IMUL             Integer Multiply, Signed            Flags: O D I T S Z A P C
                                                            *       ? ? ? ? *
IMUL source

          Logic:    AX    . AL * source          ; if source is a byte
               or
                    DX:AX . AX * source          ; if source is a word

    IMUL performs signed multiplication. If source is a byte, IMUL
    multiplies source by AL, returning the product in AX. If source is a
    word, IMUL multiplies source by AX, returning the product in DX:AX.
    The Carry Flag and Overflow Flag are set if the upper half of the
    result (AH for a byte source, DX for a word source) contains any
    significant digits of the product, otherwise they are cleared.

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   Operands                  Clocks   Transfers  Bytes   Example
   reg8                      80-98        -        2     IMUL CL
   reg16                    128-154       -        2     IMUL BX
   mem8                  (86-104) + EA    1       2-4    IMUL BITE
   mem16                 (138-164) + EA   1       2-4    IMUL WORD[BP][DI]
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