What does the MOVZBL instruction do in IA-32 AT&T syntax?

Question

What exactly does this instruction do?

movzbl  0x01(%eax,%ecx), %eax

Answer 1

AT&T syntax splits the movzx Intel instruction mnemonic into different mnemonics for different source sizes (movzb vs. movzw). In Intel syntax, it's:

movzx eax, byte ptr [eax+ecx+1]

i.e. load a byte from memory at eax+ecx+1 and zero-extend to full register.

BTW, most GNU tools now have a switch or a config option to prefer Intel syntax. (Such as objdump -Mintel or gcc -S -masm=intel, although the latter affects the syntax used when compiling inline-asm). I would certainly recommend to look into it, if you don't do AT&T assembly for living. See also the x86 tag wiki for more docs and guides.