ACOE2511 Assembly Language Arithmetic and Logic Instructions.

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Assembly Language

Arithmetic and Logic Instructions

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Arithmetic Instructions: (Addition)

A. Addition: (Flags affected: A,C,O,P,S,Z)

– ADD AL,BL ; AL AL + BL , BL unchanged

– ADD CX,DI ; CX CX + DI , DI unchanged

– ADD AH,45H ; AH AH + 45H

– ADD [BX],AL ; [BX] [BX] + AL

– ADD CX,[BX] ; CX CX + [BX]

– ADD AL,CX ; INVALID

B. Add with Carry: (Flags affected: A,C,O,P,S,Z)

– ADC AH,BH ; AH AH + BH + Carry

– ADC AX,CX ; AX AX + CX + Carry

– ADC AL,[BX+SI] ; AL AL + [BX+SI] + Carry

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Arithmetic Instructions: (Subtraction)

C. Subtraction: (Flags affected: A,C,O,P,S,Z)

– SUB AL,BL ; AL AL - BL ;BL unchanged

– SUB CX,DI ; CX CX - DI ;DI unchanged

– SUB AH,45H ; AH AH - 45H

– SUB BL,ARRAY ; BL BL - [ARRAY]

– SUB [BX],AL ; [BX] [BX] - AL

– SUB AL,CX ; INVALID

D. Subtract with Borrow: (Flags affected: A,C,O,P,S,Z)

– SBB AH,BH ; AH AH - BH - Carry

– SBB AX,CX ; AX AX - CX - Carry

– SBB AL,[BX+SI] ; AL AL - [BX+SI] - Carry

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Arithmetic Instructions: (Increment, Decrement)

E. Increment: (Flags affected: A,O,P,S,Z)

– INC AL ; AL AL + 1

– INC SP ; SP SP + 1

– INC COUNT1 ; [COUNT1] [COUNT1] + 1

– INC BYTE PTR[BX] ; [BX] [BX] + 1

– INC WORD PTR[BX] ; [BX] [BX] + 1

F. Decrement: (Flags affected: A,O,P,S,Z)

– DEC AL ; AL AL - 1

– DEC SP ; SP SP - 1

– DEC COUNT1 ; [COUNT1] [COUNT1] - 1

– DEC BYTE PTR[BX] ; [BX] [BX] - 1

– DEC WORD PTR[BX] ; [BX] [BX] - 1

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Examples• What will be the values of the carry, overflow,

sign and zero flags after the execution of each of the following instructions:

MOV DX,0

DEC DX

MOV AX,720H

SUB AX, 0E6H

MOV DX,0

DEC DX

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Example: Fill up the trace table given below.

Instructions AX

MOV AX,847AH

BX CX SI Cy S Z Address Data

0100

0101

0102

0103

0104

0105

0106

0107

0108

0109

010A

010B

010C

010D

010E

3F

78

5A

C8

93

59

4F

A3

7E

F4

09

8A

5C

6A

45

SUB CX,CX

MOV BX,5CE8H

ADD AL,BH

ADC AH,BL

MOV CL,AL

ADD CH,BL

MOV SI,0108H

SUB BL,[SI]

SBB BH,[SI+4]

ADD AL,[010CH]

MOV AX,[0106H]

INC AL

DEC BX

ADD CX,[S1+4]

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Arithmetic Instructions: (Multiplication)

G. Multiplication: (Flags affected: C,O, (A,P,S,Z are undefined))

Unsigned multiplication:

– MUL CL ; AX AL * CL

– MUL CX ; DX,AX AX * CX– MUL BYTE PTR [BX] ; AX AL * [BX] – MUL WORD PTR [SI] ; DX,AX AX * [SI]

Signed multiplication (2's complement):

– IMUL BL ; AX AL * BL

– IMUL BX ; DX,AX AX * BX

– IMUL BYTE PTR [BX] ; AX AL * [BX]

– IMUL WORD PTR [SI] ; DX,AX AX * [SI]

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Arithmetic Instructions: (Division)

H. Division: (Flags affected: A,C,O,P,S,Z (all undefined))

Unsigned Division:

– DIV CL ; AL Quotient of AX/CL

; AH Remainder of AX/CL

– DIV CX ; AX Quotient of DX,AX/CX

; DX Remainder of DX,AX/CX

Signed Division:

– IDIV CL ; AL Quotient of AX/CL

; AH Remainder of AX/CL

– IDIV CX ; AX Quotient of DX,AX/CX

; DX Remainder of DX,AX/CX

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Instructions AX

MOV AX,42C1H

BX DX SI Cy S Z Address Data

0100

0101

0102

0103

0104

0105

0106

0107

0108

0109

010A

010B

010C

010D

010E

3F

78

5A

C8

93

59

4F

A3

7E

F4

09

8A

5C

6A

45

SUB DX,DX

MOV BX,2456H

MUL BL

MUL BX

MOV AX,6A42H

MOV CH,32H

DIV CH

MOV SI,0103H

MUL BYTE PTR [SI]

IMUL BYTE PTR [SI]

MOV AX,[0109H]

IMUL AH

MOV BL,2

DIV BL

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Arithmetic Instructions: (BCD and ASCII Operations)

I. BCD and ASCII Arithmetic:

– DAA ; Decimal Adjust for Addition

– DAS ; Decimal Adjust for Subtraction

– AAA ; ASCII Adjust for Addition

– AAS ; ASCII Adjust for Subtraction

– AAM ; ASCII Adjust for Multiplication

– AAD ; ASCII Adjust for Division

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Logic Instructions:(AND, OR, XOR, NOT, NEG and TEST)

Logic Instructions:

• AND AL,BL ; AL AL AND BL (Always clears C and O flags)

• AND CL,33H ; CL CL AND 33H

• AND AX,[DI] ; AX AX AND [DI]

• OR AL,BL ; AL AL OR BL

• OR AX,1234H ; AX AX OR 1234H

• XOR AL,CL ; AL AL EX-OR CL

• XOR BH,0FH ; BH BH EX-OR 0FH

• NOT CH ; CH 1's complement of CH (No flags affected)

• NOT AX ; AX 1's complement of AX

• NEG CH ; CH 2's complement of CH (ALWAYS SETS CF)

• NEG BX ; BX 2's complement of BX

• TEST AL,30H ;Perform AL AND30H and set the flags. AL is unchanged.

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Instructions AX

MOV AX,847AH

BX CX SI Cy S Z Address Data

0100

0101

0102

0103

0104

0105

0106

0107

0108

0109

010A

010B

010C

010D

010E

3F

78

5A

C8

93

59

4F

A3

7E

F4

09

8A

5C

6A

45

XOR BX,BX

MOV CX,5CE8H

AND AL,BH

AND AH,92H

OR CL,AL

XOR CH,BL

MOV SI,0108H

OR BL,[SI]

AND BH,[SI+2]

XOR AL,[010CH]

NOT AX

NEG BL

TEST BL,14H

TEST AL,3FH

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Program Control Instructions :(Jump and Call)

Unconditional jump (JMP):

– The JMP instruction specifies the address of the next instruction to be executed. There are three types of unconditional jump instructions: the SHORT, the NEAR, and the FAR. A SHORT jump is specified with only one byte which represents the displacement between the current instruction to the next instruction. The next instruction can be located at a distance from +127 to -128 memory locations away from the current instruction. A NEAR jump specifies the address of the next instruction within the current Code Segment. A FAR jump specifies the exact address of the next instruction by specifying the values of the CS and IP registers.

– Examples:

• JMP NEXT

• JMP SHORT AGAIN

• JMP NEAR AGAIN

• JMP FAR AGAIN

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Conditional jump:

– Conditional jumps are executed only if the specified conditions are true. Usually the condition specified by a conditional jump instruction is the state of a flag. A list of the conditional jump instructions that check the state of flags is given below:

Instruction Flags tested Action

JC C = 1 Jump if carry set JZ Z = 1 Jump if equal or zero

JS S = 1 Jump on sign (Negative)

JO O = 1 Jump on overflow

JNC C = 0 Jump if not carry

JNZ Z = 0 Jump if not equal or 0

JNS S = 0 Jump if not sign (Positive)

JNO O = 0 Jump if not overflow

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Instructions AX

MOV AX,547AH

BX Cy S Z

MOV BX,9C8FH

ADD AL,BH

JC SKIP1

INC AL

ADD AH,BL

JC SKIP2

DEC AL

SUB BH,AL

JC SKIP3

DEC AX

SUB BL,AH

JNC SKIP4

INC BX

SKIP1:

SKIP2:

SKIP3:

SKIP4:

INC BX

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Conditional jump using the Compare instruction:

– Conditional jump instruction can be used after the compare ( CMP ) instruction.

– Comparison of unsigned numbers is done using the Above or Below conditions. For example 81H is Above 7EH since 129>128.

– Comparison of unsigned numbers is done using the Greater or Less conditions. For example 81H is Less than 7EH since -127<128.

– The programmer can choose between the Above/Below or Greater/Less according to the application. If the values used in a program are always positive then these values are treated as unsigned numbers, and the Above /Below conditions are used, otherwise signed numbers are used and the Greater/Less conditions are used..

Hex

00

Unsigned

00

01

02

126

127

128

129

130

253

254

255

01

02

7E

7F

80

81

82

FD

FE

FF

Signed

00

+01

+02

+126

+127

-128

-127

-126

-03

-02

-01

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A list of the conditional jump instructions used with the Compare instruction is given below:

Instruction Flags tested Action

JA C = 0 & Z = 0 Jump if above

JAE C = 0 Jump if above or equal

JB C = 1 Jump if below

JBE C = 1 or Z = 1 Jump if below or equal

JE or JZ Z = 1 Jump if equal or zero

JG O = Z AND S Jump if greater

JGE S = O Jump if greater or equal

JL S = O Jump if less

JLE Z = 1 or S = O Jump if less or equal

JNE or JNZ Z = 0 Jump if not equal or 0

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Instructions AH

MOV AX,NUM

CMP AH,77H

JA SKIP1

INC AL

JA SKIP2

INC AL

JL SKIP3

INC AH

JL SKIP4

INC AL

SKIP1:

SKIP2:

SKIP3:

SKIP4:

INC AL

NUM=4500H

AL

CMP AH,89H

CMP AH,77H

CMP AH,97H

NUM=9500H NUM=C500H

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Loops:

– The LOOP instruction is a combination of the conditional jump and the decrement CX instructions. It will decrement the contents of CX and, if CX is not zero, jump to the label associated with the LOOP. If CX becomes zero, then the next sequential instruction is executed.

– The LOOP instruction can also have conditional forms LOOPE (LOOPZ), and LOOPNE (LOOPNZ).

– The conditional jump instruction JCXZ (Jump if CX = 0) can also be used.

Procedures:

– Procedures are implemented by using the PROC directive. The last instruction in a procedure must be the RET instruction. A procedure can be called by using the CALL instruction.

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Shift instructions• SHL AL, BL ;Shifts the contents of AL to the

left as many times as the value of BL, filling the lowest bit with zero and moving the highest to the carry flag

• SHR AL, BL ; The same principle, but now a right shift

CF0

CF 0

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Arithmetic shifts

• SAL AL, BL ;Identical to SHL

• SAR AL, BL ;Shift right, but the most significant bit fills the leftmost position again

CF

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Rotate instructions

• ROL

• ROR

CF

CF

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Rotate with carry instructions

• RCL

• RCR

CF

CF

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Example: Fill up the trace table given below. Instructions AX

MOV AX,547AH

BX Cy S Z

MOV BX,9C8FH

SHR BL, 1

SHL AL, 1

MOV AL, 4H

SHL BH, AL

ROL AH, 1

ROR BL, 3

SUB BH,AL

SAR BH, 1

RCR BH, AL

SHL AH, AL

SUB AL, 3

RCL BL, AL

INC AL

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IN AND OUT INSTRUCTIONS

• IN AL, 3C ;Input byte from port 3Ch

• OUT 3Ch, AL ;Output byte to port 3Ch

• Use AX if you want to transfer 16 bits and EAX if you want to transfer 32 bits.

ACOE2511 Assembly Language Arithmetic and Logic Instructions.

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