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Chapter 17 -- 80386 Instruction Set

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# MUL -- Unsigned Multiplication of AL or AX

Opcode Instruction Clocks Description
F6 /4 MUL AL,r/m8 9-14/12-17 Unsigned multiply (AX := AL * r/m byte)
F7 /4 MUL AX,r/m16 9-22/12-25 Unsigned multiply (DX:AX := AX * r/m
word)
F7 /4 MUL EAX,r/m32 9-38/12-41 Unsigned multiply (EDX:EAX := EAX * r/m
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 multiplier 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 <> 0 then max(ceiling(log{2}(m)), 3) + 6 clocks;
Actual clock = if = 0 then 9 clocks

where `m` is the multiplier.
## Operation

IF byte-size operation
THEN AX := AL * r/m8
ELSE (* word or doubleword operation *)
IF OperandSize = 16
THEN DX:AX := AX * r/m16
ELSE (* OperandSize = 32 *)
EDX:EAX := EAX * r/m32
FI;
FI;

## Description

MUL performs unsigned multiplication. Its actions depend on the size
of its operand, as follows:
- A byte operand is multiplied by AL; the result is left in AX. The
carry and overflow flags are set to 0 if AH is 0; otherwise, they are
set to 1.
- A word operand is multiplied by AX; the result is left in DX:AX.
DX contains the high-order 16 bits of the product. The carry and
overflow flags are set to 0 if DX is 0; otherwise, they are set to 1.
- A doubleword operand is multiplied by EAX and the result is left in
EDX:EAX. EDX contains the high-order 32 bits of the product. The
carry and overflow flags are set to 0 if EDX is 0; otherwise, they are
set to 1.

## Flags Affected

OF and CF as described above; SF, ZF, AF, PF, and CF 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 exceptions as in Real Address Mode; #PF(fault-code) for a page
fault

**up:**
Chapter 17 -- 80386 Instruction Set

**prev:** MOVZX Move with Zero-Extend

**next:** NEG Two's Complement Negation