Identify the constituent parts of a total computer system
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Hardware - input and output units
Backup storage; central processing unit;
Software - applications package;
The main function of a computer system is to process data. When data
has been processed and output it becomes information. The main stages
of this procedure are:
1. The Input Stage - where data is put into the computer.
2. The Processing Stage - where the data is processed.
3. The Output Stage - where the results are displayed.
A computer also needs to store data for later use. This is done
on a backing storage device.
Hardware - input and output units
A barcodeBarcodes are different groups of vertical bars that can be
read by an optical scanner. Barcodes are printed on nearly every
product that you can buy. Shops use barcodes because they are cheap to
produce and very durable.
A concept keyboard is a flat board that contains a grid of buttons.
Each button can be programmed to follow instructions. An overlay sheet
is placed on the grid with an explanation for each button. They are
used in primary schools with young children. Many modern fast food
restaurants have overlays with either a description or picture of the
meals that are available to make ordering easier.
A digital camera allows you to take pictures and to store a digital
photographic image that can be read by a computer. You can then
transfer the images directly from your camera on to your computer.
Digital cameras are often used by estate agents for producing
advertising and sales material.
A graphics tabletA graphics tablet consists of a flat pad (the tablet)
on which the user draws with a special pen. As the user draws on the
pad the image is created on the screen. Using a graphics tablet a
designer can produce very accurate on-screen drawings.
MICR (Magnetic Ink Character Reader)
Magnetic ink characters are the strange looking numbers that appear at
the bottom of cheques. These characters are used because they are very
difficult to forge or damage. Banks use MICR to read the numbers from
the bottom of cheques to obtain data such as account numbers and bank
A magnetic strip on a credit cardMagnetic strips are built into many
plastic cards such as cheque guarantee cards, cash-point cards and
personal identity cards. The magnetic strip on the back of the card
can hold the personal details of the card owner and, with the
necessary PIN, will allow access to secure information e.g. bank
MIDI (Musical Instrument Digital Interface)
A MIDI system allows you to attach a musical instrument to your
computer, for example a musical keyboard. You can then store a musical
sequence on a computer and then have the computer play it back to you.
OMR (Optical Mark Reader)
An optical mark reader reads marks made by pencil on a printed form
into the computer. OMR systems are suited to reading pre-printed forms
and check-boxes such as National Lottery number selection sheets and
multiple-choice exam papers.
Scanners are now a cheap and common way of getting images into a
computer. They can also be used with OCR (Optical Character
Recognition) software to scan in text.
A touch screen is a special type of VDU, which has a grid of light
beams or fine wires criss-crossing the screen. When the screen is
touched (usually to choose an on-screen option), the computer senses
where you have pressed. Due to the vast amount of information they can
store, touch screen operated computers are generally used in the
provision of public information and are found in places such as
A video digitiser takes an image from a video camera or television and
converts it so that it can be used by, and stored on, a computer.
Video sequences captured using a video digitiser are often used in
Voice recognition (microphone)
A microphone is used with a voice recognition system. This can be used
with a word processing program to enter text. It can also be used as
part of security systems - only certain digitally stored voices have
Other input devices
* Light pen
* QWERTY keyboard
* Tracker ball
Monitors or Visual Display Units (VDUs)
Monitors or VDUs are the most common output device and include desktop
monitors and Liquid Crystal Displays (LCDs).
* An ink-jet printerLaser printers produce a very high quality
output, are very quiet and very fast although quite expensive to
* Ink-jet printers are cheaper to buy, offering black and white or
colour printing with reduced levels of quality and speed.
* Dot-matrix printers are not so common today. They are
comparatively noisy and low quality but are cheap to run and ideal
for draft copies.
A plotter can be used to produce high quality, accurate, A3 size or
bigger drawings. They are usually used for Computer Aided Design (CAD)
and Computer Aided Manufacture (CAM) applications such as printing out
plans for houses or car parts.
* LCD projectors
Backing store - magnetic disks
Magnetic disks are the most common backing storage medium. Magnetic
disks include 3.5-inch portable floppy disks and also the hard disk
drive fixed inside your computer.
A floppy diskYou can read data from and write data on to a floppy
disk. Floppy disks can be moved between computers and usually store up
to 1.44 Mb. They have three main uses:
* to transfer small files of data from one machine to another
* to back up important small files that are stored on your hard disk
* to store restricted files that you don't want all other users of
your computer seeing
Hard disks or hard drives
Hard disks have storage capacities thousands of times larger than
floppy disks. Data can be transferred to and from a hard disk much
more quickly than from a floppy disk.
Your hard disk will be where you store:
* the operating system
* the majority of your data files
Backing store - optical disks
A CD-ROMCD-ROM (Compact Disk Read Only Memory) disks are optical disks
that use the same technology as musical compact disks. They store up
to 650 Mb of data and a laser beam is used to read the data off the
Data is written onto the CD-ROM disk before it is sold and cannot be
changed by the user. As CD-ROMs are much bigger than floppy disks,
they can be used for multimedia applications such as encyclopaedias
and can store pictures, sounds and video clips.
It is now possible to record on to CDs with a CD writer.
Backing store - magnetic tape
Magnetic tapes or cartridges are principally used for backup. As they
are small, robust and portable, with the ability to store far more
than a floppy disk, they are often used to back up important documents
and files from the hard-drive. Magnetic media is also used for long
term storage and the archiving of data.
CENTRAL PROCESSING UNIT
CPU stands for Central Processing Unit. It is the 'brain' of the
computer. In fact, it is a microprocessor (chip) fixed to a circuit
board inside the computer.
There are three main parts of the CPU -
1. The A.L.U. (Arithmetic and Logic Unit), which does all the
arithmetic and logical operations.
2. The Main Store Memory. This is where data and programs are stored.
3. The Control Unit. This is the part, which controls the sequence of
all computer operations.
Arithmetic Logic unit is the part of the computer where all
calculating and testing happens. Data is sent to and received from the
the ALU by the control unit.
Computer memory can be divided into two main types, main store and
backing store. Main store consists of a large number of electronic
circuits and all data and information that is processed by the
computer has to go through main store.
When a sequence of instructions (program) is being executed by the
computer system the control unit ensures that the data that is used
within the computer is collected and sent from the peripherals and the
special registers (special areas of RAM). As long as the data is
controlled as it travels around the computer system the computer works
ok. If the data is not controlled or sequenced correctly then the
computer will crash.
Software - Applications Package
A computer system is made up of hardware and software. The hardware is
the physical components like input, output and backing store devices.
Software is the computer program that instructs the computer to
perform particular operations.
Software consists of very simple instructions that tell the computer
what to do with the data that it is processing. These instructions are
carried out so quickly the computer seems to be doing complicated
The computer user will have bought the computer system to perform one
or more tasks. The software that makes the computer perform these
user-related tasks is called applications software.
Some applications software may be written to perform a particular
specialised task, such as stock control or payroll for particular
company. Other applications such as a word processor program or
spreadsheet may be general purpose or generic. Generic software can be
used in a variety of ways for different purposes. A word processor
program could be used to produce posters, prepare legal documents or
write a school assignment.
Text Box: Applications Software Non-generic (task specific) Generic (general purpose) Payroll Game Stock-control Customer Billing Word processor Desk-top-publisher Database Spreadsheet
Nearly all applications programs can be customised. This allows the
user to set up the application in a way that suits them. Examples of
customisation include setting up special dictionaries for a word
processor, choosing the format for currency or dates and setting
default fonts and background colours.
Software needs to be customisable because it will usually be used by a
large number of users each having different needs. If the software can
be customised then the user can configure it to in a way that allows
them to work efficiently. For this reason generic packages such as
word processors, spreadsheets and databases usually include a large
number of configuration options. A non-generic application, written
for one specific purpose, will normally have fewer configuration
options because there will be a more limited user base and the tasks
that the users will carry out are more clearly defined.
Sometimes a customer's software needs cannot be met by an existing
package. In this case software may be specially written for the
customer. This is called bespoke software and the customer should end
up with an application that exactly matches his or her needs. However
software development costs are high so this is an expensive option. In
addition it can take a months or even years for software to be
developed. Once the software has been produced it is likely to have a
number of bugs that will need to be sorted out.
OPERATING SYSTEMS ON A MICROCOMPUTER
An operating system is a program that controls and organises the
general operation of the computer. A computer cannot work without an
operating system. Remember, without an operating system you cannot use
On a microcomputer an operating system will:
* deal with the transfer of programs in and out of memory
* allow the user to save files to a backing store
* control the transfer of data to peripherals such as printers
* provide the interface between user and computer e.g. Windows 98,
Acorn RISC OS
* issue simple error messages
THE OPERATING SYSTEM ON LARGER COMPUTERS IS VERY DIFFERENT, THOUGH
MANY OF THE JOBS ARE SIMILAR IN PRINCIPLE.
Larger computers such as mainframes operate differently from the
simple PC. They can process two or more programs at the same time and
may have to deal with many users at the same time. This sort of
operation requires very complex operating systems.
Some general features of operating systems are:
* they control the backing store and peripherals such as disk units
* they control the loading and running of programs
* they organise the use of memory between programs
* they organise processing time between programs and users
* they organise priorities between program and users
* they maintain security and access rights of users
· they deal with errors and user instructions
MODES OF OPERATION
Computers can operate in many different ways requiring different and
complex operating systems.
Real time processing
A real time system responds to incoming data immediately. Computers
that run in real time have to be very fast and have a lot of
Computers operating in real time are often dedicated to the control of
systems such as industrial processes, planes and space flights. The
computer has to react immediately to input data by issuing output
signals to the systems being controlled.
Other examples of real time processing are holiday and travel booking
Multiprogramming is a method of operating such that several programs
appear to be running at once. The term usually applies to mainframe
operation when the computer is switching between a number of different
programs or dealing with a number of different users.
The jobs are usually given priorities. The operating system switches
jobs in and out of processor time according to priority. For example,
while one job is being allocated printer time, another will be being
processed in memory. The processor is so fast that it seems that many
jobs are being processed at the same time.
Multitasking usually applies to microcomputers whereby the computer is
running a number of applications apparently at the same time.
Multi-access or multi-user
A multi-access (or multi-user) system is one where a number of users
work with terminals on-line to a central computer. The central
processor deals with users in turn; clearly the more users, the slower
the response time. Generally, however, the processor is so fast that
the response time at the most is a fraction of a second and the user
feels they are being dealt with immediately.
A batch processing system is where programs or data are collected
together in a batch and processed in one go. Typically the processing
of payrolls, electricity bills, invoices and daily transactions are
dealt with this way.
This method of operation lends itself to jobs with similar inputs,
processing and outputs where no human intervention is needed. Jobs are
stored in a queue until the computer is ready to deal with them. Often
batch processed jobs are done overnight.
Text Box: Operating System Tasks Memory Management Allocate memory to programs and make sure that one program does not interfere with memory allocated to another Communication Handle communication between the different parts of the system so that they work together Data Transfer Moving data between the different parts of the system “ e.g. transfer of date from hard drive to memory. Handling input and output operations. Processor management Deciding which program should be given processor time and for how long. Security Detecting, handling and reporting error situations to minimise loss of data. Handling log-in for password protected systems