Can a LLM convert C, to ASM to specs and then to a working Z/80 Speccy tape? Yes

Re:Can a LLM convert C, to ASM to specs and then to a workin... by beagle3 on Monday March 17, 2025 @08:09AM (#43387623)

Interesting. Surprisingly, it decided to encode the multiplication and division as addition/subtraction loops, which is incredibly inefficient - multiplying e.g. 32,000 by 32,000 (ignoring the overflow ...) will take 1,024,000,000 iterations, so thousands of seconds on the speccy's humble 4Mhz Z80 (8 instructions, each taking at least 2 T-states ...)

Here is the multiplication loop (division is similar but in reverse, subtracting instead of adding).

    mult_loop:
     ; Check if BC is zero
     ld a, b
     or c
     jr z, mult_done
     
     ; Add HL to result
     ex de, hl           ; DE = multiplier, HL = result
     add hl, de          ; Add multiplier to result
     ex de, hl           ; DE = result, HL = multiplier
     
     ; Decrement counter
     dec bc
     jr mult_loop

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Re:Can a LLM convert C, to ASM to specs and then to a workin... by ghuntley on Monday March 17, 2025 @08:13AM (#43387651)

oh interesting. One thing about the specy, is it's incredibly hard to hook in the eval technique (see middle/bottom of https://ghuntley.com/specs/ ) as deployment is manual/human. So I had to drive it all by hand and just accept it. If this was another programming language I would have taken the approach of creating a `cargo bench` over the application and then looping _that_ back into the LLM to identify and resolve the performance issues. I've done it; it works well. Just not on the speccy :P

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Re:Can a LLM convert C, to ASM to specs and then to a workin... by Joker_vD on Monday March 17, 2025 @02:36PM (#43391467)

Well, can you really expect an LLM to regurgitate e.g. the quarter-of-square method [0] without specifically prompting it for it?

[0] https://www.nature.com/articles/040573c0

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Re:Can a LLM convert C, to ASM to specs and then to a workin... by beagle3 on Monday March 17, 2025 @10:35PM (#43395190)

No, but I would expect to do binary long multiplication and division, which have been used since before the intel 4004, and which it has surely seen in many Z80 and 6502 code bases fed into it.

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Re:Can a LLM convert C, to ASM to specs and then to a workin... by rep_lodsb on Tuesday March 18, 2025 @03:01PM (#43403457)

"Who cares if it's slow, at least the code is correct" - someone who never wrote 8-bit code in their life

It at least managed to regurgitate the fast multiply-by-10 algorithm, which of course exists in countless examples all over the web. But then, instead of maybe repeating that twice to multiply by 100, it produced absolutely insane code which - even at a glance - can't be correct unless the input happened to be zero:

    ; No decimal point, multiply by 100
    ld b, h
    ld c, l
    add hl, hl          ; * 2
    add hl, hl          ; * 4
    add hl, hl          ; * 8
    add hl, hl          ; * 16
    add hl, hl          ; * 32
    add hl, hl          ; * 64
    add hl, bc          ; * 65
    add hl, hl          ; * 130
    ld b, h
    ld c, l
    srl b
    rr c                ; * 65
    add hl, bc          ; * 195
    add hl, hl          ; * 390
    add hl, hl          ; * 780
    srl h
    rr l                ; * 390
    add hl, hl          ; * 780
    srl h
    rr l                ; * 390
    srl h
    rr l                ; * 195
    srl h
    rr l                ; * 97.5
    add hl, hl          ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    add hl, hl          ; * 390
    srl h
    rr l                ; * 195
    jr done_convert

First a bunch of random left shifts and additions that overshoot the target, then an even more directionless attempt of correcting the result that seems stuck in a loop until it just gives up. Kind of reminds me of "PanicSort" (xkcd.com/1185)

And as you mentioned, it ignores overflow, which might be a real problem since even if it worked correctly, it could only handle amounts up to $655.35. A better solution would have used BCD arithmetic, which the Z80 provides some dedicated instructions for.

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