x86 Instruction Set Reference

CMPXCHG8B - Compare and Exchange 8 Bytes

Opcode Mnemonic Description
0F C7 /1 m64 CMPXCHG8B m64 Compare EDX:EAX with m64. If equal, set ZF and load ECX:EBX into m64. Else, clear ZF and load m64 into EDX:EAX.
Description

Compares the 64-bit value in EDX:EAX with the operand (destination operand). If the values are equal, the 64-bit value in ECX:EBX is stored in the destination operand. Otherwise, the value in the destination operand is loaded into EDX:EAX. The destination operand is an 8-byte memory location. For the EDX:EAX and ECX:EBX register pairs, EDX and ECX contain the high-order 32 bits and EAX and EBX contain the low-order 32 bits of a 64-bit value.

This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.

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.)
Operation 
if(EDX:EAX == Destination) {
	ZF = 1;
	Destination = ECX:EBX;
}
else {
	ZF = 0;
	EDX:EAX = Destination;
}
Flags affected

The ZF flag is set if the destination operand and EDX:EAX are equal; otherwise it is cleared. The CF, PF, AF, SF, and OF flags are unaffected.

IA-32 Architecture Compatibility

This instruction is not supported on Intel processors earlier than the Pentium processors.

Protected Mode Exceptions
#UDIf the destination operand is not a memory location.
#UDIf the destination operand is not a memory location.
#GP(0)If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a null segment selector.
#SS(0)If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)If a page fault occurs.
Real-Address Mode Exceptions
#UDIf the destination operand is not a memory location.
#UDIf the destination operand is not a memory location.
#GPIf a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
Virtual-8086 Mode Exceptions
#UDIf the destination operand is not a memory location.
#UDIf the destination operand is not a memory location.
#GP(0)If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0)If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)If a page fault occurs.