Specific Objectives

COMPUTER SYSTEM NOTES FREE , By the end of the topic the learner should be able to:

  1. a) Describe a computer system;
  2. b) Explain the functional organization of the elements of a computer system;
  3. c) Describe input devices of a computer system;
  4. d) Describe the central processing unit (CPU);
  5. e) Describe the output devices of a computer system;
  6. f) Describe the types of secondary storage devices and media;
  7. g) Distinguish between power and interface cables;
  8. h) Explain basic computer set-up and cabling;
  9. i) Distinguish between system software and application software;
  10. j) Evaluate the criteria for selecting a computer system.


  • Description of a computer system
  • Functional organization of the elements of a computer system
  • Hardware
  • Software
  • Live-ware
  • Input devices e.g.
  • Keying devices
  • Pointing devices
  • Scanning devices
  • Speech recognition devices
  • Other digital devices
  • Central Processing Unit (CPU)
  • Control Unit
  • Arithmetic and Logic Unit (ALU)
  • Memory
  • Processors

(i) types

(ii) clock speeds

  • Output Device
  • Soft copy output devices e.g.
  1. i) Visual display unit – Liquid Crystal Display (LCD), flat panel, cathode tube (CRT)
  2. ii) Sound output

iii) Light emitting

  • Hard copy output devices e.g.
  1. i) Printers (impact, non-impact)
  2. ii) Plotters
  • Secondary/Auxiliary Storage Devices and Media
  • Fixed e.g. Hard disk
  • Removable e.g.
  1. i) Floppy disks
  2. ii) Tape

iii) Optical disks (CD-R, WORM, CD-RW, DVDs)

  1. iv) Zip disks
  • Power and Interface Cables
  • Power cable
  • Parallel cable
  • Serial cable
  • Basic computer Set-up and Cabling
  • Connecting basic computer components
  • Connecting other computer peripherals
  • Classification of software
  • Purpose

– System software

  1. i) Firmware
  2. ii) Networking software

iii) Operating system

  1. iv) utilities
  • Application software
  • Acquisition

(i) standard software

(ii) user developed (in-house)

  • Criteria for selecting a computer system (specifications)
  • Hardware considerations


Definition of computer system

  • Collection of entities that work together to process and manage data to information.

Elements of a Computer System

  1. Hardware- Physical or tangible computer components e.g. keyboard, mouse etc.
  2. Software- Programs installed.
  3. Liveware- The computer user.

Central Processing Unit (CPU)

  • It processes data and controls all computer operations. Elements of the CPU are:

Arithmetic and logic unit (ALU)- Performs the actual data manipulation.

Control Unit– 1. Fetch phase-Access instruction in sequence

  1. Decode phase-Interprets instructions
  2. Execution phase-execution of commands

Main memory

It forms an integral element of the processor. It’s tasks are;

  •    Holds data awaiting processing
  • Holds data being processed
  •   Holds data that has been processed awaiting to be output

Read Only Memory (ROM)

  • Its content can be read but cannot be changed during normal computer operations.
  • It is normally used for permanent storage of data and instructions that remains unchanged for a long period of time.

Types of ROMs

  1. Mask Read Only Memory (MROM): contents cannot be changed once written on it by manufacturers.
  2. Programmable ROM (PROM): the user rather than the manufacturers can after store data but once only.
  • Erasable Programmable ROM (EPROM): the content can be reprogrammed by exposing it to ultraviolet light.
  1. Electrically Erasable Programmable ROM (EEPROM): the content can be erased and programmed using electricity.

Characteristics of ROM

  •    The content cannot be changed unless written in a special type.
  •    Non-volatile: data cannot be lost when the computer is switched off.

Random Access Memory (RAM)

  1. Static RAM: the data doesn’t need to be refreshed, it is capable of handling data extremely faster. It is also reliable.
  2. Dynamic:  The data must be refreshed i.e.  data must be read and written periodically. It stores more information than static RAM.

Types of DRAM

1)  Fast Page Mode (FPM)

2)   Extended Data Out (EDO): improvement on FPM

3)  Extended Correcting Code (ECC): Special DRAM, corrects errors, and used on savers.

4)   Synchronous Dynamic Random Access Memory (SDRAM):  twice fast as EDO. Run at a speed of  system bus up to 100 – 133 MHz

5)   Rhombus Dynamic RAM (RDRAM): has a data transfer rate of 800 MHz

6)   Double-Data  rate (DDR): fast DRAM with module speed at which  SRAM can transfer data to the processor.

7)   Video RAM (VRAM): Special type of memory that is used only with video adapter.

8)   Window RAM (WRAM):  a faster version of video memory.

DRAM in system unit on memory chamber of laptops

1)   Dual-inline Package (DIP)

2)   Single-inline Memory Module (SIMM)

3)   Dual Inline Memory Module (DIMM)

4)   Small Outline DIMM (SODIMM)

5)   Micro DIMM


Characteristics of RAM

  •   Information can be accessed at any time (random)
  •   Data can be changed

Special Purpose in RAM

  1. Buffers: They provide temporary storage for finished tasks so that the CPU is set free to carry out other activities instead of waiting for all data to be entered in information to be output.
  2. Registers: They hold a piece of data at a time and they are inside the CPU. Examples a)An accumulator: temporarily holds the result of the last step of the Arithmetic and

Logic Unit.

  1. b) An address register: Temporarily holds the next piece of data waiting to be processed.

c)Instruction register: Temporarily holds an instruction just before it is interrupted into a form that the CPU can understand.

  1. d) Storage register: It temporary holds a piece of data that is on its way to and from the CPU and the main memory.

Cache Memory

It helps the processor to access data and instructions faster due to incorporation of small high- speed type of Static RAM than it would have taken to fasten it from the relatively slow Dynamic RAM.

Types of Cache Memory

  •     Level 1 (Primary cache):  located inside microprocessors.
  •    Level 2 (Extended cache): may be inside microprocessors or mounted on the motherboard.
  •     Level 3 (Latest cache): works like level 2 to optimize system performance.


Terms used in the CPU

  1. Chip: A small rectangular piece of silicon on which most circuits are implemented by extension.
  2. Integrated Circuit (IC): It is a device that is integrated or joint to connect a circuit of several electronic components in a single package.
  3. Large Scale Integration (LSI): Refers to the technology that integrates thousands of transistors on a single Integrated Circuit (IC). If more than 20000 transistors are integrated the Very Large Scale Integration (VLSI) is used.
  4. Microprocessor: This is the brain of the PC.
  5. Microcomputer system: This refers to computer set of devices require using and operating the computer. This includes peripheral devices

Input Devices

Devices that accept data from external environment and encode it into a form understandable to the computer for processing.

Classification of Input Devices

  1. Keying devices: use buttons to make entries e.g. keyboard and keypad
  2. Pointing devices: Mouse, trackball, joystick, lightpen
  3. Scanning: capture data directly from source

Optical scanners

  1. Optical Mark Recognition (OMR): Detects marks made on a piece of paper using a soft pen or pencil.


  •    Low error rates
  •     Conspicuous errors are corrected
  •     Minimize manual work



  •    Relatively slow
  •     Difficult verification of worked data
  •     Not widely used in business


  •    Making of multiple choice exams
  •     Market research questionnaires
  •     Payroll entry
  •     Traffic surveys
  1. Optical Bar Recognition (OBR): Used to read bar codes.


  •     Barcodes can be printed by normal printing methods
  •    Type of item or price doesn’t have to be repeatedly written or retyped


  •     Bar codes can be read by machines
  •    Only numbers can be coded this way
  •    Are relatively fixed, so they are unsuitable for recording prices which can change frequently


  •   On labels and on shelves for stock taking
  •    Printed on shop goods e.g. supermarkets
  •     In libraries
  1. Optical Character Recognition (OCR): Not only does it scan characters but also can scan real objects, pictures and drawings


  •     Limited errors
  •   Early methods
  •     Document design is less complicated
  •    Character can be read by people


  •     Expensive or costly
  •    May require redesigning quite frequent
  •     Only certain font is applicable


Magnetic scanners

  1. Magnetic Ink Character Recognition (MICR): Allows special devices to read magnetic characters written in a special format.


  •     Fast, accurate and automatic
  •     Codes are both machine and human readable
  •     Reduces forgery


  •    Limited to only 14 characters
  •    Based on magnetism and can easily


Banking industry for cheques

  1. Magnetic Strip Recognition (MSR):


  •   Simple to produce
  •     Not easily damaged
  •    Stores fairly large number of characters


    Data can be changed or erased by magnetic fields



  •    On credit card
  •    On bank cards
  •     As tags on clothes
  •     As a phone card

Keying devices

  1. Traditional Keyboard
  2. Flexible keyboard: can be folded
  3. Ergonomic Keyboard: designed to provide comfort and allocate wrist strain.
  4. Keypad: Miniature keyboard used in PDAs, laptops and mobile phones.
  5. Braille keyboard: designed for the blind.

Pointing devices


  1. Standard mouse: has a ball underneath, two buttons and optical scroll button located between left and right buttons.
  2. Optical mouse
  3. Cordless/ wireless mouse: battery powered mouse that uses radio or infrared waves instead of being physically connected to the system unit



It’s advantage over mouse is that it neither requires an extra space nor a flat surface for movement.


An input device like a car gear that can be moved sideways, upwards or downwards to control the

position of the cursor.

Light pen and style

The lightpen is a hand held  device that has just a light sensitive point. It can make selections,

place images, draw and indicate dimensions by simply touching the screen. Doesn’t emit light but reacts to light emitted by the display using a photosensitive detector at its base.

Speech Recognition/ Voice input: This is where microphone is used to enter data/information or spoken words into a computer. Situations for speech recorgnition:

  •    Where hands are busy handling documents
  •    Due to movement
  •   Disability e.g. blind without hands


  1. Electronic fund transfer
  2. House/ car security i.e. voice activated locks
  • Offer security for room access
  1. Voice activated toys and scanners
  2. Quality control and automation in factories
  3. Automated material handling
  • ComputerAid Design



  1. Existence of homophones
  2. Word separation is difficult
  • Slow recognition
  1. Speaker variability
  2. It is still at the early stage of development
  3. Limited vocabularies

Touch screen, Digitizers and Cameras

Touch screen: Use touch sensitive screen where the computer detects the position of the finger on the screen and responds accordingly.

Digitizers: Almost similar to light pen but has a graph tablet on which user writes using a pen like device called a style

Cameras: Store images in digital form.

Other Examples of Input Devices

Point of sale (POS) Terminal

This consists of numeric keypads and a few controls or function keys. They are normally used in computerized wholesale or retail organizations like supermarkets.

Input Facility Considerations

  •   Cost
  •    Input Speed
  •    Volume
  •    Accuracy
  •    Reliability
  • Mode of operation
  •    Appropriateness

Problems faced while collecting data

  •     Preparation of source document is slow hence prone to mistakes.
  •    Low typing speed leads to keyboard bottleneck.
  •    It is wasteful to use a media which can only be used once like punched cards.
  •    Mistakes can be made when copying data from one medium to another.
  •    Translation delays or losses can occur if data is physically transferred to the computer.
  •     Data entry can be expensive


The Main methods of data entry

  1. Source document reader: Orders, sheets and forms are batched and converted into computer acceptable medium.
  2. Online data entry (keyboard entry): Data is entered directly into computer one translation at a time under program control.
  3. Direct data capture: Data capture directly without conversion stage.

Outputs Devices

Devices that accept information from internal environment and then decodes it in a form understandable to user 

Classification of Output Devices

  1. Soft copy: includes monitors/ VDU and the speakers
  2. Hard copy: Printers and plotters
  3. Microfiche: Computer Output on Microform (COM)


Visual Display Unit (VDU)/ Screen


  1. a) Monochrome- Display images and text in only one colour
  2. b) Color- Display in multiple colours

Examples of Graphic Adapters

  1. Monochrome Display Adapter (MDA)- display text in only one color.
  2. Hercules Graphic Adapter (HGA)- Support monochrome image and text
  3. Color Graphic Adapter (CGA)- Display using sixteen colours.
  4. Enhanced Graphic Adapter (EGA)- An improvement over colours but use 16 colours.
  5. Video Graphic Array (VGA) – Offers at most 256 colors.
  6. Super Video Graphic Array (SVGA) – an enhanced VGA

Common Types of Flat panel Display

  1. Liquid Crystal Display (LCD): Doesn’t emit light of their own instead have tiny liquid crystals that reflect light falling on them from the environment.
  2. Electro Luminescent: Improved LCD. Emit light when electrically charged.
  3. Gas plasma: Use gas that emits light in presence of electric charge.
  4. Thin Film Transistor (TFT): This is the latest technology advancement that provides high quality resolution.


How printers are classified

  1. a) Speed
  2. b) How the produce the print; character, line or page printers
  3. c) How they provide prints on the stationery; impact or non-impact printers


  1. a) Impact printers: Dot matrix, daisy wheel, golf ball and chain printers.
  2. b) Non-impact printers: Thermal, inkjet, electrostatic and laser printers
Impact Non-impact
Slow Fast
Use ink ribbon Thermal/electrostatic principle
Cheaper Costly due to technology involved
Noisy Quiet
Multiple copies produce is possible Multiple copies produce almost impossible


Printers Consideration

  1. Printers initial price, subsequent maintenance cost and cost of consumable items.
  2. Volume of printing expected.
  3. Nature of report to be generated.
  4. Range of capability for the selected printer.
  5. Interface with computer at hand on for family particular make.


They are used to produce graphical output e.g. maps and diagrams.

  1. Flatbed plotters: Has a flat surface which the stationery is placed for the pen to be moving over it in all directions to produce graphical output.
  2. Drum plotters: Has a drum onto which stationery is placed for the drawing to be possible.

Difference between a printer and a plotter

Printer: produces hardcopy of information on papers.

plotter: Produces big charts e.g. maps, pictures and drawings. Used for printing geographical, architectural and engineering drawings e.g. maps, advertisements posters to be placed on bill boards, machinery parts etc.

Computer Output (originated) microform (COM)

Provides photograph type of output into the microform (photographs reduced documents on films)



  1. Saves stationery and space
  2. Faster than printing

3 Non-bulky hence conveniently transportable

  1. Guarantee security as content cannot be ready by naked eyes

Computer Bus

  1. Longer lifespan compared to paper medium output


  1. Expensive
  2. Cause eyestrains
  3. Expensive additional equipment for viewing contents

Communication medium within a computer that allow different elements of the computer to communicate with one another.

  1. Address bus: Pathway used to locate storage position in memory for the data to be executed.
  2. Data bus: Pathway where the actual data transfer takes place.
  3. Control bus: Pathway for all timings and controlling functions sent by the control unit to the parts of the system

Auxiliary/secondary storage

Factors to consider when choosing a storage device

  •     Volume of data to be stored
  •    The cost
  •    The quantity or size of data it can hold


Magnetic Tapes

These are sequential access devices. Implies that information is at the end of tape the program will have to read almost entire tape which may take time to read. Data recorded as series of magnetic dots across tracks on the tape cassette when characters are stored serially down the length of the tape.

Sequential access medium: Data is stored across magnetic tape

Serial access medium: data is stored along the media


  •     Light and easy to carry
  •    Effective for batch applications since they are cheaper


Magnetic Disks

Floppy Disks (Diskettes)

  •   Slow since they are sequential access media
  •     Gaps between (called Inter Record

Gap, IRG) waste storage space.

Information is recorded on number of concentric circles called tracks. Tracks are divided into sectors. Sectors and tracks have same storage capacity because inner tracks have a high density. Diskettes are direct access storage medium (DASM) and access time is made up of 3 parameters.

  1. Seek time: Time taken by the read/write (R/W) head to locate right track that contains content.
  2. Rotational delay time: Time taken by drive mortar to spin the diskette past R/W head.
  3. Data transfer time: Time taken by the R/W head to interpret content and send it ascomputer input.


  1. Exchangeable and easy to carry
  2. Can be used with PCs with no hard disk
  3. They are cheap
  4. Used for back up


  1. Slower than hard disk
  2. Less storage capacity
  3. Damaged by light, magnet and dirt

Zip Disks

Come with portable external drive and are slightly large and thicker in size and has a capacity of up to 250MB

Jaz Disks

Come with portable external drive and have a capacity of 1GB to 2GB and have high access speeds.

Advantages of Magnetic Disks

  1. Light and portable
  2. Effective for batch application
  3. Cheap

Care of Magnetic Storage Media

  • Keep away from extreme temperature
  • Never touch the disk surface
  • Keep them away from magnets
  • Never use clips to hold disk
  • Keep them away from dust, moisture and high humidity. 

Optical Laser Disk

They are recorded using a laser beam (very strong concentrated light)that burns patterns as fine circular tracks (indentations) on the surface. Data is retrieved using relatively low power laser.


Super disk (SD) LS-120

They resemble 3 ¼ floppy disk but has a high capacity of 128 MB

Compact Disk (CD)

They have a high recording density of about 700MB. There are three types of compact disks:

  1. CD-ROM (Read Only Memory): The recorded data cannot be changed, deleted or added.
  2. CD-R (Recordable): They are initially blank but when data is recorded it becomes permanent.
  3. CD-RW (Re-writable): Data can be overwritten/ added or raised.

CD-ROM and CD-R are referred to as WORM (Write Once Read Many) since you cannot write the second time but you can read many times

Digital Versatile Disk (DVD)

They resemble the compact disks but have a greater storage capacity up to 17GB

Optical Tapes

They resemble magnetic tapes but they use optical technology

Hard disk


  1. They offer cheap storage
  2. They are well protected since they are housed in the CPU
  3. Fast in terms of access retrieval and saving of data

Care for the Hard disk

  1. a) Keep them away from smoke and dust.
  2. b) Use the right procedure to switch of the computer to allow the R/W head to move from the disk surface to avoid scratching on rotating surface
  3. c) Exposure to magnetic property

Minimal Requirements for a multimedia computer

  •  A Video Graphic Array (VGA)/ Super Video Graphic Array (SVGA) card and monitor.
  •  A sound card
  • 128MB RAM or higher
  •    CD or DVD drive
  •   FM/TV card



Power and Interface Cables

Power cables are used to connect the computer to main power outlet.

Interface cables connect all peripherals to the motherboard and transmit data signals.

Power supply unit supplies power to motherboard and other internal device

Types of Cables and Ports

  1. Parallel ports: They transmit data simultaneously using a set of connectors- 8bit parallel cables. They are faster over short distance. Printers, removable storage drives and network adapter. Port is (PT1) 25 PIN
  2. Serial ports: They transmit one bit at a time. They are slower than parallel ports but they support two way communications. They are 25 pin and 9 pin. COM 1 and COM2. Used by the serial printers.
  3. Universal Serial Bus (USB)

They transmit one bit at a time but at a very high speed and provide quality data transmission about 15m distance. – External storage devices, cameras etc.

Advantages of USB

    Provide very high speed

    Quality data transmission over long distance

    Support a wide range of peripheral devices

  1. Small Computer System Interface (SCSI) – Use parallel method but faster than parallel cables. The port connects up to 8 peripheral devices.

Other ports and connectors

  •     5 PIN DIN and 6 pin (PS/2): used by keyboard and mouse
  •    9 pin D and 15 HI-pin-D: monitors
  •   Audio connectors: jack plugs found on sound adapter for connecting speakers, microphone and other portable audio equipment’s.


System software

  Operating system

A set of computer programs that normally reside in the main memory and used to control the basic computer hardware resources & the operation of the entire system. 

Firmware (Store Logic)

A combination of both software and hardware recorded permanently on electronic chips mounted on the motherboard. May hold operating system, utility programs, language processors etc.

Utility Software

A special program that performs commonly used services that make certain computing aspects go on smoothly e.g. sorting, copying, disk management etc. There are two types:

  1. System level:  Help user to work with the operating system and its function. It tells whether the command entered is wrong and give suggestion on how the error can be corrected.
  2. Application level: Make use of an application program smoother and efficiently.

Functions of utility software

  •      Reporting of errors which occur during program execution
  •     Control input and output devices
  •     Protect use hardware and data from misuse
  •     Provide program testing routines
  •     Provides communication between computer user and each program
  •     Memory management
  •     Maintenance of internal time clock and log of system usage by all users.

Network software

Establish communication between two or more computers by linking them using a

communication channel like cables to create a computer network e.g. Novell Netware

Application (packages) software

Programs that are designed to help user accomplish specific tasks.

Software Uses Examples
Word processors Typing  long documents MS-Word, Word Pro
Spread sheet Calculating budgets MS-Excel, Quatropro
Desktop Publishing (DTP) Designing publications Adobe PageMaker, MS-


Computer Aided Design


Technical drawing AutoCAD
Database Keeping records and files MS-ACCESS, Dbase
Graphic Design Creating and manipulating


Corel Draw, Adobe Photoshop


Internet Software

Used in communication and perusing the internet.

Programming Software

Used to create programs that behave like software

Classification according to Acquisition

  1. In-house developed programs/ Custom design programs

They are designed to meet the needs of a specific user or organization

  1. Standard/ Vendor of the shelf software

Designed by software engineers to be used by different users. When closely related software are put under one software it is known as suit/ integrated/ general software. While packages are known as single purpose software e.g. Lotus Suite, MS Office etc.

Advantages of Standard Software

  •    Less expensive
  •  They’re readily available for almost any tasks
  •   Well tested before they are released hence chances of errors are minimal
  •    Easily modified (customized) to meet specific user needs
  •    Other software can be linked to it and use the same data


  •     Easily pirated
  •     Do not meet all user requirements

Consideration for a Computer System

Criteria for selecting a computer system

  •    Identify the user requirement
  •    Evaluate the hardware requirements that will meet users’ needs
  •    Evaluate the software requirements 


Hardware Considerations

  1. a) Processing speed: For faster data processing the speed of the processor must be relatively high.
  2. b) Memory capacity: At least 128MB RAM. There are also factors to be considered when buying a memory module-The type of module supported by motherboard

-Whether there is an empty slot on the motherboard

-Compatibility of the module with the existing one

-Module capacity

  1. c) Warranty: Arrangement between the buyer and the seller whereby incase the product fails during the agreed period of time it is taken care by the seller free of charge or for an agreed fee. Things to consider in a warranty –The scope of cover

-Call out response and liability agreement

-Regular preventive maintenance

  1. d) Cost: The cost is determined by the processor, size, and the memory capacity.
  2. e) Upgradability and compatibility: Whether the hardware chosen can be easily upgraded to accommodate emergent technologies.
  3. f) User’s needs: Can it meet the unique needs like for the disabled?
  4. g) Monitor: Always consider the size, resolution and the technology involved.
  5. h) Multimedia capability: It should have speakers, CD/DVD drive, sound card, VGA or


SVGA card.

Software Requirements

  1. a) Authenticity: Always consider genuine, valid and legitimacy and developers license.
  2. b) Documentation: Manual from the developers that guide the user on installation, usage and maintenance. –User guides


-Manual guide

-Installation process

-Quick reference


  1. c) Reliability and security: Consider whether the software offers good security to confidential and private information.
  2. d) User friendliness: How easily the user can interact with the software.
  3. e) Compatibility and system configuration: Consider whether the software can work with the computer at hand.
  4. f) Cost: Always consider the cost and whether it serves the purpose.
  5. g) Users’ needs: The purpose of the software.
  6. h) Portability: Consider whether it can be installed in different computers.




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