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Comment by zabzonk | Hacker News Reader

Please explain what you mean by "a separate instruction".

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.

> 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.

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?

It hurts less if you think of it like assignment:

    al = [rsi]
    [rdi] = al

A MOV and an INC, as opposed to just the MOV.

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