Student Publications

Author: Anthony Oduori Solo
Title: Computer Science

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What is Computer Science
The study of computers, including both hardware and software design. Computer science is
composed of many broad disciplines, including artificial intelligence and software
a) Computer
A computer is a device that works under the control of stored programs, automatically
accepting, storing and processing data to produce information.

Its programmable machine.

The two principal characteristics of a computer are:
a) It responds to a specific set of instructions in a well-defined manner.
b) It can execute a prerecorded list of instructions (a program).
Modern computers are electronic and digital. The actual machinery, wires, transistors, and
circuits are called hardware; the instructions and data are called software.
All general-purpose computers require the following hardware components:
Memory : Enables a computer to store, at least temporarily, data and programs.

Mass storage device: Allows a computer to permanently retain large amounts of
data. Common mass storage devices include disk drives
and tape drives.

Input device : Usually a keyboard and mouse, the input device is the conduit

through which data and instructions enter a computer.

Output device : A display screen, printer, or other device that lets you see what
the computer has accomplished.
Central processing unit (CPU): The heart of the computer, this is the
component that actually executes instructions.
In addition to these components, many others make it possible for the basic components to
work together efficiently. For example, every computer requires a bus that transmits data
from one part of the computer to another.
Computers can be generally classified by size and power as follows
Personal computer : A small, single-user computer based on a microprocessor.
In addition to the microprocessor, a personal computer has
a keyboard for entering data, a monitor for displaying
Information, and a storage device for saving data.

Workstation : A powerful, single-user computer. A workstation is like a
personal computer, but it has a more powerful microprocessor
and a higher-quality monitor.

Minicomputer : A multi-user computer capable of supporting from 10 to
hundreds of users simultaneously.

Mainframe : A powerful multi-user computer capable of supporting many
hundreds or thousands of users simultaneously.

Supercomputer : An extremely fast computer that can perform hundreds of
millions of instructions per second.
a) CPU
The CPU is the brains of the computer. Sometimes referred to simply as the processor or
central processor, the CPU is where most calculations take place. In terms of computing
power, the CPU is the most important element of a computer system.
On large machines, CPUs require one or more printed circuit boards. On personal
computers and small workstations, the CPU is housed in a single chip called a
Two typical components of a CPU are:
The arithmetic logic unit (ALU), which performs arithmetic and logical operations.
The control unit (CU), which extracts instructions from memory and decodes and
executes them, calling on the ALU when necessary.


b) Program
It's a set of instructions that is written in the language of the computer. A program is used
to make a computer perform a specific task e.g. producing a payroll.
The computer is only able to obey a program instruction if the program has been stored
within the computer thus this implies that the computer must be able to input and store
programs in addition to data hence the computer works under the control of stored
c) Hardware
This is a general term used to describe all the electronic and mechanical elements of the
compute, together with all the devices used with the computer.
d) Software
It's a general term used to describe all the various programs that may be used on a
computer system together with their associated documentation.
Types of software
Application software
Systems software
Application Software
It's software that is designed to be put to specific practical use.
There are two types of this kind of software namely:
- Specialist application software
- Applications packages
Specialist application software
It's a program with associated documentation, designed specifically to carry out particular
tasks e.g. controlling company's stock control, solving a set of mathematical equations
Application Packages
Its suites of programs with associated documentation used for a particular type of problem.
Many packages are designed in such a way that they can be used for a variety of similar
problems e.g. a payroll packages are sometimes produced in forms that enable them to be
setup and used by different companies each having slightly different ways in which they
need to produce their payroll.
Systems Software.

They are programs, with associated documentation, that control the way the computer
operates or provide facilities that extend the general capabilities of they system e.g. The
Operating System. The operating system controls the performance of the computer by
doing a variety of jobs to ensure that proper , orderly and efficient use of hardware by
application programs hence most applications programs can only work if there is an
operating system
Basic Functions of a computer illustrated by the below figures/pictures
Basic Functions of a Computer

mainframe network standalone

Enter Data

Manipulate Data

"View" Processed Data





This is a set of letters, digits and other symbols are frequently used to represent data items
e.g. names, prices this kind of a set of symbols is known as a Character set.

Keyboard devices, including ordinary typewriters have their own character sets, which are
commonly engraved onto the keys. Keyboard devices are frequently used to input data into
the computer.

When a keyboard of the computer input device is pressed the device produces an electrical
signal that represents the key's character to the computer. The transmission passes through
a cable from the device to the computer and the signal for each individual character is a
series of electrical pulses called pulse train.

The system of representing pulse levels by the symbol "0" and "1" corresponds to a system
of representing numbers called Binary numbers systems. The binary number system also
uses only the two symbols "0" and "1" hence the binary number is known as BITS

Start and stop bits � start bit it's the first bit in the pulse train to be transmitted. Its
purpose is to signify that start of a character transmission to the transmission. The start and
stop bit are always of opposite polarity i.e. if one is 1 the other is 0 hence the change is
pulse level takes place when a start bit is transmitted.


There are two modes of data transmission namely:

a) Asynchronous
b) Synchronous

Asynchronous Transmission
The ASYNCHRONOUS (ASYNC) format for data transmission is a procedure or protocol
in which each information CHARACTER or BYTE is individually synchronized or
FRAMED by the use of Start and Stop Elements, also referred to as START BITS and
STOP BITS. Intervals between characters may be uneven.
The Asynchronous Transmission Format is also known as START-STOP mode or
CHARACTER mode. Each character or byte is framed as a separate and independent unit
of DATA that may be transmitted and received at irregular and independent time intervals.
The characters or bytes may also be transmitted as a contiguous stream or series of
characters. It is the common method of transmission between a computer and a modem

The character or byte may contain the number of bits required to allow translation of the
BIT PATTERN into a group of symbols used to represent:
LETTERS (alpha characters)
NUMBERS (numerical values)

Synchronous Transmission

The transmission of data in which both stations are synchronized. Codes are sent from the
transmitting station to the receiving station to establish the synchronization, and data is
then transmitted in continuous streams.

Synchronous Transmission sends packets of characters at a time. Each packet is preceded
by a Start Frame which is used to tell the receiving station that a new packet of characters
is arriving and to synchronize the receiving station's internal clock. The packets also have
End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits
depending on the protocol. Both Start and End Frames have a special bit sequence that the
receiving station recognizes to indicate the start and end of a packet. The Start and End
Frames may be only 2 bytes each.
This method of data communication requires the source and destination to synchronize
their clocks together. This synchronization of the clocks can occur externally to the data
information or be incorporated with the data information. There are two types of
synchronous data transmission (or communication):
a. Externally clocked synchronous transmission
b. Internally encoded synchronous transmission
The advantage to having the clocks synchronized is that longer blocks of data can be sent
without loss of synchronization

Synchronous transmission is more efficient than asynchronous

Data representation on magnetic media

Invisible spots of magnetism are created in the magnetic surface of the medium .These
spots which can be magnetized in one of the two directions correspond to 0 or 1 according
to the direction of magnetization. This mode of storage is widely used by mainframes for
backing storage but mostly used to hold archive copies of data i.e. The data is recorded
onto the tape and then the tape is removed from the computer and held in a safe place for
future use.

Parity Bit
A parity bit is a binary digit that indicates whether the number of bits with value of one in
a given set of bits is even or odd. Parity bits are used as the simplest error detecting code.
There are two types of Parity bit
a) Even parity bit
b) Odd parity bit
a) Even Parity Bit
Refers to the parity-checking mode in which each set of transmitted bits must have an even
number of set bits. The parity checking system on the sending side ensures even parity by
setting the extra parity bit if necessary.

b) Odd Parity Bit
An odd parity bit is set to 1 if the number of ones in a given set of bits is even (making the
total number of ones odd).

Parity Checking

In communications, parity checking refers to the use of parity bits to check that data has
been transmitted accurately. The parity bit is added to every data unit (typically seven or
eight bits) that are transmitted. The parity bit for each unit is set so that all bytes have either
an odd number or an even number of set bits.
Assume, for example, that two devices are communicating with even parity (the most
common form of parity checking). As the transmitting device sends data, it counts the
number of set bits in each group of seven bits. If the number of set bits is even, it sets the
parity bit to 0; if the number of set bits is odd, it sets the parity bit to 1. In this way, every
byte has an even number of set bits. On the receiving side, the device checks each byte to
make sure that it has an even number of set bits. If it finds an odd number of set bits, the
receiver knows there was an error during transmission.

The sender and receiver must both agree to use parity checking and to agree on whether
parity is to be odd or even. If the two sides are not configured with the same parity sense,
communication will be impossible.
Parity checking is the most basic form of error detection in communications. Although it
detects many errors, it is not foolproof, because it cannot detect situations in which an even
number of bits in the same data unit are changed due to electrical noise. There are many
other more sophisticated protocols for ensuring transmission accuracy
Parity checking is used not only in communications but also to test memory storage
devices. Many PCs, for example, perform a parity check on memory every time a byte of
data is read.



Document file

Each document is stored in the computer individually by name in what is called a file.
Many document files take the form of a formatted sequence of characters. In their simplest
from such files a called text files because they consist of printable characters organized into
lines of text. A small number of control characters signify tabs, page breaks and the
divisions between lines.


Text files can be created and modiefied with the aid of a special program called text editor.
The most basic types of editor provide a means of editing text file on a line by line basis
with each line being identified by a line number. These line editors are rather outdated.

Modern Text

Modern text editors normally display the file's text on the screen. There are numerous text
editors in use today and they are often associated with particular operating system. i.e the
editor "Vi" is commonly found on computers using an operating called Unix.

Text editors can be used both to create new files and to edit existing files. When an existing
text file is edited the editor "reads" the characters from the file and displays them on the
screen. The editor may also display the cursor in the top left hand corner. The cursor
usually can be moved around the document using by means of the four cursor keys as

The text file will normally hold far more characters than can fit onto one screen display. A
typical screen is 80 characters wide and may only display about 24 lines of text. The
document size is not limited to the size of the screen. although they have disappeared from
view they are still retained within the computers memory. Thus the screen acts like a
window on the text, which can be moved up or down by means of the cursor keys.

In some editors the editor counts the number of spaces remaining and forces the word onto
a new line if necessary hence this feature is called word wrap. Therefore the carriage return
key is used only to force a blank line.

Most editors also incorporate cut and paste facilities whereby a section of text can be
selected and moved to a different part of the document. A variation on this is to copy the
selected section of text, without removing it from its original position. Such operations

often require the use of special function keys. The selection is normally made by first
marking a start position and then moving the cursor to the end position.

The editor may provide other functions on highlighted text i.e. it might be possible to save
the highlighted text in a separate new text file.

A search and replace facility is another frequently provided feature. It allows the user to
replace a single word or phrase wherever it occurs in the document with another word or
phrase for example on pressing "F5" key the user might be prompted the question FIND?.
There are many features provided by a text editor.

Word Processing

Most word processors are carried out using specialist word processing software on ordinary
computer/PCs. Hence the term word processor is used to refer to a word processing
program or a complete word processing software package e.g. WordPerfect, Word

A word processing program allows users to create, edit, format, store and print text
documents. A document is anything that can be type i.e. a memo, letter, a book, a report...

In most offices word processor have virtually replaced typewriters as a means of producing
documents thus compared with a typewriter, word processing has a number of advantages

a) The ability to store typed documents in a computer
b) The ability to correct mistakes
c) The ability to view the document on screen before printing
d) The ability to print the document many times
e) You are able to move sections of text to another part of the document
f) The ability to insert or delete words, sentences or paragraphs

Another common formatting technique is text justification which means the alignment of
text against a margin. Left justification means alignment against the left margin, right
justification means alignment against the right margin and full justification means both the
left and right justification.

When a document has been completed it can be saved for long term storage onto backing
store as a file. The file normally has a format much complex that that of a text file and is
usually directly usable by the word processing program which created it. The same
document can be loaded back into the word processing program at a later time without
needing to be retyped.

Within a word processing software package there may be a number of ancillary programs
which can be used in conjunction with word processed documents to enhance the standard
facilities i.e. Mail merge. In this it's possible to produce a standard letter into which the
name and address of a number of clients in turn are merger and series of individual letters
printed. Each recipient of the letter receives an original print.

Many word processing software include spell checking facilities. The spell check program
comes with its own dictionary and its able to scan through the document and report

wherever a word is found which does not exist in its dictionary. The user is then able to
correct the word or leave it as it was originally spelt.

Some spell checking programs can be left to search through a document and mark every
doubtful word for later checking by the user. Others can check against their dictionary of
words so quickly that they can provide immediate feedback to the user as each word is
typed in.

Many spell checkers provide the user with a list of alternative words which are similar in
spelling or sound to the doubtful word.

Another useful facility is thesaurus. This is a program which allows the user to choose an
alternative word to a selected word and the user points to a particular word and is presented
with a list of alternatives.

Desktop Publishing (DTP)

Desktop publishing system is a computer with the necessary hardware and software to
enable the user to carry out computerized typesetting. A desktop publishing system will be
able to handle document layouts involving not only printed text but diagrams and images
too. The monitor screens u in DTP systems are bit mapped with the grid of pixels on the
screen being used to represent both images and text characters.

Document Processing Using GUIs

The cut and paste options on the edit menu can not only be used within a single document
they can also be used to move text between one document and another. The user can be
editing one document while referring to another. Its this kind of features that help users to
improve productivity.

Graphical features can be used to good effect to aid the appearance of the word processor.
i.e. a ruler can be displayed across the top of the document window so that the user can see
the document's width and the tab marks.

The most significant additional facility of the GUI interface for document preparation is the
use of graphical packages to produce illustrations for inclusion into documents



Main Storage holds data and programs currently in use by the processor. You can also refer
to it as Main memory hence its able to retain information that may be later being recalled or

Features of Main Memory

Data must be transferred to main storage before it can be processed by the
processor. High access speeds then contribute to fast processing.
Its operation is wholly electronic and consequently very fast and reliable. In the
most modern computers the electronic memory circuits are also highly
Data is almost instantly accessible fro main memory because of its electronic
operation and close proximity to the processor.

Main Memory stores

a) Data awaiting output
b) Data currently being processed
c) Instructions awaiting to be obeyed
d) Data awaiting processing
e) Instructions currently being obeyed.

There are various types of Main memory i.e. ROM

ROMS � It's a non-volatile semi-conductor in which all cell states are set permanently
during manufacture. A set of instructions stored in this way is said to he hard wired and is
often called firmware. This technique is often used to extend the instruction set of the
computer by storing special purpose subroutines hence this are known as micro-programs.
ROM is frequently used to store data or instructions which the computer needs all the time
from the moment it is switched on.

Other types of ROM


PROM (Programmable Read Only Memory). The PROM is programmable by the user
hence the data is held permanently once the PROM is programmed.

EPROM (Erasable Programmable Read Only Memory). It's like PROM in that it can
be programmed by the user but has the advantage that it can be erased and reprogrammed.
EPROM must be removed from the computer in order to be erased. Special devices known
as EPROM eraser for this task.


EAROM (Electrically Alterable Read Only Memory) These memories can be read,
erased and rewritten without removing them from the computer. However, the erasing and
rewriting process is very slow compared with reading, thus limits their use

EEROM (Electrically Erasable Read Only Memory) This memory is the same as


Data us normally transferred between main storage and the processor along a device know
as BUS which is effectively a means of sending multiple bits of data in parallel



The wider the bus the more bits it can transfer in parallel hence the faster the rate of data
transfer. The bus width typically matches the word size i.e. an 8bit bus being used with a
computer having 8bit memory.

Cache Memory

This cheaps tend to be expensive this memory chips are used to provide a "cache" which is
a temporary holding area for data which is currently subject to repeated access. The
processor can access this data more quickly and there is therefore an overall speed
improvement. The cache is between main storage and processor hence acting as a holding
area through which all data and instructions pass

Cache Memory



Magnetic diskette unit and magnetic diskette (Floppy Disk)

Floppy disk unit/floppy diskette is extremely popular storage medium particularly when
used with microcomputer systems such as personal computers.

Features of Floppy Disk unit

Floppy disk units are single drive units able to hold a single disk. Single drive units
are often incorporated physically into the body of personal computers.

The action of inserting the diskette activates a turntable, which rotates the diskette
and also brings a read and write head into contact with the disk. The read write head
is moved to and from across the disk in order either to record data on the disk
surface (a write) or to read back data which has previously been recorded.

Each disk is inserted into a narrow slot in the front of the disk drive. The slot
sometimes has a small flap over it to protect the unit from dust. The diskette is
pushed into the slot and then automatically clips into place. A push button is used to
for releasing the diskette when its to be removed.

A typical floppy disk rotates at about 360 rpm compared with the hard disk which
rotates ten times faster.

Features of a Floppy Disk
Data is stored on tracks, sectors and cylinders in a way directly comparable to
the hard disk.

A pliable disk permanently sealed within rigid, smoothly lined, protective
plastic envelope, which keeps the whole disk surface covered and protected
when not in use.

Data is stored on tracks, sectors and cylinders in a way directly comparable to
that used for hard disk.

Computers read and write data in multiples of one or more sectors. This unit of
data transfer is called a cluster or allocation unit.


Optical Disk Unit and Optical Disk

There are three basic types of optical disk and all of them use laser to write or read
data. All three provide a means of storing very large volumes of data.

CD ROM (Compact Disk Read-Only Memory)
WORM (Write Once Read Many)
EO (Erasable Optical)

CD ROM (Compact Disk Read-Only Memory)

A type of optical disk capable of storing large amounts of data -- up to 1GB, although the
most common size is 650MB (megabytes). A single CD-ROM has the storage capacity of
700 floppy disks, enough memory to store about 300,000 text pages.
All CD-ROMs conform to a standard size and format, so you can load any type of CD-
ROM into any CD-ROM player. In addition, CD-ROM players are capable of playing
audio CDs, which share the same technology.
CD-ROMs are particularly well-suited to information that requires large storage capacity.
This includes large software applications that support color, graphics, sound, and especially
video. They are a replacement for the Vinyl LP record.

WORM (Write Once Read Many)

In computer storage media, WORM (write once, read many) is a data storage technology
that allows information to be written to a disc a single time and prevents the drive from
erasing the data. The discs are intentionally not rewritable, because they are especially
intended to store data that the user does not want to erase accidentally. Because of this
feature, WORM devices have long been used for the archival purposes of organizations
such as government agencies or large enterprises.

Its type of optical media, WORM devices were developed in the late 1970s and have been
adapted to a number of different media. The discs have varied in size from 5.25 to 14
inches wide, in varying formats ranging from 140MB to more than 3 GB per side of the
(usually) double-sided medium. Data is written to a WORM disc with a low-powered laser
that makes permanent marks on the surface.

EO (Erasable Optical)

They are Optical rewritable disks. The disk unit is similar to that used by WORM
but works on a slightly different principle. Instead of burning a pattern onto the disk
surface the laser heats spots on the disk surface which is made of a special alloy.
Magnetic molecules in the alloy surface can be aligned by a magnetic field when
warmed by the laser but cool again to leave a semi-permanent magnetic pattern
comparable to that found on a magnetic disk. There users can write to, read from,
and erase from EO disks as often as they can. EO disks require special hardware.


A workstation is a computer intended for individual use that is faster and more
capable than a personal computer. It's intended for business or professional use.
Workstations and applications designed for them are used by small engineering
companies, architects, graphic designers, and any organization, department, or
individual that requires a faster microprocessor, a large amount of random access
memory (RAM), and special features such as high-speed graphics adapters.

A computer terminal is an electronic or electromechanical hardware device that is
used for entering data into, and displaying data from, a computer or a computing
system. Typically it provides a text terminal interface over a serial line.



These devices display information that has been held or generated within a computer. Some
examples are shown below.

VDU or Monitor
Synthesized Voice
Machine Tool

VDU or Monitor

Visual Display Units (VDU) or monitors are used to visually interface with the computer
and are similar in appearance to a television.

A cathode ray monitor
A plasma monitor
Visual Display Units display images and text which are made up of small blocks of colored
light called pixels. The resolution of the screen improves as the number of pixels is
increased. Most monitors have a 4:3 width to height ratio.

You can print out information that is in the computer onto paper. By printing you create
what is known as a 'hard copy'.
There are different kinds of printers which vary in their speed and print quality.
The two main types of printer are impact and non-impact.



Plotters are used to produce graphs or diagrams.
Plotters can be of two types:
Pen plotters
Electrostatic plotters
Pen plotters have an ink pen attached to draw the images, and electrostatic plotters work
similarly to a laser printer.

Robots are able to perform a variety of tasks as a result of executing instructions contained
within a program.
We are still a long way from the science fiction robots and androids portrayed by the film

A robot arm
Robots in a business sense mean automated machines designed to perform mundane
operations which require accuracy, such as assembling cars.
They are seldom manufactured to resemble the human form.


Synthesized Voice

Voice synthesis has a robotic sound due to the difficulties of replicating the complexities of
human speech and pronunciation.
Voice answer back (VAB) is used to respond to telephone enquiries, such as the speaking

A speak and spell toy
Educational applications include "Speak & Spell" and arithmetic trainers.
Machine Tool

Computer Aided Manufacturing (CAM) tools are used to manufacture a variety of
Repetitive functions are controlled by sets of computer instructions.
Machine tools automate factory production and have the advantage over humans of high
accuracy, and they never require a coffee break!



Computer Science � C.S. French
Computer science logic - Peter G. Clote
Computer science � Kathleen Larson
IAT College - Nairobi


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