Chapter 1: Introduction to Computer System

Contents:
Introduction to Computer, Characteristics,Generations, History, Applications and Classification of Computer, Anatomy of a Digital Computer, Computer Architecture, Memory and its Classification, Input and Output Devices.

Introduction
A computer is an electronic machine that takes input from the user, processes the given input and generates output in the form of useful information. A computer accepts input in different forms such as data, programs and user reply. Data refers to the raw details that need to be processed to generate some useful information. Programs refer to the set of instructions that can be executed by the computer in sequential manner. User reply is the input provided by the user in response to a question asked by the computer.
A computer is an electronic device that processes data, converting it into information that is useful to people.
Computer is a machine that performs calculations and processes data with astonishing speed and precision. A computer can take thousands of individual pieces of data and turn them into more usable information-with blinding speed and almost unfailing accuracy.

Characteristics of Computers
Computers are what they are because of the following characteristics:
i) Speed
A computer is a fast electronic device that can solve large and complex problems in a fraction of seconds. For example, a microcomputer can execute millions of instructions per second over and over again, without any mistake. The speed of a computer generally depends upon its hardware configuration. For example, supercomputers can operate at speeds measured in nanoseconds and even in picoseconds – one thousand to one million times faster than microcomputers.
ii) Storage
A computer can store huge amounts of data in its different storage components in many different formats. Computers have main memory and auxiliary memory systems. With more and more auxiliary storage devices, which are capable of storing huge amounts of data, the storage capacity of a computer is virtually unlimited.
iii) Accuracy
The accuracy of a computer system is very high. In most cases, the errors are due to the human factor rather than technological flaws. For example, if a program is wrongly coded, the data is corrupted, or the program logic is flawed, then irrespective of which computer we run it on, we will always get the wrong result. Another area where mistakes can creep in is during data entry. So if a wrong input is given, the output also will be wrong – GIGO (Garbage In Garbage Out).
iv) Versatility
Computers are very versatile machines. They can perform activities ranging from simple calculations to performing complex CAD modeling and simulation to navigation missiles and satellites. In other words, they are capable of performing almost any type of different task.
v) Diligence
Diligence means being constant and earnest in effort application. Computers can perform repetitive calculations any number of times with the same accuracy. Computers don't suffer from human traits, such as tiredness, fatigue, lack of concentration etc.
Word Length
vi) A digital computer operates on binary digits – 0 and 1. A binary digit is called a bit. A group of 8 bits is called a byte. The number of bits that a computer can process at a time in parallel is called its word length. Commonly used word lengths are 8, 16, 32 or 64 bits. Word length is the measure of computing power of a computer. The longer the word length, the more powerful the computer is.
vii) Automation
Computers can be programmed to perform a series of complex tasks involving multiple programs. Computers will perform these things flawlessly. They will execute the programs in the correct sequence, switch on or switch off machines at the appropriate time, monitor the operational parameters, send warning signals or take corrective actions if the parameters exceed the control level, and so on. Computers are capable of these levels of automation, provided they are programmed correctly.

Application of Computer
Home: In many homes, the family computer is nearly as important as the refrigerator or the washing machine. People cannot imagine living without it. In fact, a growing number of families have multiple PCs in their homes; in most cases, at least one of those computers has an Internet connection. We need computers in home to do communications, to carry out business work done at home, to do schoolwork, for entertainment, to keep personal financial data up-to-date etc.
Education: More and more schools are adding computer technology to their curricula, not only teaching pure computer skills, but incorporating those skills into other classes. Students may be required to use a drawing program, for example, to draw a plan of Alamo for a history class, or use spreadsheet software to analyze voter turnouts during the last century's presidential elections. Educators see computers technology as an essential learning requirement for all students, starting as early as preschool.
Small Business: Many of today's successful small companies simply could not exist without computer technology. Each year, hundreds of thousands of individual launch business based from their homes or in small office locations. The rely on inexpensive computers and software not only to perform basic work functions, but to manage and grow their companies. These tools enable business owners to handle tasks – such as daily accounting chores, inventory management, marketing, payroll, and many others. As a result, small business become more self-sufficient and reduce their operating expenses.
Banks: All financial transactions in banks and financial institutions are done by computer software. They provide security, speed and convenience. Different branches are connected with the help of computer network, one can withdraw and deposit money for any branch system. In ATM system one can withdraw cash from any ATM machines which are interconnected in ATM system. Online banking allows to connect to bank using a computer via Internet. We can view our accounts, transfer money between accounts, print copies of checks, statements and pay bills online.
Engineering: Computer is used in engineering to collect and analyze data. Engineers use computer technology for designing and developing any business as well as scientific model using appropriate software tools. For example, CAD software package helps engineers and architects to prepare machine drawings, building drawings, circuit drawings, presentation of solid modeling (3D views) of machine parts on the screen. Any design can be drawn using CAD software like AutoCAD.
Health Care: Computers are making health care more efficient and accurate while helping providers bring down costs. Many different health care procedures now involve computers, from ultrasound and magnetic resonance imaging (MRI), to laser eye surgery and fetal monitoring. Surgeons now can use robotics surgical devices to perform delicate operations, and even to conduct surgeries remotely. New virtual-reality technologies are being used to train now surgeons in cutting-edge techniques, without cutting an actual patient. The information available on uncommon illnesses in expert system is useful to doctors. Junior doctors can make use material stored in CDs for learning new information.
Communication: We also have very fast and convenient access to information. Through E-mail, we can communicate with a person, sitting thousands of miles away in seconds. There is chat software that enables one to chat with another person on a real-time basis. Messages can be sent and the sent messages are received by their respective destinations in fractions of seconds no matter how many miles they are away. Audio and video conferencing tools are becoming readily available to the common man. All mobiles phones have software embedded in them so that we can communicate with others with low cost and from anywhere using technology.
Ticketing and reservation: One can book air tickets or railway tickets or bus tickets and make hotel reservations online with the help of computer and internet technology that saves time and confirmation. We can also book hall ticket for confirmation watching movies.
Defense: Computers are widely used in defense. Secured database and records are kept in computer with the help of software used in system. There is software embedded in almost every weapon. Software is used for controlling the flight and targeting in ballistic missiles. Software is used to control access to atomic bombs.
Multimedia: Audio or video composition and editing have been made much easier by using computers. Graphics engineers can use computers to generate short or full length films or even to create 3D models. Special effects in science fiction and action movies are created using computers. Desktop publishing: We can use computer for desktop publishing, for example, one can create page layouts for magazine, newspaper, book etc. with attractive designing by using designing software like Adobe PageMaker, Macromedia Freehand etc. in personal computer.
Entertainment: Computers are now the major entertainers and the primary time pass machines. We can use computers for playing games, watching movies, listening to music, drawing pictures etc.
Government: Various departments of the government use computer for their planning, control and law enforcement activities. For example, government uses computer for traffic, tourism, information and broadcasting, education, aviation, and many others.
Weather Forecasting: Computer can be used for weather forecasting like sunrise, sunset, rainfall, humidity, temperature of different places at the same around the globe using super computers and related programs. We can also predict the future weather forecasting on the basis of past record. Metrology is the tools to research for weather forecasting system.
Planning and Scheduling: Computer along with software can be used to store contact information, generating plans, scheduling appointments and deadlines as well. Evolution or History of Computer:
Evolution or History of computer is a study of past development of computer i.e. it is meant by the gradual improvement of accuracy, speed and efficiency of computers through generations.
Ancient people lived on the earth for centuries without counting. Then they started to count with their ten fingers. It became so difficult to live and to remember them more and more facts using their te fingers. These phenomena were gradually replaced by the use of stones, counting notches on sticks or marks on walls.
According to the concept and technology used on the devices, we can divide the evolution of computer roughly into three eras. The three eras are as follows:
1. The Mechanical Era
In this era several mechanical devices were invented those became basis for the development of modern computers. The devices and their inventor are as follows:
a.) Abacus : Abacus is a mechanical device that can be used as tool for performing mathematical calculations. It had its origin in ancient-china, Greece (Roman), Egypt and Great silky road in between 3000 years to 6000 years ago. It was used for performing simple calculations like counting, addition, subtraction and multiplication of numbers.
An abacus consists of a rectangular frame carrying a number of wooden rods. Mid-bar divides each of these rods into unequal – upper and lower parts. The upper part is called heaven, whereas lower part is called earth.
b.) Napier's bone: Napier bone, also called Napier's rod, was invented by Scottish mathematician John Napier. The Napier's bones are numbered rods which can be used to perform multiplication. The process of Napier's bone was published by John Napier in his book Rabdologia in 1617 AD. John Napier is also remembered as the inventor of logarithm.
c.) Slide Rule: The English Man William Oughtred invented a rectangular device-slide rule in 1620 AD. It was a calculating device, based on the principles of logarithm. A slide rule consists of two graduated scales, one of which slips upon the other. The scales are devised in such a manner that suitable alignment of one scale against the other makes it possible to find products and quotient of any numbers.
d.) Pascal's Calculator (Pascaline): Pascaline is a calculating device developed by French mathematician Blaise Pascal in 1642 AD. He was the founder of the modern theory of probability. It could perform addition and subtraction up to 8 digits, that was a great achievement at that time.
e.) Stepped Reckoner: The Stepped Reckoner, a first mechanical calculator, was invented by German mathematician Baron Gottfried Von Leibniz in 1670 AD. Leibniz extended Blaise Pascal's idea. It could perform addition, subtraction, multiplication, division and evaluate square roots by a series of stepped additions.
f.) Jacquard's Loom: The French Man Joseph Marie Jacquard was a textile manufacturer invented a mechanism for automated weaving cloths for the textile industry at Lyon in 1802 AD. This machine was used to automatically control weaving looms to facilitate the production of weaving cloth with complex patterns. This machine was controlled by punch-cards i.e. principle of present and absence of holes.
g.) Analytical Engine: The English professor and mathematician Charles Babbage invented the Difference Engine at Cambridge University in 1822 AD. Theoretically, it can solve differential equations and calculate various mathematical function, logarithmic, polynomial and trigonometric functions. The project could not be completed due to lack of funds. Babbage continued working on his difference Engine for a full ten years, but he had a better idea, His new idea was the construction of a general purpose, fully programmable automatic mechanical counting machine, Babbage called his machine an Analytical Engine in 1833 AD.
The Analytical Engine was general purpose mechanical computer. Babbage continued to refine the design until his death in 1871 AD. It contained all components as follows:
i) The store (equivalent to memory)
ii) The mill (equivalent to CPU)
iii) The input section using punched card
iv) The output section using punched card
For all his contribution and efforts guided a number of principles which have been shown to be fundamental to the design of today's digital computers. That's why he was considered as the "Father of modern computer".
h.) First programmer Ada Lovelace: Lady Augusta Ada Lovelace is considered as the world's first computer programmer. She wrote program for Analytical Engine. A programming language "Ada" was developed by US Department of Defense to honor her.
i.) Boolean Algebra: Boolean Algebra is algebra of logic. It was developed by George Boole, an English mathematician, in 1954 AD. It is the basis of design and implementation of digital system including digital computers. Besides the hardware, it is also useful in programming for logical decision.
j.) Tabulating Machine: Tabulating Machine was developed by Herman Hollerith to help in data processing of 1890 AD national census of USA. He also created a punch card technology that allowed coding for state of residence, age, gender and other information. Hollerith brought his punch card reader into the business world, founding Tabulating Machine Company 1896, later to become International Business Machine (IBM).
k.) Turing Machine: Turing Machine is an abstract, theoretical mathematical computing machine designed by Alan Turing in 1937 AD. The ideal machine is composed of a head, tape and a ribbon of paper of indefinite length. The tape is used to store data which is similar to memory and head simulates microprocessor of modern computers.

2) The Elector – Mechanical Era
a.) Mark I: A professor of Physics Howard H. Aiken designed a general purpose mechanical computer at Harvard University and IBM, while working on his doctorate in physics, in the year 1937 AD. The machine was called IBM Automatic Sequence Controlled Calculator, it was the first fully automatic calculating machine and later as Harvard Mark I. It was built by IBM. This giant computer used about 18 thousand vacuum tubes (valves) as main memory device with 7 lacks 50 thousand parts. It was about 51 feet long, 8 feet height and 3 feet wide. Punch card and card reader are used for input/output operation. Technically, it was very complicated machine, consumed huge amount of power and generated lot of heat during the operation. This computer could carry out addition, subtraction, multiplication, division and table reference.
b.) ABC (Atanasoff Berry Computer): In 1939, John Vincent Atanasoff and Clifford Berry designed Atanasoff-Berry Computer (ABC) for solving systems of mathematical simultaneous equation. It used 18000 valves and other 45 valves for internal logic and capacitors for storage. It used punched cards as input and output operation i.e. secondary storage. It is considered as the first computing machine which introduced the idea of binary arithmetic, regenerative memory, and logic circuits. The main thing is that this computer used electronic vacuum tubes and the circuitry was based on George Boole's Boolean Algebra.

3) The Electronic Computers Era
a) ENIAC: ENIAC stands for "Electronic Numerical Integrator And Calculator" which was the first general purpose electronic computer developed by John William Mauchly and J. Presper Eckert in 1946 AD. It was built to fulfill the requirements of the US armed force. It was 10 feet tall, occupied about 1000 sq. feet, weighed 30 tons and used more than 18,000 vacuum tubes. So, it also consumed a lot of electricity and produced a lot of heat. It could do 5,000 additions and 300 multiplications per second.
b) EDSAC: EDSAC stands for electronic delay storage automatic computer which was made by Maurice Wilkes in 1949 AD by applying J. V. Neumann's "stored program technique". EDSAC contained 3,000 vacuum tubes and used mercury delay lines for memory. Programs were input using paper tape and output results were passed to a tele printer.
C) EDVAC: EDVAC stands for "Electronic Discrete Variable Automatic Computer" designed by J.W. Mauchly and J.P. Eckert in 1952 AD. It was also based on J.V. Neumann's stored program technique. The EDVAC was a binary serial computer with automatic addition, subtraction, multiplication, programmed division and automatic checking with an ultrasonic serial memory capacity of 1000 44 bit words (later set to 1024 words, thus giving a memory, in modern terms of 5.5 kilobytes). d) UNIVAC: UNIVAC stands for "UNIVersal Automatic Computer". It was the first general purpose electronic digital computer made for business and administrative use. It was also made by J.W. Mauchly and J.P. Eckert in 1961 AD. It was 8 feet high, 15 feet long and weighed about 15 tons.

Generation of Computers
Over the years, various computing devices were invented that enabled people to solve different types of problems. All these computing devices can be classified into several generations. These generations refer to the phase of improvement made to different computing devices. The technological development in the field of computers not only refers to the improvements made to the hardware technologies, but also the improvements made to the software technologies.
There are five generation of computers which are as follows:
i) First Generation of Computers (1946 – 1959)
First generation computers were powered by thousands of vacuum tubes or thermionic valves and their memory was stored on magnetic storage devices such as magnetic tapes and drums. Most data were entered onto the computers via punch cards or paper tape. Output was in the same form human operators had to set switches before a program could run.
Features of First Generation Computers:

• These computers were based on vacuum tube technology.
• These computers were very large in size and required a lot of space.
• Processing or operating speed was in terms of milliseconds
• The power consumption was very high and it generated a lot of heat.
• Programming mainly at the hardware level (Binary or Machine language, in the form of 0s and 1s) hence was very difficult to program and use.
• These machines were unreliable and lacked versatility and speed. /li>
• Electrostatic tubes (internal), paper tape, punched card, magnetic tape were used for computer operation.
• Each machine component had to be assembled manually.
• It could only perform straight forward numerical calculations.

The examples of first generation computers are IBM 700 series, ENIAC, EDVAC, EDSAC and UNIVAC.
ii) Second Generation Computers ( 1959 – 1965)
In the second generation vacuum tubes were replaced by transistors. Due to the use of transistors, the computers became smaller and cheaper.
Features of Second Generation Computers:

• These computers were based on transistors technology.
• They were smaller, faster, more reliable, accurate and more energy efficient as compared to first generation computer.
• They were portable and generated less amount of heat as compared to first generation computer.
• Processing or operating speed was increased to microseconds.
• Magnetic core was used as primary and magnetic drum as secondary memory.
• Assembly languages was used to program and hence programing became more time efficient and less cumbersome./li>
• Electrostatic tubes (internal), paper tape, punched card, magnetic tape were used for computer operation.
• It could perform scientific calculations such as solving differential equations.

The examples of second generation computes are IBM 1401, IBM 7090, PDP 8 etc.
iii) Third Generation of Computers (1966 – 1971)
The development of Integrated Circuits (IC) signaled the beginning of the third generation. Transistors were replaced with integrated circuits known popularly as chips.
Features of Third Generation Computers:

• These computers were based on integrated circuit (IC) technology.
• They were smaller in size, more reliable, accurate than previous generation computer.
• Power consumption and heat generation was less than the previous generation computer.
• Computers became portable for the first time because of personal or desktop computer.
• Semi-conductor memory was used as primary memory.
• Magnetic disk was used as secondary memory.
• Keyboard and monitor were used as input and output device respectively for the first time.
• Processing or operating speed was increased to nanoseconds.
• Multiprogramming and multiprocessing facility was introduced.
• FORTRAN, BASIC and more high level languages are used.
• Database management system was developed.
• Computers were used in census calculation, military, banks and industries.
• Manual assembling of individual components was not required. Hence, commercial production became easier and cheaper.

The examples of third generation computers are IBM 360 series, NCR 395 etc.
iv) Fourth Generation of Computers (1971 – Till Date)
The invention of microprocessor chips marked the beginning of the beginning of the fourth generation computers. The invention of microprocessors led to the development of microcomputers or the personal computer. The first microprocessor called Intel 4004 was developed by American Intel Corporation in 1971
Features of Fourth Generation Computers:

• These computers were based on LSI, VLSI and microprocessors.
• The computers are powerful, compact, affordable, portable and totally reliable.
• Use of magnetic disk and optical disk with huge capacity for secondary storage device.
• Processing speed increased very fast up to picoseconds.
• Multiprogramming, multi-processing, multi-media and distributed computing are possible.
• Because of microprocessor, micro computers such as PCs, Laptop and Notebook computers were invented.
• Fourth generation language (4GL) and application software for micro computers became popular.
• Graphical user interface and further refinement in input and output devices introduced.
• Interconnection of computers leads to better communication and resource sharing.
• t is used for all scientific, engineering and commercial applications.

The examples of fourth generation computers are IBM-PC, HP, Mac Notebook etc.
iv) Fifth Generation of Computers (Present and Beyond)
The computers of this generation will use Artificial Intelligence (AI) and Biochips are made up of Biological organisms and protein fibers obtained from the living organism. So these computers will have the power of sense, logic, decision making capability and parallel processing.
Features of Fifth Generation Computers:

• These computers will have fully parallel processing capacity.
• Computers will be intelligent and knowledge base because of AI.
• Computers will use super conductor memory like bio-chips and Gallium Arsenide as memory device so that the speed will be very high.
• These machines will incorporate Ultra Large Scale Integration (ULSI).
• Instead of HLL (High Level Language), natural languages like English, Japanese etc. will be used for giving instruction to the computer and making computer programs.
• The application of AI in fifth generation computers has also enabled expert systems.
• The input and output for these machines will be in the form of speeches or graphic images.
• They are used in the field of medicine, treatment planning, monitoring and so on.
• The language of operating system will be PROLOG, LISP etc.

Classification of Computer
Computers are classified according to working principle, size, brand, and model. A single computer can be microcomputer on basis of size, IBM compatible on the basis of brand, AT computer on the basis of model and digital computer on the basis of working principle.

Classification of Computer on the basis of Working Principle

1. Analog Computer
2. Digital Computer
3. Hybrid Computer

1. Analog Computer
The computer which can process analog quantities (continuous data) is called an analog computer. These computers processes continuous values rather than discrete binary values (i.e. 0 and 1). It is a special purpose computer and is mainly used in scientific work and not for commercial or personal purposes. Generally, it has less storage capacity or no storage and its accuracy is poor compared to digital computers. It is specially designed to compute physical forces such as temperature, pressure etc.
Presley is an example of an analog computer. Examples of analog devices are thermometer, barometer, speedometer and ammeter.
2. Digital Computer
The computer which works on discrete data (discontinuous data, binary system or 0 and 1) is known as a digital computer. The basic principle of digital computers is either present (1) or absence (0) of electric pulses in the signals. In digital computers, even letters, words and whole texts are represented digitally. It is multipurpose and programmable. So, it is high cost, fast processing, more accurate and has larger storage capacity. It can perform tasks to control industrial processes and regulate the operations of machines, analyze and organize vast amounts of business data and simulate the behavior of dynamic systems.
3. Hybrid Computer
A computer, which has a combined feature of both analog and digital computers, is called hybrid computer. It can perform the tasks of an analog computer as well as a digital computer. It converts data from analog to digital and vice-versa. It has high cost. It is a special purpose computer. The practical examples of hybrid computers are:

• During the rocket launching process, the analog computer measures the speed of the rocket, temperature and pressure of the atmosphere. These measurements are then converted into digital signals and supplied to the digital computer to analyze the data for taking appropriate steps on launching.
• In hospitals, analog devices measure the temperature and blood pressure of patients. Then these measurements are converted into digital signals and fed to the digital computer that monitors the patient's vital information.

Classification of Computer on the basis of Power and Size

1. Super Computer
2. Mainframe Computer
3. Mini Computer
4. Macro Computer

1. Super Computer
Supercomputer is the largest, most powerful and fastest computer among digital computers. This computer has a special purpose and is capable of handling huge amounts of calculations that are beyond human capabilities. It can perform at billions of instructions per second (BIPS) and more. Some of today's supercomputers have the computing capability equal to that of 40,000 microcomputers. This computer is the most expensive computer (cost about 15-20-million-dollar range). Usually such computer uses parallel processors. Its word length is more than 128 bits. The primary use for supercomputers is in large corporations, universities and government agencies, mainly in the areas of defense and weaponry, weather forecasting, scientific research, aeronautics, satellite communication and space administration, nuclear research work, petroleum research work, molecular modeling, study of DNA structures.
Example: CRAY X-MP/24, NEC-500, PARAM, ANURAG etc.
2. Mainframe Computer
Mainframes are large, powerful and expensive computers. It is general purpose computing system designed for large scale data processing. It is very large in size with an approximate area of 1000 sq. ft. The largest mainframes can handle the processing needs of thousands of users at any given moment. Its word length is more than 64 bits. It is used mainly by large companies for bulk data processing, commercial data processing, and other large-scale operations such as bank transaction processing, insurance companies, airlines, railway reservation system, and air traffic control.
Example: IBM 1401, VAX 8000, CDC 6600 etc.
3. Mini Computer
Minicomputers are computers that are somewhere in between a microcomputer and a mainframe computer. For this reason, minicomputers are often called midrange computers. Like a mainframe, a minicomputer can handle much more input and output than a micro computer. Minicomputers are smaller, less expensive and more powerful than a personal computer. It can support about 50 terminals and requires an area of around 100 sq. ft. These computers are used for scientific and engineering computation,business transaction processing, file handling, database management, universities, banks and are often now referred to as small or midsize servers.
Example: Prime 9755, VAX 7500, HCL, HP 3000 series etc.
4. Macro computer
A microcomputer is a small sized personal computer (PC) that is designed for an individual having a microprocessor inside it. It requires small space, can be placed on a table or even kept inside a briefcase. This computer has a central processing unit on a single chip. It is mainly used in office, house, school, shop and store. The smallest of this category are laptop, notebook, palmtop and PDA.
Example, IBM PCs, Apple/Macintosh etc.
A microcomputer can be further categorized as: Non-portable & Portable category
a) The microcomputer in non-portable category can be further categorized into:

1. Personal Computer (PC)
2. Workstation Computer (WC)

i) Personal Computer (PC):
It is a small computer that easily fits on a normal sized office table. It is generally designed to be used by one person at a time. It is the most common computer and can be found everywhere like in the office, college, home, hospital and shop.
ii) Workstation Computer (WC):
A workstation is a specialized computer which has more power and features than a standard desktop PC. It is designed for the network environment in LAN. The processing power is typically 5 to 10 times more than that of PC. It has larger main and backup memory, larger and high resolution monitor and accelerated graphics handling capabilities, making it suitable for small engineering companies, architects, graphic designers, animation and video editing and any organization, department or individual.
b) The microcomputer in portable category can be further categorized into:

1. Laptop
2. Notebook
3. Tablet PC
4. Palm top
5. PDA (Personal Digital Assistant)
6. Ultrabook

i) Laptop:
A laptop computer is a portable personal computer light and small enough to sit on a person's lap. So, it is suitable for the person in the movie. It can be powered by a battery or plugged into the wall. It is lightweight, but incorporates all basic features of the computer like display, keyboard, touch pad, hard disk CD/DVD drives.
ii) Notebook:
Notebook is also designed for people on the move. But it is smaller in size compared to a laptop. It is a device with a reduced keyboard, screen, functionality, and software support than a laptop.
iii) Tablet PC:
A tablet PC is a wireless, portable personal computer with a touch screen interface. The tablet is typically smaller than a smartphone and primarily operated by touching the screen rather than using a physical keyboard. It often uses an onscreen virtual keyboard, a passive stylus pen or a digital pen. It has handwriting recognizing software, which recognizes the handwritten text and digitizes it.
iv) Plam top:
It is also a portable PC which is the size of our palm. Unlike laptop and notebook, it has very small memory and features. It is mainly used for storing phone numbers, addresses, pictures and such other small data. It contains an electronic writing pad and a light-sensitive electronic pen for providing input.
v) PDA (Personal Digital Assistant):
It is also a species of portable computer. It is much limited in terms of the task, it can perform. Nowadays, PDA contains facilities such as colored screens, scientific calculators, digital diary for storing telephone numbers and addresses, web browser, cell phone for communication, audio-video and games.
vi) Ultrabook:
An Ultrabook is a category of thin and light laptop computers designed to bridge the market gap between tablets and premium notebook PCs. Ultrabooks provide more business-friendly features than tablets and more portability than enterprise-class notebooks. Ultrabooks are distinguished from notebooks by offering more power in the form of faster processors and additional RAM, as well as better storage and larger screen sizes as well as long battery life, typically 8 hrs plus, features that also make these laptops significantly more expensive than most notebooks.

Classification of Computer on the basis of Brand
a) IBM Computer
IBM stands for International Business Machine. It is one of the largest computer and other electronic equipment manufacturing companies founded by an American Hermann Hollerith in 1924 AD. It is the first company that manufactured commercial personal computers. IBM computers are of two categories:

1. IBM PC
2. IBM Compatible PC

1. IBM PC
This computer is developed by IBM corporation itself, so it is also called branded or original IBM PC. It is expensive, durable and more reliable than an IBM compatible PC.
2. IBM Compatible PC
This computer is developed by companies other than IBM but uses parts and principles of IBM. It is generally called an assembled or duplicate of IBM PC and is comparatively cheaper than IBM PC.
b) Apple/Macintosh
All the computers manufactured by apple cooperation are known as Apple/Macintosh computers. These computers use their own software and hardware and are totally different than that of IBM computers. Software developed for Apple computer can't run on IBM computers and vice-versa. It uses MAC operating system. It is used for animation and graphics designing. It is usually costlier, reliable than IBM computers.

Classification of Computer on the basis of Model

1. XT Computer
2. AT Computer

1. XT Computer
XT stands for "eXtra Technology" and these computers are old technology computer.It is text based system or CUI (Character User Interface) software. They don't have large storage capacity and fast processing speed, so complex calculation and large data processing speed, so complex calculation and large data processing can't be done. Even the I/O devices used in XT computers are not very flexible and faster. Having processor Intel 8085/8086/8088 lies under the XT technology.
2. AT Computer
AT stands for "Advanced Technology" and these computers are the new technology computers. AT uses advanced GUI based software as well as CUI based software. Their I/O devices are interactive, flexible and faster. Because of their high speed processor and large storage capacity, complex calculation and large data processing can be done easily. Having processor Pentium 80286/80386/80486/Pentium I/Pentium II/ Pentium III etc.
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