Integer Arithmetic - Computer Organization and Assembly Language - Lecture Slides, Slides of Assembly Language Programming

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Computer Organization &

Assembly Languages

Integer Arithmetic

Chapter Overview

 Shift and Rotate Instructions

 Shift and Rotate Applications

 Multiplication and Division Instructions

 Extended Addition and Subtraction

 ASCII and Unpacked Decimal Arithmetic

 Packed Decimal Arithmetic

Logical vs Arithmetic Shifts

 A logical shift fills the newly created bit position

with zero:

• An arithmetic shift fills the newly created bit

position with a copy of the number’s sign bit:

CF

0

CF

SHL Instruction

 The SHL (shift left) instruction performs a logical

left shift on the destination operand, filling the

lowest bit with 0.

CF

0

• Operand types for SHL: SHL destination,count

SHL reg,imm

SHL mem,imm

SHL reg ,CL

SHL mem ,CL

(Same for all shift and

rotate instructions)

SHR Instruction

 The SHR (shift right) instruction performs a logical

right shift on the destination operand. The highest

bit position is filled with a zero.

CF

0

mov dl, shr dl,1 ; DL = 40 shr dl,2 ; DL = 10

Shifting right n bits divides the operand by 2 n

SAL and SAR Instructions

 SAL (shift arithmetic left) is identical to SHL.

 SAR (shift arithmetic right) performs a right

arithmetic shift on the destination operand.

CF

An arithmetic shift preserves the number's sign.

mov dl,- sar dl,1 ; DL = - sar dl,2 ; DL = -

ROL Instruction

 ROL (rotate) shifts each bit to the left

 The highest bit is copied into both the Carry flag

and into the lowest bit

 No bits are lost

CF

mov al,11110000b rol al,1 ; AL = 11100001b

mov dl,3Fh rol dl,4 ; DL = F3h

ROR Instruction

 ROR (rotate right) shifts each bit to the right

 The lowest bit is copied into both the Carry flag

and into the highest bit

 No bits are lost

CF

mov al,11110000b ror al,1 ; AL = 01111000b

mov dl,3Fh ror dl,4 ; DL = F3h

RCL Instruction

 RCL (rotate carry left) shifts each bit to the left

 Copies the Carry flag to the least significant bit

 Copies the most significant bit to the Carry flag

CF

clc ; CF = 0

mov bl,88h ; CF,BL = 0 10001000b

rcl bl,1 ; CF,BL = 1 00010000b

rcl bl,1 ; CF,BL = 0 00100001b

RCR Instruction

 RCR (rotate carry right) shifts each bit to the right

 Copies the Carry flag to the most significant bit

 Copies the least significant bit to the Carry flag

stc ; CF = 1

mov ah,10h ; CF,AH = 1 00010000b

rcr ah,1 ; CF,AH = 0 10001000b

CF

SHLD Instruction

 Shifts a destination operand a given number of bits

to the left

 The bit positions opened up by the shift are filled

by the most significant bits of the source operand

 The source operand is not affected

 Syntax:

SHLD destination, source, count

 Operand types:

SHLD reg16/32 , reg16/32, imm8 /CL SHLD mem16/32, reg16/32, imm8 /CL

SHLD Example

.data

wval WORD 9BA6h

.code

mov ax,0AC36h

shld wval,ax,

9BA6 AC

BA6A AC

wval AX

Shift wval 4 bits to the left and replace its lowest 4

bits with the high 4 bits of AX:

Before:

After:

SHRD Example

mov ax,234Bh

mov dx,7654h

shrd ax,dx,

Shift AX 4 bits to the right and replace its highest 4

bits with the low 4 bits of DX:

Before:

After:

7654 234B

7654 4234

DX AX

Your turn...

mov ax,7C36h

mov dx,9FA6h

shld dx,ax,4 ; DX =

shrd dx,ax,8 ; DX =

Indicate the hexadecimal values of each destination

operand:

FA67h 36FAh