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(nasm.info)Section B.4.118

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B.4.118. `IMUL': Signed Integer Multiply
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     IMUL r/m8                     ; F6 /5                [8086]
     IMUL r/m16                    ; o16 F7 /5            [8086]
     IMUL r/m32                    ; o32 F7 /5            [386]

     IMUL reg16,r/m16              ; o16 0F AF /r         [386]
     IMUL reg32,r/m32              ; o32 0F AF /r         [386]

     IMUL reg16,imm8               ; o16 6B /r ib         [186]
     IMUL reg16,imm16              ; o16 69 /r iw         [186]
     IMUL reg32,imm8               ; o32 6B /r ib         [386]
     IMUL reg32,imm32              ; o32 69 /r id         [386]

     IMUL reg16,r/m16,imm8         ; o16 6B /r ib         [186]
     IMUL reg16,r/m16,imm16        ; o16 69 /r iw         [186]
     IMUL reg32,r/m32,imm8         ; o32 6B /r ib         [386]
     IMUL reg32,r/m32,imm32        ; o32 69 /r id         [386]

   `IMUL' performs signed integer multiplication. For the single-operand
form, the other operand and destination are implicit, in the following
way:

   * For `IMUL r/m8', `AL' is multiplied by the given operand; the
     product is stored in `AX'.

   * For `IMUL r/m16', `AX' is multiplied by the given operand; the
     product is stored in `DX:AX'.

   * For `IMUL r/m32', `EAX' is multiplied by the given operand; the
     product is stored in `EDX:EAX'.

   The two-operand form multiplies its two operands and stores the
result in the destination (first) operand. The three-operand form
multiplies its last two operands and stores the result in the first
operand.

   The two-operand form with an immediate second operand is in fact a
shorthand for the three-operand form, as can be seen by examining the
opcode descriptions: in the two-operand form, the code `/r' takes both
its register and `r/m' parts from the same operand (the first one).

   In the forms with an 8-bit immediate operand and another longer
source operand, the immediate operand is considered to be signed, and
is sign- extended to the length of the other source operand. In these
cases, the `BYTE' qualifier is necessary to force NASM to generate this
form of the instruction.

   Unsigned integer multiplication is performed by the `MUL'
instruction: see *Note Section B.4.184::.