REP - Repeat String Operation

        Usage:  REP
        Modifies flags: None

        Repeats execution of string instructions while CX != 0.  After
        each string operation, CX is decremented and the Zero Flag is
        tested.  The combination of a repeat prefix and a segment override
        on CPU's before the 386 may result in errors if an interrupt occurs
        before CX=0.  The following code shows code that is susceptible to
        this and how to avoid it:

         again:  rep movs  byte ptr ES:[DI],ES:[SI]   ; vulnerable instr.
                     jcxz  next              ; continue if REP successful
                     loop  again             ; interrupt goofed count

                                 Clocks                 Size
        Operands         808x  286   386   486          Bytes

        none              2     2     2                   1

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