Bitwise operations

AND, OR, XOR, NOT

The logic instructions work bit by bit. AND masks bits off, OR masks bits on, XOR flips bits, and XORing a register with itself zeroes it in one instruction:

and eax, 0xFF ; keep the low byte or eax, 0x80 ; set the top bit of the byte xor eax, eax ; zero eax, faster than mov not eax ; flip every bit

Shifts

SHL and SHR shift left and right, SAR shifts right keeping the sign bit, and SAL is SHL by another name. A shift left by n multiplies by 2 to the n, and a shift right divides:

shl eax, 4 ; eax = eax * 16 shr eax, 1 ; eax = eax / 2, unsigned sar eax, 1 ; eax = eax / 2, signed

Where you see them

Compilers emit shifts for multiplication by constants, XOR for zeroing, and AND for masking in every non-trivial program. Reading them is most of reading a disassembly.

Next: Arrays

Article author: Arthur Isaev