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by zabzonk 610 days ago
Please explain what you mean by "a separate instruction".
2 comments
spc476 609 days ago
Some idiomatic C code to copy a string (I'm not saying this is good C code, but it's just an example):

    while(*d++ = *s++)
      ;
On the Motorola 68000 (based somewhat on the PDP-11) the code would look like:

    loop:       move.b  (a0)+,d0
                move.b  d0,(a1)+
                bne     loop
while on the x86 line, it would be:

    loop:       mov     al,[rsi]
                mov     [rdi],al
                inc     rsi     ; extra instruction!
                inc     rdi     ; extra instruction!
                cmp     al,0
                jne     loop
Yes, there are better ways to write that code for both the 68K and x86, but I hope this gets the point across.
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wang_li 609 days ago
> loop: move.b (a0)+,d0 move.b d0,(a1)+

...

> loop: mov al,[rsi] mov [rdi],al

This hurts my brain. When we invent time machines I'm going to use it to go back and slap whoever at intel came up with that operand order.

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int_19h 607 days ago
memcpy etc also take the destination as their first argument, and it mirrors the usual way we write assignments. Personally I always found Intel syntax to be more straightforward, but I think it's ultimately down to whatever one was exposed to first.

I wonder sometimes why we keep insisting on the "OP ARG, ARG" format in general for assembly. Why not something like `MOV X -> Y` that would make it absolutely clear and unambiguous? For that matter, why not `COPY X -> Y`, since that's what it actually does?

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spc476 607 days ago
It hurts less if you think of it like assignment:

    al = [rsi]
    [rdi] = al
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wpollock 609 days ago
A MOV and an INC, as opposed to just the MOV.
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