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(nasm.info)Section B.4.31

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B.4.31. `CMPXCHG8B': Compare and Exchange Eight Bytes
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     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.