Reverse Engineering for Beginners

CHAPTER 19. MANIPULATING SPECIFIC BIT(S) CHAPTER 19. MANIPULATING SPECIFIC BIT(S)

ANDgot executed:

Figure 19.4:OllyDbg:ANDexecuted

The 10th bit was cleared (or, in other words, all bits were left except the 10th) and the final value now is
0x12344 478 (1001000110100010001 0 001111000).

Optimizing MSVC

If we compile it in MSVC with optimization turned on (/Ox), the code is even shorter:

Listing 19.11: Optimizing MSVC

_a$ = 8 ; size = 4
_f PROC
mov eax, DWORD PTR _a$[esp-4]
and eax, -513 ; fffffdffH
or eax, 16384 ; 00004000H
ret 0
_f ENDP

Non-optimizing GCC

Let’s try GCC 4.4.1 without optimization:

Listing 19.12: Non-optimizing GCC
public f
f proc near

var_4 = dword ptr -4
arg_0 = dword ptr 8

push ebp
mov ebp, esp
sub esp, 10h
mov eax, [ebp+arg_0]
mov [ebp+var_4], eax
or [ebp+var_4], 4000h
and [ebp+var_4], 0FFFFFDFFh
mov eax, [ebp+var_4]
leave
retn
f endp

There is a redundant code present, however, it is shorter than the MSVC version without optimization.

Now let’s try GCC with optimization turned on-O3: