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GST 115:
A computer system consists of two broad components, known as Hardware and
Software. The hardware refers to the physical units or achiness or functional
units which make up the computer configuration.
The hardware can be divided into these categories:
Central processing unit
Main memory
Secondary\auxiliary storage
Input devices
Output devices
Connectivity hardware
The Central Processing Unit s also called Processor. This is the brain or heart of
the computer equipment. The CPU carries out the calculations for the program
and controls the other components of the system. It does the function by
organizing circuits into two main units, called
Arithmetic Logic Unit and Control Unit.
A microprocessor (processor) is an integrated circuit designed to process
instructions. It is the most important and usually the most expensive components
of a computer. It can be more accurately described as a CPU on a chip because it
contains-on a single chip-circuitry that performs essentially the same task as the
central processing unit (CPU) of a classic mainframe computer. For example:
intel® Pentium® Extreme Edition Dual Core with HT Technology #.20GH z,
800mHz FSB, 2 MB Cache.
Microprocessors performance is affected by several factors; including clock
speed, bus speed, word size, cache size, instruction set and processing techniques.
 Clock speed: 3.20GHz indicates the speed of microprocessor clock- a
timing device that sets the pace for executing instruction. A cycle is the
smallest unit of time in a microprocessor’s universe.
A cycle is the smallest unit of time in a microprocessor’s universe.
A specification such as 3.2GHz means that the microprocessor’s clock
operates at a speed of 3.2 billion cycles per second. A computer with
3.2GHz will be faster than 1.5GHz or 933MHz processor.
 Front side Bus: FSB refers to the term that refers to the circuitry that
transports data to and from the microprocessor. A fast side bus moves data
quickly and allows the processor to work at full capacity. It ranges from
200MHz to 1250MHz.
 Word size: refers to the number of bits that a microprocessor can
manipulate at one time. Word size is based on the size of register in ALU
and the capacity of circuits that lead to those registers. For example a
processor with a 32 bit word size has 32-bit register, processor. (Will have
 Cache: it is a temporary storage area in the memory. It is a special highspeed memory that allows a microprocessor to access data more rapidly
than from memory located elsewhere on the system board.
 Instruction set: this is the collection of pre-programmed activities such as
addition, subtraction, comparison, division and multiplication. We have
complex instruction set computer with complex instructions each requiring
several clock for execution. Also there is reduced instruction set computer
(RISC), it performs most instruction faster than a CISC processor.
 Processing technique; some processor executes instruction serially; that is
one instruction at a time, this is known as serial processing. The processor
must complete all steps in the instruction cycle before it begins to execute
the next instruction.
 Pipelining: a processor can begin executing an instruction before it
completes the previous instruction.
 Parallel Processing: multiple instructions are executed at the same time.
 Dual Core Processor: it’s a computer having a single chip containing the
circuitry for two microprocessor. It is faster than single core processors.
The ALU contains circuits that do Arithmetic and Perform Logical Operations.
The Control Unit contains circuits that analysed and execute instructions.
The Arithmetic Logic Unit
The ALU contains arithmetic circuits that can subtract multiply and divide two
numbers. More complex operations such as finding the square root of a number
are done by sequence of their basic operations. To perform a calculation or logical
operation, number is transferred from primary storage to storage registers in the
These numbers are sent to the appropriate arithmetic or logic circuit. The results
are sent back to the storage registers. The results are transferred from the storage
registers to primary storage.
The microprocessor’s Control Unit fetches each instruction, Data is loaded into
the ALU’s register and finally the Control Unit gives the ALU the green light to
begin processing.
Instruction Cycle refers to the process in which a computer executes a single
instruction. Some parts of the instruction cycle are performed by the
microprocessor’s Control Unit, other parts of the cycle are performed by the
ALU. The instruction thata computer is supposed to process for a particular
program. Are held in memory. When the program begins, the memory address of
the first instruction is placed in a part of the microprocessor’s control unit called
an Instruction Pointer. The control unit can then fetch the onstruction by copying
data from that address into its instruction register. From there, the control unit can
interpret the instruction, gather the specified data or tell the ALU to begin
The Control Unit fetches instructions interprets them, fetches data and tells the
ALU which processing operations to perform. ALU uses register to hold data that
are ready to be processed. When it gets the go-ahead signal from the Control Unit,
the ALU processes the data and places the result in an accumulator from the
accumulator, the data can be sent to memory or used for further processing.
The Control Unit
The control Unit controls the whole computer system by performing the
following functions:
 Directs and Coordinates all operation called for by the program
 Activates the appropriate circuits necessary for inputs and output devices.
 Causes the entire computer system to operate in an automatic manner.
The control unit contains a temporary storage location called an instruction
register for storing the instruction being executed. It also contains circuits called
the instruction decoder – which analyse the instruction register and cause it to be
executed. The control unit executes each instruction by following the same basic
sequence of steps:
 The next instruction in the program is retrieved for primary storage and
stored in the instruction register
 The instruction is sent to the instruction decoder where it is analysed.
 The decoder sends signals to the ALU, primary storage, I/O devices, and
secondary storage that cause the actions required by the instructions to be
Primary storage is also called internal storage or memory. It is used to store
programs and data currently being processed by CPU. Primary storage circuits
like light bulbs need electricity to stay on. If the power to the computer is turned
off, all the circuits will turn off and all data in primary storage will be lost. When
computer is turned back on the data will not reappear. The data is lost forever.
Because of this characteristic primary storage is called volatile storage. This type
of primary storage is called RANDOM ACCESS MEMORY or RAM. RAM is
the main type of primary storage used with computers and it is volatile.
Several computers have another type of primary storage called ROM – Read Only
Memory. ROM is non-volatile storage. This means that when the power to the
computer is turned off, the content of the ROM is not lost. ROM can store preset programs that are always put by computer manufactures. When you turn on a
PC, you will usually see a reference on BIOS (Basic Input output System). This
is part of the ROM chip containing all the programs needed to control the
keyboard, monitor, disk drive, etc.
There are different types of ROM, including PROM, EPROM and EEPROM.
PROM – Programmable Read Only Memory – In this type of ROM, data or
program can be stored once in PROM but never changed.
EPROM – Erasable Programmable Read Only Memory – In this type of ROM,
data or program can be erased by removing the device and exposed it to ultraviolet
EEPROM – Electrically Erasable Programmable Read Only Memory – data or
program can be completely erased using a special device and new programs or
new data can be stored in it.
The Use of Primary Storage (RAM)
Main Memory has several uses:
 Input area – where the data is stored when it is read into CPU, awaiting
 Operating system – controls the operation of the computer.
 Working storage – where calculations are performed and data is stored
 Output area where the information is stored prior to output. Both the input
and output areas are buffer.
 Application program area – where the user program is held.
Secondary Storage is an optional attachment, which is cable –connected to the
CPU. Secondary is non-volatile. Any data or programs stored in secondary
storage stays there, even with the computer power turned off, unless someone
purposely erases them. Secondary storage is a permanent from of storage. They
are cheaper forms of storage that can store a lot of information until they are
needed. They can also be called offline storage media.
There are various data storage devices which are encompassed into three main
1. The magnetic media
2. The optical media
3. Semi conducting media
 THE MAGNETIC MEDIA: Examples include the floppy disk, hard
disk and magnetic tape. For years, the magnetic technology has
dominated the mass storage arena. The most common example in use
today is the magnetic disk, in which thin spinning disk with magnetic
coating is used to hold data. Read/write concentric heads are placed
above and/or below the disk so that the disks spins, each head traverses a
circle, called a track, around the disk’s upper or lower surface. By
repositioning the read/write head, different concentric tracks can be
 FLOPPY DISK: This has a square shape. The protective square case
houses a round piece of flexible plastic disk with a magnetic coating. It is
portable and has a very small size of 1.44MB. The reading and writing
speed is very slow and it is not durable. They are not common these days
because of the associated disadvantages. Though they are new types of
floppy disk with bigger capacity and better durability but they require
special drive to drive them and thus they are not popular. Examples
include Zip Disk (up to 750MB) and super disk (120MB)
 HARD DISK: Hard disk is a very huge type of memory and it is one of
the factors that determine how powerful a computer system is. It contains
a set of circular plates made of metal with a magnetic coating. All the
plates are sealed within a hard disk drive together with a read/write head.
It rotates at a very high speed. When data is to be read from a hard disk,
the computer gives the disk address and the hard disk will go to the
specific location to read the data.
 THE MAGNTIC TAPE: This is a magnetically coated strip of plastic on
which data are stored. Large amount of data can be stored on it. It is very
cheap. It requires a tape drive to read and write data on a tape. It is a slow
storage device mainly used for long term storage and backup
 OPTICAL MEDIA: These are storage media that hold content in digital
form and such content can be written and read using laser technology.
These storage media include CD and DVD variations.
 COMPACT DISK: CD is a round plastic disc with metal coating. It
implores laser technology for storing and accessing data. It can be further
classified into: CD-ROM Group, CDR (CD recordable), CD-RW (CD
rewriteable).these disks are 12 centimetres in diameter and consist of
reflective materials covered with a clear protective coating. Information is
recorded on them by creating variations in their reflective surfaces. This
information can be retrieved by means of a laser beam that monitors
irregularities on the reflective surface on the CD as it spins.
 B.DIGITAL VERSATILE DISK: this is an advanced storage media. It
has a diameter and thickness similar to the CD. It can also be further
classified into: DVD ROM, DVDR, DVD-RW, Magneto-Optical Disks.
 SEMI-CONDUCTING MATERIAL: these are often called flash
memories. They use semi-conductor clips to store data. They are silent in
operation and they are used in some portable devices such as digital
cameras, PDA and MP3. Examples include: Microdrive (MD), Secure
Digital card (SD card), Compact flash card (CF card) and Smart Media
card (SM card). A semiconductor chip may contain millions of tiny
transistors or capacitors.
Input devices are devices that allow users to enter data and command into the
computer system. Some of these devices are explained below:
behind MICR is that human readable characters reprinted on documents in an
ink that is magnetisable and are also designed in a special type of font. The ink
contains ferrous oxide, and so by magnetising it when read into MICR
reader/sorter, the characters become recognisable.
2. MOUSE: A mouse is a handheld device, which allows you to control your
computer without having to type instructions from the keyboard. It helps you to
move around your computer screen, select any object and activate commands.
The computer mouse shape resembles an actual mouse. It has rubber ball
embedded at its lower sides and buttons on top. As the user places palm over the
top of the mouse and moves it across the mouse-pad, the ball at the lower side
also moves. The movement of the ball determines the location of the arrow or
mouse-pointer on the computer screen. A mouse-pointer is a small arrow or
other symbol on the computer screen that moves as you move the mouse. There
are 3 different shapes of mouse. There are: three-button mouse, scroll mouse
and cordless mouse.
3. TRACKBALL: Trackball is an alternative to a mouse. It operates in the same
way as a mouse. It is less complicated than a mouse and is designed for the use
of children. Turning the ball with your hand moves the pointer on the screen.
Trackballs are useful when there is not enough space for a mouse, for instance
in portable computers. Most trackballs have buttons which can be used to select
options on the screen.
An output device is a hardware that presents digital information in a form that
human beings can understand. They include:
The most important output, if the data is needed for human interpretation, is on
sheets of printed continuous stationery. This is called hard copy of data. It is
produced by devices called printers. The most common type are the line
The results of the processed data or information first appear on the VDU before
it is sent to the printer or the computer system is switched. The VDU produces
only a transistor copy. This information can be text or graphical characters.
Connectivity hardware is the hardware that allows computers to be physically
connected to different types of network. For example connectors, network
interface cards, modem, repeater etc.