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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.

    The 80386 can handle 64-bit multiplications in the same way, but uses the
    EDX:EAX register pair.

    Starting with the 80186, IMUL has two additional uses that allow for 16-
    bit results:

                IMUL register16, immediate16

    In this form, register16 is multiplied by immediate16, and the
    result placed in register16. The other form looks like this:

                IMUL register16, memory16, immediate16

    Here, memory16 is multiplied by immediate16, and the result is
    placed in register16.

    In both these forms, the carry and overflow flags will be set if the
    result is too large to fit into 16 bits.

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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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