3-448
INSTRUCTION SET REFERENCE
MUL—Unsigned Multiply
Description
This instruction performs an unsigned multiplication of the first operand (destination operand)
and the second operand (source operand) and stores the result in the destination operand. The
destination operand is an implied operand located in register AL, AX or EAX (depending on the
size of the operand); the source operand is located in a general-purpose register or a memory
location. The action of this instruction and the location of the result depends on the opcode and
the operand size as shown in the following table.
:
The result is stored in register AX, register pair DX:AX, or register pair EDX:EAX (depending
on the operand size), with the high-order bits of the product contained in register AH, DX, or
EDX, respectively. If the high-order bits of the product are 0, the CF and OF flags are cleared;
otherwise, the flags are set.
Operation
IF byte operation
THEN
AX
<
AL
?
SRC
ELSE (* word or doubleword operation *)
IF OperandSize = 16
THEN
DX:AX
<
AX
?
SRC
ELSE (* OperandSize = 32 *)
EDX:EAX
<
EAX
?
SRC
FI;
FI;
Flags Affected
The OF and CF flags are cleared to 0 if the upper half of the result is 0; otherwise, they are set
to 1. The SF, ZF, AF, and PF flags are undefined.
Opcode
Instruction
Description
F6 /4
MUL r/m8
Unsigned multiply (AX
<
AL
?
r/m8)
F7 /4
MUL r/m16
Unsigned multiply (DX:AX
<
AX
?
r/m16)
F7 /4
MUL r/m32
Unsigned multiply (EDX:EAX
<
EAX
?
r/m32)
Operand Size
Source 1
Source 2
Destination
Byte
AL
r/m8
AX
Word
AX
r/m16
DX:AX
Doubleword
EAX
r/m32
EDX:EAX
