LOCK - Lock Bus

        Usage:  LOCK
                LOCK: (386+ prefix)
        Modifies flags: None

        This instruction is a prefix that causes the CPU assert bus lock
        signal during the execution of the next instruction.  Used to
        avoid two processors from updating the same data location.  The
        286 always asserts lock during an XCHG with memory operands.  This
        should only be used to lock the bus prior to XCHG, MOV, IN and
        OUT instructions.

                                 Clocks                 Size
        Operands         808x  286   386   486          Bytes

        none              2     0     0     1             1

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