F6 /5         IMUL r/m8                 AX← AL * r/m byte

F7 /5         IMUL r/m16                 DX:AX ← AX * r/m word

F7 /5         IMUL r/m32                 EDX:EAX ← EAX * r/m doubleword

0F AF / r         IMUL r16,r/m16                 word register ← word register * r/m word

0F AF / r         IMUL r32,r/m32                 doubleword register ← doubleword register * r/m

                               doubleword

6B / r ib         IMUL r16,r/m16,imm8         word register ← r/m16 * sign-extended immediate byte

6B / r ib         IMUL r32,r/m32,imm8         doubleword register ← r/m32 * sign-extended immediate

                               byte

6B / r ib         IMUL r16,imm8                 word register ← word register * sign-extended immediate

                               byte

6B / r ib         IMUL r32,imm8                 doubleword register ← doubleword register * sign-extended

                               immediate byte

69 / r iw         IMUL r16,r/m16,imm16        word register ← r/m16 * immediate word

69 / r id         IMUL r32,r/m32,imm32        doubleword register ← r/m32 * immediate doubleword

69 / r iw         IMUL r16,imm16                 word register ← r/m16 * immediate word

69 / r id         IMUL r32,imm32                 doubleword register ← r/m32 * immediate doubleword


Description

Performs a signed multiplication of two operands. This instruction has three forms, depending on the number of operands.


One-operand form. This form is identical to that used by the MUL instruction. Here, the source operand (in a general-purpose register or memory location) is multiplied by the value in the AL, AX, or EAX register (depending on the operand size) and the product is stored in the AX, DX:AX, or EDX:EAX registers, respectively.


Two-operand form. With this form the destination operand (the first operand) is multiplied by the source operand (second operand). The destination operand is a general-purpose register and the source operand is an immediate value, a general-purpose register, or a memory location. The product is then stored in the destination operand location.


Three-operand form. This form requires a destination operand (the first operand) and two source operands (the second and the third operands). Here, the first source operand (which can be a general-purpose register or a memory location) is multiplied by the second source operand (an immediate value). The product is then stored in the destination operand (a general-purpose register).


When an immediate value is used as an operand, it is sign-extended to the length of the destination operand format.


The CF and OF flags are set when significant bits are carried into the upper half of the result. The CF and OF flags are cleared when the result fits exactly in the lower half of the result.


The three forms of the IMUL instruction are similar in that the length of the product is calculated to twice the length of the operands. With the one-operand form, the product is stored exactly in the destination. With the two- and three- operand forms, however, result is truncated to the length of the destination before it is stored in the destination register. Because of this truncation, the CF or OF flag should be tested to ensure that no significant bits are lost.


The two- and three-operand forms may also be used with unsigned operands because the lower half of the product is the same regardless if the operands are signed or unsigned. The CF and OF flags, however, cannot be used to determine if the upper half of the result is non-zero.


Accumulator Multiplies

Operands        Bytes                Clocks

r8                  2                    11   NP

r16                 2                   11   NP

r32                 2                   10   NP

mem8            2+d(0-2)          11   NP

mem16           2+d(0-2)         11   NP

mem32           2+d(0-2)          10   NP


implied              operand              result

multiplicand         (multiplier)

AL           *          byte              =  AX

AX            *          word               =  DX:AX

EAX           *          dword              =  EDX:EAX


2 and 3 operand Multiplies

Operands        Bytes                Clocks

r16, imm              2+i(1,2)          10   NP

r32, imm              2+i(1,2)         10   NP

r16,r16,imm           2+i(1,2)         10   NP

r32,r32,imm           2+i(1,2)          10   NP

r16,m16,imm           2+d(0-2)+i(1,2)         10   NP

r32,m32,imm           2+d(0-2)+i(1,2)         10   NP

r16, r16              2+i(1,2)          10   NP

r32, r32              2+i(1,2)          10   NP

r16, m16              2+d(0-2)+i(1,2)         10   NP

r32, m32              2+d(0-2)+i(1,2)         10   NP


Flags

For the one operand form of the instruction, the CF and OF flags are set when significant bits are carried into the upper half of the result and cleared when the result fits exactly in the lower half of the result. For the two- and three-operand forms of the instruction, the CF and OF flags are set when the result must be truncated to fit in the destination operand size and cleared when the result fits exactly in the destination operand size. The SF, ZF, AF, and PF flags are undefined.

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