Basic Unit of Computer System

7/29/2019 Basic Unit of Computer System

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Chapter 2

The Basic Units of a Computer SystemSystem is a group of components, consisting of subsystems or procedures that work in a

coordination fashion to achieve some objective.

A computer system composed of two main components:

Computer hardwareand Computer software

Computer Hardware

Computer hardware is the physical part of the computer that you can see, you use and the parts

you can touch.

The hardware part of a computer system is composed of a number of interacting physical parts basedon the need of the information flow.Information flows in the computer hardware. Based on

information processing, we can divide computer hardware into four:

1) Input Device2) Storage Device3) Output Device

4) Central Processing Unit (CPU)

Fig computer system

Input DevicesInput devices are used to enter information into computer. They convert the data we give them

into the form that can be manipulated in the computer (electronic format).

Keyboard

is an input device consisting of a set of typewriter-like keys that enable one to enter data into acomputer. The keys on the keyboard are often classified as follows

Alphanumeric keys - Letters and numbers Punctuation keys - comma, period, semicolon, etc. Special keys- function keys, control keys, arrow keys, caps lock, etc.

In detail the following are descriptions of the keys of keyboard.


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Escape key - Generally used to abort system actions.

Function keys - functions vary from program to program. For example F1 is used to get help

most of the times

Enter key - used to enter commands or to return to blank (new) lines in a document.

Backspace key - used to move the cursor backwards, deleting backward.Tab key - used to access tab stop.

Caps lock keyused to write small or capital letters.Shift key - used to produce upper case or lower case letters and sometimes to invoke commandsControl key & Alt key - commonly used in combination with other keys to enter commands e g.

Control home.

Insert key - used to enter characters.Arrow keys - used to move the cursor around the display screen.

Delete keys - used to delete characters.

Num lock keys - used to activate/deactivate the numeric keypad.

Print screen keys - used to print the contents of the screen.

The arrangement of the keyboard's key may differ. The most common in English- speaking

countries is the QWERTY keyboard. It is called QWERTY keyboard because the first five keyson the top row of letters spell Q, W, E, R, T, and Y. The arrangement of keys in this case isbased on typewriter (alphabets and punctuation) and calculator (numeric pad). This is to make

data entry easy.

The other type of keyboard layout is the Dvorak Keyboard. In this layout, the most commonly

used letters are placed in the most accessible places. The home keys are the five English vowel

letters A, O, E, U, I (for the left hand) & the five most often used consonants (D, H, T, N, S) are

the home keys for the right hand. The next most frequently used letters are on the keys one rowup, the next easier position to reach.

MouseMouse is a hand-operated device that controls the movement of the cursor or pointer on a display

screen. It is a small object that can be rolled along a rough flat surface.

Its name is derived from its shape, which looks a bit a mouse; its connecting wire that one can

imagine to be the mouses tail. Mouse was invented by Douglas Engelbart of Stanford Research

center in 1963.

Mouse can have as many as three buttons whose function depends on what program is runningLeft-button: for left click. E.g selecting text

Right-button: for right click. E.g see property of file

Middle button: for scrolling purpose. This button is sometimes missing.

Most mouse are connected to the system unit by a cable or cord. There are also mouse that are

not connected to system unit by a cable/cord. They are called cordless mouse (wireless mouse).They communicate with system unit using infrared light, similar technology used in remote

controls, or radio waves.

Mouse has roller ball at the bottom. The roller ball converts mouse movements into electronicsignals.


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Mouse pad - is a pad on which you can move a mouse. It provides more traction than smooth

surfaces such as glass & wood, so they make it easier to move a mouse accurately.

Advantage: -easy to use

Use: making menu selection editing documents moving documents (i.e. dragging data from one location to another location) Opening and closing programsOptical Recognition SystemsOptical recognition system uses light to read characters, codes, and marks, and convert them into

computer understandable format. There many types of optical recognition systems: OCR (Optical Character Recognition) OMR (Optical Mark recognition) Barcode Reader

I) OCROCR are scanners that read typewritten, computer printed, and in some cases hand printed characters

from ordinary documents. They scan the shape of character and compare it with predefined font shapesfound in memory. Based on the comparison, they can convert the characters into computer code.

Use:

Enter documents found on paper into computer without the need to type it using keyboard. Thismakes the job easy.

II) OMROMR senses the presence or absence of a mark, such as pencil marks. It doesnt utilize letters of the

alphabet. Instead, with the use of electronic scanners, marks and symbols are converted into

appropriate electronic signals.It is used to process questionnaires or exams using computer. The read mark is interpreted and matched

against previously entered answer key. Example, the ESLCE is corrected using this device.

III) Magnetic Ink Character Recognition (MICR)

It is intensively used by U.S. banking industry to input information on checking accounttransactions. It has been in use since the 1950s, allows checks and deposit slips to be read both

by people and by machines. The machines, called magnetic ink character readers, read and

convert the characters into machine code by detecting the presence of magnetized particles in theink on the checks or deposit slips. As many as 1500 checks per minute can be read and stored.

MICR uses a special ink that can be magnetized during processing. It is almost exclusively used bybanking industry for processing checks. When the check is processed, the amount is written in the

lower right corner.


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Fig Check that uses magnetic ink

The numbers you see at the bottom of the check uses magnetic ink.

Advantage:It allows both computer and human being to read data

IV)Bar Code Reader

Bar code reader is a peripheral that reads bar codes printed on products. Bar code uses a patternor symbols to represent data. It consists of vertical lines and spaces of different width. There are

different kinds of bar codes; the most commonly used is called UPC (Universal Product Code). Itis used to identify product manufacturer, product number, etc.

Fig bar code and bar code reader respectively

Use:

used in super market, grocery, etc to identify products and their price.Speech Recognition SystemsSpeech recognition is the ability of computers to recognize human speech. We use microphone

to enter the speech into computer. The computer then processes the speech and performs the

action ordered by speech.It allows:

To issue/enter commands to the computer. For example, instead of going tostart button->programs->Microsoft Wordto open Microsoft Word, we simply speak into the microphoneof the computer Open Microsoft Word. Then the computer opens the program.

To enter data using dictation instead of typing from keyboard. This makes data entry veryeasier.

This is the latest technology and is still under research.

Light Pen


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Light pen is a light sensitive pen like device that is used in conjunction with computer monitor. Itis used by touching the screen with this device to create or modify graphics. A light cell in the tip

of the pen senses light from the screen to determine the pens location on the screen.

Fig light pen and monitor

Use:

to draw directly on the screen by using the screen as paper to select options for processing (to select from menu)Joystick

Joystick uses the movement of vertical stem to direct pointer on the screen.

Often used with computer games

It has buttons you can press to activate events.

Touch ScreenTouch screen allow you to use finger instead of mouse or related input device. It lets you to enter

data or command (e.g. button click) by touching areas of the screen. It enables you perform

activities you perform with mouse using your finger. It is not used to enter large amounts of data.

Used:

To issue a command to software to perform a particular task like menu selection, buttonclick, etc

To choose from a list of optionsAdvantage: They are easy to use They are easy to learn even by no technical peopleThey are used to provide information in hotels, airports, and other public areas.

Output Devices

Output devices are usedto get data out of a computer so that it can be examined, analyzed or


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distributed to others. It converts information from machine-understandable form to a humanunderstandable form.

Two types of output:

Soft copy: displayed on monitor, projector, or similar devices

Hard copy: printed on paper

ExamplesThe Visual Display Unit (VDU) or monitor or screenPrinters (dot matrix, daisy wheel, laser printers)PlottersVoice (audio) response unitDisk drives

Monitor

Monitor is the most frequent type of output device. It consists of a display surface called screenwhich displays output to the user. Image is displayed on monitors using pattern of lighted dots.

Each of these lighted dots is called pixel a short form of picture element.

There are three types of monitor:1) CRT (Cathode Ray Tube)

2) LCD (Liquid Crystal Display)

3) Plasma display

CRT Monitor

CRT is similar in size and technology to televisions. It uses electron guns that emit electrons.

These electrons are directed towards phosphorous coated screen. The phosphorous coat glows(emits light) when hit by electron from electron guns, thus creating visible things that we see.

Disadvantage: -large size (bulky)- consumes much power


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Fig CRT monitor and electron gun

Yoke: moves electron beams across and down the screen by generating electromagnetic field

Electron beams: negatively charged electrons moving towards the screen form electron guns

Crystal DisplayCrystal display is most commonly used in digital watches, clocks, calculators, etc. In computer,

they are commonly used for portable computers. E.g. laptop.

Liquid crystal uses liquid crystal for display purpose, not electron guns. The liquid crystal is

deposited between sheets of polarizing material.

Advantage over CRT: -small size which makes it easy to transport-consumes less power which makes it suitable for laptop.

Gas Plasma

Gas plasma substitutes neon gas for liquid crystal. The neon gas glows and produces pixels thatform picture. It is used for portable computers

Advantage:Better display quality than LCD but more expensive than LCD

Printer

There are two types of output:Soft copy: is information that is displayed on monitor, projector, etc.

Hard copy: a data that is printed on paper.

Printer is a device that enables us to produce a hard copy of data/information. They have varying

speeds, capabilities, and printing methods. Based on the way they print, printers are divided intotwo:

Impact printers Non-impact printer

Impact printers

Impact printer transfer printable material onto paper by striking paper, ribbon, and charactertogether. Most of such printers take continuous form of paper. Some of impact printers are:

Dot-matrix PrinterThey print by striking wire pins against an inked ribbon. The print head of such printers contains

pins. When this pin is pressed against a ribbon and paper, it prints small dots. The combinationof small dots printed closely forms a character.

Daisy-wheel PrinterIt has a plastic or metal wheel on which the shape of each character stands out in relief. A

hammer presses the wheel against a ribbon producing characters on paper.Disadvantage:

They cant print graphicsNon-Impact PrintersThey print without having a mechanism that strikes a paper. They print by spraying ink or by


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using heat and pressure to fuse black powder onto paper. The various types of non-impact printerinclude:

Ink Jet PrintersAn ink jet printer sprays tiny drops of ink onto paper. The print head of such printers contains anozzle which has many holes. Ink is propelled by heat or pressure through the nozzle holes and

form character or image on the screen.

Produce high quality print Quiet because the paper is not struck Heavy weight paper is recommended

Laser PrinterThey work similarly to photocopying machine. They convert data from computer into a laser

beam (kind of light), why they are called laser printer. They use mirrors to direct light onto adrum which in turn produce characters on paper. The drum is light sensitive material that

becomes charged when it is hit by light. The charged areas of the drum attracts ink from toner

and the ink is transferred onto paper. A heating material heats the ink and attach it onto therpaper. It produces high quality print.

Thermal printersThermal printers use heat to transfer inks from ink sheets onto printing surface. They producehigh-quality print. Because of this, they are used to produce high quality color artwork and text.

They can use plain paper but produce best result on chemically treated papers.

PlottersPlotters are used to produce high-quality drawings such as bar charts, maps, architectural

drawings, and three-dimensional drawings.

Can produce high quality multi-color documents Can print on large size papers, which most printers cant handle.

Storage Devices

Two types of storage devices:

Primary storage device: data is lost when computer is off. Secondary storage device: stores data permanently

Secondary Storage

Secondary storage (also called auxiliary storage), takes many forms. It includes punched cards,punched paper tape, magnetic tape, magnetic disk and optical disk. Based on information access,

secondary storage devices are divided into two: Sequential Access: information is accessed sequentially. To access information on such

media, we start from the beginning and read through to the end. Jumping to some part is not

possible. E.g. tape recorder cassette

Random Access: information can be accessed in any order. You can access the first or thelast part of information by jumping others. E.g. floppy disk


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Sequential Access Medias

1) Punched CardsPunched cards are one of the oldest and most familiar data storage devices. There are two types

of punched cards, 80 columns and 96 columns, and the first one is a standard punched card. Data isstored on punched cards by cutting parts of the paper out and leaving other parts intact. The cut part

(hole) may represent binary 1 and the intact part represents binary 0.

Fig punched card

2) Punched Paper Tape

It is similar to punched card, except it is a continuous strip of paperPunched paper tape is a continuous strip of paper used to store data in much the same manner aspunched cards. This storage media is less expensive and more compact than punched cards.

3) Magnetic Tapes

During 1950s and 1960s, magnetic tape was the primary method of storing large amount of data.

Today they are used primarily for back up purposes.

It consists of thin ribbon plastic called tape. One side of the tape is coated with iron-oxide, amaterial that can be magnetized. By magnetizing and demagnetizing the tape, computer stores

data on it.

Fig magnetic tape

Disadvantage:

Sequential accessBack up is a data that is stored in a different place than the original computer where data is used.

This is to avoid the danger of data loss due to disasters. If the original data is lost, the copy willbe used.

Read-Write head: writes data on the tape or reads data from tape.

Random Access Storage MediaData can be accessed in any order.


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Magnetic DisksMagnetic disk is the most widely used storage medium on all computers. It consists of a round piece

of plastic or metal the surface of which is covered with magnetic material. Magnetic disks offer: High storage capacity Reliability Fast date access during reading or writing to disk.

Magnetic disks are metals or plastic platters coated with ferrous oxide, an easily magnetizable

material. They allow for random access of information and overcome the slow access time

commonly found in tape files.Examples: floppy disk, hard disk

Floppy Disks

They are also called diskette. It is made up of thin Mylar plastic which is coated with metaloxide. Data is stored as electromagnetic charges on the plastic coat. It stores data by means of

magnetizing and demagnetizing the metal oxide.

Floppy is an easily removable storage used to transport data from one computer to another. Itis used to store programs and data for a longer period outside the computer system. Often used

with mini and micro computers. Data is stored in concentric circles called tracks.

Fig sector and track

Each track is further divided into sectors. A sector stores the amount of information that can be

read or written in single operation.


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The most widely used type of floppy disks is the 5.25 inch and the 3.5 inch wide floppy disks. Each has

two different storage capacity.

31/2 inch: -can store 1.44MB of data on double side high density-can store 720kb of data on single side low density

51/4 inch: -can store 1.2MB of data on double side high density-can store 360kb of data on single side low density

Density refers to the number of bits that can be recorded on one-inch of track.High density: increased storage capacity

Low density: low storage capacity

Rules for handling Floppy Disks

Floppy disks are magnetic devices which stores data on them. Improper handing of floppy disks

may result in loosing the data stored on the disk. It is therefore necessary to know the following

rules to protect your disks and/or your data on it from damage.

Never place the disk near magnetic devices, keep the disk away from your telephone sincetelephone boxes contain magnetic units.

Always place disks back into their envelope when you are not using them. Never touch your floppy disk media. Never smoke near floppy disks. Store your floppies in a safe location.Keep at room temperature in the range of 10 to 50

oc

Always make a backup copy of your floppy disk.

Disk drive: is part of a computer system which reads from and writes data on a disk.

Hard DiskA high capacity magnetic disk made up of metal which can be fixed in the system unit of the

computer. It serves as a secondary storage and it enables very fast accessibility of data. Accessing

data from hard disk is faster than from floppy disks.

The disks, which are usually grouped together into a disk packare separated by small air spaces to

allow access for read-write head. Each disk is subdivided into tracks and sectors on which

information is stored. The disk pack is mounted on a magnetic disk drive which rotates the disk at

speed up to 1,000 revolutions per second.


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Fig hard disk

Platters are grouped together to form hard disk. The amount of time it takes to retrieve (or store)data from (or to) disk is called the disk access time. It rotates at high speed and this is measured

in terms of revolutions per second.

Data is stored on hard disks by magnetizing and demagnetizing the surface each cylinder. They

store data on both sides of the cylinder i.e two sided. It is read-write head that reads or writes

data from hard disk.

Optical DiskOptical disks use laser light to read or write data from optical disk.Laser: light amplified

stimulated emission of rays.Optical disks use high powered laser light to burn microscopic holes on the surface of the disk to

store data. Burned out part is called pit, and the non-burnt part is called land. Land may represent

binary 0 and pit binary 1.

Fig CD pit and land

Optical disks use low powered laser to read data.

There are two most common types of optical disks:

CD (Compact Disc) DVD (Digital Versatile Disc)CD (Compact Disc)CD can store 700BM of information. Data is stored only on one side of the disc.There are threebasic types of CDs:

1) CD-ROM (Compact Disc Read Only Memory)

2) CD-R (Compact Disc Recordable)


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3) CD-RW (Compact Disc Read-Write)

CD-ROMThis kind of is read only i.e. you cant write data to such CDs or you cant erase from them.

Data is only accessed/read from these CDs.

CD-RIt is also called WORM (Write Once and Read Many). These CDs can be written on once. Butthen after, you cant rewrite on it or erase data from it. After you first wrote data on such discs,

then only thing you can do to read data from them.

CD-RWThey are also called erasable optical discs. You can write data as many times as you want on

such CDs. You can also erase the content of such CDs.

DVDDVD stands for Digital Versatile Disc. But some people mistakenly call it Digital Video Disc.It is a new technology that is introduced recently. It is similar to CD except that it can store large

amounts of data and it has narrow tracks than CD.

It can store 4.7-17GB of information.

Central Processing Unit

It is also called microprocessors or processors. A microprocessor is a semiconductor device

consisting mostly one software controlled LSI chip which performs the functions of automatic dataprocessing. Processors are not optimized to perform complex numerical calculations and other

special tasks (graphics manipulations). For this purpose specialized coprocessorswhich are alsocalled math-coprocessorshave been developed to increase the processing capability of a computerand speed up complex mathematical floating-point calculations. The coprocessors operate in parallelwith CPUs.

CPU executes instructions and performs the computer's processing activities. It is also known as

processor or microprocessor. It functions the same purpose as the human brain for human being.It is called the brain of the computer.

CPU has three sub-components: Control Unit (CU) Arithmetic Logic Unit (ALU) Memory

The three sub components are connected by a cable like device called bus. It facilitates

communication between the parts. It is used to transfer data between these sub components.


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Control UnitAs human brain control the body, control unit controls the computer. Control Unit does not

execute instruction by itself, i.e. does not carry out instruction, but it directs other processingelements to execute instruction.

It retrieves instruction from memory and this process is called fetch instruction. After fetchinginstruction, it translates that instruction into computer understandable format and sends signals toother computer hardware units to carry out the instruction. This process (the process of

translating instruction) is called decoding.

Arithmetic Logic UnitThe purpose of ALU is to execute instruction. It performs two operations:

Arithmetic operation Logic Operation

Arithmetic operation: this includes mathematical operations like addition, subtraction,

multiplication, division, etc. If you give your computer the instruction 2+3, this will be includedin arithmetic operation and it is executed by Arithmetic Unit.

Logical Operation: this is concerned with the comparison of data and it is called logical

operation. It includes operators like less than, greater than, equal to, less or equal to, greater or

equal to, different from, etc. e.g. if mark>80, grade is 'A'.

Arithmetic Logic Unit has two parts:

Arithmetic Unit: perform arithmetic operationsLogic Unit: performs logical operations

Registers

When instruction is loaded from main memory, it is placed first in registers to wait instructionfrom control unit. Registers are temporary storage location for data prior to execution in ALU.

There are different types of registers. E.g. data register, program counter, address register, etc.

How CPU WorksCPU operates by performing the following four operations, i.e. instruction execution has four

steps:

1) Fetch instruction2) Decode instruction

3) Execute instruction

4) Store result

Fetch Instruction: fetching instruction is getting instruction from memory. Instructions are stored

in memory and wait execution. The Control Unit gets the instructions from memory and brings it

for execution. It keeps track of which instruction is to be executed next.Decode Instruction: translating instruction into commandsthat the computer can process.

Decoding understands what the instruction is about and what it tells the computer to do.

Execute Instruction: the actual processing of the commands i.e carrying out what the instructionorder the computer to do.

Instruction cycle

Execution cycle


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Store Result: writing the result of executed instruction into memory.

Example: suppose we give the following two instructions to the computer2+3

3*4

When we write these instructions, first they are stored in memory (RAM) and wait for execution.The first step in carrying out instruction is getting the instruction from memory. The control unit

gets the first instruction 2+3 and sets pointer to next program to be executed i.e. 3*4. This is the

fetching process.Then the CU translates the instruction and it finds out that the instruction tells the computer to

add 2 and 3 together. This is translating or decoding instruction.

After decoding CU sends control message to ALU based on the interpretation done in step 2. The

ALU adds the numbers and gets the result which is 5. This process is called execution.The last step is storing the result of execution, 5 in this case, in memory back. This process is

called storing the result.

CPU speed is measured in Hertz. Hertz is the number of cycles per second.

1Hz=1cycle per second

20Hz=20 cycle per second

Larger units are KHz (Kilo Hertz), MHz (Mega Hertz), GHz(Giga Hertz), etc.1KHz=1000Hz

1MHz=1000 KHz

1GHz=1000MHZCurrent CPUs are as fast as 2-3GHz (2-3 billion cycles per second)

Based on the number of instruction set they can execute, we have two types of CPU: RISC(Reduced Instruction Set Computer) CISC(Complex Instruction Set Computer)

CISC: uses large number of basic instruction i.e larger number of instruction set could be

executed in such CPUS.This is the most commonly used type of CPU. E.g. Intel Pentium

RISC: Use fewer instruction sets. The design is simpler and less costly. Faster than CISC

Main MemoryMain memory refers to integrated circuit that stores program instructions and data. Memory

stores three things:

Operation system software instructions Application software instruction Data that is being processed

There are three types of primary memory:

RAM (Random Access Memory)

ROM (Read Only Memory)


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CMOS (Complementary Metal Oxide Semiconductor)

RAMRAM is temporary storage i.e. the data is lost when the computer is off unlike secondary storage.

Because of this it is called volatile memory. Why is it volatile? It uses electric power to store

data. When you write anything on your computer, first it is stored on RAM. When you save thefile, it is transferred into secondary storage. RAM has differing capacity, the common ones being128, 256, and 512.

It is directly accessible by CPU. It is called RAM because each memory location can be accessedrandomly using memory address. Each unit in RAM has memory address by which it can be

easily accessed/referenced.

ROMROM store information permanently. They have programs built into them at the factory and that

program could not be changed or erased by the user, but read.

It is non-volatile, read-only (not changeable). Read-only means data can't be altered or erased butread.

ROM contains special instruction that the computer uses when it is turned on. E.g instruction that

starts the computer, instruction that displays information on screen, etc.

There are three kinds of ROM: ROM (Read Only Memory) PROM (Programmable ROM) EPROM (Erasable Programmable ROM) EEPROM (Electrically Erasable Programmable ROM)

PROM: this kind of ROM can be written by user once. But, you can't erase or write to it morethan once; you can only read from.

EPROM: this kinds of ROM can be written to or erased as many times as the user wants. It is

like CD-RW.EEPROM: similar to EPROM, it can be re-written to or erased. But we use electrical power to

erase the contents here.

CMOSCMOS contains essential information that is required when the computer is turned on. E.g. RAM

size, type of mouse, etc.

Non-volatile, but modifiable to incorporate changes. It is powered by battery when computer isoff.

Computer Software

A computer hardware is directed by a set of instructions. Without these instructions, computercan do nothing. These set of instructions are called software (also called programs).

We use programming languages to write these instructions. Examples of programming language

includes C, C++, Visual Basic, Java, etc. Examples of instruction are:


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Coutx;

The first instruction prints This line sends information to screen on the monitor.The second lineaccepts input from user.

Softwares are categorized into two: System Software Application Software

System Software

System software consists of programs that are related to controlling the actual operations of the

computer equipment/resource. There are three types of system software: Operating System Utility Software Language translators

Operating system manage resources, provides a user interface, and run application softwares. It

organizes resources such as keyboard, mouse, printer, monitor, etc. It also presents GUI

(Graphical User Interface) to the user for easy use of computer. It makes complex hardware more

user friendly i.e. it acts between the user and hardware.

A) Operating system

Operating system coordinates the activity between the user and the computer. An operatingsystem has four major functions.

I Process managementIn operating system terms, a process is a program or part of a program that can be executedseparately. The operating system does the task of managing these processes that are being

executed and waiting to be executed. There are different ways of managing processes: Single tasking Multitasking multiprocessing

I I Input/output ManagementThe I/O manager coordinates the computers communication with outside world, flow of data to

the display screen and other output devices (printers/ plotters) and from the key board or other

input devices. Handles the flow of data to and from the disk drives (file management).Handles the process of preparing a disk for use, the copying, renaming, erasing task of a file.

I II Memory ManagementDuring processing, some area of memory are used to store the operating system instructions,application program instructions, and data to be processed. It is the task of operating system to

allocate or assign each of these items to a memory area.

IV System Administration


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Operating system as a system administrator manages the performance of the system (e.g

response time), system security (username and password), and storage device management

(deleting files, renaming files, formatting disks, creating directories, etc).

Types of Operating System

1)

Based on the number of programs they can handle at a timeBased on the number of programs they can run a time, operating systems are divided intotwo:

Single tasking os: on such system, only one program can be run on the computer at a time. Torun another program, you have to close the first program and load another program. For

example, if you are working on Microsoft Word and in addition if you want to open

Microsoft Excel, first you have to close Word first because you cant open two programs at

the same time. Then you can open Excel. Single tasking os cant handle more than oneprogram at a time.

Examples of such operating systems are MS-DOS, and PC-DOS which were used in 1980s.

Multi-tasking os: on such systems you can run as many programs as you like. You can work

on Word and Excel at the same time. Such systems can handle more than one program at a

time. Examples Windows 2000, Windows XP, Macintosh, etc.

2) Based on the number of users they can serve at a timeBased on the number of users they can serve at a time, we divide os into two:

Single User: they can serve only one user at a time. They cant serve more than one user at atime. E.g. Windows 95, Windows 98/2000/xp, etc

Multi User: lets more than one computer to access the computer at one time. E.g. Unix This is

done via network that connects terminals to the multi user computer.

B) Utility ProgramsUtilities are programs that make computing easier. They perform specific tasks related tomanaging computer resources or files. There are different utility programs:

i) Troubleshooting programs: enable us to recognize and correct computer problems beforethey become serious.

ii) Anti-virus programs: they protect your computer against viruses or other maliciousprograms that damage computer. E.g MacAfee, Sophos, Norton, etc.

iii) File compression programs: are used to reduce the size of files or data so that it takesless storage space or network band. E.g WinZip, WinRAR, etc.

iv) Uninstall programs: these softwares enable us to safely and completely removeunneeded programs/softwares from your computer.

v) Back up softwares: with the help of this software, we can make copies of files to beused in case of the original data is lost/damaged. This copy is called back up.

vi) Screen savers: helps to prevent your work from being seen by others if you leave yourcomputer idle for some time.

C) Language translators


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Language translators are used to convert the programming instruction written by users into

binary code that the computer can understand. They are written for specific programming

languages and computer system.

Application Software

Application software performs useful work for the user. These useful work could be: Word processing-document creation Spreadsheet-electronic calculation Email/communicating-email sending and reading

Users use these softwares to perform different activities like calculation, video editing, wordprocessing, presentation, etc.

Some of the uses of application software:

i) Word ProcessingThis is the most widely used computer application. Word processing is the use of computerto produce documents that consist primarily of text. Such documents can also contain

pictures, drawing, photograph, etc.

Used for: Writing letter Writing memos Producing reports

Some of the softwares used for word processing include: Microsoft word Corel WordPerfect Lotus WordPro WordStar

ii) Spreadsheet SoftwareSpreadsheet software is used to organize, manipulate, and graph numeric information. Itenables us to do financial analysis, and other complicated mathematical calculation

electronically. E.g interest rate, payroll, etc.

Some of the softwares used for this purpose include: Lotus 1-2-3 Microsoft Excel Quatropro

iii)Database Management system (DBMS)Database is a collection of related data that is stored in computer. DBMS organizes the

collection of data so that information can be retrieved easily. Database softwares allow youto create a database and to retrieve, manipulate, and update the data. Database may contain

one or more tables. The following table and other related tables together form databse.

Employee table

Name ID DOB Department Salary

1

2


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Softwares that are used for DBMS:

Microsoft Access Oracle SQL FoxPro

Dbase IV

iv)Presentation GraphicsPresentation graphics helps you to create professional and exciting presentations that could

be used in meeting, group discussion, etc. Such softwares enbable you to incorporate charts,graphics, sound, etc in your presentation.

Softwares used for this purpose include:

Microsoft PowerPoint Lotus Freelance Graphics Corel Presentations

v)

Accounting softwareAccounting software enables companies to record and report their financial transactions.

Tasks done by these software include:

Invoicing Accounts Receivable Accounts Payable Payroll Inventory General Ledger Purchase orders, etc

Softwares used for this purpose: Peachtree accounting Intuit QuickBooksvi)Computer Aided DesignIt is used to create design for products, new buildings, etc. They are used by engineers.

Example of softwares used for this purpose include:

AutoCAD, ArchiCAD, Parametric

Vii) Communication

They are used to transfer data from one computer to another. They enable computers to send

and receive data of any kind such as text, graphics, fax documents, etc.Some of the softwares used for this purpose are:

Netscape Navigator-to work on internet Internet Explorer-to work on internet Procom Plus Crosstalk


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Viruses, Trojan Horses and WormsVirus

Virus is a malicious (destructive) program/software that damages computer. It copies itselfon to other programs and spreads through multiple computer system. They may damage the

files on that computer or attack the system in general. Some of actions performed by virus

include: Duplicating themselves on your computer to consume storage space your computerruns out of memory.

Delete or modify your files(documents) Damage your software Damage your hardware, etc.

Worm

Worm is a malicious program like virus. But it does not need help to move from one computer toanother which virus cant do. Virus need help to move from one computer to another. It destroys

the computer system like virus.

Trojan Horse

Trojan horse software that seems to perform useful activity but who has malicious programs in it.

It may damage files, and perform other harmful actions on your computer. It does not replicate

itself. For example, you may think that the program is computer game and you start playing.While you are playing, the game starts to affect your computer inside.

Documents

Basic Unit of Computer System