CENTRAL PROCESSING UNIT - Little Flower College

Central Processing Unit

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CENTRAL PROCESSING UNIT

Ms.Sindhu K.S

Dept of Computer science

Computer Organization

Computer Architectures Lab

Central Processing Unit

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Introduction

MAJOR COMPONENTS OF CPU

? Storage Components

Registers

Flags

? Execution (Processing) Components

Arithmetic Logic Unit(ALU)

Arithmetic calculations, Logical computations, Shifts/Rotates

? Transfer Components

Bus

? Control Components

Control Unit

Register

File

ALU

Control Unit

Computer Organization

Computer Architectures Lab

Central Processing Unit

3

REGISTERS

? In Basic Computer, there is only one general purpose register,

the Accumulator (AC)

? In modern CPUs, there are many general purpose registers

? It is advantageous to have many registers

¨C Transfer between registers within the processor are relatively fast

¨C Going ¡°off the processor¡± to access memory is much slower

? How many registers will be the best ?

Computer Organization

Computer Architectures Lab

Central Processing Unit

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General Register Organization

GENERAL REGISTER ORGANIZATION

Input

Clock

R1

R2

R3

R4

R5

R6

R7

Load

(7 lines)

SELA

{

3x8

decoder

MUX

MUX

A bus

SELD

OPR

} SELB

B bus

ALU

Output

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Computer Architectures Lab

Central Processing Unit

5

Control

OPERATION OF CONTROL UNIT

The control unit

Directs the information flow through ALU by

- Selecting various Components in the system

- Selecting the Function of ALU

Example: R1 ? R2 + R3

[1] MUX A selector (SELA): BUS A ? R2

[2] MUX B selector (SELB): BUS B ? R3

[3] ALU operation selector (OPR): ALU to ADD

[4] Decoder destination selector (SELD): R1 ? Out Bus

Control Word

3

SELA

3

SELB

3

SELD

5

OPR

Encoding of register selection fields

Binary

Code

000

001

010

011

100

101

110

111

Computer Organization

SELA

Input

R1

R2

R3

R4

R5

R6

R7

SELB

Input

R1

R2

R3

R4

R5

R6

R7

SELD

None

R1

R2

R3

R4

R5

R6

R7

Computer Architectures Lab

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