4 ms·
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
by spc476 2y 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.
- wang_li 2y 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.
- spc476 2y agoIt hurts less if you think of it like assignment: al = [rsi] [rdi] = al
- int_19h 2y agomemcpy 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?