Question

How to compare two words in NASM assembly? consider this code:

global start
start:
    mov eax,array
    mov edx,4
    mov ecx,2987074
.LOOP1:
    cmp word [eax],ecx
    je .FOUND
    add eax,4
    sub edx,1
    jz .NOTFOUND
    jmp .LOOP1
.FOUND:
    xor ebx,ebx
    jmp .EXIT
.NOTFOUND:
    mov ebx,1
.EXIT:
    mov eax,1
    int 0x80

array:  
    dd 1137620
    dd 3529469
    dd 2987074
    dd 1111111
    dd 2222222

returns

foo.asm:7: error: mismatch in operand sizes

and changing cmp from cmp word [eax],ecx to cmp word [eax],word ecx

returns:

foo.asm:7: warning: register size specification ignored
foo.asm:7: error: mismatch in operand sizes

I have no idea how to fix this. Can someone explain it?

Answer 1

cmp word [eax],ecx is wrong because operand sizes don’t match (ecx is a dword, not word). Most x86 instructions with two operands can only work with operands of the same size.

cmp word [eax],word ecx is wrong because ecx is a dword, not word.

If you’re coming from the (g)as/gcc world, it’s worth noting that their .word is the machine word and on a 32-bit machine it’s 32-bit. NASM’s word is always 16-bit and its dword is always 32-bit.

You probably want just cmp [eax], ecx. Since the two operands of cmp must be of the same size, NASM deduces here that the memory operand at address in eax is the same size as the register operand ecx, 32-bit (dword).