CMPXCHG - Compare and Exchange

        Usage:  CMPXCHG dest,src  (486+)
        Modifies flags: AF CF OF PF SF ZF

        Compares the accumulator (8-32 bits) with dest.  If equal the
        dest is loaded with src, otherwise the accumulator is loaded
        with dest.

                                 Clocks                 Size
        Operands         808x  286   386   486          Bytes

        reg,reg           -     -     -     6             2
        mem,reg           -     -     -     7             2

        - add 3 clocks if the mem,reg comparison fails


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