The CMPXCHG8B instruction compares the 64-bit value in EDX:EAX with DEST. EDX contains the high-order 32 bits, and EAX contains the low-order 32 bits of 64-bit value. If they are equal, the 64-bit value in ECX:EBX is stored into DEST. ECX contains the high-order 32 bits and EBX contains the low-order 32 bits. Otherwise, DEST is loaded into EDX:EAX.