Dave's nasmjf Dev Log 01

Created: 2022-07-22

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This is an entry in my developer’s log series written between December 2021 and August 2022 (started project in September). I wrote these as I completed my port of the JONESFORTH assembly language Forth interpreter.

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    First log session to test what I've got so far. GNU Debugger recorded in
    GNU screen for the full GNU experience.
    I'll clean up a lot of the gdb prompts and stuff for clarity.

Reading symbols from nasmjf...
Breakpoint 1 at 0x804900e: file nasmjf.asm, line 80.
Breakpoint 1, _start () at nasmjf.asm:80
80          cld    ; Clear the "direction flag" which means the string instructions (such
82          mov [var_S0], esp ; save the regular stack pointer (used for data) in FORTH var S0!
84          mov ebp, return_stack_top ; Initialise the return stack pointer

    Trying a defined "function" in GDB to cut down on the typing. I always
    have to cast the NASM labels to (int) since the debugging info has no
    way of telling GDB what I'm storing there. "int" in this case just means
    I've got a 4-byte (32 bit) value. GDB has a strong C heritage.
        p - displays the VALUE of the label, which is an address
        x - displays the memory at the address
        p/x and x/x displays as hexadecimal
        *(int) uses the address stored AT the memory referenced by the label
            (again, strong C heritage in this syntax)
    All three of these won't always be relevant, but it saves a lot of typing.

(gdb) define foo
Type commands for definition of "foo".
End with a line saying just "end".
>p/x (int)$arg0
>x/x (int)$arg0
>x/x *(int)$arg0

    Initial nonsense over. now we use the main mechanism that drives the Forth
    instructions: the NEXT macro is inlined at the end of every word and here
    to bootstrap the action. cold_start contains the address of the "QUIT" word.
    (quit is a silly name - it doesn't quit Forth, it "quits" TO the interpreter)
    (side note: i'd like everything to be lowercase except assembly macros. But
    after 'quit' and 'docol', I haven't been good about converting them. Will
    probably do a couple rounds of cleanup at some point...)

    NEXT loads the address of the next instruction and we jump to it, executing
    the machine code there.

_start () at nasmjf.asm:88
88          mov esi, cold_start       ; give next forth word to execute
27          lodsd     ; NEXT: Load from memory into eax, inc esi to point to next word.
28          jmp [eax] ; Jump to whatever code we're now pointing at.

    Since QUIT is defined with the DEFWORD macro, it begins with a call to
    the 'DOCOL' word - which, in essense, sets up the rest of the Forth word
    to be executed (QUIT, in this case) for another call to NEXT.

docol () at nasmjf.asm:40
40          lea ebp, [ebp-4]   ; "load effective address" of next stack position
41          mov [ebp], %1      ; "push" the register value to the address at ebp
70          add eax, 4      ; eax points to docol (me!) in word definition. Go to next.

    Here I use that 'foo' function to see if that's true about the eax register.
    Note that the add 4 instruction has NOT yet executed. GDB always shows the
    next instruction before you tell it to step forward to that instruction!

(gdb) foo $eax
$9 = 0x804a010
0x804a010:      0x08049000
0x8049000 <docol>:      0x89fc6d8d

    Yup! It points to DOCOL all right. Now we step and add 4 to eax:

(gdb) s
71          mov esi, eax    ; Put the next word pointer into esi
(gdb) foo $eax
$10 = 0x804a014
0x804a014:      0x0804a12c
0x804a12c:      0x08049218

    Every single Forth word ends with NEXT, which executes the next word.
    In this case, it's happening at the end of DOCOL (and DOCOL's job is
    to get everything set up to have NEXT execute the rest of the word...)

(gdb) s
27          lodsd     ; NEXT: Load from memory into eax, inc esi to point to next word.
28          jmp [eax] ; Jump to whatever code we're now pointing at.

    Double-checking that the instructions in QUIT are what we'll be running

(gdb) foo $eax
$12 = 0x804a12c
0x804a12c:      0x08049218
0x8049218 <code_R0>:    0x04c30868

    Yes! The 'R0' constant is the first thing we run in QUIT! It's really wild
    how constants in Forth are actually words with a single instruction that
    pushes a value onto the stack! In this case, R0 is the top of the return

    The push %5 line is from the DEFCONST macro, which, in turn, calls the
    DEFCODE macro because consts are words. Then the NEXT macro continues to
    the next word in QUIT...

code_R0 () at nasmjf.asm:568
568             push %5
code_R0 () at nasmjf.asm:27
27          lodsd     ; NEXT: Load from memory into eax, inc esi to point to next word.
28          jmp [eax] ; Jump to whatever code we're now pointing at.

    ...which happens to be RSPSTORE, which puts a value on the return stack.

code_RSPSTORE () at nasmjf.asm:201
201             pop ebp
code_RSPSTORE () at nasmjf.asm:27
27          lodsd     ; NEXT: Load from memory into eax, inc esi to point to next word.
28          jmp [eax] ; Jump to whatever code we're now pointing at.

    ...and then QUIT runs INTERPRET, which takes words on STDIN and then
        ...calls _WORD to get a word from input which
            ...calls _KEY to get a character ("key") of input

code_INTERPRET () at nasmjf.asm:209
209             call _WORD              ; Returns %ecx = length, %edi = pointer to word.
_WORD.skip_non_words () at nasmjf.asm:310
310             call _KEY               ; get next key, returned in %eax
_KEY () at nasmjf.asm:351

    _KEY first checks to see if it needs input (currkey has reached
    bufftop). On first run, they're both zero, so yeah, we need more

    Aside: again, "key" isn't how we would normally describe this in
    a modern environment - it's the next "character" (and even that's
    becoming a thing of the past now that Unicode is pretty much standard

    Anyway, comparing currkey (ebx = 0) and bufftop (0) sets the Zero
    Flag (ZF) because the difference between them is the same. As we
    can see in the 'info reg' display below:

351             mov ebx, [currkey]
352             cmp ebx, [bufftop]
353             jge .get_more_input
(gdb) info reg
ebx            0x0                 0
eflags         0x246               [ PF ZF IF ]

    We get more input by telling Linux to give us input from
    STDIN into a fixed-size buffer:

_KEY.get_more_input () at nasmjf.asm:361
361             xor ebx,ebx             ; 1st param: stdin
362             mov ecx,buffer          ; 2nd param: buffer
363             mov [currkey],ecx
364             mov edx,buffer_size     ; 3rd param: max length
365             mov eax,__NR_read       ; syscall: read
366             int 0x80                ; syscall!

    Now I type "foo<enter>":


    We check to make sure the input isn't zero-length.
    I don't think it would ever be - the <enter> key would
    always give us at least '\n'?

367             test eax,eax            ; If %eax <= 0, then exit.
368             jbe .eof
369             add ecx,eax             ; buffer+%eax = bufftop
370             mov [bufftop],ecx

    We can see how long the input string is. Yup, 4 bytes is
    right: "foo\n".

(gdb) foo $eax
$15 = 0x4

    Now we're back to _KEY, having gathered some input.
    We repeat the check...

371             jmp _KEY
_KEY () at nasmjf.asm:351
351             mov ebx, [currkey]
352             cmp ebx, [bufftop]
353             jge .get_more_input

    This time we have input (and bufftop is at a higher
    address than currkey), so we continue on by grabbing
    the current "key" (character):

354             xor eax, eax
355             mov al, [ebx]           ; get next key from input buffer

    If that worked, the al register now has the first
    character of "foo\n". Yup, there's the "f"! (p/c means
    print as a character. We can also p/s to print a C-style

(gdb) p/c $al
$19 = 102 'f'

    Now we set currkey to the next character and return...

356             inc ebx
357             mov [currkey], ebx        ; increment currkey
358             ret

    Back at _WORD, we check to see if we've hit a character
    to skip. Forth is so syntactically simple, I just love it.

    NOTE that the jbe instruction is "jump if compared value is
    before (less than) or equal", so any character smaller
    than an ASCII space (0x20) will cause us to keep seeking in the
    .skip_non_words loop. This is a clever way to skip spaces,
    tabs, newlines, returns, form feeds, etc. I'll improve the
    comments for these instructions in the actual program now.

_WORD.skip_non_words () at nasmjf.asm:311
311             cmp al,'\'              ; start of a comment?
312             je .skip_comment        ; if so, skip the comment
313             cmp al,' '              ; space?
314             jbe .skip_non_words     ; if so, keep looking

    Nope, character looks good. So we add it to word_buffer
    in memory. The stosb instruction implicitly copies what's
    in the al register (the 'b' is for byte) to memory at
    the address stored in the edi register.

    Then edi is incremented so that the next time this happens,
    the next byte will go to the next position, and so forth.
    It turns out, this is the sort of thing we're guaranteeing
    when we cleared the direction flag at the very beginning.

317             mov edi,word_buffer     ; put addr to word return buffer in edi

    Now that we've established that we're past any whitespace
    and are gathering the actual input, we're in .collect_word.
    I'll snip the stepping through _KEY for 'o', 'o', and '\n'

_WORD.collect_word () at nasmjf.asm:319
319             stosb                   ; add character to return buffer
320             call _KEY               ; get next key, returned in %al

    After every call to _KEY, we check to see if we're done
    collecting the word. The ja instruction is "jump if the
    compared value is after (greater than)," which is the
    exact opposite of the jbe check above.
    To put it straight: before we were looping WHILE the
    character was whitespace, now we loop UNTIL the character
    is whitespace.

321             cmp al,' '              ; is blank?
322             ja .collect_word        ; if not, keep looping

    Now _WORD returns the length and address of the collected word.

325             sub edi, word_buffer    ; hmm, the len?
326             mov ecx, edi            ; return it
327             mov edi, word_buffer    ; return address of the word
328             ret

    Then we return to _INTERPRET from _WORD:

code_INTERPRET () at nasmjf.asm:212
212             xor eax,eax             ; back from _WORD...zero eax

    Let's check the return values now:

(gdb) p $ecx
$1 = 3
(gdb) x/3c $edi
0x804a068 <word_buffer>:        102 'f' 111 'o' 111 'o'

    Yay! There's the "foo" string that was input.
    Even though I've got some of the _FIND word that tries to
    match the input word, I think this has been quite enough
    for one log. :-)
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