July 16, 2013

Computer Basics - must read for beginners - helpful for APPSC and Bank PO Exams


Definition : A Computer is an electronic device that can perform activities that involve Mathematical, Logical and graphical manipulations. Generally, the term is used to describe a collection of devices that function together as a system.

It performs the following three operations in sequence.


1.   It receives data & instructions from the input device.


2.   Processes the data as per instructions.


3.   Provides the result (output) in a desired form.


Data :  It is the collection of raw facts, figures & symbols.


Ex :  Names of students and their marks in different subjects listed in random order.


Information :  It is the data that is processed & presented in an organized manner.


Ex : When the names of students are arranged  in alphabetical order, total and average marks are calculated & presented in a tabular form, it is information.

Program Set of instructions that enables a computer to perform a given task.


Advantages of computers :


1.   High speed : Computers have the ability to perform routine tasks at a greater speed than human beings. They can perform millions of calculations in seconds.

2.   Accuracy : Computers are used to perform tasks in a way that ensures accuracy.


3.   Storage : Computers can store large amount of information. Any item of data or any instruction stored in the memory can be retrieved by the computer at lightning speeds.

4.   Automation : Computers can be instructed to perform complex tasks automatically ( which increases the productivity).

5.   Diligence : Computers can perform the same task repeatedly & with the same accuracy without getting tired.

6.   Versatility : Computers are flexible to perform both simple and complex tasks.


7.   Cost effectiveness : Computers reduce the amount of paper work and human effort, thereby reducing costs.

Limitations of computers :

1.   Computers need clear & complete instructions to perform a task accurately. If the instructions are not clear & complete, the computer will not produce the required result.


2.   Computers cannot think.


3.   Computers cannot learn by experience.


Generations of computers :


Component used

First Generation

(1946-1954 )

Vacuum tubes

Second Generation



Third Generation

(1968-1975 )

Integrated Circuits (IC)

Fourth Generation

( 1976-1980)

Very Large Scale Integrated Circuits


Fifth Generation

(1980 till today )

Ultra Scale Integrated Circuits (ULSI)

Micro  Processor (SILICON CHIP)



The computer system consists of three units:


1.   Input device    2. Central Processing Unit (CPU)                   3. Output device



   Unit                                                Function

1.Input  device                               :    Reads information from input media and enters to the computer in a coded form




(a) Memory unit                          :    Stores program and data


(b) Arithmetic Logic unit            :    Performs arithmetic and logical functions


(c) Control Unit                           :    Interprets prograinstructions and controls the input and output devices

3. Output device                           :    decodes information and presents it to the user


 Central Processing Unit: It is the part of the computer that carries out the instructions of a computer program. It is the unit that reads and executes program instructions. Hence it is known as the

brain‖ of the computer. The CPU consists of storage or memory unit, Arithmetic Logic Unit (ALU)


and control unit.


 (a). Memory Unit: It is also known as the primary storage or main memory. It stores data, program instructions, internal   results and final output temporarily before it is sent to an appropriate output device. It  consists of  thousands of cells called storage locations‖.   These cells activate with off-on‖ or binary digits(0,1) mechanism.  Thus a character either a letter or numerical digit is stored as a string of (0,1) Binary digits ( BITS).  These bits are used to store instructions and data by their combinations.

 (b)  Arithmetic and Logical Unit(ALU) It is the unit where all   Arithmetic operations (addition, subtraction etc.) and logical functions such as true or false, male or female are performed.  Once data are fed into the main memory from input devices, they are held and transferred as needed to ALU where

processing takes place.  No process occurs in primary storage. Intermediate generated results in ALU are temporarily placed in memory until needed at later time. Data may move from primary memory to


ALU and  back again to storage many times before the process is finalized.

  (c).Control Unit  :     It acts as a central nervous system and ensures that the information is stored correctly and the program instructions are followed in proper sequence as well as the data are selected from the memory as necessary.  It also coordinates all the input and output devices of a system .

Input Devices


Devices used to provide data and instructions to the computer are called Input devices. Some important input devices are

Key board,  MouseScanner,  MICR,  Web camera,  Microphone etc.


1.   Keyboard: The Key board is used for typing text into the computer. It is also known as standard Input device. A computer keyboard is similar to that of a type writer with additional keys. The most commonly available computer keyboard has 104 keys.


There are different types of keys on the keyboard. The keys are categorized as :


·    Alphanumeric keys , including letters & numbers.


·    Punctuation keys, such as colon (:), semicolon (;) Question mark (?), Single & double quotes


·    Special keys such as arrow keys, control keys, function keys (F1 to F12), HOME, END etc.


2 Mouse:    It is a device that controls the movement of the cursor on a monitor. A mouse will have 2 buttons on its top. The left button is the most frequently used button. There will be a wheel between the left and right buttons. This wheel enables us to smoothly scroll through screens of information. As we move the mouse, the pointer on the monitor moves in the same direction. Optical mouse is another advanced pointing device that uses a light emitting component instead of the mouse ball. Mouse cannot be used for entering the data. It is only useful to select the options on the screen.

3. Scanner:It is an input device that can read text or illustrations printed on paper and translate into digital form. The main advantage of these scanners is that the data need not be entered separately

resulting in saving lot of time.


Scanners are of two types: i) optical scanners      ii) MICR


i)  Optical scanners:


a. Optical character Recognition(OCR): In this, characters are read with the help of a light. This is used in office atomization, documentation in library etc.

b. Optical mark recognition(OMR): It is a technology where an OMR device senses the presence or absence of a mark such as a pencil mark. OMR is used in tests such as aptitude tests.

c. Optical barcode recognition(OBCR): Barcode readers are photoelectric scanners that read the bar codes or vertical zebra striped marks printed on product containers. This is used in super markets, book shops etc.

ii. MICR: This is widely used in banks to process the cheques. This allows the computer to recognize characters printed using magnetic ink.

4. Magnetic Ink Character Recognition(MICR):   : It is a character recognition technology used primarily by the banking industry to facilitate the processing of the cheques. MICR characters ( cheque No., Acc.No.etc) are printed  in  special  ink  usually containing  iron  oxide. When  a document  that contains the ink needs to be read, it passes through a machine which magnetizes the ink and there will be a reader sorter unit which translates the magnetic information into characters. MICR provides a secure, high speed of scanning and processing information. It scans about 2600 cheques/min.


Output devices



Any device that is capable of representing information on a computer is called an Output device. Output devices receive information from the CPU and  present it to the user in the desired form.


Some important Output devices are : Monitor,  Printer


 1 .Terminal/Monitor: It is similar to TV screen- either a monochrome (black & white) or colour and it displays the output.  It is also referred as Visual  Display Unit(VDU).  Several types of monitors are in use.  Some of them are Colour Graphic  Adapter(CGA),  Enhanced Graphics Adaptor(EGA) , Video Graphics Adapter (VGA) and Super Video Graphics  Adapter (SVGA).

The screen sizes differ from system to system.  The standard size is 24 lines by 80 characters. Most systems have provision for scrolling which helps in moving the text vertically or horizontally on the screen.

2 . Printer: A printer is used to transfer data from a computer onto paper. The paper copy obtained


from a printer is often referred as ―printout‖.  The different printers and their speeds are as follows:


S. No.


Mode of Printing



Dot – Matrix



Prints the character in dotted pattern


through printer ribbon using either 24 pin or 9 pin

200/300 to 700




Ink Jet printer

Work by spraying ionized ink

Slow, 90 CPS


Laser printer

Also called page printer. Uses laser


beam to produce an image.

6 to 12 PPM


Line printer

Prints lines at a time instead of single



300 to 600 LPM



Produces drawings or graphs through


pens which are filled with different colours.


( CPS: Characters Per Second;   PPM: Pages Per Minutes;   LPM : Lines Per Minute)



Memory or  storage capacity is one of the important components of a computer.  Any storage unit of a computer system is classified on the basis of the following criteria:


1.  Access time: This is the time required to locate and retrieve stored data from the storage unit in response to program instructions.


2.  Storage capacity: It is the amount of data that can be stored in the storage unit.


3.   Cost per bit of storage.


Units of memory: The computer stores a character in the storage cells with binary (0,1) mechanism. Thus the basic unit of memory is  a  bit (binary digit 0,1).  To store a character, a computer requires 8 bits or 1 byte.   This  is called the  word length‖ of the storage unit.  Hence the storage capacity of the computer is measured in the number of words it can store and is expressed in terms of bytes.   The different units of measurement are


8 Bits               =      1 Byte


210  (or) 1024  Bytes   = 1 Kilo Byte (KB)


210  (or)1024 KB         = 1 Mega Byte (MB)


210  (or)1024 MB        = 1 Gega Byte (GB) Conversion :

ASCII – American Standard Code for Information Interchange.This code has given alphabets like some numbers which can be converted to Binary form. A- 65 …….Z – 90     and    a – 97  ………. z – 121

By using these codes the alphabets can be converted to digital  & hence to Binary form.


Types of Memory : A computer memory is of two types


1. Primary Memory ( Internal storage)        2. Secondary Memory ( External storage)


Primary Memory :   Primary memory is also called internal memory and is an important part of a computer. It is the main area in a computer where the data is stored. The stored data can be recalled instantly and correctly whenever desired. This memory can be quickly accessed by the CPU for reading or storing information. Primary memory is further classified into two types:

 Random Access Memory (RAM) and Read- Only Memory (ROM ) RAM:

RAM is also known as read/write memory as information can be read from and written onto it. RAM is a place in a computer that holds instructions for the computer, its programs and the data. The CPU can directly access the data from RAM almost immediately. However, the storage of data and instructions in RAM is temporary, till the time the computer is running. It disappears from RAM as soon as the power to the computer is switched off. i.e it is volatile memory.

 ROM:It is called Read-only memory as information can only be read from and not written or changed onto ROM. ROM is the ‗built-in' memory of a computer. It stores some basic input output instructions put by the manufacturer to operate the computer. The storage of data and instructions in ROM is permanent. It does not depend on the power supply. i.e it is non-volatile memory.


Secondary memory: The primary memory which is faster (and hence expensive) is generally not sufficient for large storage of data.  As  a result, additional memory, called the auxiliary‖ or secondary memory is used.  It is also referred as backup storage as it  is used to store large volume of data on a permanent basis which can be transferred to the primary memory whenever required for processing. Data are stored in secondary storage in the same binary codes as in the main (primary memory) storage. Some of  the devices of secondary storages are  Floppy Disk, Hard Disk, CD-ROM, DVD and Flash drive.


1. Floppy Disk: It is also referred as Diskette: and is made of  flexible Vinyl material.  It has a small hole  on  one  side  called  Right  protect  notch,   Which  protects  accidental  writing/deleting  the information from the disk. There is a hole in the centre through which the spindle of drive unit rotates the disk.   The disks are available in two sizes of 5.25 and 3.5 inches and these could be either low- density or high-density floppies.   Storage capacity of floppies are measured in kilobytes (KB) and

megabytes (MB).  The details about the storage capacities of the floppies are presented below:


Floppy Disk

Storage Capacity

Size (Diameter)

Low Density

360 KB

5.25 inches

High Density

1.2 MB

5.25 inches

High Density

1.44 MB

3.5 inches


2.8 MB

3.5 inches


2. Hard Disk:   The hard disk can hold more information than the floppy disk and the retrieval of information from hard disk is faster when compared to floppies or tapes. A hard disk is fixed inside the CPU and its capacity ranges from 20 MB onwards. The hard disk is made up of a collection of discs (one below the other) known as platters on which the data is recorded. These platters are coated with magnetic material.  It is less sensitive to external environmental disorders and hence the storage in hard disk is safe.  A small hard disk might be as much as 25 times larger than a floppy disk. Storage Capacity of hard disks varies from 20 MB to several Gega bytes like 80GB, 160GB.

3.  CD-ROM: CD-ROM stands for Compact Disk–Read Only Memory. It is used to store a wide variety of information. Its main advantage is that it is portable and can hold a large amount of data.. The storage capacity of most CD-ROMs is approximately 650 MB or 700 MB.

CD-ROMs have the following variations:


(i) CD-R(Compact disc Recordable): Data can be written onto it just once. The stored data can be read. Data once written onto it cannot be erased.

(ii) CD-RW(Compact disc Rewritable): It is also called erasable CD. Data once written onto it can be erased to write or record new information many times.To use a CD-ROM, a device called CD drive is needed.



4. DVD: DVD stands for Digital Versatile Disc. It is similar to a CD-ROM, except that it can store larger amounts of data. The storage capacity of a DVD is at least 4.7MB. DVDs that can store up to

17GBs are also available. Because of their capacity, DVDs are generally used to store a very large multimedia presentations and movies that combine high quality sound and graphics.


5. Flash Drive: It is a small, portable device that can be used to store, access and transfer data. Due to its small size, it is commonly called Pen drive. It is also called USB drive. We can read, write, copy, delete, and move data from computer to pen drive or pen drive to computer. It comes in various storage capacities of 2GB, 4GB, 8GB etc. It is popular because it is easy to use and small enough to be carried in a pocket. This device is plugged into the USB port of the computer and the computer automatically detects this device.





Hardware: The physical components of the computer are known as Hardware. It refers to the objects that we can actually touch.


Ex:   input and output devices, processors, circuits and the cables.


Software: Software is a program or set of instructions that causes the Hardware to function in a desired way.  The basic difference between the Hardware and Software is just the same as that exists between TV and TV studio.   Without TV studio (software) from where the programs are telecast, the TV (Hardware) is a dead machine.

 There are five categories of software.  They are:


1.   Operating System            2. Translators           3. Utility programs         4. Application programs


5. General purpose programs


 1.   Operating System (OS) : The   software that manages the resources of a computer system and schedules its operation is called Operating system. The operating system acts as interface between the hardware and the user programs and facilitates the execution of programs.


Generally the OS acts as an interface between the user and the Hardware of the computer. i.e It is a bridge between the user and the  Hardware.

The User interface provided by the OS can be character based or graphical.


CUI  --  Character  user Interface


GUI  --  Graphical user Interface


CUI  It is operated with keyboard only.  Ex: MS-DOS,   UNIX


GUI :   The system can be operated with mouse and keyboard.   Ex: Windows 95,  Windows XP etc


·     Disk Operating System (DOS):  It was developed as early as 1980 by Bill Gates at the age of 19.  It is suited  for personal computers. Dos is a single user and single task operating system



·     WINDOWS :     It works with DOS and it supports single user and multitask system.   Irequires a powerful PC with a minimum RAM of 8 MB .



·     UNIX AND XENIX It is suited for multi-user and multi-task system



2.   Translators  : Computers can understand instructions only when they are written in their own language the machine language . Therefore, a program written in  any other language should be translated into machine language The software that translates‖ the instructions of different languages is known as translators .

There are two types of translators. They are  compilers and  Interpreters


A Compiler checks the entire user written program (known as the source program) and if it is error free, produces a complete program in machine language (known as object program).  The source program is retained for possible modifications and corrections and the object program is loaded into the computer for execution.  If the source program contains errors, the compilers produce a list of errors athe end of the execution of the program. i.e  a compiler translates the whole program before execution. An interpreter does a similar job but in a different style. The interpreter translates one statement at a time and if it is error free, executes.  This continues till the last statement.  Thus an interpreter translates or executes the first instruction before it goes to the second, while a compiler translates the whole program before execution.


The major difference between compiler and interpreter is


1.   Error correction  is  very much simpler in  the case of interpreter as  it  translates  the statements in stages.  The compiler produces an error list of the entire program at the end.

2.   Interpreter takes more time for the execution of the program compared to compilers as it translates one statement at a time


Programming Languages:  There are three types of programming languages.



1.   Machine Languages: Computers respond only to machine language.  This language is in terms of binary codes (0,1).  i.e. all programs should be written with these codes, which is difficult, time consuming  and  leading  to  errors  while  writing  the  programs.  There  is  no  unique  standard machine language. Rather there are many machine languages. These are machine dependent. These are referred  as the first generation languages.


2.   AssemblLanguages  :  It  uses mnemonic  codes  rather than  numeric  code(as  in  machine languages). Ex. Add or A is used as a symbol for addition.  It requires translators to convert into machine language.  Like machine language, writing program in assembly language is also  time consuming. These are also machine dependent.


3.   High Level Languages (HLL): These are referred as problem oriented languages (POL). These are referred as third generation languages. The advantages of these languages are


·    The  high level languages are convenient for writing programs as they can be written without any codes.  These languages follow rules likeEnglishlanguage.



·    Because of their English like nature, less time is required to write a program.



·    They are machine independent.  A program written in any HLL can be run on computers of different types without any modifications.


Several High Level Languages which are in common use: 

  1. FORTRAN :   FORmula TRANslation
  2. COBOL :       Common Business Oriented Language
  3.  BASIC :        Beginner's All purpose Symbolic Instruction Code
  4.  PROLOG:     PROgramming in LOGic
  5.  ALGOL :     ALGOrithmic Language


3.  Utility Programs: These are pre-written programs supplied by the manufacturer for maintaining day to day activities of computer system.


Example: COPY, SORT, MAILING, virus scanning software etc.,



4. Application Programs: These are user written programs to do a specific job which can be changed to meet the individual needs.  These programs are written in different languages such as BASIC or C or busing database packages like dBASE, OracleExample:  Payroll,   Billing,  Railway Reservation etc.

5. General Purpose Packages: These packages are developed to suit the needs of  research workers scientists in different fields.  These packages are categorized as :



i) Data Analysis    ii) Word Processing  iii) Spread Shee iv) Graphics an v) Databases


Data Analysis


Ex:   SPSS (Statistical Package for Social Science), MSTAT,      MICROSTAT,    GENSTAT SAS etc.

Word Processing



Spread Sheet


Ex: LOTUS,   Qpro,  VP-PLANNER,  SYMPHONY,  MS-Excel  etc.








Ex:  dBASE,  FOX-BASE,  FOX-PRO, ORACLE, MS-Access  etc.



Functions of  Operating System Today most operating systems perform the following important functions :



1.   Processor management : It manages the assignment of processor to different tasks being performed by the computer system.


2.   Memory management : It manages the allocation of main memory and other storage areas to the system programmes as well as user programmes and data.


3.   Input / Output management : It manages the co-ordination and assignment of different Input and Output devices while one or more programmes are executed.


 4.   File management : It allows all files to be easily changed and modified through the use of text editors or some other file manipulation routines.


5.   Establishment and enforcement of a priority system : It determines and maintains the order in which jobs are to be executed in the computer system.


6.   Interpretation of commands and instructions.



7.   Facilitates  easy communication between the computer system and the computer operator.


Classification of Computers



Computers are classified according to the storage capacity, speed and the purpose for which they are developed.  These can be classified into three types:


1.   Analog Computes       2. Digital Computers          3. Hybrid Computers


1. Analog computers: They operate by measuring instead of counting. The name (derived from greek word analog) denotes that the computer functions by establishing similarities between the two quantities.  They are powerful tools for solving differential equations.


2.   Digital Computers: These computers operate by counting.  All quantities are  expressed as discrete digits or numbers.   These are useful for evaluating arithmetic expressions and manipulations of data.


3.   Hybrid Computers: Computers which combine the features of analog and digital computers are known as Hybrid computers.

A majority of the computers that are in use are digital.  These computers were essentially developed for computations.  Later, the developments in the computers led to the use of digital computers in variety of applications.  Depending on the use of applications, the  digital computers are classified into   1) Special Purpose Computers  and   2) General Purpose Computers


1.   Special Purpose Computers:  These are developed with a specific purpose.  Some of the areas where these computers are being used are soil testing, drip irrigation, medical scanning, traffic signals, spacecraft, rocket technology etc.,


2.   General Purpose Computers: These are developed to meet the requirements of several areas such as simulation, solving mathematical equations, payroll and personnel database. These computers are available in different sizes and capabilities and are further classified (based on memory, speed, storage) as follows.


a) Super Computers       b) Mainframe Computers       c) Mini Computers     d) Micro Computers



a)   Super Computers: These have extremely large storage capacities and computing speeds which are atleast 10 times faster than other computers.  These are used for large scale numerical problems in scientific and engineering disciplines such as electronics, weather forecasting  etc.,.     The  first  super  computer  was  developed  in  U.S.A.  by  CRAY computers.   In India the indigenous super computer was  developed under the name Param.

b)   Mainframe Computers: They also have large storag and high computing speed (but relatively lower than the super computers).  They are used in applications like weather forecasting, space applications etc., they support a large number of terminals for use by a variety of users simultaneously, but are expensive

c) Mini Computers: It is   a medium sized computer with moderate cost, available indigenously  and  used  for  large  volume  applications.     It  can  serve  multi-usersimultaneously


d)   Micro Computers:  A micro computer is the smallest general purpose processing system.Micro  computers    are  also  referred  as  personal  computers(PC).     These  are  self contained units and usually developed for use by one person at a time but can be linked to very large systems. They are cheap, easy to use even at homes and can be read for variety of applications from small to medium range.  These are available in three models:


1.  PC : Personal Computer


2.  PC-XT : PC with Extended Technology


3.   PC-AT : PC with Advanced Technology



Personal Computer:  A personal computer has a Monitor (VDU), a keyboard , Disk Drive (s), printer and CPU.   The CPU of PC has a mother board with several chips mounted on a circuit board.   The major components of the circuit board are : Microprocessor, RAM and ROM chips and other supporting circuits.


Microprocessor:  The Microprocessor chip is like a brain of human being which contains circuits and registers to perform arithmetic, logic and control functions. i.e it contains ALU & CU.   These chips will be able to retrieve data from the input output devices, store, manipulate and process a byte of data at a time.  There is an address bus which is built into these chips to determine the storage locations (of

RAM) of the data and the instructions of the program. Over the years, different microprocessors were developed and the first in the series is INTEL


8080. The other procesors are 8088, 80286, 80386, 80486, Pentium I, II, III and Pentium 4. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5000 times faster.


Clock Speed: The speed with which the instruction is executed or number of pulses per second is called the clock speed.  It is measured in terms of million pulses per second (MHZs).

The following table gives the differences among the different processors that intel has introduced over the years :






Clock speed

Data width






8 bits





5  MHZ

16 bits

8-bit bus






16 bits





16 MHZ

32 bits





25 MHZ

32 bits


Pentium I



60 MHZ

32 bits

64-bit bus


Pentium II



233 MHZ

32 bits

64-bit bus


Pentium III



450 MHZ

32 bits

64-bit bus


Pentium 4



1.5 GHZ

32 bits

64-bit bus


Pentium 4




3.6 GHZ

32 bits

64-bit bus


Peripherals: The input, output and secondary storage units like floppy drives, keyboard, mouse, etc., are called peripherals.

Classification of PCs: PCs are mainly classified on the basis of primary memory (RAM), Microprocessor, clock


speed and peripherals. The classification is presented in the following table:






Clock speed




640 KB



Floppy    Drives,    key

board and printer



1 MB


18 MHZ

Floppy  Drives,  Hard-

Disk Drive, Key board and printer



1 MB to

32 MB

80286 to 80486

25 MHZ and


Floppy   drives Hard

disk,            Keyboard, Mouse and Printer

Working with personal computers



Drives of the computer:  A computer can have  Floppy Disk Drive along with Hard Disk drives in the system.  These are referred as follows:


Floppy Disk Drive   :   or B


 Hard Disk                :   C


 Booting of   the Computer: Booting imply activating the computer for working. Thus, when the computer is switched on, the operating system residing in the floppy or in the hard disk is transferred into the primary memory. After this, the computer is ready to accept the commands at user terminal. If OS is not existing either in floppy drive or in the hard disk drive,  non-system or disk error is flashed on the screen. The booting of the computer is carried out with a software which resides on ROM chip called BIOS ( Basic Input Output System).  Thus when a PC with system disk either in Hard-Disk Drive or Floppy Drive is switched on, the screen finally displays one of the following symbols with a blinking cursor at its end:

a)  A:\>  This imply that the computer is booted through Floppy drive `A' and is ready to accept DOS commands.  The symbol \> is referred as DOS prompt from where DOS commands can be entered.

b)  C:\> It imply that the PC is booted through the Hard-Disk Drive  `C'.


Booting is essential whenever the system is locked or switched-off.  A PC can be booted through two ways:         a) Cold Booting           b) Warm Booting.

a) Cold Booting:    It is only switching OFF the system and after some time again switching    ON.


b)   Warm Booting:   it  is  carried  out  by  pressing  the  following  three  keys  simultaneously : Ctrl  +  Alt  + Del.    It is preferable to cold booting as it safeguards the system from sudden power fluctuations resulting out of switching ON and OFF the system.



MS-DOS is a collection of programs and other files. It is a single user and single programming environment.

MS-DOS is designed to provide a method of organizing and using the information stored on disks, application programs, system programs and the computer itself.


Files and File names: A file is a collection of related information.  The files should have suitable names for their identification in later use.

Rules for naming the files:


1.   File names should be of one to eight characters in length with an option of one to three character extension

2.   File names can include any one of the following characters: A to Z (or a to z) 0 to 9, $, &, #, @. %, ( ), { } The characters which are not allowed are:, ; + / \ * as these have special meaning


3.   A period (.) is used to separate the first part of a file name from the extension.  (LETTER.TXT, RAMA.WK1, etc.,)

4.   When a file name includes an extension, it should be referred along with its extension and not only with the first part

5.   Reserved devices names are not allowed.  (COMMAND.COM, DATE, etc.,


Types of files in DOS: -


There are different types of files you can create in your computer, according to the usage.  To differentiate each file, the computer will give an extension to identify the different types of files you have created.


Executable Files: The files with extension of either .COM or .EXE are called executable files.  These files are directly executed by typing their names (without the extension) by the computer.  A file with

.COM  extension  refers  to  COMMAND  file  name  and  the  other  with  .EXE  extension  refers


EXECUTABLE program file.


Batch Files: the file containing a series of DOS commands, which are executed automatically one at a time in the order entered, is known as the batch file.  A batch file has the extension .BAT.  If the batch file is given a special name AUTOEXEC.BAT, it is automatically executed when the computer is switched on or during booting.


Backup Files: These are backup files of a file and have extension .BAK.  These files are such that their contents can not be altered (edited).  These files are created while working with WORDSTAR package for typing a document.

System Files: These are program files developed for system control.  They have .SYS extension. Directory: It is a collection of files, size, date and time of creation of files.  The relation between files, directories and disk is similar to the relation between papers, filing folders and filling cabinets.  Just as a cabinet contain papers and other folders, a directory may contain directories also.  The main directory of

a drive is called Root Directory into which several directories and sub-directories may exist.


Types of DOS commands:   There are two types of commands:  i) interna and   ii) external


i) Internal DOS commands: Internal commands are those commands which manage files. They are loaded into the memory of the PC when PC is booted.  These are stored in a large file with file name COMMAND.COM,  when a PC  is  booted.

1. DIR:     Dir displays continuously the directory of file names, file size, date and time of creation.


C:\>dir a:   This command displays the list of files and directories from the drive A   i.e. the floppy drive.


It  can be specified with two options:


i) Dir/p: It pauses the listing of the directory whenever the screen is full.  The next screen load of files can be   displayed by pressing any key.

ii) Dir/w: displays the files of the  directory in a five column format.  Only file names and their extensions will be displayed without the date and time of creation.

The total number of files and remaining bytes are listed.


In addition,  DIR can  also be specified with wildcard characters (such as * or ? to  list files sharing a common element in the file name or extension.

Eg: DISTATS.* : lists all files with  STATS and other combinations along with the extensions. DIR *.COM   :  lists all files with extension .COM

2. DEL:    Deletes a specified file.                       

Syntax:  DEL {filename} Del stats  deletes the file ―stats‖.

Del  stats.*  deletes all extensions of the filestats‖.


Suppose you want to delete a file COMPUTER.DOC on A drive, the command will be: C:> DEL A: COMPUTER.DOC ¿

3. COPY:   The COPY command  is used to copy files from one place to another.  The way to execute


copy command  is to type COPY leave a space followed by the source file name again a space followed by destination file.

Syntax: copy {Drive name} <source file> {Drive name} <target file name>


Ex: 1. COPY STATS MATHS copies a file STATS with the name MATHS on the same drive


2. COPY PATH A:ENTO  copies a file PATH‖ from the current drive to driveA‖ with the file name ENTO‖.

3. COPY A:MATHS C:STATS  copies a fileMATHS form drive A‖ to driveC‖  with the file name STATS

4. COPY A:MATHS.* C:STATS.*  copies all extensions of fileMATHS form drive A to drive C with name STATS‖.

4. REN:  The REN command is used to rename the old file with another name. The way to execute


Rename command is to type REN leave a space followed by the old file name again followed by a space and  new file name.

Ex: Suppose if you want to change the name of a file MATHS.TXT to STATS.TXT the


following command  will work:




5.DATE:    It displays current date and asks for new date in (mm)-(dd)-(YY) format.  If no date is to be changed, the old date can be retained by pressing enter key.

Syntax: C:\> Date


6.TIME:   displays current time and asks for new tine and if no new time is to be entered, pressing


enter retains the old time


Syntax: C:\> Time


7.VER:      displays the version of DOS


8.CLS :  when you want to clear the screen, type in the following command:


C:\> CLS 


ii) External commands: External commands are small file programs used for doing specific jobs.


Some of the useful commands :


1.   SORT: It sorts the files in alphanumeric order: either A to Z or Z to A


DIR| SORT  (or) DIR/ON :  Displays all the files in A to Z order


DIR| SORT/R (or) DIR/O-N:   Displays all the files in reverse order (i.e Z to A)


2.   FORMAT: Formatting a (new) disk imply organizing the new disk into magnetic tracks and sectors that are readable by DOS.  The PC copies files only on a formatted disk. However, if an old disk is formatted, all the existing files on it would be erased. Hence this command should be  used carefully.

FORMAT a:  formats disk in driveA‖




This command displays the volume label of the current disk



It is used to create, change or delete the volume label on the disk



It is used to provide access to files located on other directories or on other disks.


Thus while in C‖ drive, one can directly invoke files of DOS, WORDSTAR (WS)



LOTUS directories by giving the following command:




This command is generally listed in the AUTOEXEC.BAT files  for its execution when the PC is booted


6.   MD: It is used to make a new directory (or sub-directory) which is subordinate to the current

(or   root) directory.  It is specified as follows:


C:\> MD\STATS   (or) MD STATS : This command makes (creates) a new directory with


the name ―  STATS‖.


7.   CD  It is used to change from one directory to the other.


Iwe have to work with files in the directory STATS, the directory has to be changed to ―STATS‖

Ex:  C:\>CD\ STATS  (or) CD STATS : This command changes the root directory to the

directory STATS.


8.   RD It removes or deletes a directory.  To remove a directory, it is essential to first delete all the files in the directory.

To remove the directory STATS, the command is            C:\> RD STATS




9.   TREE: It displays the directories and sub-directories existing in a drive with aTREE‖ diagram. The difference between TREE and DIR commands is that: DIR displays all files with its size, date and time of creation.  It also displays only directories with symbol <DIR>

(without files and sub directories in that directory) whereas, TREE displays directories and sub directories in a drive without files.


Computer viruses and Vaccines:


Def: A VIRUS is a set of instructions (or program) or software prepared to destroy a package and data.


A virus kills data and program files.  Although the purpose for developing such softwares is not


known, they appear to be basically developed for a check against the piracy of program packages.


Functioning of viruses: viruses reside in executable files such as .COM or .EXE.  Thus when PC is switched on, the virus enters the RAM through  .COM (COMMAND.COM) file and starts destructive work of killing data and program files.  Whenever the PC is switched off, they again go back to the executable files.


More than 1000 computer viruses have been identified.  They are classified as :



File Allocation Table (FAT) based viruses

ii )Key board viruses and


File based viruses


Some of the well known viruses are:


Joshi, Die_Hard-2,   Jerusalem,  Generic,  Brain,  May_5th,  Gumnam,  World peace etc.,


VACCINE: It is a software developed for removing the viruses.


In DOS version, there are in-built programs to guard against computer viruses.  These are anti-virus programs for both DOS and WINDOWS and a memory resident virus-safe program that guards against the introduction of files that may be affected by virus.


Some of the anti-virus vaccines are: NASHSOFT, UTSCAN, AVG, AVIRA, Meccaf etc.




Salient features of Windows Operating System:


1.   It converts the plain character based user interface provided by DOS into a Graphical User Interface(GUI) such as pictures, symbols and words on your screen that can be controlled by the mouse.

2.   It provides a multitasking capabili