lock cmpxchg指令

CMPXCHG - Compare and Exchange
0F B0/ r CMPXCHG r/m8,r8 Compare AL with r/m8. If equal, ZF is set and r8 is loaded
into r/m8. Else, clear ZF and load r/m8 into AL.
0F B1/ r CMPXCHG r/m16,r16 Compare AX with r/m16. If equal, ZF is set and r16 is
loaded into r/m16. Else, clear ZF and load r/m16 into AL
0F B1/ r CMPXCHG r/m32,r32 Compare EAX with r/m32. If equal, ZF is set and r32 is
loaded into r/m32. Else, clear ZF and load r/m32 into AL

Description
Compares the value in the AL, AX, or EAX register (depending on the size of the operand) with the first operand (destination operand). If the two values are equal, the second operand (source operand) is loaded into the destination operand. Otherwise, the destination operand is loaded into the AL, AX, or EAX register.

This instruction can be used with a LOCK prefix to allow the instruction to be executed atomi-cally. To simplify the interface to the processor’s bus, the destination operand receives a write cycle without regard to the result of the comparison. The destination operand is written back if the comparison fails; otherwise, the source operand is written into the destination. (The processor never produces a locked read without also producing a locked write.)