Computer Organization

1. INTRODUCTION

The Second half of the twentieth century is usually known as the Age of Computers

The term ‘Computer’ originates from the word ‘Compute’ which means “to calculate”. So a computer is normally considered to be a calculating device that can perform arithmetical operations at an enormous speed.

Computers can be used in various practical life applications such as :

Airline / Railway Reservation

Banking management system

Library management

Transport system

Satellite launching/communication

Traffic control etc.

It will be, thus, wrong to say that a computer is only a calculating device. In a general way, the term 'COMPUTER' can be understood as:

C - Commonly O - Operated M - Machine P - Primarily U - Used for

T - Trade

E - Education and

R - Research

2. COMPUTER ORGANIZATION

Computer Organization covers the internal architectural design of a computer

and the basic functions that are performed by each of its components.

A block diagram of the basic Computer Organization is as shown in Fig. 1

3. BASIC COMPONENTS OF COMPUTER A digital computer essentially consists of five basic components: I) Input Unit

II) Arithmetic Logic Unit (ALU)

III) Control Unit (CU)

IV) Memory Unit (MU)

V) Output Unit

The Arithmetic Logical Unit (ALU), the Control Unit (CU), and the Memory Unit (MU) of a computer system, jointly constitute the Central Processing Unit (CPU).

Fig. 1 Computer organization

In the light of Computer Organization, we can redefine the term "Computer" as:

"Computer is an electronic device whichCan accept data in a specified form (through Input

Unit)- Stores it (in a Memory Unit)- Process data at a very high speed (through CPU) and- Produces required results in the prescribed format

(through Output Unit)"

These five components of computer architecture correspond to the five basic operations, generally performed by a computer system.

I) Input Unit

Input unit is that part of a computer through which user communicates with the computer.

Input unit acts as an interface between user (programmer) and computer.

For any type of computation, to be performed through computer, data/instruction/command must be entered into it.

There are many types of input devices e.g.

(i) Keyboard

(ii) Mouse

(iii) Light pen

(iv) Joystick

(v) Punch-card-reader

(vi) Touch sensitive screen etc.

Input device transforms the data (currently in human-understandable-form) into binary codes (computer-understandable-form). This transformation is accomplished by the unit called input interface.

In short, input unit performs following functions:

(i) It accepts (reads) the data, the programmes and the instructions from the external world (user).

(ii) It converts the data and instructions etc. into computer understandable - form.

(iii) It supplies the converted data and instructions etc. to the computer for further processing.

CENTRAL PROCESSING UNIT (CPU)

The arithmetic logical unit (ALU), the memory unit (MU) and the control unit (CU) together constitute the Central Processing Unit (CPU). The Central Processing Unit is also known as the BRAIN OF COMPUTER.

All the major functions like calculations, comparisons, activations and control of operations of all the units of the computer are made inside CPU.

II) Arithmetic Logical Unit (ALU)

As is clear from its name, arithmetic logical unit (ALU) is the unit used for carrying out the execution of instructions involving mathematical calculations and logical decisions. It is a place where the actual execution of instructions takes place during processing of operation.

The basic arithmetic computations include addition, subtraction, multiplication, division etc and logical comparisons such as less than, equal to, greater than etc.

Data (operands) are taken from main memory, operations are performed on the data and result is stored back onto main memory.

III) Control Unit (CU) Because of its operational power, control unit is also known as the

BRAIN OF THE CPU. Control unit generates the electrical signals to control and coordinate the

functioning of four other units. One by one, instructions are fetched from the main memory, interpreted in this

unit and control signals are generated to various units accordingly. The major functions of control unit are the decisions :

- about storage allocations of devices. - about storage deallocation of devices. - attachment of new hardware. - proper functioning of software - user interaction.

Broadly computer memory may be classified into two categories :

(i) Primary memory

(ii) Secondary memory.

Diagrammatically the classification is as shown in Fig. 2

Fig 2 : Classification of memory

IV) Memory Unit (MU)The data and the instructions entered into the computer system through Input Unit have to be stored inside the computer before actual processing starts. The intermediate results produced during processing and the final results of the computations are also needed to be stored on to memory, before passing them to the output units.

i) Primary Memory : It is also known as main memory or internal memory or essential memory. It is very fast in nature. All the programs and the data that are to be executed, should be put first into this memory.

This memory is further subcategorized in two types: (a) Random Access Memory (RAM) (b) Read Only Memory (ROM)

(a) Random Access Memory (RAM): It is also known as Read-Write-memory because any information on

it can be written, read and finally deleted, when no longer required. RAM is volatile in nature. It means that when power of the computer

is switched off, its contents get lost.

(b) Read Only Memory (ROM): This memory is used to store the programs that do not need regular alteration. Data stored is non-volatile in nature i.e. data is not lost when power

of computer is switched off.

ii) Secondary Memory It is also known as External or

Auxiliary memory. It stores the data and programs permanently.

Hence, it is non-volatile in nature i.e. it retains its contents even when the computer is switched off.

It has much larger capacity (usually in gigabytes) than that of primary memory and are not very fast in access.

It is cheaper than primary memory. Secondary memories can be magnetic tapes, magnetic discs, magnetic drum, floppy, hard disc etc.

V) Output Unit

The job of output unit is reverse to that of input unit. It links the computer with

the external environment/world. This unit converts the information from

computer-understandable-form (binary form) to human-understandable-form.

This transformation is accomplished by the units called output interfaces.

There are many types of output devices e.g.(i)Visual display units (VDU)(ii) Printers(iii) Graph plotters(iv) Card punch(v)Paper tape punch(vi) Magnetic tapes etc.In short, output unit performs following functions:• It accepts the results produced by the computer which are in coded

form and hence, cannot be understood by the reader.• It converts these coded results in human acceptable (readable) form.• It supplies the converted results to the outside world.