Mastering Assembly Programming

The conditional assembly

Sometimes we may want a macro instruction or a code fragment to be assembled differently depending on certain conditions. Both MASM and GAS provide this functionality too, but let's get back to FASM (as the most convenient one) and consider the following macro instruction:

macro exordd p1, p2{   if ~p1 in <eax, ebx, ecx, edx, esi, edi, ebp, esp> &\      ~p2 in <eax, ebx, ecx, edx, esi, edi, ebp, esp>      push eax      mov  eax, [p2]      xor [p1], eax      pop  eax   else      if ~p1 in <eax, ebx, ecx, edx, esi, edi, ebp, esp>         xor [p1], p2      else if ~p2 in <eax, ebx, ecx, edx, esi, edi, ebp, esp>         xor p1, [p2]      else         xor p1, p2      end if   end if}

It may appear a bit complicated at first, but the purpose of the macro is rather simple. We extend an XOR instruction ...