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CMPXCHG8B       Compare and Exchange 8 Bytes         Flags: O D I T S Z A P C
                                                            - - - - - * - - -
CMPXCHG8B destination                                CPU: Pentium+

        Logic   if EDX:EAX = destination
                   ZF . 1
                   destination . ECX:EBX
                else
                   ZF . 0
                   EDX:EAX . destination
                endif

    CMPXCHG8B compares the quadword in EDX:EAX with a 64-bit memory
    operand. If they are equal, the quadword in the ECX:EBX register
    pair is loaded into destination and the zero flag is set;
    otherwise, the destination operand is loaded into the EDX:EAX
    register pair and the zero flag is cleared.


    Opcode      Format
    0F C7 /r    CMPXCHG8B mem64


    Length and timing
    Operands    Bytes   8088    186     286     386     486     Pentium
    mem, reg    3+d(0-2)                                        10   NP