Computer is an electronic data processing machine which takes the input, stores the data in its memory then process it with the help of Arithmetic Logic Unit (A.L.U) under the supervision of Control Unit (C.U) and then give us the output according to the instructions (Program).

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The word computer is derived from a Latin word "Computare" which means to count or to calculate. The computer is an extension of human mind which can perform several tasks with a great speed. Computers are just like a dumb servant who cannot take any decision by itself they only do what they are told to do so.

GENERAL PURPOSE COMPUTERS

General purpose computers are designed to solve a large variety of problems. The different programs can be used to solve many problems. Most digital computers are general purpose computers and used in business and commercial data processing.

SPECIAL PURPOSE COMPUTERS

A computer designed for machine control or process control would be different than a general purpose computer. The special purpose computers are designed to solve specific problems. The computer program for solving a specific problem is built right into the computer. Most analog computers are special purpose computers. These special purpose computers are widely used in industrial robotics.

SUPER COMPUTERS

Large scientific and research laboratories as well as the government organizations have extra ordinary demand for processing data which required tremendous processing speed, memory and other services which may not be provided with any other category to meet their needs. Therefore very large computers used are called Super Computers. These computers are extremely expensive and the speed is measured in billions of instructions per seconds (B.I.P.S).

Computer is important in this contemporary world of human efforts. The frequent use of computer rapidly changing the nature of work. Its impact is being felt in industries, banks, education etc. to solve the complicated and time consuming problems. Computer can improve the efficiency of an organization. The business person employing a computer can trim unnecessary over time reducing dependency upon unreliable person or outside agencies.

The importance and capabilities of computer can be discussed under three heads.

• Speed 
• Accuracy 
• Reliability 

Computer is an electronic data processing machine which can perform arithmetic and logical operations with very much speed. It can perform million instructions per second. That would take a person year to complete. To classify the speed of different computers the manufacturer has the criteria of (MIPS) million instructions per second.

ACCURACY

The computer must process the data accurately. Accuracy is the prime consideration of installing computer. Computers are universally accused of making mistakes on bills, statements and etc. The probability of computer errors are quite often and traceable.

RELIABILITY

Computer system are widely accept because of there exceptional reliability. Computer constantly provide the same accurate results under all operating conditions.

ANALOG COMPUTERS

Analog computers are used to process continuous data. Analog computers represent variables by physical quantities. Thus any computer which solve problem by translating physical conditions such as flow, temperature, pressure, angular position or voltage into related mechanical or electrical related circuits as an analog for the physical phenomenon being investigated in general it is a computer which uses an analog quantity and produces analog values as output. Thus an analog computer measures continuously. Analog computers are very much speedy. They produce their results very fast. But their results are approximately correct. All the analog computers are special purpose computers.

DIGITAL COMPUTERS

A computer which process information represented by combination of *** or discontinuous data. It is a device for performing sequence of arithmetic and logical operations, not only on data but on its ***. Digital computer that is capable of performing sequences of internally stored instructions for calculation. Digital computers are not very much fast but their results are very much accurate. All the digital computers are general purpose computers.

HYBRID COMPUTERS

Various specifically designed computers are with both digital and analog characteristics combining the advantages of analog and digital computers when working as a system. Hybrid computers are being used extensively in process control system where it is necessary to have a close representation with the physical world.

The hybrid system provides the good precision that can be attained with analog computers and the greater control that is possible with digital computers, plus the ability to accept the input data in either form.

CLASSIFICATION OF COMPUTERS

MAINFRAME COMPUTERS

The most expensive, largest and the most quickest or speedy computer are called mainframe computers. These computers are used in large companies, factories, organizations etc. the mainframe computers are the most expensive computers, they cost more than 20 million rupees. In this computers 150 users are able to work on one C.P.U. The mainframes are able to process 1 to 8 bits at a time. They have several hundreds of megabytes of primary storage and operate at a speed measured in nano second.

MINI COMPUTERS

Mini computers are smaller than mainframes, both in size and other facilities such as speed, storage capacity and other services. They are versatile that they can be fitted where ever they are needed. Their speeds are rated between one and fifty million instructions per second (MIPS). They have primary storage in hundred to three hundred megabytes range with direct access storage device.

MICRO COMPUTERS

These are the smallest range of computers. They were introduced in the early 70’s having less storing space and processing speed. Micro computers of todays are equivalent to the mini computers of yesterday in terms of performing and processing. They are also called "computer of a chip" because its entire circuitry is contained in one tiny chip. The micro computers have a wide range of applications including uses as portable computer that can be plugged into any wall.

DATA PROCESSING

Data processing often referred as "D.P" is the process collecting data together and converting the data into information. The method used for collecting data may be manual, semi manual, mechanical or electronic.

Data processing is a term mostly associated with business and commercial words. Since computer are being used in the processing of data. The term E.D.P (Electronic Data Processing) may also be used.

ELECTRONIC DATA PROCESSING

Data processing means transformation of data for producing meaningful results for carrying out scientific, business activities. The result of data processing is called "INFORMATION".

The transformation of data consist of a sequence of operations. The sequence is called "PROCEDURE". A data processing is a system which takes data as an input, carries out the required processing on data and produces the information. The system is called "MANUAL" when processing is performed by human beings and "AUTOMATIC" when machines are used. When computers are used for data processing.

The electronic logic of micro computer is based on a single printed circuit board which is stored in a case and provided with a keyboard for data entry and a monitor. The I/O logic may support a range of peripherals such as printer, disk, mouse etc. Microcomputer have operating system and peripheral connectivity to support microcomputers.

Since the computer hardware is capable of understanding only machine level instructions so it is necessary to convert the instruction of a programme written in high level language to machine instruction before the programme can be executed by the computer. This job is carried out by compiler.

A compiler is translating programme that translates the instruction of a high level language into machine language. A high level programme is called a source programme. After the source programme has to be converted into machine language by a compiler. It is referred to as an object programme.

A compiler can translate only those programme which has been written in the language for which the computer is meant e.g., FORTRAN compiler is only capable of translating source programme which have been written in FORTRAN. Each machine requires a separate compiler for each high level language.

The compiler analysis each statement in the source programme and generate a sequence of machine instruction. As the compiler analysis each statement it uncovers certain types of errors. Compiler can diagnose the following kinds of errors in a source programme.

1. Illegal characters. 
2. Improper sequence of instruction in a programme. 

It is a collection of programs reside on the disk. DOS is a single user operating system used with microcomputers, which provides the communication link between user and hardware. DOS plays a vital role in operation of a computer system. It facilitates the user to copy, print, delete, rename etc a file. The DOS contains three main files COMMAND>COM, IO.SYS, MSDOS.SYS and two types of commands.

• Internal commands 
• External commands 

Internal commands are those which are transferred into the memory at the time of booting the computer. These commands are the part of a program called COMMAND.COM. These commands are always available to the user although they cannot be displayed, delete, rename. Examples are CLS, TIME, DATE, COPY, VER.

EXTERNAL COMMANDS

External commands are the conventional programs which are reside on the disk they do not transferred into the memory at the time of booting the computer. These commands can be displayed on the screen by taking the directory of a disk. External commands can be deleted, copy and even rename. Examples are diskcopy, format, xcopy, chkdsk, diskcomp, backup.

Errors are the mistakes made by the programmer during program writing. A program relays run perfectly for the first time and each program needs to be tested to ensure that it is correct and contains no errors. There are three types of errors generally encountered.

1. Syntax Error 
2. Logical Error 
3. Run Time Error 

The syntax of a programming language is the set of rules to be followed when writing a program in that language. These rules are similar to the grammar rules of Urdu or English language. When a program violates these rules, computer generates a syntax error. Syntax must be correct before program execution.

LOGICAL ERROR

These errors are the most difficult ones to find, if a programmer enters a sign of multiply instead of addition at the time of making the program, the answer will not be correct because interpreter will not be able the detect these types of errors. A logical error will not stop the execution of the program but the result will not be accurate.

RUN TIME ERROR

These types of errors stop the execution of the program. It may be due to entering the invalid data at the time of executing a program. For example, if a program is expecting the user to enter the numeric data and the user enters a string data, computer will display an error message.

Input devices are used to provide data or information to the computer. The computer follows the instructions given to it by and input device. A variety of input devices are used with the computer depending on the type and purpose of input information. For example, a keyboard is commonly used to transfer data or information from human readable form to machine readable form. Other examples of input devices are: mouse, joystick, trackball, light pens, digitizers, scanners, optical character reader (OCR), touch window, etc.

KEYBOARD

A keyboard is the most commonly used input device which helps us in simply keying in required information in a computer. This information is subsequently stored in the computer’s memory. A keyboard can be used effectively to communicate with the computer but considered to be relatively slow as compared to other input devices. The keyboard is divided into following divisions:

ALPHABETIC KEYPAD

These keys are similar to a standard typewriter and is used to type general information.

NUMBERIC KEYPAD

These keys are used to input numeric data only. These are very useful in case of large numeric data input because all numeric keys can be accessed by one hand only. These keys can also be used as an alternative to the screen navigation and editing keys.

FUNCTION KEYS

These are keys marked as F1 - F12, located normally at the top of the keyboard. These are special keys provided to a programmer which allow him to attach special functions to each key. Each of these function keys are also given some special function in different packages.

SCREEN NAVIGATION AND EDITING KEYS

These keys are provided to move around in the screen. May programs use these keys to let the user move around the screen display. In some keyboards these keys are also provided inside the numeric keypad as alternate keys.

An output device is used to display the data or information that we receive from the computer. An output device can be used to display or print the intermediate or final results performed by computer. A variety of output devices are used with computer. The use of these devices depends on the type and purpose of output. Some examples of output devices are: Monitors, Printer, Plotters, Visual display unit (VDU), liquid crystal display (LCD), etc.

MONITOR

To display result or output from computer, a T.V like device is used which is called monitor. The monitors are also referred as C.R.T (Cathode Ray Tube), V.D.U (Visual Display Unit). The monitor assist during input from the keyboard, this display is called a soft copy. The monitor can be of various kinds, depending on the type of application. Monitors are categorized into two groups:

• Monochrome monitors 
• Colour monitors 

Monochrome monitors are used specially for text editing purpose. These monitors can display only one colour. Normally amber, green or paper white.

COLOUR MONITORS

Colour monitors serves a wide range of selection according to the application. Such as red, green, blue, Enhance Graphic Array (EGA), Colour Graphic Array (CGA), Video Graphics Array (VGA), Super Video Graphics Array (SVGA).

IMPACT PRINTING

Is the method used by the conventional type writers. In some type of impact printing a metal "hammer" embossed with a character strikes a print ribbon, which presses the characters image into paper. In other types the hammer strikes the paper and presses it into the ribbon characters created through impact printing can be formed by either a solid font or dot matrix printing mechanism.

NON – IMPACT PRINTING

Does not depend on the impact of metal on paper. In fact no physical contact at all occurs between the printing mechanism and the paper. The most popular non-impact methods today utilize thermal transfer, ink-jet.

DOT MATRIX PRINTER

Any printer that produces character made up of dots using a wire pin printed head. The quality of output from a dot matrix printer depends largely on the number of dots in the matrix, which might be low enough to show individual dots or might be high enough to approach the look of fully formed characters. Dot matrix printers are often categorized by the number of pins in the printer head typically, 9 or 24.

LINE PRINTERS

Any printer that prints one line at one time, as opposed to one character at a time or one page at a time. Line printer typically produce the 11 by 17 inch "computer" printout. They are high speed devices and are often used with mainframes, minicomputers, or networked machines rather than with single user system. Types of line printers include chain printers and band printer.

LASER PRINTERS

An electrophotographic printer that is based on the technology used by photocopiers. A focussed laser beam and a rotating mirror are used to draw an image of the desired page on a photosensitive drum. This image is converted on the drum into an electrostatic charge, which attracts and holds toner. A piece of electrostatically charged paper is rolled against the drum, which pulls the toner away from the drum and onto the paper. Heat is then applied to fuse the toner to the paper. Finally, the electrified charge is removed from the drum and the excess toner is collected. By omitting the final step and repeating only the toner application and paper handling steps, the printer can make multiple copies.

DAISY WHEEL PRINTER

Daisy wheel printer are some times called letter quality printer because they are often used to produce attractive correspondence. The D.W.P is a flat circular device made of metal with character embossed on it. As this wheel spins at a very high speed the hammer hits the specific character against the ribbon which presses against the paper.

THERMAL TRANSFER PRINTER

It is a kind of non-impact printer. In electrothermal printing, characters are burned on to a special paper by heated rods on a print heat. They transfer ink from a wax-based ribbon onto plain paper. These printer can support high quality graphic.

INK – JET PRINTER

It is a kind of non-impact spray small dots of electrically charged ink onto a paper to form images. Ink jet printer are flexible enough to be used as plotters.

Secondary storage devices are also called backup storage because it is used to store data. Volume of data on permanent basis which can be partially transferred to the primary storage, when required for data processing. Afterwards these devices are comparatively cheap and provide greater space to store the data /instructions are stored on secondary storage devices in the same binary codes as in primary storage.

RANDOM ACCESS DEVICES

Random Access Devices are those devices on which we can directly access the data. These devices are comparatively provide the fast communication.

For example, hard disk, floppy disk, optical disk.

SEQUENTIAL ACCESS DEVICES

Sequential Access Devices are those in which we can access the data one by one in a sequence. These devices provide slow communication as compared to Random Access Device.

NEEDS OF SECONDARY STORAGE DEVICE

The storage capacity of the primary storage of today’s computer is not sufficient. To store a large volume of data as a result additional memory called secondary storage is needed with most of the computer system.

These devices also provides the fast communication than I/O devices. The internal memory of a computer is a volatile memory. Therefore, we cannot save the data permanently. In that case we require secondary storage device which provide the facility to store the data for future use.

FLOPPY DISK

A floppy disk, also called simply a diskette or disk, is a small flexible Mylar disk coated with iron oxide on which data are stored. The floppy disk has been around since early 1970s, today it is available in three 3½ inch, 5¼ inch and 8 inch sizes. The 5¼ and 8 inch diskettes are covered by stiff protective jacket with different holes. The central big hole called hub ring which is used to hold by disk drive during rotation. The elongated read write window is used to read and write data through read/write head. The small hole next to the hub ring is called index hole which is used to locating data through computer. The cut out on the side of the floppy disk is called write protect notch. If we cover this opening with a piece of paper then we can’t write data on to disk.

In small diskette a hard plastic cover and protective metal is used to protect disk. Before using a disk we have to format a disk in which disk is divided into tracks and sectors for storing the data. Diskettes may be double sided and single sided while the storage capacity become less or more.

Floppy diskettes are more convenient to use with microcomputers. A floppy disk which is a random access device can access data fast than magnetic tape.

MAGNETIC TAPE

Magnetic tape is a sequential access device about one half or one fourth inch in size and made of Mylar (a plastic material) coated with a thin layer of iron oxide. Data can be read and write through a device which is called tape drive. The read/write head of tape drive which is an electromagnetic component read, write and erase data from magnetic tape. Magnetic tape is divided into nine separate strips or tracks in which eight tracks are used to store data and ninth track is used for error checking bit.

Magnetic tape can store large quantities of data therefore they are erasable, usable and durable secondary storage device. But it can use with large computers. 

Hard disk Hard disk is a thin circular metal plate coated both side with a magnetic material. A hard disk pack consist of a number of disk mounted on central shaft which rotate at a speed of 2400 rpm or more. In a hard disk information is stored on both the surfaces of each disk plate except the upper and the lower surfaces of the bottom plate which are not used. Information is recorded on the track of the disk surfaces in the form of invisible tiny magnetic spot. The presence of a magnetic spot represent 1 bit and its absence represents 0 bit. Hard disk are potentially very high capacity storage devices typically in the range of 20 megabyte to 1 gigabyte. Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. There are two basic types of disk system. Moving head  Fixed head  MOVING HEAD The moving head consist of one read/write head for each disk surface mounted on an axis and which can be moved in and out. In this system each read/write head moves horizontally across the surface of the disk. So that it is able to access each track individually. Information stored on the tracks which constitute a cylindrical shape through the disk pack are therefore accessed simultaneously. FIXED HEAD In the fixed head system the axis are non-movable. A large number of read/write heads are distributed over the disk surfaces. One head for each track as a result no head movement is required and therefore information is accessed more quickly.

Flow chart is a symbolic representation of flow of programme. It can graphically represent data processing procedure Flow chart serves two purposes, i.e., the planning of the program structure as an aid in writing the computer program. Secondly the documentation of the program logic and work flow for the purpose of the communication with other person and to recall a program at a later time. If the program needs to be changed. The second purpose is very important because it is very difficult to check the logic of the program.

SYMBOLS OF FLOW CHART

TERMINAL

The terminal symbol is used to indicate the beginning, ending of the program logic flow. It is the first and last symbol of the flow chart.

INPUT / OUTPUT BOX

The input/output box is used to denote any function of an input and output in the program. If there is a program instruction to input or output the data from any I/O device that step will be indicated on this symbol.

PROCESS BOX

A processing symbol is used in a flow chart to represent arithmetical and data movement instructions.

FLOW LINES

Flow lines are used to indicate the flow of operations. The exact sequence in which the instructions are to be executed. The flow of flow chart is normally from top to bottom and left to right.

DECISION BOX

The decision box is used in a flow chart to indicate a point at which a decision has to be made and to branch to one or more alternative points that is possible during execution. The appropriate path followed, depends on the result of the decision.

CONNECTOR

A flow chart becomes complex, when the number and direction of flow lines is confusing or it spread over more than one page. It is useful to utilize the connector symbol as a substitute for flow lines.

SYSTEM FLOW CHART

System Flow Chart is used to describe complete data processing cycle including the hardware devices and media used. A System Flow Chart shows flow of data in a system where and in what form it is received by the system, how it is input to the computer and what storage media are used to hold it. The symbol used in system flow chart indicates the peripherals to be used in data processing procedure, where output will be produced. System flow chart do not explain the logic of the programs.

There are two modes of operation for BASIC. The mode that you are in determines what BASIC will do with the instruction you give it. When you start BASIC you receive the OK prompt. You then have two modes available to you immediately.

DIRECT MODE

In the direct mode BASIC acts like a calculator. No line numbers are required. Direct mode is not of course the main purpose of BASIC, but it is useful at times particularly when you are debugging program or short problems in which you want to perform quick calculation e.g., PRINT 3+4.

INDIRECT MODE

In this mode you first put a line number on each statement. Once you have a program you can run it and get your results. The indirect mode saves your instructions in the computer along with their line number, you can execute the program as many times as you wish simply by typing RUN.