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Vol. 2A 3-695
INSTRUCTION SET REFERENCE, A-M
MUL—Unsigned Multiply
MUL—Unsigned Multiply
Description
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 Table
3-61.
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.
In 64-bit mode, the instruction’s default operation size is 32 bits. Use of the REX.R
prefix permits access to additional registers (R8-R15). Use of the REX.W prefix
promotes operation to 64 bits.
See the summary chart at the beginning of this section for encoding data and limits.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
F6 /4 MUL r/m8 Valid Valid Unsigned multiply (AX ← AL ∗ r/m8).
REX + F6 /4 MUL r/m8
*
Valid N.E. Unsigned multiply (AX ← AL ∗ r/m8).
F7 /4 MUL r/m16 Valid Valid Unsigned multiply (DX:AX ← AX ∗
r/m16).
F7 /4 MUL r/m32 Valid Valid Unsigned multiply (EDX:EAX ← EAX ∗
r/m32).
REX.W + F7 /4 MUL r/m64 Valid N.E. Unsigned multiply (RDX:RAX ← RAX ∗
r/m64.
NOTES:
* In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is
used: AH, BH, CH, DH.
Table 3-61. MUL Results
Operand Size Source 1 Source 2 Destination
Byte AL r/m8 AX
Word AX r/m16 DX:AX
Doubleword EAX r/m32 EDX:EAX
Quadword RAX r/m64 RDX:RAX