03 Basic Computer Organization

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INTRODUCTION

Computer is potentially a very powerful data processing tool consisting of the

units known as peripheral devices, which serve either a means of feeding

data into the system or receiving results from the system.

The objective of this chapter is to provide an overview of the working units ofcomputer system to the user. In this chapter, the key terms, such as logical

components, organizations, technologies employed in main memory systems

are described.

3.1 BASIC COMPUTER ORGANIZATION

A digital computer primarily comprises of three units as shown in Fig 3.1,

such as input unit, processing unit and output unit. Each unit is described

below in the text.

3.2 INPUT UNIT

In the beginning of a computer-based information system, the term input is

used to describe the process of collecting raw data into a form as quickly and

efficiently as possible, that is usable by computer for processing. Thus, an

input unit is responsible for accepting data and instruction in the form, that

depends upon the device used. It is generally an electromechanical device

that accesses the data that computer uses, which is communicated from outside.

This is the actual means by which data is converted into electronic pulses and

further transformed and stored as binary codes. This task is performed by

BASIC COMPUTER

ORGANIZATION

3

[3.1]

In This Chapter, we will

Learn functions of computer system and its various units

Learn about CPU, its components, its features, processing speed

Learn various types of storage and its characteristics

Learn variations of RAM, ROM, VRAM, and Flash memory

OBJECTIVES


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Computer Fundamentals3.2

input interface. Input unit functions can be summarized as follows :(1) It provides a means for the machine to communicate with its external

environment by accepting (or reading) data and the list of instructions.

The common input devices are: Card reader, Paper tape reader, Magnetic

tape drive, Disk drive, Keyboard, etc.

(2) It converts these instructions and data in computer acceptable form.

(3) It supplies the converted instructions and data to the computer system

for further processing.

CPU

Figure 3.1: Basic organization of a computer system

3.3 OUTPUT UNIT

The output devices of the computer enable it to communicate results to the

outside world or user. It accepts the results produced by the computer which

are in coded form and hence cannot be easily understood. It converts these

coded results to human acceptable (readable) form. This activity is performed

by output interfaces, which are designed in compatible with output devices. It

supplies the converted results to the outside world.Output devices fall into two categories:

1. Devices which produce output that is understood by human beings,

e.g.,Printers and Monitors (VDU)

2. Auxiliary storage devices that hold data may be intended for further

processing(e.g.,Magnetic disks)

Input/output is a piece of hardware that can be used for both providing data to

a computer and for receiving data from it, e.g.,disk drive.


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Basic Computer Organization 3.3

3.4 CENTRAL PROCESSING UNIT

The CPU is known to be heart or brain of the computer system as it

follows the instructions of the software/program to manipulate data into

information.

A digital computer consists of various types of units, all interconnected and

functioning under central control. The central processor or Central

Processing Unit (CPU) carries out a variety of essential data manipulation

(including, calculations, comparisons, sorting, etc.) and controlling tasks in

the computer which is also known as microprocessor i.e., microprocessor

= ALU + CU. The architecture of the CPU determines the way of processing

of data. Modern computers transmit data by employing multi-staged pipelines.Generally, CPU includes some sort of error correction mechanism to ensure

proper flow of data through these lines.

In this unit actual execution of the instruction takes place. Every micropro-

cessor contains a system clock. The speed at which processor executes

instruction is known asclock speedand it is measured in Megahertz (MHz).

Where mega means millions and hertz means cycles. Therefore 550 MHz

processor performs 550 million cycles per second. Generally, clock speed

determines the system performance. Technology advances for higher clock

speed. The latest generation of processors operate in gigahertz(GHz)i.e.,a

billion cycles per second.

System clockdetermines and controls the speed of operations taking place

within a computer.

Clock speed is the speed at which system clock oscillates. For

microcomputers it is measured in Megahertz (MHz) and Gigahertz (GHz).

Megahertz (MHz):a measure of frequency equivalent to 1 million cycles

per second Gigahertz (GHz): a billion cycles per second.

A computer performs its operation in fraction of a second. The increasing

speed given below are:

Millisecond = 10-3 second

Microsecond = 10

-6

secondNanosecond = 10-9 second

Picosecond = 10-12 second.

CPUconsists of two parts Control Unit (CU) and an Arithmetic-Logic Unit

(ALU). Both of them contain high storage areas and all are linked by electronic

path known asbus.


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3.4-1 Arithmetic and Logical Unit

ALU is the principal element of theCPU. It performs the basic four arithmetic

operations of addition, subtraction, multiplication, division, etc. To determine

whether one number is larger than another, or whether the number is positive

or negative, or zero, it performs logical (comparison) operations on data.

Additionally it also performs logical boolean operators such as and, or,

not. When not is applied to binary operands, it forms complement of that

operand. These operations are carried out at very high speeds. The arithmetic

logic unit also allows the control unit to make logical decisions that affect the

flow of the program. The arithmetic and control units heavily depend on third

element of a computer, the main storage or memory unit. The arithmetic unit

needs numbers to operate on and to store the immediate results until the endof calculation.

Arithmetic/Logic unit, performs arithmetic and logical operations and

controls the speed of these operations.

3.4-2 Control Unit

The other important element of the CPU is control unit, which is required to

supervise the functioning as whole. Control unit acts as automatic operator, it

provides the means of communication within the machine, by moving,

advancing or transferring information. CU does not perform any actual

processing on data. Rather, it manages and co-ordinates the entire computersystem. Basically, control unit is incharge of decoding instructions and

sequencing the actions of various functional units. It fetches program

instructions one by one in sequence, stored in main memory. It interprets (or

decodes) them, and sends appropriate control signals to the various units. It

resolves conflicts between on chip resources and responds to external and

internal interrupts.

Control unit refers to a functioning unit which carries out four basic

operations, these are: (1) fetches an instruction, (2) decodes the

instructions, (3) executes the instruction, and (4) it stores the result.

3.5 STORAGE UNIT

Different types of memory (or storage) are usually used in a computer system.

Any storage unit of a computer may have following characteristics:

(i) Access timeis the amount of time needed for locating and delivering

data from memory (storage device) to the processor after the address

for the data has been selected. Access time is usually measured in milli-

second and is used as a performance measure for disks and CDs.


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(il) Storage capacityis the amount of information/ data a storage unit canhold. Higher storage capacity devices are desirable.

(iii) Cost per bit of storage is generally higher with fast devices, though

the objective is to minimize the cost.

These may be fast or slow. The availability of fast, inexpensive memory has

had enormous impact on computer technology. Fast is relatively small and

expensive storage units, used as fast access by the central processing unit.

Slow is significantly large and inexpensive storage units. Storage unit of

computer system consist of four types of storage, namely, Registers, Cache

memory, Primary memory and Secondary memory. The description of each

memory is classified according to its performance and cost as given in Fig.3.2.

Since the 1970s, memory performance and cost have reduced at the rate of27% per year. Over this period, the price per bit of main memory has decreased

at the astonishing rate of approximately 27% per year. These advances have

taken place primarily, due to improvement in the integrated microelectronic

technologies. The commonly used memory types are described as per the

hierarchy(seeFig. 3.2) further in this chapter.

Registers

Cache

Main memory

Secondary storage (disks)

Off-line storage (tape)

IncreasingPerformanceand cost

Slow and inexpensive

Figure 3.2: The Memory Hierarchy

(Source: Compu ter Science En cyclopaedia:Memory, Nature Publishing House- 2001)

3.5.1 Register

Registersare high speed storage areas that temporarily store data during

processing to enhance computers performance

The size of the internal registers indicates howmuch information the processor

can operate on at one time and how data moves around internally within the

chip. A Register is capable of holding one number and its location is with an

execution unit. For example, the processor can add numbers in two different

registers storing the result in a third register. The register size determines the


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size of data the processor can operate on. It is usually a small chip data arraythat is addressable by program instructions. Registers are in cluded in all major

microprocessor and provide storage with fastest possible access time.

3.5.2 Cache Memory

Cache memory is a small high speed buffer memory used to hold

instructions temporarily that the processor is likely to use frequently, to

speed up processing.

The CPU of a computer system commonly use Cache memory where it holds

or buffers the contents of the main memory because CPU runs much faster

than main memory RAM. Thus to reduce the waiting time of CPU cache isused. Cache memory also holds the contents of the secondary storage (viz:

disk or tape). Caching is used in various purposes such as for look up files in

directory system and web pages on a web server. Cache memory reduces

traditional system bottleneck because system RAM is much slower than CPU.

This prevents the processor from having to wait for program and data from

slower main memory.

C.P.U Cache

MemoryMain memory

(Large in capacity andslow in speed than

cache memory)

Figure 3.3: Illustrates The Cache Memory

3.5-3 Main Memories or Integrated Circuits (Semi-conductor memories)

The characteristics of main memory lie between the two extremes of slow and

fast memories. In general, the main memory will contain instructions and data

that are accessed by a program while it is executing. Main memory stores data

and instructions for the CPU. It has four basic purposes:

1. To hold data and instructions for a given job, from an input unit

(keyboard, terminal, etc.) before processing.

2. To hold intermediate results, while processing.

3. To hold data after processing, until they are transferred to output device

(for example, to a printer).

4. It also holds program and instructions not only received from input

devices but also from secondary storage devices.

5. It also stores complex programs such as operating system.

Thus, primary storage or main memory consists of mainly four areas:

(1) Input Storage Area : It holds data and instruction required for

processing.

(2) Working Storage Area : It holds the data being processed and


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intermediate results of such processing.(3) Output Storage Area:This holds the final result of processing.

(4) Program Storage Area:This area holds the sequence of instructions

for the computer.

A word is the largest amount of data that can be handled by the

microprocessor in one operation, word size of 16 bits and 32 bits are

common.

Physically, this memory is thin silicon chip which contains a number of small

storage cells that hold data. Each of these cells or locations can store fixed

number of bits calledword length.Thus, memory is divided into N (0 to N-l)

words (where N is power of 2). Each location holds either data item or aninstruction. Typically, each memory address refers to more than one bit of

data in the memory system. The minimum uniquely addressable unit of data

in main memory is typically eight bits, or a byte. However, the size of this

addressable quantum, is often called Word. Word is usually of 4 bytes. This

chip is an integrated circuits containing number of transistors on a single, thin

silicon wafer. The built-in memory allows the CPU to store and retrieve data

quickly, that it is 100 times faster than secondary storage. Memory is like

catalog system in the library, where each drawer contains number of references

as number of bits in each cell with a particular address. Even small computer

systems now have relatively large memories. Since memory addresses are

binary numbers, the memory size is typically related to a power of two anddescribed in units of Kilobytes (KB), Megabytes (MB), Gigabytes (GB), or

(for mass storage) Terabytes (TB).

A B it = each 0 or 1, short for binary digit

A Byte = group of 8 bit, represent one character

where 8 bits = 1 byte = 1 character

A Kilobyte = 210 or 1,024 bytes

A Megabyte = 1024 Kilobytes

A Gigabyte = 1024 Megabytes

A Terabyte = 1024 Gigabytes and

A Petabyte = 1024 Terabytes

Main memory sizes ranged from 64 MB-l GB on an inexpensive personal

computer to several GB on a mainframe.

There are two types of built-in memory, permanent and temporary, known as

ROM and RAM, respectively, details of each given below:

3.5.3-1 Read Only Memory

As we know in computer terminology readmeans transferring data instruction


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from an input source to the computers main memory (or CPU) and writeistransferring data/instruction fromcomputers main memory to an output device.

Therefore, Read only means data/instruction can be retrieved from the ROM

Chip but cannot be modified.

Read Only Memory (ROM) is simple type of memory, whose contents cannot

be changed even after loss of electrical power. There are two types of read

only memory, namely, (a) manufacturer programmed, and (b) user programmed.

(a) Manufacturer programmed ROM: ROM is one in which the contents

are programmed by the manufacturer during the construction of the

chip, it is unalterable. Usually, ROMs are used for high volume

controlled applications such as the set of instructions known as system

boot program that is needed to make the computer system ready foruse when its power is switched on. These instructions are startup

instructions, commonly found in ROM. ROM-BIOS means Read only

Memory Basic Input Output Services. These services or instructions

perf orm the basi c cont rol over hardware including I/O devices .

Generally, ROM chip of a computer stores Operating System, Disk

Drivers, Printers drivers, etc. Thus, ROM stores program and data which

do not change and are frequently used.

ROM (read only memory) chip contain fixed start-up instructions. It

cannot be written on or erased by the computer user, only trained

programmers can write by using special equipment.

(b) User-programmed ROM: The user-programmed ROM is one in which

user can load and store read only programs and data. The user-

programmed ROM include the following:

Programmable Read only Memory (PROM) : It can be written

oncevia an irreversible process. PROM is a non-volatile storage,

i.e.,the stored information cannot be changed even if the power is

switched off.

Erasable Programmable Read only Memory (EPROM) :Data

on EPROM can be erased to store new information. However, the

information on this chip can only be read. When EPROM is in use,

information in the chip remains until it is erased. EPROM chipsare generally used by R&D personnel who test the efficiency of

the system with new programs. EPROMs are of two types namely,

Electrically Erasable Programmable Read Onl y Memory

(EEPROM) andUltraVioletErasableProgrammable ReadOnly

Memory (UVEPROM). Data on EEPROM can be erased by using

high electric voltage and due to this ease with which stored program

can be altered, EEPROM is also known as flash memory. Whereas

data on UVEPROM can be reset by exposure to intenseultra violet


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rays. Flash memory : Flash memory is small variant of Electrically

Erasable programmable Read Only Memory. These memory chips

are the latest known storage devices, available in capacities ranging

from 512 MB to 2GB. They are non-volatile memories as they

retain the data stored in them without a power source. They may

be used to replace magnetic storagei.e.,floppy diskette. The other

advantages are that the read time of flash memory is quite less than

the write time. Currently, flash memories are used to store images

from digital cameras also in pagers, cell phones, MP3 players. The

recent advancements are that flash memory cards would replace

tape and disk storage. The size of flash memory cards is compactof 1 I. Compared to hard disk storage, flash memory cards are

expensive. Flash memory chips can be erased and reprogrammed

more than once

3.5.3-2 Random Access Memory

RAM (random access memory) chips are meant for primary storage.

They hold temporarily (a) software/program instructions and (b) data

before and after processing.

Random access means that any location can be referenced in the same time

and in same manner, as it is independent of the address or location in the

memory. It is a volatile memory. It holds data and instructions, during

TABLE 3.2: COMPARISON BETWEEN RAM AND ROM

ROM

Rea d On ly M emo ry i.e.,

information can only be read and

cannot be written on to it.

It stores information permanently.

Information is not lost even if

power supply is switched off.

Known as non-volatile memory.

Holds system softwares such as

operating systems.

Types of ROMs are PROM,

EPROM EEPROM.

RAM

Random A ccess M e mo r y a s i t

randomly selects and uses location of

the memory to access the information.

It holds information temporarily.

Information is lost when power supply

is switched off.

Known as volatile memory.

Holds application programs which are

currently in use.

Types of RAMs are Dynamic RAM

and Static RAM.


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3.5.3-3 Video RAM (VRAM)

Video RAM is a special type of dynamic RAM (DRAM) used in high

speed video applications. VRAM uses special circuitry that transfers

pixel from video RAM to screen serially (bit by bit)

Monitor has its own memory known as VRAM that is used to accelerate the

display of graphics on the screen. It does this by the use of two ports, one port

is connected to the screen and other to CPU. Data from CPU flow to the

screen VRAM very smoothly. That is why we see clean graphic display on the

screen. A variation of this is Window RAM (WRAM) that supports memory.

3.5-4 Secondary Storage

Secondary storage is required to supplement the limited capacity and volatile

characteristics of primary storage. The secondary storage is inexpensive, having

large capacity and is non-volatile in nature. It stores program and data, which

may be required at later date. The most commonly used secondary storage

devices are magnetic disks, tapes, or optical disks (refer Chapter 8).

Characteristics Of Primary And Secondary Storage

Primary Storage

1. It is the main memory as part of

the CPU.

2. It is most expensive.

3. Storage capacity is generally in

Megabytes (MB) Now available

in few GB.

4. Information can be retrieved and

processed very very fast.

5. Program and data are processed

directly on RAM componentof

primary memory.

6. Bas ed o n s em i c on du cto r

Technology.

7. Access is random

8. It holds system software such as

operating system, Application

programs and user programs that

are currently in use

Secondary Storage

It is auxiliary memory which works

under the control of CPU.

Relatively less expensive than pri-mary memory.

Storage Capacity is in Giga bytes

(GB).

Retr ie va l a nd p ro ce ss ing o f

information is relatively higher than

primary memory.

Program and data cannot be

processed directly but first it is

placed on RAM of main memory.

Made up of plastic coated with iron

oxide material.

Access can be sequential or random.

It holds program and data other than

those which are not currently in use

i.e.,used for backup purposes.

S.N


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SUMMARY

Digital computer consist of three units such as input unit, processing

unit and output unit.

Input/output unit is nothing but hardware, which is used for both sending

to and receiving from computer.

CPU follows the instruction to manipulate data into information.

The speed at which processor executes instruction is known as clock

speed.

ALU performs arithmetic and logical operations and also control

operations. Cache memory is high speed buffer memory.

Built-in memory is of two types ROM and RAM which are permanent

and temporary respectively.

Video RAM is used to accelerate the display of graphics on the screen.

REVIEW QUESTIONS

1. Fill in the blanks with one of the words given in the bracket.

(a) Results are obtained from the computer through its ......... unit.

(Input/ Output/CPU)(b) . .. . .. . .. . . ..unit provides communication from the main

machine.(Input/ Output/ Control)

(c) ...................... is the name given to main computer equipment and

its peripheral devices. (Software/Hardware/Firmware)

(d) Instructions to the computer are given through .....................

(Hardware/Program/Input)

(e) Calculations are made in the computer with the help of its

.................... (Memory/Arithmetic and Logic Unit/Control Unit)

2. Draw a Block diagram to illustrate the basic organization of a computer

system and explain the functions of various units.3. Distinguish between primary memory and secondary memory.

4. CPU is the heart and brain of computer. Comment.

5. What type of calculations are performed in ALU?

6. Define the architecture of the CPU.

7. Describe the main functions performed by control unit.

8. What are the basic operations performed by the computer system?


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9. Why computer is called as computer system?10. What do you understand by bus in computer system? What are the

primary functions it performs?

11. Identify the functions of main memory.

12. What is system clock? How clock speed can be determined?

13. What tasks are performed by main memory while ALU is performing

calculations?

14. A computer has 64 MB memory. How many characters can be stored in

its memory?

15. Differentiate between static and dynamic RAM. Which is more

commonly used?16. Explain the storage hierarchy of a computer system.

17. What are the basic functions performed by output unit?

18. What type of information is generally stored in ROM? Discuss the

variations of ROM.

19. Differentiate between RAM and ROM.

20. Main memory contains four types of storage area. Specify the purpose

of each.

21. What are registers? State its use.

22. What is cache memory? What is the purpose of this memory with the

help of a diagram.

23. Differentiate between register and cache memory.

24. Arrange the following four measures of time from smallest to largest:

(i) Microsecond

(ii)Millisecond

(iii) Picosecond

(iv) Nanosecond

25. Distinguish between

(a) Main and Cache Memory

(b) PROM and EPROM(c) Control Unit and Arithmetic Logic Unit

26. What is execution cycle?

27. Give one or two reason for cache memory in a processor.

28. How memory chips affect on system performance?

29. State What happens to RAM when: -

(a) Double-click on application icon

(b) When more than one application windows are opened?


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REVIEW AND THINKING TASK

Start your computer and take a note of the capacities of RAM, ROM,

Cache Memory, Speed. Differentiate the same characteristics with

calculator and mobile phone memory.

Try to change the wallpaper, screen saver, style of your computer system

etc.

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03 Basic Computer Organization