COMPUTER ORGANIZATION AND APPLICATION
COMPUTER GENERATIONS
Classification of computers into generations is based on fundamental technology. It provides a
framework for growth in the computer industry.
First generation computers
i)
ii)
iii)
iv)
v)
vi)
vii)
They were bulky in size and required a large space for installation
Thousands of vacuum tubes were for processing which emitted large amounts of heat
They consumed a lot of power
Had frequent hardware failures
Operating speed was slow
Commercial production was difficult and costly
used machine language for programming making id difficult to program
Examples include:
ENIAC- electronic numerical integrator
UNIVAC- universal automatic computer
EDVAC- electronic discrete variable automatic computer
Second generation computers
A new electronic switching device called transistor was invented.
i) They were more reliable than first generation hence less prone to hardware failures
ii) It easier to program since assembly language was used for programming
iii) The memory of this computer was composed of magnetic cores.
iv) They were faster, smaller, consumed less power, and emitted less power than first generation
computers.
Examples include:
IBM 3070, Honeywell 400
Third generation computers
The use of integrated circuit chip(IC) was introduced. IC’s means circuits consisting of several
electronic components such as transistors, capacitors, and resisters grown on a simple chip of silicon
eliminating wired integration between components.
i) They were more powerful, reliable, less expensive, smaller, and cooler to operate than the first
generation of computers,
ii) They were easier to upgrade
iii) They had faster and larger capacity secondary storage
iv) Mini computers became affordable even to small companies
v) Time sharing operating system allowed interactive usage, simultaneous use many users and online
systems.
vi) They have standardization of high level programming languages for programming
vii) They employed the use of small scale integration technology
Examples include:
IBM 360, IBM 370, PDP-8
Fourth generation computers
The average number of electronic components packed on a silicon chip doubled each year. They
employed the use of large scale integration where it is possible to integrate more than 30,000
electronic components on a single chip.
i)
They were more powerful,smaller,consumed less power, emitted less heat than the 1st, 2nd and 3rd
generation computers
ii) They were more reliable and less prone to hardware failures
iii) They had faster and larger primary and secondary storage compared to 3rd generation.
iv) Use of standard high-level programming languages allowed portability of programs
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v)
vi)
Graphical user interface made the systems easier for users to learn how to use
Personal computers even more affordable to individuals
Fifth generation computers
This was followed by ultra large scale integration where it was possible to integrate about one
million electronic components in a single chip. This progress led to creation of a micro-processor. A
micro-processor contains all the circuits needed to perform arithmetic logic and control
functions/core activities of the computer on a single chip.
i) They employ the use of a large memory with a very fast access time. Hard disks used for secondary
storage are cheaper, smaller, and larger in capacity.
ii) Floppy disks, compact disks are used as portable medium for porting programs and data from one
computer to another.
iii) There is a spread of high speed computer networking which has enabled multiple computers to be
connected to enable them to communicate and share data.
iv) Companies have developed graphical user interfaces to enable individuals who are not computer
professionals to use them by making computers more user friendly.
v) Portable computers are more smaller allowing users to use computing facility while traveling
vi) They are more powerful, consume less power, more reliable than the earlier generations.
vii) Many types of computers , in all price ranges have come up
CLASSIFICATION OF COMPUTER SYSTEMS
a) Super computers
Computers are classified according to size, processing speed, cost e.t.c they are classified as
They are very powerful (have high processing power). They are used for processing large amounts
of data related to a single complex problem. They are optimized for those applications that are
beyond the reach of the contemporary computers. They are used for scientific purposes, weather
forecasting, space missions, nuclear research and parallel computing. It has limited use because of
the price tag. Mostly research centre’s and government agencies with scientific/ engineering
functions have them. Only a few are operational.
b) Main frame computers
It is a large general purpose computer to which other computers can be connected to so that they
can share the facilities of the information processing facility e.g ATM’S they are designed to be
used by hundred’s of users simultaneously and handle vast amounts of data. It has a sophisticated
operating system that can run both batch and online processing programs. It allows multi user
online use, multi tasking and multi processing. The primary function of mainframes today is to
support large databases. They are owned by large businesses and government organisations that
need a central depository of data that can be managed and controlled centrally.
c) Mini computers
Hardware components of these are often similar to those of mainframes in terms of functionality
although their features and capabilities are less powerful. Depending on the system design, a mini
computer can support a large number of terminals either attached locally or using communication
lines. It is an intermediate size computer that can run the same kinds of applications as mainframes
but lack the speed and storage capacity.
d) Micro computers
This is a computer whose C.P.U is a micro-processor. A micro processor is one whose components
are on a single integrated circuit chip. Hardware components are also similar to those of mainframes
and mini computers. However they are generally of a smaller scale e.g portable computer,
embedded computers e.g calculators, watches
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Advantages of a Computer
Computer systems have many advantages, some of which are listed as follows:
1. Computers operate at high speed, hence there may be cost savings e.g. staff costs,
equipment costs etc.
2. Computers produce accurate results (information).
3. Computers can work continuously without getting bored or tired.
4. Computers can work on voluminous data items.
5. Computers can work on any problem (solve any problem), provided a relevant
instruction set (program) are availed to them.
6. Computers are flexible, (i.e. they can adapt to any workload without much strain).
7. Computers produce reliable information.
8. Large volumes of data can be conveniently stored, assessed and altered.
9. Computers can provide useful information to management for control and decision
making.
10. Computers help to reduce paper work significantly.
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11. Computer systems can reduce the number of persons required for performing
various organizational activities.
12. The use of computers for office activities reduces the requirements of office space.
Notes:
The above advantages may bring several benefits into the organization. Such benefits may
be classified as cost savings, improved data processing quality and better management
information.
1.3.4 Disadvantages of Using a Computer
As much as we say that computers are useful to humans, they come with some
disadvantages are given as follows:
a) Computers are costly, if it is more efficient and effective to carry out the
processing tasks without employing the use of computers, then computerization
is needless. Computer prices have gone down tremendously, however, but they
are still out of reach for many poor people
b) Due to rapid change in the computer technology, the computer and related
facilities can become outdated very fast, hence posing a risk of capital loss.
c) Changing from the old system, the manual system, to a computerized system is
a gradual process during which there is interruption in the normal working
environment.
d) There is usually the fear that installing the computer into an organization might
result in replacing some human employees.
e) In case the computer system fails, it might be extremely difficult to revert back
to the old manual system. For this reason, stand by procedures are necessary,
but expensive.
f) Controlling a computer file's contents is a difficult exercise due to the fact that
the contents of files of computer based systems are none-human sensible. There
is further fear that such contents may suffer from fraudulent acts by the people
who know them.
g) Manual systems, though slow, are more flexible as compared to the
computerized systems.
h) Employees may resist the adoption of computers by organizations in which they
work
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1.3.4 Resistance from Employees
It is human nature to fear the unknown; we are always comfortable doing work the way we are
used to sometimes, there is resistance from employees/users to adopt new technology. The main
reasons for this resistance are:
 Fear of change: People are creatures of habit and hence are afraid of change
 Fear of losing their jobs: People usually associate the computer with loss of jobs
and hence they will be afraid that they might end up losing their jobs.
 Fear of failure: Since the computer is very new in a given working environment,
the people will be afraid that they might never adapt to it
 Loss of control: The management will be afraid that once a computer is implemented,
they might lose control of the organization.
 Lack of Understanding: The user may not understand the benefits of the computer
system in their jobs. Thus this will create resistance since the computer will be looked
upon as an intruder.
COMPUTER SOFTWARE
Software is a set of computer programs, procedures and associated documents which describe the
programs and how they are to be used. A computer program is a sequence of logical instructions,
written in a language, which can be understood by the computer which controls the activity of processing
by the computer.
Most software can be divided into two major categories
 System software
 Application software
Application software
A set of one or more programs, designed to solve a specific problem or do a specific task. E.g.
 Word processing – enables us to make use of a computer system for editing, viewing, formatting,
storing, retrieving, and printing documents e.g. Microsoft word, word perfect, word pro, WordStar.
 Spreadsheet software – a numeric data analysis tool, which allows us to create a kind of
computerized ledger e.g. Microsoft excel, lotus 1-2-3
 Database software – a set of one or more programs, which enable us to create a database, maintain it,
organize its data in desired fashion and to selectively retrieve useful information from it. Microsoft
access, Quattro pro, VISIC ALC
 Graphics software – enables us to use a computer system for creating, editing, viewing, storing,
retrieving and printing designs, drawings, pictures, graphs and anything else that can be draw in the
traditional manner.
 Education software – allows a computer system to be used as a teaching and learning tool.
 Entertainment software – allows a computer system to be used as an entertainment
System software
System software is a set of one or more programs, designed to control the operation and extend the
processing capability of a computer system. In general it performs the following functions:
 Supports the development of other application software
 Supports the execution of other application software
 Monitors the effective use of various hardware resources e.g. CPU, memory, peripheral
 Communicates with and controls the operation of peripheral devices e.g. printers, hard disk
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Some of the most commonly known types of system software are:
 Operating systems – it takes care of the effective and efficient utilization of all the hardware and
software components of the computer system e.g. Microsoft DOS, Windows, UNIX, LINUX
 Programming language translators – they transform the instructions prepared by programmers
in a programming language into a form which can be interpreted and executed by a computer
system e.g. Assemblers, Compilers, Interpreters
 Communication software – they enable transfer of data and programs from one computer
system to another in a network environment
 Utility programs – a set of programs which help user in system maintenance tasks and in
performing tasks of routine nature. E.g. formatting hard disks
Operating System
It’s an integrated set of programs that controls the resources (CPU, Memory, Input/output devices) of a
computer system and provides its users with an interface or virtual machine that is more convenient to use than
the bear machine.
The two primary objectives of Operating System use.
(i)
Making a computer system convenient to use i.e. hides details of Hardware resources from the
programmer and provides him with a convenient interface of using computer system. It acts as an
intermediary between hardware and software providing a high level interface to low level hardware
and making it easier for the software to access the use of those resources.
(ii)
Managing computer resources. This involves performing such tasks as keeping track of who is using
which resource, granting resource requests, accounting for resource usage, and mediating
conflicting requests from different programs and users.
Main functions of Operating System.
(i)
(ii)
(iii)
(iv)
(v)
Process Management
Memory Management
File Management
Security
Command Interpretation.
Operating System arose historically as people needed to solve problems associated with using computers.
Much of operating system history is driven by relative cost factors of hardware and people. Hardware started
out fantastically expensive, relative to people and the relative costs has been decreasing ever since.
(i) Process Management
A process (job) is a program in execution. The main objective of the process management module of
operating system is to manage the process submission to the system in a manner to minimize the idle time of
various processors (CPU, I/O processors e.t.c) of the computer system.
It takes care of creation and deletion of processes, scheduling of various system resources to different
processes requesting them and providing mechanisms for synchronization and communication among
processes
Multiprogramming: This is the interleaved execution of two or more different and independent programs by
same computer. Two or more user programs are placed in the main memory and executed consecutively.
Note: The CPU is capable of conducting / executing only one instruction at a time.
Requirements of Multi-programming System
(a) Large Memory – Required to accommodate a good number of user programs along with the operating
system.
(b) Memory Protection- To prevent a job in one memory partition from changing information or instructions
of a job in another memory partition.
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(c) Job status preservation- preserves the job’s complete status information when the CPU is taken away
from it and restores this information back, before the CPU is given back to it again.
(d) Proper job mix- to effectively overlap the operations of the CPU and Input/output devices.
(e) CPU scheduling – To decide which of the ready jobs should be allocated the CPU for execution.
Multitasking: Technically speaking it is the same as multiprogramming. It is system’s capability to
concurrently work on more than one task/process. Some authors prefer using multiprogramming for multi-user
systems and multitasking for single-user systems. The progress of different tasks can be viewed on different
windows in a multitasking system.
Multiprocessing: Used to describe interconnected computer configurations or computers with two or more
CPUs which have the ability to simultaneously execute several programs. Multiprocessing systems have better
performance and reliability.
Note:
These systems however require a very sophisticated operating system to schedule, balance and coordinate
input, output and processing activities of multiple processors. The design of such operating system is a
complex and time taking job. They are expensive to procure and maintain.
(ii) Memory Management
Its job is to keep tracks of which parts of memory are in use and which are not in use, to allocate memory to
processes when they need it and de allocate when they are done.
Virtual Memory – Extension of memory which allows execution of processes that might not be completely
loaded in memory. It is often described as hierarchy of two storage systems-one is a low cost, large capacity,
low speed system (hard disk) the other is a high cost, small capacity high speed system (RAM). The operating
system manages the two storage systems in such as a way that the users of the system feel that they have
access to a single, large, directly addressable and fast main memory.
(iii) File Management
A file is a collection of related information. Every file has a name, data and attributes. The name of a file
uniquely identifies it in the system and is used by its users to access it. The file management module of an
operating system takes care of file related activities such as accessing, naming, sharing and protection files.
File Access Methods
To use information stored in a file, it must be accessed and read into computer memory. The two commonly
supported file access methods at operating system level are:
1. Sequential Access.
2. Random Access.
1. Sequential Access. A process can read the bytes or records in the file in the order in which they are stored,
starting at the beginning. A sequential file can however be rewound and read as often as needed.
2. Random Access. Information stored in a random access file can be accessed randomly, irrespective of the
order in which the bytes or records are stored.
File Operation
An operating system provides a set of operations to deal with files and their contents. A typical set of file
operations provided by an operating system may be as follows;
1. Create-Is used to create a new file.
2. Delete- Is used to delete an existing file, which is no longer needed.
3. Open-Is used to open an existing file, when a user wants to start using it.
4. Close-Is used a close a file, when a user has finished using it.
5. Read-Is used to read data stored in a file.
6. Write-Is used to write new data in a file.
7. Seek-. Is used with random access file to first position the read/ write pointer to a specific place in the
file, so that data can be read from, or written to, that position.
8. Get attributes- Is used to access the attributes of a file.
(iv)
Security Management
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Security in a computer system deals with protecting the various resources and information of a computer
system against destruction and unauthorized access.
A total approach to computer security involves both external and internal security;
 External security deals with securing the computer system against external factors such as fires, floods,
earthquakes, stolen disks, leaking out of stored information by a person who has access to the information.
For external security, the commonly used methods include; maintaining adequate backup copies of stored
information at places far away from the original, information using security guards to allow entry of only
authorized persons into the computer centre. Allowing access to sensitive information to only trusted users
etc.
 Internal security deals with:
(a) User Authorization – Once a user is allowed physical access to the computing facility, the users
identification must be checked by the system before he/she can actually use the facility. E.g. Voice
recognition systems, Finger prints, Eye scanning, ATMs.
(b) Access Control – Even when a user passes the authorization stage and is allowed to use the computing
facility, a way is needed to prohibit the user from accessing the system’s resources/information that he or she
is not authorized to access e.g. pin number, password.
(c) Cryptography – Even if a user manages to gain access to the information which he/she is not authorized
to access, a way is need to ensure that the user cannot make use of that information. Cryptography works on
the idea that if it is not possible to ensure access control, it is better to prevent comprehension of information.
(v)
Command Interpretation
The command interpretation module of an operating system provides a set of commands using which the
user can give instructions to the computer for getting some job done by it. When a user gives instructions to
the computer, the interpreter takes care of interpreting the command and directing the system resources to
handle the request hence the command interpreter provides a user interface to hide the hardware details of
the system from the user. The two broad categories of user interface supported by various operating systems
are;
(i)
Command Line Interface
(ii) Graphical user interface(GUI)
(a) Command Line interface: This is a textual user interface in which the user gives instructions to a
computer by typing commands. To enter a command, the user uses the keyboard to key in words and
symbols e.g UNIX and DOS.
(b) Graphical user interface (GUI); It is much easier to learn and use than command line interface. A GUI
provides the screen full of graphical icons and menus and allows the user to make a rapid selection from the
displayed icons or menus to use instructions to the computer.
A point and-draw device is normally used to rapidly point to and select a particular graphical icon or menu
item from the multiple options displayed on the screen. E.g. Window, LINUX
Popular Operating Systems.
1. UNIX.
It is a multi- user, timesharing operating system. It was developed in the early 1970s at the Bell labs by Ken
Thompson and Dennis Ritchie for a small PDP- 11 computer. It was the first operating system to be written
in a high level language (C – Language) since UNIX was written in C- language moving it to a new machine
was much easier. This was an important reason for its large popularity and availability on a wide variety of
systems. It has multi- processing capability but is also used in single – processor computers. Additionally, it
has a command driven interface. It has been the most widely used multi-user network Operating System.
There are many versions that can run on many different designs of computer i.e. mainframe computers,
micro and mini- computers.
2. DOS
Stands for “Disk Operating System”. It is a single user operating and IBM compatible Pcs. It was introduced
in 1981 by Microsoft and IBM and was the most popular Operating System for personal computers in the
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1980s. Its popularity stared reducing in the 1990s with the launch of Ms Window Operating System. It is a
single user operating single tasking and command operating system.
3. Microsoft Windows:
Developed by Microsoft to overcome the limitation of its own Dos operating system was initially developed
as a graphical user interface for Dos not as an operating system. The first success version of this operating
System was Windows 3.00 released in1990. Other versions were Windows 95, 98, 2000, XP, Millennium,
NT, and Vista.
Windows is a single- user- multi- tasking operating system. Progress is represented by icons which provide
the user with ease of manipulating of file and programs i.e. clicking, double – clicking, drag, drop, etc.
4. LINUX
An open –source operating system enhanced and backed by thousands of programmers worldwide. Its
multitasking operating systems which was originally designed for Pcs.
The Linux is derived from its inventor called Linus Torvalds. He was a student at the University of Helsinki,
Finland in the early 1990s when he wrote the first version of Linux as a toy project. He later posted the code
on the internet and asked programmers to help him build it into a working system. The result was Linux.
Torvalds holds the copyright but permits free distribution of the source code. He oversees the development
of the Kernel and its own trademark. When someone submits a change or a feature, torvalds and his core
team of kernel developers review the merit of adding it to the source code
Ways of acquiring computer software
An organization can develop proprietary software that is unique, customized programs for a specific
application area and on the other hand a company can purchase and use an existing software package (off-the –
shelf software).
Customized/proprietary software
Programmers determine the type of software to be developed according to users specific problems/
needs. The software can be developed by an in-house development team or by a contracted software company
but based on the specific user needs.
Advantages
i)
You can get exactly what you need in terms of reports, features e.t.c
ii)
Being involved in development offers a further level in control over results
iii)
There’s more flexibility in making modifications that may be required due to changes in technology or
environment.
iv)
You don’t pay for features which you won’t use
v)
The user get in-house’s a competitive advantage
Disadvantages
i)
The cost of developing the software is high/expensive
ii)
It takes more time to develop
iii)
It is vulnerable or prone to errors because it has not been thoroughly tested
Off –the –shelf software
They are readily available across the shops or from software vendors.
Advantages
i)
They are cheap because development and maintenance costs are shared among many customers
ii)
They take less time to develop
iii)
A user can start using the software almost immediately he purchases it
iv)
Possibility of errors is low because it has been properly tested
i)
ii)
Disadvantages
The user pays for features he/ she may never use
It does not solve all user problems
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iii)
iv)
It is difficult to make alterations and modifications
A user may be locked/forced to using the products of a particular supplier/manufacturer especially
upgrades
COMPUTER HARDWARE
These are the physical or tangible components of a computer system. They perform five functions including;
input, output, storage, processing and communication.
INPUT DEVICES
Input is any data or instructions you enter into the memory of a computer. Once input is in memory, the CPU
can access it and process the input into output. Four types of input are data, programs, commands, and user
responses:
An input device is any hardware component that allows you to enter data, programs, commands, and
user responses into a computer. Input devices include the keyboard, pointing devices, scanners and reading
devices, digital cameras, audio and video input devices, and input devices for physically challenged users
THE KEYBOARD
You enter data into a computer by pressing the keys on the keyboard. Desktop computer keyboards
usually have from 101 to 105 keys, while keyboards for smaller computers such as laptops contain fewer keys.
A computer keyboard includes keys that allow you to type letters of the alphabet, numbers, spaces, punctuation
marks, and other symbols such as the dollar sign ($) and asterisk (*). A keyboard also contains special keys
that allow you to enter data and instructions into the computer.
Function keys are special keys programmed to issue commands and accomplish certain tasks.
Keyboards also contain keys that can be used to position the insertion point on the screen. The insertion point
is a symbol that indicates where on the screen the next character you type will display .Many of the newer
keyboards include buttons that allow you to access your CD/DVD drive and adjust speaker volume.
POINTING DEVICES
A pointing device is an input device that allows you to control a pointer on the screen. In a
graphical user interface, a pointer is a small symbol on the display screen. A pointer often takes the shape of a
block arrow, an I-beam (T) or a pointing hand. Using a pointing device, you can position the pointer to move
or select items on the screen. For example, you can use a pointing device to move the insertion point; select
text, graphics, and other objects; and click buttons, icons, links, and menu commands.
Common pointing devices include the mouse, trackball, touchpad, pointing stick, joystick, touch
screen, light pen, and graphics tablet.
Trackball
Some users opt for alternative pointing devices other than a mouse, such as a trackball.
Whereas a mechanical mouse has a ball mechanism on the bottom, a trackball is a stationary pointing device
with a ball mechanism on its top.
Touchpad
A touchpad or trackpad is a small, flat, rectangular pointing device that is sensitive to pressure and motion. To
move the pointer using a touchpad, you slide your fingertip across the surface of the pad. Some touchpads have
one or more buttons around the edge of the pad that work like mouse buttons; on others, you tap the pad’s
surface to simulate mouse operations such as clicking.
Stick
A pointing stick is a pressure-sensitive pointing device shaped like a pencil eraser that was first developed by
IBM for its laptop computers. Because of its small size, the pointing stick is positioned between keys on the
keyboard. To move the pointer using a pointing stick, you push the pointing stick with your finger. The pointer
on the screen moves in the direction that you push the pointing stick.
Joystick
Users running game software such as a driving or flight simulator may prefer to use a joystick as their pointing
device. A joystick is a vertical lever mounted on a base. You move the lever in different directions to control
the actions of a vehicle or player. The lever usually includes buttons called triggers that you can press to
activate certain events. Some joysticks also have additional buttons that you can set to perform other actions.
Touch Screen
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A monitor that has a touch-sensitive panel on the screen is called a touch screen. You interact with the
computer by touching areas of the screen with your finger, which acts as an input device. Because they require
a lot of arm movements, touch screens are not used to enter large amounts of data. Instead you touch words,
pictures, numbers, or locations identified on the screen.
Pen Input
Many input devices use an electronic pen instead of a keyboard or mouse for input. Some of these devices
require you to point to onscreen objects with the pen; others allow you to input data using drawings,
handwriting, and other symbols that are written with the pen on a surface.
LIGHT PEN
A light pen is a handheld input device that contains a light source or can detect light. Some light pens require a
specially designed monitor, while others work with a standard monitor. Instead of touching the screen with
your finger to interact with the computer, you press the light pen against the surface of the screen or point the
light pen at the screen and then press a button on the pen. Light pens are used in applications where desktop
space is limited such as in the health-care field or when a wide variety of people use the application, such as
electronic voting.
GRAPHICS TABLET
A graphics tablet, also called a digitizer or digitizing tablet, consists of a flat, rectangular, electronic plastic
board used to input drawings, sketches, or other graphical data. Each location on the graphics tablet
corresponds to a specific location on the screen. When you draw on the tablet with either an electronic pen or a
puck, the tablet detects and converts the movements into digital signals that are sent into the computer.
SCANNERS AND READING DEVICES
These devices capture data from a source document, which is the original form of the data. When using a
keyboard or pointing device to enter data, the source document might be a timecard, order blank, invoice, or
any other document that contains data to be processed.
Devices that capture data directly from source documents include optical scanners, optical character
recognition devices, optical mark recognition devices, bar code scanners, and magnetic-ink character
recognition readers. Examples of source documents used with these devices include advertisements, brochures,
photographs, inventory tags
Optical Scanner
An optical scanner, usually simply called a scanner, is a light-sensing input device that reads printed text and
graphics and then translates the results into a form the computer can use. The density of the dots, known as the
resolution, determines sharpness and clearness of the resulting image. Resolution typically is measured in dots
per inch (dpi), and is stated as the number of columns and rows of dots
OPTICAL CHARACTER RECOGNITION
Optical character recognition (OCR) is a technology that involves reading typewritten, computer-printed, or
handwritten characters from-ordinary documents and translating the images into a form that the computer can
understand. Most OCR devices include a small optical scanner for reading characters and sophisticated
software for analyzing what is read.
Optical Mark Recognition (OMR)
Optical mark recognition (OMR) devices read hand-drawn marks such as small circles or rectangles. A person
places these marks on a form, such as test, survey, or questionnaire answer sheet
Bar Code Scanners
A bar code scanner uses leaser beams to read bar codes (Figure 3-4). A bar code scanner is an identification
code that consists of a set of vertical lines and spaces of different widths. The bar code, which represents some
data that identifies the item, is printed on a product’s package or on a label that is affixed to a product so it can
be read by a bar code scanner. The bar code scanner uses light patterns form the bar code lines to identify the
item.
Magnetic Ink Character Recognition Reader
A Magnetic-ink character recognition (MICR) reader is used to read text printed with magnetized ink. MICR is
used almost exclusively by the banking industry for check processing. Each check in your checkbook has
precoded MICR characters on the lower-left edge; these characters represent the bank number, your account
number, and the check number.
DIGITAL CAMERAS
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A digital camera allows you to take pictures and store the photographed images digitally instead of on
traditional film. With some digital cameras, you download, or transfer a copy of, the stored pictures to your
computer by connecting a cable between the digital camera and your computer and using special software
included with the camera. With other digital cameras, the pictures are stored directly on storage media such as
a floppy disk, PC Card, or flash card
Audio Input
Audio input is the process of entering (recording) music, speech, or sound effects. To record high quality
sound, your personal computer must have a sound card. (Most new computers today come equipped with a
sound card.) Sound is entered via a device such as a microphone, tape player, or audio CD player, each of
which plugs into a port on the sound card. External MIDI devices such as an electric piano keyboard also can
connect to the sound card for audio input.
Speech Recognition
Another use for a microphone is speech recognition. Speech recognition, also called voice recognition, is the
computer’s capability of distinguishing spoken words. Speech recognition programs do not understand speech;
they only recognize a vocabulary of certain words. The vocabulary of speech recognition programs can range
from two words (such as Yes and No) to more than sixty thousand words.
Video Input
Video input or video capture is the process of entering a full-motion recording into a computer and storing the
video on a hard disk or some other medium. To capture video, you plug a video camera, VCR, or other video
device into a video capture card, which is an expansion card that converts the analog video signal into a digital
signal that a computer can understand. (Most new computers are not equipped with a video capture card.) Once
the video device is connected to the video capture card, you can begin recording. After you save the video on a
hard disk, you can play it or edit it using video-editing software.
Videoconferencing
A videoconference is a meeting between two or more geographically separated individuals who use a network
or the Internet to transmit audio and video data. To participate in a videoconference, you must have a
microphone, speakers, and a video camera mounted on your computer. As you speak, members of the meeting
hear your voice on their speakers. Any image in front of the video camera, such as a person’s face, displays in
a window on each participant’s screen.
OUTPUT DEVICES
WHAT IS OUTPUT?
Output is data that has been processed into a useful form called information. That is, a computer processes
input into output. Computers generate several types of output, depending on the hardware and software being
used and the requirements of the user. You may choose to display or view this output on a monitor, print it on
a printer, or listen to it through speakers or a headset. Four common types of output are text, graphics, audio,
and video.
An output device is any computer component capable of conveying information to a user. Commonly used
output devices include display devices, printers, speakers, headsets, data projectors, facsimile machines, and
multifunction devices
1. PRINTERS
A printer is an output device that produces text and graphics on a physical medium such as paper or
transparency film. Printed information is called hard copy because the information exists physically and is a
more permanent form of output than that presented on a display device (soft copy).Hard copy, also called a
printout, can be printed in portrait or landscape orientation.
Generally, printers can be grouped into two categories: impact and no impact. Printers in each of these
categories are discussed in the following sections.
i) Impact Printers
An impact printer forms characters and graphics on a piece of paper by striking a mechanism against an ink
ribbon that physically contacts the paper. Because of the striking activity, impact printers generally are noisy.
Many impact printers do not provide letter quality print.
Impact printers also are ideal for printing multipart forms because they easily can print through many layers of
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paper. Finally, impact printers are used in many factories and at retail counters because they can withstand
dusty environments, vibrations, and extreme temperatures.
Two commonly used types of impact printers are dot-matrix printers and line printers. Each of these printers
is discussed in the following sections.
Dot-matrix printers
A dot-matrix printer is an impact printer that produces printed images when tiny wire pins on a print head
mechanism strike an inked ribbon. When the ribbon presses against the paper, it creates dots that form
characters and graphics. Most dot-matrix printers use continuous-form paper, in which each sheet of paper is
connected together. The pages generally have holes punched along two opposite sides so the paper can be fed
through the printer
Line Printer
A line printer is a high-speed impact printer that prints an entire line at a time. The speed of a line printer is
measured by the number of lines per minute (1pm) it can print. Capable of printing up to 3,000 lines per
minute (1pm), these printers often are used with mainframes, minicomputers, or with a network in applications
such as manufacturing, distribution, or shipping.
ii) Non impact Printers
A non impact printer forms characters and graphics on a piece of paper without actually striking the paper.
Some spray ink, while others use heat and pressure to create images. Because these printers do not strike the
paper, they are much quieter than the previously discussed impact printers.
Three commonly used types of non impact printers are ink-jet printers, laser printers, and thermal printers.
Each of these printers is discussed in the following sections.
Inkjet printers
An ink-jet printer is a type of non impact printer that forms characters and graphics by spraying tiny drops of
liquid ink onto a piece of paper. Ink-jet printers usually use individual sheets of paper stored in a removable or
stationary tray. These printers can produce letter-quality text and graphics in both black-and-white and color
on various materials such as envelopes, labels.
Laser printers
A laser printer is a high-speed, high-quality no impact printer. Laser printers for personal computers usually
use individual sheets of paper stored in a removable tray that slides into the printer case. Some laser printers
have trays that can accommodate different sizes of paper, while others require separate trays for letter- and
legal-sized paper. Most laser printers have a manual feed slot where you can insert individual sheets and
envelopes. You also can print transparencies on a laser printer.
Laser printers can print text and graphics in very high quality resolutions, ranging from 600 dpi to 1,200 dpi.
While laser printers typically cost more than ink-jet printers, they also are much faster, printing text at speeds
of four to thirty pages per minute.
Thermal printers
A thermal printer generates images by pushing electrically heated pins against heat-sensitive paper. Standard
thermal printers are inexpensive, but the print quality is low and the images tend to fade over time. Thermal
printers are, however, ideal for use in small devices such as adding machines.
iii) Portable Printers
A portable printer is a small, lightweight printer that allows a mobile user to print from a laptop or handheld
computer while travelling. Barely wider than the paper on which they print, portable printers easily can fit in
a briefcase alongside a laptop computer.
iv)Plotters and Large-Format Printers
Plotters and large-format printers are sophisticated printers used to produce high-quality drawings
such as blueprints, maps, circuit diagrams, and signs. Because blueprints, maps, and other such drawings can
be quite large, these printers typically can handle paper with widths up to 60 inches. Some plotters and largeformat printers use individual sheets of paper, while others take large rolls. These printers are used in
specialized fields such as engineering, drafting, and graphic art and usually are very costly.
Two basic types of plotters are pen plotters and electrostatic plotters. A pen plotter uses one or more
color pen light beams, or a scribing device to draw on paper or transparencies. Pen plotters differ from other
printers in that they produce continuous lines, whereas most printers generate lines by printing a closely spaced
series of dots.
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An electrostatic plotter uses a row of charged wires (called styli) to draw an electrostatic pattern on
specially coated paper and then fuses toner to the pattern. The printed image is composed of a series of very
small dots, which provide high-quality output.
2. DISPLAY DEVICES
A display device is an output device that visually conveys text, graphics, and video information.
Information shown on a display device often is called soft copy, because the information exists electronically
and is displayed for a temporary period of time.
CRT Monitors
A CRT monitor, or monitor, is a display device that consists of a screen housed in a plastic or metal
case. A color monitor displays text, graphics, and video information in color.
Monitors that display only one color are considered monochrome. A monochrome monitor displays text,
graphics, and video information in one color (usually white, amber, or green) on a black background
Monochrome monitors are less expensive than color monitors. Most monitors are referred to by their
viewable size, which is the diagonal measurement of the cathode ray tube inside the monitor and is larger than
the actual viewing area provided by the monitor.
LCD displays
LCD displays commonly are used in laptop computers, handheld computers, digital watches,’ and calculators
because they are thinner and more lightweight than CRT monitors.
Gas Plasma Monitors
For even larger displays, some large business or power users prefer gas plasma monitors, which can measure
more than 42 inches and hang directly on a wall
MONITOR QUALITY
The quality of a monitor’s display depends largely on its resolution, dot pitch, and refresh rate. The resolution,
or sharpness and clarity, of a monitor is related directly to the number of pixels it can display. Resolution is
expressed as two separate numbers: the number of columns of pixels and the number of rows of pixels a
monitor can display. A monitor with a higher resolution displays a greater number of pixels, which provides
a smoother image.
Another factor that determines monitor quality is dot pitch, which is a measure of image clarity. The dot
pitch is the vertical distance between each pixel on a monitor. The smaller the distance between the pixels, the
sharper the displayed image. Text created with a smaller dot pitch is easier to read. To minimize eye fatigue,
you should use a monitor with a dot pitch of .28 millimeters or smaller.
3. AUDIO OUTPUT (speakers, headsets)
Audio is music, speech, or any other sound. Audio output devices are the components of a computer that
produce music, speech, or other sounds, such as beeps. Two commonly used audio output devices are speakers
and headsets
To boost the low bass sounds, you can add a woofer (also called a subwoofer). The stereo speakers and
woofer are connected to ports on the sound card. Most speakers have tone and volume controls so you can
adjust these settings.
4. DATA PROJECTORS
A data projector takes the image that displays on a computer screen and projects it onto a screen so that an
audience of people can see the image clearly. Data projectors can be large devices attached to a ceiling or wall
in an auditorium, or they can be small portable devices Two types of smaller, lower-cost units are LCD
projectors and DLP projectors.
LCD projector
An LCD projector, which uses liquid crystal display technology, attaches directly to a computer and uses its
own light source to display the information shown on the computer screen. Because LCD projectors tend to
produce lower-quality images, some users prefer to use a DLP projector for sharper, brighter images.
A digital light processing (DLP) projector uses tiny mirrors to reflect light, producing crisp, bright, colorful
images that remain in focus and can be seen clearly even in a well-lit room.
5. FACSIMILE (FAX) MACHINE
A facsimile (fax) machine is a device that transmits and receives documents over telephone lines. The
documents can contain text, drawings, or photographs, or can be handwritten. When sent or received via a fax
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machine, these documents are known as faxes. A stand-alone fax machine scans the original document,
converts the image into digitized data, and transmits the digitized image. A fax machine at the receiving end
reads the incoming data, converts the digitized data into an image, and prints or stores a copy of the original
image.
Fax capability also can be added to your computer using a fax modem. A fax modem is a communications
device that allows you to send (and sometimes receive) electronic documents as faxes. A fax modem transmits
computer-prepared documents, such as a word processing letter, or documents that have been digitized with a
scanner or digital camera. A fax modem is like a regular modem except that it is designed to transmit
documents to a fax machine or to another fax modem.
6. VIDEO CARDS
To display color, a color monitor works in combination with a video card, which is included with today’s
personal computers. A video card, also called a graphics card or video adapter, converts digital output into an
analog video signal that is sent through a cable to the monitor. The monitor separates the video signal into red,
green, and blue signals. Electron guns then fire the three color signals to the front of the monitor. These three
dots - one red, one green, and one blue are combined to make up each single pixel.
The number of colors that a video card can display is determined by the number of bits it uses to store
information about each pixel
THE COMPONENTS IN THE SYSTEM UNIT
1. System Unit (CPU)
Arithmetic Logic Unit (ALU)
Performs basic arithmetic calculations
And logical comparisons
CONTROL UNIT (CU)
selects and interprets program ins
tructions and then sees that they
are executed
REGISTERS
Used to hold information on a temporary basis
MAIN MEMORY
RAM and ROM
SECONDARY STORAGE DEVICES
 Floppy disk
 Compact disk
 Hard disk
The system unit is a box-like case housing the electronic components of a computer that are used to process
data. System unit components include the processor, memory module, expansion cards, and ports and
connectors. Many components reside on a circuit board called the motherboard or system board. The
motherboard contains different types of chips, or small pieces of semi conducting material on which one or
more integrated circuits (IC) are etched. One of the more important chips is the central processing unit.
2. The Central Processing Unit
The system unit, sometimes referred to as the processor, interprets and carries out the basic instructions that
operate a computer. That is, most of the devices connected to the computer communicate with the CPU in
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order to carry out a task. The CPU contains the control unit and the arithmetic/logic unit.
The Control Unit
The control unit, one component of the CPU, directs and coordinates most of the operations in the
computer. For every instruction, the control unit repeats a set of four basic operations called the machine
cycle
(1) fetching the instruction or data item from memory,
(2) decoding the instruction into commands the computer understands,
(3) executing the commands, and, if necessary,
(4) storing, or writing, the result to memory.
Arithmetic/Logic unit
The arithmetic/logic unit, another component of the CPU, performs the arithmetic, comparison, and logical
operations. Arithmetic operations include addition, subtraction, multiplication, and division. Comparison
operations involve comparing one data item to another to determine if the first item is greater than, equal to,
or less than the other item. Logical operations work with conditions and logical operators such as AND, OR,
and NOT
Registers
The CPU uses temporary storage locations called registers, to hold data and instructions. The function
include storing location of where instruction was fetched storing an instruction while it is being decoded,
storing data while the ALU processes it, and storing the results of a calculation.
4. Memory
In the system unit, a computer’s memory stores data, instructions, and information. Memory stores three
basic items: The operating system and other system software that control the usage of the computer
equipment; Application programs designed to carry out a specific task such as word processing; and The data
being processed by application programs.
RAM
RAM (random access memory) is a memory chip that the processor can read from and write to. RAM is
volatile memory, meaning that its contents are lost when the computer’s power is turned off. Multiple
programs can be loaded into RAM simultaneously, provided you have enough RAM to accommodate all the
programs. The program with which you are working currently displays on the screen. Basic types of RAM
include dynamic RAM and static RAM, RDRAM (Rambus DRAM), Synchronous DRAM (SDRAM)
ROM
ROM (read-only memory) is a memory chip that only can be read and used; that is, it cannot be modified.
ROM is non-volatile memory, meaning that its contents are not lost when the computer’s power is turned off.
Commonly used for storing of program instructions that are not subjected to change. Generally provided by
manufacturer. ROM chips that contain permanently written data, instruction, or information are called
firmware. Another type of ROM chip, called programmable read only memory (PROM) clip, is a blank ROM
chip on which you can permanently place items. The instructions used to program a PROM chip are called
micro-code. Once the micro-code is programmed into the PROM chip, it functions like a regular ROM chip
and cannot be erased or changed.
Flash memory
Another type of non-volatile memory is called flash memory or flash ROM. Unlike a PROM clip that can
be programmed only once, flash memory can be erased electronically and reprogrammed. Flash memory is
used to store programs on personal computers, as well as cellular telephones, printers, digital cameras, pagers,
and personal digital assistant. Flash memory is available in sizes ranging from 1 to 40 MB.
CMOS
Another of memory clip in the memory unit is complementary metal-oxide semiconductor (CMOS).
CMOS memory is non-volatile memory used to store configuration information about the computer, such as
the type of disk drives, keyboard, and monitor; the current date and time; and other start-up information
needed when the computer is turned off. CMOS use battery power to retain information even when the power
to the computer is turn off. Battery-backed CMOS memory thus keeps the calendar, date, and time current
even when the computer power is off. Unlike ROM, information stored in CMOS memory can be changed,
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such as when you change from standard time to daylight saving time
Expansion Slots and Expansion Boards
An expansion slot is an opening, or socket, where a circuit board can be inserted into the motherboard.
These circuit boards, sometimes referred to as expansion boards or expansion cards, add new devices or
capabilities to the computer, such as a modem or more memory. Plug and Play refers to a computer’s
capability to automatically configure expansion boards and other devices as they are installed.
Port
A cable often attaches external devices to the system unit. The interface or point of attachment, to the
system unit is called a port. Ports have different types of connectors used to join a cable to a device.
i)Serial Port
A serial port is one types of interface used to connect a devices to the system unit. Because a serial port
transmits only one bit of data at a time, it usually is used to connect devices that do not require fast data
transmission rates, such as a mouse, keyboard, or modem
ii) Parallel Port
Unlike serial port, a parallel port is an interface used to connect devices that are capable of transferring
more than one bit at a time. Parallel ports originally were developed as an alternative to the slower speed
serial ports.
Many printers connect to the system unit using a parallel port with a 25-pin female connector. This parallel
port can transfer eight bits of data simultaneously through eight separate lines in a single cable
Types of Processors
CISC Processors (Complex instruction set computing)
One of the earlier goals of CPU designers was to provide more and more instructions in the instruction set of
the CPU, to ensure it directly supports more features making it easier to translate high level language programs
to machine language and to ensure machine language programs run more effectively. However every
additional instruction in the instructions set of the CPU requires the necessary hardware circuitry to handle
such instructions, adding more complexity to CPU’s hardware circuitry.
Another goal of CPU designers was to optimize the usage of expensive memory, to achieve this they
tried to pack more instructions in the memory by introducing the concept of variable length instructions. CISC
processors possess many processing features whereby making the job of machine programmers easier. They
are complex and expensive to produce.
RISC Processors (Reduced instruction set computing)
In early 1980 some CPU designers discovered that several of the instructions supported by CISC CPU’s are
rarely used hence came out with an idea that the complexity of the CPU design can be greatly reduced by
implementing only a bare minimum basic set of instructions plus some of the more frequently used instructions
in the hardware circuitry of the CPU. Other complex instruction did not need to be supported in the instruction
set in the CPU because they can always be implemented in software by using the basic set of instructions.
The designers came up with the idea of making all instructions of uniform length so that the decoding
and execution of all instructions becomes simple and fast. This design resulted in producing a faster and less
expensive processor. They have a small instruction set thereby placing extra demand on programmers who
must consider how to implement complex computations by combining simple instructions. Due to a simple
design RISC processors are faster for most applications, less complex and less expensive to produce than CISC
processors.
MEMORY (STORAGE DEVICES)
While performing a processing operation, the CPU needs a place to temporarily hold instructions to be
executed and data to be used with those instructions. Memory, which is composed of one or more chips on the
motherboard, holds data and instructions while they are being processed by the CPU.
The two basic types of memory are volatile and non-volatile. The contents of volatile memory, such as
RAM, are lost (erased) when the power to the computer is turned off. The contents of non-volatile memory,
however, are not lost when power is removed from the computer. For example, once instructions have been
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recorded onto a non-volatile ROM chip, they usually cannot be erased or changed, and the contents of the
chip are not erased when power is turned off.
SECONDARY STORAGE
Storage, also called secondary storage, auxiliary storage, or mass storage, holds items such as data,
instructions, and information for future use. Think of storage a filing cabinet used to hold file folders, and
memory as the top of your desk. When you need to work with a file, you remove it from the filing cabinet
(storage) and place it on your desk (memory). When you are finished with the file, you return it to the filing
cabinet (storage).
Storage is non-volatile, which means that items in storage are retained even when power is removed from
the computer. A storage medium (media is the plural) is the physical material on which items are kept. One
commonly used storage medium is a disk, which is a round, flat piece of plastic or metal with a magnetic
coating on which items can be written. A storage device is the mechanism used to record and retrieve items to
and from a storage medium.
The speed of a storage device is defined by its access time, which is the minimum time it takes the device to
locate a single item on a disk. Compared to memory, storage devices are slow.
The size, or capacity, of a storage device, is measured by the number of bytes (characters)
1. FLOPPY DISKS
A floppy disk, or diskette, is a portable, inexpensive storage medium that consists of a thin, circular, flexible
plastic disk with a magnetic coating enclosed in a square-shaped plastic shell. When discussing a storage
medium, the term portable means you can remove the medium from one computer and carry it to another
computer. A floppy disk drive is a device that can read from and write to a floppy disk.
2. HARD DISKS
When personal computers were introduced, software programs and their related files required small amounts
of storage and fit easily on floppy disks. As software became more complex and included graphical user
interfaces and multimedia, file sizes and storage requirements increased. Today, hard disks - which provide
far larger storage capacities and much faster access times than floppy disks - are one of the primary media for
storing software programs and files. Current personal computer hard disks can store from 4 GB and above of
data, instructions, and information.
A hard disk usually consists of several inflexible, circular disks, called platters, on which items are stored
electronically.
3. COMPACT DISCS
A compact disc (CD) is a flat, round, portable, metal storage medium that usually is 4.75 inches in diameter
and less than one-twentieth of an inch thick (Figure 5-8). Compact disks store items such as data, instructions,
and information by using microscopic pits (indentations) and land (flat areas) that are in the middle layer of
the disc.
CD-ROMs
CD-ROM (pronounced SEE-DEE-Rom, is an abbreviation for compact disc read-only memory) The
contents of standard CD-ROMs are written, or recorded, by the manufacturer and only can be read and used.
That is, they cannot be erased or modified- hence, the name read-only. A CD-ROM can hold up to 700 MB of
data, instructions, and information, or about 450 times that which can be stored on a high-density 3.5-inch
floppy disk.
CD-R (compact disc-recordable)
Is a technology that allows you to write on a compact disc only once using your own computers? Once you
have recorded the CD-R, you can read from it as many times as you wish.
CD-RW (compact disc-rewriteable)
Is an erasable disc that you can write on multiple times CD-RW overcomes one of the disadvantages of CDR disks - that you can write on them only once.
DVD-ROMs
A DVD-ROM (digital video disc-ROM) is an extremely high capacity compact disc capable of storing from
4.7 GB to 17 GB - more than enough to hold a telephone book containing every resident in the United States.
Not only is the storage capacity of a DVD-ROM greater than a CD-ROM, a DVD-ROM’s quality also far
surpasses that of a CD-ROM.
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4. MAGNETIC TAPES
One of the first storage media used with mainframe computers was magnetic tape, a magnetically coated
ribbon of plastic capable of storing large amounts of data and information at a low cost. Tape storage requires
sequential access, which refers to reading or writing data consecutively. Like a music tape, you must forward
or rewind the tape to a specific point to access a specific piece of data.
5. PC CARDS
A PC Card is a thin, credit card-sized device that fits into a PC Card expansion slot on a laptop or other
personal computer. Different types and sizes of PC Cards are used to add storage, additional memory,
communications, and sound capabilities to a computer.
6. Picture card or compact flash card
Some digital cameras also use a matchbook-size card, sometimes called a picture card or compact flash
card, to store pictures, which are then transferred to a computer by inserting the card into a card reader or
slot. These compact flash cards have storage capacities ranging from 2 MB to 256 MB.
7. Smart Cards
A smart card, which is similar in size to a credit card or ATM card, stores data on a thin microprocessor
embedded in the card. An intelligent smart card contains a CPU and has input, process, output, and storage
capabilities. In contrast, a memory card has only storage capabilities. When the smart card is inserted into a
specialized card reader, the information on the smart card is read and, if necessary, updated.
8. Microfilm and Microfiche
Microfilm and microfiche are used to store microscopic images of documents on roll or sheet film.
Microfiche uses a small sheet of film, usually about four inches by six inches. The images are recorded onto
the film using a device called a computer output microfilm (COM) recorder. The stored images are so small
they can be read only with a microfilm or microfiche reader.
Applications of microfilm and microfiche are widespread. Libraries use these media to store back issues
of newspapers, magazines, and genealogy records. Large organizations use microfilm and microfiche to
archive inactive files. Banks, for example, use it to store transactions and cancelled checks, and the U.S.
Army uses it to store personnel records. Using microfilm and microfiche provides a number of advantages: it
greatly reduces the amount of paper firms must handle; it is inexpensive; and it has the longest life of any
storage medium.
Relationship among the hardware, system software, application software and users of a
computer system
HARDWARE
(Physical devices/ components of the
computer system)
SYSTEM SOFTWARE
(Software that constitute the operating and programming
Environment of the computer system)
APPLICATION SOFTWARE
(Software that do a specific task or solve a specific problem)
USERS
(Normally interact with the system via the user
Interface provided by the application software
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COMMUNICATIONS
When referring to computers, communications describes a process in which one computer transfers data,
instructions, and information to another computer(s). Communications requires: a device that initiates the
transfer (a sending device); a communications device (such as a modem) that converts the sent material into
signals capable of being carried by a communications channel; a communications channel over which the
signals are sent; a communications device that receives the signals and converts them into a form understood
by the receiving device; and a device that accepts the sent material (a receiving device).
COMMUNICATIONS CHANNELS
An important aspect of communications is the channel, which is the communications path between two devices.
A communications channel is composed of one or more transmission media. Transmission media consists of
materials or techniques capable of carrying one or more signals.
TRANSMISSION MEDIA
Transmission media are one of two types: physical or wireless. Physical transmission media use wire,
cable, and other tangible (touchable) materials to
send communications signals; wireless transmission media send communications signals through the air or
space using radio, Microwave, and infrared signals.
1. Physical transmission media used in communications include twisted-pair cable, coaxial cable, and fibreoptic cable. These cables typically are used within buildings or underground.
2. wireless transmission media
Wireless transmission media used in communications include broadcast radio, cellular radio, microwaves,
communications satellites, and infrared. Wireless transmission media are used when it is impractical or
impossible to install cables. Many wireless devices that access the Internet now use the WAP (Wireless
Application protocol), which is a standard set of communications specifications that help to ensure successful
communications.
TRANSMISSION CHARACTERISTICS
Any transmissions sent during communications can be categorized by a number of characteristics including the
signal type, transmission mode, transmission direction, and transmission rate.
Signal Type: Analog or Digital
Depending on the devices and media involved, that signal is either analog or digital. Telephone equipment
originally was designed to carry only voice transmission in the form of an analog signal, which consists of a
continuous electrical wave. Computers, however, process data as digital signals, which are individual electrical
pulses that represent the bits grouped together into bytes. For telephone lines to carry digital signals, a special
piece of equipment called a modem converts between. Digital signals (0s and Is) and analog signals.
Categories of channels
Broadband vs. Baseband
You have learned that several different types if communications media exist, including both physical (twistedpair cable, coaxial cable, fibre-optic cable) and wireless (broadcast radio, cellular radio, microwave, satellite,
infrared). Some types of these media can transmit only one signal at a time, called base-band transmission,
while others can transmit multiple signals simultaneously, called broad-band transmission. Media that use
broadband transmission transmit signals it a much faster speed than those that use baseband transmission.
Satellite is an example of a media that uses broadband transmission. Two other widespread applications of
broadband transmission are digital subscriber lines (DSL) and cable television networks.
Transmission Direction: Simplex, Half-Duplex, and Full-Duplex
The direction in which data flows along transmission media is classified in one of three types: simplex, halfduplex, or full-duplex (Figure 6-7).
 Simplex transmission,
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Data flows only in one direction - from the sending device to the receiving device. Simplex transmission is
used only when the sending device does not require a response from the receiving device. Security systems and
fire alarms that contain a sensor, for example, use simplex transmission.
 Half-duplex transmission
In half-duplex transmission, data can flow in either direction - from sender to receiver and back - but only in
one direction at a time. Citizens band (CB) radio, for example, uses half-duplex transmission; you can talk or
listen, but you cannot do both at the same time. Many fax machines, credit card verification systems, and
automatic teller machines also use half-duplex transmission.
 Full-duplex transmission
In full-duplex transmission, data can flow in both directions at the same time. A regular telephone line, for
example, supports full-duplex transmission, meaning both parties can talk at the same time. Full-duplex transmission is used for applications with intensive computing requirements or those with heavy traffic.
Transfer Rates
The speed with which a transmission medium carries data is its transfer rate. Transfer rate usually is expressed
as bits per second (bps) - that is, the number of bits that can be transmitted in one second. Today's transmission
media transmit data at rates ranging from millions of bits per second (Mbps) to billions of bits per second
(Gbps).
Communications software
For two computers to communicate, they must have compatible communications software. Some
communications devices are pre- programmed to accomplish communications tasks. Other communications
devices require a separate communications software program to ensure proper transmission of data.
Communications software helps you establish a connection to another computer and manage the
transmission of data between computers; it resides in the main memory of the sending and receiving computers
while the connection is established.
To help you establish a connection to another computer, communications software uses wizards,
dialog boxes, and other onscreen messages to prompt you for information and automate tasks where possible.
Communications software usually includes one or more of the following features: dialing, file transfer,
terminal emulation, and Internet access.
The Internet access feature allows you to use the computer to connect to the Internet to send entail,
participate in chat rooms, visit World Wide Web sites, and so on. When you purchase a modem, it usually
includes a basic communications software package so you can connect to other computers, such as ones used
for a bulletin board system. When you subscribe to an online service or Internet service provider, the provider
typically sends you a separate communications software package, which you must use to connect to their
computers so you can access the Internet, send e-mail, and use their other services.
Communications devices
A communications device is any type of hardware capable of transmitting data, instructions, and information
between a sending device (sender) and a receiving device (receiver). At the sending end, a communications
device converts the data, instructions, or information from the sender into signals understood by the
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transmission media on the communications channel. At the receiving end, the communications device receives
the signals from the communications channel and converts the signals into a form understood by the receiver.
Recall that sending and receiving devices include a variety of types of hardware such as handheld
computers, laptop computers, desktop computers, minicomputers, mainframes, digital cameras, and fax
machines. The type of communications device used in a communications system depends on the type of
sending and/or receiving devices, as well as the type of transmission media.
Some of the more common types of communications devices are modems, cable modems,
multiplexers, and network interface cards.
Today, thousands of computer networks exist, ranging from small networks operated by home users to
global networks operated by numerous telecommunications firms. To interconnect the many types of networks
that exist, various types of communications devices are used, including hubs, repeaters, bridges, gateways, and
routers.
NETWORKS
A network is a collection of computers and devices connected by communications channels that allows users
to share data, information, hardware, and software with other users. Individuals and organizations connect
computers in a network for a variety of reasons, including the ability to share hardware, data and information,
and software; and to facilitate communications. Networks reduce costs and increase efficiency and allowing
sharing of programs, data and peripheral devices.
Local Area Network (LAN)
A local area network (LAN) is a network that connects computers in a limited geographical area, such as a
school computer laboratory, office, or group of buildings. Two popular types of LANs are peer-to-peer
networks and client/server networks.A LAN is made of two or more computers connected to each other within
the same geographical area but not necessarily within the same building.
Metropolitan Area Network (MAN)
A MAN is a network of computers spread over a metropolitan area such as a city and its suburbs. This city
bridges its LANs with a series of backbones, making one large network for the entire city. The MAN may be
operated by one organisation or be shared resources used by several organisations in the same city.
Wide Area Network (WAN)
A wide area network (WAN) is a network that covers a large geographical area (such as a city or country)
using a communications channel that combines telephone lines, microwave, satellites, or other transmission
media.
Network Topologies
The configuration, or physical arrangement, of the device in a communications network is called network
topology or network architecture. Three commonly used network topologies are bus, ring, and star. Network
also combinations of these topologies.
 Bus Network
A node must be intelligent enough to listen to the bus and recognise own addresses in order to receive
incoming data
 Ring Network
Intelligent central controller. Each node has an equal amount of intelligence
 Star Network
All nodes are joined at a single central point using one link for each node. Has central node, employs switching
techniques to interconnect the outlying stations. Affects the functionality of network if central host is not
operating
Disadvantages of networks
 Networks are expensive to set up
 Security problems
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PROTOCOLS
A protocol is a set of rules and procedures for exchanging information among computers. Using the same
protocols, different types and makes of computers can communicate with each other.
Ethernet
Ethernet is a LAN protocol that allows personal computers to contend for access to a network.
Token Ring
A token ring protocol controls access to a network by requiring that a special signal called a token is shared
or passed among network devices.
TCP/IP
TCP/IP (Transmission control protocol/Internet protocol) is a set of protocols used to manage data
transmission by breaking it up into packets.
File transfer protocol (FTP)
A set of rules used to govern the sending and receiving of files on the internet.
Telnet
One of the ways that a person can access a remote computer over a network such as the internet. It’s often used
by network administrators to remotely control and/or troubleshoot the operation of a web server. Telnet
programs act as if your computer is a terminal that is connected to the remote computer.
Intranets
Intranets are internal networks that use Internet and Web technologies. Intranets make company information
accessible to employees and facilitate working in groups.
Extranet
An extranet is an intranet that extends to authorized users outside the company. Extranets facilitate
communications among a company’s customers or suppliers.
Videoconferencing involves using video and computer technology to conduct a meeting between
participants at two or more geographically separate locations.
INTERNET
The word internet is derived from two words: interconnection and networks. The Internet, a
worldwide collection of networks, offers the World Wide Web and such popular Web-based activities as ecommerce and Internet telephony. The internet, the world’s biggest network, is actually a collection of smaller
networks located all over the world. It can be used foe a variety of purposes, such as checking e-mail, chatting,
downloading files searching for information, listening to music e.t.c in order to gain access to the vast
resources found on the internet you must first become part of one of the networks which it comprises. The
networks of large companies and universities are directly connected to the internet, so if you are part of one of
these networks and you have appropriate network privileges, you can access the internet.
The average person however, gains access through a local internet service provider (ISP). An ISP is a
company with a direct connection to the internet that grants subscribers access to the various internet services.
World Wide Web
It is basically a collection of text and multimedia documents called web pages, some of which are linked. A
web page is an electronic document written in a computer language called HTML. Web pages are usually
encoded in a special language called hypertext Mark-up language (HTML) that allows one web page to
provide links to several others. A group of related web pages on the same web server is known as a website.
Each web page and website has an address called a uniform resource locator (URL). Web pages are viewed
using special software called web browsers e.g. internet explorer, Netscape or opera. The transmission of web
pages is governed by a protocol known as Hypertext transfer protocol (HTTP).
E-mail
E-mail (electronic mail) is the exchange of text messages and computer files via a communications
network. Each person using e-mail must have an e-mail address. The e-mail service provider has a special
server to handle the e-mail for its customers. E-mails are mainly text messages that may (Not always) have a
little formatting similar to a word processing document. It may be used to send files attached to the e-mail
message called attachments. Before mail is read, it is downloaded from the mail server to the user’s computer.
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Advantages
1) It is faster than normal mail.
2) It is cheaper and more convenient .e-mails are free.
3) It’s very easy to send files.
4) The same message can be easily and quickly sent to several people.
Disadvantages
1) It cannot be used to send parcels
2) You cannot send e-mail to people who do not have an e-mail account.
3) E-mail makes it easy for computer viruses to be spread.
COMPUTER PROGRAMMING LANGUAGES
1St Generation Programming Languages (Machine Language)
the machine language of a computer is written as strings of binary (ones and zeros).the circuitry of a computer
is wired in a manner that it immediately recognizes the machine language instructions and converts them in the
electrical signals needed to execute them.
Advantages
1. Programs written in machine language can be executed very fast
2. The program can directly address and control the internal circuitry of the computer making it more
effective in hardware usage and control
Disadvantages
1. it is machine dependent
2. difficult to program
3. it is error prone
4. it is difficult to modify
5. the programmer has to have expert knowledge of the computer
6. it is extremely difficult to learn and use
2nd Generation programming languages (Assembly Language)
They use symbols and codes to represent program instructions instead of binary digits. The resulting program
still directly instructs the computer hardware therefore not easily portable.
Advantages
1. It is easier to correct errors and modify program instruction
2. They have the same execution efficiency as machine language
3. Easy to understand and save a lot of time and effort of programmer
4. There is no worry about address
Disadvantages
1. They are still machine dependent
2. Knowledge of hardware is required
3. They are still time consuming
4. Program execution takes longer than machine language
3rd generation programming languages (Procedural language)
They are easier to learn and use than were the earlier generations. While not directly manipulating specific
hardware components, they must still give the computer detailed instructions of how to reach the desired
results. They incorporate great use of english like instructions which are easier to learn and use. Examples
include COBOL, FORTRAN, C, and PASCAL.
Procedural programming languages offer very little control and flexibility. They are portable.
4th generation programming languages (non-procedural language)
They are much easier to use and more English like than procedural languages. Its programs tell the computer
what it needs to accomplish but do not provide detailed instructions as to how to accomplish it. They
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concentrate on the output and not procedural details. They can be used by people who are not computer
specialists.
e.g. C++, VISUAL BASIC, POWER BUILDER.
Object oriented programming languages
OOP was first introduced in 1967 by developers of a programming language named SIMULA 67. The basic
idea behind OOP is that programming languages are used for simulating real world problems on computers.
OOP’s consist of objects that include descriptions of data relevant to the object as well as the operations that
can be done on that data.e.g query languages, report generators, application generators
Features of OOP’s
1. There is extensive use of graphical objects
2. There are features that allow easy program coding, error detection and correction
3. In order to write code, one double clicks on an object and selects the event for which they are writing code.
Advantages
1. Program coding is made easier
2. Error correction is made easy
3. Writing a program or application takes less time compared to procedural.
4. It is easier to learn than procedural
5. They are portable
6. The programmer does not need to be an expert on the internal workings of a computer
7. Programs made using OOP’s are user friendly
Disadvantages
1. Interface design takes more time
2. One has to master all usable objects and event procedures in order to create an effective application
3. Programs made using OOP’s occupy much more storage space than procedural languages
4. In some cases they are difficult to learn and use
DATA PROCESSING
Definitions



Data – Basic facts about the activities of the business. E.g. number of hours worked by
an employee on a particular machine.
Information – Data which is summarized in the way you want it, so that it is useful to
you in your work. It’s the end product of data processing. Must be accurate, timely,
complete, concise and relevant.
Data Processing – Collection and manipulation of items of data to produce meaningful
information. It’s the process of transforming raw data into meaningful output i.e.
information.
Data processing can be done manually using pen and paper, mechanically using simple
devices like typewriters or electronically using modern data processing tools such as
computers.
Electronic data processing has become so popular that the manual and mechanical
methods are being faced out.
Factors determining the methods of data processing
a. Size and type of business.
Methods of producing the information largely depend on the size and type of business. In a
very small company, one person can be able to produce all the necessary information.
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However as the company increases in size, more people and aids e.g. calculators and small
computers will be needed. Large volumes of data and information will require the use of
large computers.
b. Timing aspects.
Some information requirements are less time critical than others. The timing requirement for
information will have a considerable bearing on the methods and equipment needed to
provide it.
c. Link between applications.
Where data is needed for more than one information requirement, a different method of
processing it may be suggested.
Stages of data processing
From the source documents.
a. Preparation of data for input
Data is checked for correctness and validity i.e. transcription and they are also sorted.
b. Input of data
The collected data is converted from human readable to machine readable form. The
conversion takes place in the input device.
c. Processing
The transformation of input data by the C.P.U to a more meaningful output (information).
d. Output
The production of the end product. The information can then be distributed to the target
group or stored for future reference.
Data Collection
Data collection is getting the data from the point of its origin (source documents) to the
computer in a form suitable for processing. Also referred to as data gathering or fact finding.
Tools that help in data collection include source documents such as forms, data capture
devices e.g. digital camera e.t.c.
Stages of data collection
a. Data creation
It’s the process of putting together facts in an organized manner.
There two basic alternatives;
 Source documents – A great deal of data still originate in the form of manually prepared
documents.
 Data capture – Data is produced in machine-sensible form at source and is read directly by
a suitable device e.g. a bar code reader.
b. Data transmission
This depends on the method and medium of data collection adopted:
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 If computer is located at a central place, the documents will be physically transmitted i.e.
by the postman or courier services.
 It’s also possible for data to be transmitted by means of telephone lines to central
computer. In this case no source documents would be involved in the transmission process.
c. Data preparation
This is the transcription (conversion) of data from the source documents to a machinereadable form.
Data collected using devices that directly capture data in digital form don’t require
transcription.
d. Data conversion
Data may need to be converted from one media to another e.g. from floppy disk to hard disk
for input to the computer.
e. Input validation
Data entered into the computer is subjected to validity checks by a computer program before
being used for processing.
f. Sorting
This stage is required to re-arrange the data into a sequence required for processing. It’s a
practical necessity for efficient processing of sequentially organized data in many
commercial and financial applications.
g. Control
It’s not really a stage because control is applied throughout the whole collection.
Data collection media and methods
a. On-line transmission of data from source e.g. Data Direct Entry (DDE)
b. Source document keyed directly keyed directly into diskette (key-to-diskette).
c. The source document itself prepared in machine-sensible form using character recognition
techniques.
d. Data capture devices.
e. Portable encoding devices.
f. Source data captured from tags, plastic badges or strips.
g. Creation of data for input as a by-product of another operation.
Controls
Objectives of controls
i.
To ensure that all data is processed.
ii. To preserve the integrity of maintained data.
iii. To detect, correct and re-process all errors.
iv.
To prevent and detect data.
Importance of controls
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They must be instituted early as possible in the system. The quality of input data is of vital
importance to the accuracy of output. Everything possible must be done to ensure that the data
are complete and accurate just before input to the computer.
Types of controls
a. Manual controls
b. Data preparation
c. Validation checks
d. Batch controls
e. Other controls
a. Validation checks
These are attempts to build into the computer program powers of judgement so that incorrect
items of data are detected and reported.
These check s can be made in two stages:
 Input– When data is first input into the computer, different checks can be applied to
prevent errors going forward for processing. For this reason, the first computer run is often
referred to as validation or data vet.
 Updated – Further checking is possible when the data input are being processed.
The following are the main types of validation checks that may be used:
 Presence - Data are checked to ensure that all fields are present.
 Size - Fields are checked to ensure that they contain the right number of characters.
 Range - Number of codes are checked to ensure that they are within the permissible range.
 Character check - Fields are checked to ensure that they contain only characters of the
correct type e.g. no letters in numeric field.
 Format - Fields are checked to ensure that they are of the correct format.
 Reasonableness - Quantities are checked to ensure that they are not abnormally high or
low.
 Check digits – Use of a check digit enables a number to be self-checking. It’s a means of
ensuring that a number (e.g. a customer account number) maintains its validity.
b. Manual controls
Even in advance systems, considerable checking of the source documents is often necessary.
Such checks may be:
 Scrutiny to detect:
i. Missing entries.
ii. Illegal entries.
iii. Illogical or unlikely entries.
 Reference of the document to stored data to verify entries.
 Re-calculating to check calculations made on the document.
Types of errors during data collection.
a. Missing source documents.
b. Source documents on which entries are omitted, illegible or dubious.
c. Transcription errors e.g. errors in copying data from one form to another.
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d. Data preparation errors e.g. errors made when keying onto diskette.
e. Program faults.
f. Machine hardware faults.
Types of data or computer files
Computer files – a collection of information initially created in memory and then stored with a
file name on a secondary storage.
Are of two types:
- Command files - Contains program executable code that computer reads and uses as
instructions to prepare a task.
- Text/data files - Contain standard alphanumeric information that has been entered into
the file via an application program such as word processor, spreadsheet, database e.t.c.
Types of files
 Master file
Main file that contains permanent data against which transactions are processed.
Contain:
o Reference data- tends to be relatively permanent and which is processed by amending
i.e. making occasional changes in the form of insertion of new records, deletion or
alteration of existing records e.g. personnel details.
o Dynamic data – Changes frequently and is processed by updating i.e. changing values
of various fields e.g. earnings and deductions.
 Transaction/movement files
Includes input and output files for holding temporary incoming and outgoing data.
Used to update dynamic data on master files.
 Reference file
It’s a file with a reasonable amount of permanency.
Used for look up or reference purposes e.g. PAYE deductions.
 Report file
Set of records extracted from data in master file used to prepare report which can be printed
at a later date.
Stores relatively permanent records extracted from the master files or generated after
processing.
 Back up/History file
Holds copies of data or information from computer storage.
Retained for historical use or back up.
 Dump/work file
Used to hold data for security and recovery purposes or for temporary storage whilst other
work is being carried out e.g. total earnings and deductions calculations (payroll processing).
 Sort file
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Mainly used where data is to be processed sequentially in a sequential process.
Data Storage Hierarchy
It normally consists of the following six levels;
1. Bit. The smallest item of data is a single binary
digit (a bit), either 0 or 1.
2. Character. Multiple related bits are combined
to form a character. A bit is the basic unit of
primary and secondary storage, and a character
is the basic unit for human conception.
3. Field. It’s a meaningful collection of related characters.
4. Record. Multiple related fields are combined to form a record.
5. File. Multiple related records are combined to form a file. A file is a number of related
records which are treated as a unit. Every record in a file has the same set of fields. Each
record in a file is identified, for storage and retrieval purpose, by a key field whose contents
are unique for each record in a file. In case of an employee file, employee-code field may
serve as the key. In case of an employee file, employee-code field may serve as the key
field.
6. Database. Multiple related files are integrated to form a database. A database is a collection
of logically related data elements from multiple files.
Standard methods of organizing data
Two standard methods for organizing data are:
- File-oriented approach
- Database-oriented approach
File-oriented approach was the traditional method and has largely been replaced today by the
database-oriented approach.
 File-oriented approach
In this method, an application’s data is organized into one or more files and the application
program processes the data stored in these files to generate the desired output.
This method of organizing data is simple, inexpensive and usually easy to use.
It however suffers the following limitations:
1. Limited query flexibility. In file-oriented approach, the records are organized
according to a key field. As long as we search records based on key field value, the
query can be processed quickly.However, when the key field is not relevant to the
information needed, the entire field has to be searched.
2. Data redundancy. This is the repetition of the same data items in more than one file.
In file-oriented approach, the same data items are often included in many different
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files. This leads to increase in cost of data entry and data storage. It also leads to data
integrity problem.
3. Data integrity problem. Data integrity is the consistency of the data in all files, i.e.
when some changes occur in a data item, every file which contains that field, should
be updated to reflect the change for consistency. For example, when a data item
changes, if all files are not updated correctly, there may be frequent discrepancies
among reports produced from different files, causing confusion.
4. Lack of program/data independence. In file-oriented approach, the application
programs usually contain data format statements that precisely define each data field
to be processed. This often results in different files having the same data item stored
using different data formats. Data dependence occurs when the data is dependent on
the application. Due to data dependence problem, whenever there is need to add,
delete, or change data formats, the application program must also be changed. Data
dependence problem also causes incompatibility among data files from different
applications due to which these data files cannot be linked, if such a need arises.
5. Limited data security flexibility. A file-oriented approach normally offers file-level
data security feature i.e. data access restrictions can be enforced only for an entire
file, not for a record or a record or a field of data item.
Database-oriented approach
In this approach, data from multiple related files are integrated together in the form of
a database, with the following properties:
1. It provides greater query flexibility.
2. It reduces data redundancy.
3. It solves data integrity (inconsistency) problem.
4. It makes data independent of the application programs i.e. fields can be
added, changed and deleted from the database without affecting existing
programs.
5. It also includes data security features at database level, record level, and
even at field level to provide greater flexibility of restricted data access.
File organization1
This is the arrangement of records within a particular file.
File organization is very important because it determines the methods of access efficiency and
flexibility.
File organization methods
a. Sequential file organization
Records are stored and accessed in a particular order using a key field. Retrieval requires
searching sequentially through the entire file word for word from the beginning to the end.
e.g. if the record required is the twentieth record on the file, in order to get it into storage to
process it the computer will first have to read in all nineteen preceding records. The principle
storage medium for sequential files is magnetic tapes.
1
Very important
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Sequential file organization is the most efficient and economical file organization in case of
application in which there is a large number of file records to be updated at regular scheduled
intervals.
Applications such as payroll processing and monthly bill processing are processed in this
manner.
Advantages of sequential files
1. They are conceptually simple to understand and use.
2. They are also easy to organize and maintain.
3. They need relatively inexpensive input- output media and devices for their storage
and processing.
4. They are most efficient and economical to use in applications in which the activity
ratio is high(most records are changed during update runs)
Disadvantages of sequential files
1. They are very inefficient and uneconomical for applications in which the activity
ratio is very low.
2. Since an entire sequential file may need to be read just to retrieve and update few
records, accumulation of transactions into batches is recommended before
processing them. Hence, the use of sequential files is limited to a batch-processing
environment.
3. Because of the need to accumulate transactions into batches before processing them,
sequential processing precludes the possibility of up-to-the-minute data.
4. Sequential processing require the files to be sorted before processing i.e. both the
transaction and master files must be sorted and placed in the same sequence before
processing.
5. Sequential processing often leads to data redundancy problem, since the same data
may be stored in several files sequenced on different keys.
b. Serial file organization
Records in a file are accessed one after another.
c. Random/direct file organization
Records are stored in the disk at random but are accessed directly.
Many applications require up-to-the-minute timeliness of data. The users of such applications
cannot afford to wait for the transactions to be accumulated to batches and then processed
together. Such applications require a transaction to be processed immediately as and when it
occurs. Examples of such applications are airlines or railway reservation systems, teller
facility in banking applications, systems for enquiring whether a certain item is in stock in a
store e.t.c. For such applications, the use of random/direct file organization is recommended,
in which the desired record pertaining to the current transaction at hand can be directly
located by its key field value, without having to search through a sequence of other records.
The principal storage medium is magnetic/optical disc.
Direct file organization uses an address generating function to convert a record key value into
a storage address on the disk on which the file is stored.
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Advantages of direct files
1. Given the key, any record can be quickly located and retrieved directly, without the
need for a sequential search of the file.
2. Transactions need not be sorted and placed in sequence before processing.
3. Accumulation of transactions into batches is not required before processing them.
They may be processed as and when generated.
4. It can support interactive online applications, which need to provide up-to-theminute information in response to inquiries from users.
5. If required, it’s also possible to process direct file records sequentially in a record
key sequence.
Disadvantages of direct files
1. They require relatively expensive hardware and software resources because they
must be stored on a direct-access storage device, such as a disk.
2. Due to address generation overhead involved, they are less efficient and economical
than sequential files for use in sequential applications with high activity ratio (The
ratio of the total number of records in a transaction file and the total number of
records in the master file).
3. Special security measures are often necessary for online direct files, which are
simultaneously accessible from multiple online stations.
d. Indexed sequential files
There two types of files involved:
- The data file which contains records stored in the file and
- The smaller index file which contains the key and disk address of each record stored in
the data file.
The records of the file can be stored in a random sequence, but the index table is in sorted
sequence on the key value. This provides the user with a very powerful tool. Not only can the
file be processed randomly, but it can also be processed sequentially.
Advantages of indexed sequential files
1. They can be quite efficiently used for sequential processing of high activity ratio
applications.
2. They can also be used quite efficiently for direct access processing of low activity
ratio appliances.
Disadvantages of indexed sequential files
1. They require relatively expensive hardware and software resources because they
must be stored on a direct-access basis, such as a disk.
2. They require more storage space than other types of files because of the need for
index file.
3. When used for direct access online applications, access to records may be slower
than direct files.
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