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CAVITE STATE UNIVERSITY
T3 CAMPUS
Department of Information Technology COSC 65 – Programming Languages

Second Group: MUL, IMUL, DIV, IDIV
These types of operands are supported:
REG
memory

REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc.…

MUL and IMUL instructions affect these flags only: CF, OF.

When the result is over operand size, these flags are set to 1, when result fits in operand size,
these flags are set to 0.

For DIV and IDIV flags are undefined.

 MUL – Unsigned Multiply:  IMUL – Singed Multiply:
When operand is a byte: When operand is a byte:
AX = AL * operand. AX = AL * operand.

When operand is a word: When operand is a word:
(DX AX) = AX * operand. (DX AX) = AX * operand.

 DIV – Unsigned Divide:  IDIV – Signed Divide:
When operand is a byte: When operand is a byte:
AL = AX / operand AL = AX / operand
AH = remainder (modulus) AH = remainder (modulus).

When operand is a word: When operand is a word:
AX = (DX AX) / operand AX = (DX AX) / operand
DX = remainder (modulus) DX = remainder (modulus).

MUL Instructions

Multiplicand Multiplier Product
MUL reg/mem8 ; 8 bit reg to 8 bit mem
MUL reg/mem16 ; 16 bit reg to 16 bit mem AL reg/mem8 AX
MUL reg/mem32 ; 32 bit reg to 32 big mem AX reg/mem16 DX:AX

; Sample code for MUL Instruction
.model small
.data
.code
main proc
mov ax, 2000h
mov bx, 100h
mul bx
endp
end main

Note: You can use the single step process on emu8086 to check what’s happening on our code.

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