Chapter 3: Organization of a computer system

3.1 Computer System ComponentsA computer system is a system composed of components that are classified either as Computer

hardware or Computer software.

o Computer software is the set of instructions, called programs, that tells the computer

what to do and when to do it.

o Computer hardware is the physical parts of the computer that you can see, use, touch,

and that may be dropped and broken.

3.1.1 Computer HardwareHardware refers to the physical components of the computer. It is that part of the computer or

any accessory attached to the computer called “peripheral” which you can touch or feel. The

computer case, monitor, keyboard, mouse, printer, modem and scanner all constitute hardware.

The hardware of a computer is infrequently changed, in comparison with software and data

which are "soft" in the sense that they are readily created, modified or erased on the computer.

The hardware part of a computer system is composed of a number of interacting physical parts

based on the need of the information flow.

The following diagram shows a representative classification of a computer hardware, which

shows the various parts responsible for each of these operations.

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3.1.1.1 Input Devices It is the unit used to enter data into the computer so that it can be processed.

It converts information from a form suitable to human beings to one understandable by

the computer.

Examples of input devices:

- Keyboard (which is the most widely used input devices). It is the standard input

device. It has about 100 keys. The keys are arranged as follows:

1. Alphanumeric keys – used for typing, similar to that of a typewriter.2. Numeric keys – found at the right side of the keyboard, it is used for entering

numbers and moving the pointer.3. Function keys – these are the twelve function keys, F1 – F12, at the first row of

the keyboard. They may be programmed to perform different tasks.4. non printable control keys – these are used for backspacing, going to next line,

tabulation ,moving cursor up or down , insert delete characters etc. there is also a space bar at the bottom for leaving

5. Special character keys – these are characters such as <, >, ? , / , {, }, [ ,] ,(, ) ,. , “, \ ,!,@,#,$,%,^,&,*. Most of these are printed when the shift key in the keyboard is pressed down and the key on which it is written is pressed. For example, when shift key and the key with digit 2 are pressed together, @ is printed.

- Disk drives (which reads & writes data on disks)

- Mouse

It is a pointing device used to move a graphical pointer on the screen. It can also be used to issue commands, draw, and perform other types of input tasks. Variations of the mouse include the trackball, track pad and integrated pointing device.

Trackball – is similar to a mouse except the ball is on top where it can be moved with the thumb.

Track pad – is a touch-sensitive pad that is used to move the pointer by gliding your finger across its surface.

Integrated pointing device – is a small joystick built into the keyboard. You can move it in the direction that you want to move the pointer

- Scanner

- Light pen (a photo cell to choose a displayed response to request further

information).

- Microphone

- Touch screen

- Joystick

- Optical recognition system

1. OCR (Optical character recognition) OCR (Optical character recognition), which is capable to recognize hand-

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printed and typewritten characters.

2. OMR (Optical Mark recognition). It doesn’t utilize letters of the alphabet.

Instead, with the use of electronic scanners, marks and symbols are

converted into appropriate electronic signals. A typical example of an

OMR application would be computerized test forms. OMR systems are

also used to read bar codes such as the Universal Product Code (UPC).

3.1.1.2 Output Devices Used to get data out of a computer so that it can be examined, analyzed or distributed to

others.

Convert the result of the only-machine understandable form to a form understandable by

human beings.

Examples:

The Visual Display Unit (VDU) or Monitor or Screen

The monitor displays the output of the computer. Monitors are categorized by the colors they produce and by the technology they

use. By the colors they produce, monitors are classified as follows:

Monochrome - one color text/image on single color background, i.e. white on blue, or green on black.

Grayscale – shades of gray on a white background. Color – can display different colors, from 16 to 16 million colors.

By the technology they use, monitors are classified as follows: Cathode ray tubes (CRT) – in these monitors, electrons are fired at phosphor

dots on the screen. The dots are grouped into picture elements called pixels, which glow when struck by electrons. In color CRT monitors, each pixel contains a red, green and blue dot. These glow at varying intensities to produce color images.

Liquid Crystal Display (LCD) – it is an example of flat panel display. The screen is still made of dots but is quite flat. LCD displays are made of two layers of a polarizing material with a liquid crystal solution in between. An electrical signal makes the crystals line up in a way that keeps light from going through entirely or just partly. A black screen has all the crystals lined up so that no light gets through. A color LCD screen uses groups of 3 color cells instead of 3 phosphor dots. The signal for a picture cleverly lets just the right spots show their colors. Your eye does the rest.

When comparing monitors, consider the following: Size – the diagonal measurement of its face in inches. 17” is common. Resolution – number of pixels on the screen. This is important especially for

graphics, page layout and CAD. o VGA (Video Graphics Array) monitors have resolution of 640 X 480 pixels.

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o SVGA (Super VGA) monitors have resolutions ranging from 800 X 600 pixels to 1024 X 768 pixels or more.

Refresh rate – indicates how often the picture is redrawn on the monitor. If the rate is low, the picture will appear to flicker. Flicker is not only annoying but also causes eye strain and nausea. So, a high refresh rate is desirable. 60 times per second is tolerable at low resolutions for most people. 75 times per second or more is better and is necessary for high resolutions. 

Dot pitch – is the distance between phosphor dots that make up a single pixel. A dot pitch of .28 is very common and .26 should be good for nearly all purposes.

PC Projector – connects to a PC and is used to project images on a large screen.

Printers

Printers are used to produce “hard copy” of documents, programs and data. Some printers use serial communication (RS232 standard) while others use parallel communication to transfer information from the PC to the printer. In serial communication, data is transferred one bit at a time while in the case of parallel communication; one byte (8 bits) is transferred at one time. The printing method (the actual forming of characters) also varies. Based on the printing method used, printers are categorized as impact printers and non-impact printers. Impact printers strike an inked ribbon, pressing ink from the ribbon onto the paper while non-impact printers use different methods to place ink on paper.

Dot–matrix printers are common types of impact printers. The print head has an assembly of tiny pins that are activated in a row and physically “hit” the paper through the ribbon to produce the desired impression. Speeds are expressed in characters per second (cps) and vary between 180cps and 400cps. These printers are loud, inexpensive and slow. The quality of the printout is not as good as that of inkjet or Laser printers. But, we can have carbon copies and printouts on stencils from these printers.

Inkjet printers are non-impact printers. Tiny particles of ink are electrically charged after leaving the nozzle. The droplets are then guided to the right position on paper by electrically charged deflection plates. The print quality is very good. We cannot use them with stencils nor can we produce carbon copies using these printers. Inkjet printers are available for color or black and white.

LASER (Light Amplification by Simulated Emission of Radiation) printers are the most advanced of all. In this printer the laser beam forms an image of the characters to be printed on to the light sensitive drum. Ink then sticks to the character images and is transferred from the drum and fused onto the paper. Laser printer speeds are specified in pages per minute (PPM), e.g. 6PPM, 12PPM or 24PPM.

Factors to be considered while selecting printers include: speed (measured in PPM or CPS), quality (measured in dots per inch – dpi), initial cost, cost of operation and special features like support for color and large page formats.

Sound systems – multimedia PCs come with a sound card, speakers and a CD-ROM

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or DVD drive. A sound card translates digital signals into analog that drive the

speakers.

Plotters

Disk drives, etc

3.1.1.3 Computer Memory and Storage MediumThe data and instructions that are entered into the computer system through input units have to

be stored inside the computer before the actual processing starts. Similarly, the results produced

by the computer after processing must also be kept somewhere inside the computer system

before being passed on to the output units. Moreover, the intermediate results produced by the

computer must also be preserved for ongoing processing. The storage unit of a computer system

is designed to cater to all these needs. It provides space for storing data and instructions, space

for intermediate results, and also space for the final results.

The storage unit comprises of two types of Storage (Memory): Primary Memory & Secondary

Memory.

Primary Memory (Storage)

It is also known as internal or main memory

It is used to store data and program

It is directly accessible by the CPU

It also used to hold intermediate and final results of the execution

It can be visualized as an ordered sequence of storage locations or array of cells labeled

from zero up, which is called the address.

It is divided in to two:

Random Access Memory (RAM)

o Temporally stores the program and data currently being used, so that the CPU can

quickly access the information

o Its contents change rapidly and often

o It has limited storage capacity, since it is very expensive

o It is volatile memory, i.e. it loses its content when a computer loses electric

power.

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o It is classified as DRAM (Dynamic Random Access Memory) and SRAM (Static

Random Access Memory).

Read Only Memory (ROM)

o It is static, non volatile memory

o It allows data to only read from, which was recorded by the computer

manufacturer.

o It contains minimum instructions that the computer needs to get started

o Used to store programs and instructions that the computer used frequently and

permanently.

o It is classified as, ROM, PROM (Programmable ROM), EPROM (Electronically

Programmable ROM), EEPROM (Erasable Electronically Programmable ROM)

External Memory (Secondary Memory (Storage))

When you create data on a computer or write programs, you can not save them permanently in

computer’s primary memory, because:

o It is not large enough to store all of the programs & data that are used, and

o It loses its content when the computer is turned off.

The secondary storage, also known as auxiliary storage, is used to take care of these

limitations of the primary storage. That is, it is used to supplement the limited storage

capacity and the volatile characteristic of primary storage.

It is much cheaper than primary storage and it can retain information even when the

computer system is switched off or reset.

It is normally used to hold permanently the program instructions, data, and information of

those jobs on which the computer system is not working on currently, but needs to hold them

for processing at a later time.

It can be erased and reused

The physical material that actually holds data is called storage medium. The hardware that

writes data to or reads data from a storage medium is called a storage device. And Storage

capacity is the amount of data the storage media can hold.

Examples:

Magnetic tape: fallen in popularity & now used as a backup device (it is also

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known as tertiary storage)

Floppy disk (3 ½ & 5 ¼): the most common external storage device ( can hold up

to 1.44MB)

Hard disk: become popular due to its capacity & access speed (can hold up to

120 GB)

Compact disk (CD): optically high capacity disks (can hold up to 700 MB, and

found in different varieties, such as CD-ROM, CD-R, CD-RW)

Digital Versatile Video (DVD): also known as digital video disk, and optically

high density disk. (can hold up to 8.5 GB in single side, and can be found in

different varieties, such as DVD-ROM, DVD-R, DVD-RW)

There are three technologies exist for secondary storage: magnetic, optical and semi conductor.

Examples:

magnetic storage: Floppy disks, hard disk, magnetic tapes

Optical storage: CD-ROM, DVD-ROM, CD-R, CD-RW

Semi conductor storage: Flash disk

Magnetic Storage

The medium contains iron particles, which can be polarized, i.e. given magnetic charge in one of

two directions. Each particle’s direction represents a 1 or a 0, representing each bit of data that

the CPU can recognize. A disk drive uses read/write heads containing electromagnets to create

magnetic charges on the medium.

Disk Formatting: It maps the disk’s surface and determines how data will be stored.

During formatting, the drive creates circular tracks around the disk’s surface, and then

divides each into sections called sectors. The OS organizes sectors into groups, called

clusters, and then tracks each file’s location according to the cluster it occupies.

Formatting also deletes any information that exists on the disk.

Hard disk drive

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Write-protect Window

3 ½”, 1.44MB Diskette

Inside the case of the hard disk drive are a set of rigid rotating disks, called platters,

stacked on a spindle. As the platters rotate at high speed, Read/Write heads move

back and forth recording or reading magnetic information on both sides of the

platters. The speed with which the heads can find data is called “average access

time”.

Hard disk drives are normally installed inside the computer case, although external

hard disk drives that are connected to the computer by a cable are also available.

Hard disk drive is selected based on its storage capacity (measured in “Gigabytes”

(GB)) and its access time (measured in milliseconds (ms)). The larger the capacity

and the smaller the access time, the better. Today, it is not uncommon to have hard

disks with capacity in excess of 200GB.

Operating system and application software normally reside in hard disk drives.

Documents created using application software may also be placed on hard disk

drives.

Floppy disk drive

Floppy disk drives store information onto a removable media

known as diskettes or floppy disks. Floppy disk drives operate

slower and have much less capacity (1.44MB) than hard disk drives.

Diskettes store information on the magnetic surfaces of a thin,

flexible disk inside a protective plastic cover.

Although some years back, we had two standards as far as physical size is concerned, namely

3½” and 5¼”, the standard size these days is invariably 3½”. With respect to the amount of

information that a 3½” diskette can store, a few years back diskettes were coming with sizes

of 720KB and 1.44MB; now they come only with 1.44MB.

It is possible for someone to format a diskette, or write information on a diskette

unintentionally and important information could be lost in certain situations. To

avoid such unintentional loss of information, diskettes are write–protected. To write–

protect a 3½” diskette, open the write–protect window by sliding the plastic tab

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covering the window. We can read information that is stored in write–protected

diskettes, but we cannot write onto them.

Software used to come in floppies some years ago. Now, floppies are rarely used to

carry software unless the software happens to be very small. Floppies are mainly

used to carry personal files, to transfer information from one PC to another or as

backup media for small files.

Magnetic Tape

Information stored in a hard disk or floppy disk may be damaged for various reasons.

To protect against losing any of your files, you should make a backup copy of all

your data. Small files (1MB) can be backed up to a floppy disk. For larger files or

groups of files, it is more convenient to use CD–R/W or magnetic tape unit.

Magnetic tape systems offer very slow data access, but provide large capacities and

low cost. Thus, they are particularly convenient if we want to backup the entire hard

disk.

Tape backup units are available as either internal or external models. Tapes must be

formatted before you can use them. Backed up information could be compressed,

where compression ratios vary for different types of data.

Optical Storage

COMPACT DISK (CD)

It is a flat, metallized plastic disc with digital information encoded on it in a spiral

from the center to the outside edge.

The standard CD 650 to 700MB of data

CD-ROM Drive used only for reading from CDs

CD Writer: also known as CD-Burners which can record information’s (like data,

movies, audios and other) in combination with relevant Burning software.

The most common type of removable media, cheap but fragile. It is portable.

o CD–R (it can be recorded once and read many times):WORM(Write Once Read Many)

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o CD –RW (write and read many times)

o CD-ROM (an abbreviation for "Compact Disc Read-Only Memory") is non-

volatile optical data storage medium using the same physical format as audio

CDs, readable by a computer with a CD-ROM drive.

DVD (DIGITAL VERSATILE DISC)

It is also known as "Digital Video Disc"

It is an optical disc storage media format that can be used for data storage, including

movies with high video and sound quality.

DVD-ROM Drive used only for reading from DVD

DVD Writer: also known as DVD-Burners which can record information’s

DVDs resemble compact discs as their physical dimensions are the same (12 cm or

occasionally 8 cm in diameter) but they are encoded in a different format and at a

much higher density.

o DVD-ROM: read only, manufactured by a press

o DVD-R [DL]: recordable once [dual layer record once]

o DVD-RW [DL]: rewritable [dual layer rewritable]

The disc may have one or two sides, and one or two layers of data per side; the

number of sides and layers determines the disc capacity.

o DVD-: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gigabytes (GB)

o DVD-: single sided, double layer, 8.5 GB (7.92 GB)

o DVD-: double sided, single layer on both sides, 9.4 GB (8.75 GB)

o DVD-: double sided, double layer on one side, single layer on other, 13.3 GB

(12.3 GB)

o DVD-: double sided, double layer on both sides, 17.1 GB (15.9 GB)

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Semiconductor Storage

Flash disk

It is developed using IC technology, which becoming popular and accessed through

USP (Universal Serial Port) port

It can handle data in Giga bytes

3.1.1.4 The Central Processing Unit (CPU)

Fig: a typical processor (CPU)

It is also known as processor

The CPU and other components necessary to make it work are mounted on a main circuit

board called a motherboard.

It is the heart of the computer system, also known as a “brain of the computer”

It performs two major functions:

Coordinates and controls the computer’s activities

E.g. retrieving files from a disk, interpreting data & commands, and sending data to

output devices, etc.

Performs arithmetic and logical operations on a binary number system

It consists of three major parts:

1. Arithmetic logic unit (ALU)

o It is referred as the computers "number crunchier".

o It performs the arithmetic and the logical operation, having a set of electronic

circuit which performs the required operation.

- The data and instructions, stored in the primary storage prior to processing are

transferred to registers when needed to the ALU, where processing takes

place.

- Intermediate results generated in the ALU are temporarily transferred back to

the primary storage until needed at a later time.

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- So data may thus move from primary storage to ALU and back again to

storage many times before the processing is over.

2. Control unit (CU)

o It acts as a central nervous system for the other components of the computer

system.

o It is responsible for controlling the overall operation of the computer system, i.e.

it directs the operation of the entire system

o It is responsible to fetch, interpret and control the execution of program

instructions stored in a memory

o It directs and coordinates all units of the computers, such as telling:

- input devices what data to input,

- the memory unit where to store the data,

- the ALU where to find the data, what operation to perform, and where to store

the result, and

- output devices what information to record and where to store it

3. Registers

o A memory element in CPU

o It temporarily store data and memory locations

o It retains an intermediate results required by the ALU

o It acts as a paths or conduits that connect the Arithmetic Logical Unit to the main

memory.

3.1.1.5 The Bus It is an electronic circuit which produces communication path between the different

components of a computer system along which data are transferred.

Its size determines the speed or efficiency of the computer.

It is classified in to two types, depends on which component it connects:

o The bus, which communicates the different parts of the CPU is called Internal

Bus.

o The bus, which communicates the CPU with memory and peripheral devices, is

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called External Bus.

It is classified in to three types, depends on the type of information communicated with:

o Address Bus: - is a unidirectional bus over which digital information is

transferred to identify either a particular memory location or particular I/O

address.

o Data Bus: - a bidirectional bus system which interconnects the CPU, memory

and all the peripheral Input / Output devices of a computer system for the

purpose of exchanging data.

o Control Bus: - a bus used to select and enable an area of main storage and

transmit signals required to regulate the computer operation. (W/R operation)

Note: Factors that affect the processing speed of a computer: Registers: The size of the registers, also called word size, determines the amount of

data with which the computer can work at one time. Most PCs have 32-bit registers.

RAM: has direct effect on system’s speed. The more the RAM size, the faster. These

days, PCs come with RAM sizes of 128MB – 2GB.

System clock – sets the pace for the CPU by using a vibrating quartz crystal. A

single “tick” of the clock is the time required to turn a transistor off and back on.

This is a clock cycle measured in hertz (Hz). The faster the PC’s clock runs, the more

instructions the PC can execute each second. Today, PC clock speeds exceed 3GHz

(3 X 109 Hz).

Bus width: determines how many bits can be transmitted between the CPU and other

devices. The larger the bus width, the faster the computer.

Cache memory: cache is a high speed memory that holds the most recent data and

instructions that have been loaded by the CPU. It is located directly on the CPU (L1

cache) or between the CPU and RAM (L2 cache). Amount of cache memory has

tremendous impact on the computer speed. The larger the cache size, the faster the

computer.

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3.1.2 Computer SoftwareWe have seen previously that; however, one has to give precise instructions to the hardware in

order to solve any problem.

Recall that the computer hardware is an electronic device which has the potential of performing

the task of solving a problem. However one has to give precise instructions to the hardware in

order to solve problem. The finite set of instructions (steps) that the computer follows to perform

a given job is called a program.

Note: Any program to be executed, first it should be reside / loaded/ in the primary memory.

Software:- is a collection of programs and routines that support the operations of performing a

task using a computer.

It includes documentations, rules and operational procedures.

It makes the interface between the user and the electronic components of the

computer.

It is classified into two:

o System software: helps the computer to manage its own tasks and devices.

o Application software: enables us to perform a particular task like writing letters,

analyzing data, sorting files, drawing diagrams or playing games.

3.1.2.1 System Software It constitutes those programs which facilitate the work of the computer hardware.

It organizes and manages the computer resources, handles the input/output devices, and

enables the system to understand programming languages

It lies between the hardware & the application software.

It acts as intermediate between the user and the hardware system. I.e. it serves as means

of communication between user and a computer.

It controls the hardware by performing functions that users shouldn’t have to or are

unable to handle, which makes complex hardware to be user friendly.

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It contains three main categories:

Operating system: tells the computer how to interact with the user and its own

devices. Examples include Windows, UNIX, Linux, etc.

Language software: used to write programs that run on computer.

Utility Programs: programs that provide additional utilities to users of a computer.

Some are included with the operating system

Operating System

It is the most vital part which interfaces & coordinates the activities of users, application

software and computer hardware.

It have three major functions:

i. Controlling operations (control program)

Coordinates, or supervises the activity of the computer system.

Decides where programs and data should be stored in the computer memory.

Handles communications among the computer components, applications software

and the user.

Controls the saving and retrieving of files to and from disks in the disk drive.

It performs all its controlling tasks without the involvement or awareness of the

user.

ii. Input/output Management

The 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.

iii. Command Processing ( Command Interpreter)

It interprets the commands or what you enter using the keyboard or other input

devices.

If you write an internal command it carries out the function of that

command

If it is external command or other executable file

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It searches for the corresponding file in the default (current disk) or

the user specified disk

Loads the file into memory and transfers control to that program.

Once this program is terminated, control returns to command

COM and the program for that command or file is discarded from

memory

It is classified into three types: the classification is based on the number of programs they

can handle at a time and the number of users they serve at once at one or different

stations.

Single user and single tasking. E.g. MS DOS

Single user and multi tasking. E.g. MS Windows

Multi user and multi tasking. E.g. UNIX

Language Software (Programming Language)

It is used by programmers to develop application software’s and translate programs to

machine code.

It is a generic name consisting of various programs that serve as editors & translators to

develop programs in a number of programming languages.

Translator: a program that converts one or more languages to another language.

There are three types of translators, namely assembler, Compiler & interpreter.

Assembler: a program that translates assembly languages into machine

code.

Compiler: a program that translates entire code written in high level

language into a machine code. (FORTRAN, C++, COBOL)

Interpreter: a program that translates each line of a program for

immediate execution. (Java)

It can generally be classified as: low-level languages and high-level languages.

Low-level language: classified as machine language and assembly language.

Machine language

It is the natural language of the computer.

A program written in machine language is ready for immediate

execution.

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It is very difficult to understand, and a program written using

machine language, takes the form of strings of zeros and ones.

Even the simplest program is complex and lengthy.

Each computer type has its own machine language and thus a

program written in machine language for one computer may not

run in another computer, i.e. machine language programs are not

portable.

Assembly language

It consists of English-like abbreviations.

It is easier to understand than machine language.

Programs written in assembly language need to be translated into

machine language before they can be executed.

The program that translates assembly language programs to

machine language is called an assembler.

High-level languages

They are English-like instructions.

They are easier to learn than low-level languages.

Programs written in high-level language are called source code and they

must be translated into machine language before they can run on a

computer. After the translation, the program in machine language is called

object code.

Examples: C, C++, Basic, Pascal, FORTRAN, COBOL, JAVA, etc.

Utility programs

Includes programs that provide additional utilities to users of a computer.

Programs that perform tasks related to the maintaining of your computer's health -

hardware or data.

Examples: file de-fragmentation, data configuration, backup, antivirus, screen saver

programs, calculator, calendar, notepad, etc.

3.1.2.2 Application Software17

It is software that is designed to perform tasks for the specific area or areas.

It is usually called application packages as they may include a number of programs along

with operating instruction, documentation and so forth.

Depending on their function or task they are categorized in to the following types:

Word Processors/ Word processing

It is a computerized typewriter which permits the electronic creation, editing, formatting,

filing and printing of documents.

Is the most common application of microcomputers

Example: WordStar, WordPerfect, Microsoft word

Spreadsheet

It is an electronic worksheet display on the VDU.

It is useful for accounting, financial analysis and other mathematical calculation.

Example: Lotus 1-2-3, Microsoft Excel

Database management system

It allows you to store information on a computer, retrieve it when you need it and update

it when necessary.

Example: Dbase IV, FoxPro, Microsoft Access, Oracle.

Computer graphics

It is used to create, edit, display, and print graphs, diagrams, charts and image in a

computer system.

Example: AutoCAD, Photoshop, 3D Max.

Statistical packages

Applications that are especially designed for statistical work

Example: SPSS, MStat.

Accounting software

Software’s designed specifically to perform financial statements, ledgers, etc.

Example: Decease, Scala, Peachtree

3.1.3 Computer Physical and Operational Setup

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Printer

Mouse

Keyboard

System Unit

Monitor

3.1.3.1 Physical setup Of the ComputerWhen you look at a computer externally, it simply consists of the system unit and the most

common peripheral devices, namely the monitor, keyboard, mouse and (optionally) printer.

The Computer Case

It houses all the major components of the computer.

o On the front end of the case may be found on/off switch, reset button (optional),

status indicators (power and HDD indicators) and openings to floppy disk drives,

CD–ROM drives and tape cassettes.

o On the rear end of the case are found ports for connecting peripheral devices with the

PC and a power socket for connecting the PC to the electrical power source.

Expansion Slots and Cards

Expansion cards are small circuit boards that are used to upgrade a computer. These

include: enhanced graphics cards, modems, networking cards, sound cards, etc.

Expansion cards are inserted into expansion slots in the motherboard (a board into which

most of the basic components of the computer are built).

PCI (Peripheral Component Interconnect) is a local bus system that allows devices to be

inserted quickly and easily (plug–and–play compatible). Most of the computers today

come with PCI slots and cards.

AGP (Accelerated Graphics Port) is a new platform bus specification that enables high

performance graphics capabilities, especially used for 3D graphical display applications.

USB (Universal Serial Bus) defines a class of hardware that makes it easy to add

serial devices to your computer. It provides an instant, no-hassle way to connect a new

digital joystick, a scanner, a set of digital speakers, a digital camera or a PC telephone to

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their computer. To install a USB device, plug the cord from the device into any USB Port

 on your computer.

Power Supplies

It converts normal household electricity, alternating current (AC), into electricity that can

be used by the computer, direct current (DC).

o In Ethiopia, the normal household voltage is 220V/50Hz AC.

It also lowers the voltage level.

Its capacity is measured in watts (W). A power supply of 150–200 W is large enough for

most computers

3.1.3.2 Operational Setup of The Computer

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