CMPXCHG8B
Usage: CMPEXHG8B dest,src Modifies flags: ZF
Compares the accumulator (64 bits) with 'dest'. If equal the 'dest' is loaded with 'src', otherwise the accumulator is loaded with 'dest'.
Compare and Exchange Eight Bytes
CMPXCHG8B mem ; 0F C7 /1 [PENT]
This is a larger and more unwieldy version of CMPXCHG: it compares the 64-bit (eight-byte) value stored at [mem] with the value in EDX:EAX. If they are equal, it sets the zero flag and stores ECX:EBX into the memory area. If they are unequal, it clears the zero flag and stores the memory contents into EDX:EAX.
CMPXCHG8B can be used with the LOCK prefix, to allow atomic execution. This is useful in multi-processor and multi-tasking environments.
EXAMPLE:
mov D$Test 15
mov eax 55
mov ecx 20
cmpxchg8b Q$Test ; eax = 15d > 0Fh