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IMUL -- Signed Multiply

Opcode      Instruction            Clocks      Description

F6  /5      IMUL r/m8              9-14/12-17  AX= AL * r/m byte
F7  /5      IMUL r/m16             9-22/12-25  DX:AX := AX * r/m word
F7  /5      IMUL r/m32             9-38/12-41  EDX:EAX := EAX * r/m dword
0F  AF /r   IMUL r16,r/m16         9-22/12-25  word register := word
                                               register * r/m word
0F  AF /r   IMUL r32,r/m32         9-38/12-41  dword register := dword
                                               register * r/m dword
6B  /r ib   IMUL r16,r/m16,imm8    9-14/12-17  word register := r/m16 *
                                               sign-extended immediate byte
6B  /r ib   IMUL r32,r/m32,imm8    9-14/12-17  dword register := r/m32 *
                                               sign-extended immediate byte
6B  /r ib   IMUL r16,imm8          9-14/12-17  word register := word
                                               register * sign-extended
                                               immediate byte
6B  /r ib   IMUL r32,imm8          9-14/12-17  dword register := dword
                                               register * sign-extended
                                               immediate byte
69  /r iw   IMUL r16,r/m16,imm16   9-22/12-25  word register := r/m16 *
                                               immediate word
69  /r id   IMUL r32,r/m32,imm32   9-38/12-41  dword register := r/m32 *
                                               immediate dword
69  /r iw   IMUL r16,imm16         9-22/12-25  word register := r/m16 *
                                               immediate word
69  /r id   IMUL r32,imm32         9-38/12-41  dword register := r/m32 *
                                               immediate dword

Notes

The 80386 uses an early-out multiply algorithm. The actual number of clocks depends on the position of the most significant bit in the optimizing multiplier, shown underlined above. The optimization occurs for positive and negative values. Because of the early-out algorithm, clock counts given are minimum to maximum. To calculate the actual clocks, use the following formula: 
  Actual clock = if m <> 0 then max(ceiling(log{2}(m)), 3) + 6 clocks  
  Actual clock = if m = 0 then 9 clocks   
  (where m is the multiplier)
Add three clocks if the multiplier is a memory operand.

Operation

result := multiplicand * multiplier;

Description

IMUL performs signed multiplication. Some forms of the instruction use implicit register operands. The operand combinations for all forms of the instruction are shown in the "

Description

" column above.

IMUL clears the overflow and carry flags under the following conditions: 

   Instruction Form    Condition for Clearing CF and OF
   r/m8                AL := sign-extend of AL to 16 bits
   r/m16               AX := sign-extend of AX to 32 bits
   r/m32               EDX:EAX := sign-extend of EAX to 32 bits
   r16,r/m16           Result exactly fits within r16
   r/32,r/m32          Result exactly fits within r32
   r16,r/m16,imm16     Result exactly fits within r16
   r32,r/m32,imm32     Result exactly fits within r32

Flags Affected

OF and CF as described above; SF, ZF, AF, and PF are undefined

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exeptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

When using the accumulator forms (IMUL r/m8, IMUL r/m16, or IMUL r/m32), the result of the multiplication is available even if the overflow flag is set because the result is two times the size of the multiplicand and multiplier. This is large enough to handle any possible result.

up: Chapter 17 -- 80386 Instruction Set
prev: IDIV Signed Divide
next: IN Input from Port