Table of Contents
Chapter One
7
1. Overview of Computer science
7
1.1. Introduction to Information and Communication Technology (ICT)
7
1.2. Data versus Information
7
1.3. Definition of Computer and Computer Science
8
1.4. Characteristics of computers
9
1.5. Types of computers
10
Uses of Mainframes
18
1. Desktop Computers
22
Single Unit Systems
23
Net top Systems
23
Single Board Computers
24
Thin Clients
25
2. Mobile Computers
26
Laptops
27
Ultra books
27
Chrome books
27
Net books
28
Tablets
28
Smart phones
Chapter 2: Development of Computers
2.1 History of Computers
29
32
32
Mainframes to PCs The 1960s saw large mainframe computers become much more
common in large industries and with the US military and space program. IBM
became the unquestioned market leader in selling these large, expensive, error-prone,
and very hard to use machines.
40
2.2 Generation of Computers
41
2.3. Current trends
46
Chapter Five
48
3. ORGANIZATION OF COMPUTER SYSTEMS
48
3.1. Introduction to Computer Systems
48
3.2. Computer hardware
48
How the CPU Executes Program Instructions
60
3.3. Computer software
61
Chapter Four
64
4.Data Representation in Computers
64
4.1. Units of Data Representation
64
4.2. Concept of Number Systems and Binary Arithmetic
65
4.2.1 The Decimal Number System
65
4.2.2. The Binary number system
66
4.2.3 Octal number system
66
4.2.4. Hexadecimal number system
66
4.3. Conversion between Number Systems
66
4.3.1 Binary to Decimal
68
Binary (base2) to Octal (base 8) or hexadecimal (base16) and vice versa
69
4.3.2. Octal to hexadecimal and Vise versa
70
4.3.3. Converting Decimal Number with Fractions to Binary
70
4.3.4. Converting Binary with Fraction to Decimal
70
4.3.5. Conversion from Binary with Fraction to Octal/Hexadecimal
71
4.3.6 Conversion from Octal or Hexadecimal with Fraction to Binary
71
4.3.7. Conversion from Octal with Fraction to Hexadecimal
71
4.3.8. Conversion from Hexadecimal with Fraction to Octal
72
4.3.9. Conversion from Octal/Hexadecimal with Fraction to Decimal.
72
4.4. Binary Arithmetic
72
4.5. CODING METHODS
74
4.6. Unicode
79
4.7. Representation of Negative Numbers and Arithmetic
79
Chapter Five
87
5.Data Communications and Computer Networks
87
5.1. Basics of Data Communication
87
5.2. Data Transmission
88
5.3 Mode of transmission
89
5.4. Data Transmission Protocols
91
5.6. Computer Networks
92
5.6.1 Basic of Networking
92
5.7. Types of Networks
94
5.8. Network Topology
99
• Protocol: Specifies a common set of rules and signals the computers on the
network use to communicate. Protocols define the format, timing, sequence, and error
checking used on the network.
99
Advantages of Bus Topology
100
Disadvantages of Bus Topology
100
Advantage of Star Topology
101
Disadvantages of Star Topology
101
Figure. 5.10. Ring topology
102
4. Mesh Topology
103
Transmission Cables
104
• The kind of cable or other medium that is used to connect the various computers in
a network
104
•
Common types:
104
–
Twisted-pair cable,
104
–
Coaxial cable, and
104
–
Fiber optic cable
104
1.
Twisted-pair wire
104
2.
Coaxial cable
104
3.
Fiber Optic Cables
104
–
2km without the use of repeaters.
104
–
one fiber could replace hundreds of copper cables
104
5.9. Brief Introduction to Internet
105
About the Web
105
E-mails
107
Chapter Six
109
5. Computer Security
109
5.1 Introduction to Computer Security
109
6.2. Why Computer Security
110
6.3. Security Threats
110
1.
Fraud and Theft
111
2.
Loss of Physical and Infrastructure Support
111
3.
Malicious Hackers
111
4.
Threats to Personal Privacy
112
5.
Malicious Code
112
Viruses
112
Virus is self-duplicating computer program that interferes with a computer's
hardware or operating system (the basic software that runs the computer). Viruses
are designed to duplicate or replicate them to avoid detection. Like any other
computer program, a virus must be executed for it to function—that is, it must be
located in the computer's memory, and the computer must then follow the virus's
instructions. These instructions are called the payload of the virus. The payload may
disrupt or change data files, display an irrelevant or unwanted message, or cause the
operating system to malfunction.
112
There are five categories (types) of viruses, they are: parasitic or file viruses,
bootstrap sector, multi-partite, macro, and script viruses.
112
Worms
113
Worm is a program that propagates itself across computers, usually by spawning
copies of itself in each computer's memory. A worm might duplicate itself in one
computer so often that it causes the computer to crash. Sometimes written in separate
“segments,” a worm is introduced surreptitiously into a host system either for “fun”
or with intent to damage or destroy information. The term comes from a sciencefiction novel and has generally been superseded by the term virus. Worms can form
segments across a network and damage the network by using its resources (memory
space) highly. The segments of worms across a network can communicate strengthen
their damage.
113
Trojan Horses
114
There are other harmful computer programs that can be part of a virus but are not
considered viruses because they do not have the ability to replicate. These programs
fall into three categories: Trojan horses, logic bombs, and deliberately harmful or
malicious software programs that run within Web browsers, an application program
such as Internet Explorer and Netscape that displays Web sites.
114
A Trojan horse is a program that pretends to be something else. A Trojan horse may
appear to be something interesting and harmless, such as a game, but when it runs it
may have harmful effects. The term comes from the classic Greek story of the Trojan
horse found in Homer’s Iliad.
114
Bombs
114
A bomb infects a computer’s memory, but unlike a virus, it does not replicate itself.
A logic bomb delivers its instructions when it is triggered by a specific condition,
such as when a particular date or time is reached or when a combination of letters is
typed on a keyboard. A logic bomb has the ability to erase a hard drive or delete
certain files.
114
6.4. Techniques to Reduce Security problems
Virus Detection
114
117
Chapter One
1. Overview of Computer science
1.1. Introduction to Information and Communication Technology (ICT)
1.2. Data versus Information
Data:




Simple facts and figures
It is raw and unprocessed and therefore meaningless to us.
Usually it is the result of experience , observation or experiment
Numbers, characters, symbols, images etc., which can be processed by a
computer.
 Data must be interpreted, by a human or machine, to derive meaning
Information:





is data that has been processed for use
is a human interpretation of knowledge
It is processed and selected so that it is useful and applicable
has meaning in some context for its receiver
When information is entered into and stored in a computer, it is generally referred
to as data. After processing (such as formatting and printing), output data can
again be perceived as information.
 When information is packaged or used for understanding or doing something, it is
known as knowledge. For example, there are a plenty of items listed on a MenuCard, in a hotel (data). You don't order everything, you just order the dish you
want to eat, (information).
Technology
 Is a way of solving problems by the application of knowledge from multiple
disciplines.
An ICT is a set-up consisting of hardware, software, data and the people who use them. It
commonly includes communications technology, such as the Internet.
ICT and computers is not the same thing. Computers are the hardware that is often part of
an ICT system. ICT Systems are used in a number of environments, such as: offices,
shops, factories, aircraft, and ships. They're also used in fields such as: communications,
medicine, and farming.
The importance of ICT systems are:







more productive - we can complete a greater number of tasks in the same time at
reduced Cost by using computers than we could prior to their invention
able to deal with vast amounts of information and process it quickly
able to transmit and receive information rapidly
The three main types of ICT systems
Information systems (are focused on managing data and information.)
Control systems (mainly control machines).
Communications systems (transport data from one place to another).
1.3. Definition of Computer and Computer Science
A computer is a programmable machine designed to perform arithmetic and logical
operations automatically and sequentially on the input given by the user and gives the
desired output after processing.
Computer components are divided into two major categories namely hardware and
software. Hardware is the machine itself and its connected devices such as monitor,
keyboard, mouse etc. Software is the set of programs that make use of hardware for
performing various functions.
What is computer Science?
Computer Science is the study of computers and computational systems. Unlike electrical
and computer engineers, computer scientists deal mostly with software and software
systems; this includes their theory, design, development, and application.
Principal areas of study within Computer Science include artificial intelligence, computer
systems and networks, security, database systems, human computer interaction, vision
and graphics, numerical analysis, programming languages, software engineering,
bioinformatics and theory of computing.
Although knowing how to program is essential to the study of computer science, it is only
one element of the field.
Computer scientists design and analyze algorithms to solve programs and study the
performance of computer hardware and software.
The problems that computer scientists encounter range from the abstract-- determining
what problems can be solved with computers and the complexity of the algorithms that
solve them – to the tangible – designing applications that perform well on handheld
devices, that are easy to use, and that uphold security measures.
1.4. Characteristics of computers
The characteristics of computers that have made them so powerful and universally
useful are speed, accuracy, diligence, versatility and storage capacity. Let us discuss them
briefly.
Speed
Computers work at an incredible speed. A powerful computer is capable of performing
about 3-4 million simple instructions per second.
Accuracy
In addition to being fast, computers are also accurate. Errors that may occur can
almost always be attributed to human error (inaccurate data, poorly designed
system or faulty instructions/programs written by the programmer)
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.
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.
Storage Capacity
Today’s computers can store large volumes of data. A piece of information once
recorded (or stored) in the computer, can never be forgotten and can be
retrieved almost instantaneously.
1.5. Types of computers
These days, computers are available in many sizes and types. You can have a computer
that can fit in the palm of your hand to those that can occupy the entire room; single user
computers can be used by hundreds of users simultaneously. Computers also differ on
their data processing abilities. Hence, computers can be classified according to purpose,
Basis of Work (processing), and Size.
Figure 1.1 Types of computer
Classification Basis of Work
Different types of computers process the data in a different manner. According to the
basic data handling principles, computers can be classified into three categories: analog,
digital, and hybrid.
1. Analog Computer:
The computer that work with natural phenomena and physical values like earthquake
measurement, speed of the wind, weight light etc is known as Analog computers. It is
especially used in scientific work, medical and industrial field. These are special purpose
computers. It measures physical values such as temperature or pressure that fall along a
continuous scale in temperature or pressure. For example, you can see a system on petrol
pump that contains an analog processor and analog device that converts flow of petrol
into quantity. Speedometer in cars and your watch are other examples of analog
computer.
Examples:
 Thermometer
 Voltmeter
 Speedometer
 Gasoline pomp
Features of Analog computer





It is specific to the particular task so we cannot use
it works for multiple applications.
It works on continuous data and gives continuous output.
It works on real time and no storage capacity.
It gives in the form of a graph, signals, table etc
Figure 1.2 Analog Computers
2. Digital Computer:
A computer that work with digital value 0 and 1. Where 0 is OFF and 1 is ON. It works
with discrete data. Digital computer does not measure the continuous data for continuous
output. Most of the electronic system is based on the digital system. Digital computers
are very popular for actual computers are very popular for actual computer works like
preparation of the report, documentation, billing and other graphical work etc. The entire
PC (Personal Computer) used today on different fields are digital computers.
Examples:
 Abacus
 Desk top & pocket computers
 The general purpose computers
Features of Digital computer
 It works on the discontinuous or discrete data.
 It is applicable for the general purpose so this is very versatile for the application.
It is based on the digits 0 and 1.
 It is faster. It has storage section also.
 It is highly accurate and reliable than an analog system.
Figure 1.3 Digital Computer
3. Hybrid computer:
It is the combination of analog and digital computer system. It works with continuous and
discrete value. The good qualities of analog and digital computers are combined on this
computer and made the hybrid computer. These are used in ICU (Intensive Care Unit) of
the hospital, jet planes, and other data analysis terminals. Hybrid computer transfers the
data from analog to digital and digital to analog and vice-versa.
Example:
Intensive Care Unit (ICU) section, in the hospital, uses analog devices to measure the
patient’s heart function, temperature and other vital signs. These measurements may then
be converted into numbers and supplied to a digital component in the system. This
component is used to monitor the patient’s vital signs and to send an immediate signal to
the nurse’s station if any abnormal readings are detected.
Features of hybrid computer




It is an expensive system.
It is designed for a special purpose so it is not versatile.
It works on discrete and continuous value.
It has limited storage.
It is complex than other computer systems.
Figure 1.4 Hybrid Computers
Classification According to Purpose
Computers can be applied or used for different purposes. They can be used either for
special
1. Special Purpose Computers
They are designed to solve a single type of problem, that is, their components and
function are uniquely adapted to a specific situation involving specific application.
These computes cannot be used for other applications unless their circuits are redesigned,
that is, they lacked versatility. However, being designed for specific tasks, they can
provide the result very quickly and efficiently.
Example:
 The public telephone box
 Traffic control system
 Ticket machines (used in grocery, super market etc.)
 Pocket calculators etc.
 Counters
Most analog computers are special purpose computers.
2. General-purpose computers
They are designed to solve a range of problems through the use of “store program
concept”.
These machines can be used for various applications, ranging from scientific as well as
business purpose applications. Though such computers are versatile and flexible, they
generally lack in speed and efficiency.
Examples: Micro, Mini Mainframe and Super computers
Classification Based on Size
The different types of computers can be grouped into six major categories according to
size. Each category excels at specific functions.

Supercomputers

Mainframes

Mini-computers

Servers

Personal computers

Embedded systems
Generally, sizes of computers determine the processing abilities. Larger computers have
higher processing speeds while smaller ones offer the better experience for personal
computing.
1. Supercomputers
These are arguably the most powerful in terms of speed and accuracy. They are types of
computers used in solving complex mathematical computations. They are capable of
executing trillions of instructions per second, which is calculated in floating point
operations per second (FLOPS).
The typical personal computer used at home and the office is only capable of calculating
millions of instructions per second (MIPS). Supercomputers can go even faster with the
rate of peta FLOPS (or PFLOPS). This could bring up their processing numbers up to the
quadrillion.
Supercomputers were made popular in the 1960s by Seymore Cray. They soon became
the choice for complex projects. They have evolved from the grid to cluster systems
of massively parallel computing.
Cluster system computing means that machines use multiple processors in one system,
rather than arrays of separate computers in a grid.
The operating systems that run in supercomputers vary depending on the manufacturer
but are generally based on the Linux Kernel. A few popular ones include,

CNK OS used in Blue Gene from IBM

Cray Linux Environment used in Titan

Sunway Raise OS in Sunway TaihuLight
These computers are the largest in terms of size. They can occupy anything from a few
feet to hundreds of feet. They also don’t come cheap as they can be priced between
$200,000 to over $100 million.
Year
Name of
Supercomputer
Manufacturer
Speed in
PFLOPS
2008
Roadrunner
IBM - USA
1.105
2009
Jaguar
Cray - USA
1.759
2010
Tianhe - 1A
NUDT - China
2.566
Year
Name of
Supercomputer
Manufacturer
Speed in
PFLOPS
2011
K Computer
Fugitsu - Japan
10.51
2012
Titan
Cray - USA
17.59
2013
Tianhe – 2
NUDT - China
33.86
2014
Tianhe – 2
NUDT - China
33.86
2015
Tianhe – 2
NUDT - China
33.86
2016
Sunway TaihuLight
NSC -China
93.01
2017
Sunway TaihuLight
NSC -China
93.01
Table 1.1 the Top Supercomputers Since 2008
Figure 2.5 Tianhe-2 was the fastest supercomputer in 2013 - 2015
Uses of Supercomputers
Because of their superiority, supercomputers are not intended for your everyday tasks.
They handle exhaustive scientific applications that require complex and real-time
processing.










In the field of science, researchers use these machines to compute and model
properties of biological compounds like protein and human blood.
They are also used to interpret new diseases and strains, and predict illness behavior
and treatment.
The military use supercomputers to test new aircraft, tanks, and a host of weaponry
and camouflage.
They also use them to understand the effects they will have on soldiers and wars.
These machines are also used to help encrypt and decrypt sensitive data.
In entertainment, supercomputers are used to help make a flawless online gaming
experience. Games like World of Warcraft demand intense processing. When
thousands of gamers around the world are playing, supercomputers help stabilize the
game performance.
Meteorologists use them to simulate weather behavior. They can also be used to
predict earthquakes.
Scientists use them to simulate and test the effects of nuclear weapon detonation.
Scientists also use them to simulate the events of the Big Bang and other space
related projects.
Hollywood uses supercomputers to create realistic animations.
The famous supercomputers Deep Blue and Watson defeated chess Grandmaster
Gary Kasparov and quiz expert Ken Jennings respectively.
Mainframes
Mainframe computers are large sized computer types. They are equally powerful but fall
short in terms of the computation ability in supercomputers. They are like big file servers,
enabling multiple users from nearby and remote locations to access resources at the same
time. Also known as big iron, these systems can handle massive amounts of data going in
and out simultaneously. This makes them popular with businesses.
They are also resilient as they are capable of operating for over 10 years without failing.
Users access the mainframe using terminals or personal computers. This can happen
within the same building or via wide area network (WAN).
Most of these systems run the z/OS (operating system) on 64bit architecture.
Figure 2.6-IBM System z9 mainframe is a large size computer type
Uses of Mainframes
They are used in large organizations where thousands of clients have to access data
simultaneously.
For examples:

Performing ATM cash withdrawals and deposits. During the process,
communication between the mainframe and remote computer will help accomplish
the financial transactions at hand.

Business transactions that use credit cards or pre-paid cards.

Online electronic transactions.

Cloud storage.

Handling of patient records in major hospitals.

Making reservations and travel schedules for airline companies.

Manipulation and tallying of data for census and electoral purposes.
The prices of mainframe computers, especially from IBM, start at $75,000 and can go up
to $1 million.
System z9, Fujitsu-ICL VME and Hitachi’s Z800 are examples of Mainframes.
Minicomputers
Minicomputers are general purpose devices without the monumental expenses associated
with a larger system. Their processing power is below that of mainframe systems but
above the capabilities of personal computers.
Also known as mid-range computers, these became popular in the late 1960s but have
become almost extinct because of the popularity of personal computers. The latter can
now perform most of the tasks reserved for minis.
The first minicomputer was unveiled in 1967 by Digital Equipment Corporation and was
followed later by designs from IBM and other companies.
They became popular for control related functions as opposed to computing prowess.
Over the years, their usage was limited to dedicated control assignments in mid-range
organizations.
Figure 2.7- Prototyp 1990, MicroVAX II Clone Minicomputer is a mid-sized computer
Minicomputers were intended for a number of activities listed below:

Switchboard control.

Dedicated applications for graphics and computer design.

Time-sharing, to allow multiple users to interact concurrently on a single system.

Control and monitoring of manufacturing activities.

Monitoring and control of laboratory equipment.
Texas Instrument TI-990, K-202 and MicroVAX II are examples of minicomputers.
Servers
These are types of computers used to provide resources, services, and functionality to
client computers in a server-client network model. Resources provided are based on the
functions of a particular server, which may fall under these categories:

File server

Database server

Print server

FTP servers

Application server

Web server
Their sizes will depend on purpose and tasks in the network. Of course bigger and more
multitasking installations will require multiple system and storage installation.
A common errant is that desktop systems can be used as servers. Far from it, true server
systems are specialized computers with abilities far beyond what personal computers can
deliver.
Servers are optimized to run 24 hours and are capable of hot swapping of storage and
other hardware without having to shut down the system.
Figure 2.8- Hot swap disk drive bay in a server system
Microcomputers/Personal Computers
Microcomputers are the smallest, least expensive and the most used types of computers.
They have a small memory, less processing power, are physically smaller, and permit
fewer peripherals compared to super and mainframe computers. They are more
commonly known as personal computers or simply PCs. The term was initially used to
refer to IBM compatible computers.
They became popular in the 70s and 80s, at the dawn of the microprocessor chips. These
chips meant that a machine used by one individual was now feasible.
The advent of PCs meant cheaper alternatives to more expensive and centralized systems.
They were more affordable for office use and created cheaper networking environments.
By the mid-1990s, they became the de facto computer of choice for offices and homes.
The last 20 years have seen the proliferation of even smaller systems.
This signaled the start of the mobile age, which continued to go with the trend of smaller
devices as the new century progressed. This ultimately gave birth to wearable computers
and gadgets.
The operating system used in personal computers vary, but the common ones include,

Windows

Mac OS X

Linux

IOS

Android
Categories of personal computers include:

Desktop computers

Mobile computers

Wearable computers
1. Desktop Computers
Desktop computers are made up of separate components such as:

The system unit; a rectangular case that contains important parts like the
motherboard, microprocessor, memory modules, disk drive, and optical drive.

The monitor.

A mouse.

A keyboard.
Figure 2.9- A typical desktop computer fall under the small size computer that sit on desk
Single Unit Systems
Single unit computers, also known as all-in-one PCs, are a sub-type of desktop
computers. They integrate the monitor and system unit within a single unit.
They also have connectivity to a mouse, keyboard, and other peripherals, usually through
USB ports.
Figure 2.10- All in one single unit computer
Net top Systems
Net top, which are sometimes called mini PCs, are small and cheap system units. They
use less power and perform less processing.
Common features of Nettops include the Intel Atom microprocessor, 1-2 GB memory,
and Wi-Fi connectivity.
Just like any other desktop, they attach to peripheral accessories via USB ports and the
monitor via VGA or DVI ports.
Figure 2.11-A net top computer
Single Board Computers
These are the smallest possible computers which mimic the shape and functionality of
full-size desktop motherboards. They fit on miniature circuit boards, the size of an ATM
card and spot numerous input/output ports for connectivity to external peripherals.
Standouts are USB ports for a keyboard and mouse, HDMI output to monitors, Ethernet
ports, and Bluetooth/wireless capability.
A single board computer (SBC) is an integrated piece of hardware which is called so
because it only spots one board, unlike the desktop computer which features additional
circuitry like memory chips and processor.
It is also a low power, fan-less circuitry, low-cost system, and popular with hobbyists and
developers.
An SBC can easily be confused with an embedded system because of its size but is not,
because it permits general purpose functionalities synonymous with microcomputers.
Raspberry Pi3, Arduino and BeagleBone Blue are popular examples of SBC
Figure 2.12- Raspberry Pi2 Single Board Computer (SPB)
Thin Clients
These are low-cost computer types which rely on server systems in order to provide
computing services to attached monitors. They communicate to the server via the remote
desktop protocol and are part of the networking implementation setup known as
client/server model.
While a thin client depends entirely on the availability of a server, a desktop based client
(the typical desktop computer), sometimes called fat client, can operate independently of
a server in case of transmission downtime.
A typical thin client features most input/output ports for connectivity to peripherals.
Stand out are VGA or DVI ports to the monitor, PS/2 or USB ports for keyboard and
mouse, and audio input/output ports.
Figure 2.13 -Ncomputing thin client
2. Mobile Computers
Mobile devices have become the norm in recent years. Most users opt for laptops and
tablets due to ease of use on the go, and battery power.
Particular features that make mobile systems a favorite include:

Extended battery use.

Wi-Fi capabilities.

Mobility.
The most common types of mobile computers include:

Laptop computers.

Tablets.

Smartphones.

Personal Digital Assistants (PDA).
Figure 2.14 A 17 inch laptop and 10 inch netbook side by side.
Laptops
Laptops are lightweight mobile PCs with a thin screen. They were initially called
notebook computers because of their small size. They operate on batteries.
Unlike desktops, these systems combine the microprocessor, screen, and keyboard in a
single case. The screen folds down onto the keyboard when not in use.
Ultra books
Ultra books are special laptops specifically designed to be thin and lightweight. They
usually have longer lasting batteries (5 hours minimum) and have strong hardware and
processing power to run any software around.
Ultra books also ship with the faster SSD storage in place of the slower hard disk drives
that are commonly used.
Chrome books
Chrome books are low-end laptops that only runs the web-based Chrome operating
system. After the installation of Chrome OS, additional software can only be installed via
the Chrome Web Store.
The OS allows you to achieve traditional PC functionality online. You can type
documents, edit them, implement group discussions, have teleconferencing, and use basic
online tools like search engines and e-mail.
These devices are increasingly targeted for users that spend most of their time online for
social activities. Their hardware includes the Intel Atom microprocessor, Wi-Fi and
wired network connectivity, solid state disks (SSD), and an average of five hours of
battery life. They usually do not have optical drives.
Net books
Net books can be thought of as mini laptops. They are smaller in size, price, and
processing power. Just like Chrome books, they are primarily designed for web browsing,
electronic communication, and cloud computing. They are catered to users who require
less powerful client computers.
Their specs are similar to Chrome books. The biggest difference is that they can run the
lightweight Linux operating system.
Tablets
A tablet is a mobile computer equipped with a touch screen or hybrid screen, which
allows the user to operate it by use of a digital pen or fingertip.
Most tablets today are both multi-touch and multi-tasking, making it possible to
manipulate them using multiple fingers and accomplishing multiple tasks simultaneously.
Tablets are handy, especially when normal notebooks and laptops are simply too bulky
for the mobile user.
Figure-2.15 Tablet
Smart phones
Back in 1996, a company called Palm Computing developed a gadget called Palm 1000.
It was revolutionary in conception but did not actually build consumer excitement.
While the idea of a miniaturized computer was not new, the fact that someone had
actually been able to make a device with an operating system that could work within its
limitations was a huge leap forward. It was one of the biggest innovations in the tech
industry.
The iPhone, released in 2007, was the first true smartphone. It became an instant hit with
consumers worldwide. It started the smartphone industry that still persists today.
Most smartphones today use an operating system such as IOS and Android. They often
have the ability to add applications. This is in contrast to regular cellular phones which
only support sandboxed applications like Java games. In terms of features, smartphones
support full email capabilities as well as multiple functions to serve as a complete
personal organizer.
Depending on the manufacturer, other functions might include additional interfaces such
as miniature QWERTY keyboards, touch screens, built-in cameras, contact management,
built-in navigation software, ability to read office documents in PDF and Word file
formats, media software for playing music, browsing photos, and viewing video clips.
Embedded Systems
These are computer-based systems which are standalone electronic hardware designed to
perform dedicated computing tasks. They are not general purpose installations like the
personal computer. Actually, they are computers which may not always seem to be
computers!
They include a combination of the outer hardware, microprocessor chip, and software.
The core of such systems is the microprocessor or micro-controller which execute the
assigned task.
The embedded software, usually firmware, is but not always fixed onto volatile memory
which may not always require post-installation configurations. In any case, the hardware
does repetitive assignments.
The old cell phones used well before the smartphones became a phenomenon, could
easily fall under the category of embedded systems since their sole purpose was to make
and receive calls. Smartphones today, however, have evolved into general purpose
mobile computers.
Firmware on these systems is written in the read-only memory (ROM) or flash memory
chips. Despite the seemingly persistent firmware which is deemed unaltered, they can be
re-programmed to suit evolving demands.
Popular devices that may be categorized under embedded systems are listed below:

Set-top boxes

MP3 players

DVD players

Drones

Printers

Antilock braking systems

USB devices like internet dongles

Streaming players like Google Chromecast and Roku

Thermostats

Calculators

Toys

Digital cameras

ATM machines

Video game consoles

Routers and network peripherals

Computer add-on cards and peripherals

Digital watches
Some useful technical terms
1. The computer which possess continuous data. (Analog computer)
2. The computers which are used in offices for general purpose. (Personal computer)
3. The fastest and most expensive computer system. (Super computer)
4. The computers that use the microprocessor as their CPU. (Microcomputer)
5. A computer that uses analog and digital device. (Hybrid computer)
6. Picture element on the monitor. (Pixel)
7. The computer that uses discrete on/off system. (Digital computer)
8. The portable computer which can be used where these is on direct electricity. (Laptop
computer)
9. The original computers developed by IBM Company. (IBM PC)
10. The device which converts analog signal to digital and vice-versa. (Modem)
Chapter 2: Development of Computers
2.1 History of Computers
We have all heard stories of primitive peoples counting their sheep by moving sticks or
stones. Our base-ten number system undoubtedly grew from the use of 10 fingers as
counting objects.
Together with the development of people, the need to calculate and keep track of
information had become popular issue. So they soon develop a simple computing device
and had a power of storing small information. However, many thousands of years elapsed
before developing mechanical calculator.
Some of the calculating devises are mentioned bellow:
a) The Abacus: - It is one of the earliest mechanical computational devices. It was in
use in the Middle East as early as 2500 BC. The familiar Chinese abacus (dating
approximately 1200 AD ) is composed of a frame and a number of wires. The wires
correspond to position of digits in decimal number units: tens, hundreds, and so onand the beads represent digits. Beads above the cross bar represent 5 and that bellow
represent 1.
The abacus shows zero, if all the bead bellow the cross bar are at the lower frame and
above are at the upper frame.
Addition of two numbers on the abacus can be performed by representing the first
number and the second number without resetting the first. On any wire showing 10 or
more, the two beads above the cross bar are moved back, and an extra 1 (the Cary) is
added two the wire on the left.
This process can be easily generalized to addition and subtraction of more than two
numbers.
Figure 2.2. Abacus Parts
b) Pascal’s Calculator: - It is the first true mechanical calculator. In 1642, at the age of
19, the French philosopher and mathematician Blaise Pascal developed a rotating
wheel calculator, the predecessor of the latter popular desktop calculator. He built
largely to assist his father, who was a tax collector in the town of Rouen, Pascal’s
calculator has one wheel corresponding to each power of 10; each wheel has 10
position, one for each of the digits (0,..9). Although, Pascal’s calculator could only
add and subtract, it could be used indirectly for multiplication (by successive
addition) and division (by successive subtraction) as well.
Figure 2.3. Pascal’s Calculator
c) The Difference Engine: - It is the forerunner of the modern computer. Charles
Babbage (1792-1871), a British mathematician and engineer, is considered by many
to be the real father of today’s computer was the developer of the difference engine
and designer of the analytical engine. The difference engine also based on the rotating
wheels principle and it was operated by means of a single crank. This devise has a
power of calculation and print the output without human intervention. He finally
designed significantly improved version of the difference engine (but not built) called
Analytic engine. It has different key components
- The store: A memory wheel consisting of set of counter wheels
- The mill: An arithmetic unit capable of performing the four basic arithmetic
operations. It operated on pairs of mechanical registers and produced a result
stored in another register, all of which were located in the store.
- Operation cards: These cards selected one of the four arithmetic operations by
activating the mill to perform the selected function.
- Variable cards: These cards selected the memory locations to be used by the mill
for a particular operation (a source of operand and the destination of the result).
- Out put: was to print or a card punch device.
But finally the design halt largely due to the technology of the day is not far enough
too supply the required raw materials.
Figure 2.4. Proposed Difference Engine
d) Herman Hollerith’s Tabulating Machine: - Herman Hollerith was a statistician that
in 1880 and develop his machine commissioned by the U.S. Census Bureau to
develop a technique for speeding up the processing of census data that took at least 8
years before. He develops his machine that uses the punched card to punch the census
data and tabulated by using his machine. This machine processes the 1890 American
census data within 3 years. It was really a great development. He finally began the
tabulating Machine Company, which later becomes the International Business
Machine Corporation (IBM).
Figure. 2.5. Herman Hollerith’s Tabulating Machine
e) Mark I: - Developed by Howard Aiken at Harvard University (1944) which was the
first electromechanical computer. Instruction was provided by means of punched
paper tape, which combined the functions of Babbage’s operation cards and variable
cards. Each instruction had the format A1 A2 OP where A1 and A2 are registers
storing the operand, OP is the operation to be performed (e.g. +, -, x, ÷). Mark I could
do a single addition in 6 seconds and division in 12 seconds.
Figure. 2.6. Mark I Howard Aiken at Harvard University (1944)
f) ENIAC (Electronic Numerical Integrator And Computer): - Developed by Eckert
and Mauchly at the university of Pennsylvania. This was the first electronic calculator
and first general purpose digital computer. This machine was enormous, weighing 30
tones. Occupying 15,000 square feet of floor space and containing over 18,000
vacuum tubes. When operating, it consumed over 140 KWPH of power. It had a
capability of performing 5,000 additions per second. Its memory consisted of 20
“accumulators” each capable of holding a 10 digit decimal number. Each digit was
represented by a ring of 10 vacuum tubes. At any time, only one of the 10 tubes was
in ON state, representing one of the 10 digits.
 ENIAC did not use internally stored programs. Programs were wired on boards
similar to a telephone switch board.
 One of the major drawbacks of ENIAC was that it had to be programmed
manually by setting switches and plugging and unplugging cables.
Figure 2.7. ENIAC Computer
The first substantial computer was the giant ENIAC machine by John W. Mauchly and J.
Presper Eckert at the University of Pennsylvania. ENIAC (Electrical Numerical
Integrator and Calculator) used a word of 10 decimal digits instead of binary ones like
previous automated calculators/computers. ENIAC was also the first machine to use more
than 2,000 vacuum tubes, using nearly 18,000 vacuum tubes. Storage of all those vacuum
tubes and the machinery required to keep the cool took up over 167 square meters (1800
square feet) of floor space. Nonetheless, it had punched-card input and output and
arithmetically had 1 multiplier, 1 divider-square rooter, and 20 adders employing decimal
"ring counters," which served as adders and also as quick-access (0.0002 seconds) readwrite register storage.
The executable instructions composing a program were embodied in the separate units of
ENIAC, which were plugged together to form a route through the machine for the flow of
computations. These connections had to be redone for each different problem, together
with presetting function tables and switches. This "wire-your-own" instruction technique
was inconvenient, and only with some license could ENIAC be considered
programmable; it was, however, efficient in handling the particular programs for which it
had been designed. ENIAC is generally acknowledged to be the first successful highspeed electronic digital computer (EDC) and was productively used from 1946 to 1955.
A controversy developed in 1971, however, over the patentability of ENIAC's basic
digital concepts, the claim being made that another U.S. physicist, John V. Atan as off,
had already used the same ideas in a simpler vacuum-tube device he built in the 1930s
while at Iowa State College. In 1973, the court found in favor of the company using Atan
as off claim and Atan as off received the acclaim he rightly deserved.
g) The Von Neumann Machine: - The task of entering and altering programs for the
ENIAC was extremely tedious. Von Neumann was the consultant on the ENIAC
project and forward the stored program concept, i.e. designing the computer to get its
instruction by reading them from memory alongside the data and a program could be
set or altered by setting the values of a portion of a memory. Based on this concept,
the first true electronic computers were developed by the name EDVAC (Electronic
Discrete Variable Automatic Computer) and EDSAC (Electronic Delay Storage
Automatic Computer).
Figure.2.12. Von Neumann Machine Architecture
h) Commercial Computers: - The 1950s saw the birth of computers industry with two
companies, Spery and IBM, dominating the market place. In 1947, Eckert and
Mauchly develop their successful commercial computer called UNIVAC I (Universal
Automatic Computer). UNIVAC was division of Remington Rand (later Sperry Rand
Corporation). IBM also the major manufacturer of punched card processing
equipment, delivered its first electronic stored program computer, the IBM 701, in
1953.
Figure.2.13. Commercial computers
Progression of Hardware
In the 1950's two devices would be invented that would improve the computer field and
set in motion the beginning of the computer revolution. The first of these two devices was
the transistor. Invented in 1947 by William Shockley, John Bardeen, and Walter Brattain
of Bell Labs, the transistor was fated to oust the days of vacuum tubes in computers,
radios, and other electronics.
 The vacuum tube, used up to this time in
almost all the computers and calculating
machines, had been invented by American
physicist Lee De Forest in 1906. The
vacuum tube, which is about the size of a
human thumb, worked by using large
amounts of electricity to heat a filament
inside the tube until it was cherry red. One
result of heating this filament up was the
release of electrons into the tube, which
could be controlled by other
Vaccum Tubes
Figure 2.11.vacum tubes
elements within the tube. De Forest's original device was a triode, which could
control the flow of electrons to a positively charged plate inside the tube. A zero
could then be represented by the absence of an electron current to the plate; the
presence of a small but detectable current to the plate represented a one.
Transistors
 Vacuum tubes were highly inefficient,
required a great deal of space, and needed to be
replaced often. Computers of the 1940s and 50s
had 18,000 tubes in them and housing all these
tubes and cooling the rooms from the heat
produced by 18,000 tubes was not cheap. The
transistor promised to solve all of these problems
and it did so. Transistors, however, had their
problems too. The main problem was that
transistors, like other electronic components,
needed to be soldered together. As a result,
Figure. 2.12. Transistors
the more complex the circuits became, the more complicated and numerous the
connections between the individual transistors and the likelihood of faulty wiring
increased.
 In 1958, this problem too was solved by Jack St. Clair Kilby of Texas
Instruments. He manufactured the first integrated circuit or chip. A chip is really a
collection of tiny transistors which are connected together when the transistor is
manufactured. Thus, the need for soldering together large numbers of transistors
was practically nullified; now only connections were needed to other electronic
components. In addition to saving space, the speed of the machine was now
 Increased since there was a diminished distance that the electrons had to follow.




Mainframes to PCs
The 1960s saw large mainframe computers become much more common in large
industries and with the US military and space program. IBM became the
unquestioned market leader in selling these large, expensive, error-prone, and
very hard to use machines.
A veritable explosion of personal computers occurred in the early 1970s, starting
with Steve Jobs and Steve Wozniak exhibiting the first Apple II at the First West
Coast Computer Faire in San Francisco. The Apple II boasted built-in BASIC
programming language, color graphics, and a 4100 character memory for only
$1298. Programs and data could be stored on an everyday audio-cassette recorder.
Before the end of the fair, Wozniak and Jobs had secured 300 orders for the Apple
II and from there Apple just took off.
Also introduced in 1977 was the TRS-80. This was a home computer
manufactured by Tandy Radio Shack. In its second incarnation, the TRS-80
Model II, came complete with a 64,000 character memory and a disk drive to
store programs and data on. At this time, only Apple and TRS had machines with
disk drives. With the introduction of the disk drive, personal computer
applications took off as a floppy disk was a most convenient publishing medium
for distribution of software.
IBM, which up to this time had been producing mainframes and minicomputers
for medium to large-sized businesses, decided that it had to get into the act and
started working on the Acorn, which would later be called the IBM PC. The PC
was the first computer designed for the home market which would feature
modular design so that pieces could easily be added to the architecture. Most of
the components, surprisingly, came from outside of IBM, since building it with
IBM parts would have cost too much for the home computer market. When it was
introduced, the PC came with a 16,000 character memory, keyboard from an IBM
electric typewriter, and a connection for tape cassette player for $1265.
By 1984, Apple and IBM had come out with new models. Apple released the first
generation Macintosh, which was the first computer to come with a graphical user
interface(GUI) and a mouse. The GUI made the machine much more attractive to
home computer users because it was easy to use. Sales of the Macintosh soared
like nothing ever seen before. IBM was hot on Apple's tail and released the 286AT, which with applications like Lotus 1-2-3, a spreadsheet, and Microsoft Word,
quickly became the favourite of business concerns.
 That brings us up to about ten years ago. Now people have their own personal
graphics workstations and powerful home computers. The average computer a
person might have in their home is more powerful by several orders of magnitude
than a machine like ENIAC. The computer revolution has been the fastest
growing technology in man's history.
2.2 Generation of Computers
Although computer professionals do not agree on exact dates or specifics, computer
developments are often categorized by generations. Actually there are four generations
and major characteristics that distinguish these generations are the following:
 Dominant type of electronic circuit elements used.
 Major secondary storage media used.
 Computer language used.
 Types or characteristic of operating system used.
 Memory access time (time to store or retrieve a word or data from memory).
Computer generations are usually categorized by dramatic improvement in the hardware,
typically refold or better increases in speed and reliability.
First generation (1950s)
 Used vacuum tubes as components for the electronic circuit.
 Punched cards were the main source of inputs, and magnetic grams were used for
internal storage.
 Operate in a speed of milliseconds (thousands of a second) and could handle more
than 10,000 additions each second.
 Most applications were scientific calculations.
Figure.2.14. First Generation Computers
Second generations (early 1960s)
 Transistors were the main circuit components. (Transistors are a solid state device
made from silicon which is smaller, cheaper, faster, dissipate less energy and
more reliable than vacuum tube but work in the same way with the vacuum tube.)
 Invented by Bell Labs.
 Magnetic tapes (similar with home tape caste), used for main storage,
 Operate in microseconds (millionths of a second) with more than 200,000
additions possible each second.
 Business applications become more commonplace, with large data files stored on
magnetic tape and disk. (Magnetic disk: is a circular platter constructed of metal
or plastic materials coated with magnetizable substance.)
 High-level languages COBOL and FORTRAN were introduced during this
period. Batch operating systems are used that permitted rapid processing of
magnetic tape files.
Figure. 2.15. Second Generation of computers
Third generation (late 1960s, early 1970s)
 Characterized by solid-state logic and integrated circuit (IC). (A single, selfcontained transistor is called discrete component. In early 1960 electronic
equipment composed of discrete components transistors, capacitors, resistors,
They are:
 manufactured separately
 Packed in their own containers and soldered (wired together) on a circuit board.
So the entire manufacturing process was cumbersome and expensive. Do to these
and other problems in 1958 the achievement that revolutionized electronics
started the era of microelectronics: the invention of integrated circuit.
 Computer storage switched from magnetic cores to integrated circuit boards that
provide modularity (expandable storage) and compatibility (interchangeable
equipment
 New input/output methods such as optical scanning and plotters.
 Software become more important with sophisticated operating systems, improved
programming languages,
Figure.2.15. Third Generation Of computers
Fourth generation (late 1970s, early 1989s)
 Greatly expanded storage capabilities and improved circuitry.
 Has a large-scale integrated circuits (LSI) which has several hundred thousand
transistors placed on one tiny silicon chip.
 Computer memory operates at speeds of nano seconds (billionths of a second)
with large computers capable of adding 15 million numbers per second.
Figure .2.16.Fourth Generation of computers
The fifth generation computer is in progress. An architecture, which makes use of the
changes in technology and allows a simple and natural methodology for solving
problems, is being sought. These computers will have intelligent processors i.e.,
processors which can draw inferences. Users will also be able to interact with them in
natural languages such as English, German etc. Japans are working intensively on the
project for developing the 5th generation.
Figure. 2.17.Five generation of computers
Summary of generation of computers
Generation
1st
3rd
2nd
4th
Circuit element
Vacuum tube
Transistor
IC
LSI and VLSI
SSD
Punched card
Magnetic Tape
Magnetic disk
Mass storage
device
Language
Machine &
assembly
Fortran,
COBOL etc
Structured
language
Application
oriented
Operating
system
Operator control
Batch system
Application
oriented
Time sharing
Memory
Access time
1ms
10μs
10ns
1ns
Approx. date
1946-57
1958-64
1965-71
From 1971 above
examples
ENIAC, UNIVAC,
UDVAC
IBM7090, 7094
IBM system
Late IBM product
2.3. Current trends
in hardware and software include the increasing use of reduced instruction-set computing,
migration to the UNIX operating system, the development of large software libraries,
microprocessor-based smart terminals that allow remote validation of data, speech
synthesis and recognition, application.
The most advance trends in computer science is Machine learning, cloud computing and
Artificial Intelligence. There are so many multi-disciplinary courses emerge as
bioinformatics, biomedical computing and data analytics, obviously statistics and
theoretical computer science play a big role in it.
A current trend in computing is for sure the Computer Data Security and this is
because computers are not only used in the office or at home but in almost
every field. Computers control telephones, information on the Internet,
distribution of electrical power, monitors operations in nuclear power plants among other
very important applications; as it is mentioned by David Salomon in his
“Elements of Computer Security”. A huge cybercrime started in 2009 and was
discovered in 2010 by Net witness Corporation, a Virginia based network Security
Company. “More than 75,000 computers belonging to about 2500 companies around
the world (374 in the United States) have been compromised. The list of victims includes
Fortune500 companies, US local, state, and federal government agencies,
energy companies, ISPs, and educational institutions. The perpetrators lured
company employees by free (infected) software and baited them into opening
infected email attachments. Once compromised, computer was searched for sensitive
corporate documents, login information, IPs and URLs of friends and
colleagues, and passwords. The computer was then added to a botnet (dubbed
Kneber), and employed to spread its“message” to other machines.” Salomon, D.
(2010). Elements of computer security. New York: Springer-Verlag. The Kneber botnet
was named like this because of the username linking the affected machines around the
globe. The names of the companies affected were never revealed but “the Wall Street
Journal listed Merck & Co., Cardinal Health Inc., Paramount Pictures and Juniper
Networks Inc. as four companies that had been affected.” (Network World – 2015). It
was also mentioned that federal companies were also part of the attack. This was one
of many other cyber-attacks that make us think about the importance of
Computer S ecuri t y. C om put er ex perts predi ct t hat c yber securit y wi ll
concent rat e i n t he near fut ure in Smart phones, tablets, and many other
personal devices that are now part of our lives and that have replaced personal
computers; and this is because we do not use them only for social networks or to chat
(which also covers the fact of sending sensitive personal information) but to make
banking transactions, make shopping etc.
Chapter Five
3. ORGANIZATION OF COMPUTER SYSTEMS
3.1. Introduction to Computer Systems
System is a group of components, consisting of subsystems or procedures that work in a
coordination fashion to achieve some objective.
A computer system composed of components that are classified either as Computer
hardware or Computer software.
3.2. Computer hardware
When you go near computer, you see that different devices like the monitor, CPU,
mouse, printer etc. Are connected to each other through wires.
These devices are the physical parts of computer. So we can say that the Computer is a
collection of many physical parts which are connected to each other with the help of
wires and cables. All these physical parts of a computer are computer hardware. Some
parts are used to give input, some are used to get output, some are for processing and
storing data etc. According to these uses, we group the parts as:
Input Devices,
Output devices,
Storage Devices,
Processing device
Processing
Device(CPU
)
Input Device
Output Device
Storage
Device
Figure: 3.1 Hardware Basic
When we see the computer from inside, we can find that there are many parts. The figure
below shows the inside of the computer. We can categorize these parts in a more detailed
way.
1) Input Devices
2) Output Devices
3) Processing Device
-CPU
4)Storage Devices
-Primary Memory (RAM & ROM))
-Secondary Storage(floppy Disk, Hard Disk, CD-ROM)
Ports are used for connecting hardware from outside like input and output devices.
Buses are used for moving the information from one part to another part inside computer
Input Devices
Output Devices
PORTS
RAM
BUS
Mother Board
Processor
HARD DRIVE
(CPU)
ROM
FLOPPY
CD_ROM
DRIVE
Figure 3.2: Computer from Inside
Now we will discuss each part in detail.
1) Input Devices:
The data and instructions that a computer receives are called input. An input device is
used to give instructions to the computer. It works like our sense organs (nose, eyes, ears
etc) which takes in information (by smelling, seeing, listening etc) and send it to our
brain. Without the input devices we cannot do anything with computer.
There are many input devices like keyboard, mouse, scanner, touch-screen, joystick, light
pen, MICR, OCR and OMR. We can group these input units into following types.





Text input devices: Keyboard
Pointing devices: Mouse, touch screen, light pen
Gaming devices: Joystick
Image video input devices: scanner, web cam, OCR, MICR, OMR
Audio input devices: microphones
Keyboard: Keyboard is the way to enter data or instruction into a computer. It has
different keys for letters of alphabet, numbers, function keys, other keys which move
cursor on the screen and special keys. The standard keyboard has 104 keys. Keyboard
can be used to type documents and to send instructions to the CPU.
Mouse: It is another common input device. It is also known as pointing device. It can be
used to move cursor (a pointer) around the screen, to draw shapes or to make a choice
from the menu.
It is made up of small plastic box with buttons on top. On the bottom, some of the mouse
have small rubber ball which rolls and control the direction of the pointer on the screen.
Another type of mouse uses an optical system to track the movement of the mouse.
As the mouse is dragged on the surface, the movement is converted into data which is
sent to the CPU. The CPU uses this data to plot the direction of the mouse’s movement. It
sends the signals to the cursor on the monitor screen, which follows the movement of the
mouse exactly. The functions of the two buttons on the top of mouse are different. Left
one is used to select objects and text and the right one is used to access menus.
Light Pen: It is a pointing device like mouse. Its shape is like a pen. With a light pen, we
can move the pointer and select the object by directly touching the object by pen on the
screen. It has a light sensitive element on the tip of pen, which detects the location of the
pen on the screen. We can also draw on the screen using light pens.
Joystick: Joysticks are similar to mouse. They are mainly used in computer games. They
are sometimes used in CAD/CAM systems and other applications. It has a stick that
pivots on a base and reports its angle or direction to the device it is controlling. It has one
or more buttons which are called trigger and are used to trigger the action.
Scanners: Scanners are like eyes. They see the images or printed text and translate them
into binary code. To do this, scanner sends a beam of light to the page and then measures
how much light has reflected back. Then the digital code is generated according to the
light or dark image and is given to the amount of light for each portion. This code is sent
to the CPU, which then creates an image. They are used to input pictures or small portion
of a page.
Touch Screen : A touch screen is becoming very popular input device these days. By
using touch screen monitors, user can operate a computer by simply touching the
monitor. Touch input is suitable for a wide variety of computing applications. We can
select options and give commands by pressing different areas on the screen. A touch
screen are sensitive to the touch of a finger. A touch-screen system is made up of a
touch sensor, a controller card, and a software driver. When we touch the screen area, the
touch screen panel registers this touch events and passes these signals to the controller.
Then the controller processes the signals and sends the data to the processor. The
software driver translates touch events into mouse events.
OCR (Optical Character Reader) : This device is used for the data entry purposes. It
uses one of the two following methods:Matrix matching method and feature extraction
method.
In matrix matching method, When the OCR scanner reads data, Matrix matching takes
the data as a character and compares this character with a set of stored character matrices
or templates. When an image matches one of these prescribed matrices of dots within a
given level of similarity, the computer labels that image as the corresponding ASCII
character.
In Feature extraction method, When the OCR scans the letters, scanned letters are
condensed into their basic features. Then the basic features of a letter are compared to a
list of features stored in a program's code. If the features matches, the letter is recognized.
For example: the letter "a" is made from a circle, a line on the right side and an arc over
the middle. The arc over the middle is optional. So, if a scanned letter had these
"features" it would be correctly identified as the letter "a" by the OCR program.
MICR: It stand for Magnetic Ink Character Recognition. It is pronounced as my-ker. It
is character recognition technology used mainly by the banks to facilitate the processing
of cheques. You must have seen a cheque having a number at the bottom. That number
uses special magnetic ink. Writing style is also different. Magnetic ink is used so that
character can be read into the system even when they are overprinted by the other marks
like cancellation stamp.
Characters in magnetic ink are read by a special machine. This machine changes these
characters into code, so that the computer can verify the characters for its duplication.
OMR: It stands for Optical Mark Recognition. It reads different marks, codes, and then
convert them into computer readable code. The most common application of this device
is the use of 2 HB pencil and bubble optical answer sheet in multiple choice question
examinations. Students mark their answers, or other personal information, by darkening
circles marked on a sheet. Afterwards the sheet is automatically graded by OMR
scanning machines.
Web Cam: It is a small camera, which gives an image as an input.
Micro phone: It is used to give voice input to the computer. It is connected to the sound
card in the computer It provides input by converting the sound into electrical signal.
Output Devices:
Anything that provides us with information is output device. In a computer, the Output is
an information that it produces. In the computer, the output is in the form of binary code.
Output devices, such as printer, monitor and speaker displays it in a way that we can
understand. Output can be displayed on the screen or printed on a paper or can be heard
on the speaker. Printed output is called hard copy. Output on the screen is called soft
copy.
Output devices can be categorize into following types:
Image/video output devices: Printer, Monitor
Audio output devices: Speaker, headphones
Printer: Its an output device It takes what you see on the computer and prints it on the
paper or transparency. Printed output is also called a hard copy . Printer uses ink to put
data on paper or transparency. There are many types of printers.
1)Dot Matrix Printer
2)Ink Jet Printer
3)Daisy Wheel Printer
4) Laser Printer
Speed of the printer is measured in cps(character per second),lpm(line per
minute),ppm(pages per minute). The quality of the print is measured in dpi(dots per
inch). Some printers can print in color also. Dot matrix, ink jet and laser printers are most
commonly used printers. Printer can be classified into two categories according to the
technologies used in them. Impact printer and non-impact printer. An impact printer uses
a device to press/strike something against the ribbon to put a character on the page. Dot
matrix, daisy and line printer are impact printers.
Non-impact printers print without having a mechanism to strike against a sheet of paper.
Ink jet, laser and thermal printer are non-impact printer.
Dot Matrix Printer: It uses dots to form a characters on a paper. Dots are made by pins.
Dot matrix printer is made up of steel pins which strike the paper through a inked ribbon
to create a pattern of tiny dots. The quality of print depends upon the number of steel pins
in the machine.
Daisy Wheel printer: In this type of printer, the part that puts the characters looks like
a wheel. Numbers and letters are arranged in a wheel. And the daisy wheel spins until
the correct letter is in position. Then hammer strikes the character against the ribbon,
printing it on the paper.
Line Printer: It prints a complete line of text at a time. It uses drum or chain with all the
characters in the character set on it. The drum or chain moves and prints the necessary
characters in the right place.
Ink jet printer: It prints the character using fine jet of ink which comes out from the
tiny nozzles onto the paper. The ink is kept in reservoir and fed into the firing chambers
just below each nozzle.
Laser Printer: It uses a beam of light (laser beam) to convert binary data into print. The
laser puts an electric charge in the shape of a character on the rotating drum. The dry ink
or toner stick only to the drum where it has been charged. These printers are very fast.
They can print a whole page at once.
Thermal Printer: It uses heat to put the characters on the paper. The paper has a special
coating on it. The printer uses heated wires to turn the paper black. It is basically used for
taking ECG in hospital or lab.
Plotter: Plotter is a special type of output device designed to produce high quality
graphics in a variety of colors.
Monitor: It is a visual display unit (VDU), often called a monitor. Computer generates
data or output which can be displayed on the monitor. It is an electrical device and looks
like a television screen. There are two types of monitors: CRT monitor and LCD
Monitors. CRT Monitor uses picture tubes also called cathode ray tube ( picture tube is
used in television) to display information or image on the screen. Generally these
monitors are large and bulky due to this picture tube. It also require relatively high
voltage power supply.
Because of the high voltage requirement and bulky size, A newer monitors have been
developed and used these days with new technology. They use flat panel LCD (liquid
crystal display) display in place of cathode ray tube. They are very thin and light weight.
Speaker: Speakers are output devices which produce sound. They are just like a stereo
speakers.
Storage Devices
The most important feature of computer is its storing capacity. Memory is the place in the
computer where information can be stored. Every computer has two kinds of memory.
Primary memory and secondary memory. When we use any program, the computer
loads the program from hard disk (secondary memory)to faster memory RAM (Primary
memory). We perform the operations while the program is in RAM. The data gets written
back to hard disk(secondary memory) when we save or quit the program,
Primary memory: Primary storage, or memory, is directly accessible to the CPU. The
CPU continuously reads instructions stored in the memory and executes them. It has two
parts: Read Only Memory(ROM) and Random Access Memory(RAM).
RAM: When we enter data from keyboard, the data is first read by the RAM. The
output is also stored in RAM. But the information will be stored in it as long as computer
is on. When the computer is switched off, all the data stored in it is lost. That is why it is
called temporary or volatile memory. It is called Random Access Memory because the
computer can pick out or access any piece of data from any location of memory.
ROM: This memory is used to store things which are never going to change. In ROM
data or program cannot be written or stored but only can be read. The instructions cannot
be changed, and they won't be wiped out when the computer is turned off. That is why
this memory is called permanent or nonvolatile memory. This memory is used to store
the program like operating systems . When the computer is switched on and it is starting
up, the instructions stored in ROM tells the CPU how to go ahead.
Secondary Memory: Primary memory or memory is a temporary memory and when we
turn off the computer, programs and data present in the memory get erased. When we
are sharing the computer, we must yield the memory to someone else to run the program.
Also, we want to store data and programs we have processed in computer. So we need to
have some permanent storage.To store the data permanently, secondary memory is used.
Secondary memory is used to store large amount of data permanently as compared to
primary memory. There are various types of secondary storage devices available like,
floppy disk, hard disk, CD-ROM, pen drive etc.
Floppy Disk: Floppy disk is a circular thin plastic jacket coated with magnetic material.
It is covered with a hard plastic cover to protect this disk. It stores information
magnetically on one or both sides. This is very useful in transferring data from one
computer to another. Most floppy disks are 3.5 inches in size. Floppy disk drive should
be present in computer for reading or writing data in floppy disk. These days, floppy
disks are not used much.
Hard Disk: Hard disk is a solid, rounded disk made up of magnetic material and placed
inside the computer. They do exactly the same thing as floppy disk, except that its
storing capacity is more. The information stored in hard disk is permanent until we erase
it.
Optical Disk Storage: Optical disk technology is categorized by its read/write
capability. It uses the following technology: In these kind of disks, a metallic material is
spread over the surface of the disk. A laser heats the surface and when the data is
entered, due to the laser heat, tiny spots are created on the surface of disk. To read the
data from disk, laser scans the disks, and the lens picks up different light reflections from
the various spot. CDs( compact Disk) come into this category.
CD-ROMs are one type of CD, in which we can't write or change anything. We can only
read. CD-ROM is recorded on by the manufacturer. Such a disk cannot be used for our
files, but manufacturer’s supply the software’s in CD-ROM. It is like a compact disk we
buy from music store and tt is a read-only disk. It means that it can't be erased or written
by the user.
It can store much more information than floppy disk. We need to have a CD-drive
installed on computer to read from CD-ROM.
In CD drive, we get write CD drive or CD-RW as an option. CD-RW is a write once
and read many times media. With a CD-RW drive, we can store our data. This is an
convenient, inexpensive and safe way to store large volume of data.
DVDs: Digital Versatile disk(DVD) are now widely used in computers. Data are stored
just like in CDs. DVD disks are read by laser beam of shorter wave length than used by
the CD-ROM, which allows increased storage capacity in DVDs.
USB Pen Drive: It is a flash memory data storage device integrated with USB( Universal
serial port) interface. It is removable and rewritable, much smaller than a floppy disk
(size:1 to 4 inches or 2.5 to 10 cm), and weighs less than 2 ounces (60 g). Its storage
capacity ranges from 64 MB to 32 GB. It is compact, faster, and holds much more data.
Its design is more durable. To access the data stored in a flash (pen) drive, the drive
must be connected to a USB port built into a computer.
Processor:
Secondary memory
Programsecondary memory
Central Processing Unit
output
Input
Control
Arithmetic
Unit
Logic Unit
Memory
Figure: 3.3. Central Processing Unit
Central Processing Unit (CPU): There is a main part of computer that takes input,
processes the data and gives output. This part is called central processing unit (CPU). It
is a highly complex electronic circuitry. All the computers have a central processing unit.
CPU has direct relationship with Primary memory. The CPU interacts closely with
primary storage, or main memory, for instructions and data. Data is stored in main
memory temporarily, while the CPU is executing a program. But, memory is not part of
the CPU.
CPU has two parts: The control unit and the arithmetic/logic unit. Each part has a specific
function.
Control Unit: They are the in charge of computer. The control unit directs the flow of
information to entire computer system or to execute the instructions. Control unit is like
a leader of the music concert. In the music concert, leader does not plays the music but
controls and instruct the group to perform. Similarly, the control unit does not execute
program instructions; it directs the other parts of the system to do so. The control unit
communicates with both the arithmetic/logic unit and memory.
Arithmetic/Logic unit (ALU):The arithmetic/logic unit executes all the arithmetic and
logical operations.
The arithmetic/logic unit can do all the mathematical calculations: addition, subtraction,
multiplication, and division. It also performs logical operations. A logical operation is
basically a comparison of values. It can compare numbers, letters, or special characters.
some common logic comparison symbols are
= equal to
< less than
> greater than
<= less than or equal to
>= greater than or equal to
<> not equal
The computer can take action based on the result of the comparison. This is a very
important capability. By comparing computer can tell, for instance, whether there are
seats available on Train to go to particular place, whether person has exceeded the quota
of issued books in library.
How the CPU Executes Program Instructions
Before the execution of a program, the program data and instructions are moved into the
memory from input device or from secondary storage device. Then the CPU performs the
following steps.
1. The control unit first gets the instructions from memory. It checks what the
instruction means and accordingly sends the necessary data from memory to
arithmetic/logic unit.
2. The arithmetic/logic unit then performs the actual operation on data and stores the
result back in the memory.
When the results are in memory, then the control unit tells the memory to send the result
to an output device or a secondary storage device. The above process is shown in the
figure 3.4.
Control Unit
Arithmetic/Logic
2) Decode
and send to ALU
1)Get the instruction
Memory
Figure: 3.4 Executions in CPU
Unit
3)execute
4)store
3.3. Computer software
Computer software is the set of programs that makes the hardware perform a set of tasks
in Particular order. Hardware and software are complimentary to each other. Both have to
work Together to produce result Computer software is classified into two broad
categories; system software and application software
System Software:
System software consists of a group of programs that control the operations of a
computer Equipment including functions likes managing memory, managing peripherals,
loading,
Storing, and is an interface between the application programs and the computer. MS DOS
(Microsoft’s Disk Operating System), UNIX is examples of system software.
Application software:
Software that can perform a specific task for the user, such as word processing,
accounting, Budgeting or payroll, fall under the category of application software.
Word processors, spreadsheets, database management systems are all examples of
general purpose application software.
Types of application software are:
Word processing software: The main purpose of this
software is to produce documents. MS-Word, Word Pad,
Notepad and some other text editors are some of the
examples of word processing software.
Database software: Database is a collection of related
data. The purpose of this software is to organize and
manage data. The advantage of this software is that you can
change the way data is stored and displayed. MS access,
dBase, FoxPro, Paradox, and Oracle are some of the
examples of database software.
Spread sheet software: The spread sheet software is used to
maintain budget, financial statements, grade sheets, and
sales records. The purpose of this software is organizing
numbers. It also allows the users to perform
Simple or complex calculations on the numbers entered in rows and columns. MS-Excel
is one of the examples of spreadsheet software.
Presentation software: This software is used to display the information in the form of
slide show. The three main functions of presentation software is editing that allows
insertion and formatting of text, including graphics in the text and executing the slide
shows. The best example for this type of application software is Microsoft
PowerPoint.
Multimedia software: Media players and real players are the examples of multimedia
software. This software will allow the user to create audio and videos. The different
forms of multimedia software are audio converters, players, burners, video encoders
and decoders.
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Chapter Four
4.Data Representation in Computers
We enter data into a computer or review (see) output data from a computer using the letter of
alphabet, various special symbols, and the numerals in the decimal number system. But since
computer is an electronic device, which understands electrical flow (signal), there is no letter,
symbol or number inside the computer. Computer works with binary numbers.
4.1. Units of Data Representation
When data is stored, processed or communicated within the computer system, it is packed in
units. Arranged from the smallest to the largest, the units are called bit, byte, and word; these
units are based on the binary number system.
BIT:

Bits are the smallest units and can convey only two possible states 0 or 1;

Bit stands for Binary digits;

A bit is a single element in the computer, on a disk that stands for either “ON” indicating
1 or “OFF” indicating 0;
In the computer “ON” is represented by the existence of current and “OFF” is represented by
the non-existence of current. On a magnetic disk, the same information is stored by changing
the polarity of magnetized particles on the disk’s surface.
BYTE:
Bits can be organized into large units to make them represent more and meaningful information.
This large unit is called a byte and is the basic “unit of data representation” in a computer
system. The commonly used byte contains 8 bits. Since each bit has two states and there are 8
bits in a byte, the total amount of data that can be represented using a single byte is 28 or 256
possible combinations. Each byte can represent a character.
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A byte is then used as a unit of measurement in the computer memory, processing unit, external
storage and during communication. If the computer memory is 524288 byte, this is expressed in
short by saying 512KB, where KB stands for kilobyte.
-
1 Kilobyte (1KB) is 210 or 1024 bytes
-
1 Megabyte (MB) is 220 bytes or 210 kilobytes
-
1 Gigabyte (GB) is 230 bytes or 220 kilobytes or 210 megabytes
WORD:
Word refers the number of bits that a computer process at a time or a transmission media
transmits at a time. Although bytes can store or transmit information, the process can even be
faster if more than one byte is processed at a once. A combination of bytes, then form a “word”.
A word can contain one, two, three, four or eight bytes based on the capacity of the computer.
Word length is usually given in bits. We say that a computer is an 8-bit, a 16 bit, a 32 bit or a 64
bit computer to indicate that the amount of data it can process at a time. The larger the word
length a computer has the more powerful and faster it is.
4.2. Concept of Number Systems and Binary Arithmetic
A number system defines a set of values used to represent quantity. There are various number
systems e.g. decimal, binary, octal, hexadecimal, etc each differs one another by the number of
symbols used in the system. Each numbering system used different symbols to represent a given
quantity.The number systems that are generally used by computers are: decimal, binary, octal,
and hexadecimal.
4.2.1 The Decimal Number System
The primary number system used is a base ten number system or decimal number system. The
Decimal number system is based on the ten different digits or symbols (0,1,2,3,4,5,6,7,8,9).
Starting at the decimal point and moving to the left, each position is represented by the base
(radix) value (10 for decimal) raised to power. The power starts at Zero for the position just to
the left of the decimal point. The power incremented for each positions that continues to the left.
Moving to the right of the decimal point is just like moving to the left except that we will need
to place a minus sign in front of each power.
For example: (8762)10 = (8*103)+ (7*102) + (6*101)+ (2*100)
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(0.475)10= (4*10-1) + (7*10-2) + (5*10-3)
4.2.2. The Binary number system
Computers do not use the ten digits of the decimal system for counting and arithmetic. Two
digits, 0 and 1, are used to refer for these two states.
Binary number system is based on the two different digits; 0 and 1. With binary number system,
it is very easier for the hardware to represent the data. Binary number system is base two
number system.
For example: (01100)2,(10110.011)2 , etc
4.2.3 Octal number system
The octal number system with its eight symbols (0, 1, 2, 3, 4, 5, 6, and 7) is a base 8 system.
For example: (322)8, (10.25)8, etc
4.2.4. Hexadecimal number system
Hexadecimal number system is another number system that works exactly like the decimal and
binary number systems, except that the base is 16. It uses 16 symbols (0-9, and A-F characters
to represent 10-15).
For example: (8F0)16, (D.45)16, etc
4.3. Conversion between Number Systems
Computers use binary numbers for internal data representation whereas they use decimal
numbers externally. Therefore, there should be some conversion between number systems in
order to represent data in a computer that is originally represented in other number systems.
Some conversion methods are discussed below.
Decimal to Binary
It is important to note that every decimal number system has its equivalent binary number. For
example:
Binary
Decimal
0
0
01
1
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10
2
101
5
11
3
110
6
100
4
111
7
1000
8
1001
9
Binary
Decimal
etc
Conversion from binary to its equivalent decimal and from decimal to its equivalent binary is
possible. The method, which is used for the conversion of decimal into binary, is often called as
the remainder method. This method involves the following steps.
-
Begin by dividing the decimal number by 2 (the base of binary number system)
-
Note the remainder separately as the rightmost digit of the binary equivalent
-
Continually repeat the process of dividing by 2 until quotient is zero and keep writing the
remainders after each step of division (these remainders will either be 0 or 1)
-
Finally, when no more division can occur, write down the remainders in reverse order (last
remainder written first)
Example: Determine the binary equivalent of (44)10
2
44
2
22
0 LSB (List Significant Bit)
2
11
0
2
5
1
2
2
1
2
1
0
1
1
Remainder
MSB (Most Significant Bit)
Taking the remainder in reverse order we have 101100. Thus the binary equivalent of (44)10 is
(101100)2
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In general to convert a decimal number X to a number in base M, divide X by M, store the
remainder, again divide the quotient by M, store the remainder, and continue until the quotient
is 0. And concatenate (collect) the remainders starting from the last up to the first.
Example: Convert 7810 to base eight (Octal)
7810=1168
Example: Convert 3010 to base sixteen (hexadecimal)
3010=1E16
4.3.1 Binary to Decimal
In the binary to decimal conversion, each digit of the binary number is multiplied by its
weighted position, and each of the weighted values is added together to get the decimal number.
Example: Determine the decimal equivalent of (100100)2
1*25 + 0*24 + 0*23 + 1*22 + 0*21 +0*20 = 32+4 =36
Therefore, the decimal equivalent of (100100)2 is 36
In general to convert a number X consists of digits X1 X2 X3 …Xn in base m to decimal; simply
expand the number with base m. That is
(X1X2X3…Xn) m =X1*mn-1+X2*mn-2 +X3*mn-3+...+ Xi*mn-i+… Xn-1m1+Xn*m0
=Y10
Example: convert (234)8 to decimal
=2*82 + 3*81 + 4*80 = 128+24+4 = 156
Example: convert (A1B) 16 to decimal
=A*162 + 1*161 + B*160 = 2587
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Binary (base2) to Octal (base 8) or hexadecimal (base16) and vice versa
To convert a number in binary to octal group three binary digits together starting from the last
digit (right) and if there are no enough digits add zeros to the front end (left) and find the
corresponding Octal of each group.
Example: Convert 1001001 to octal
Convert 101101001 to octal
1001001=001,001,001
101101001
= 1118
=101,101,001
=5518
To convert binary to hexadecimal group four binary digits together starting from right
and if there are no enough digits add zeros at the left.
Example:
Convert
111100100
hexadecimal
to
Convert 111001111 to Hexadecimal
111001111 =0001 1100 1111
111100100 =0001 1110
0100
=1
14
4
=1
E
4
=1
12
15
=1
B
F
=(1BF)16
= (1E4)16
To convert from Octal to binary, convert each octal digit to its equivalent 3 bit binary
starting from right.
Example: Convert (675) eight to binary
675eight =110 111 101
=(110111101)two
Convert 231eight to binary
231eight = 010 011 001
=(10011001)two
To convert from Hexadecimal to binary convert each hex. Digit to its equivalent 4-bit
binary starting from right.
Example: Convert 23416 to binary
23416 =0010 0011 0100
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= 10001101002
2AC16 =0010 1010 1100
Convert 2AC to binary
=10101011002
4.3.2. Octal to hexadecimal and Vise versa
To convert from Octal to hexadecimal, first we have to convert to binary and the binary
to hexadecimal. To convert from hexadecimal to Octal, first we have to convert to binary
and then the binary to Octal.
Example: Convert 2358 to hexadecimal
Convert (1A)16 to Octal
2388=010 011 101
1A=0001 1010
=0000 1001 1101
=000 011 010
=0
= 0
9
13
=9D16
3
2
=328
4.3.3. Converting Decimal Number with Fractions to Binary
- First change the integer part to its equivalent binary.
-
Multiply the fractional part by 2 and take out the integer value, and again multiply
the fractional part of the result by 2 and take out the integer part, continue this
until the product is 0.
-
Collect the integer values from top to bottom & concatenate with the integer part.
Example.
A) Convert 12.2510 to binary
B) Convert3.1875 to binary
1100.01
11.0011
4.3.4. Converting Binary with Fraction to Decimal
To convert a binary number Y1Y2Y3Y4Yn.d1d2d3…dm to decimal
-
first convert the integer part to decimal by using
y1 y2 y3 y4…yn=y1*2n-1+y2*2n-2+….yj*2n-j+….+yn-1*21+yn*20=Q and
-
Convert the fractional part to decimal by using
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d1d2d3…dm=d1*2-1+d2*2-2+d3*2-3+…+dj*2-j+..+dm*2-m=R
-
Then decimal equivalence of y1 y2 y3 y4…..yn.d1d2…dm will be Q+R where Q
is the integer part and R is the fractional part.
Ex1 : Convert 11001.0101 to decimal
11001
=
1x24
+
1x23
Ex 2: Convert 1000.1 to decimal
1000 = 1+23 +0+0+0=8
+0x22+0x21+1x20= 16+8+1= 25= Q
1= 1x2-1=½ = 0.5
0101 =0x2-1+1x2-2+0x2-3+1x2-4
1000.1 = 8.510
= 0+¼+0+1/16 = 0.3125 = R
=>11001.0101 = 25.3125.
4.3.5. Conversion from Binary with Fraction to Octal/Hexadecimal
 Group three/four digits together starting from the last digit of the integer part, and if
there is less number of digits add some zeros in the beginning.
 Group three/ four digits together starting from the first digit of the fractional part, and if
there is less number of digits add some zeros to the end.
 Covert each group of the integer and the fractional part to their equivalent
Octal/hexadecimal and collect the results by adding point (.) to separate the integer part
from the fractional part.
Ex 1:- Covert 1010.01112 to octal
Ex2:- Covert 1110101.101112 to hexadecimal
4.3.6 Conversion from Octal or Hexadecimal with Fraction to Binary
 Convert each Octal/hexadecimal digit to its equivalent 3/4-bit binary digit.
 Collect the binary sequences by separating the integer part binaries from the fractional
part binaries with point (.)
4.3.7. Conversion from Octal with Fraction to Hexadecimal
 To convert from Octal to hexadecimal, first convert the Octal to binary and then the
binary to hexadecimal
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4.3.8. Conversion from Hexadecimal with Fraction to Octal
 To convert from hexadecimal to Octal, first convert the hexadecimal to binary and then
the binary to Octal.
4.3.9. Conversion from Octal/Hexadecimal with Fraction to Decimal.
 To convert from Octal/hexadecimal to decimal, first convert to binary and –then the
binary to decimal.
4.4. Binary Arithmetic
Arithmetic in binary is much like arithmetic in other numeral systems. Addition,
subtraction, multiplication, and division can be performed on binary numerals. The simplest
arithmetic operation in binary is addition. Adding two single-digit binary numbers is
relatively simple:
0+0=0
0+1=1
1+0=1
1 + 1 = 10 (carry:1)
When the result of an addition exceeds the value of the radix, the procedure is to "carry the
one" to the left, adding it to the next positional value. Carrying works the same way in
binary:
1 1 1 1 1 (carried digits)
01101
+10111
------------=100100
Subtraction works in much the same way:
0−0=0
0 − 1 = 1 (with borrow)
1−0=1
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1−1=0
One binary numeral can be subtracted from another as follows:
** *
(starred columns are borrowed from)
100101
−
1011
---------------=011010
The following steps are involved:
-
First, for the least significant bit(the right most bit) , 1-1 is 0
-
For the next bit, 0-1 cannot be computed since the subtrahend is smaller than the
minuend. Borrow 1 from the third bit to form the binary number 10 (decimal 2)
and do the subtraction. The operation is 10-1=1 which in decimal number system
is 2-1=1
-
For the third bit, since we borrowed 1 for the second bit, we have 0-0 that is 0
-
For the forth bit again, we cannot perform the subtraction. However the fifth bit
in the minuend is zero, so we must borrow from the sixth bit. This makes the
fifth bit 10 (decimal 2). Borrowing from the fifth bit makes it 1 and the fourth bit
become 10 (decimal 2). Now the subtraction in binary is 10-1=1 which is the
result of the fourth bit.
-
For the fifth bit, we now have 1-0=1
-
Since we borrowed 1 from the sixth bit for the fourth bit, so for the sixth bit, the
subtraction is 0-0=0
Multiplication in binary is similar to its decimal counterpart. Two numbers A and B can be
multiplied by partial products: for each digit in B, the product of that digit in A is calculated
and written on a new line, shifted leftward so that its rightmost digit lines up with the digit
in B that was used. The sum of all these partial products gives the final result.
For example, the binary numbers 1011 and 1010 are multiplied as follows:
1 0 1 1 (A)
× 1 0 1 0 (B)
--------0 0 0 0 ← Corresponds to a zero in B
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+
1011
← Corresponds to a one in B
+ 0000
+1011
--------------=1101110
Binary Division is again similar to its decimal counterpart:
__________
101 |11011
Here, the divisor is 1012, or 5 decimal, while the dividend is 110112, or 27 decimal. The
procedure is the same as that of decimal long division; here, the divisor 1012 goes into the
first three digits 1102 of the dividend one time, so a "1" is written on the top line. This result
is multiplied by the divisor, and subtracted from the first three digits of the dividend; the
next digit (a "1") is included to obtain a new three-digit sequence:
1
_________
101 |11011
−101
----011
The procedure is then repeated with the new sequence, continuing until the digits in the
dividend have been exhausted:
Thus, the quotient of 110112 divided by 1012 is 1012, as shown on the top line, while the
remainder, shown on the bottom line, is 102. In decimal, 27 divided by 5 is 5, with a
remainder of 2.
4.5. CODING METHODS
In today’s technology, the binary number system is used by the computer system to
represent the data in the computer in understandable format. There are a lot of ways to
represent, numeric, alphabetic, and special characters in computer’s internal storage area. It
is possible to represent any of the character in our language in a way as a series of electrical
switches in arranged manner. These switch arrangements can therefore be coded as a series
of equivalent arrangements of bits. In this way, every character can be represented by a
combination of bits that is different from any other combination.
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There are different coding systems that convert one or more character sets into computer
codes. Some are: EBCDIC, BCD, ASCII-7 & ASCII-8, Unicode, etc.
In these encodings, binary coding schemes separate the characters, known as character set,
in to zones. Zone groups characters together so as to make the coding scheme to decipher
and the data easier to process. With in each zone, the individual characters are identified by
digit code.
EBCDIC: Pronounced as “Eb-see-dick” and stands for Extended Binary Coded Decimal
Interchange Code.
It is an 8-bit coding scheme: (00000000 – 11111111), i.e. it uses 8 bits to represent each
character. It accommodates to code 28 or 256 different characters. This provides a unique
code for each decimal value 0 to 9 , each upper and lower case English letter (for total of
52), and for a variety of special characters. Since it is an 8-bit code, each group of the eight
bits makes up one alphabetic, numeric, or special character.
Coding Examples
Character
zone (4 Bit)
digit (4 Bit)
Character
Zone
0-9
15
0-9
a
1000
0001
a-i
8
1-9
b
1000
0010
j-r
9
1-9
A
1100
0001
s-z
10
2-9
B
1100
0010
A-I
12
1-9
0
1111
0000
J-R
13
1-9
9
1111
1001
S-Z
14
2-9
Digit
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BCD (Binary Coded Decimal)
There were two types of BCD coding techniques used before. The 4 bit BCD, which represent
any digit of decimal number by four bits of binary numbers.
If you want to represent 219 using 4 bit BCD you have to say 001000011001
 4 bits BCD numbers are useful whenever decimal information is transferred into or out of a
digital system. Examples of BCD systems are electronic ousters, digital voltmeter, and
digital clocks; their circuits can work with BCD numbers.
 BCD’s are easy for conversion but slower for processing than binary. And they have limited
numbers because with BCD we can represent only numbers 0000 for 0 and 1001 for 9.
BCD (6-bits)
It uses 6-bits to code a Character (2 for zone bit and 4 for digit bit) it can represent 26 = 64
characters (10 digits, 26 capital characters and some other special characters).
Some Coding Examples
Character
zone (2 Bit)
0-9
0
0-9
A-I
3
1-9
Character
Zone
A
110001
Q
101000
8
001000
9
digit (4 Bit)
digits
00
1001
ASCII-7
ASCII stands for American Standard Code for Information Interchange. ASCII-7 used
widely before the introduction of ASCII-8 (the Extended ASCII). It uses 7 bits to
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represent a character. With the seven bits, 27( or 128) different characters can be coded
(0000000-1111111). It has 3 zone and 4 digit bits positions
Coding examples:
Charter
zone (3 bit)
digit(4 bit)
$
010
0100
0-9
3
0-9
%
010
0101
A-O
4
1-15
A
100
0001
P-Z
5
1-10
a
110
0001
b
110
0010
The ASCII System
Also referred as ASCII-8 or Extended ASCII. It is commonly used in the transmission of
data through data communication and is used almost exclusively to represent data
internally in microcomputers. ASCII uses 8-bits to represent alphanumeric characters
(letters, digits and special symbols). With the 8-bits, ASCII can represent 28 or 256
different characters (00000000-11111111). It assigns 4 bits for the zone and the rest for
the digit.
Coding Examples:
Character
zone (3 BIT) digit (4 BIT)
0-9
3
0-9
a
0110 0001
A-O
4
1-15
b
0110 0010
P-Z
5
0-10
A
0100 0001
a-o
6
1-15
B
0100 0010
p-z
7
0-10
?
0011 1111
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+
0010 1011
1
0011 0001
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4.6. Unicode
Unicode has started to replace ASCII and other coding methods at all levels. It enables users to
handle not only practically any script and language used on this planet; it also supports a
comprehensive set of mathematical and technical symbols to simplify scientific information
exchange. Unicode provides a unique number for every character, no matter what the platform,
no matter what the program, no matter what the language. Unicode was originally designed to be
a 16-bit code, but it was extended so that currently code positions are expressed as integers in the
hexadecimal range 0..10FFFF (decimal 0..1 114 111).
4.7. Representation of Negative Numbers and Arithmetic
There are different ways of representing negative numbers in a computer.
I. Sign- magnitude representation.
In signed binary representation, the left-most bit is used to indicate the sign of the number.
Traditionally, 0 is used to denote a positive number and 1 is used to denote a negative number.
But the magnitude part will be the same for the negative and positive values. For example,
11111111 represents-127 while, 01111111 represents + 127. We can now represent positive and
negative numbers, but we have reduced the maximum magnitude of these numbers to 127.
In a 5- bit representation we use the first bit for sign and the remaining 4- bits for the magnitude.
So using this 5 bit representation the range of number that can be represented is from -15
(11111) to 15 (01111)
Example 1: Represent-12 using 5-bits sign magnitude representation
-
first we convert 12 to binary i. e 1100
-
Now -12 = 11100
Example 2: Represent –24 using 8-bits sign magnitude representation
24=00011000
-24 = 10011000
In general for n-bit sign magnitude representation the range of values that can be represented
are –(2 n-1-1 ) to (2 n-1-1).
Note: In sign magnitude representation zero can be represented as 0 or -0
This representation has two problems one is it reduces the maximum size of magnitude, and the
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second one is speed efficiency to perform arithmetic and other operations. For sign magnitude
representation, correct addition and subtraction are relatively complex, involving the comparison
of signs and relative magnitude of the two numbers. The solution to this problem is called the
complement representation.
II. One’s Complement
In one’s complement representation, all positive integers are represented in their correct binary
format. For example +3 is represented as usual by 00000011. However, its complement, -3, is
obtained by complementing every bit in the original representation. Each 0 is transformed into a
1 and each 1 into a 0. In our example, the one’s complement representation of -3 is 11111100.
Example: +2 is 00000010
-2 is 11111101
Note that in this representation positive numbers start with a 0 on the left, and negative numbers
start with a 1 on the left most bit.
Example 1: add –3 and 3 with word size 4
3 = 0011
-3=1100
sum =1111 (=0)
Ex2. Add -4 and +6
- 4 is 11111011
+ 6 is 00000110
The sum is (1) 00000001
Where 1 indicates a carry. The correct result should be 2 or 00000010.
In one’s complement addition and subtraction, if there is an external carry it should be added to
get the correct result. This indicates it requires additional circuitry for implementing this
operation.
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III. Two’s Complement Representation
In two’s complement representation, positive numbers are represented, as usual, in singed binary,
just like in one’s complement. The difference lies in the representation of negative numbers. A
negative number represented in two’s complement is obtained by first computing the one’s
complement and then add one.
Example: +3 is represented in signed binary as 00000011
Its one’s complement representation is 11111100.
The two’s complement is obtained by adding one.
It is 11111101.
Example: let’s try addition.
(3) 00000011
+ (5) +00000101
(8) 0001000
The result is correct
Example: Let’s try subtraction
(3)
00000011
(-5) + 111111011
11111110
Example : add +4 and -3(the subtraction is performed by adding the two’s complement).
+4 is 00000100
-3 is 111111101
The result is [1] 000000001
If we ignore the external carry the result is 00000001 ( i. e 1 In decimal). This is the correct
result. In two’s complement, it is possible to add or subtract signed numbers, regardless of the
sign. Using the usual rules of binary addition, the result comes out correct, including the sign.
The carry is ignored. One’s complement may be used, but if one’s complement is used, special
circuitry is required to “correct the result”.
Carry and overflow
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Ex
(128) 10000000
+(129) 10000001
[257]=(1) 00000001
Where 1 indicates a carry. The result requires a ninth bit (bit 8, since the right- most bit is 0). It
is the carry bit.
The two’s complement representation has one anomaly not found with sign magnitude or one’s
complement. The bit pattern 1 followed by N-1 zeros is its own 2’s complement. To maintain
sign bit consistency, this bit pattern is assigned the value –2Nfor example, for 8-bit word,
-128 = 10000000
its 1’s complement
=01111111
+1
=100000000 = -128
Overflow will occur in four situations, including: 1.
The addition of large positive numbers.
2.
The addition of large negative numbers.
3.
The subtraction of a large positive number from a large negative numbers.
4.
The subtraction of a large negative number from a large positive number.
Overflow indicates that the result of an addition or subtraction requires more bits than are
available in the standard 8-bit register used to contain the result.
Fixed format representation: We now know how to represent signed integers: however, we
have not yet resolved the problem of magnitude. If we want to represent large integers, we will
need several bytes. In order to perform arithmetic operations efficiently, it is necessary to use a
fixed number of bytes, rather than a variable number. Therefore, once the number of bytes is
chosen, the maximum magnitude of the number that can be represented is fixed.
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Subtraction by Use of Complements.
 Complements are mainly used for representing negative numbers and subtraction.
 In performing binary subtraction or addition of negative number by use of
complements only one procedure,
binary
addition, is needed as one can subtract by adding its
complements.
 To subtract any number, positive or negative, substitute the required complement for the
numbers to be subtracted and then add.
If the result is
-
An (n+1)-bit number, and the arithmetic is in Ones complement the (n+1)
th
bit, a carry, is
added to the right most bit of the result. This process is called an end-around carry. If it is in
Two’s complement discard the (n+1) th bit.
-
An n-bit number and the arithmetic is in Ones complement, to read the binary value calculate
the ones complement of the magnitude bits and place a minus sign in front of it.
-
Two’s complement, to read the binary value calculate the two’s complement of the
magnitude bits and place a minus sign in front of it.
Example:
Perform the following in ones and two’s complements in 5-bits.
A. 12-6
B. 6-12
C. -12-6
A= 12
B=6,
A=01100
B=00110
Ones complement of -A=10011 & -B=11001
Two’s complement of - A= 10100 & -B= 11010
Example c: Is wrong this is because the occurrence of overflow. Arithmetic overflow is that
part of the result of an operation which is lost because of the resulting value exceeds the capacity
of the intended storage location.

Arithmetic overflow occurs when the sign bits of A and B are the same but the sign bit of
the result is different.
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Floating-point representation
In this representation decimal numbers are represented with a fixed length format. In order not
to waste bits, the representation will normalize all the numbers. For example, 0.000123 wastes
three zeroes on the left before non -zero digits. These zeroes have no meaning except to indicate
the position of the Decimal point. Normalizing this number result in .123x10-3.123 is the
normalized mantissa; -3 is the exponent.
We have normalized this by eliminating all the
meaningless zeroes to the left of the first non-zero digit and by adjusting the exponent.
Example: 22.1 is normalized as .221x102.
The general form of floating point representation is Mx10E where M is the mantissa, and E is
the exponent. It can be seen that a normalized number is characterized by a mantissa less than 1
and greater than or equal to.1 all cases when the number is not zero.
To represent floating numbers in the computer system it should be normalized after converting to
binary number representation system.
Example: 111.01 is normalized as .11101x23.
The mantissa is 11101. The exponent is 3.
The general structure of floating point is
Sign
Exponent
Mantissa (significand)
In representing a number in floating point we use 1 bit for sign, some bits for exponent and the
remaining bit for mantissa. In floating point representation the exponent is represented by a
biased exponent (Characteristics).
Biased exponent = true exponent + excess 2n-1, where n is the number of bits reserved for the
exponent.
Example:
Represent –234.375 in floating point using 7 bit for exponent and 16 bit for mantissa.
First we have to change to normalized binary
i. e 234 = 11100010
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0.375= 0.011
234.375 = 11100010.011 = 0.11100010011x28
true exponent = 8
excess 2 n-1 = 2 7-1= 26= 64
Biased exponent = 8+26 =8+64 = 72 = (100 1000) 2
Therefore –234.375 is represented as
1 1001000
Sign
1110001001100000
7-bits
16 bits
Example: Represent 34.25 in floating point using 7 bit for exponent and 24 bits for mantissa.
34.25 = 100010.012
The normalized form of 34.25 = .10001001x 26
True exponent = 6
Excess 2 n-1 = 2 7-1= 6+26
Biased exponent
=6+64=70
70 = 10001102
Therefore, 34.25 is represented as
0
1000110
100010010000…..0
To represent a number in floating point:
 Represent the number in normalized binary form.
 Find the biased exponent
 Change the biased exponent to binary
 Write the sign, the exponent in the exponent part and the mantissa in the mantissa part
 If there are fewer digits in the exponent add zeros to the left and for mantissa add zeros to the
right.
Floating-point Arithmetic
To perform floating-point arithmetic:
 First correct the numbers to binary with the same exponent (the highest)
 Apply the operator on the mantissa and take one of the exponent
 Normalize the result
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Example: Find 23.375 + 41.25 using 7-bit for exponent and 10 bit for mantissa.
23.375 = 10111.011 = 0.1011101x25 = 0.010111011x26
41.25 = 111001.01
= 0.11100101x26
23.37+41.25 = 0.01011101 x 26+0.1110010x26
= (0.010111011+0.11100101) x 26
= 0.1010000101x26
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Chapter Five
5.Data Communications and Computer Networks
5.1. Basics of Data Communication
Data communication: is the process of sharing ideas, information and messages with others in a
particular time and place. It is exchange of data between two or more parties.
For data communications to occur, the communicating devices must be part of a communication
system made up of a combination of hardware (physical equipment) and software (programs).
The effectiveness of a data communications system depends on four fundamental characteristics:
delivery, accuracy, timeliness, and jitter.
1. Delivery. The system must deliver data to the correct destination. Data must be received by
the intended device or user and only by that device or user.
2. Accuracy. The system must deliver the data accurately. Data that have been altered in
transmission and left uncorrected are unusable.
3. Timeliness. The system must deliver data in a timely manner. Data delivered late are useless.
In the case of video and audio, timely delivery means delivering data as they are produced, in the
same order that they are produced, and without significant delay. This kind of delivery is called
real-time transmission.
4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the
delivery of audio or video packets.
Components
Figure 5.1. Components of data communication
1. Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television, and so on.
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4. Transmission medium. The transmission medium is the physical path by which a message
travels from sender to receiver. Some examples of transmission media include twisted-pair wire,
coaxial cable, fiber-optic cable, and radio waves
5. Protocol. A protocol is a set of rules that govern data communications. It represents an
agreement between the communicating devices.
5.2. Data Transmission
The need of information has increased from time to time. This leads to the need of sharing of
information among different agents (individual), which may be at different places or locations.
Data communication is the exchange of information between two agents. For exchange of
information, the information should be transmitted from one point to another through a
transmission media called Channel. The following figure shows the different components of data
communication.
Agent
1
Input
device
2
Transm
itter
3
Source System
Transmis
sion
medium
Receiver
4
5
Output
device
6
Destination system
1. Input information (m)
2. Input data q or signal q(t)
3. Transmitted signal s(t)
4. Received signal r(t)
5. Output data q or signal q (t)
6. Out put information m
Information is transmitted in a form of packets the information is divided into packets and one
packet is transmitted at a time. When a packet of information is transmitted the sender must be
sure that the receiver receives the information and the receiver must check that it receives correct
information. The information is transmitted successfully the receiver must send an
acknowledgment to the sender.
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5.3 Mode of transmission
The transmission medium may be physical (it connects the transmitter and receivers through
wire) or logical (There are different mode of transmission)
 Simplex transmission:
In this transmission, signals are transmitted in only one direction: One station is transmitter and
the other is receiver.
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the
two devices on a link can transmit; the other can only receive. Keyboards and traditional
monitors are examples of simplex devices. The keyboard can only introduce input; the monitor
can only accept output. The simplex mode can use the entire capacity of the channel to send data
in one direction.
 Half-duplex transmission
In this transmission signals are transmitted in both directions, both stations may transmit, but
only one at a time.
In half-duplex mode, each station can both transmit and receive, but not at the same time. When
one device is sending, the other can only receive, and vice versa. In a half-duplex transmission,
the entire capacity of a channel is taken over by whichever of the two devices is transmitting at
the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The halfduplex mode is used in cases where there is no need for communication in both directions at the
same time; the entire capacity of the channel can be utilized for each direction.
 Full-duplex transmission
In this transmission signals transmitted in both directions, The medium carries signals in both
directions at the same time.
In full-duplex both stations can transmit and receive simultaneously. The full-duplex mode is
like a two way street with traffic flowing in both directions at the same time. In full-duplex
mode, signals going in one direction share the capacity of the link: with signals going in the other
direction. One common example of full-duplex communication is the telephone network. When
two people are communicating by a telephone line, both can talk and listen at the same time. The
full-duplex mode is used when communication in both directions is required all the time. The
capacity of the channel, however, must be divided between the two directions.
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Figure 5.2. Data flow
Data Transmission Channels
A channel is a medium that carries a signal from the transmitter to the receiver. The range of
frequencies that can be transmitted over a transmission medium is called band Width of a
channel. The rate of data transmission is directly proportional to the band width.
There are three types of data transmission channels:A. Narrow-band:- It is the smaller band and has slow data transmission rate. E.g. telegraph
line.
B. Voice- band: It is the wider band and has better data transmission rate than the narrow
band. Ex telephone lines are used for voice-band channel.
C. Broad-band: - It is the widest band and has used to transmit large volume of data with high
speed.
Information is transmitted in the form of analog or digital. Most communication lines are
designed to carry analog signals. Digital transmissions are used for telecommunications.
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Therefore, technique must be used to represent a digital signal in analog carrier. The process of
modifying the carrier signal to transmit digital information is called “Modulation”. When the
transmitted signal is received, the information must be reconverted into digital data. This
process is called “Demodulation” .These conversions between digital data and analog data are
handled by a device called a modem, an acronym for modulator and demodulator.
5.4. Data Transmission Protocols
Two different methods are used for transmitting data, namely, asynchronous and synchronous.
1. Asynchronous data transmission protocol
-
Data is transmitted character by character.
Data is transmitted at irregular time interval
-
A start bit is transmitted directly before each character.
-
To signify the end of the transmission, 1 or 2 stop bits are transmitted directly after each
character.
-
The start bit and stop bit are always of opposite polarity.
-
Usually, 0 is the start bit and 1 is the stop bit.
-
Between the start bit and the stop bits, the data bits are transmitted at uniformly spaced
time interval.
2. Synchronous Data Transmission Protocol
-
The transmission occurs at fixed intervals and fixed rates.
-
The need for start and stop bits is eliminated in synchronous transmission.
-
Allows for continues sending of characters
-
Each character is combined with others into a data packet.
-
The data packet is prefixed with a header field, and suffixed with a trailer field. Which
includes a checksum value (used by the receiver to check for errors in sending)
-
The header field: used to address information (sender and receiver), packet type and
control data.
-
The data field – contains checksum information
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5.6. Computer Networks





The purpose of the networking guidelines are as follows:
to assist students in understanding the benefits of networking
to help students place in context their current stage of networking
Development in their school.
to assist students in planning the next stage of network development
in their school.
to provide standard networking ‘models’ and best practice to students
That will assist students in their network planning.
5.6.1 Basic of Networking
A computer network consists of a collection of computers, printers and other
Equipment that is connected together so that they can communicate with each other.
It is a collection of computers and peripheral devices connected by communication links that
allow the network components to work together.
Importance of Networking
1. Resource sharing ( To share hardware such as the server To share computer CPU and
hard disk)
2. To share databases
3. To share application programs
4. To undertake parallel processing
5. High reliability by having alternative sources of supply.
6. Money saving
7. Increase system performance.
8. Powerful communication medium among widely separated people.
Hardware requirements to establish simple computer network
1. Computers
- A minimum of two computers is required to establish a computer network.
2. Network Interface Card/Network Adapter Card
-
It is an expansion card that physically connects a computer to the network.
-
Each computer in the network must have a network card.
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-
It takes the data from the computer’s internal bus and converts it into standard packets of
information that it then sends along the cable.
-
It takes the data from the computer and adds header.
-
When receiving data, it looks at each packet and checks the destination address in the
header.
-
If it recognizes its own address, it checks that it has no errors and signals the CPU that
there is data to be processed.
Figure 5.3.Network Interface Cards (NICs)
3. Cables
-
Cables are used to physically connect the computers on the network
-
Types of cables
i. Co-axial cables
ii. Twisted Pair Cables
iii. Fiber Optics
4. Modem- Modulator/Demodulator
-
It lets computers exchange information though telephone lines.
-
When transmitting information, the modulator changes the computers digital signal to
analog signal.
-
When receiving information, the demodulator translates the analog signal
-
Back to a digital signal.
Modulator – Digital to Analog
Demodulator – Analog to Digital
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5.7. Types of Networks
Based on geographical span of coverage, computer networks are broadly classified
into three major categories.
1. Local Area Network (LAN)

Computers are connected close together to each other with in the same ‘local’ area (ex.
building, office, school, Lab …). Computers are connected directly on-premises -usually
through wiring and sometimes with infrared signals (similar to your TV remote control) or
low-powered radio signals. It is the basic building block of any computer network. It is
confined in a limited geographical area.
Characteristics of LAN:
1. Physically limited
2. High bandwidth
3. Inexpensive cable media (Co-axial or twisted pair)
4. Used for data and hardware sharing
Advantages of LAN





Speed
Cost
Security
E-mail
Resource Sharing
Disadvantages of LAN




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Expensive To Install
Requires Administrative Time
File Server May Fail
Cables May Break
Figure 5.4. Local Area Network (LAN)
2. Metropolitan Area Network (MAN)
o A MAN is optimized for a larger geographical area than a LAN, ranging from
several blocks of buildings to entire cities.
o A MAN might be owned and operated by a single organization, but it usually will
be used by many individuals and organizations , Uses Fiber Optics cables
For example: Colleges, Universities, banks etc.
A MAN often acts as a high speed network to
Allow sharing of regional resources.
o A MAN typically covers an area of between 5 and 50 km diameter.
Examples of MAN: Telephone Company
o network that provides a high speed DSL to
o Customers and cable TV network.
Metropolitan Area Network (MAN)
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Figure 5.5. Metropolitan Area Network
3. Wide Area Networks (WAN)

Communication is established through telephone lines, microwave links and satellites etc.

It has no geographical limit

It is made up of a number of interconnected LANs. E.g. The internet

The computers are attached spread apart geographically such as a state, the country, or the
world

Computers in a network are situated in a wider geographical area.

It may contain a number of local area network

uses external communication facilities such as phone lines, cable, television lines, or satellite
transmission to carry data over longer distance.
Figure 5.6. Wide Area Network
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Personal Area Network (PAN)
A PAN is a network that is used for communicating among computers and computer
Devices (including telephones) in close proximity of around a few meters within a room
• It can be used for communicating between the devices themselves, or for connecting to a larger
Network such as the internet.
• PAN’s can be wired or wireless
is a computer network used for communication among computer devices, including telephones
and personal digital assistants, in proximity to an Individual’s body.
• The devices may or may not belong to the person in question. The reach of a PAN is
Typically a few meters.
Figure 5.7.Personel Area Network
Based on the architecture, networks are divided into two broad categories :
1. Peer-to-peer Networks
2. Server –based Networks
The type of network you choose to implement depends on
i. Size of the organization
ii. Level of security required
iii. Level of administrative support available
iv.
Amount of network traffic
v. Needs of the networks use
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vi.
Network budget
1. Peer-to-peer Networks
In the peer-to-peer configuration, only two computers are directly connected by cable
and one computer can directly access the resources located in the other computer.

No dedicated server

Every computer acts as both a client and a server

Good for 10 or few users

Less security

User at each computer determines what data on that computer is shared on the network.,
also called work groups

all computers on the network belong to users and are equal as far as the network is
concerned

Computers simply connect with each other in a workgroup to share files, printers and
Internet access.

This is most commonly found in home configurations and is only practical for workgroups
of a dozen or less computers
2. Server-Based

In the workgroup configuration, more than two computers are connected but the maximum
number of computers is not greater than ten.

In server based, one computer can directly access the resources located in other computer
provided that the other computer gives permission to use the resources.

There is usually a Server Machine to which all of the computers log on

This server can provide various services, including
– centrally routed Internet Access,
–
mail (including e-mail),
– file sharing and printer access,
– Ensuring security across the network.

Supports large number of users

Needs dedicated server (acts only as a server, but not as a client)

Security is an issue
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
Size is limited by a server and network hardware

Requires at least one professional administrator.
5.8. Network Topology
•
Topology: specifies the geometric arrangement of the network. Common topologies are:
bus, ring, star, mesh
•
The physical topology of a network refers to the layout of cables, computers and other
peripherals.
•
Logical topology is the method used to pass the information between the computers
•
Protocol: Specifies a common set of rules and signals the computers on the network use to
communicate. Protocols define the format, timing, sequence, and error checking used on
the network.
Standard Topologies
1. Bus Topology
•
All workstations are connected directly to the main backbone that carries the data.
•
Consists of devices connected to a common, shared cable.
•
Traffic generated by any computer will travel across the backbone and be received by all
workstations.
•
This works well in a small network of 2-5 computers
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•
Messages are detected by all nodes and are accepted by the nodes to which they are
addressed.
•
Relies on collision detection or token passing to regulate traffic.
•
If one node fails, the rest of the network can continue to function normally.
Figure. 5.8. Bus topology
Advantages of Bus Topology
-
Easy to implement
Low cost
Disadvantages of Bus Topology
-
Limited cable length and workstation
Difficult to isolate network fault.
Cable fault affects all workstations
2. Star Topology
•
•
One of the most common network topologies found in most offices and home networks.
Cable segments from each computer (node) are connected to a centralized component
called a hub.
Hub - is a device that processes and switches the messages from one incoming line to another.
- Signals are transmitted from the sending computer through the hub to all computers on
the network.
- Failure of one computer doesn’t affect the network
- Failure of the hub affects the network
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-
Cabling cost is high
Figure: 5.9. Star topology
Advantage of Star Topology
-
Easy to add new workstations
Centralized control
Centralized network
Easy to modify
-
If one computer on the star topology fails, then only the failed computer is unable to send
or receive data
Disadvantages of Star Topology
-
Hubs are expensive
Each computer is connected to a central hub or switch, if this device fails, the entire
network fails!
3. Ring Topology
•
Computers are connected on a single circle of cable. Unlike the bus topology, there are no
terminated ends.
•
Signals travel around the loop in one direction and pass through each computer
•
The method by which the data is transmitted around the ring is called token passing
•
A token is a special series of bits that contains control information

Each computer acts as a repeater to boost the signal and send it to the next computer.

Failure of one computer can have an impact on the entire network.
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
Allows signals to travel farther by regenerating signals

Cable failures affect limited users.

Logically, a ring topology is a circular arrangement of computers where the signals from
one node travel around the ring in clockwise direction .Because the signals pass through
each computer, the failure of one computer or a break in a cable could bring the entire
network failure.
Figure. 5.10. Ring topology
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4. Mesh Topology
•
Each computer is connected to every other computer by a separate cable.
•
Provides redundant paths through the new work
Hybrid Topology Types
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Transmission Cables
•
The kind of cable or other medium that is used to connect the various computers in a
network
•
Common types:
– Twisted-pair cable,
– Coaxial cable, and
– Fiber optic cable
1. Twisted-pair wire
•
Is a copper wire similar to the common telephone line
•
Each of the pair of wire are twisted
•
It can be Shielded (STP) or Unshielded (UTP)
•
UTP:
– the most popular cable around the world
– used not only for networking but also for the traditional telephone
2. Coaxial cable
•
A type of wire that consists of a center wire surrounded by insulation and then a grounded
shield of braided wire.
•
The shield minimizes electrical and radio frequency interference.
•
more expensive than standard telephone wire much less susceptible to interference and can
carry much more data
3. Fiber Optic Cables
•
can be used over greater distances
– 2km without the use of repeaters.
– one fiber could replace hundreds of copper cables
•
high bandwidth
•
Low loss of signals
•
The diameter could be millionths of a meter.
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5.9. Brief Introduction to Internet
Sometime in the mid 1960's, during the Cold War, it became apparent that there was a
need for a bombproof communications system.
A concept was devised to link computers together throughout the country. With such a
system in place large sections of the country could be linked and messages could still get
through.
Basically the Internet was an emergency military communications system operated by the
Department of Defense's Advanced Research Project Agency (ARPA). The whole
operation was referred to as ARPANET.
In time, ARPANET computers were installed at every university in the United States that
had defense related funding. Gradually, the Internet had gone from a military pipeline to
a communications tool for scientists. As more scholars came online, the administration of
the system transferred from ARPA to the National Science Foundation.
Years later, businesses began using the Internet and the administrative responsibilities
were once again transferred.
At this time no one party "operates" the Internet, there are several entities that "oversee"
the system and the protocols that are involved.
The speed of the Internet has changed the way people receive information. It combines
the immediacy of broadcast with the in-depth coverage of newspapers...making it a
perfect source for news and weather information.
About the Web
Each web site has an address, or Uniform Resource Locator (URL). The URL contains a
set of instructions that are read by the browser.
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The beginning of the URL contains the protocol. This is usually "http" (Hypertext
Transfer Protocol) or "ftp" (File Transfer Protocol). The second section of the URL
reveals the domain. Directories follow the domain. Lastly is the name of the document.
(If no document is named the browser will automatically open any document in the
directory named "default" or "index."
Internet Services:
•
E-mail transfer
•
USENET
•
World Wide Web (WWW,
Web, W3)
•
File transfer/access (FTP)
•
Remote login/ execution
(Telnet)
•
Video Conferencing
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E-mails
Even with the multimedia excitement of the Web, Electronic Mail (email) is the
most frequently used application of the Internet. Many people who have access to
the Internet at school, home, and work, use the Internet for no other purpose than
to send and receive email.
It's all very easy. You create the message, log onto the Internet, and send it. The
message first goes to your Internet Service Provider's mail server, which in
turn sends it to the recipient's mail server. On the way your message may go
through several servers, each reading the domain name in order to route it to the
appropriate server.
The message then remains in the recipient's mail server until he requests it by
"checking his mail."
Each email address you send is made up of certain components that help route it
to the proper recipient:
•
Every user, which belongs to the network, has his/her account and computer
system that provides the account.
•
To send a message
–
User name: identifies the sender or the recipients.
–
Domain name: identifies the computer system on which the user has
an account.
Examples: Xamza120@gmail.com
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World Wide Web (WWW)

The WWW allows you to combine text, a video, graphics, and even animation to
make a document a viewed easy.

Links within WWW documents can take you quickly to other related documents.
WWW is a set of sites that you can go o for information.

The process of sharing common information of the world by the help of the Internet
services.

It requires special software programs like Netscape, Internet Explorer, or others.
Use Net

It is one of the Internet services which allow users from any where on the Internet
to participate a discussion groups (News groups). It is an organized electronic mail
(e-mail) system, except there is no single user that mail is sent to.

A world wide distributed discussion group consists of a set of newsgroups.

Articles or messages are posted to news groups and the articles are then broadcasted
to other interconnected computer systems.
Telnet:- it is a program that lets you log into a remote computer directly through the
Internet and you can work on that computer.
File Transfer Protocol (FTP)

This enables you to examine the files of remote hosts on the Internet and to
transfer files between your hosts and the others.

Using FTP programs we can upload or download files. But to do this there should
be an admission from the remote computer.

Helps to transfer files and programs from one system to another.
Video Conferencing

The internet is, in its raw form, communication.

Video conferencing means making a conference on the internet by individuals
who live in different locations.

The individuals speak and see each other.
 It is similar to conference in a hall except they are at distant.
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Chapter Six
5. Computer Security
5.1 Introduction to Computer Security
Security: The prevention and protection of computer assets from unauthorized access,
use, alteration, degradation, destruction, and other threats.
 Computer systems should have a set of protection policies to restrict and control
the system resources.
 Considering:
o Unauthorized access
o Malicious modification or destruction
o Accidental introduction of inconsistency
Security Goals
 Data Confidentiality
o It is concerned with having secret data remain secret
 Data Integrity
o Unauthorized users should not be able to modify any data without the
owner’s permission
o Includes removing data and adding false data
 System Availability
o Means nobody can disturb the system to make it unusable
Security is thus based on the following independent issues:
 Privacy - the ability to keep things private/confidential
 Trust - do we trust data from an individual or a host? Could they be
used against us?
 Authenticity - are security credentials in order? Are we talking to
whom? We think we are talking to, privately or not.
 Integrity - has the system been compromised/altered already?
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6.2. Why Computer Security
Computer security is required because computer systems are vulnerable to many threats
that can inflict various types of damage resulting in significant losses. This damage can
range from errors harming database integrity to fires destroying entire computer centers.
There may be several forms of damage, which are obviously interrelated. These include:

Damage or destruction of computer systems.

Damage or destruction of internal data.

Loss of sensitive information to hostile parties.

Use of sensitive information to steal items of monetary value.

Use of sensitive information against the organization's customers, which may
result in legal action by customers against the organization and loss of customers.

Damage to the reputation of an organization.

Monetary damage due to loss of sensitive information, destruction of data, hostile
use of sensitive data, or damage to the organization's reputation.

Losing the ability to use the system
6.3. Security Threats
A threat is a potential violation of security. The effects of various threats vary
considerably: some affect the confidentiality or integrity of data while others affect the
availability of a system.
A computer security threat can be any person, act, or object that poses a danger to
computer security. Generally, environments can be hostile because of

Physical threats - weather, natural disaster, bombs, power failures, etc.

Human threats - stealing, trickery, bribery, spying, sabotage, accidents.

Software threats - viruses, Trojan horses, logic bombs, denial of service,
trapdoor.
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1. Fraud and Theft
Computer systems can be exploited for both fraud and theft both by "automating"
traditional methods of fraud and by using new methods. For example, individuals may
use a computer to skim small amounts of money from a large number of financial
accounts, assuming that small discrepancies may not be investigated. Financial systems
are not the only ones at risk. Systems that control access to any resource are targets (e.g.,
time and attendance systems, inventory systems, school grading systems, and longdistance telephone systems). Insiders or outsiders can commit computer fraud and theft.
Insiders (i.e., authorized users of a system) are responsible for the majority of fraud.
Since insiders have both access to and familiarity with the victim computer system
(including what resources it controls and its flaws), authorized system users are in a
better position to commit crimes. Insiders can be both general users (such as clerks) and
technical staff members. An organization's former employees, with their knowledge of an
organization's operations, may also pose a threat, particularly if their access is not
terminated promptly.
2. Loss of Physical and Infrastructure Support
The loss of supporting infrastructure includes power failures (outages, spikes, and
brownouts), loss of communications, water outages and leaks, sewer problems, lack of
transportation services, fire, flood, civil unrest, and strikes.
3. Malicious Hackers
The term malicious hackers, sometimes called crackers, refer to those who break into
computers without authorization. They can include both outsiders and insiders. Much of
the rise of hacker activity is often attributed to increases in connectivity in both
government and industry. One 1992 study of a particular Internet site (i.e., one computer
system) found that hackers attempted to break in at least once every other day. The
hacker threat should be considered in terms of past and potential future damage.
Although current losses due to hacker attacks are significantly smaller than losses due to
insider theft and sabotage, the hacker problem is widespread and serious.
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4. Threats to Personal Privacy
The accumulation of vast amounts of electronic information about individuals by
governments, credit bureaus, and private companies, combined with the ability of
computers to monitor, process, and aggregate large amounts of information about
individuals have created a threat to individual privacy. The possibility that all of this
information and technology may be able to be linked together has arisen as a specter of
the modern information age.
5. Malicious Code
Malicious code refers to viruses, worms, Trojan horses, logic bombs, and other
"uninvited" software. Sometimes mistakenly associated only with personal computers,
malicious code can attack other platforms.
Viruses
Virus is self-duplicating computer program that interferes with a computer's hardware or
operating system (the basic software that runs the computer). Viruses are designed to
duplicate or replicate them to avoid detection. Like any other computer program, a virus
must be executed for it to function—that is, it must be located in the computer's memory,
and the computer must then follow the virus's instructions. These instructions are called
the payload of the virus. The payload may disrupt or change data files, display an
irrelevant or unwanted message, or cause the operating system to malfunction.
There are five categories (types) of viruses, they are: parasitic or file viruses, bootstrap
sector, multi-partite, macro, and script viruses.
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Worms
Worm is a program that propagates itself across computers, usually by spawning copies
of itself in each computer's memory. A worm might duplicate itself in one computer so
often that it causes the computer to crash. Sometimes written in separate “segments,” a
worm is introduced surreptitiously into a host system either for “fun” or with intent to
damage or destroy information. The term comes from a science-fiction novel and has
generally been superseded by the term virus. Worms can form segments across a network
and damage the network by using its resources (memory space) highly. The segments of
worms across a network can communicate strengthen their damage.
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Trojan Horses
There are other harmful computer programs that can be part of a virus but are not
considered viruses because they do not have the ability to replicate. These programs fall
into three categories: Trojan horses, logic bombs, and deliberately harmful or malicious
software programs that run within Web browsers, an application program such as Internet
Explorer and Netscape that displays Web sites.
A Trojan horse is a program that pretends to be something else. A Trojan horse may
appear to be something interesting and harmless, such as a game, but when it runs it may
have harmful effects. The term comes from the classic Greek story of the Trojan horse
found in Homer’s Iliad.
Bombs
A bomb infects a computer’s memory, but unlike a virus, it does not replicate itself. A
logic bomb delivers its instructions when it is triggered by a specific condition, such as
when a particular date or time is reached or when a combination of letters is typed on a
keyboard. A logic bomb has the ability to erase a hard drive or delete certain files.
6.4. Techniques to Reduce Security problems
Backup
Storing backup copies of software and data and having backup computer and
communication capabilities are important basic safeguards because the data can then be
restored if it was altered or destroyed by a computer crime or accident. Computer data
should be backed up frequently and should be stored nearby in secure locations in case of
damage at the primary site. Transporting sensitive data to storage locations should also be
done securely.
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Encryption
Another technique to protect confidential information is encryption (Encryption, process
of converting messages or data into a form that cannot be read without decrypting or
deciphering it. The root of the word encryption—crypt—comes from the Greek word
kryptos, meaning “hidden” or “secret.”)
Computer users can scramble information to prevent unauthorized users from accessing
it. Authorized users can unscramble the information when needed by using a secret code
called a key. Without the key the scrambled information would be impossible or very
difficult to unscramble.
Approved users
Another technique to help prevent abuse and misuse of computer data is to limit the use
of computers and data files to approved persons. Security software can verify the identity
of computer users and limit their privileges to use, view, and alter files. The software also
securely records their actions to establish accountability. Military organizations give
access rights to classified, confidential, secret, or top-secret information according to the
corresponding security clearance level of the user. Other types of organizations also
classify information and specify different degrees of protection.
Passwords
Passwords are confidential sequences of characters that allow approved persons to make
use of specified computers, software, or information. To be effective, passwords must be
difficult to guess and should not be found in dictionaries. Effective passwords contain a
variety of characters and symbols that are not part of the alphabet. To thwart imposters,
computer systems usually limit the number of attempts and restrict the time it takes to
enter the correct password.
A more secure method is to require possession and use of tamper-resistant plastic cards
with microprocessor chips, known as “smart cards,” which contain a stored password that
automatically changes after each use. When a user logs on, the computer reads the card's
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password, as well as another password entered by the user, and matches these two
respectively to an identical card password generated by the computer and the user's
password stored in the computer in encrypted form. Use of passwords and "smart cards"
is beginning to be reinforced by biometrics, identification methods that use unique
personal characteristics, such as fingerprints, retinal patterns, facial characteristics, or
voice recordings.
Firewalls
Computers connected to communication networks, such as the Internet, are particularly
vulnerable to electronic attack because so many people have access to them. Using
firewall computers or software placed between the networked computers and the network
can protect these computers. The firewall examines, filters, and reports on all information
passing through the network to ensure its appropriateness. These functions help prevent
saturation of input capabilities that otherwise might deny usage to legitimate users, and
they ensure that information received from an outside source is expected and does not
contain computer viruses.
Disaster Recovery Plans
Organizations and businesses that rely on computers need to institute disaster recovery
plans that are periodically tested and upgraded. This is because computers and storage
components such as diskettes or hard disks are easy to damage. A computer's memory
can be erased or flooding, fire, or other forms of destruction can damage the computer’s
hardware. Computers, computer data, and components should be installed in safe and
locked facilities.
Anti-viral Tactics
 Preparation and Prevention
Computer users can prepare for a viral infection by creating backups of legitimate
original software and data files regularly so that the computer system can be restored if
necessary. Viral infection can be prevented by obtaining software from legitimate sources
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or by using a quarantined computer to test new software—that is, a computer not
connected to any network. However, the best prevention may be the installation of
current and well-designed antiviral software. Such software can prevent a viral infection
and thereby help stop its spread.
Virus Detection
Several types of antiviral software can be used to detect the presence of a virus. Scanning
software can recognize the characteristics of a virus's computer code and look for these
characteristics in the computer's files. Because new viruses must be analyzed as they
appear, scanning software must be updated periodically to be effective. Other scanners
search for common features of viral programs and are usually less reliable. Most antiviral
software uses both on-demand and on-access scanners. On-demand scanners are launched
only when the user activates them. On-access scanners, on the other hand, are constantly
monitoring the computer for viruses but are always in the background and are not visible
to the user. The on-access scanners are seen as the proactive part of an antivirus package
and the on-demand scanners are seen as reactive. On-demand scanners usually detect a
virus only after the infection has occurred and that is why they are considered reactive.
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