Assembly Language for Beginners

1.22. MANIPULATING SPECIFIC BIT(S)

ANDgot executed:

Figure 1.97:OllyDbg:ANDexecuted

The 10th bit has been cleared (or, in other words, all bits were left except the 10th) and the final value
now is
0x12344 478 (0b1001000110100010001 0 001111000).

Optimizing MSVC

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

Listing 1.272: 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 1.273: 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]