Bitwise

Chapter 11 - Bitwise Instructions

11.1 Bitwise

Bitwise operators work at the binary level. They allow you to manipulate individual bits within an integer. They are used for low-level programming of device drivers, graphics, and communications protocols. C++ and Java also have the bitwise operators. Bitwise operations would allow you to implement a large number of Boolean variables without using much space. A 32-bit integer couple be used for 32 Boolean variables.

Here is an online bitwise calculator to help visualize: http://bitwisecmd.com/

Bitwise Operators
Name	C++/Java Operator	Example	x86 Example - Results placed in first operand
Bitwise AND	&	42 00101010 & 27 00011011 = 10 00001010	mov eax, 42 and eax, 27
Bitwise OR	\|	3 00000011 & 17 00010001 = 19 00010011	mov [A], 3 or [A], 17
Bitwise XOR	^	3 00000011 ~ 17 00010001 = 18 00010010	mov eax, 3 mov ebx, 17 xor al, bl
Bitwise NOT	~	~ 42 00101010 = 213 11010101	mov edx, 42 not edx
Shift Left	<<	42 << 1 00101010 = 84 01010100	mov eax, 42 shl eax, 1
Shift Right	>>	100 >> 2 01100100 = 25 00011001	mov [A], 100 shr [A], 2

The AND is a good way to isolate bits. If you AND binary 10101001 with 00001111 you will get the right 4 digits: 00001001.

The shift left is a fast way to multiply and integer by 2. The shift right is a fast way to divided an integer by 2. If bits slide off the end, the carry flag is set. You shouldn't use the shift left and right for negative numbers.

bitwise.asm

format PE console
include 'win32ax.inc'

;=======================================
section '.code' code readable executable
;=======================================
start:
        mov eax, 42
        and eax, 27
        mov [A], eax
        cinvoke printf,"42 & 27 = %d %c",[A],10
        mov [A], 3
        or [A], 17
        cinvoke printf,"3 | 17 = %d %c",[A],10
        mov eax, 3
        mov ebx, 17
        xor al, bl
        mov [A], eax
        cinvoke printf,"3 ^ 17 = %d %c",[A],10
        mov [A], 100
        shr [A], 2
        cinvoke printf,"100 >> 2 = %d %c",[A],10
        invoke Sleep,-1

;======================================
section '.data' data readable writeable
;======================================
A       dd 0

;====================================
section '.idata' import data readable
;====================================
library msvcrt,'msvcrt.dll',kernel32,'kernel32.dll'
import msvcrt,printf,'printf'
import kernel32,Sleep,'Sleep'

Output

42 & 27 = 10
3 | 17 = 19
3 ^ 17 = 18
100 >> 2 = 25