Started writing.

[jonesforth.git] / jonesforth.S

/*      A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*-
        By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth

        gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S

        INTRODUCTION ----------------------------------------------------------------------

        FORTH is one of those alien languages which most working programmers regard in the same
        way as Haskell, LISP, and so on.  Something so strange that they'd rather any thoughts
        of it just go away so they can get on with writing this paying code.  But that's wrong
        and if you care at all about programming then you should at least understand all these
        languages, even if you will never use them.

        LISP is the ultimate high-level language, and features from LISP are being added every
        decade to the more common languages.  But FORTH is in some ways the ultimate in low level
        programming.  Out of the box it lacks features like dynamic memory management and even
        strings.  In fact, at its primitive level it lacks even basic concepts like IF-statements
        and loops.

        Why then would you want to learn FORTH?  There are several very good reasons.  First
        and foremost, FORTH is minimal.  You really can write a complete FORTH in, say, 2000
        lines of code.  I don't just mean a FORTH program, I mean a complete FORTH operating
        system, environment and language.  You could boot such a FORTH on a bare PC and it would
        come up with a prompt where you could start doing useful work.  The FORTH you have here
        isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making
        it a good tutorial). It's possible to completely understand the system.  Who can say they
        completely understand how Linux works, or gcc?

        Secondly FORTH has a peculiar bootstrapping property.  By that I mean that after writing
        a little bit of assembly to talk to the hardware and implement a few primitives, all the
        rest of the language and compiler is written in FORTH itself.  Remember I said before
        that FORTH lacked IF-statements and loops?  Well of course it doesn't really because
        such a lanuage would be useless, but my point was rather that IF-statements and loops are
        written in FORTH itself.

        Now of course this is common in other languages as well, and in those languages we call
        them 'libraries'.  For example in C, 'printf' is a library function written in C.  But
        in FORTH this goes way beyond mere libraries.  Can you imagine writing C's 'if' in C?
        And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict
        yourself to the usual if/while/for/switch constructs?  You want a construct that iterates
        over every other element in a list of numbers?  You can add it to the language.  What
        about an operator which pulls in variables directly from a configuration file and makes
        them available as FORTH variables?  Or how about adding Makefile-like dependencies to
        the language?  No problem in FORTH.  This concept isn't common in programming languages,
        but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not
        the lame C preprocessor) and "domain specific languages" (DSLs).

        This tutorial isn't about learning FORTH as the language.  I'll point you to some references
        you should read if you're not familiar with using FORTH.  This tutorial is about how to
        write FORTH.  In fact, until you understand how FORTH is written, you'll have only a very
        superficial understanding of how to use it.

        So if you're not familiar with FORTH or want to refresh your memory here are some online
        references to read:

        http://en.wikipedia.org/wiki/Forth_%28programming_language%29

        http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm

        http://wiki.laptop.org/go/Forth_Lessons

        Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html

        SETTING UP ----------------------------------------------------------------------

        Let's get a few housekeeping things out of the way.  Firstly because I need to draw lots of
        ASCII-art diagrams to explain concepts, the best way to look at this is using a window which
        uses a fixed width font and is at least this wide:

 <------------------------------------------------------------------------------------------------------------------------>

        ASSEMBLING ----------------------------------------------------------------------

        If you want to actually run this FORTH, rather than just read it, you will need Linux on an
        i386.  Linux because instead of programming directly to the hardware on a bare PC which I
        could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux
        process with a few basic system calls (read, write and exit and that's about all).  i386
        is needed because I had to write the assembly for a processor, and i386 is by far the most
        common.  (Of course when I say 'i386', any 32- or 64-bit x86 processor will do.  I'm compiling
        this on a 64 bit AMD Opteron).

        Again, to assemble this you will need gcc and gas (the GNU assembler).  The commands to
        assemble and run the code (save this file as 'jonesforth.S') are:

        gcc -m32 -nostdlib -static -Wl,-Ttext,0 -o jonesforth jonesforth.S
        ./jonesforth

        You will see lots of 'Warning: unterminated string; newline inserted' messages from the
        assembler.  That's just because the GNU assembler doesn't have a good syntax for multi-line
        strings (or rather it used to, but the developers removed it!) so I've abused the syntax
        slightly to make things readable.  Ignore these warnings.

        ASSEMBLER ----------------------------------------------------------------------

        (You can just skip to the next section -- you don't need to be able to read assembler to
        follow this tutorial).

        However if you do want to read the assembly code here are a few notes about gas (the GNU assembler):

        (1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator.  The registers
            available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them
            have special purposes.

        (2) Add, mov, etc. take arguments in the form SRC,DEST.  So mov %eax,%ecx moves %eax -> %ecx

        (3) Constants are prefixed with '$', and you mustn't forget it!  If you forget it then it
            causes a read from memory instead, so:
            mov $2,%eax         moves number 2 into %eax
            mov 2,%eax          reads the 32 bit word from address 2 into %eax (ie. most likely a mistake)

        (4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards"
            and '1b' (etc.) means label '1:' "backwards".

        (5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc.

        (6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and
            less repetitive.

        For more help reading the assembler, do "info gas" at the Linux prompt.

        Now the tutorial starts in earnest.

        INDIRECT THREADED CODE ----------------------------------------------------------------------

        








*/

/* NEXT macro. */
        .macro NEXT
        lodsl
        jmp *(%eax)
        .endm

/* Macros to deal with the return stack. */
        .macro PUSHRSP reg
        lea -4(%ebp),%ebp       // push reg on to return stack
        movl \reg,(%ebp)
        .endm

        .macro POPRSP reg
        mov (%ebp),\reg         // pop top of return stack to reg
        lea 4(%ebp),%ebp
        .endm

/* ELF 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 $cold_start,%esi    // Initialise interpreter.
        NEXT                    // Run interpreter!

        .section .rodata
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

/*----------------------------------------------------------------------
 * Fixed sized buffers for everything.
 */
        .bss

/* FORTH return stack. */
#define RETURN_STACK_SIZE 8192
        .align 4096
        .space RETURN_STACK_SIZE
return_stack:

/* Space for user-defined words. */
#define USER_DEFS_SIZE 16384
        .align 4096
user_defs_start:
        .space USER_DEFS_SIZE






/*----------------------------------------------------------------------
 * Built-in words defined the long way.
 */
#define F_IMMED 0x80
#define F_HIDDEN 0x20

        // Store the chain of links.
        .set link,0

        .macro defcode name, namelen, flags=0, label
        .section .rodata
        .align 4
        .globl name_\label
name_\label :
        .int link               // link
        .set link,name_\label
        .byte \flags+\namelen   // flags + length byte
        .ascii "\name"          // the name
        .align 4
        .globl \label
\label :
        .int code_\label        // codeword
        .text
        .align 4
        .globl code_\label
code_\label :                   // assembler code follows
        .endm

        .macro defword name, namelen, flags=0, label
        .section .rodata
        .align 4
        .globl name_\label
name_\label :
        .int link               // link
        .set link,name_\label
        .byte \flags+\namelen   // flags + length byte
        .ascii "\name"          // the name
        .align 4
        .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
        .align 4
var_\name :
        .int \initial
        .endm

        // Some easy ones, written in assembly for speed
        defcode "DROP",4,,DROP
        pop %eax                // drop top of stack
        NEXT

        defcode "DUP",3,,DUP
        pop %eax                // duplicate top of stack
        push %eax
        push %eax
        NEXT

        defcode "SWAP",4,,SWAP
        pop %eax                // swap top of stack
        pop %ebx
        push %eax
        push %ebx
        NEXT

        defcode "OVER",4,,OVER
        mov 4(%esp),%eax        // get the second element of stack
        push %eax               // and push it on top
        NEXT

        defcode "ROT",3,,ROT
        pop %eax
        pop %ebx
        pop %ecx
        push %eax
        push %ecx
        push %ebx
        NEXT

        defcode "-ROT",4,,NROT
        pop %eax
        pop %ebx
        pop %ecx
        push %ebx
        push %eax
        push %ecx
        NEXT

        defcode "1+",2,,INCR
        incl (%esp)             // increment top of stack
        NEXT

        defcode "1-",2,,DECR
        decl (%esp)             // decrement top of stack
        NEXT

        defcode "4+",2,,INCR4
        addl $4,(%esp)          // increment top of stack
        NEXT

        defcode "4-",2,,DECR4
        subl $4,(%esp)          // decrement top of stack
        NEXT

        defcode "+",1,,ADD
        pop %eax
        addl %eax,(%esp)
        NEXT

        defcode "-",1,,SUB
        pop %eax
        subl %eax,(%esp)
        NEXT

        defcode "*",1,,MUL
        pop %eax
        pop %ebx
        imull %ebx,%eax
        push %eax               // ignore overflow
        NEXT

        defcode "/",1,,DIV
        xor %edx,%edx
        pop %ebx
        pop %eax
        idivl %ebx
        push %eax               // push quotient
        NEXT

        defcode "MOD",3,,MOD
        xor %edx,%edx
        pop %ebx
        pop %eax
        idivl %ebx
        push %edx               // push remainder
        NEXT

        defcode "=",1,,EQU      // top two words are equal?
        pop %eax
        pop %ebx
        cmp %ebx,%eax
        je 1f
        pushl $0
        NEXT
1:      pushl $1
        NEXT

        defcode "<>",2,,NEQU    // top two words are not equal?
        pop %eax
        pop %ebx
        cmp %ebx,%eax
        je 1f
        pushl $1
        NEXT
1:      pushl $0
        NEXT

        defcode "0=",2,,ZEQU    // top of stack equals 0?
        pop %eax
        test %eax,%eax
        jz 1f
        pushl $0
        NEXT
1:      pushl $1
        NEXT

        defcode "AND",3,,AND
        pop %eax
        andl %eax,(%esp)
        NEXT

        defcode "OR",2,,OR
        pop %eax
        orl %eax,(%esp)
        NEXT

        defcode "INVERT",6,,INVERT
        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).

        defcode "EXIT",4,,EXIT
        POPRSP %esi             // pop return stack into %esi
        NEXT

        defcode "LIT",3,,LIT
        // %esi points to the next command, but in this case it points to the next
        // literal 32 bit integer.  Get that literal into %eax and increment %esi.
        // On x86, it's a convenient single byte instruction!  (cf. NEXT macro)
        lodsl
        push %eax               // push the literal number on to stack
        NEXT

        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

        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

        defcode "!",1,,STORE
        pop %ebx                // address to store at
        pop %eax                // data to store there
        mov %eax,(%ebx)         // store it
        NEXT

        defcode "@",1,,FETCH
        pop %ebx                // address to fetch
        mov (%ebx),%eax         // fetch it
        push %eax               // push value onto stack
        NEXT

        defcode "+!",2,,ADDSTORE
        pop %ebx                // address
        pop %eax                // the amount to add
        addl %eax,(%ebx)        // add it
        NEXT

        defcode "-!",2,,SUBSTORE
        pop %ebx                // address
        pop %eax                // the amount to subtract
        subl %eax,(%ebx)        // add it
        NEXT

/* ! and @ (STORE and FETCH) store 32-bit words.  It's also useful to be able to read and write bytes.
 * 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.
 */
        defcode "!b",2,,STOREBYTE
        pop %ebx                // address to store at
        pop %eax                // data to store there
        movb %al,(%ebx)         // store it
        NEXT

        defcode "@b",2,,FETCHBYTE
        pop %ebx                // address to fetch
        xor %eax,%eax
        movb (%ebx),%al         // fetch it
        push %eax               // push value onto stack
        NEXT

        // The STATE variable is 0 for execute mode, != 0 for compile mode
        defvar "STATE",5,,STATE

        // This points to where compiled words go.
        defvar "HERE",4,,HERE,user_defs_start

        // This is the last definition in the dictionary.
        defvar "LATEST",6,,LATEST,name_SYSEXIT // SYSEXIT must be last in built-in dictionary

        // _X, _Y and _Z are scratch variables used by standard words.
        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

        defcode "DSP@",4,,DSPFETCH
        mov %esp,%eax
        push %eax
        NEXT

        defcode "DSP!",4,,DSPSTORE
        pop %esp
        NEXT

        defcode ">R",2,,TOR
        pop %eax                // pop parameter stack into %eax
        PUSHRSP %eax            // push it on to the return stack
        NEXT

        defcode "R>",2,,FROMR
        POPRSP %eax             // pop return stack on to %eax
        push %eax               // and push on to parameter stack
        NEXT

        defcode "RSP@",4,,RSPFETCH
        push %ebp
        NEXT

        defcode "RSP!",4,,RSPSTORE
        pop %ebp
        NEXT

        defcode "RDROP",5,,RDROP
        lea 4(%ebp),%ebp        // pop return stack and throw away
        NEXT

#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
        xor %eax,%eax
        mov (%ebx),%al
        inc %ebx
        mov %ebx,(currkey)
        ret

1:      // out of input; use read(2) to fetch more input from stdin
        xor %ebx,%ebx           // 1st param: stdin
        mov $buffer,%ecx        // 2nd param: buffer
        mov %ecx,currkey
        mov $buffend-buffer,%edx // 3rd param: max length
        mov $__NR_read,%eax     // syscall: read
        int $0x80
        test %eax,%eax          // If %eax <= 0, then exit.
        jbe 2f
        addl %eax,%ecx          // buffer+%eax = bufftop
        mov %ecx,bufftop
        jmp _KEY

2:      // error or out of input: exit
        xor %ebx,%ebx
        mov $__NR_exit,%eax     // syscall: exit
        int $0x80

        defcode "EMIT",4,,EMIT
        pop %eax
        call _EMIT
        NEXT
_EMIT:
        mov $1,%ebx             // 1st param: stdout

        // write needs the address of the byte to write
        mov %al,(2f)
        mov $2f,%ecx            // 2nd param: address

        mov $1,%edx             // 3rd param: nbytes = 1

        mov $__NR_write,%eax    // write syscall
        int $0x80
        ret

        .bss
2:      .space 1                // scratch used by EMIT

        defcode "WORD",4,,WORD
        call _WORD
        push %ecx               // push length
        push %edi               // push base address
        NEXT

_WORD:
        /* Search for first non-blank character.  Also skip \ comments. */
1:
        call _KEY               // get next key, returned in %eax
        cmpb $'\\',%al          // start of a comment?
        je 3f                   // if so, skip the comment
        cmpb $' ',%al
        jbe 1b                  // if so, keep looking

        /* Search for the end of the word, storing chars as we go. */
        mov $5f,%edi            // pointer to return buffer
2:
        stosb                   // add character to return buffer
        call _KEY               // get next key, returned in %al
        cmpb $' ',%al           // is blank?
        ja 2b                   // if not, keep looping

        /* Return the word (well, the static buffer) and length. */
        sub $5f,%edi
        mov %edi,%ecx           // return length of the word
        mov $5f,%edi            // return address of the word
        ret

        /* Code to skip \ comments to end of the current line. */
3:
        call _KEY
        cmpb $'\n',%al          // end of line yet?
        jne 3b
        jmp 1b

        .bss
        // A static buffer where WORD returns.  Subsequent calls
        // 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

        NEXT

        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

        // Parse a number from a string on the stack -- almost the opposite of . (DOT)
        // Note that there is absolutely no error checking.  In particular the length of the
        // string must be >= 1 bytes.
        defcode "SNUMBER",7,,SNUMBER
        pop %edi
        pop %ecx
        call _SNUMBER
        push %eax
        NEXT
_SNUMBER:
        xor %eax,%eax
        xor %ebx,%ebx
1:
        imull $10,%eax          // %eax *= 10
        movb (%edi),%bl
        inc %edi
        subb $'0',%bl           // ASCII -> digit
        add %ebx,%eax
        dec %ecx
        jnz 1b
        ret

        defcode "FIND",4,,FIND
        pop %edi                // %edi = address
        pop %ecx                // %ecx = length
        call _FIND
        push %eax
        NEXT

_FIND:
        push %esi               // Save %esi so we can use it in string comparison.

        // Now we start searching backwards through the dictionary for this word.
        mov var_LATEST,%edx     // LATEST points to name header of the latest word in the dictionary
1:
        test %edx,%edx          // NULL pointer?  (end of the linked list)
        je 4f

        // Compare the length expected and the length of the word.
        // Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery
        // this won't pick the word (the length will appear to be wrong).
        xor %eax,%eax
        movb 4(%edx),%al        // %al = flags+length field
        andb $(F_HIDDEN|0x1f),%al // %al = name length
        cmpb %cl,%al            // Length is the same?
        jne 2f

        // Compare the strings in detail.
        push %ecx               // Save the length
        push %edi               // Save the address (repe cmpsb will move this pointer)
        lea 5(%edx),%esi        // Dictionary string we are checking against.
        repe cmpsb              // Compare the strings.
        pop %edi
        pop %ecx
        jne 2f                  // Not the same.

        // The strings are the same - return the header pointer in %eax
        pop %esi
        mov %edx,%eax
        ret

2:
        mov (%edx),%edx         // Move back through the link field to the previous word
        jmp 1b                  // .. and loop.

4:      // Not found.
        pop %esi
        xor %eax,%eax           // Return zero to indicate not found.
        ret

        defcode ">CFA",4,,TCFA  // DEA -> Codeword address
        pop %edi
        call _TCFA
        push %edi
        NEXT
_TCFA:
        xor %eax,%eax
        add $4,%edi             // Skip link pointer.
        movb (%edi),%al         // Load flags+len into %al.
        inc %edi                // Skip flags+len byte.
        andb $0x1f,%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

        defcode ":",1,,COLON

        // 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

        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.

        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
        rep movsb               // Copy the word
        pop %esi
        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.
        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

        defcode ",",1,,COMMA
        pop %eax                // Code pointer to store.
        call _COMMA
        NEXT
_COMMA:
        movl var_HERE,%edi      // HERE
        stosl                   // Store it.
        movl %edi,var_HERE      // Update HERE (incremented)
        ret

        defcode "HIDDEN",6,,HIDDEN
        call _HIDDEN
        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
        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

        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
        NEXT

/* This definiton of ' (TICK) is strictly cheating - it also only works in compiled code. */
        defcode "'",1,,TICK
        lodsl                   // Get the address of the next word and skip it.
        pushl %eax              // Push it on the stack.
        NEXT

/* This interpreter is pretty simple, but remember that in FORTH you can always override
 * it later with a more powerful one!
 */
        defword "INTERPRETER",11,,INTERPRETER
        .int INTERPRET,RDROP,INTERPRETER

        defcode "INTERPRET",9,,INTERPRET
        call _WORD              // Returns %ecx = length, %edi = pointer to word.

        // Is it in the dictionary?
        xor %eax,%eax
        movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...)
        call _FIND              // Returns %eax = pointer to header or 0 if not found.
        test %eax,%eax          // Found?
        jz 1f

        // In the dictionary.  Is it an IMMEDIATE codeword?
        mov %eax,%edi           // %edi = dictionary entry
        movb 4(%edi),%al        // Get name+flags.
        push %ax                // Just save it for now.
        call _TCFA              // Convert dictionary entry (in %edi) to codeword pointer.
        pop %ax
        andb $F_IMMED,%al       // Is IMMED flag set?
        mov %edi,%eax
        jnz 4f                  // If IMMED, jump straight to executing.

        jmp 2f

1:      // Not in the dictionary (not a word) so assume it's a literal number.
        incl interpret_is_lit
        call _SNUMBER           // Returns the parsed number in %eax
        mov %eax,%ebx
        mov $LIT,%eax           // The word is LIT

2:      // Are we compiling or executing?
        movl var_STATE,%edx
        test %edx,%edx
        jz 4f                   // Jump if executing.

        // Compiling - just append the word to the current dictionary definition.
        call _COMMA
        mov interpret_is_lit,%ecx // Was it a literal?
        test %ecx,%ecx
        jz 3f
        mov %ebx,%eax           // Yes, so LIT is followed by a number.
        call _COMMA
3:      NEXT

4:      // Executing - run it!
        mov interpret_is_lit,%ecx // Literal?
        test %ecx,%ecx          // Literal?
        jnz 5f

        // Not a literal, execute it now.  This never returns, but the codeword will
        // eventually call NEXT which will reenter the loop in INTERPRETER.
        jmp *(%eax)

5:      // Executing a literal, which means push it on the stack.
        push %ebx
        NEXT

        .data
        .align 4
interpret_is_lit:
        .int 0                  // Flag used to record if reading a literal

        // NB: SYSEXIT must be the last entry in the built-in dictionary.
        defcode SYSEXIT,7,,SYSEXIT
        pop %ebx
        mov $__NR_exit,%eax
        int $0x80

/*----------------------------------------------------------------------
 * Input buffer & initial input.
 */
        .data
        .align 4096
buffer:
        // XXX gives 'Warning: unterminated string; newline inserted' messages which you can ignore
        .ascii "\
\\ Define some character constants
: '\\n'   10 ;
: 'SPACE' 32 ;
: '\"'    34 ;
: ':'     58 ;

\\ CR prints a carriage return
: CR '\\n' EMIT ;

\\ SPACE prints a space
: SPACE 'SPACE' EMIT ;

\\ Primitive . (DOT) function doesn't follow with a blank, so redefine it to behave like FORTH.
\\ Notice how we can trivially redefine existing functions.
: . . SPACE ;

\\ DUP, DROP are defined in assembly for speed, but this is how you might define them
\\ in FORTH.  Notice use of the scratch variables _X and _Y.
\\ : DUP _X ! _X @ _X @ ;
\\ : DROP _X ! ;

\\ The 2... versions of the standard operators work on pairs of stack entries.  They're not used
\\ very commonly so not really worth writing in assembler.  Here is how they are defined in FORTH.
: 2DUP OVER OVER ;
: 2DROP DROP DROP ;

\\ More standard FORTH words.
: 2* 2 * ;
: 2/ 2 / ;

\\ [ and ] allow you to break into immediate mode while compiling a word.
: [ IMMEDIATE           \\ define [ as an immediate word
        0 STATE !       \\ go into immediate mode
        ;

: ]
        1 STATE !       \\ go back to compile mode
        ;

\\ LITERAL takes whatever is on the stack and compiles LIT <foo>
: LITERAL IMMEDIATE
        ' LIT ,         \\ compile LIT
        ,               \\ compile the literal itself (from the stack)
        ;

\\ condition IF true-part THEN rest
\\   compiles to:
\\ condition 0BRANCH OFFSET true-part rest
\\   where OFFSET is the offset of 'rest'
\\ condition IF true-part ELSE false-part THEN
\\   compiles to:
\\ condition 0BRANCH OFFSET true-part BRANCH OFFSET2 false-part rest
\\   where OFFSET if the offset of false-part and OFFSET2 is the offset of rest

\\ IF is an IMMEDIATE word which compiles 0BRANCH followed by a dummy offset, and places
\\ the address of the 0BRANCH on the stack.  Later when we see THEN, we pop that address
\\ off the stack, calculate the offset, and back-fill the offset.
: IF IMMEDIATE
        ' 0BRANCH ,     \\ compile 0BRANCH
        HERE @          \\ save location of the offset on the stack
        0 ,             \\ compile a dummy offset
;

: THEN IMMEDIATE
        DUP
        HERE @ SWAP -   \\ calculate the offset from the address saved on the stack
        SWAP !          \\ store the offset in the back-filled location
;

: ELSE IMMEDIATE
        ' BRANCH ,      \\ definite branch to just over the false-part
        HERE @          \\ save location of the offset on the stack
        0 ,             \\ compile a dummy offset
        SWAP            \\ now back-fill the original (IF) offset
        DUP             \\ same as for THEN word above
        HERE @ SWAP -
        SWAP !
;

\\ BEGIN loop-part condition UNTIL
\\   compiles to:
\\ loop-part condition 0BRANCH OFFSET
\\   where OFFSET points back to the loop-part
\\ This is like do { loop-part } while (condition) in the C language
: BEGIN IMMEDIATE
        HERE @          \\ save location on the stack
;

: UNTIL IMMEDIATE
        ' 0BRANCH ,     \\ compile 0BRANCH
        HERE @ -        \\ calculate the offset from the address saved on the stack
        ,               \\ compile the offset here
;

\\ BEGIN loop-part AGAIN
\\   compiles to:
\\ loop-part BRANCH OFFSET
\\   where OFFSET points back to the loop-part
\\ In other words, an infinite loop which can only be returned from with EXIT
: AGAIN IMMEDIATE
        ' BRANCH ,      \\ compile BRANCH
        HERE @ -        \\ calculate the offset back
        ,               \\ compile the offset here
;

\\ BEGIN condition WHILE loop-part REPEAT
\\   compiles to:
\\ condition 0BRANCH OFFSET2 loop-part BRANCH OFFSET
\\   where OFFSET points back to condition (the beginning) and OFFSET2 points to after the whole piece of code
\\ So this is like a while (condition) { loop-part } loop in the C language
: WHILE IMMEDIATE
        ' 0BRANCH ,     \\ compile 0BRANCH
        HERE @          \\ save location of the offset2 on the stack
        0 ,             \\ compile a dummy offset2
;

: REPEAT IMMEDIATE
        ' BRANCH ,      \\ compile BRANCH
        SWAP            \\ get the original offset (from BEGIN)
        HERE @ - ,      \\ and compile it after BRANCH
        DUP
        HERE @ SWAP -   \\ calculate the offset2
        SWAP !          \\ and back-fill it in the original location
;

\\ With the looping constructs, we can now write SPACES, which writes n spaces to stdout.
: SPACES
        BEGIN
                SPACE   \\ print a space
                1-      \\ until we count down to 0
                DUP 0=
        UNTIL
;

\\ .S prints the contents of the stack.  Very useful for debugging.
: .S
        DSP@            \\ get current stack pointer
        BEGIN
                DUP @ .         \\ print the stack element
                4+              \\ move up
                DUP S0 @ 4- =   \\ stop when we get to the top
        UNTIL
        DROP
;

\\ DEPTH returns the depth of the stack.
: DEPTH S0 @ DSP@ - ;

\\ .\" is the print string operator in FORTH.  Example: .\" Something to print\"
\\ The space after the operator is the ordinary space required between words.
\\ This is tricky to define because it has to do different things depending on whether
\\ we are compiling or in immediate mode.  (Thus the word is marked IMMEDIATE so it can
\\ detect this and do different things).
\\ In immediate mode we just keep reading characters and printing them until we get to
\\ the next double quote.
\\ In compile mode we have the problem of where we're going to store the string (remember
\\ that the input buffer where the string comes from may be overwritten by the time we
\\ come round to running the function).  We store the string in the compiled function
\\ like this:
\\   LITSTRING, string length, string rounded up to 4 bytes, EMITSTRING, ...
: .\" IMMEDIATE
        STATE @         \\ compiling?
        IF
                ' LITSTRING ,   \\ compile LITSTRING
                HERE @          \\ save the address of the length word on the stack
                0 ,             \\ dummy length - we don't know what it is yet
                BEGIN
                        KEY             \\ get next character of the string
                        DUP '\"' <>
                WHILE
                        HERE @ !b       \\ store the character in the compiled image
                        1 HERE +!       \\ increment HERE pointer by 1 byte
                REPEAT
                DROP            \\ drop the double quote character at the end
                DUP             \\ get the saved address of the length word
                HERE @ SWAP -   \\ calculate the length
                4-              \\ subtract 4 (because we measured from the start of the length word)
                SWAP !          \\ and back-fill the length location
                HERE @          \\ round up to next multiple of 4 bytes for the remaining code
                3 +
                3 INVERT AND
                HERE !
                ' EMITSTRING ,  \\ compile the final EMITSTRING
        ELSE
                \\ In immediate mode, just read characters and print them until we get
                \\ to the ending double quote.  Much simpler than the above code!
                BEGIN
                        KEY
                        DUP '\"' = IF EXIT THEN
                        EMIT
                AGAIN
        THEN
;

\\ While compiling, [COMPILE] WORD compiles WORD if it would otherwise be IMMEDIATE.
: [COMPILE] IMMEDIATE
        WORD            \\ get the next word
        FIND            \\ find it in the dictionary
        >CFA            \\ get its codeword
        ,               \\ and compile that
;

\\ RECURSE makes a recursive call to the current word that is being compiled.
\\ Normally while a word is being compiled, it is marked HIDDEN so that references to the
\\ same word within are calls to the previous definition of the word.
: RECURSE IMMEDIATE
        LATEST @ >CFA   \\ LATEST points to the word being compiled at the moment
        ,               \\ compile it
;

\\ ALLOT is used to allocate (static) memory when compiling.  It increases HERE by
\\ the amount given on the stack.
: ALLOT HERE +! ;


\\ Finally print the welcome prompt.
.\" OK \"
"

_initbufftop:
        .align 4096
buffend:

currkey:
        .int buffer
bufftop:
        .int _initbufftop