
+ different from theirs, because they have an extra indirection).
+
+ You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA.
+*/
+
+ defword ">DFA",4,,TDFA
+ .int TCFA // >CFA (get code field address)
+ .int INCR4 // 4+ (add 4 to it to get to next word)
+ .int EXIT // EXIT (return from FORTH word)
+
+/*
COMPILING ----------------------------------------------------------------------
Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this:
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
+ 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.
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).
+ 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:
: DOUBLE DUP + ;
^
|
- Next byte returned by KEY
+ 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,
- gets its codeword pointer, and appends it:
+ 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.
|
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_.
- I hope I don't need to explain that ; (SEMICOLON) just such a word, flagged as IMMEDIATE.
+ 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.
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
being compiled. This prevents FIND from finding it, and thus in theory stops any
chance of it being called.
- Compared to the description above, the actual definition of : (COLON) is comparatively simple:
+ 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 ":",1,,COLON
+ defcode "CREATE",6,,CREATE
- // 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
+ // 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
/*
- , (COMMA) is a standard FORTH word which appends a 32 bit integer (normally a codeword
- pointer) to the user data area pointed to by HERE, and adds 4 to HERE.
+ 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
ret
/*
- ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag.
+ 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,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 "[",1,F_IMMED,LBRAC
+ xor %eax,%eax
+ movl %eax,var_STATE // Set STATE to 0.
NEXT
+ defcode "]",1,,RBRAC
+ movl $1,var_STATE // Set STATE to 1.
+ NEXT
+
+/*
+ Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets
+ the word HIDDEN and goes into compile mode.
+*/
+
+ defword ":",1,,COLON
+ .int WORD // Get the name of the new word
+ .int CREATE // CREATE the dictionary entry / header
+ .int LIT, DOCOL, COMMA // Append DOCOL (the codeword).
+ .int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition).
+ .int RBRAC // Go into compile mode.
+ .int EXIT // Return from the function.
+
+/*
+ ; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag.
+*/
+
+ defword ";",1,F_IMMED,SEMICOLON
+ .int LIT, EXIT, COMMA // Append EXIT (so the word will return).
+ .int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition).
+ .int LBRAC // Go back to IMMEDIATE mode.
+ .int EXIT // Return from the function.
+
/*
EXTENDING THE COMPILER ----------------------------------------------------------------------
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
+ 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,
*/
defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE
- call _IMMEDIATE
- NEXT
-_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 rarely used.
+ 'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the
+ most recently defined word (used above in : and ; definitions) you would do:
+
+ LATEST @ HIDDEN
+
+ 'HIDE word' toggles the flag on a named 'word'.
+
+ Setting this flag stops the word from being found by FIND, and so can be used to make 'private'
+ words. For example, to break up a large word into smaller parts you might do:
+
+ : SUB1 ... subword ... ;
+ : SUB2 ... subword ... ;
+ : SUB3 ... subword ... ;
+ : MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ;
+ HIDE SUB1
+ HIDE SUB2
+ HIDE SUB3
+
+ After this, only MAIN is 'exported' or seen by the rest of the program.
*/
defcode "HIDDEN",6,,HIDDEN
- call _HIDDEN
- NEXT
-_HIDDEN:
- movl var_LATEST,%edi // LATEST word.
+ pop %edi // Dictionary entry.
addl $4,%edi // Point to name/flags byte.
xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit.
- ret
+ NEXT
+
+ defword "HIDE",4,,HIDE
+ .int WORD // Get the word (after HIDE).
+ .int FIND // Look up in the dictionary.
+ .int HIDDEN // Set F_HIDDEN flag.
+ .int EXIT // Return.
/*
' (TICK) is a standard FORTH word which returns the codeword pointer of the next word.
BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the
top of stack is zero).
- The diagra below shows how BRANCH works in some imaginary compiled word. When BRANCH executes,
+ 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):
+---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+
NEXT
/*
- PRINTING STRINGS ----------------------------------------------------------------------
+ LITERAL STRINGS ----------------------------------------------------------------------
- LITSTRING and EMITSTRING are primitives used to implement the ." operator (which is
- written in FORTH). See the definition of that operator below.
+ 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 %eax // push it on the stack
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 "EMITSTRING",10,,EMITSTRING
+ defcode "TELL",4,,TELL
mov $1,%ebx // 1st param: stdout
- pop %ecx // 2nd param: address of string
pop %edx // 3rd param: length of string
+ pop %ecx // 2nd param: address of string
mov $__NR_write,%eax // write syscall
int $0x80
NEXT
/*
- COLD START AND INTERPRETER ----------------------------------------------------------------------
+ QUIT AND INTERPRET ----------------------------------------------------------------------
- COLD is the first FORTH function called, almost immediately after the FORTH system "boots".
+ 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.
- INTERPRETER is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate
+ 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)
- // 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!
+/*
+ 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
-
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
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.
+ 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
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
+ 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
-/*
- START OF FORTH CODE ----------------------------------------------------------------------
+ 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
- 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.
+ 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
- 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.
+ defcode "SYSCALL0",8,,SYSCALL0
+ pop %eax // System call number (see <asm/unistd.h>)
+ int $0x80
+ push %eax // Result (negative for -errno)
+ NEXT
- Some notes about the code:
+/*
+ DATA SEGMENT ----------------------------------------------------------------------
- \ (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.
+ 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.
- Similarly, any backslashes in the code are doubled, and " becomes \" (eg. the definition of ."
- is written as : .\" ... ;)
+ 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.
- 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.
+ 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).
- FORTH is case-sensitive. Use capslock!
+ 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)).
- Enjoy!
+ 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
- .data
+/*
+ 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:
- // 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 @ ;
-\\ : 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.
-.\" JONESFORTH VERSION \" VERSION @ . CR
-.\" 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