Question

I just opened a file in IDA Pro and I found some code that looks completely useless. However, I thought it might have some use. Doesn’t the sub eax,0 just subtract 0 from eax?

The code:

hinstDLL= dword ptr  4  
fdwReason= dword ptr  8  
lpReserved= dword ptr  0Ch  

mov     eax, [esp+fdwReason]  
sub     eax, 0  
jz      short loc_10001038  

Answer 1

The sub instruction sets flags (OF, SF, ZF, AF, PF, and CF, according to the documentation) – the mov instruction does not. The jz will jump only if the zero flag (ZF) is set, so if you want to jump based on the value in eax that flag has to be set appropriately.