lecture 5 arithmetic instructions

Prepared by: Khurram Tanvir 1 CS203 HO#5

Kingdom of Saudi Arabia د��ا �� ا�� ا�Royal Commission at Yanbu �� ا ��ا University College – Yanbu ��ا ��ا-�� Department of ACS & AIT Yanbu Al-Sinaiyah

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2ND Semester 2007-08

CS-203 HANDOUT 1 (HO - 5)

ARITHMETIC INSTRUCTIONS IN 8086

5. Introduction

This unit deals with the arithmetic instructions used with the 8086 processor.

5.1 Instructions to Add and Subtract

8086 uses same instructions to add and subtract signed and unsigned numbers. These are as follows:

ADD used for adding

INC used for incrementing

SUB used for subtracting

DEC used for decrementing

All these instructions operate on values in single registers, memory or constants. They can also be combined to handle larger values that require two registers for storage. The syntax for Add and subtract are as follows: ADD Register, Immediate Number

Register, Register Register, Memory Location Memory Location, Register

Flags Affected Operation Opcode Code Example Meaning OF SF ZF AF PF CF

ADD ADD AX,5 AX ← AX + 5 * * * * * * Addition

INC INC BX [BX]←[BX]+1 * * * * * - SUB SUB CL,AH CL ← CL – AH * * * * * * Subtraction

DEC DEC VAR1 [VAR1] ← [VAR1] – 1 * * * * * - Example: ADD AL, 34; 34 is added with data of register AL and result is stored in AL ADD AH, AL; Data of AL is added with data of AH and result is stored in AH ADD AX, Number1; Data stored in memory location ‘Number1’ added with AX ADD Number1, BX; Data of register BX is added with Memory Location


Note: Destination cannot be an immediate number. In an instruction, source and destination cannot be memory.

ADD 34, AL; -> WRONG

ADD NUM1, NUM2 -> WRONG (memory to memory addition not allowed in 8086)

5.2 Instructions of Multiplication and Division

To multiply and divide, 8086 has different instructions for signed and unsigned numbers.

Multiplication and division instructions have also special requirements depending on the size of the operands and the processor the code runs on.

• MUL used to multiply the unsigned numbers

• IMUL used to multiply the signed numbers

• DIV used to divide the unsigned numbers

• IDIV used to divide the unsigned numbers

The syntax for multiply and divide are as follows:

• MUL Register example MUL CX

• IMUL Register example IMUL CX

• DIV Register example DIV CX

• IDIV Register example IDIV CX Note that before performing the multiplication, we need to store the first number in the accumulator register. Also to perform the division, we need to store the number to be divided (dividend) in data register. Following table shows the example codes, their meaning, and flags affected after the execution of the instruction.

• * means flag changed

• - means flag not affected by instruction

• ? means flag is undefined after executing of instruction

The next examples show 8-bit signed and unsigned addition and subtraction.

8-bit signed and unsigned addition .DATA

MEM8 DB 3

.CODE

; ADDITION ;

MOV AL, 2 ; START WITH REGISTER

INC AL ; AL + 1 -> AL

ADD AL, 4 ; AL + 4 -> AL

ADD AL, MEM8 ; AL + 3 -> AL

MOV AH, AL ; COPY TO AH

ADD AL, AH ; AL + AH -> AL


8-bit signed and unsigned subtraction ; SUBTRACTION

;

MOV AL, 9 ; AL <- 9

DEC AL ; AL – 1 ->AL

SUB AL, 2 ; AL – 2 ->AL

;

;

SUB AL, MEM8 ; AL – 3 ->AL

;

;

MOV AH, 4 ; AH <- 4

SUB AL, AH ; AL – AH ->AL

;

;

Example 1

TITLE "PROGRAM 1 EXPERIMENT 4" ; This program reads two numbers from the keyboard and ; gives their sum. This program uses internal registers ; to store the variables.

.MODEL SMALL

.STACK 200

.DATA

CRLF DB 0DH,0AH,'$'

PROMPT1 DB 'Enter the first positive integer: ','$'

PROMPT2 DB 'Enter the second positive integer: ','$'

PROMPT3 DB 'The sum of the two numbers is: ','$'

.CODE

.STARTUP

LEA DX,PROMPT1 ;DISPLAY PROMPT1

MOV AH,09H

INT 21H

MOV AH,01H ;READ FIRST NUMBER

INT 21H

SUB AL,30H ;Convert character to number

MOV CL,AL ;SAVE THE NUMBER IN CL

LEA DX,CRLF ;MOVE CURSOR TO NEXT LINE

MOV AH,09H

INT 21H


MOV AH,09H

INT 21H

MOV AH,01H ;READ SECOND NUMBER

INT 21H


ADD AL,CL ;PERFORM ADDITION AND SAVE RESULT IN CL

MOV CL,AL

ADD CL,30H ;CONVERT DIGIT TO CHARACTER



MOV AH,09H

INT 21H


MOV AH,09H

INT 21H

MOV DL,CL ;DISPLAY SUM

MOV AH,02H

INT 21H

.EXIT

END

Example 2 TITLE "PROGRAM 2 EXPERIMENT 4"

; This program reads two numbers from the keyboard and

; displays their sum. This program uses the memory to

; store the variables.

.MODEL SMALL

.STACK 200

.DATA

CRLF DB 0DH,0AH,'$'

PROMPT1 DB 'Enter the first positive integer: ','$'

PROMPT2 DB 'Enter the second positive integer: ','$'

PROMPT3 DB 'The sum of the two numbers is: ','$'

NUM1 DB ?

NUM2 DB ?

RES DB ?

.CODE

.STARTUP


MOV AH,09H

INT 21H

MOV AH,01H ;READ FIRST NUMBER

INT 21H


MOV NUM1,AL ;SAVE NUM1


MOV AH,09H

INT 21H


MOV AH,09H

INT 21H

MOV AH,01H ;READ SECOND NUMBER

INT 21H


MOV NUM2,AL ;SAVE NUM2

ADD AL,NUM1 ;PERFORM ADDITION

MOV RES,AL ;SAVE RESULT IN RES


MOV AH,09H

INT 21H


MOV AH,09H

INT 21H

;DISPLAY SUM

MOV DL,RES ;RETREIVE RES FROM MEMORY

ADD DL,30H ;CONVERT DIGIT TO CHARACTER


MOV AH,02H

INT 21H

.EXIT

END

5.3 Using Multiplication Instructions:

The MUL instruction multiplies unsigned numbers. IMUL multiplies signed numbers. For both instructions, one factor must be in the accumulator register (AL for 8-bit numbers,

Flags Affected Opcode Code Example Meaning OF SF ZF AF PF CF

Operation DEC DEC VAR1 [VAR1] ← [VAR1] – 1 * * * * * -

Multiplication MUL MUL CL MUL CX

AX ← AL * CL (DX,AX) ← AX* CX

* ? ? ? ? *

AX for 16-bit numbers). The other factor can be in any single register or memory operand. The result overwrites the contents of the accumulator register. Multiplying two 8-bit numbers produces a 16-bit result returned in AX. Multiplying two 16-bit operands yields a 32-bit result in DX:AX. The following examples illustrate multiplication of unsigned 8 integers.

multiplication of unsigned 8-bit integers ; 8-bit unsigned multiply

mov al, 3 ; Load AL

mov bl, 2 ; Load BL

mul bl ; BL * AL -> AX

; Product in AX

; overflow and carry set

multiplication of unsigned 16-bit integers ; 8-bit unsigned multiply

mov ax, 3 ; Load AL

mov bx, 2 ; Load BL

mul bx ; BX * AX -> (DX:AX)

; DX has high 16 bits

; AX has lower 16 bits

5.4 Using Division Instructions:

The DIV instruction divides unsigned numbers, and IDIV divides signed numbers. Both


Flags Affected Operation Opcode Code Example Meaning OF SF ZF AF PF CF

Division DIV DIV CX AX ← Q(([DX,AX])/CX) DX ←R(([DX,AX])/CX)

? ? ? ? ? ?

return a quotient and a remainder. Table summarizes the division operations. The dividend is the number to be divided, and the divisor is the number to divide by. The quotient is the result. The divisor can be in any register or memory location except the registers where the quotient and remainder are returned.

AX ← Quotient(([DX,AX])/CX)

DX ←Remainder(([DX,AX])/CX)

Size of Operand Dividend Register Size of Divisor Quotient Remainder

16 bits AX 8 bits AL AH

32 bits DX:AX 16 bits AX DX

Table: Division Operations

Unsigned division does not require careful attention to flags. The following examples illustrate signed division, which can be more complex.

unsigned division ; Divide 16-bit unsigned by 8-bit

mov ax, 700 ; Load dividend 700

mov bl, 36 ; Load divisor DIV 36

div bl ; Divide BL ------

; Quotient in AL 19

; Remainder in AH 16


Example Test Questions

Fill in the blanks

1. DEC VAR1 means ___________________________________. 2. ___________statement decrements the register by 1.

Specify True or False

1. ADD instruction is used for both signed and unsigned numbers (True/False) 2. IMUL instruction is used for both signed and unsigned numbers (True/False) 3. As a result of multiplication the higher 16 bits go to AX (True/False)

Choose the best answer.

As a result of DIV instruction, the remainder goes to

a. CX register b. AX register c. DX register d. BX register

AS a result of MUL instruction, the higher 16 bits are saved in

a. CX register b. AX register c. DX register d. BX register

Question: How division of unsigned numbers is carried out in 8086.

lecture 5 arithmetic instructions

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