Slide 15.3: IMUL instruction

IMUL Instruction

The IMUL (signed multiply) instruction performs signed integer multiplication by preserving the sign of the product.

IMUL — Signed Multiply

Usage:

IMUL src
IMUL src, immed (286+)
IMUL dest, src, immed8 (286+)
IMUL dest, src (386+)

Flag	O	D	I	S	Z	A	P	C
Result	*			?	?	?	?	*

Signed multiplication of accumulator by “src” with result placed in the accumulator. If the source operand is a byte value, it is multiplied by AL and the result stored in AX. If the source operand is a word value it is multiplied by AX and the result is stored in DX:AX. Other variations of this instruction allow specification of source and destination registers as well as a third immediate factor.

	Clocks
Operands	286	386	486	Size Bytes
reg8	13	9-14	13-18	2
reg16	21	9-22	13-26	2
reg32		9-38	12-42	2
mem8	16	12-17	13-18	2-4
mem16	24	12-25	13-26	2-4
mem32		12-41	13-42	2-4
reg16, reg16		9-22	13-26	3-5
reg32, reg32		9-38	13-42	3-5
reg16, mem16		12-25	13-26	3-5
reg16, immed	21	9-22	13-26	3
reg32, immed	21	9-38	13-42	3-6
reg16, reg16, immed	21	9-22	13-26	3-6
reg32, reg32, immed	21	9-38	13-42	3-6
reg16, mem16, immed	24	12-25	13-26	3-6
reg32, mem32, immed	24	12-41	13-42	3-6

It has three formats:

One-operand formats: Store the product in the accumulator:
```
   IMUL  r/m8          ; AX = AL × r/m byte
   IMUL  r/m16         ; DX:AX = AX × r/m word
   IMUL  r/m32         ; EDX:EAX = EAX × r/m doubleword
```
The storage size of the product makes overflow impossible. The Carry and Overflow flags are set if the upper half of the product is not a sign extension of the lower half.