位运算与移位指令

Low-level code constantly manipulates individual bits: setting flags, packing fields, masking, and fast math. x86 provides dedicated bitwise and shift instructions.

AND keeps only the bits set in both operands, the standard way to mask out unwanted bits.

OR forces specific bits to 1 while leaving others unchanged.

XOR flips bits where the mask is 1. A famous idiom zeroes a register quickly:

NOT flips every bit (ones complement).

SHL shifts bits left, filling with zeros. Each shift left multiplies an unsigned value by two.

SHR shifts right with zero fill, dividing an unsigned value by powers of two.

SAR shifts right but preserves the sign bit, so it divides signed values correctly.

ROL and ROR rotate bits around, wrapping the bit that falls off back in on the other side, used in hashing and crypto.

BT copies a chosen bit into CF so you can test it, while BTS/BTR test and set/reset it.

Handy patterns:

• xor reg, reg to zero
• shl/shr for power-of-two math
• and reg, mask to extract fields

You learned bitwise and shift instructions:

• AND mask, OR set, XOR toggle, NOT invert
• SHL/SHR for unsigned power-of-two math
• SAR preserves sign for signed division
• ROL/ROR rotate; BT tests a bit into CF