After using IDA Pro to disassemble a x86 dll, I found this code (Comments added by me in pusedo-c code. I hope they're correct):
test ebx, ebx ; if (ebx == false)
jz short loc_6385A34B ; Jump to 0x6385a34b
mov eax, [ebx+84h] ; eax = *(ebx+0x84)
mov ecx, [esi+84h] ; ecx = *(esi+0x84)
mov al, [eax+30h] ; al = *(*(ebx+0x84)+0x30)
xor al, [ecx+30h] ; al = al XOR *(*(esi+0x84)+0x30)
jnz loc_6385A453
Lets make it simpler for me to understand:
mov eax, b3h
xor eax, d6h
jnz ...
How does the conditional jump instruction work after a xor instruction?
Answer
Like most instructions, xor sets the processor condition flags depending on the result of the previous operation. In this case, the Z flag will be set if the result of the xor is zero. The jnz instruction tests the Z flag and branches if it is not set.