COMPUTER DEFINITION:A computer is an electronic device, which executes software programs. It is made up of two parts - hardware and software . The computer processes input through input devices like mouse and keyboard. The computer displays output through output devices like a monitor and printer.

USES:

1. Word Processing - Word Processing software automatically corrects spelling and grammar mistakes. You can use the copy and paste features. You can print documents and make several copies. It is easier to read a word-processed document than a handwritten one. You can add images to your document.

2. Internet - It is a network of almost all the computers in the world. You can browse through much more information than you could do in a library. That is because computers can store enormous amounts of information.

3. Banks - All financial transactions are done by computer software.

They provide security, speed and convenience.

4. Travel - One can book air tickets or railway tickets and make hotel

reservations online.

5. Telecommunications - Software is widely used here. Also all mobile

phones have software embedded in them.

6. Defence - 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.

7. E-Learning - Instead of a book it is easier to learn from an E-learning

software.

CHARACTERISTICS :1. Speed : Computers work at an incredible speed. A powerful

computer is capable of performing about 3-4 million simple instructions per second.

2. Accuracy : In addition to being fast, computers are also accurate. Errors that may occur can almost always be attributed to human error

3. Diligence :Unlike human beings, computers are highly consistent. They do not suffer from human traits of boredom and tiredness resulting in lack of concentration. Computers, therefore, are better than human beings in performing voluminous and repetitive jobs.

4. Versatility :Computers are versatile machines and are capable of performing any task as long as it can be broken down into a series of logical steps. The presence of computers can be seen in almost every sphere – Railway/Air reservation, Banks, Hotels, Weather forecasting and many more.

5. No IQ: - Computer is a dumb machine and it cannot do any work without instruction from the user. It performs the instructions at tremendous speed and with accuracy.

Computer Classification: Computers can be generally classified

1. Personal computer: A small, single-user computer based on a

microprocessor. In addition to the microprocessor, a personal

computer has a keyboard for entering data, a monitor for

displaying information, and a storage device for saving data.

2. Workstation: A powerful, single-user computer. A workstation is

like a personal computer, but it has a more powerful

microprocessor and a higher-quality monitor.

3. Minicomputer: A multi-user computer capable of supporting from

10 to hundreds of users simultaneously.

4. Mainframe: A powerful multi-user computer capable of

supporting many hundreds or thousands of users simultaneously.

5. Supercomputer: An extremely fast computer that can perform

hundreds of millions of instructions per second.

Basic computer Operations:1. Input: This is the process of entering data and programs in to the

computer system. It takes as inputs raw data and performs some

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

data from us to the computer in an organized manner for processing.

2. Storage: The process of saving data and instructions permanently is

known as storage. Data has to be fed into the system before the

actual processing starts. It is because the processing speed of Central

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

CPU with the same speed. Therefore the data is first stored in the

storage unit for faster access and processing.

3. Processing: The task of performing operations like arithmetic and logical

operations is called processing. The Central Processing Unit (CPU) takes data

and instructions from the storage unit and makes all sorts of calculations

based on the instructions given and the type of data provided. It is then sent

back to the storage unit.

4. Output: This is the process of producing results from the data for getting

useful information. Similarly the output produced by the computer after

processing must also be kept somewhere inside the computer before being

given to you in human readable form. Again the output is also stored inside

the computer for further processing.

5. Control: The manner how instructions are executed and the above

operations are performed. Controlling of all operations like input, processing

and output are performed by control unit. It takes care of step by step

processing of all operations inside the computer.

FUNCTIONAL UNITS:The computer system is divided into three separate units for its operation. They

are :

1) arithmetic logical unit

2) control unit.

3) central processing unit.

Arithmetic Logical Unit (ALU) :

1. Logical Unit :After you enter data through the input device it is stored in

the primary storage unit. The actual processing of the data and instruction are

performed by Arithmetic Logical Unit. The major operations performed by the

ALU are addition, subtraction, multiplication, division, logic and comparison. 2. Control Unit (CU) : The next component of computer is the Control Unit,which acts like the supervisor seeing that things are done in proper fashion.Control Unit is responsible for co coordinating various operations using timesignal. The control unit determines the sequence in which computer programsand instructions are executed

3. Central Processing Unit (CPU): The ALU and the CU of a computer

system are jointly known as the central processing unit. You may call CPU as

the brain of any computer system. It is just like brain that takes all major

decisions, makes all sorts of calculations and directs different parts of

the computer functions by activating and controlling the operations.

Computer basically consists of two parts:

1. Hardware

2. Software

Hardware:- Hardware refers to the physical devices of the computersystem. Thus the various input, output, storage, processing devicesconnected in a computer system constitute the hardware. Somecommon hardware devices are the monitor, the floppy disk drive, harddisk drive etc.

Software:To make the hardware work properly we require a set of

instructions. These set of instructions or programs which enhance

the capability of the computer machine is called software. Software

is a group of programs that solve specific problems. It acts as an

interface between the user and the hardware.

Types of software:

Software can be classified into two main categories:

1. The Application Software

2. the system Software

1.Application Software:- Application Software is a set of programsdesigned to solve specific functions of the user. The set of programsincluded in the application software packages are called applicationprograms and the programmers who prepare the applicationsoftware are called application software programmers.

It can be broadly classified into following categories:

1. Word processing software

2. Spreadsheet software

3. Database software

4. Graphics and presentation software

5. Entertainment and Educational software

6. Communication software

1. Word Processing Software: It is most widely used application

in the computer system to create, edit, view, store, retrieve and

print all kind of text based documents. The various documents

includes newspapers, magazines, letters and advertisements.

We can also bold, italic and underline a particular character

using this application software. Some commonly used word

processing softwares available are Microsoft Word, Word Pad

etc.

2. Spreadsheet Software: Spreadsheets are the applications that can hold

large amount of numerical data arranged in rows and columns. The

Spreadsheet software allows the user to prepare the computerized ledger. It

is used for financial results, maintain accounts etc.. Each box in which data

is entered is called cell. Different cells are combined to form a worksheet,

changes can also be made easily in each cell.

3.Database Software:- A database software is a set of programs

that can helps the user to create the database, maintains it by

performing modifications, organize it and relatively retrieve

useful information from it. It permits users to create and

maintain database to extract information from the database

whenever required.

4.Graphics and Presentation software :- It permits the users to

create charts, line drawing and graphically portray the data in

an electronic spreadsheet or database.

5.Communication software:- It permits users to send and

receive transmission of data to/from remote computers, and to

process and store the data as well.

2.)System Software:- The system software is a set of programs that

coordinate the operations of various hardware components connected to a

computer. It acts an interface between the user and the computer. The

programs that constitute the system software are called the system

programs and the people who prepare the system software are called

system programmers. Examples of system software are Operating system,

Compilers , Linkers , Assemblers and Interpreter.

Features:-

1.It allows the use of system programming.

2.Fast in speed.

3.It is machine dependent that is same system software cannot run on

different machine.

4.Smaller in size.

5.Difficult to design.

Operating System:-

An operating system is the program that, after being initially loaded into the

computer by a boot program, manages all the other programs in a computer. The other

programs are called applications or application programs. The application programs

make use of the operating system by making requests for services through a defined

application program interface (API). In addition, users can interact directly with the

operating system through a user interface such as a command language or a graphical

user interface (GUI)

USER

An operating system performs these services for applications:

1.)In a multitasking operating system where multiple programs can be running at the

same time, the operating system determines which applications should run in what

order and how much time should be allowed for each application before giving

another application a turn.

2.)It manages the sharing of internal memory among multiple applications.

3.)It handles input and output to and from attached hardware devices, such as hard

disks, printers, and dial-up ports.

o.s

HARDWARE

4.)It sends messages to each application or interactive user (or to a system

operator) about the status of operation and any errors that may have occurred.

5.)On computers that can provide parallel processing, an operating system can

manage how to divide the program so that it runs on more than one processor at a

time.

Linux, Windows 2000, OS/400 are all examples of operating systems.

Types:

1. Real-Time Systems:- A real-time operating system is a multitasking operating

system that aims at executing real-time applications.. The main objective of real-

time operating systems is their quick and predictable response to events. They

have an event-driven or time-sharing design and often aspects of both.

2.Multiuser :- A multi-user operating system allows multiple users to

access a computer system concurrently. Time-sharing system can be

classified as multi-user systems as they enable a multiple user access to

a computer through the sharing of time. Single-user operating systems,

as opposed to a multi-user operating system, are usable by a single user

at a time. Being able to have multiple accounts on a Windows

operating system does not make it a multi-user system.

3.Multitasking:- When only a single program is allowed to run at a

time, the system is grouped under a single-tasking system. However,

when the operating system allows the execution of multiple tasks at

one time, it is classified as a multi-tasking operating system. Multi-

tasking can be of two types: pre-emptive or co-operative. In pre-

emptive multitasking, the operating system slices the CPU time and

dedicates one slot to each of the programs. Unix-like operating systems

such as Solaris and Linux support pre-emptive multitasking.

4.Distributed:- A distributed operating system manages a group of

independent computers and makes them appear to be a single computer.

The development of networked computers that could be linked and

communicate with each other, gave rise to distributed computing.

Distributed computations are carried out on more than one machine.

When computers in a group work in cooperation, they make a

distributed system.

5. Embedded:- Embedded operating systems are designed to be used in

embedded computer systems. They are designed to operate on small

machines like PDAs with less autonomy. They are able to operate with

a limited number of resources. They are very compact and extremely

efficient by design. Windows CE and Minix 3 are some examples of

embedded operating systems.

Booting Process:-

Any computer becomes operational when some software takes

control of it. The process of starting or switching on the

computer is called booting of the computer. The disk with the

help of which the system boots itself is called bootable

diskand the program or set of instructions which help to

start the computer are called bootable programs.

There are mainly two types of booting:

1. Hot booting

2. Cold booting

1.Hot booting:-In this booting, we boot the system when the

computer halts or crashes due to some reasons by either pressing

the reset button or by pressing ctrl+alt+del keys simultaneously

from the keyboard. It is also known as resetting. On pressing the

restart button the computer restarts itself. If it doesn’t do so then

user should turn the power off, wait for few seconds, then again

switch on the power button and resume the operation.

2. Cold booting:-Switching on the computer by pressing ON/OFF

switch on the computer’s cabinet is known as cold booting. This

type startup is slow process and programs are loaded from the disk.

Programming LanguagesThere are two major types of programming languages. These are Low Level

Languages and High Level Languages. Low Level languages are further divided in to

Machine language and Assembly language.

Low Level Languages:- The term low level means closeness to the way in which the

machine has been built. Low level languages are machine oriented and require

extensive knowledge of computer hardware and its configuration.

(a) Machine Language:- Machine Language is the only language that is directly

understood by the computer. It does not needs any translator program. We also call it

machine code and it is written as strings of 1′s (one) and 0’s (zero). When this

sequence of codes is fed to the computer, it recognizes the codes and converts it in to

electrical signals needed to run it. For example, a program instruction may look like

this:

1011000111101

It is not an easy language for you to learn because of its difficult to understand. It is

efficient for the computer but very inefficient for programmers. It is considered to the

first generation language. It is also difficult to debug the program written in this

language.

Advantage:The only advantage is that program of machine language run very fast because no translation

program is required for the CPU.

Disadvantages:1.It is very difficult to program in machine language. The programmer has to know details of

hardware to write program.

2.The programmer has to remember a lot of codes to write a program which results in program

errors.

3.It is difficult to debug the program.

(b) Assembly Language:-It is the first step to improve the programming structure. You

should know that computer can handle numbers and letter. Therefore some combination of

letters can be used to substitute for number of machine codes.

The set of symbols and letters forms the Assembly Language and a translator program is

required to translate the Assembly Language to machine language. This translator program is

called `Assembler’. It is considered to be a second-generation language.

Advantages:1.The symbolic programming of Assembly Language is easier to understand and saves a lot of

time and effort of the programmer.

2.It is easier to correct errors and modify program instructions.

3.Assembly Language has the same efficiency of execution as the machine level language.

Because this is one-to-one translator between assembly language program and its

corresponding machine language program.

Disadvantages:

1.One of the major disadvantages is that assembly language is machine

dependent. A program written for one computer might not run in other

computers with different hardware configuration.

HIGH LEVEL LANGUAGES:-Higher level languages are simple

languages that use English and mathematical symbols like +, -, %, / etc. for its

program construction. Higher level languages are problem-oriented languages

because the instructions are suitable for solving a particular problem. For

example COBOL (Common Business Oriented Language) is mostly suitable

for business oriented language where there is very little processing and huge

output. There are mathematical oriented languages like FORTRAN (Formula

Translation) and BASIC (Beginners All-purpose Symbolic Instruction Code)

where very large processing is required.

Advantages :Higher level languages have a major advantage over machine

and assembly languages that higher level languages are easy to learn and use.

It is because that they are similar to the languages used by us in our day to

day life.

Assemblers

Assemblers performs the following functions:-

1.translate assembly instructions and pseudo-instructions

into machine instructions

2. Convert decimal numbers, etc. specified by

programmer into binary

Assemblers make two passes over the assembly file:-

a) First pass:- It reads each line and records labels in a

symbol table

b)Second pass:- It uses information in symbol table to produce

actual machine code for each line

Assembly Language

ProgramAssembler Machine Level Language

Program

INPUT OUTPUT

Linker

Linker is a tool that merges the object files produced by separate

compilation or assembly and creates an executable file

It performs the following tasks:-

1.Searches the program to find library routines used by program,

e.g. printf(), math routines.

2.Determines the memory locations that code from each module

will occupy and relocates its instructions by adjusting absolute

references.

Prog1.obj Prog2.obj Progn.obj

Linker

Executable Program File

Compilers

A compiler is a program that reads a program in one language, the

source language and translates into an equivalent program in another

language, the target language.

The translation process should also report the presence of errors in the

source program.

Source Program → Compiler → Target Program

↓

Error Messages

There are two parts of compilation:-

1.)The analysis part breaks up the source program into constant piece

and creates an intermediate representation of the source program.

2.)The synthesis part constructs the desired target program from the

intermediate representation.

LoaderA loader is the part of an operating system that is responsible for loading

programs. It is one of the essential stages in the process of starting a program,

as it places programs into memory and prepares them for execution. Loading

a program involves reading the contents of executable file, the file containing

the program text, into memory, and then carrying out other required

preparatory tasks to prepare the executable for running. Once loading is

complete, the operating system starts the program by passing control to the

loaded program code.

All operating systems that support program loading have loaders, apart from

systems where code executes directly from ROM or in the case of highly

specialized computer systems that only have a fixed set of specialised

programs.

In the case of operating systems that support virtual memory, the loader may

not actually copy the contents of executable files into memory, but rather may

simply declare to the virtual memory subsystem that there is a mapping

between a region of memory allocated to contain the running program's code

and the contents of the associated executable file.

Computer Memory

DEFINITION:-Memory is something that stores, preserves and recalls data

when needed. Computer memory stores data that is accessed by the Central

Processing Unit (CPU). Thus, memory acts as storage for uncompleted tasks and

other information needed to accomplish those tasks.

TYPES OF MEMORY:-

1) RAM (Random-Access Memory): This is the same as main memory. When

used by itself, the term RAM refers to read and write memory; that is, you can

both write data into RAM and read data from RAM. This is in contrast to ROM,

which permits you only to read data. Most RAM is volatile, which means that it

requires a steady flow of electricity to maintain its contents. As soon as the

power is turned off, whatever data was in RAM is lost.

TYPES OF RAM:

1. Dynamic RAM (DRAM): Most computers incorporate two types of

volatile RAM: static and dynamic. Although both types require constant

electrical current to function, they have some important differences.

Dynamic RAM is less expensive, and therefore it is the most widely used.

DRAM stores each bit of information in a separate capacitor on the

integrated circuit. The DRAM chip requires only one transistor and one

capacitor for each bit of storage. This makes it both cheap and space

efficient. DRAM also suffers periodic access limitations, because it

cannot be read during the refresh cycle. One specialized type of DRAM

is more common is embedded DRAM, or EDRAM.EDRAM is DRAM

integrated onto the same chip as the processor and used as cache memory.

2. Static RAM (SRAM): Static RAM (SRAM) has the advantage of

being faster than DRAM, although the disadvantage is that it is more

expensive.

SRAM generally requires less power than DRAM, although its

power requirements vary depending on clock speed. At higher

clock speeds, it can use as much power as DRAM; however, at

more moderate speeds, it requires only a fraction of what DRAM

uses. When idle, SRAM power requirements are low. Each bit in a

SRAM chip requires a cell of six transistors, although DRAM

needs only one transistor and one capacitor. This means that

SRAM cannot achieve the storage densities of the DRAM family.

As with DRAM, SRAM chips are mostly large arrays of these cells

of transistors. The two primary applications of SRAM are

1. Embedded use and in computers. Embedded use refers to

SRAM use in automotive and consumer electronics, industrial

equipment, and almost all appliances or toys with an electronic

user interface.

2. Devices, such as cell phones and music synthesizers, can

incorporate several megabytes of SRAM.

2)ROM (read-only memory): Computers almost always

contain a small amount of read-only memory that holds

instructions for starting up the computer. Unlike RAM, ROM

cannot be written to.

TYPES OF ROM:1) PROM(programmable read-only memory): A PROM is a

memory chip on which you can store a program. But once the

PROM has been used, you cannot wipe it clean and use it to store

something else. Like ROMs, PROMs are non-volatile.

2) EPROM(erasable programmable read-only memory): An

EPROM is a special type of PROM that can be erased by

exposing it to ultraviolet light.

3)EEPROM(electrically erasable programmable read-only

memory): An EEPROM is a special type of PROM that can be

erased by exposing it to an electrical charge.

CACHE MEMORY:

Cache memory is random access memory (RAM )that a

computer microprocessor can access more quickly than it can

access regular RAM. As the microprocessor processes data, it

looks first in the cache memory and if it finds the data there

(from a previous reading of data), it does not have to do the more

time-consuming reading of data from larger memory.

Cache that is built into the CPU is faster than separate cache,

running at the speed of the microprocessor itself. However,

separate cache is still roughly twice as fast as Random Access

Memory (RAM). Cache is more expensive than RAM, but it is

well worth getting a CPU and motherboard with built-in cache in

order to maximize system performance.

Disk caching applies the same principle to the hard disk that memory

caching applies to the CPU. Frequently accessed hard disk data is

stored in a separate segment of RAM in order to avoid having to

retrieve it from the hard disk over and over. In this case, RAM is faster

than the platter technology used in conventional hard disks.

Main Memory

(RAM)

Cache

Memory

Processor(CPU)

SECONDARY STORAGE DEVICES

The primary memory stores all the information in the form of electric

signals. So whenever power fails or you switch off the computer all the

information that has been stored is lost. Therefore there was requirement of

some other memory medium where large data and information can be stored

permanently. This requirement of memory generated the secondary storage

devices or Secondary Memory of the computers. The Secondary Memory is

also known as Auxiliary Memory or Backup Memory.

TYPES OF SECONDARY STORAGE DEVICES:

i) Floppy disk

ii) Hard disk

iii) CDROM

iv) Magnetic tape

v) DVD

i) Floppy Disk: A floppy disk is a secondary storage device. It is a

circular piece of plastic material coated with particles, which are

magnetized. This thin plastic sheet is protected from outside by a plastic

cover to prevent the sensitive data stored on them. The commonly used

floppy disks are of 3.5 ” diameter. Floppies are used to store data and

transfer them from one computer to another. Due to their size and portability

they are the most popular storage mediums in offices and at homes.

The data inside the floppies are stored in tracks and sectors. The entire

floppy is divided into circular segments called tracks. Each track is given a

unique number. The outermost track is referred as 0 and the track inner to

them is 1 and so on. Each track is further divided into segments called

sectors. The number of segments in each track has the same capacity. In a

typical 3.5” floppy disk the number of tracks and sectors and their storage

capacity is denoted below:

1.44 MB = 1.474560 bytes = 512 bytes X 2 sides

ii) HARD DISKS: A hard disk is fixed inside the cabinet of CPU (Central

Processing Unit). It is made up of many rigid metal platters coated to store

data magnetically. The hard disk rotates while recording data. This rotation

speed is measured in the unit of revolutions per minute (rpm). The normal

speed of hard disks is 3600 revolutions per second. The read/write head of

the hard disks moves across its surface. The storage capacity of the hard

disks is many times more than the floppy disks.

The normal storage capacity of hard disks installed inside the Pentium-IV

machines now a day is 40 Gigabytes. Due to large storage capacity it is

preferred to store all important data into the hard disks of the computers.

The data stored in the hard disks are retrieved faster as compared to the

floppy disks as they are installed inside the computers.

diskDisk head

iii) CD-ROM (COMPACT DISK READ ONLY MEMORY)

Compact Disks most popularly known as CDs are the most attractive

storage device these days. They are Read only memory device as without

the CD writers we cannot store our files and folders on them. The diameter

of the CD ranges from 10 cms to 30 cms. A typical CD is of 13 cm diameter.

One CD can store 600 MB to 750 MB data on it. Laser reads the normally

used CDs. So once the data is stored into the CD ROM it cannot be erased.

Recently writable CDs are also introduced in the market but they are not so

common. The read/write speed of a CD-ROM is average access tune is 125

milliseconds.

CD comprises of a transparent thin layer of aluminium covering the pitted

surface for reflectivity. The data may be in the form of text, image, sound,

video clips, animated movies or their combinations. The CD Rom drive are

mostly used to keep back ups & access the data for reading.

iv) Magnetic Tape: Magnetic Tape is a recording medium consisting of

a thin tape with a coating of a fine magnetic material, used for recording

analogue or digital data. A device that stores computer data on magnetic

tape is a tape drive. The capacity of tape media are generally on the same

order as hard disk drives . Magnetic Tapes Generally transfer data a bit

slower than hard drives, however magnetic tapes are cheaper and are more

durable

Organization of data on disks:

I)TRACKS: A circle on the disk surface on which data are contained.

II)SECTORS:1. That portion of a track falling along a predefined pie-shaped portion of

the disk surface.

2. The number of bytes stored in a sector is the same, regardless of where

the sector is located; thus, the density of bits is greater for sectors near the

centre of the disk

3. The rotational speed is constant; i.e., constant angular velocity

4. Thus, the transfer rate is the same for inner sectors and outer sectors

III) CYLINDERS: A set of tracks simultaneously accessible from the

heads on the head assembly.

IV) HEADS:A transducer attached to an arm for writing/reading data

to/from the disk surface

Head assembly: A mechanical unit holding the heads and arms. All the

head/arm units move together, via the head assembly

42Figure 11.1 Moving-head disk mechanism (HDD)

Access time : The access time or response time of a rotating drive is

a measure of the time it takes before the drive can actually transfer data. The

factors that control this time on a rotating drive are mostly related to the

mechanical nature of the rotating disks and moving heads. It is composed of

a few independently measurable elements that are added together to get a

single value when evaluating the performance of a storage device. The key

components that are typically added together to obtain the access time are:

1. Seek time

2. Latency time

1. Seek Time: The seek time measures the time it takes the head assembly

on the actuator arm to travel to the track of the disk where the data will be

read or written.[7] The data on the media is stored in sectors which are

arranged in parallel circular tracks and there is an actuator with an arm that

suspends a head that can transfer data with that media. When the drive needs

to read or write a certain sector it determines in which track the sector is

located.

It then uses the actuator to move the head to that particular track. If the

initial location of the head was the desired track then the seek time would be

zero. If the initial track was the outermost edge of the media and the desired

track was at the innermost edge then the seek time would be the maximum

for that drive.

2. Latency Time: It is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head. It depends on the rotational speed of a disk , measured in revolutions per minute (RPM).For most magnetic media-based drives, the average rotational latency is typically based on the empirical relation that the average latency in milliseconds for such a drive is one-half the rotational period. Maximum rotational latency is the time it takes to do a full rotation excluding any spin-up time . Therefore the rotational latency and resulting access time can be improved (decreased) by increasing the rotational speed of the disks.

45

Seek Time Latency Time Transfer Rate

Desired

track

Seek

Head

TransferLatency

Note: Access time = seek time + latency