Unit III Computers

8/22/2019 Unit III Computers

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COM PUTER FUNDAM ENTAL S

Unit II I

Computers and information processing Classification of Computer input device output device

storage device Batch and online processing Hardware, Software Database Management

System

I NTRODUCTION TO COMPUTER

WHAT IS A COMPUTER?

The term Computer is derived from the term Compute which means to

reckon or to calculate something. Initially human being used computersjust to do arithmetic calculations. That is why, they have named the

device which is doing calculations as computer. But now, the computer

can do more than just calculation. It can do logical decisions, data

processing etc.

DEFINITI ON FOR COMPUTER

Computer can be defined as an Electronic device which can do both arithmetic and logical

operations at a faster rate which has memory to store and operates automatically under the control

of instructions stored in the memory

So, a computer is an electronic device which may use either valves or transistors or ICs (integrated

circuits) which can perform arithmetic operations like addition, subtraction, multiplication and

division and can do logical operations like checking whether a condition is true or false at a faster

rate. It is having memory to store and process.

Computer is a devicethat transforms data. Data can be anything like marks obtained by you

in various subjects. It can also be name, age, sex, weight, height, etc. of all the students in

your class or income, savings, investments, etc., of a country.

Computer i) accept data ii) store data, iii) process data as desired, and iv) retrieve the stored

data as and when required and v) print the result in desired format.

TERM S ASSOCI ATED WITH THE SPEED OF COM PUTERS

M illi Second One thousand instructions/second

M icro Second Million Instruction/Second

Nano Second Thousand Million Instructions/Second

PicoSecond Millions Million Instruction/Second

Computers are very fast

The unit of measurement of Computers Speed is MIPS (Million Instructions

Per Second)

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CHARACTERI STICS OF COMPUTER

Speed

Computer can work very fast. It takes only few seconds for calculations that we take hours

to complete. Suppose if you have to calculate and write a report of average monthly income

of say 1500 persons manually, it will take at least one or two weeks if you work hard but for

the computer, it will be less than 5 minutes to calculate and some extra time to print

depending on the printer you use. You will be surprised to know that computer can perform

millions (1,000,000) of instructions and even more per second. Therefore, we determine the

speed of computer in terms of microsecond (10-6 part of a second) or nano-second (10-9

part of a second). From this you can imagine how fast your computer performs work.

Accuracy

The degree of accuracy of computer is very high and every calculation is performed with

the same accuracy. The accuracy level is determined on the basis of design of computer.

The errors in computer are due to human and inaccurate data.

Diligenceor consistency

A computer is free from tiredness, lack of concentration, fatigue, etc. It can work for hours

without creating any error. If millions of calculations are to be performed, a computer will

perform every calculation with the same accuracy. Due to this capability it overpowers

human being in routine type of work.

Versatility

It means the capacity to perform completely different type of work. You may use your

computer to prepare payroll slips. Next moment you may use it for inventory management

or to prepare electric bills.

Power of Remembering

Computer has the power of storing any amount of information or data. Any information can

be stored and recalled as long as you require it, for any numbers of years. It depends entirely

upon you how much data you want to store in a computer and when to lose or retrieve these

data.

Neatness

The execution & the reports generated by the computer will be 100% neat as designed by

us through our program. The neatness will be maintained from the initial to final stage of

execution which is not possible in the case of manual jobs.

Automation

With the help of stored program concept, wherein a instructions will be executed one after

the another automatically by the system. This eliminates unnecessary manual intervention

Storage& retrieval of information

The Computer has an in-built memory where it can store a large amount of data. You can

store data in secondary storage devices such as floppies, hard disks etc., which can be kept

outside your computer and can be carried to other computers. Lateron if you wasnt to

retrieve the data from

LI M ITATI ONS OF THE COMPUTERS



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Cannot think by its own

Cannot detect any logical errors (GIGO)

Human beings have the potential to try out various alternatives to solve the unexpected

which computers cannot.

Computers have no Intuition power

BASIC COMPUTER OPERATIONS

A computer performs basically five major operations or functions irrespective of their size and

make. These are

1) it accepts data or instructions by way of input,

2) it stores data,

3) it can process data as required by the user,

4) it gives results in the form of output, and

5) it controls all operations inside a computer.

We discuss below each of these operations.

1. Input

This is the process of entering data and programs in to the computer system. Computer

is an electronic machine like any other machine which takes as inputs raw data and

performs some processing giving out processed data. Therefore, the input unit takes data

from user/operator to the computer in an organized manner for processing.

2. Storage:

The process of saving data and instructions permanently is known as storage. Data has

to be fed into the system before the actual processing starts. It is because the processing

speed of Central Processing Unit (CPU) is so fast that the data has to be provided to

CPU with the same speed. Therefore the data is first stored in the storage unit for faster

access and processing. This storage unit or the primary storage of the computer system

is designed to do the above functionality. It provides space for storing data and

instructions.

The storage unit performs the following major functions:

All data and instructions are stored here before and after processing.

Intermediate results of processing are also stored here.

3. Processing:

The task of performing operations like arithmetic and logical operations is called

processing. The Central Processing Unit (CPU) takes data and instructions from the

storage unit and makes all sorts of calculations based on the instructions given and the

type of data provided. It is then sent back to the storage unit.

4. Output:

This is the process of producing results from the data for getting useful information.

Similarly the output produced by the computer after processing must also be kept



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somewhere inside the computer before being given to you in human readable form.

Again the output is also stored inside the computer for further processing.

5. Control:

The manner how instructions are executed and the above operations are performed.

Controlling of all operations like input, processing and output are performed by control

unit. It takes care of step by step processing of all operations in side the computer.

FUNCTIONAL UNITS

In order to carry out the operations mentioned in the previous

section the computer allocates the task between its various

functional units. The computer system is divided into three

separate units for its operation. They are :

1) Arithmetic Logical Unit,

2] Main Memory Unit,

3) Control Unit,

4] Input Units and

5] Output Units

Arithmetic L ogical Unit (AL U)

After you enter data through the input device it is stored in the primary storage unit.

The actual processing of the data and instruction are performed by Arithmetic

Logical Unit. The major operations performed by the ALU are addition, subtraction,

multiplication, division, logic and comparison. Data is transferred to ALU from

storage unit when required. After processing the output is returned back to storage

unit for further processing or getting stored.

Main Memory Unit (MMU)

All instructions and data will be temporarily stored here for further processing.

RAM is called as MMU in a computer. This is highly volatile and cannot retain

anything if power goes off. Using Input Units, necessary data and instruction will

be sent to MMU for temporary storage and then will be sent to ALU or CU

according to the instructions that are given at that time. This is also called as primary

memory.Control Unit (CU)

The next component of computer is the Control Unit, which acts like the supervisor

seeing that things are done in proper fashion. The control unit determines the

sequence in which computer programs and instructions are executed. Things like

processing of programs stored in the main memory, interpretation of the instructions

and issuing of signals for other units of the computer to execute them. It also acts as

a switch board operator when several users access the computer simultaneously.

Thereby it coordinates the activities of computers peripheral equipment as they

perform the input and output. Therefore it is the manager of all operationsmentioned in the previous section.

Input Units



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These are secondary storage units which supply instructions and data to the CPU

whenever needed. There are various input devices like Card Rader, Magnetic Tape

Reader, MICR, Magnetic Disks, Optical Disks, and Flash Drives (like Pen Drives,

MP3 or MP4 players, Wrist Watches or Pens or Spy camera with Pen Drives, Cell

phones with flash drives etc.).

Output Units

These are Secondary storage devices and Printers. Printers are exclusive output

device wherein you can take hard copy of any reports you want to print. Apart from

secondary storage devices, we are having line printers, inkjet printers, Thermal

Printers, Laser printers etc.

Central ProcessingUnit (CPU)

The ALU, MMU and the CU of a computer system are jointly known as the central

processing unit. You may call CPU as the brain of any computer system. It is just

like brain that takes all major decisions, makes all sorts of calculations and directs

different parts of the computer functions by activating and controlling theoperations. It can also be called as heart of a computer. If any one part of the CPU

got failed, we cannot use the configuration as computer.

CLA SSIFICAT ION OF COMPUTER

Computers can be classified as under:

PurposeWise like Special Purpose or General Purpose

TypeWise like Analog, Digital or Hybrid computersand

Sizewise like micro Computer, Mini Computer, Mainframe Computer or SuperComputer

Purposewiseclassification

Computers can be classified into General Purpose and Special Purpose computers.

General PurposeComputers

These are nothing but the computers which we are using for our daily use.

Here, different software of different purposes can be executed one after

another like executing Ms-Word, then Ms-Access, Oracle application, Visual

BASIC application, Payroll Application, Inventory Control Application,Financial Accounting System, DTP applications etc. with the same

computer. This is not specifically allotted for any special or designated

purpose.

Special PurposeComputers

These are dedicated for some specialized tasks. Mostly they will be used for

the purpose for which they are dedicated. CT Scanners, Endoscope,

equipments (using computers) used for doing Laser operations computers

dedicated for launching for rockets and missiles etc., are some examples.Typewiseclassification

Computers can be classified into Analog, Digital or Hybrid Computers.



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Analog Computer

Analog Computer works with the qualitative data or physical force. Some

physical force is used to operate analog computer.

For ex., in the case of Thermometer, the physical force is heat of human

body. The mercury kept inside the thermometer will be expanding in

proportion to the heat of human body. To have correct measurement, onemust use good quality instrument. Otherwise the result is highly

unpredictable. In the case of spring balance or normal balance, the physical

force is gravitation force of the earth. In the case of Speedo meter, the speed

of rotation of wheel is the physical force. In the case of mechanical watch,

the physical force is tension of the spring which rotates the teeth wheel.

The results given by the analog computer is not highly dependable unless

until we use good and quality instruments

The analog computer processes work electronically by analogy. It uses an

analog for each variable and produces analogs as output. It, thus, measurescontinuously. It does not produce number but produces its results in the

form of graph. It is more efficient in continuous calculations. The analog

computer accepts variable electrical signals (analog values) as inputs, and its

output is also in the form of analog electrical signals

Digital Computer

Digital Computers work with quantitative data. There is some element of

counting number of low or high pluses (Electronic pulses or low voltage or

high voltage) emitted by electronic components which will be represented

normally by 0s & 1s and also the output in On-Off signal.

Most of the computers available to-day is digital computers and now we

use the term Computers to refer digital computers only.

Some crude examples for digital computers are digital watches, Digital

EB meters, Glucose meters, digital thermometers etc.,

Hybrid Computers

It is a a combination of both Analog & Digital Computers. A part of the

processing is done by Analog computer and a part by digital computers. Ahybrid computer combines the benefits of both Analog Computers and

Digital Computers. It provides greater precision that can be attained with

Analog Computers and the speed and greater control by Digital Computer. It

can accept input data in both analog and digital form. It is used for

simulation applications. Now, in most of the big concerns entire production

process will be carried on by hybrid computers wherein human intervention

is very low and only few staffs will be maintained just to observe what is

going on. Only in the case of emergency they will stop the machine and

report the matter to the management and they dont know what is going on in

between. Examples are laparoscope, CT scanners, endoscopes etc.

Sizewiseclassifications



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Computers can be classified into micro Computer, Mini Computer, Mainframe

Computers and Super Computers.

Micro Computers

Micro Computers are General purpose computers. It is an outcome of 4th

generation with the birth of Microprocessor. A Microprocessor is called as the

miniature of chips. Computers size, cost, weight etc., has reduced to a greaterextend. Microcomputer is at the lowest end of the computer range in terms of

speed and storage capacity. Its CPU is a microprocessor. This is a single user

oriented system.

When they were introduced, they were costing around 1.5 lakhs to 5 lakhs.

Initially they came with two 8 inch floppy drives and used CP/M operating

system which is Character User Interface based operating system. The first

microcomputers were built of 8-bit microprocessor chips. The languages used at

that time were only BASIC, COBOL, Fortran and lower end text editors.

Now, micro computers are so powerful and it can be called as micro computer,PCs, Workstations, Clients or Nodes.

In Personal Computers (PCs), Pc-Dos or Ms-Dos or OS2 operating system is

used. Now, we are using multi-user operating systems like UNIX and Linux

operating system in PCs. With the introduction of Windows operating system

GUI concepts, Multi-tasking, multi-programming concepts were introduced in

PCs. Other most common modern micro computer is Apple computer which is

using GUI operating system OS2.

Mini Computer

When the computer is used for data entry or printing purposes, the CPU will be

keeping idle. If that time is used by another user, there will be effective usage of the

CPU. This idea gave birth to multi-user oriented approach. Here, several users can

use the idle time of the CPU effectively. When micro computers were introduced,

they were used for data processing. Offline data entry machines were used for data

entry. Off line Data entry machines were costing so high. This gave birth to

attachment of multi terminals and using of CPUs time by several users effectively.

Mini computer is designed to support more than one user at a time. It possessed

large storage capacity and operates at a higher speed than microprocessors. Severaldumb terminals around maximum 20 terminals will be attached to a CPU. Once the

main terminal is booted other dumb terminals will be booted and attached to the

CPU and can use the system. In this environment each and every user will be feeling

as if he is using an independent system. Here, all requests will be sent in queue to

the CPU and will be executed on fist come first served basis. But the user may not

feel all these since the CPU is very fast

MainframeComputers

These are general purpose computers. Mainframe was a term originally referring

to the cabinet containing the central processor unit or "main frame" of a room-

filling Stone Age batch machine. After the emergence of smaller

"minicomputer" designs in the early 1970s, the traditional big iron machines



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were described as "mainframe computers" and eventually just as mainframes.

Nowadays, a Mainframe is a very large and expensive computer capable of

supporting hundreds, or even thousands, of users simultaneously. These types of

computers are generally 32-bit microprocessors. They operate at a very high

speed, have very large storage capacity and can handle the work load of many

users. They are generally used in centralized databases. They are also used as

controlling nodes in Wide Area Networks (WAN).

Example of mainframes are DEC, ICL and IBM 3000 series.

Supercomputer:

They are the fastest and most expensive machines. They have high processing

speed compared to other computers. They are using multi micro processors

whereas other computer types will have single processors. This facilitated

multiprocessing. One of the ways in which supercomputers are built is by

interconnecting hundreds of microprocessors. Supercomputers are mainly being

used for whether forecasting, biomedical research, remote sensing, aircraftdesign and other areas of science and technology.

Examples of supercomputers are CRAY YMP, CRAY2, NEC SX-3, CRAY

XMP and PARAM from India.

ADVANTAGES OF COMPUTERS USING EL ECTRONIC DEVI CES

Highly reliableand accurate

Due to the usage of electronic circuits in place of mechanical gears and wheels, the

problems of wear and tear, backlash, hysteresis, etc. are totally eliminated.

Electronic computers are therefore very highly reliableand accurateVery Fast

They are very fast since the computer operates at electronic speed i.e., speed of light

where as the manual mechanical computers are very slow.

Automatic Execution

Automatic operation is carried on due to stored program concept. So, frequent

Manual intervention is very low. Multi-usage of the same program is also possible

which are not possible in the case of manual mechanical computers.

Highly Reliableand do Complex Operations

Mechanical can perform only limited arithmetic operations and we can not fully

depend on the results given by them whereas the results of digital computers are

more versatile and can perform logical operations also. By writing relevant

programs we can execute even complex arithmetic operations.



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HISTORY OF COMPUTERS

In olden days, human being used fingers in their hands and legs. to count. When they felt the

insufficiency of fingers, they tried to use stones and pebbles and they found it very inconvenient to

carry wherever they go.

During 3000 BC, Chinese & Japanese used ABAUS mechanical calculator which is still widelyused in Asia. Meanwhile in Asia, the Chinese were becoming very involved in commerce with the

Japanese, Indians, and Koreans. Businessmen needed a way to tally accounts and bills. Somehow,

out of this need, the abacus was born. The abacus is the first true precursor to the adding machines

and computers which would follow.

Then Napiers Bones was developed in the year 1617. John Napier, a Scotsman, invented

logarithms which use lookup tables to find the solution to otherwise tedious and error-prone

mathematical calculations. Logarithm is a technology that allows multiplication to be performed

via addition. The magic ingredient is the logarithm of each operand, which was originally obtained

from a printed table. But Napier also invented an alternative to tables, where the logarithm values

were carved on ivory sticks which are now called Napier's Bones.

Napier Bones

The Slide Rule was first built in England in the year 1630. In 1960's, it was used by the NASA

engineers of the Mercury, Gemini, and Apollo programs which landed men on the moon. . The

slide rule works on the basis of Napiers rules for logarithms. It was used until 1970s



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Pascal designed the first mechanical calculator (Pascaline) based on gears. It performed addition

and subtraction. In 1642 Blaise Pascal, at age of 19 invented the Pascaline as an aid for his father

to collect tax. Pascal built 50 of this gear-driven one-function calculator (it could only add) but

couldn't sell many because of their exorbitant cost and because they really weren't that accurate (at

that time it was not possible to fabricate gears with the required precision). Up until the present age

when car dashboards went digital, the odometer portion of a car's speedometer used the very same

mechanism as the Pascaline to increment the next wheel after each full revolution of the priorwheel.

Pascal's Pascaline[photo 2002 IEEE]

Just a few years after Pascal, the German Gottfried Wilhelm Leibniz (co-inventor with Newton of

calculus) managed to build a four-function (addition, subtraction, multiplication, and division)

calculator was called him as the stepped reckoner because, instead of gears, it employed fluted

drums having ten flutes arranged around their circumference in a stair-step fashion. Although the

stepped reckoner employed the decimal number system (each drum had 10 flutes), Leibniz was the

first to advocate use of the binary number system which is fundamental to the operation of moderncomputers. Leibniz is considered one of the greatest of the philosophers but he died as poor.

Leibniz's Stepped Reckoner (haveyou ever heard" calculating" referred to as

" reckoning"?)

In 1801 the Frenchman Joseph Marie Jacquard invented a power loom that could base its weave

(and hence the design on the fabric) upon a pattern automatically read from punched wooden cards,

held together in a long row by rope. Descendents of these punched cards have been in use ever

since (remember the "hanging chad" from the Florida presidential ballots of the year 2000?).



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Jacquard's technology was a real boon to mill owners, but put many loom operators out of work.

Angry mobs smashed Jacquard looms and once attacked Jacquard himself. History is full of

examples of labor unrest following technological innovation yet most studies show that, overall,

technology has actually increased the number of jobs.

In the year 1822, Charles Babbage developed Differential Engine. With this machine, he

compiled statistics of life and saved around two years of processing time but he was unable todevelop this machine successfully.

In the year 1833, he conceived the idea Analytical Engine. According to him this machine will be

using punched cards for storage purpose and working with steam engines. Babbage called the two

main parts of his Analytic Engine the "Store" and the "Mill", as both terms are used in the weaving

industry. The Store was where numbers were held and the Mill was where they were "woven" into

new results. In a modern computer these same parts are called the memory unit and the central

processing unit (CPU). He was the first person who gave the concepts of storage, processing and

input and output. Still we are following the same concept. So he is called as Father of Moedern

Computers

The following diagrams will illustrate this:

If you compare the above two diagrams, one can understand that still we are following the concept

of Charles babbage and we are unable to deviate from the his idea even after lot of developments in

computer field. That is why he is called as Father of M odern Computers.

Babbage is largely remembered because of the work ofAugusta Ada (Countess of L ovelace) who

was the first computer programmer. She was fascinated by Babbage's ideas and through sending

and receiving letters and meetings with Babbage, she learned enough about the design of the



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Analytic Engine. While Babbage refused to publish his knowledge for another 30 years, Ada wrote

a series of "Notes" wherein she detailed sequences of instructions she had prepared for the Analytic

Engine. The Analytic Engine remained inbuilt (the British government refused to get involved with

this one) but Ada earned her spot in history as the first computer programmer. Ada invented the

subroutine and was the first to recognize the importance of looping.

A step towards automated computing was the development of punched cards, which were first

successfully used with computers in 1890 by Herman Hollerith and James Powers, who worked for

the US. Census Bureau. They developed devices that could read the information that had been

punched into the cards automatically, without human help. Because of this, reading errors were

reduced dramatically, work flow increased, and, most importantly, stacks of punched cards could

be used as easily accessible memory of almost unlimited size. Furthermore, different problems

could be stored on different stacks of cards and accessed when needed. Herman Hollerith's

technique was successful and the 1890 census was completed in only 3 years at a savings of 5

million dollars. Hollerith was the first American associated with the history of computers. He was

also the first to make a bunch of money at it. His company, the Tabulating Machine Company,

became the Computer Tabulating Recording Company in 1913 after struggling in the market andmerging with another company that produced a similar product. The company hired a gentleman

named Thomas J. Watson in 1918 who was primarily instrumental in turning the company around.

In 1924, the company was renamed International Business machines (IBM) Corporation.

A closer look at theCensusTabulating Machine

Aiken thought he could create a modern and functioning model of Babbages Analytical Engine.

He succeeded in securing a grant of 1 million dollars for his proposed Automatic Sequence

Calculator; the Mark I for short from IBM. In 1944, the Mark I was "switched" on. Aiken's

colossal machine spanned 51 feet in length and 8 feet in height. 500 meters of wiring were required

to connect each component.

GENERATI ONS OF COMPUTERS

First Generation - 1946-1958: Vacuum Tubes



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First Generation computers used vacuum tubes as electronic components for circuitry and

Magnetic Drum for storage. They were very big in size. They were costlier. Power

consumption is heavy. They produced heavy noise but the speed of processing is very slow.

First Generation Started with the introduction of ENIAC [Electronic Numerical Integrator And

Calculator] in the year 1946 by , John P. Eckert, John W. Mauchly , and their associates at the

Moore school of Electrical Engineering of University of Pennsylvania. Main purpose of

developing ENIAC was to assist the Word War II by preparing Firing Table to decide at what

velocity, at which direction, at which force, at which height if bombs exploded, heavy calamity

could be effected to the enemy.

ENIAC Features

1. It was using 18,000 valves 70,000 resistors and 5 million soldered joints.

2. It was weighing around 30 tons and occupied around 300 cu. Ft.

3. It computed at speeds 1,000 times faster than the Mark I was capable o f only 2 years

earlier

4. It could do nuclear physics calculations (in two hours) which it would have taken 100

engineers a year to do by hand.

5. The system's program could be changed by rewiring a panel.

6. It consumed about 180,000 watts of electrical power.

7. It had punched card I/O, 1 multiplier, 1 divider/square rooter, and 20 adders using

decimal ring counters , which served as adders and also as quick-access (.0002 seconds)

read-write register storage.

8. Used vacuum tubes for circuitry and magnetic drums for memory

9. Very Big, consumed more space.

10. Very expensive to operate & maintain.

11. Used more electricity, generated a lot of heat, which was often the cause of

malfunctions.

12. Relied on machine language, the lowest-level programming language understood by

computers, to perform operations, and they could only solve one problem at a time.



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13. Input was based on punched cards and paper tape, and output was displayed on

printouts.

The First Stored Program Computer was EDSAC [Electronic Discrete Storage Automatic

Computer] in 1949. The first American Stored Program Computer was EDVAC (Electronic

Discrete Variable Automatic Computer) by John von Neumann in 1950. Ecker and Mauchly

produced UNIVAC I (UNIVersal Automatic Computer) in 1951. The UNIVAC was the first

commercial computer delivered to a business client, the U.S. Census Bureau in 1951. Then

UNIVAC II came into existence.

Characteristicsof First Generation computers:

1. Used valves for data processing and storage.

2. They had a memory size of 20 bytes and speed of 5 mbps.

3. They produced heavy noise

4. They consumed enormous power.

5. They generated lot of heat due to the used of more valves.

6. They were very slow and unreliable.

7. They used punched cards for data storage.

8. They used binary language.

Second Generation - 1959-1964: Transistors

Transistors replaced vacuum tubes

The transistor was invented in 1947 but did not see widespread use in computers until the

late 50s.

The transistor was far superior to the vacuum tube.

With the usage of transistors, computers became smaller, faster, cheaper, more energy-

efficient and more reliable than their first-generation computers.

Though the transistor still generated a great deal of heat that subjected the computer to

damage, it was a vast improvement over the vacuum tube.

Second-generation computers still relied on punched cards for input and printouts for

output.

Second-generation computers moved from cryptic binary machine language to symbolic

or assembly languages, which allowed programmers to specify instructions in words.



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High-level programming languages were also being developed at this time, such as early

versions of COBOL and FORTRAN.

These were also the first computers that stored their instructions in their memory, which

moved from a magnetic drum to magnetic core technology.

The first computers of this generation were developed for the atomic energy industry.

IBM 1620: Its size was smaller as compared to First Generation computers and mostly

used for scientific purpose. IBM 1401: Its size was small to medium and used for business

applications. CDC 3600: Its size was large and is used for scientific purposes.

Third Generation - 1965-1970: Integrated Circuits

This is very important generation and lot of activities happened in

computer field during this generation. The development of the

integrated circuit was the hallmark of the third generation of

computers. ICs are called as miniature of Valves and Transistors i.e.,several valves and transistors functions were put into a small IC. In

1958, Jack Kilby who is an engineer with Texas Instruments,

developed the Integrated Circuit (IC). The Integrated Circuit combined

three electronic components onto a small silicon disc, which was made

from quartz rock. Scientist later managed to fit more components on a single chip, called

semiconductor. As a result of it, more components were able to squeeze onto the chip and thereby

computers became ever smaller. Transistors were miniaturized and placed on silicon chips which

drastically increased the speed and efficiency of computers. Semiconductors are nothing but ICs

which used to conduct the electronic signals partially. Instead of punched cards and printouts, users

interacted with third generation computers through keyboards and monitors and interfaced with an

operating system, which allowed the device to run many different applications at one time with a

central program that monitored the memory. Another third generation computer development

included the use of an OS (operating system) that allowed computers to run multiple programs

together with a central program that monitored & coordinated the memory of the computer.

Computers for the first time became accessible to a mass audience because they were smaller and

cheaper than their predecessors. Mini computers were developed during this period. During this

period BASIC [Beginners All purpose Symbolic Instruction Code] language was developed by

Prof. John Kemny and Thomas Kurtz in the year 1964 for the benefits of beginners and students.

Till then, programming was meant for experienced programmers and scientists. This gave birth to

multiprogramming and Timesharing concepts.

Characteristics of Third Generation Computers

Characteristics ofThird Generation Computers in comparison with that of previous generationcomputers are

1. Third Generation Computers were based on integrated circuit (IC) technology.

2. Third Generation Computers were able to reduce computational time from microseconds to

nanoseconds

3. Third Generation Computers devices consumed less power and generated less heat. In somecases, air conditioning was still required.4. The size of Third Generation Computers was smaller as compared to previous computers



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5. Since hardware of the Third Generation Computers rarely failed, the maintenance cost for it

was quite low.

6. Extensive use of high-level language became possible in Third Generation Computers.7. Manual assembling of individual components was not required for Third Generation

Computers, so it reduced the large requirement of labor & cost. However, for themanufacture of IC chips, highly sophisticated technologies were required

8. Commercial production became easier and cheaper.

Fourth Generation - 1970-Present: Microprocessors

The microprocessor brought the fourth generation of computers. Microprocessors are called as

miniature of chips. In Microprocessor, thousands of integrated circuits were built onto a single

silicon chip. What in the first generation filled an entire room could now fit in the palm of the hand.

All modern day computers are Fourth Generation Computers. All of us are using Fourth Generation

Computers for our day-to-day activities. With the improvement in the IC (Integrated Circuit), the

size of the computers started to go down. Invention of VLSI (Very Large Scale Integration)

squeezed hundreds of thousands of components onto a single chip, where as the ULSI (Ultra-large

Scale Integration) increased that number into the millions by the year 1980. Advancement in

technology makes Fourth generation computers cheaper in price and best in quality than all other

generation of computers.

The Intel 4004 chip, developed in 1971, located all the components of the computer - from the

central processing unit and memory to input/output controls - on a single chip. Whereas the IC used

in previous computer generations, the IC had had to be manufactured to serve a special purpose,

now a single microprocessor could be manufactured & then programmed to meet any number of

demands. Soon everyday household items such as Televisions, Music Systems, Washing Machines,

Micro Ovens, and Automobiles incorporated microprocessors.

In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the

Macintosh. Microprocessors also moved out of the realm of desktop computers and into many areas

of life as more and more everyday products began to use microprocessors.

As these small computers became more powerful, they could be linked together to form networks,

which eventually led to the development of the Internet. Fourth generation computers also saw the

development of GUIs, the mouse and handheld devices.

Integration Types

Small scale integration Up to 100 devices on a chip

Mediumscaleintegration 100 - 3,000 devices on a chip 100

Largescaleintegration 3,000 - 100,000 devices on a chip

Very largescale integration 100,000 -100,000,000 devices on a chip

Ultra largescale integration Over 100,000,000 devices on a chip

Characteristics of Fourth Generation Computers

1. Fourth generation computers are microprocessor based systems.2. Fourth generation computers are very small.

3. Fourth Generation computers are the cheapest among all other computer generations.4. Fourth generation computers are portable and quite reliable.



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5. Fourth generation computers do not require air conditioning since they generate negligible

amount of heat.

6. Minimum maintenance is required for Fourth generation computers since hardware failureis negligible for them.

7. The production cost of Fourth generation computers is very low8. GUI and pointing devices enables users to learn to use the computer quickly.

9. Interconnections of computers leads to better communication and resource sharing.

10. Fourth generation computers are very powerful than previous generations and can easily domore calculation or can run more programs at a time and for more hours.

Fifth Generation - Present and Beyond: Artificial Intelligence

Fifth generation computing devices, based on artificial intelligence, are still in development, though

there are some applications, such as voice recognition, that are being used today. The idea of fifth

generation computer was introduced by Japans Ministry of International Trade and Industry in

1982. The term fifth generation was stretched out to convey the system as being a leap beyond

existing computer machines. But the fifth generation computer system (FGCS) project of Japan

was failed since the Ministry of International Trade and Industry ( MITI ) of Japan stopped funding

for it. The use of parallel processing and superconductors is helping to make artificial intelligence a

reality. Quantum computation and molecular and nanotechnology will radically change the face of

computers in years to come. The goal of fifth-generation computing is to develop devices that

respond to natural language input and are capable of learning and self-organization

Advances in science behind the creation of fifth generation computer

Many advances in the science of computer-design and technology are coming together to enable the

creation of fifth generation computers. Two such engineering advances are give below

1. parallel processing, which replaces von Neumanns single central processing unit design

with a system harnessing the power of many CPUs to work as one.

2. the technology of superconductors which is another great advantage, allows the flow of the

electricity with very less or even no resistance, greatly improving the information flowspeed.

MEMORY OF COMPUTERS

There are two kinds of computer memory: primary and secondary. Primary memory is accessible

directly by the processing unit. RAM is an example of primary memory. As soon as the computer is

switched off the contents of the primary memory is lost. You can store and retrieve data much

faster with primary memory compared to secondary memory. Secondary memory such as floppy

disks, magnetic disk, etc., is located outside the computer. Primary memory is more expensive than

secondary memory. Because of this the size of primary memory is less than that of secondary

memory. We will discuss about secondary memory later on.

Computer memory is used to store two things:

i) instructions to execute a program and

ii) data.

When the computer is doing any job, the data that have to be processed are stored in the primary

memory. This data may come from an input device like keyboard or from a secondary storage



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device like a floppy disk.

As program or the set of instructions is kept in primary memory, the computer is able to follow

instantly the set of instructions. For example, when you book ticket from railway reservation

counter, the computer has to follow the same steps: take the request, check the availability of seats,

calculate fare, wait for money to be paid, store the reservation and get the ticket printed out. The

program containing these steps is kept in memory of the computer and is followed for each request.

But inside the computer, the steps followed are quite different from what we see on the monitor or

screen. In computers memory both programs and data are stored in the binary form. You have

already been introduced with decimal number system, that is the numbers 1 to 9 and 0. The binary

system has only two values 0 and 1. These are called bits. As human beings we all understand

decimal system but the computer can only understand binary system. It is because a large number

of integrated circuits inside the computer can be considered as switches, which can be made ON, or

OFF. If a switch is ON, it is considered 1 and if it is OFF, it is 0. A number of switches in different

states will give you a message like this: 110101....10. So the computer takes input in the form of 0

and 1 and gives output in the form 0 and 1 only. Is it not absurd if the computer gives outputs as 0s

& 1s only? But you do not have to worry about.

Every number in binary system can be converted to decimal system and vice versa; for example,

1010 meaning decimal 10. Therefore it is the computer that takes information or data in decimal

form from you, convert it in to binary form, process it producing output in binary form and again

convert the output to decimal form.

The primary memory as you know in the computer is in the form of ICs (Integrated Circuits).

These circuits are called Random Access Memory (RAM). Each of RAMs locations stores one

byteof information. (One byte is equal to 8 bits). A bit is an acronym forbinary digit, which stands

for one binary piece of information. This can be either 0 or 1. You will know more about RAM

later. The Primary or internal storage section is made up of several small storage locations (ICs)

called cells. Each of these cells can store a fixed number of bits called word length.

Each cell has a unique number assigned to it called the address of the cell and it is used to identify

the cells. The address starts at 0 and goes up to (N-1). You should know that the memory is like a

large cabinet containing as many drawers as there are addresses on memory. Each drawer contains

a word and the address is written on outside of the drawer.

UNIT SY MBOL POWER

OF 2

Number of bytes

Byte0

21

K ilobyte (1Thousand) K B10

21,024

M egabyte (1Million) MB20

21,048,576

Gigabyte (1 Billion) GB 30 1,073,741,824



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2

Terabyte(1Trillion) TB40

21,099,511,627,776

Petabyte(1quadrillion)

PB 50

21,125,899,906,842,624

Exabyte(1 quintillion) EB60

21,152,921,504,606,846,976

Zettabyte(1sextillion) ZB70

21,180,591,620,717,411,303,424

Yottabyte(1septillion) Y B80

21,208,925,819,614,629,174,706,176

1024 YB = 1 (Bronto Byte)

1024 Brontobyte = 1 (Geop Byte)

Geop Byte is The Highest Memory Measurement Unit!!!

Capacity of Primary Memory

You know that each cell of memory contains one character or 1 byte of data. So the capacity is

defined in terms of byte or words. Thus 64 kilobyte (KB) memory is capable of storing 64 X 1024= 32,768 bytes. (1 kilobyte is 1024 bytes). A memory size ranges from few kilobytes in small

systems to several thousand kilobytes in large mainframe and super computer. In your personal

computer you will find memory capacity in the range of 64 KB, 4 MB, 8 MB and even 16 MB

(MB = Million bytes). Now It is in GB.

The following terms related to memory of a computer are discussed below:

Different types of memory

1. Random Access M emory (RAM ): The primary storage is referred to as random access

memory (RAM) because it is possible to randomly select and use any location of the

memory directly store and retrieve data. It takes same time to any address of the memory as

the first address. It is also called read/write memory. The storage of data and instructions

inside the primary storage is temporary. It disappears from RAM as soon as the power to the

computer is switched off. The memories, which loose their content on failure of power

supply, are known as volatile memories .So now we can say that RAM is volatile memory.

2. Read Only Memory (ROM): There is another memory in computer, which is called Read

Only Memory (ROM). Again it is the ICs inside the PC that form the ROM. The storage of

program and data in the ROM is permanent. The ROM stores some standard processing

programs supplied by the manufacturers to operate the personal computer. The ROM can

only be read by the CPU but it cannot be changed. The basic input/output program is stored

in the ROM that examines and initializes various equipment attached to the PC when the

switch is made ON. The memories, which do not loose their content on failure of power



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supply, are known as non-volatile memories. ROM is non-volatile memory.

3. PROM There is another type of primary memory in computer, which is called

Programmable Read Only Memory (PROM). You know that it is not possible to modify or

erase programs stored in ROM, but it is possible for you to store your program in PROM

chip. Once the programmes are written it cannot be changed and remain intact even if

power is switched off. Therefore programs or instructions written in PROM or ROM cannot

be erased or changed.

4. EPROM: This stands for Erasable Programmable Read Only Memory, which over come

the problem of PROM & ROM. EPROM chip can be programmed time and again by

erasing the information stored earlier in it. Information stored in EPROM exposing the chip

for some time ultraviolet light and it erases chip is reprogrammed using a special

programming facility. When the EPROM is in use information can only be read.

5. CacheMemory: The speed of CPU is extremely high compared to the access time of main

memory. Therefore the performance of CPU decreases due to the slow speed of main

memory. To decrease the mismatch in operating speed, a small memory chip is attached

between CPU and Main memory whose access time is very close to the processing speed of

CPU. It is called CACHE memory. CACHE memories are accessed much faster than

conventional RAM. It is used to store programs or data currently being executed or

temporary data frequently used by the CPU. So each memory makes main memory to be

faster and larger than it really is. It is also very expensive to have bigger size of cache

memory and its size is normally kept small.

6. Registers: The CPU processes data and instructions with high speed, there is also

movement of data between various units of computer. It is necessary to transfer the

processed data with high speed. So the computer uses a number of special memory units

called registers. They are not part of the main memory but they store data or information

temporarily and pass it on as directed by the control unit.

7. Flash Memory: A solid-state, nonvolatile, rewritable memory that functions like a

combination of RAM and hard disk. Flash memory is durable, operates at low voltages, and

retains data when power is off. Flash memory cards are used in digital cameras, cell phones,

printers, handheld computers, pagers, and audio recorders.

8. Virtual Memory :

In the early years, computer memories were small and more expensive. Programmers were

using a total memory size of only 4096 18-bit words for the both user programs and

operating system in PDP-1. So, the programmer had to fit his program in this small

memory. Nowadays, computers have some gigabytes of memory but the modern programs

need much more memory. To solve this problem, operating systems use secondary

memories such as disk as main memory.

In the first technique, the programmer divided the program up into a number of pieces

called overlays. At the start of the program, first overlay was loaded into memory. When it

finished, loads next overlay. Programmers must manage overlays between memory and

disk. He was responsible to find it from disk and load it to memory. It was difficult for

programmers.

In 1961, a group of researchers from Manchester established automatic overlay

management system called virtual memory.



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Virtual memory is organized into "pages". A page is the memory unit typically a few

Kbytes in size. It is mostly 4-Kbytes. You can learn page size by typing page size

command. When a program references to an address on a page not present in main memory,

a page fault occurs. After a page fault, the operating system seeks for the corresponding

page on the disk and loads it onto main memory by using a page replacement algorithm

such as LRU. We can start a program when none of the program is in main memory. When

the CPU tries to fetch the first instruction of the program, it gets a page fault, because thememory doesn't contain any piece of the program in the main memory. This method is

called demand paging.

If a process in main memory has low priority or is sleeping, that means it won't run soon. In

this case, the process can be backed up on disk by the operating system. This process is

swapped out. The swap space is using for holding memory data.

Processes use virtual addresses for transparency. They don't know about physical memory.

CPU has a unit called Memory Management Unit which is responsible for operating

virtual memory. When a process makes a reference to a page that isn't in main memory, the

MMU generates a page fault. The kernel catches it and decides whether the reference is

valid or not. If invalid, the kernel sends signal "segmentation violation" to the process. If

valid, the kernel retrieves the page process referenced from the disk.

SECONDARY STORAGE

You are now clear that the operating speed of primary memory or main memory should be as fast

as possible to cope up with the CPU speed. These high-speed storage devices are very expensive

and hence the cost per bit of storage is also very high. Again the storage capacity of the main

memory is also very limited. Often it is necessary to store hundreds of millions of bytes of data for

the CPU to process. Therefore additional memory is required in all the computer systems. This

memory is called auxiliary memory or additional memory or attached memory or secondary

memory.

In this type of memory the cost per bit of storage is low. However, the operating speed is slower

than that of the primary storage. Huge volume of data are stored here on permanent basis and

transferred to the primary storage as and when required. Most widely used secondary storage

devices are magnetic tapes and magnetic disk.

1.Magnetic Tape: Magnetic tapes are used for large computers like mainframe computers where

large volume of data is stored for a longer time. In PC also you can use tapes in the form of

cassettes. The cost of storing data in tapes is inexpensive. Tapes consist of magnetic materials thatstore data permanently. It can be 12.5 mm to 25 mm wide plastic film-type and 500 meter to 1200

meter long which is coated with magnetic material. The deck is connected to the central processor

and information is fed into or read from the tape through the processor. It is similar to cassette tape

recorder.

M agnetic Tape



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10.5 inch computer magnetic tape.

Advantages of Magnetic Tape:

Compact: A 10-inch diameter reel of tape is 2400 feet long and is able to hold

800, 1600 or 6250 characters in each inch of its length. The maximum capacity of

such tape is 180 million characters. Thus data are stored much more compactly on

tape.

Economical: The cost of storing characters is very less as compared to other

storage devices.

Fast: Copying of data is easier and fast.

L ong term Storage and Re-usability: Magnetic tapes can be used for long term

storage and a tape can be used repeatedly with out loss of data.

2.Magnetic Disk: You might have seen the gramophone record, which is circular like a disk and

coated with magnetic material. This is a Random Accessing Device. That is, here you can pick out

any files or records at random or your choice immediately and accessing time will be low to access

any record or file you require. These are non-volatile storage device. Magnetic disks used in

computer are made on the same principle of gramophone record player. It rotates with very high

speed inside the computer drive. Data is stored on both the surface of the disk. Magnetic disks are

most popular for direct access storage device. Each disk consists of a number of invisible

concentric circles called tracks. Information is recorded on tracks of a disk surface in the form of

tiny magnetic spots. The presence of a magnetic spot represents one bit and its absence represents

zero bit. The information stored in a disk can be read many times without affecting the stored data.

So the reading operation is non-destructive. But if you want to write a new data, then the existing



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data is erased from the disk and new data is recorded.



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Hard Disk

A hard disk drive (often shortened as "hard disk", "hard drive", or "HDD"), is a non-volatile

storage device which stores digitally encoded data on rapidly rotating platters with magnetic

surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a

tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had

removable media; however, an HDD today is t ypically a sealed unit which is called as

Winchester Disk. Here the hard disk will be kept in an airtight box and the user cannot

tamper with it normally.

HDDs record data by magnetizing ferromagnetic material directionally, to represent either a

0 or a 1 binary digit. They read the data back by detecting the magnetization of the material.

A typical HDD design consists of a spindle which holds one or more flat circular disks

called platters, onto which the data are recorded. The platters are made from a non-magnetic

material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic

material. Older disks used iron(III) oxide as the magnetic material, but current disks use a

cobalt-based alloy.

Howfiles arestored in a disk



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data is stored in blocks

blocks a set of sectors

tracks are divided into various sectors

Actual files or information are stored in available sectors

files will have names

files are indefinite in size

files may be updated (in part or whole)

directory entries record file data

file allocation table keeps track of file

pieces and contains File name, size,

date and time of creation, Attributes of

a file and the address of the starting

cluster. At the end of each cluster you

will have the address of next cluster

and your file will be stored as a sequence of links.

If there is any break in the links you will come across Lost clusters or chains

Problem. Actually the data of a file is existing in the disk but your OS is unable to

fetch the lost links because of improper shutdown or closing. So all details of your

files will not be listed.

A Cylinder is nothing but same numbered concentric circle or track of nth the

sides. ie. 1st

track of head 0 and 1st

track of head 1

Floppy Disk: It is similar to magnetic disk discussed above. The floppies are made up of

thin, flexible polythene film on which high quality magnetic oxide is coated. Since it is

flexible in nature, it is called as floppy. It is encased in a square plastic shell to protect it

from mishandling, dust, Moisture. They are 8 inch, 5.25 inch or 3.5 inch in diameter. They

come in single or double density and recorded on one or both surface of the diskette. The

capacity of a 5.25-inch floppy is 1.2 mega bytes whereas for 3.5 inch floppy it is 1.44 mega

bytes [High Density]. It is cheaper than any other storage devices and is portable. The

floppy is a low cost device particularly suitable for personal computer system. Now, they

have been largely superseded by USB flash drives, External Hard Drives, CDs, DVDs, andmemory cards (such as Secure Digital).

8 Floppy 5 Floppy 3 Floppy



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Floppy Disk

3.Optical Disk:

CD ROM and DVD are optic readable media, contrary to hard disks, floppy disks and tapes, which

are magnetic. The optic storage media are read with a very thin and very precisely aimed laser

beam. They supplement the magnetic media.

They have clear advantages in the areas of data density and stability: Data can be packed much

more densely in an optic media than in a magnetic media. And they have much longer life span. It

is presumed that a magnetic media, such as a hard disk or DAT (digital audio tape) can maintain

their data for a maximum of five years. The magnetism simply fades away in time. Conversely, the

life span of optic media is counted in tens of years.

Let us take a closer look at these disks, which are becoming increasingly popular for all types of

information, education and entertainment.With every new application and software there is greater demand for memory capacity. It is the

necessity to store large volume of data that has led to the development of optical disk storage

medium. Optical disks can be divided into the following categories:

1.Compact Disk/ Read Only Memory (CD-ROM): The compact disk (CD) was introduced

by Philips and Sony in 1980 to replace LP records. It is a small plastic disk with a reflecting

metal coating, usually aluminum. Myriad's of tiny indentations are burned into this coating.

These indentations contain the music in millions of bits. The CD is organized in tracks.

Each track is assigned a number.

The big advantage of the CD is its high-quality music reproduction and total absence of

back ground noise as well as a great dynamic. During operation, the software in the drive

can correct errors caused by such things as finger marks on the disk. All in all, the CD is an

excellent music storage media.

CD-ROM disks are made of reflective metals. CD-ROM is written during the process of

manufacturing by high power laser beam. Here the storage density is very high, storage cost

is very low and access time is relatively fast. Each disk is approximately 4 1/2 inches in

diameter and can hold over 600 MB of data. As the CD-ROM can be read only we cannot

write or make changes into the data contained in it.2.WriteOnce, Read Many (WORM): The inconvenience that we can not write any thing in

to a CD-ROM is avoided in WORM. A WORM allows the user to write data permanently

on to the disk. Once the data is written it can never be erased without physically damaging



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the disk. Here data can be recorded from keyboard, video scanner, OCR equipment and

other devices. The advantage of WORM is that it can store vast amount of data amounting

to gigabytes (109 bytes). Any document in a WORM can be accessed very fast, say less

than 30 seconds.

3.Erasable Optical Disk: These are optical disks where data can be written, erased and re-

written. This also applies a laser beam to write and re-write the data. These disks may be

used as alternatives to traditional disks. Erasable optical disks are based on a technology

known as magnetic optical (MO). To write a data bit on to the erasable optical disk the MO

drive's laser beam heats a tiny, precisely defined point on the disk's surface and magnetises

it.

TheCompact Disk

The CD-ROM is designed differently. It has only one track, a spiral winding its way from the

center to the outer edge:

This long spiral track holds up to 650 MB data in about 5.5 billion dots (each is one bit). The

incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you

could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide

and almost 3.5 miles (5 km) long!

A CD has a long, spiraled data track. If you were to unwind this

track, it would extend out 3.5 miles (5 km).

CD Basics: T heBumps

If you've read How CDs Work, you understand the basic idea of

CD technology. CDs store music and other files in digital form -

- that is, the information on the disc is represented by a series of

1s and 0s (see How Analog and Digital Recording Works for

more information). In conventional CDs, these 1s and 0s are

represented by millions of tiny bumps and flat areas on the disc's

reflective surface. The bumps and flats are arranged in a

continuous track that measures about 0.5 microns (millionths of a meter) across and 3.5 miles (5

km) long.

To read this information, the CD player passes a laser beam over the track. When the laser passes

over a flat area in the track, the beam is reflected directly to an optical sensor on the laser

assembly. The CD player interprets this as a 1. When the beam passes over a bump (pit), the light

is bounced away from the optical sensor. The CD player recognizes this as a 0.



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Courtesy : http://computer.howstuffworks.com/cd-burner1.htm

A CD player guides a small laser along the CD's data track.

In conventional CDs, the flat areas, or lands, reflect the light back to the laser assembly; the bumps

deflect the light so it does not bounce back.

What theCD Player Does: T racking

The hardest part is keeping the laser beam centered on the data track. This centering is the job ofthe tracking system. The tracking

system, as it plays the CD, has to

continually mo ve the laser

outward. As the laser moves

outward from the center of the

disc, the bumps move past the

laser faster -- this happens because

the linear, or tangential, speed of

the bumps is equal to the radius

times the speed at which the disc is

revolving (rpm). Therefore, as the

laser moves outward, the spindle

motor must slow the speed of the

CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at

a constant rate.

Data read from CD-ROM

Data are usually read from the CD-ROM at a constant speed. The principle is called CLV (ConstantLinear Velocity). It implies that the data track must pass under the read head at the same rate,

whether in inner or outer parts of the track. This is accomplished by varying the disk rotation speed,

based on the read head's position. The closer to the center of the disk is the faster the rotation speed.

In the music CD, data are read sequentially. Therefore, rotation speed variation is not necessary.

The CD-ROM disk on the other hand has to read in random pattern. The read head must jump

frequently to different parts of the disk. Therefore, it forever has to change rotation speed. You can

feel that. It causes pauses in the read function. That is a disadvantage of the CD-ROM media. Also

the faster versions can be rather noisy.

Theconstruction of a CD

The CD itself is made up of one continuous track about 0.5 microns wide and around 5km in

length. This track is a small groove spiralling round and round the CD from the centre to the edge.

http://computer.howstuffworks.com/cd-burner1.htmhttp://www.alientools.com/http://www.alientools.com/http://computer.howstuffworks.com/cd-burner1.htm


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The materials used to make a CD are at the top we have the label, then a layer of acrylic, a layer of

aluminium ad finally a thicker layer of plastic to protect the CD.

When manufacturing a production CD like what you buy in the shops, A heavy duty stamp is used

with microscopic bumps arranged as a single track of data. This is then stamped on a disc of

polycarbonate plastic. Then the Aluminium coating is applied for its properties as a reflective

surface. Acrylic is then applied for protection, and the label is then placed on. This is obviously a

large volume solution and the technique is no good for home use.

CD-R and CD-E

In 1990, the CD-ROM technique was advanced to include home burning. You could buy your own

burner. That is a drive, which can write on special CD-ROM disks. These have a temperature

sensing layer, which can be changed by writing. You can only write on any given part of these

disks once. This CD-R disk is also called a WORM disk (Write OnceRead Many). Once the CD-R

is burnt, it can be read in any CD drive for sound or data.

There is also a type called CD-erasable (CD-E), where you can write multiple times on the same

disk surface. This technique is promising. However, not all CD drives can read these CD's. Thelatest drives, which can adjust the laser beam to match the current media, are called multi read.

Look for that, when you buy a new CD-ROM drive.

DVD

The next optic drives we will see in the next few years is the DVD (Digital Video Disks)drive.

They are being developed by several companies (Philips, Sony, and others) and represent a

promising technology. DVD stands for Digital Versatile Disk.

They are thought of as a future all-round disk, which will replace CD-ROM and laser disks. In the

future, DVD might also replace VHS tapes for videos. Certain DVD drives can both read and writethe disks. There are also read only, designed for playing videos.

The DVD is a flat disk, the size of a CD - 4.7 inches diameter and .05 inches thick. Data are stored

in a small indentation in a spiral track, just like in the CD. DVD disks are read by a laser beam of

shorter wave-length than used by the CD ROM drives. This allows for smaller indentations and

increased storage capacity.

The data layer is only half as thick as in the CD-ROM. This opens the possibility to write data in

two layers.The outer gold layer is semi transparent, to allow reading of the underlying silver layer.

The laser beam is set to two different intensities, strongest for reading the underlying silver layer.

Here you see a common type DVD ROM drive:

Other DVD types

We have the following DVD versions:



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DVD-ROM is for read-only, like the CD-ROM. This media is usable for distribution of

software, but especially for multimedia products, like movies. The outer layers can hold 4.7 GB,

the underlying 3.8 GB. The largest version can hold a total of 17 GB.

DVD-R (recordable) are write once-only like CD-R. This disk can hold 3.9 GB per side .

DVD RAM can be written and read like a hard disk. Capacity is 2.6 GB per side or whatever

the agree on. There are many problems with this format.

4. Flash Drives

Use flash memory media

No moving parts so more resistant to shock and vibration, require less power, makes

no sound

Solid-state storage system

Most often found in the form of:

Flash memory cards

USB flash drives

Solid-state drives

Hybrid hard drives

Very small and so are very appropriate for use with digital cameras, digital music

players, handheld PCs, notebook computers, smart phones, MP4 Players etc.

Now, the most common media of storage is Pen Drive using USB port. It replaced Floppy

drives. Now floppies become extinct.



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INPUT OUTPUT DEVICES

A computer is only useful when it is able to communicate with the external

environment. When you work with the computer you feed your data and instructionsthrough some devices to the computer. These devices are called Input devices.

Similarly computer after processing, gives output through other devices called output

devices.

For a particular application one form of device is more desirable compared to others.

We will discuss various types of I/O devices that are used for different types of

applications. They are also known as peripheral devices because they surround the CPU

and make a communication between computer and the outer world.

Input Devices

Input devices are necessary to convert our information or data in to a form which can be

understood by the computer. A good input device should provide timely, accurate and

useful data to the main memory of the computer for processing followings are the most

useful input devices.



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1. Card Reader :

This is a sequential accessing device where in the information are entered in

the form of punching holes on cards

The IBM 80-column punching format, with rectangular holes, eventually wonout over the competing UNIVAC 90-character format, which used 45

columns (2 characters in each) of 12 round holes. Punch cards were widely

known as just IBM cards, even though other companies made cards and

equipment to process them. The rectangular bits of paper punched out are

called chad (recently, chads) orchips (in IBM usage).

IBM punch card format

The IBM card format held 80 columns with 12 punch locations each,

representing 80 characters. The top two positions were called zone punches,

12 (top) and 11. These often encoded plus and minus signs. The remaining ten

positions represented (from top to bottom) the digits 0 through 9.

Originally only numeric information was coded, with 1 or 2 punches per column:

digits (digit [0-9]) and signs (zone [12,11] sometimes over-punching the Least

Significant Digit). Later, codes were introduced for upper-case letters and special

characters. A column with 2 punches (zone [12,11,0] + digit [1-9]) was a letter; 3

punches (zone [12,11,0] + digit [2-4] + 8) was a special character. The introduction

of EBCDIC in 1964 allowed columns with as many as 6 punches (zones[12,11,0,8,9] + digit [1-7]). For computer applications, binary formats were

sometimes used, where each hole represented a single binary digit (or "bit").



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Information are entered by punchingholes on cards using coding for each

characters

A card punching machine was used to punch information on cards. A Card

verifier should be used to verify the information punched on card. With out

verifying the information punched on a card, we cannot use it in a card reader.A Card sorter is used to arrange the cards in the desired sorting order. A card

Reader will be used to read information on cards. There will a passage in a

card reader for the flow of cards. On the top that passage, there will be 12

source of light and at the bottom there will be 12 photo cells. When the card

is flowing inside the passage the light will be following on the respective

photocell and that cells where light falling will be activated and this is

considered as 1 . If the cells are not activated, It will be considered as 0 .

In this way all information will be sent to the computer in binary format.

Now this cards have beco me obsolete one because :

1. They are very costlier i.e., a card may cost 50 paise and only 80

characters can be entered in a card. If you want to enter 80000

characters you have to use 1000 cards. So, the total cost will be Rs.500.

A floppy will be costing only Rs. 25 where you can enter 1.2MB

information. A 4 MB pen drive will be costing Rs. 500 only.

2. Cards are not reusable where as the floppies or pen drives are reusable.

If you make any mistake, the card is not reusable. You have to take fresh

card and re-enter the information on it.

3. The cost of maintaining the cards will also be a prohibitive one. Youhave to protect them from dust, moisture and insects.

4. They occupy huge storage space but now we are having Floppy disks,Hard disks, CDs, DVDs, Pendrives etch which may not occupy a lesser

amount of space.

2. M agnetic Tapediscussed earlier

3. M agnetic Disk discussed earlier

Floppy Disk

Hard Disk



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4. Optical Disk discussed earlier

5. K eyboard: - This is the standard input device attached to all computers. The

layout of keyboard is just like the traditional typewriter of the type QWERTY. It

also contains some extra command keys and function keys. It contains a total of 101

to 104 keys. A typical keyboard used in a computer is shown in Fig. 2.6. You have

to press correct combination of keys to input data. The computer can recognise the

electrical signals corresponding to the correct key combination and processing is

done accordingly.

TheM altron K eyboard

The Maltron keyboard is designed to lessen user fatigue and perhaps carpal tunnel

syndrome. Note the angles of the keys and the many keys that are operated with

your thumbs.

6. Mouse: - Mouse is an input device shown in Fig. 2.7 that is used with your

personal computer. It rolls on a small ball and has two or three buttons on the top.

When you roll the mouse across a flat surface the screen censors the mouse in the

direction of mouse movement. The cursor moves very fast with mouse giving you

more freedom to work in any direction. It is easier and faster to move through a

mouse.

7. Scanner: The keyboard can input only text through keys provided in it. If we

want to input a picture the keyboard cannot do that. Scanner is an optical device that

can input any graphical matter and display it back. The common optical scanner

devices are Magnetic Ink Character Recognition (MICR), Optical Mark Reader

(OMR) and Optical Character Reader (OCR).

8. Magnetic Ink Character Recognition (MICR): - This is widely used by banks to process

large volumes of cheques and drafts. Cheques are put inside the MICR. As they enter the reading



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unit the cheques pass through the magnetic field which causes the read head to recognise the

character of the cheques.

9. Optical Mark Reader (OMR): This technique is used when students have

appeared in objective type tests and they had to mark their answer by darkening a

square or circular space by pencil. These answer sheets are directly fed to a

computer for grading where OMR is used.

10.Optical Character Recognition (OCR): - This technique unites the direct

reading of any printed character. Suppose you have a set of hand written characters

on a piece of paper. You put it inside the scanner of the computer. This pattern is

compared with a site of patterns stored inside the computer. Whichever pattern is

matched is called a character read. Patterns that cannot be identified are rejected.

OCRs are expensive though better the MICR.

Output Devices

1. Visual Display Unit: The most popular input/output device is the Visual Display Unit

(VDU). It is also called the monitor. A Keyboard is used to input data and Monitor is

used to display the input data and to receive massages from the computer. A monitor has

its own box which is separated from the main computer system and is connected to the

computer by cable. In some systems it is compact with the system unit. It can be colour

or monochrome. It can be a Monochrome where only black and white colour are

available. It can also be CGA ( Colour Graphics Adapter) where 4 colours are available

or can be EGA (Enhanced Graphics Adaptor) or can be VGA (Video Graphics Array)

where more colours are available or Super VGA.

Concepts and terminology : When we talk about screens, there are currently three

different types to choose from:

CRT (Cathode Ray Tube) the common type screens. They are found in

different technologies, suchas Invar and Trinitron.

LCD (Liquid Crystal Display) flat and soft displays. TFT is the most

expensive display of this type.

The TFT screen is also called a "soft" screen, since the images appear softer

than from Cathode Ray Tubes..

Common principles

The principles in these screen types are quite different, but the screen image design

rests on the same concepts:

Pixels. The screen image is made of pixels (tiny dots), which are

arranged in rows across the screen. A screen image consists of between

480,000 and 1,920,000 pixels.

Refresh rate. The screen image is "refreshed" many times per second.Refresh rates are measured in Hertz (HZ), which means "times per

second".



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Color depth. Each pixel can display a number of different colors. The

number of colors, which can be displayed, is called color depth. Color

depth is measured in bits.

Video RAM . All video cards have some RAM. How much depends on

the desired color depth. Video cards usually have 1, 2 or 4 MB RAM

for normal usage.

These concepts are central to the understanding of the video system. Since the CRT

screens are still by far the most common, they will form the basis for this review.

3. Terminals: It is a very popular interactive input-output unit. It can be divided

into two types: hard copy terminals and soft copy termin

Unit III Computers

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