Reverse Engineering for Beginners

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

Listing 19.44: C/C++

if (value & (1<<n))

This is usually implemented in x86 code as:

Listing 19.45: x86

; CL=n
MOV REG, 1
SHL REG, CL
AND input_value, REG

19.6.3 Set specific bit (known at compile stage)

Listing 19.46: C/C++

value=value|0x40;

Listing 19.47: x86

OR REG, 40h

Listing 19.48: ARM (ARM mode) and ARM64

ORR R0, R0, #0x40

19.6.4 Set specific bit (specified at runtime).

Listing 19.49: C/C++

value=value|(1<<n);

This is usually implemented in x86 code as:

Listing 19.50: x86

; CL=n
MOV REG, 1
SHL REG, CL
OR input_value, REG

19.6.5 Clear specific bit (known at compile stage)

Just applyANDoperation with the inverted value:

Listing 19.51: C/C++

value=value&(~0x40);

Listing 19.52: x86

AND REG, 0FFFFFFBFh

Listing 19.53: x64

AND REG, 0FFFFFFFFFFFFFFBFh

This is actually leaving all bits set except one.

ARM in ARM mode hasBICinstruction, which works like theNOT+ANDinstruction pair:

Listing 19.54: ARM (ARM mode)

BIC R0, R0, #0x40