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Student Publications
Author: Anthony Oduori Solo
Title: Computer Science
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INTRODUCTION TO COMPUTERS AND
COMPUTER SCIENCE
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
engineering
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
microprocessor.
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
programs.
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
i)
Application software
ii)
Systems software
i)
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.
ii)
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
Input
Enter Data
Processing
CPU
Manipulate Data
Output
"View" Processed Data
Storage
DATA REPRESENTATION AND
TRANSMISSION
Characters
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.
Transmission
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)
PUNCTUATION MARKS
CONTROL ELEMENTS
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
Efficiency
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 PROCESSING
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.
Editors
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
shown
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
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
accessed.
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
miniaturized.
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
EPROM
EAROM
EEROM
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
EAROM
BUSES
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
BUS
MAIN MEMORY
PROCESSOR
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
MAIN MEMORY
Cache Memory
PROCESSOR
BACKING STORAGE
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.
WORKSTATIONS AND TERMINALS
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.
OUTPUT DEVICES
These devices display information
that has been held or generated
within a computer. Some
examples are shown below.
VDU or Monitor
Printers
Plotters
Robot
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.
Printers
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
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.
Robot
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
industry.
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
clock.
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
products.
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!
REFERENCE
Computer Science � C.S. French
Computer science logic - Peter G.
Clote
Computer science � Kathleen Larson
IAT College - Nairobi
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