A level-computing-9691-paper-1-notes

A Level. Computing (9691/1)
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CPT1: Computing Fundamentals

1.1 COMPONENTS OF A COMPUTER SYSTEM

Candidates should be able to:
1. Define the terms hardware, software, input device, storage
device and output device.
2. Describe the purpose of input devices, storage devices and
output devices.

Hardware
Hardware are the physical components of a computer – eg the input devices, output devices,
primary storage (memory) and secondary storage (backing store), Central Processing Unit etc.
Note that input and output devices are collectively known as peripherals.

Software
Software are the sets of instructions/programs that are loaded into the memory of the computer in
order to perform a task or to control the operation of the computer.

Peripheral
A peripheral is a device that is external to the computer’s ‘box’ and connected to it via one of the
Input/Output (I/O) ports.

Common peripherals include:
• mouse;
• keyboard;
• VDU;
• printer;
• scanner.

Input device
An input device is hardware that allows data to be entered into a computer.

Common input devices include:
• keyboard;
• mouse;
• scanner;
• digital camera

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Storage device
A storage device is hardware that allows data to be saved, long-term, after it has been inputted into
the computer.

Common storage devices include:
• Hard disk drive;
• CD-ROM (or CD-R, CD-RW)
• USB Flash ‘pen’

Output device
An output device is hardware that allows a computer to present data to a user.

Common output devices include:
• Visual display unit (VDU/monitor);
• Printer;
• LCD projector.

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Categories of Software

3. Describe the different roles and functions of systems software
and applications packages.

Systems software
The set of programs needed to control and monitor the operation of the hardware (and applications
software) of a computer.
Systems software consists of the following:
• Operating System software
• Utility programs
• Programming tools
• Library programs

Operating System software
The Operating System is the software that controls the operation of the hardware and hides its
complexities from the user.

The operating system is loaded into main memory during start-up.
Examples of Operating Systems are DOS, Windows, Unix, Linux, Mac OS. Note that the operating
system is not just one large program – it consists of a set of many programs all of which are
necessary to get the computer to work.

Utility programs
Utility programs are non-essentials small programs that are designed to perform common tasks that
thousands of computer users benefit from at one time or another.
Some utility programs help maintain the functioning of the system and others make life easier for
the computer users.
Utility programs include:
• file backup;
• file compression;
• disk formatters;
• disk defragmentation;
• file recovery;
• virus detection and cleaning etc..
Performance monitoring programs can also be classified as utility software – this is software that is
used to monitor disk, memory and processor use.

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Programming tools
Programming tools include language translators such as compilers and assemblers that are
necessary to ‘build’ programs. They also include interpreters which are necessary to run programs
written in languages such as Visual Basic.
Note that the MS DOS versions of DEBUG and QBASIC were clearly systems software although
some people argue that Visual Basic is closer to applications software.

Library programs
Library programs are files that contain program code, which is available to all applications to share.
They allow different applications to communicate and share resources. MS Windows™ uses many
library files known as dynamic link libraries (*.dlls – pronounced dee-el-el’s).

Applications packages
Application packages (software) are the programs that are consciously used by the user to solve
problems or perform work related tasks – writing a letter, keeping accounts, printing invoices, etc.

Small-scale applications
Even though they are complex in their programming and can contain millions of lines of code,
some applications are designed to be installed on a single computer for one user to use. These
applications include:
• word processing;
• spreadsheets;
• desktop publishing (DTP);
• presentation software;
• drawing packages.

Large-scale applications
Some of the large scale applications can be used by hundreds of people at the same time and store
millions upon millions of records. Such applications are often central to large organisations such as
banks, supermarkets and other types of large business organisations. These applications include:
• stock control;
• payroll;
• order processing and tracking;
• utility billing.

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1.2 THE SYSTEMS DEVELOPMENT LIFE CYCLE

4. Describe the stages of the systems life cycle.

The stages in the systems life-cycle
Most IT projects use the System’s Life-cycle approach to developing a new system. This approach
consists of several distinct stages, which follow one after the other.
During the development life-cycle, a team is not permitted to go back to a previous stage – this
could cause the project to over-run in terms of both cost and time.
The stages in the System’s Life-Cycle are as follows:
• Problem identification
• Feasibility Study (Initial investigation)
• Analysis (detailed investigation)
• Design
• Coding (software development)
• Testing
• Conversion
• Review (Evaluation)
• Maintenance

Note that each stage of the System’s Life-cycle has a distinct end-point, which can be shown to the
customer and ‘signed off’. This helps to ensure that the final product is what the customer actually
wanted!

Problem identification
The problem identification is a statement of the existing problems and description of user
requirements as outlined by the customer.

Feasibility Study
A feasibility study is an initial investigation of a problem in order to ascertain whether the proposed
system is viable, before spending too much time or money on its development.

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Analysis
The analysis is a detailed, fact-finding, investigation of the existing system in order to ascertain its
strengths and weaknesses and to produce the list of requirements for the new system.

Design
Design is the production of diagrams, tables and algorithms, which show how the new system is to
look and work.
The design will show:
• how the interfaces and reports should look;
• the structure of and relationships between the data;
• the processing to be used to manipulate/transform the data;
• the methods to be used for ensuring the security and validity of the data.

Coding
Coding is the creation and editing of the interfaces, code and reports so they look and work as
indicated in the design stage.

Note that user and technical documentation will also be produced during the coding stage.

Testing
Testing is the process to ensure that the system meets the requirements that were stated in the
analysis and also to discover (and eliminate) any errors that might be present.

Conversion
Conversion is the process of installing the new system into the customer’s organisation and training
the employees to use it.

Review
Post-implementation review (also known as evaluation) is a critical examination of a system after it
has been in operation for a period of time.

Maintenance
Maintenance is the process of making improvements to a system that is in use.

The reasons for maintenance could be to fix bugs, to add new features or to make the system run
quicker.

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Defining the problem

5. Explain the importance of defining a problem accurately.

An accurate problem definition is needed so that the developers know exactly what is expected
from the system. This means that the system that is delivered is going to be what the customer
expected. Without an accurately defined problem, it is likely that the software that is developed will
not fully satisfy the needs of the end users.

Note that there has to be a two-way dialogue between the analyst and the users because:
• The users do not know a lot about computers and their capabilities;
• Programmers will not know very much about the way the business works, for which they
are developing the software

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Feasibility Study

6. Describe the function and purpose of a feasibility study.

After the problem/task has been defined and before a complete detailed study of exactly what is
needed within a new system, a feasibility study is undertaken to verify that the system that is
required is, in fact, viable and that it is worth proceeding.

There are five factors that are considered in a feasibility study:
• Technical feasibility – this investigates whether the hardware and
software exists to create the system that is wanted.
• Economic feasibility – this investigates the cost of developing a
new system (including the purchase of new hardware) and then determines whether the
benefits of a new system would outweigh the costs.
• Legal feasibility – investigates if there is a conflict between what is wanted and the law.
For example, would the new system satisfy the requirements of the Data Protection Act?
• Operational feasibility – investigates whether the current working practises within the
organisation are adequate to support the new computer system. It is possible that the new
system would require employees to perform duties in a different way –this may not be
acceptable!
• Schedule feasibility – this investigates the amount of time that the new system is likely to
take to develop and determines whether it can be developed within the timescale that is
available.

Estimating cost effectiveness

Costs Benefits

New Hardware Reduced Staffing

The New software Better service to customers

Training (time and money) Improved management information

Faster processing that speeds up payments from
Conversion (Time)
customers

Maintenance (Money)

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Fact finding during analysis

7. Explain the importance of determining the information
requirements of a system and describe different methods of
fact finding, including questionnaires, observation, and
structured interviews, highlighting the advantages and
disadvantages of each method.

When attempting to create a new IT system, it is vital that sufficient information is gained about the
way the present system operates. The usual methods of obtaining this information include:
• Interviewing staff
• Observation of current procedures
• Examination of paperwork
• Surveying (with questionnaires)

Interviewing staff
All levels of staff from end-user to senior management need to be involved during the analysis
stage. A systems analyst should try to interview as many of them as possible so that all their needs
can be ascertained. Interviews are time-consuming, but very effective.
It should be remembered that many of the staff may not be sure exactly what they require and so the
system analyst may have to ‘tease’ out some of their requirements with carefully thought out
questions.

Benefits Drawbacks
User can express their opinions in a detailed Time consuming for the analyst
way
Extension questions can be asked as a result Users may feel intimidated and not tell the truth
of the user’s answers to the original questions about what they feel is lacking in the existing
system
Users may feel valued and involved with the
new system

Observation of current procedures
Time and motion studies could be undertaken to see how data and documents move around the
existing system and to detect where bottlenecks occur and determine where procedures could be
made more efficient. This could be done by using a test document that has its movement tracked
through all its stages as it moves around the system.

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Observing the staff at work can often prove more advantageous than just interviewing them,
because it may uncover processes that they do sub-consciously and do not remember to tell the
analyst about during interview.

Benefits Drawbacks
Allows the analyst to see the current system at Users may work differently if they know that
first hand and what processes that are often they are being watched.
done sub-consciously
Analyst can get a feel for user-competence
Analyst can measure the time taken to do
tasks

Examination of paperwork
This will help to show the inputs and outputs of the current system and so help determine the inputs
and outputs required by the new one. The paperwork will include documents that are received from
an organisation’s customers – such as membership application forms or orders. They will also
include documents that are produced by the current system – such as the invoices sent to customers
and the current reports that are produced for the management team.

Benefits Drawbacks
Analyst can get an idea of the volume of data Can be time consuming if there is a large
being stored and processed volume of files to go through
Analyst can see what output is required
Analyst can see how data is currently validated

Surveying (with questionnaires)
If there are many users of the system then surveying staff by asking them to complete a
questionnaire would be a more efficient method of gathering information than conducting personal
interviews. The analyst could then choose to follow up some of the responses with an interview.

Benefits Drawbacks
Efficient in terms of time Responses to questions are less flexible than
in an interview
Answers can be anonymous There is often a low return rate
It can be difficult to design a good
questionnaire

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Requirements analysis

8. Describe what is involved when analysing the requirements of
a system, explaining the nature of the requirements
specification and its content, including current data structures,
inputs, outputs and processing represented in diagrammatic
form (data flow diagrams, system flowcharts), identify
inefficiencies/problems in the current system.

The deliverable at the end of analysis is documentation that shows an investigation into the current
system and a list of system requirements for the new one. These requirements need stated in a clear,
specific and measurable way.

In order to ascertain these requirements, the systems’ analyst needs to examine the current data
structures and relationships between them. They must also trace the flow of data through the
existing system – this will begin by determining the source of the various data, identifying the ways
in which the data is processed; and finish by identifying the destinations of the final outputs.

The analysis documentation will contain the following:
• Identification of existing and prospective users;
• Identification of current data and its structure;
• Identification of inputs, outputs and processes within the current system;
• Identification of data flows including the sources and destinations of the data;
• Identification of the strengths and the weakness of the current system;
• Listing of objectives.

Current data and its structures
This is the description of the current data within the system – its data type, validation techniques
used, relationships with other data within the system. The analysis data dictionary is usually
presented in tabular form, with each different category of data being described in its own table.

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Data flow diagrams
A data flow diagram is a drawing that depicts the transformation of data within an existing system
by using three different graphical symbols connected by labelled, directed lines.

A data flow diagram is an analysis tool that represents what a system does, not how it does it. They
are:
• graphical – eliminating thousands of words;
• logical representations – not physical models;
• hierarchical – showing systems at any level of detail; and
• jargonless – allowing user understanding and reviewing.

An example of a data flow diagram depicting the sale of turkeys on a farm:

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System flowcharts

A system flowchart shows an outline of how a system operates.

The following shows the system flowchart for a school’s registration system that stores student
attendance data on a magnetic disk in a database called Register.

KEY:

Input/output

On-line storage

Process

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Design

9. Describe a design specification including input design,
diagrammatic depiction of the overall system, processing, data
structure design and output design.

The deliverable at the end of the design stage is documentation that could be used, unaided, by a
third-party programmer to create the system as the designer intends.

This means that each section of the design must be detailed and clear. Explanations of what must be
done and why this method is chosen need to be included.

The design documentation will contain the following:
• user interface designs (input forms and menus);
• specification of data structures (including the relationships between different types of data);
• validation procedures;
• output/report designs;
• algorithms;
• security methods.

User interfaces
Humans will need an interface that allows them to:
• give instructions – Print, Save, Open, Delete, Copy, Paste etc.
• enter data – file names, number of pages to print;
• make choices – Yes, No, Cancel etc.

Computers need ways to:
• inform of errors – illegal operation, invalid data input, printer out of paper, wrong password:
• tell on progress – copying, deleting, installing, downloading:
• display the results of processing
• ask for options – e.g. number of pages to be printed, which file to open.
• provide help with performing tasks – this help could be in the form of Status bar text, yellow
Tip boxes, an Office Assistant or even a full-blown help file accessed via the Help menu.

The user interface designs will need clearly annotated drawings to tell the programmer exactly what
is to be done.

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Example of an interface for data entry:

Processing (algorithm) design
These algorithms outline the programming methods that need to be used to process the data.
Algorithms are usually written in pseudo-code, which are instructions that are half-way between
English and a programming language. The advantage of pseudo-code is that it can be used to
describe an algorithm in a way that is not specific to any particular programming language.

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Data structures and validation
Data can be of many types and can have many properties. Typically, a table is constructed for each
category of data. The properties of the data and the necessary validation techniques are stated as
shown:

Field Name Data Type Validation Comments
MemberID AutoNumber Uniqueness Automatically generated
(Integer) check Primary Key
FirstName Text (size 25) Presence check Enter members first name –
maximum 25 characters)
LastName Text (size 25) Presence check …
DateOfBirth Date (Short) Data-type + …
Presence check
Gender Text (size 1) Existence check Default – M; choice of M/F
(on ‘M’ or ‘F’) select from radio-button.
---
---

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Output design
This needs to be detailed in the same way as the input designs. Example of a mail-merge letter
output:

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Testing

12. Explain the importance of system testing and installation
planning.

Testing is undertaken to ensure that a system satisfies the user’s requirements and to discover any
errors that might be present.

Testing needs to be undertaken by both the programming team and by the end-users. Note that
testing a program can never adequately prove or demonstrate the correctness of the system – it can
only reveal the existence of errors.

Testing by the programming team
Testing by the programming team is ongoing as the system is developed, but it is still very
important to undertake tests at the end, after the programming team believe that the system is
finished. This formal testing at the end of development is known as alpha-testing.

Alpha-testing
This is the formal testing at the end of development. It:
• is undertaken by the programming team;
• uses data that the programmers perceive to be realistic;
• is designed to ensure that the requirements/objectives have been met.

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Testing by the end-users
Testing by the end-users is either beta-testing or acceptance testing.

Beta-testing
Beta testing is when the software is tested under real conditions, using real data, by a selection of
real end-users.

This phase of testing is necessary because software developers will probably not anticipate all the
combinations of conditions that will occur when the software is in use in its ‘real’ environment.
This is the stage of testing where problems with different hardware combinations are usually
discovered, as are problems with ‘clashing’ software.
During beta testing, users generally agree to report problems and bugs to the developers. These will
then be corrected and the software may then undergo a second round of beta testing before the
package is eventually released.

These end-users who test the system during beta-testing:
• use the system with realistic volumes of real data;
• use the system with a variety of different hardware and configurations;
• report faults/errors back to analyst;
• check that there is a reasonable response time;
• ensure that the user interface is clear;
• ensure that the output as expected.

Acceptance testing
Acceptance testing is where the customer specifies tests to check that the supplied system meets
his/her requirements as specified at the analysis stage and that the system works in their own
environment.

Note that the tests and data are specified by the customer, but the testing itself may be carried out
by the customer or by the developer under the scrutiny of the customer

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Conversion

Conversion is the process of changing from an existing system to a new system.

Note that conversion may take place within a day, or it might be that it is several months before all
parts of the new system have replaced the old.

Problems arising when converting/changing over
Converting from an existing system to a new one is not always smooth. The following problems
could arise:
• Data may have to be converted because the format in the new system may be different
to the format of the old system.
• Users will have to be trained so that they will be able to use the new system – the
organisation may even need to employ additional staff.
• Data may be lost during conversion – must make sure there is a full backup made
before changing to the new system;
• Hardware may need to be replaced/upgraded – if it does not satisfy the demands of
the new system;
• System software may need to be replaced/upgraded – i.e. new system may be created
to take advantage of the features within Windows XP and so will not run correctly on an
earlier operating system.
• Old data may need to be archived;

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Methods of conversion/changeover
The change from the use of an existing system to the use of a new system cannot always be
instantaneous. There are several different methods of converting to a new system, the four most
common are:
• Parallel;
• Direct;
• Pilot;
• Phased.

Parallel
Parallel conversion is when the old system and the new system operate along side each other for a
period of time, until all issues with using the new system have been resolved.

Parallel conversion allows an organisation to revert to the old system if the new system fails.

Direct
Direct changeover is when the old system is stopped being used one day and is replaced, in full, by
the new system the next day.

There is no going back when direct changeover is used.

Pilot
Pilot conversion is one department within an organisation changes to the new system before the
others.

This department will discover any problems with the use of the systems and these problems can be
ironed out before the rest of the organisation converts to the new system.

Phased
Phased conversion is when the old system is gradually replaced, in stages, by the new system.

This type of changeover is convenient when the system comprises of several different modules.
This will allow the organisation to convert to one of the new modules first, but maintain the use of
the other existing ones. This type of conversion means that training can be concentrated on one new
module at a time.
Note that if phased conversion is used, it is vital that the new system and the old system can share
data.

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Evaluation (review)

10. Explain the importance of evaluating the system, and how to
identify the criteria used for evaluation.

Evaluation (also known as post-implementation review) is a critical examination of a system after it
has been in operation for a period of time.

Purpose
The purpose of an evaluation is to assess the success of a system. Specifically, it will assess the
suitability, effectiveness, usability and maintainability of the system.
The evaluation will ask many questions including:
• can it carry out the all the requirements that were set?
• is it an improvement on the existing system?
• is it cost effective?
• is it easy to use?
• is the new system compatible with the existing systems?
• is the system easy to maintain?

The evaluation will also consider:
• what limitations there are in the system;
• what enhancements could be made to the system in the future.

Note that feedback from the end-users should also be included.

Timing
The evaluation will occur after a new system has been in operation for some time – usually a period
of between three and six months.
The waiting period allows users and technical staff to learn how to use the system, get used to new
ways of working and understand new procedures required. It allows management a chance to
evaluate the usefulness of the reports and on-line queries that they can make and go through several
month-end periods when various routine reports will have been produced. Shortcomings of the
system, if they exist, will be becoming apparent at all levels of the organisation.

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System documentation

11. Explain the content and importance of different types of
documentation at different stages in the system life cycle,
including the technical and user manuals.

There are two types of documentation that are necessary:
• Technical documentation – aimed at a future system developer
• User documentation/Manual – aimed at the end user.

Technical documentation:
Note that technical/system documentation is very valuable for the maintenance process. This is
because it will show how each part of a system has been constructed and the reasons why certain
choices have been made. The technical documentation should include:
• Annotated program listing – if the system is coded.
• Data flow diagram
• System flow diagram
• Structure charts/pseudo code/algorithm designs
• Test plan
• Data dictionary – i.e. the field definitions (including data-type, field length, validation)
• Entity relationship diagrams

User Documentation
As well as a contents page and index, the user documentation should include:
• Overview of the system
• Instructions on how to install
• Instructions on how to backup the data
• Instructions on how to operate the program
• Details on possible errors and how to deal with them
• Glossary of terms used within the documentation

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Maintenance

13. Explain the purpose of maintaining the system, and explain the
need for system review and reassessment, understanding that
software has a limited life span.

Maintenance is the process of making improvements to (or modifying) a system that is in use.

The need for maintenance
Maintenance is needed because:
• Bugs are discovered in the software code – these bugs will have been identified only after
the system is in full use. They will be fixed and a ‘patch’ will be issued that changes the
appropriate lines of code within the end-users’ programs.
• The user requirements may change – this often happens after a system has been in
operation for some time and the users see further uses of the data that the system produces.
In some cases, the additional requirements may have been identified during the original
development, but they were not implemented because the system’s life-cycle approach to
projects does not allow a change in requirements once they have been agreed (such a change
would extend development time and cause a project to miss its deadline).
• Some in-built parameters change – e.g. VAT rate;
• Hardware is changed – the system will be updated to take advantage of new hardware
developments. This could be a new input device, output device or even communications
device.
• The performance needs tuning – often some of the original code, although working
without error, uses some quite cumbersome routines that are slow to execute. System
performance can often be improved by finding more efficient algorithms for such routines.
• Operating system is upgraded – the system will be modified to take advantage of the
additional capabilities of the operating system.

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Types of maintenance
There are three types of maintenance:
• corrective maintenance;
• adaptive maintenance;
• perfective maintenance.

Corrective maintenance
Corrective maintenance is the removal of some of the known bugs in a program.

Software that has been released to the public will still contain bugs. Some of these bugs will be
previously unknown to the developers while others will be known to exist, but solutions will not yet
have been discovered. The software will be released with these ‘known bugs’ because of the
necessity to meet pre-set target dates and the need to gain some income from the software to
continue to pay the developers.
Eventually some of the bugs will be fixed and the solutions will need to be incorporated into the
public’s version of the programs. This type of maintenance is often done by releasing a ‘patch’
which is a very small program that actually changes lines of code within the main program. These
patches are available from Internet sites or from the CD-ROMs that are provided with computer
magazines.

Adaptive maintenance
Adaptive maintenance is the addition of new features to a program because of a change in users’
requirements.

The new version of the program may contain an added (or modified) feature or it may contain a
change in the interface. Adaptive maintenance could be needed because of:
• a change in the organisations/users requirements;
• a change in the law;
• a change in processes such as the method of tax calculation;
• to take account of new technologies.

Perfective maintenance
Perfective maintenance is when internal routines are changed to make them more efficient, so that
the application operates faster.

In the initial release of the software some of the processes, although error-free, may have used long
and slow routines. Perfective maintenance will make improvements in the way that the software
performs by ‘tidying up’ some of the internal routines. Changes to the interface may also be made.

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Prototyping

14. Describe prototyping to demonstrate how a solution will
appear.
15. Describe the spiral and waterfall models of the systems life
cycle.

A prototype is a simplified working model of a proposed system used as a rough indicator of how
the new system will work.
The prototype will consist of a set of screens and processes that show the user (and developer) what
might be possible. It will help a customer to gain a clearer idea of a proposed system so that they
can give feedback before the development has gone too far.
In the prototyping approach, the analysis establishes an outline specification. A model is then built
in order to evaluate it or have it approved before building the production model.

Prototyping can involve the repeated re-development of a system with new features being added as
the initially vague requirements are refined.

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1.4 SYSTEMS SOFTWARE

Operating system

20. Describe the purpose of operating systems.

The operating system is an integrated set of programs that is used to control and manage the
resources and overall operations of a computer.

Its role is to provide a ‘virtual machine’ by hiding the complexities of the hardware from the user. It
does this by providing a ‘buffer’ between the user and the hardware allowing the user to deal with a
simplified system, but without loosing any of its computational powers.
In addition to providing the HCI, the Operating System manages the hardware resources in order to
provide for an orderly and controlled allocation of the memory, storage media, processor time, and
I/O devices among the various processes competing for them.

The function of an operating system
The operating system (OS) for a standalone computer will be much simpler than that of a
supercomputer which is controlling hundreds of networked terminals and executing many different
kinds of job simultaneously. Nevertheless, all operating systems perform the same basic functions.
These include:
• memory management;
• file management (sometimes known as backing-store management);
• allocation of processor time;
• input and output management.

An operating system also manages:
• interrupts;
• errors;
• the human/computer interface.

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Memory management
The operating system has to allocate memory to each running application and to itself. Each
application is loaded into its own memory space – this means that if one program freezes, the others
can, in theory, keep working normally. It also means that you can terminate a ‘frozen’ program by
pressing [Ctrl]+[Alt]+[Delete]. Doing this will not effect any other programs.
To manage memory effectively, the operating system must:
• assign programs their own area of memory;
• prevent two programs from using the same area of memory;
• reallocate memory when a program is quit.

File management
The OS controls the transfer of data from disk to memory and back again. It also has to maintain a
directory of the disk so that files and free space can be quickly located. The directory is called the
File Allocation Table (FAT for short). To manage files effectively, when files are written to a disk
the OS ensures that:
• existing files are not over-written;
• when files are deleted from the disk the storage blocks are made available for new files.

Allocation of processor time
When several processes are executing on a computer at the same time (eg downloading a file,
printing and listening to an .mp3), then they will all need to receive time from the processor. Some
processes, such as printing, require very little processor time, while other processes require the
processor continuously. In such situations the Operating System would place a high priority on
printing requests to get them cleared and then allow it to concentrate on the more demanding
processes.

Input output device management
When two programs want to print to the same printer, the operating system has to ensure that the
two ‘jobs’ do not interfere with each other. It would normally do this by putting the jobs into a
queue and then pass them to the printer when the printer is ready.

ADDITIONAL NOTES:

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Interrupt handling
The OS detects and responds to many different kinds of interrupt such as:
• a user pressing a key on the keyboard;
• a printer sending a message that it is out of paper;
• a hardware or software malfunction.

In the case of a keyboard interrupt the operating system may simply display the appropriate
character on the VDU, but in the case of the printer sending an ‘out of paper’ interrupt, the
operating system will display a message to inform the end-user.

Error handling
Application software should normally include routines to deal with their own errors. When this is
not done, it is necessary for the operating system to deal with them or else the whole computer
could crash.
The operating system should be able to ‘freeze’ the program that causes an error and display a
message to the end user. A message such as ‘General Protection Fault’ might be displayed when a
badly-written application tries to use an area of memory that has been assigned to the operating
system itself.

The human/computer interface
The HCI allows a user to communicate with the computer. In early operating systems, users gave
instructions to the computer by typing command words. Most modern operating systems provide a
Graphical User Interface (GUI), which allows a user to choose commands by moving a pointer and
‘clicking’ on menus.

ADDITIONAL NOTES:

57



Types of operating system

21. Describe the characteristics of different types of operating
systems and their uses: batch, real-time, single-user, multi-
user, multi-tasking and distributed systems.
22. Describe a range of applications requiring batch processing,
and applications in which a rapid response is required.

Batch operating system
A batch operating system is one which allows input of data as batches and processes the data only
when all input has been collected so that the processing is carried out from beginning to end
without user interaction.

In a batch system the data is collected and input into the computer over a period of time. It is stored
as a ‘job’ to be processed later. Several batch jobs are usually executed at the same time so that the
processor and other resources are kept as busy as possible by switching between the different jobs.

Batch processing is typically used for:
• processing OMR forms such as those that contain answers to a multiple-choice exam;
• payroll;
• utility billing

In the case of processing multiple-choice OMR forms, the batch operating system can process the
forms incredibly fast, but it needs to reject forms that it cannot read – marks may be too faint, two
marks may exist on the same line etc. It does this by rejecting unreadable forms into a ‘hopper’ and
the data from these forms then need to be handled manually.

In the case of payroll

ADDITIONAL NOTES:

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Interactive operating system
An interactive operating system is one that allows the user and the computer to be in direct two-way
communication.

The user enters commands and data as the application is executed and the results of processing are
displayed immediately.

Categories of interactive system are single-user, multi-user, multi-tasking and real-time.

Single-user
A single-user operating system is one that can only be used by one person at a time.

Multi-user
A multi-user operating system is one that allows two or more users to communicate with the
computer at any one time, with each user interacting with the computer via separate terminals.

Multi-tasking
Multitasking is the apparent concurrent execution of two or more programs, on the same computer,
in such a way that communication and data sharing is possible.

Real-time
A real-time operating system is one in which requests are executed immediately and can therefore
produce a response within a specified, short, interval of time.

Some definitions sate that:
‘A real-time operating system (RTOS) is an operating system that guarantees a certain
capability within a specified time constraint’.

Almost any general purpose operating system such as Microsoft Window or MacOS can be
considered real-time to some extent – even if an operating system doesn’t fully qualify as real-time,
it may have characteristics that enable it to be considered as a solution to a particular real-time
problem.

ADDITIONAL NOTES:

59



Real-time operating systems are characterised by their ability to:
• deal with events which happen at unpredictable moments in time;
• deal with multiple events that occur simultaneously;
• support application programs which are non-sequential in nature – i.e. programs which do
not have a START:PROCESS:END structure;
• carry out processing and produce a response within a specified interval of time.

Note that the results of processing may be returned in milliseconds – as in the guidance systems of a
cruise missile – or if the processing is complex, it might take a couple of seconds – as in a
temperature control system in a large greenhouse.

Examples of real-time operating systems:
• Airline flight reservation;
• Missile guidance;
• Temperature/pressure control;
• Process control.

Flight reservation
The booking needs to be processed quickly and confirmation given to the customer straight away.
When a seat has been booked on a flight, the system needs to be updated before the next transaction
occurs in order to avoid the possibility of double-booking.

Missile guidance
Various sensors that detect the altitude, latitude, speed etc. constantly provide up-to-date
information to the guidance system. If the missile is slightly off course then adjustments must be
made immediately – a one second delay could result in the missile hitting the wrong target.

Temperature/pressure control
Sensors in a nuclear power station will be providing data on the current temperature of the reactor.
If the reactor starts to overheat, then initiating cooling will have to happen instantly to avoid a
potentially dangerous situation or meltdown.
Sensors in a greenhouse will be providing data on the current temperature and humidity. If either
goes outside the preset range then an action such as opening/closing the greenhouse windows will
need to occur straight away.

Process control
In manufacturing processes 1000s of signals per second can arrive from sensors attached to the
system being controlled. Because such systems are extremely fast moving, the response time may
have to be less than one thousandth of a second.

ADDITIONAL NOTES:

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Distributed system
A distributed system is one in which file storage (or processing) is shared between different
computers at different locations.

Users of a distributed system will be unaware of the exact physical location of the data that they are
accessing – it is likely that one set of records is retrieved from one remote system and the next set
of records is retrieved from another!

An example is a banking system where the data for each customer is held on the server at their local
branch, but is accessible from any branch.

Network operating system
A network operating system (NOS) is one that is able to share its resources (such as hard disk or
printer) and use the resources of others.

Note that in sharing its resources the network operating system is able to control what other users
are permitted access to and whether that access should be read & write or read-only.

A NOS often consists of exactly the same files as a standard operating system, but has an extra
layer of software. This software (known as the redirector) intercepts commands for file storage and
printing and, in a manner completely transparent to the user, redirects the requests to the appropriate
server. In this way files, printers and application programs resident on the server can be used by the
client exactly as if they were resident on the user’s own system.

Using a network printer
When a computer is set up to share its printer (known as a ‘print server’), the NOS will:
• allow the computer directly attached to the printer to make its printer visible to the other
computers;
• allow the other computers to be aware of the existence of the shared printer;
• allow the print requests from the other computers to be redirected to the print server’s
printer;
• allow this printer server to control the printing requests of the other computers by putting all
the requests into a print queue;

Note that it is usually possible for the print server to use access rights to control which users are
allowed to use the printer and to assign users priority in the queue.

ADDITIONAL NOTES:

61



Human-computer interface (user interface)

23. Identify and describe the purpose of different types of user
interface: forms, menus, GUI, natural language and command
line, suggesting the characteristics of user interfaces that
make them appropriate for different types of user.

The user interface is the hardware and software that provides the means for communication
between the user and the computer.

Different interfaces have been developed for different needs. Common types of HCI are:
• Forms
• Menus;
• Graphical (GUI);
• Natural Language;
• Command Line.

ADDITIONAL NOTES:

62



Forms
This kind of interface presents the user with an on-screen form into which they enter or view data.

The form will often be arranged into different sections and will consist of text-boxes, checkboxes,
radio buttons, drop down lists and other input ‘controls’ to help the user enter data quickly and to
help with validation.

Travel agents and other booking systems would typically use a form interface.

ADDITIONAL NOTES:

63



Menu interface
A menu interface is one that provides a list of choices from which the user can choose by pointing
to/clicking on. Each choice that the user selects will display a screen with other choices and
ultimately the desired choice.

ATMs and mobile phones often use menu driven interfaces and so do the ticket machines on the
London underground and many tourist information systems.

The benefits of a menu interface is:
• less human error – the user can only choose from the options available;
• user is restricted from accessing other parts of the system.

The drawbacks include:
• there may be no shortcuts for accessing common choices.

Tourist information systems would typically use a menu interface with a touch-screen to act as
both an input and output device. This:
• avoids the need for additional peripherals such as a mouse;
• allows ease of use by indicating possible choices with icons (not just text);
• provides an enclosed system with protection against vandals and the weather (can be used
outside).

ADDITIONAL NOTES:

64



Graphical User Interface (GUI)
A graphical user interface (GUI) is one that provides a means of interaction using windows, icons,
menus and a pointer to control the programs and operating system.

This kind of interface is sometimes called a WIMP and consists of the following:

• Windows;
• Icons;
• Menus;
• Pointer.

ADDITIONAL NOTES:

65



Windows
A window is a bounded area of the screen within which a specific task is executing – e.g. word
processing, web browsing, file management etc.

Icons
An icon is a small image that represents a program, folder, a device or a file.

Menus
A menu is a listing of options from which a user may choose – menus in a GUI are usually ‘pop-up’
or ‘drop-down’.

Pointer
A pointer is an on-screen ‘arrow’, usually controlled by mouse, used for navigation and to select
appropriate options by clicking a button.

Note that it is possible to set the pointer to be an image other than an arrow, but doing this often
makes the system harder to use.

Benefits and drawbacks of a GUI

Benefits Drawbacks
Easy for a novice because a GUI is usually Powerful hardware is required – fast
intuitive – the screen is arranged as a processor, high quality graphics card and VDU,
metaphor of a desktop with icons used to RAM and HDD with large capacity.
represent familiar objects.
User does not have to remember the precise Can be frustrating for an experienced user to
format of the instructions – instructions are perform certain tasks because a greater
initiated by selecting icons or menu number of operations may be required.
commands.
There is likely to be comprehensive, easy to Not all instructions are available through the
navigate, on-line help available. GUI – the command line will still need to be
used for many technical tasks.
Modern GUIs allow very easy execution of
some commands by ‘drag and drop’.

ADDITIONAL NOTES:

66



Natural Language
A natural language interface is one which allows a user to communicate with the computer by
speaking or typing in their normal way.

Ask Jeeves was a natural language search engine – it will allow you to type a question in the normal
way and it will interpret the question and provide the answer if it can. Below is an example:

The image part with relationship ID rId47 was not found in the ﬁle.

Type the question in your
usual language – ‘what is the
capital of France?’

The image part with relationship ID rId48 was not found in the ﬁle.
Jeeves will tell you the
answer if he knows it!

ADDITIONAL NOTES:

67



Command Line interface
A command-line interface is one in which the operating system provides a ‘prompt’ and the user
types a command to start program execution or to perform a housekeeping task.

For example:

The computer might prompt as follows:
C:>

The command prompt is the > character and the C: is the pathname for the current directory.

The user typing the following command:
C:>Del *.doc

will cause all files in the current directory with the extension .doc to be deleted.

Another command with MS DOS is:
C:>copy MyFile.doc C:BackupsMyFile.bak

This command copies the file called MyFile.doc into the directory called Backups and renames the
file MyFile.bak.

Sometimes ‘switches’ can be used with commands:
C:>xcopy C:Backups A:Backups /s /e

This command copies the directory named Backups from disk C: onto disk A: the /s means that
subdirectories are copied too, and the /e means that empty directories are also copied.

A command line interface is not for novice users, but is often used by an IT technician who needs to
perform tasks that are difficult when using a GUI.

When using a command line, the user must only type valid commands and they must be typed in
the correct format – omitting a space or a ‘’ will usually cause the command to fail.

ADDITIONAL NOTES:

68



Benefits and drawbacks of a command line interface

Benefits Drawbacks
Only low specification hardware is required – Difficult for a novice because they have to
monotone VDU, basic processor, small RAM remember a large number of commands and
and HDD. their exact format.
Experts can perform complex tasks using a The syntax of the command is vital and so the
single (although lengthy) command. instruction will not execute if the command is
typed incorrectly
An instruction can be executed from any part Easy to make mistakes.
of the system (do not have to have a certain
window open).

ADDITIONAL NOTES:

69



1.5 DATA REPRESENTATION

Number representation

26. Express numbers in binary, binary-coded decimal (BCD), octal
and hexadecimal.

Computers can only process binary data – i.e. 1’s and 0’s. If numeric data is to be processed then it
cannot be processed in its usual base10 form, it must be converted into its base2 form – known as
binary.

There are two ways that can be used to represent numbers in binary:
• ‘pure’ binary;
• binary-coded decimal (BCD).

There are also two ‘half-way’ stages that are sometimes used by programmers because they are
easier to understand than a string of 1s and 0s:
• octal;
• hexadecimal.

‘Pure’ binary
Pure binary represents numbers using just two digits (‘0’ and ‘1’) and columns, which increase by a
factor of two.

This is in contrast to our normal number system (denary), which uses ten digits (0-9) and columns,
which increase by a factor of ten.

In denary the number one hundred and ninety seven is represented as:

100 10 1
1 9 7

In binary, it is represented as:

128 64 32 16 8 4 2 1
1 1 0 0 0 1 0 1

ADDITIONAL NOTES:

77



Counting in binary
In binary, the first 15 numbers are as follows:

128 64 32 16 8 4 2 1
0 0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0 1
2 0 0 0 0 0 0 1 0
3 0 0 0 0 0 0 1 1
4 0 0 0 0 0 1 0 0
5 0 0 0 0 0 1 0 1
6 0 0 0 0 0 1 1 0
7 0 0 0 0 0 1 1 1
8 0 0 0 0 1 0 0 0
9 0 0 0 0 1 0 0 1
10 0 0 0 0 1 0 1 0
11 0 0 0 0 1 0 1 1
12 0 0 0 0 1 1 0 0
13 0 0 0 0 1 1 0 1
14 0 0 0 0 1 1 1 0
15 0 0 0 0 1 1 1 1

Can you see the pattern?

Converting from binary to denary
A binary number, such as 1001 0101, can be converted into its denary equivalent as follows:

1. Write the binary number with the appropriate column headings:

128 64 32 16 8 4 2 1
1 0 0 1 0 1 0 1

2. Add the column headings under which there is a binary ‘1’:
= 128 + 16 + 4 + 1
= 149

ADDITIONAL NOTES:

78



Converting from denary to binary
A denary number, such as 107, can be converted into binary as follows:

1. Write down the binary column headings:

128 64 32 16 8 4 2 1

2. Then, starting from the left, ‘take out’ the values in the column headings, if possible:

128 cannot be taken out of 107 so that column contains a ‘0’:

128 64 32 16 8 4 2 1
0

64 can be taken out of 107 so that column contains a ‘1’; this leaves 107 – 64 = 43:

128 64 32 16 8 4 2 1
0 1

32 can be taken out of 43 so that column contains a ‘1’; this leaves 43 – 32 = 11:

128 64 32 16 8 4 2 1
0 1 1

3. keep repeating the above process until the whole number has been converted to pure binary:

128 64 32 16 8 4 2 1
0 1 1 0 1 0 1 1

ADDITIONAL NOTES:

79



Binary Coded Decimal (BCD)
Binary Coded Decimal is one of the early memory encodings. Rather than converting the entire
denary value into its pure binary form, it converts each digit, separately, into its 4-bit binary
equivalent. The table below shows the 4-bit BCD equivalents of the ten denary digits:

8 4 2 1
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1

Note that the higher codes are not used in BCD because they do not represent a denary digit. These
are:

1 0 1 0
1 0 1 1
1 1 0 0 Not used in BCD because their
denary equivalents are higher
1 1 0 1
than ‘9’.
1 1 1 0
1 1 1 1

ADDITIONAL NOTES:

80



Converting from denary to BCD
Each digit is converted to its 4-bit BCD equivalent. Thus, the number 319 would be represented in
12-bits as follows:

8 4 2 1 8 4 2 1 8 4 2 1
0 0 1 1 0 0 0 1 1 0 0 1
(3) (1) (9)

Converting from BCD to denary
Each group of 4-bits are converted into the equivalent denary digit. Thus, the 12-bit binary coded
decimal number 0110 1000 0011 is denary 683 as shown:

8 4 2 1 8 4 2 1 8 4 2 1
0 01 1 0 1 0 0 0 0 0 1 1
6 8 3

Octal
The octal number system uses eight digits (0 to 7) to represent numbers, and columns which
increase by a factor of eight.

… … … 4096 512 64 8 1

Converting from octal to denary
The octal number 652 would be converted to denary as shown:

… … … 4096 512 64 8 1
6 5 2

(6 × 64) + (5 × 8) + (2 × 1) = 426

ADDITIONAL NOTES:

81



Converting between octal and binary
The octal number 652 (426 in denary) is represented in binary as:

256 128 64 32 16 8 4 2 1
1 1 0 1 0 1 0 1 0

If we combine the bits in groups of three and label with the appropriate column headings…

4 2 1 4 2 1 4 2 1
1 1 0 1 0 1 0 1 0
6 5 2

… we can see that converting from octal to binary converts each digit into its 3-bit binary
equivalent (very similar as converting between denary and BCD).

Thus the octal equivalent of binary number 10011101 will be:

4 2 1 4 2 1 4 2 1
0 1 0 0 1 1 1 0 1
2 3 5

Note the extra ‘0’ added at the
front because the original
binary was only 8-bits.

ADDITIONAL NOTES:

82



Hexadecimal
The hexadecimal number system uses 16 digits to represent numbers. The denary digits 0 – 9 are
used together with the first six letters of the alphabet (A – F).

With hexadecimal, instead of column headings doubling, as they do in binary, or increasing by a
factor of 10 as they do in denary, each column heading in hexadecimal increases by a factor of 16.
The column headings are:

… … … 65,536 4096 256 16 1

Examples of hexadecimal numbers include: 3FC2, CFF8, 92B0, EE4D, ACDC.

Note that the number 9375 could either be ordinary denary or hexadecimal – to make it clear the
symbols ‘h’, ‘#’ or ‘&’ are often used. Thus, if the number was in hexadecimal, it would be written
as 9375h, #9375 or &9375.

Converting from hexadecimal to denary
The hexadecimal number 2C5A can be converted into its denary equivalent as follows:
1. Write the hexadecimal number with the appropriate column headings:

… … 4096 256 16 1
2 C 5 A

2. Noting that A ≡ 10 and C ≡ 12, convert in the same way as conversion from binary to denary:
= (4096 x 2) + (256 x 12) + (16 x 5) + (1 x 10)
= 11 354

ADDITIONAL NOTES:

83



Converting from hexadecimal to binary
This uses the same method as octal to binary, except each hexadecimal digit is represented by 4-
bits. Thus, the hexadecimal number B7C can be converted into a 12-bit binary as follows:

8 4 2 1 8 4 2 1 8 4 2 1
1 0 1 1 0 1 1 1 1 1 0 0
(B) (7) (C)

Converting from denary to hexadecimal
Convert to binary first and then to hexadecimal. For example, 462 can be converted as follows:

Convert to binary:

2048 1024 512 256 128 64 32 16 8 4 2 1
0 0 0 1 1 1 0 0 1 1 1 0

Convert each group of four into their hexadecimal equivalent:

8 4 2 1 8 4 2 1 8 4 2 1
0 0 0 1 1 1 0 0 1 1 1 0
1 12 (C) 14 (E)

= 1CE

Uses of hexadecimal
Hexadecimal is often used by Assembly language programmers to reference memory. It is also used
within HTML property values – specifically background and font colours.
There are three advantages of using hexadecimal:
• hexadecimal is quicker for a programmer to enter into a computer than binary;
• hexadecimal is easier for a programmer to understand and remember – 8F8B is easier to
remember than 1000111110001011.
• it is very easy to convert between binary to hexadecimal.

ADDITIONAL NOTES:

84



Summary
Expressing the denary number 195 as eight-bit binary, BCD, octal and hexadecimal:

Binary

… … … 128 64 32 16 8 4 2 1
1 1 0 0 0 0 1 1

BCD

8 4 2 1 8 4 2 1 8 4 2 1
0 0 0 1 1 0 0 1 0 1 0 1

Octal

… … … 512 64 8 1
0 3 0 3

or:
Pure Binary: 1 1 0 0 0 0 1 1
Group in threes: 0 1 1 0 0 0 0 1 1
Octal: 3 0 3

Hexadecimal

4096 256 16 1
C 3

or:
Pure Binary: 1 1 0 0 0 0 1 1
Group in fours: 1 1 0 0 0 0 1 1
Hexadecimal: C 3

ADDITIONAL NOTES:

85



Negative binary numbers

27. Describe and use two’s complement and sign and magnitude
to represent negative integers.

Two’s compliment
Two’s compliment is a method of representing negative numbers in binary, whereby the most
significant bit maintains its magnitude, but is made negative.

If, for example, one byte is used to represent a ‘signed’ integer using the two’s compliment method,
the column headings would become:

– 128 64 32 16 8 4 2 1

Thus, the denary integer 18 would be represented as:

– 128 64 32 16 8 4 2 1
0 0 0 1 0 0 1 0

And the negative integer – 18 would be represented as:

– 128 64 32 16 8 4 2 1
1 1 1 0 1 1 1 0

Notes:
• negative numbers will always start with a ‘1’ and positives will start will a ‘0’;
• the range of integers that can be represented using one byte is from – 128 up to + 127.

– 128 64 32 16 8 4 2 1
1 0 0 0 0 0 0 0 = – 128
0 1 1 1 1 1 1 1 = + 127

ADDITIONAL NOTES:

86



Converting a negative denary integer into two’s complement
Due to the way that binary numbers work, there is an easy ‘trick’ that allows very fast conversion.
Taking the denary integer – 52 as an example, you can use the three stages shown below:

Stage one
Convert the positive form of the number into unsigned binary:

– 128 64 32 16 8 4 2 1
0 0 1 1 0 1 0 0 = + 52

Stage two
Starting at the right hand side, copy each bit, up to and including the first ‘1’:

– 128 64 32 16 8 4 2 1
0 0 1 1 0 1 0 0
1 0 0

Stage three
Reverse all the other bits:

– 128 64 32 16 8 4 2 1
0 0 1 1 0 1 0 0
1 1 0 0 1 1 0 0 = – 52

This will always work – even if you do not understand why!

ADDITIONAL NOTES:

87



Converting a two’s complement number into denary
This is the same as converting any binary number into denary, as long as you remember that the
most significant bit is negative. For example the ‘signed’ binary number 1 1 0 1 0 1 0 1 is converted
as follows:

– 128 64 32 16 8 4 2 1
1 1 0 1 0 1 0 1

= – 128 + 64 + 16 + 4 + 1
= – 43

Sign and magnitude
The alternative to using two’s complement to represent negative numbers is to use the ‘sign and
magnitude’ method – here, the most significant bit is used as a sign bit without a numerical value.

– 64 32 16 8 4 2 1
1 1 0 0 1 1 0 0

= – (64 + 8 + 4)
= – 76

Notes:
• the range of integers that can be represented using one byte is from – 127 up to + 127.
• although the sign and magnitude method is easier for humans it is much harder to use for
computers performing arithmetic.

SPOT CHECK
1. Assuming a single byte is used, convert the following numbers into two’s compliment binary:
(a) – 5 (b) – 10 (c) – 20

2. What is the denary value of 1010 1011 if the binary codes represent:
(a) a two’s compliment number (b) a sign and magnitude number

ADDITIONAL NOTES:

88



Binary arithmetic

28. Perform integer binary arithmetic, that is addition and
subtraction.

Addition
Computers will only ever add two numbers at a time – if three numbers need to be added, a
computer will add the first two and then add the third number will be added to the result.

Since only two numbers are added at a time, there are limited outcomes:

0+0=0
0+1=1
1+0=1
1 + 1 = 2 (‘10’ in binary – this is 0 ‘down’ and ‘carry’ 1)

Note that when you add a ‘carry’ to the next column, it is possible for:

1 + 1 + 1 (the carry) = 3 (‘11’ in binary – this is 1 ‘down’ and ‘carry’ 1)

This is better shown if we add 1010 1110 1100 and 0011 1010 1010:

1 0 1 0 1 1 1 0 1 1 0 0
0 0 1 1 1 0 1 0 1 0 1 0
Carried bits  1 1 1 1 1 1

1 1 1 0 1 0 0 1 0 1 1 0

ADDITIONAL NOTES:

89



Subtraction
To perform subtraction, the number to be subtracted is converted into its two’s compliment
negative and then added.

For example to subtract 12 from 25:

1. Convert the 12 into two’s compliment –12

– 128 64 32 16 8 4 2 1
12 0 0 0 0 1 1 0 0

– 12 1 1 1 1 0 1 0 0

2. Now add this to the 25:

– 12 1 1 1 1 0 1 0 0
25 0 0 0 1 1 0 0 1
Carried bits  1 1 1 1

0 0 0 0 1 1 0 1

Note that there is still a ‘carry’ bit, but this is ignored.

SPOT CHECK
1. Work out the following using binary addition and subtraction:
(a) 34 + 73 (b) 67 – 96

ADDITIONAL NOTES:

90



Text (character) representation

29. Explain the use of code to represent a character set (ASCII,
EBCDIC and UNICODE).

Character Set
A character set are the characters that can be recognised by a computer.

Character encoding
A computer is able to represent four types of characters:
• alphanumeric characters – letters A – Z and a – z and the digits 0 – 9.
• punctuation characters and other ‘special’ symbols such as , . ; : “ ‘ ! @ £ $ % & * ( ) + <
• graphical characters such as ♣, ♦, ♥, ♠, Ψ, , , ✘, ☛, ✻ (and many more…);
• control characters – [Return], [Esc], [Space], [Alt], etc.

Within a computer, each character is represented using a unique binary code. Although there are
many different methods of encoding the characters, three of the most common are ASCII, EBCDIC
and Unicode.

ASCII
American Standard Code for Information Interchange (ASCII) is used for character encoding by
most Windows™ PCs. ASCII can be used to translate alphanumeric characters into a 7-bit binary
code that represents all the characters available from the keyboard including punctuation and some
special symbols such as ‘@’, # and $:

A development of ASCII, known as Extended ASCII, uses an 8-bit code that also defines codes for
additional characters, including some graphical ones. Note that using an 8-bit code means the
maximum number of characters that can be represented is 256.

ADDITIONAL NOTES:

91



How character encoding works
The diagram below shows how the message “Hello World” is stored in the memory of a computer
using the ASCII codes:

The message is typed at the keyboard. Electronics in the keyboard convert the typed characters into
ASCII binary codes that are sent from the keyboard along a cable to the computer. The computer
stores these codes in its internal memory. The computer also provides a visual display of the
characters as they are typed. To be able to do this, electronics inside the computer convert the
stored binary codes back into their character equivalents.

EBCDIC
Extended Binary Coded Decimal Interchange Code (EBCDIC) was developed by IBM for use in
their mainframe systems. It has the same limitation as ASCII in that its 8-bit code can only define
256 different characters.

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Notice how the EBCDIC codes are completely different to ASCII – if a message was sent that had
been encoded using ASCII, but received by a system that used EBCDIC, then the resulting message
would not make sense.

Unicode.
Unicode is an international system of representing characters using 16 bits. Using 16 bits means
that 216 = 65 536 different characters can be represented (thus overcoming the limitation of ASCII
and EBCDIC).
Unicode allows every character from most alphabets to have a code of its own – Chinese, Russian,
Greek, Urdu etc, including Egyptian Hieroglyphics. Note that there are plenty of spare codes that
are used for mathematical symbols, common graphics and even the Braille symbols

Some of the Mongolian characters: Some of the Hebrew characters

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ADDITIONAL NOTES:

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1.6 DATA CAPTURE

Standard input devices

30. Describe manual and automatic methods of gathering and
inputting data into a system, including form design, keyboard
entry, voice recognition, barcodes, optical mark recognition
(OMR), optical character recognition (OCR), magnetic ink
character recognition (MICR), touch screens; image capture,
chip and pin, sensors and remote data logging.

An input device is the hardware that is used to enter commands or data into a computer.

In many cases this requires the data to be converted into machine-readable form. Some input
devices – such as a keyboard, mouse and flat-bed scanner – require a human-user to be present to
control the input. Other devices – such as sensors and optical mark readers – can obtain data
automatically without the need for a human to be present. These latter devices are often referred to
as data capture devices.

Keyboard
The keyboard remains the most common input device although in terms of speed, it is one of the
most limited. It is, however, suitable for entering a wide range of data and it is a device that is
familiar to every office worker. Every key on a keyboard has a switch underneath it – when a key is
pressed the switch is closed and a signal is sent to the computer. Keyboards are wired so that the
signal which is sent is determined by the row and the column in which the key is. Most keyboards
have the keys arranged in the same way as the once popular typewriter – this arrangement is known
as QWERTY because it is the order of the first characters. This QWERTY layout was designed to
reduce the risk of jamming on early mechanical typewriters by spreading the most commonly used
letters around the keyboard – this effectively slowed down the typist. Improved layouts have been
designed such as the Dvorak layout which rearranges the keys in an attempt to distribute typing
more evenly among the fingers of both hands. Using these improved layouts increases the speed at
which data can be entered. Such layouts have never become popular, however, because there has
been no general agreement on a standard layout and also because of the time that it takes to get used
to a new arrangement.
Modern keyboards have in excess of 105 keys that
include 12 or more function keys, some of which
can be programmed by the user. Other keys have
specialist functions that can be used for navigation
within a document, to copy an image of the screen,
to put the computer into energy saving mode or to

ADDITIONAL NOTES:

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display the Start Menu if using Microsoft Windows™. Note that standard text entry using a
keyboard is usually between 20 and 60 words per minute (wpm), but a trained operator can reach
speeds in excess of 100 wpm.

Graphics Tablet
A graphics tablet is a specialist input device that is used to trace (or create original) drawings,
pictures and designs into a computer. It consists of a flat tablet and a pen-like stylus. Software
detects the movement of the stylus on the tablet’s surface and the changes to the image are
immediately displayed upon the screen of the VDU. This provides an input device that is used in a
similar way to paper and pencil.

Light Pen
This is used in a similar way to a graphics tablet except the light pen is moved over the VDU screen
itself. The pen is triggered by the raster scan of the VDU – it detects when the electron beam
building up the screen image has just passed the point where the pen is positioned. Knowing the
instant at which the beam passed the pen, the software can calculate whereabouts on the screen it
has been placed.

Touch Sensitive Screen
Touch sensitive screens are now often used as an alternative to a mouse. The user places their finger
(or a stylus device) on the screen and the position is detected – the command/option that relates to
the screen display is then processed. One method of detecting the position of the figure is to have a
series of horizontal and vertical infrared beams directed across the screen. Position is calculated by
detecting which beams have been interrupted.
Touch sensitive screens are generally used for navigation in data display applications. Here the user
can select one of a limited number of menu/navigation choices by pointing at the screen.
These screens have started to become popular in restaurants, ATMs, London Underground ticket
booths and some British Telecom payphones. The Science Museum in London is now full of touch
screen terminals that are used to help visitors find the location of exhibits and also to provide them
with related information. They are particularly suited to this type of environment since they are
easily enclosed within a damage resistant casing.
Touch sensitive screens offer flexibility in that it is easy to redesign the screens when
improvements or modifications are made – for example an extra button can be added by modifying
the computer program that controls the interface. Systems can be made user-friendlier because the
buttons can contain relevant graphics as well as words.

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Scanner – flatbed or hand-held
Scanners work by shinning a light
onto (or through) the material to be Choosing the resolution
scanned. The scanner then detects the Many people use the very high
image by measuring the reflected (or default manufacturer.set by the
scanner
resolutions
This results
transmitted) light – the most common in slow processing and a very large
sensor used to detect the reflected light is the charge coupled disk image (over 20+Mbytes in
device (CCD) which is also used in digital cameras. When some cases). The resolution
using a flatbed scanner the image to be scanned is placed chosen for the scan should reflect
upon a glass plate and a bright light source moves underneath the final print of the image. If you
intend to
use
the final image at the
the glass. This is in essence the same as a photocopier. Some same size then the scanning
flatbed scanners can be fitted with a transparency adapter so resolution should be set at the
that they can scan photographic negatives or slides. same as the resolution of the
printer; if the final printout will be
Scanning technology has increased tremendously over the half the size then the scanning
last few years. A typical flatbed scanner can produce images resolution should be halved.
with a resolution of more than 1200 dpi. If a page of text is If the image is to be viewed on
scanned into the computer, then Optical Character screen – within a web page for
Recognition (OCR) software can be used to convert the example – then the scanning
scanned image into an editable file. A decent flatbed scanner resolution should be set at 70/72
dpi.
can be bought for about £100 (Apr 2002).
If text is being scanned for
Hand-held scanners are smaller than flatbed scanners and conversion using OCR, then the
cannot scan a whole page of A4 in one go. The scanner is resolution must be 200 or 300 dpi.
moved across the image by hand and so is liable to jogging –
the image quality is therefore not as good as the flatbed.
Some hand-held scanners have the appearance of a pen and are designed to scan just one line of text
at a time.
Digital document scanners are high-speed flatbed devices that are capable of scanning books and
double-sided documents at 50 pages per minute.

Digital camera
These capture ‘still’ images. There are some available that will capture short movie sequences onto
a flash memory card. The resolutions of these devices has improved significantly in the last few
years and now images can be captured that are barely
distinguishable from a traditional photograph caught on film. Digitisers
Digitiser is the name given to any
Sound card with microphone device that changes analogue data
into digital data that can be
A sound card is the only method of getting analogue sound processed by a computer. A video
into a computer. A microphone or another sound device digitiser can convert the signal from
needs to be plugged into the sound card. Sound is an a television or videotape; a digital
analogue signal that needs to be converted into a digital form camera converts light and a sound
card converts sound.
before it can be processed by a computer. A sound card has
an analogue to digital converter that does this.

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Music can be captured through a sound card where it can then be edited and mixed.
Voice recognition
Voice recognition is software that will either allow a user to give a computer commands or it will
translate the user’s spoken words into editable, word processing text. Modern voice recognition
systems can translate with over 90% accuracy. Note that when attempting to use voice recognition
software the user should use a microphone that filters out background noise.

Benefits Drawbacks
It’s a natural way for humans to communicate It will be hard for a computer to understand a
user with a strong accent.
People with disabilities can use this system Background noise might make the system
unusable in certain environments
Its “hands free” – allows users to do other
tasks
Not much training is needed for the user

Uses of voice recognition include:
• fast entry of text into a word processing document – e.g. the first draft of an essay;
• used with telephones so that a user can pay bills using a credit card, without having a human
at the other end of the line.

Digital camcorder
Although Digital Video (DV) camcorders capture moving images onto a magnetic tape, the data is
digitally encoded. This means that the movie can be transferred to a computer (usually via a
firewire or USB port) and then edited. After all editing has finished the video can be transferred
back to the camera tape, recorded onto a standard VHS video tape or even transferred onto a DVD.
In either case, the quality of the final movie is very high and does not reduce in quality after being
played a number of times. Some digital camcorders will record video straight onto a DVD-R.

Specialist reading/scanning devices
Specialist reading and scanning devices are used for automatic data capture. This is often as part of
a transaction – eg withdrawing cash from a bank; borrowing a book from a library or buying food
from a supermarket. The process of data capture requires that the data can be stored in machine-
readable form. Data capture methods remove the need for data to be input from a keyboard and they
therefore eliminate the most likely source of error – the human operator. Also, since there is no
need to transcribe the data using a keyboard, data capture methods produce faster and cheaper input
of data.

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Data capture can be divided into two broad methods:
• Document reading and scanning methods – here the data is present on the source
document in computer readable format. The data may be pre-printed (as in a bar code) or
added to the document before input (as when buying a lottery ticket). In some cases the
whole document is fed into the computer and the data is read from it, but in other cases, only
the section of the document that contains the data is scanned into the computer.
• Sensing methods – special sensors are used to obtain the data and it is transmitted straight
to the computer. Examples include automatic weather stations; speed cameras; ten-pin
bowling or even PIRS on alarm systems.

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Magnetic Ink Character Recognition (MICR)
Specially formatted characters are printed
in a magnetic ink on the document. When
the characters are passed through a
magnetic field each of them produces a
unique effect on the field and so they are
detected. MICR is very accurate and it is
unaffected if the source document is dirty
or creased. It is difficult to tamper with data
written using magnetic ink.
It is, however, expensive to print the magnetic characters on the source document. This high cost
makes its use prohibitive in most cases except for on cheques and paying in slips. These are the
only significant use of this method.
A chequebook is pre-printed with bank sort code, customer account number and cheque number.
The value of the cheque is printed in MICR characters when the clearing bank processes the
cheque, before it is read by the computer

Optical Mark Reader (OMR)
OMR uses an accurately laid out pre-printed source document. For input,
this source document is marked in pre-determined positions. Normally the
mark made is a (soft) pencil or black biro line joining two dots on the
document. The completed document is passed through a light beam. The
dark marks that have been made on the source document reflect very little
of the light and so the position of mark is recognised. The document itself is
printed in ink that will not be seen by the reader (eg red). Lines down the
edge of the document allow the reader to determine the orientation and
position of the document as it passes through. Several thousand documents
can be read each hour.
OMR provides an easy way for users to record data where there are a
limited number of possible inputs. OMR is used for selecting numbers in The National Lottery and
it can be used to capture answers to multiple-choice examinations. Further possible uses would be
collecting survey information and to collect certain types of numerical data. It has been used for
collecting meter readings but is now superseded by hand held data recorders that store the meter
readings in RAM.
A major advantage of OMR is that no special equipment is needed to produce the marks.
Disadvantages of OMR are that creased or dirty documents cannot be read. If the user alters an
entry the resultant smudging may make the document unreadable. It can only collect a limited range
of responses that can be indicated in advance on the form.
When batch processing is used with OMR (e.g. when processing multiple-choice answer sheets)
there must be a ‘hopper’ for rejected documents that cannot be read or which have entries that were
not permitted – e.g. two answers to a question. These documents would need to be examined by the
operator and the data input manually.

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OMR is useful for turnaround documents. These are documents that originate from the computer
with some data already present, the missing parts are then completed and the document is used to
input additional data. In the case of multiple choice answer sheets the candidate number and the
centre number would be printed in OMR form on the answer sheet before being sent to the school.
Candidates would then complete the test by marking the answer sheets in the appropriate places.
The sheets would then be returned to the exam board and processed one after the other – as a batch.

Optical Character Recognition (OCR)
Printed or even hand-written characters on the document are
compared with stored character shapes. Up to 300 pages per
minute can be scanned. Note that there may be problems with
distinguishing between ‘O’ and ‘0’, ‘I’ and ‘1’ unless special
fonts are used.
OCR is useful for inputting large volumes of text – perhaps for
later checking and editing. An example might be to input a long list of addresses that are available
in printed form only.
OCR is heavily used in turnaround documents in applications such as gas, electric and water utility
billing and insurance premium renewal. The bill is prepared with OCR customer identification and
posted. When the customer pays the bill they return part of the original document with their
payment and this is fed back into the system via an OCR reader.
The post office use OCR to recognise the hand-written post codes on envelopes – this allows fast
sorting of the mail.
As with OMR, provision must be made for collecting and inputting data from rejected documents –
these details will have to be entered manually.

Bar Code Reader
A bar code consists of a series of lines. A standard EAN (European
Article Number) format is used on most products. This consists of 13
digits with two lines representing each of the digits. The thickness and
spacing of the lines determines the digit represented. A wand, a gun or
a fixed scanner can read the bar code. Note that the bar code number is usually printed underneath
the bar code itself so that it can be manually entered if the bar code does not scan correctly.
The choice of device is usually determined by the application – a wand is often used in libraries
because a book has to be both scanned and stamped and it is easy for the librarian to hold both the
wand and the stamp in the same hand; a scanner is used in supermarkets for speed; a gun is used in
DIY stores because some goods will be too large to pass over a fixed scanner.
Bar codes are limited to data that can be coded numerically – e.g. product code, stock number,
library number. They have the advantage that a computer using a normal printer can print them so
that ‘in house’ systems are possible with an application generating its own bar codes. This makes it
a cost-effective solution for libraries, stock and production control etc. Movement of stock through
a warehouse could be captured using bar codes attached to stock items. The major application of bar
codes is in combination with Point of Sales Terminals in supermarkets

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In some applications it will be necessary to have additional data capture or input to allow for items
that have no bar code – e.g. vegetables at the supermarket

Magnetic Stripe Cards
This is any plastic card with a magnetic strip on the back. The strip
stores data that is read when the card is wiped through a reader. In the
case of credit and cheque cards the magnetic strip will contain the
account number, the type of card and the expiry date. Another use of
these cards includes security passes – where the card opens an electronic
lock and the data read from the card can be used to keep track of where
people have been. Magnetic stripe cards are commonly used in sales
promotions and loyalty card schemes. These are where customers collect points on their card every
time they buy something (Note that with the Nectar card and Tesco’s Club card, the number of
points that a customer has earned is stored on a central computer whereas in the Premier Points
scheme, the points are stored on the card itself (ie data on the card is both read and written).
In security and banking applications the user will have a changeable PIN number that is used to
authorise the card’s use. The PIN number is no longer stored on the card but it is held in encrypted
form on the processing device.

Smart Cards
A smart card is a card that is the same size as a standard credit card but is embedded with a
microprocessor and a memory chip.
This chip can hold a lot more information that a small strip of
magnetic tape and the chip is very hard to copy. It is also very
hard to damage a chip on a smart card. Smart cards are seen by
many people as the method of eliminating the need for real
money – some schools already use smart cards to pay for school
meals instead of using cash. In these schools, each student pays
£10 (or so) for a smart card that is ‘loaded’ with £10 of cash.
Whenever they buy a school meal, the card is taken by one of the dinner supervisors where it is read
by a special machine and the cost of the dinner is taken from the card. When the card runs low on
funds, the student takes it to the school office together with payment and the appropriate amount of
cash is then added to the card. Note that many mobile phones have some kind of smart card in them.

Sensors
In specialised applications sensors may be used to detect events and feed data directly into a
computer. Examples involve most control applications such as traffic lights, burglar alarms,
washing machines etc. The sensors used will usually be specific to the application – e.g. detectors
embedded in the road for traffic light systems. The sensors may be remote and in certain situations
they may store the data locally for later transmission to the computer or perhaps collection. This
type of data collection is often called data logging. The data logger may transmit data to the
computer via cable or radio link or it may write data to a cassette.

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1.8 DATA MANAGEMENT

Backup and archiving

33. Explain the procedures involved in backing up data and
archiving, including the difference between data that is backed
up and data that is archived.

Backing up
Backing up is making a copy of a file(s) onto a separate removable storage medium so that data can
be recovered if the original is deleted or corrupted.
Possible media for backup include floppy disk, zip disk, CD-RW, DVD-RW, USB Pen or magnetic
tape. The copy must be stored safely - away from the original. In a commercial environment the
copy would be stored in a fire-proof safe or sometimes taken off-site.
Sometimes a whole hard disk is copied, other times it is only the contents of a particular folder or
sometimes an incremental backup is done which only copies the files that have changed.

Archiving
Archiving is removing old data from an on-line system and storing it off-line onto a separate,
removable storage medium.
The purpose of archiving is to free up online storage space and still retain a copy of the old data in
case it is needed some time in the future.
As well as freeing up storage space, archiving will also speed up the online system.

Backup strategies
Aim of a backup strategy
The aim of backup strategy is to have a secure, up-to-date, copy of data so that a file or even a
whole hard disk can be recovered if necessary.

Backup phases
A backup strategy consists of two phases:
• copying the data when the computer system is working normally
• restoring part, or all of the data in the event of loss or corruption.
In some scenarios, it may be essential to implement phase two immediately after data loss, but in
other situations a couple of days may be sufficient.

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Simple backup
The simplest backup strategy is to copy the entire contents of a computer’s hard disk onto a tape at
the end of each day. The tape will then be stored in a fireproof safe or even taken off-site by a
manager. A different tape is used each day of the week.
This backup strategy is not practical in large organisations that have vast amounts of data to copy –
the copying might continue into the next day and all the data may not fit onto a single tape. Large
organisations need to consider their backup strategies more carefully.
For example a large organisation will store its data-files in a separate directory (or on a separate
disk) to the software programs. This allows the data-files to be backed up every day, whilst
allowing backup of the software programs to occur only when there have been changes.
Also, large organisations usually only backup the entire data-file directory once each week – on the
other days there will be an ‘incremental backup’ of only those files that have changed since the last
backup.

Factors in a backup strategy
In any backup strategy, the following factors need to be considered:
• frequency;
• medium used;
• location of storage;
• time of day;
• what files/folders/directories;
• how long to keep backups.

Frequency of backups
Once a day should be a minimum, but if the data is of sufficient value, more frequent backups may
be necessary.

Backup medium
There are several options available:
• Magnetic DAT tape – this is often the preferred medium for larger backups because they can
store large amounts of data on a very compact media.
• (DVD)CD-R/RWs – these are a good medium onto which to backup or archive personal
data. They do not have sufficient storage capacity for backing up all the data in a large
organisation (e.g. a school), but are a very good for individuals who are backing up their
personal files.
• USB Pens – these are another option that could be used by an individual to back up their
personal files.
• Removable hard disks – these are an option which many large organisations now use for
backup because of their speed of writing and access. It is important though, that if hard disks
are used, they can (and are) removed from the main data store and kept securely elsewhere.

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Locations of backup storage
The backed up data needs to be held in a secure location in case of fire or burglary. There is little
point in keeping the backup tapes in the same room as the server from which the data is copied – a
fire would destroy both the original and the copy. The backed up data should either be stored in a
separate building within the organisation, or taken off-site – in either case, the tapes/disks should be
stored in a fire-proof safe.

Time of day/night to perform the backup
Most organisations will time a backup to occur during the night when the computer systems are not
in use. This will allow all the files to be copied successfully because most backup software will not
copy files that are in use. Another benefit of night-time backup is that the main users of the system
will not be affected by the inevitable slowing down that occurs during the backup process.

What data is to be backed up
The directory that contains the software only needs to be backed up
after an upgrade or reconfiguration. RAID (Redundant Array of
The directory that contains the users files can be backed up in two Inexpensive Disks)
ways – a full backup will copy every single file whilst an RAID is a process, which
incremental backup will only copy files that have been changed writes data simultaneously
since the previous backup. A full backup will typically occur at the to more than one disk. In its
end of a week (e.g. on a Friday) and an incremental backup will simplest form, one hard
generally be chosen for all the other days. disk is ‘mirrored’ to another
and so there is an exact
Length of time that backups are kept and 100% up-to-date copy
that can be used if one disk
Many organisations use a different tape each day of the week. This
fails.
means that a tape is rewritten after just 7 days (not much time to
spot an error). A better strategy would be: Note that RAID is not
backup because a file
• keep the Monday to Thursday incremental backups for one deleted from one disk will
week; also be deleted from the
• keep the Friday full backup for a month other and the data is not
• keep the last backup of each month for a year. copied onto a removable
This strategy allows data to be recovered even if the medium and it is possible
corruptionloss is not discovered for several months. You may end that two hard disks will fail
simultaneously, thus losing
up using about 20 tapes, but the security of the data is worth it.
all the data.

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ADDITIONAL NOTES:

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1.9 HARDWARE

Components of the processor

34. Describe the function and purpose of the control unit, memory
unit and ALU (arithmetic logic unit) as individual parts of a
computer.

Control Unit
The Control Unit manages the execution of instructions by fetching them from memory, then
decoding and executing them one at a time.

Memory unit
The memory unit is where the data and instructions are held so that they can be accessed by the
processor.

The memory unit stores:
• the parts of the operating system that are currently in use;
• the parts of the application programs that are currently in use;
• the data files that are currently in use.

Arithmetic Logic Unit (ALU)
The Arithmetic Logic Unit performs the arithmetic and logical operations on the data.

The ALU performs, for example, addition and subtraction, and logic operations such as AND, and
OR.

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Registers

35. Explain the need for, and use of, registers in the functioning of
the processor (Program Counter, Memory Address Register,
Memory Data Register, Current Instruction Register and
Accumulator).

A register is a small area of memory, within the processor itself, where data or control information
is temporarily stored.

Registers are used because transferring data to and from a register is much quicker than transferring
the data between the processor and main memory.

Special purpose registers
A processor will consist of special purpose and general purpose registers. The common special
purpose registers are as follows:

Accumulator (AX)
This accumulator is used as a temporary store for the result of the last arithmetic or logical
operation.

Program Counter (PC)
This program counter holds the address of the next instruction that will be fetched from main
memory.

Memory Data Register (MDR)
Also known as the Memory Buffer Register (MBR), this register contains the data/instruction that is
currently being copied from/to main memory.

Memory Address Register (MAR)
This register contains the memory location of the data or instruction that is being transferred
between the CPU and main memory (or another component).

Current Instruction Register/Instruction Register (CIR/IR)
This register holds the current instruction while it is being decoded and executed.

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System bus

36. Explain the need for, and describe the use of, buses to convey
information (Data, Address and Control buses).

The System bus is a set of parallel lines that connect the internal components of a computer.

The system bus is comprises of three separate elements:
• address bus;
• data bus;
• control bus.

Address bus
The address bus is the set of parallel lines that carries the memory address (location) of where data
is to be read from or written to.

The address bus is a one-way bus in that the addresses are always provided by the processor.
A wide address bus is necessary if large amounts of memory need to be used.

To address 16 Mbytes of memory the address bus must have at least 24 lines (224 = 16 Meg!)

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Data bus
The data bus is the set of parallel lines that carry the program instructions and data between the
internal components such as CPU and the main memory, or main memory and secondary storage.

This is a two-way bus in that the data can be transferred in either direction – from the CPU to main
memory, or from main memory to the CPU.
A wide data bus allows data to be transmitted quickly and is an important factor in determining the
overall speed of a computer.

The diagram below shows that a 32-bit bus transmits data four times as fast as an 8-bit data bus:

Control bus
The control bus is the set of parallel lines that carries ‘control’ signals that provide status
information and controls the flow of data.

Note that the control bus is two-way although some of the lines are only one way!

Signals carried on the control bus
The control bus will carry signals for the following:
• status – indicates whether data is to be read or written;
• timing –ensures that the data transfers are kept in sync.;
• transfer acknowledgement – confirming that a data transfer was successful;
• interrupt – both interrupt request and acknowledgement.

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Connectivity of devices

37. Describe the connectivity of devices (methods of hard wiring,
and wireless connections).

Hard wiring

Connecting computers as a network
Although wireless connections are becoming more popular, the most common method used to
connect computers together is still to use cable.
The standard types of cable are:
• Coaxial – similar to the cable that links an antenna to a television. It carries an electric
signal down a single, central wire. This type of cabling is not used very much any more.
• Unshielded twisted pair (UTP) – effectively telephone cable, but of better quality and with
8 wires. This is the most common type of cable to use with Local Area Networks and can
transmit data at speeds of up to 100 Mbps.
• Fibre-optic – the same cable as is used for cable television. This type of cable consists of a
very thin strand of glass, or plastic, that is completely surrounded by glass, or plastic, of a
different density. This allows a pulse of light to ‘bounce’ down the cable. When travelling
along fibre-optic cable, the data literally travels at the speed of light!

Connecting computers and peripherals
The two modern ways of connecting a peripheral to a computer is to use USB or FireWire:
• USB (universal serial bus) – this is the most common way of connecting peripherals (such
as printers, scanners and digital cameras) to your computer. USB originally allowed data to
be transferred at 12 Mbps, but in its latest form (known as USB2) it can transfer data at
speeds of up to 480 Mbps.
• FireWire (also known as i-link or 1394) – a fast way of connecting a peripheral device to
your PC. It is usually used with Digital Camcorders and external HDDs. Speeds of up to 800
Mbps can be achieved with FireWire 2.

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Wireless
Wireless media do not use an electrical or optical conductor. In most cases the earth’s atmosphere is
the physical path for the data. The common types of wireless media that transmit data use electro-
magnetic waves and are:
• Radio waves – the distances that can be covered by radio waves are dependent upon the
frequency, but can be hundreds of miles.
• Microwaves – these are transmitted at much higher frequencies than radio waves and will
provide better performance. Satellite dishes are often used with microwave transmission
where communication can be directly between two systems within the same geographical
area or it can make use of a satellite in geosynchronous orbit 50 000 km above the earth.
• Infrared – this uses ‘invisible’ infrared light to transmit the data. Most household remote
controls (used with televisions and videos, etc.) use infrared transmissions. Infrared
transmissions are either ‘point-to-point’ or they can be reflected off walls and ceilings; they
cannot travel more than a few metres and they cannot pass through walls.

Diagram showing the range of electromagnetic waves that can be used to transmit data:

Bluetooth
Bluetooth is one of the latest radio technologies that enables devices such as computers, mobile
phones, hands-free kits and other peripherals to be connected without cables up to 30 feet away.
Like many modern wireless devices (including portable phones and wireless computers) it uses
waves with a frequency of 2.4 GHz.

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Network topologies

Topology refers to the way in which computers and other networked devices are interconnected – it
describes the physical layout of the cables and interconnecting devices.

The three most basic topologies are the bus, ring and star.
Bus
A bus topology is one in which the nodes are connected by a single cable and the data is transmitted
in both directions along the cable, to all the nodes.

This means that all the nodes receive all transmitted data. An Ethernet bus (the most common type
of local area network) can have a maximum length of 185 metres and a maximum of 30 nodes.
Note that there must be ‘terminators’ at either end of the cable – preventing the signals being
reflected back down the cable.

The advantages of a bus network are:
• it is quick and inexpensive to install (single length of cable);
• additional computers can be added easily (up to the maximum of 30).
The disadvantages are:
• a cable fault will affect the whole network;
• network performance degrades under a heavy load (maximum computers on an Ethernet bus
is 30).

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Ring
A ring network is one in which the nodes are linked in a closed loop and the data is transmitted in a
single direction around the loop.

There are Repeaters at each node to ‘boost’ the data signals as they pass around the ring. The use of
repeaters allows a ring network to cover larger distances than other types of local area network, and
if using optical fibre cable, ring networks can span a distance of up to 100 kilometres. The first
implementation of this type of network was the Cambridge Ring, which is shown below. Now the
most important implementation is the IBM token ring.
The advantages of a ring topology are:
• there is no dependence on a central computer – each node can handle data transmission to
and from itself;
• very high transmission rates are possible;
• transmission of messages around the ring is relatively simple and avoids ‘collisions’ because
the transmission is in only one direction.

The disadvantages are:
• if one node breaks down, transmission between the other nodes might be disrupted;
• extending an existing ring can be difficult because of the rewiring required.
• not very secure because data travels all the way around the ring and so there are plenty of
opportunities for messages to be intercepted.

Note that the first two disadvantages are overcome with a Token-Ring implementation.

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Star
A star topology is one in which each node is connected directly to a central hub using its own,
dedicated cable, with all communications between workstations go through this hub.

This was originally the configuration for IBM mainframe systems, when dumb or semi-dumb
terminals were linked to a central computer. It is now the most common topology used within
LANs and many WANs.
It is usual, but not essential, to have one or more servers connected to the hub. In these server-based
networks, most of the communication is between workstations and a server.
The advantages of a star topology are:
• if there is a cable fault only one node is affected;
• the data transmission rate can vary on each link – allowing old and new computers to
communicate with each other;
• there is greater security of transmissions if a ‘switch’ is used as the hub;
• cabling faults are easy to locate because the hubs usually use LEDs to indicate working
connections;

The disadvantages of a star network are:
• hub failure affects all users – although local processing can still take in some systems;
• cabling can be difficult – individual cables must go between each computer and the hub –
this can add to the installation time and cost.

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Primary memory

38. Describe the differences between types of primary memory and
explain their uses.

ROM (Read Only Memory)
ROM is computer memory, which contains pre-recorded data that can be read, but not modified or
deleted.

Note that:
• with most ROM chips, the information is burnt in during manufacture..
• ROM is non-volatile and so retains its contents when the power is removed.

The uses of ROM are to store:
• the ‘bootstrap loader’ within most personal computers – the bootstrap loader is the first
program that is executed at start-up and its job is to load the operating system;
• fonts within laser printers;
• the ‘character pattern’ memory – this is the pattern of pixels that are lit up for each text
character, when a command-line interface is being used.

RAM (Random Access Memory)
RAM is the main memory of the computer, which can be both read and written to.

Note that:
• any part of RAM can be accessed at any time by using its memory address, rather than
having to access it sequentially, from the beginning;
• RAM is volatile – its contents are lost when the computer is turned off;
• computers have more RAM than ROM.

The uses of RAM are to store:
• the parts of the operating system that are currently in use – the operating system needs to be
stored here so that it can be updated with newer versions;
• the parts of the application programs that are currently in use;
• the data files that are currently in use.

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Secondary storage (backing store/auxiliary memory)

39. Describe the basic features, advantages, disadvantages and
uses of secondary storage media.

Secondary storage is the non-volatile, long-term store for programs and data that are not currently
in use.

Secondary storage is needed because:
• main memory is volatile – contents are lost when the power is turned off;
• main memory is finite – cannot store everything.

Measurement of storage
The common units to measure storage capacity are:
• Bytes
• Kilobytes (1024 bytes)
• Megabytes (1024 Kilobytes)
• Gigabytes (1024 Megabytes)
• Terabytes (1024 Gigabytes)

Types of Media
Secondary storage can either be magnetic, optical or electronic (although it is usually magnetic) and
the most common secondary storage device in a computer is the hard disk.

Magnetic Media Optical Media Electronic Media
Floppy Diskette CD-ROM Memory cards/sticks (for
games and cameras)
Hard disk drive (HDD) CD-R USB ‘pens’
Zip disk CD-RW
Tape/DAT DVD
Blu-Ray

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Magnetic media
Magnetic media are tapes or disks that are coated with iron oxide (or similar magnetic material).
The iron oxide particles are aligned by a read/write head so that all particles that represent a ‘1’ bit
point in one direction and all particles that represent a ‘0’ point in the perpendicular direction.
Floppy diskette
Floppy diskettes (usually called ‘floppy disks’) consist of a thin sheet of
magnetic coated plastic encased in a hard plastic casing typically of 7 cm square.
Most floppy diskettes are of the ‘High Density’ type and have a capacity of
1.44Mb.
A diskette used to be the standard for transferring very small files from one
computer to another. It was also used as a ‘boot up’ disk if a hard disk became corrupted. Floppy
disks are now effectively redundant due to their small capacity, slow access speeds and lack of
reliability.
Hard Disk Drive (HDD)
All standalone PCs come equipped with an in-built
hard disk, the capacity of which is nowadays
measured in gigabytes. A new PC typically has a
200+ Gbyte disk. The hard disk is used for storing
software including the operating system, other
systems software, application programs and the data-
files.
Magnetic disks require formatting before use. This is
essentially a process of marking out the surface into
usable areas in a way that allows the disk drive to
find its way about the medium when reading and
writing at a later time.
Hard drives have a very fast transfer rate.
Zip Disks
A zip drives is similar to floppy drives in that the individual disks are removable. The individual
disks, however, have a much larger capacity – zip disks are made with capacities of either 100 or
250 Mbytes.
Tapes and Cartridges
Digital Audio Tapes (DAT) (cartridges) are sealed units similar to an audio cassette. The main use
of DAT are to hold backup-up copies of hard disks within medium to large organisations – they can
store 1os of gigabytes of data.
Note that data on tape is stored ‘sequentially’– i.e. a tape is read/written from beginning to end.
This is okay for writing the data, but it can take a long time to access specific data that you may
want recovered.

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Optical media
Optical media, such as CD-ROM and DVD, use a laser to detect ‘pits’ (holes) in a disk – the
existence if a pit represents a ‘1’ and the non-existence of a pit represents a ‘0’.
CD-ROM
These disks are the same size as audio CDs and are read by
Magneto-Optical Storage
a laser in the same way. On CD-ROMs the data cannot be
changed. Access is slower than a hard disk because the spin Note that some devices are classed
as magneto-optical. These are disks
speed is slower. A CD-ROM can store about 700 Mbytes of that have a magnetic surface that can
data. They are used in situations where the data does not only be magnetised when heated to
age quickly. Examples include encyclopaedias, large about 200ºC. A laser beam is used to
catalogues and telephone directories. Software is usually heat a spot on the surface that is then
supplied on CD-ROM. magnetised to record the data. The
direction of magnetisation affects the
CD-R (Recordable) light reflecting off the surface, so the
data can be read optically. These
Sometimes called WORM disks (Write Once Read Many) devices provide about 125 megabytes
CD-Rs allow the user to write onto the surface of a CD- of storage on a disc the same size as
ROM. This can only be done once since the surface is a floppy. They can be used for
backup and archiving data and for
permanently changed during the writing process. The
transferring data from one system to
resulting disc can be read by a normal CD ROM drive. CD- another. In addition to their vastly
Rs are useful for archiving data or for small-scale CD- increased storage space compared to
ROM production. Capacity varies slightly between two a floppy disk, they have the
different standards – CD-R74 and CD-R80. The former can advantage that the data stored is less
vulnerable to stray magnetic fields
hold 74 minutes of audio and 650 Mbytes of computer data and temperature effects since the
while the latter holds 80minutes of audio or 700 Mbytes of surface of the disc must be heated
computer data. before the data can be changed.

CD-RW (Re-writable)
Re-writable CDs are used in a similar way to CD-Rs, but their surface is not permanently changed
when data is written to it. This means that data can be deleted and space on the disk is freed. CD-
RWs contain a crystalline compound that is made up of silver, indium, antimony and tellurium
metals. When this compound is heated and cooled slowly, it becomes reflective to light. When the
compound is heated to a hotter temperature and then cooled quickly, it become absorbent to light –
the absorbent areas are the equivalent of the pits on a standard CD. The laser that is used within
CD-rewriters has three intensities – one to make the crystalline compound reflective, one to make
the compound absorbent and the third, low intensity setting is used for reading data.
Digital versatile disk (DVD)
Originally referred to as digital video disks, DVDs have gradually replaced CD drives in computers.
DVDs are exactly the same size and thickness as a standard CD, but the tracks are much closer
together and the pits are much smaller. DVDs can store data on both sides of the disk although, in
most DVD drives, they need to be physically removed and flipped over for the second side to be
read.

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As well as storing data on both sides, DVD can also store data within two layers on each side and
this gives rise to four different standards of DVD:

Number of sides Number of layers Capacity
1 1 4.7 Gb
1 2 8.5 Gb
2 1 9.4 Gb
2 2 17 Gb

As writable DVD drives have become more readily available, these have started to replace
videotapes (although it is also common to use recording equipment containing a hard disk). DVDs
can store up to 17 Gbytes of data – this allows up to eight hours of video to be stored on one small
disk.
There are several advantages of recording onto DVD as opposed to magnetic tape:
• Disks can be randomly accessed – the user can move to any part of a film easily;
• DVDs store the data digitally – the quality of the images or sound does not deteriorate with
constant use;
• The digital storage allows the images and sound to be much higher quality than tape.

Electronic ‘flash’ media
Electronic media use millions of electronic switches to store data.

Memory cards and sticks
These are typically used to add extra storage in cameras, mobile phones and games consoles (most
of these devices have some ‘built-in’ memory). This technology has developed rapidly over the last
7/8 years and this has led to a variety of different shapes and sizes of memory card, each being
developed for a specific purpose.

USB ‘flash’ drives
These are the most recent development in file storage. They are small devices with built-in memory
cards. They can plug into the USB port of a computer and they function as a portable hard drive.
They have the advantage over portable hard disk drives in that they have no moving parts and are
therefore much more suitable for carrying around. Their disadvantage is that their capacity is much
smaller – a maximum of about 8 gigabytes at present.
These devices are known by many names including: flash drives; USB pens; USB keys and simply
as USB drives.

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Secondary storage summary
Magnetic media
Price
Media Capacity Speed Uses
[2008]
VERY RARE NOW.
Floppy Disk 1.44 Mb V Slow Backing up small amounts of data – eg word
processing files.
VERY RARE NOW.
Zip Disk 100/250 Mb Medium
Backing up work on a home computer
Storage of operating system, application
Hard Disk > 200 Gb Fast £40
programs and user produced data-files.
Backing up large amounts of data – eg on a
Tape/DAT > 70 Gb V Slow
network file server.

Optical media
Price
[2008]
CD-ROM 650/700 Mb Medium 30p each Distribution of commercial software
Backing/archiving up work and software on a
CD-R/RW 650/700 Mb Medium 50p each
home computer.
Distribution of software – particularly ones
DVD 4.7 – 17 Gb Quite fast that contain a lot of multimedia and videos;
backing up files on the hard disk
Backing/archiving up work and software on a
DVD-R/RW 4.7 – 9.4 Gb Quite fast
home computer.
Blu-ray 25 –100+ Gb ?? Distribution of High Definition movies

Electronic media
Price
[2008]
Storing digital images (from a digital camera);
Flash Card  8+ Gb Quite fast Storing contact details and diary information
(mobile phone).
Transferring files from one computer to
USB Pens  8+ Gb Quite fast another;
Backing up personal files.

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Buffers and interrupts

40. Describe the transfer of data between different devices and
primary memory, including the uses of buffers and interrupts.

Buffer
A buffer is an area of memory between two components/devices into which data is temporarily
stored during data transfers between components/devices that operate at different speeds.

Peripheral devices operate at slower speeds than the processor. To allow for these speed differences
a buffer needs to be used.

The buffer may be built into the I/O controller of the computer or of the peripheral device (or both!)

Interrupt
An interrupt is a signal from a device to the processor, to indicate that it wants attention.

Transferring/sending data to a printer
When a document is printed, it is first spooled to the hard disk and added to the print queue. When
it is the document’s turn to be printed the following process is repeated:
• the print buffer is filled;
• the buffer is then emptied to the printer;
• when the buffer is empty (or nearly empty), an interrupt is sent to processor;
• this requests the buffer to be refilled.

Note that when the buffer is being emptied, the processor can continue with other tasks.

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1.9 DATA TRANSMISSION

43. Describe the characteristics of a LAN (local area network) and
a WAN (wide area network).
44. Show an understanding of the hardware and software needed
for a LAN and for accessing a WAN, eg the internet.

Local area network (LAN)
A local area network is an interconnection of computers within a limited geographical area –
usually a single building or site (e.g. a school).

Roles of computer
Within a local area network, there are three roles for a computer:
• servers – these computers provide network resources (file-, printer-, Internet-, e-mail- or
database- are the most common types of server);
• clients – these computers use network resources, but do not provide resources to others;
• peers – these computers use network resources and they provide some of their own.

Hardware needed for a LAN
In addition to the computers, the additional hardware needed for a local area network is:
• network cards (NIC) – each computer (or other networked device, such as a printer) needs
one of these;
• specialist network cables (e.g. UTP) or wireless transceivers – these provide the means
(channels) by which the data can be transmitted between the networked devices;
• hubs, repeaters or switches – these are vital in star networks and provide the central point to
which all the cables are connected.

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Wide area network (WAN)
A wide area network is an interconnection of computers located at different geographical sites –
these may be different parts of the same town, different towns, countries or even continents.

Hardware needed for a WAN
A WAN uses:
• existing telecommunications channels to join the computers together – public telephone
lines or satellite links;
• a modem, router (or a similar device).

Note that modern ‘broadband’ connections to WANs are possible using improved technologies such
as ISDN, ADSL and digital connections through digital televisions boxes as offered by Virgin
Media.

Network card (network adapter)
A network adapter is the small electronic circuit board within a computer (or other networked
device) that controls the sending and receiving of data within a LAN.

A network adapter (sometimes referred to as a NIC – Network Interface Card) is either integrated
into the system board or inserted into one of the expansion ports inside the computer. It:
• provides a connection socket for the network cables;
• translates the signals that are generated by a computer into a form that is suitable for
transmission down a wire (or through the air) – for example, it translates the parallel signals
from the processor into serial signals that can be carried down a wire.

Differences between a LAN and a WAN

LAN WAN
Short distance connections – single site Long distance connections – different towns
(school network) (different branches of a supermarket)
Uses specialist network cables/wireless Existing telephone cables or communications
equipment links.

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Networking a printer
A network printer is one that is shared and available for ‘remote’ printing.

The printer may have its own network card or it may be connected to a networked computer using a
USB cable.

Benefits of sharing a printer:
The main benefits of networked printers include:
• each user has access to more than one printer – if one fails or runs out of supplies, another
printer can be used.
• one expensive colour laser printer can be used by people in several different rooms/offices.

Drawbacks of sharing a printer:
Drawbacks include:
• the printer may be in a different room to the person who has printed;
• the printouts will not be private;

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Networking compared to stand-alone operation

Benefits Drawbacks
Allows the sharing of an internet connection or Security of files is less than on stand-alone
other hardware such as a printer machines – this includes a greater threat of
virus infections spreading
Allows the sharing of software – available from A fault in the network will mean that the shared
an applications server hardware and software resources are no
longer available
Allows sharing of files (and databases – A poorly configured network may have
available from a file sever ‘bottlenecks’ and will probably run slower than
standalone computers
A user can access their files from any
computer
Teacher can see what is being done on any
machine
Users can communicate with each other using
e-mail or another form of electronic messaging

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148



Data transmission

45. Describe the different types of data transmission: serial and
parallel; and simplex, half-duplex and duplex modes.
46. Explain the relationship between bit rates and the time
sensitivity of the information.

Modes of data transfer
Data transmission is either between two computers, a computer and a peripheral, or between the
internal components of a computer.

Different transmission modes are suited to different purposes. The main ones being:
• serial
• parallel

Serial
Serial transmission is where data is sent one bit at a time along a single data line.

Serial transmission is used in networks and is also the method used for all USB connections to
peripherals.

Parallel
Parallel transmission is where multiple bits are sent simultaneously.

Parallel transmission is technically much faster than serial. However, it can only be used over short
distances because the data then becomes out of sync. For this reason, the only common use for
parallel transmission nowadays, is between the internal components of the computer.

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Direction of data transfer
Data can be transferred in either one or two directions. The names for the different options are:
• simplex
• duplex
• half-duplex

Simplex
A simplex communication is one which allows data to be transmitted in one direction only.

Duplex
A duplex communication is one which allows data to be transmitted in both directions at the same
time.

Network ports on servers will usually be set to duplex mode (as opposed to half-duplex).

Half-duplex
A half-duplex communication is one which allows data to be transmitted in both directions, but
only in one direction at a time.

A printer will use half-duplex (data needs to travel in both directions, but not at the same time).

Bit rate
Bit rate is the number of bits transferred in each second.

Examples of transmission speeds:
• most LANs operate at a bit-rate of 100 Mbps (megabits per second);
• broadband connections are usually between 2 and 8 Mbps;
• USB2 connections can be as quick as 480 Mbps.

Note that the above are maximum speeds. In practise, the speeds are always less because of bad
connections and interference.

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Error checking

47. Recognise that errors can occur in data transmission, and
explain methods of detecting and correcting these errors
(parity checks, the use of parity in data blocks to become self-
correcting, check sums and echoes).

Errors will occur when data bits become flipped – i.e. a ‘1’ bit flips to a ‘0’ and vice-versa. As long
as this is not a common occurrence, this type of error is easily detected by the use of a parity bit.

Parity bits
A parity bit is an extra bit that is derived from, and added to, data before it is transmitted so that
transmission errors can be detected.

Even parity
This is when the total number of ‘1’ bits transmitted is even.

Odd parity
This is when the total number of ‘1’ bits transmitted is odd.

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Parity checking
Parity checking is used in the transmission of ASCII characters. Each character is encoded as a
group of 7 bits when it is stored in memory, tape or disk. The eighth bit of the byte is used as a
parity bit. The parity bit is set as a ‘1’ or a ‘0’ so that the total number of ‘1’ bits is odd. The data
and the parity bit are sent together. The receiving computer receives the transmission and works out
what the parity bit should be – it then compares this with the parity bit it actually received – if they
are different, then there has been an error:

The data without the parity
1 1 0 0 1 0 0
bit

The parity bit is worked
0 1 1 0 0 1 0 0
out to be a ‘0’

The data (with the parity bit) is sent to the recipient computer,
0 1 0 0 0 1 0 0
but one of the bits is ‘flipped’

The recipient computer works out that the parity bit should be
0 1 0 0 0 1 0 0
a ‘1’, which is different to the parity bit that it received!

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Check sums
A check sum is a value which is computed from a block of data and is transmitted or stored with the
data in order to detect a change.

A check sum is used to verify data transmission or data decompression. Note that, in this context, a
‘block’ of data could refer to an Internet packet, a file, or even a whole directory.

The receiving system re-computes the checksum based upon the received data and compares this
value with the one sent with the data. If the two values are different then the recipient knows that
there has been an error. If they are the same, however, the receiver has some confidence that the
data was received correctly – it is possible that there has been an error, but this is extremely
unlikely.

Notice that the principle of the check sum is similar to that of a check digit. The use of a check digit
to validate input rather than verify transmission is one of the main differences – i.e. a check sum is
used to verify much larger quantities of data and so it will need to be larger itself. The checksum
may be 8 bits (modulo 256), 16, 32, or in some cases bigger. Internet packets use a 32-bit
checksum.

Echoes
An echo is a method of error-checking whereby the data that is received is sent back to the sender to
check that it is unchanged.

The echo process is as follows:
• the data that has been received is sent back to its origin;
• it is then compared to the original data;
• any differences will signify a transmission error;
• and the data will be resent.

ADDITIONAL NOTES:

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Packet switching and circuit switching

48. Describe packet switching and circuit switching.
49. Explain the difference in use of packet switching and circuit
switching.

Circuit Switching
Circuit switching is where two computers that want to exchange data establish a path through a
network, which is then used for the entire transmission.

This path is called a circuit and a different path may be established for each different
communications session.
This approach resembles a telephone connection in which a circuit is established to enable
communication to take place from one telephone to another.

An advantage of circuit switching is that the data travels through the network with very little
processing overhead – data segments arrive in the order in which they are dispatched and so very
little effort is required to reconstruct the original message.
The main disadvantage of circuit switching occurs when the connected devices do not transfer data
continuously. Under these circumstances, the connection will sometimes be idle and so bandwidth
is wasted.
Another disadvantage is that a circuit switched network cannot connect computers or devices that
operate at different transfer rates.

ADDITIONAL NOTES:

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Packet Switching
Packet switching is where two computers that want to exchange data break it down into discrete
‘blocks’ called packets. These packets are then ‘routed’ individually through the network.

In packet switching networks, it is possible that each packet takes a different path to the
previous one.
It is therefore possible that the recipient computer will receive the packets in a different
order to that in which they were sent. Thus, packet switching requires that each packet is
numbered so that the recipient can reconstruct the original message.

Packet A is sent to node 1. Node 1 stores the packet and determines the next leg of the
route – node 2, in this case. The packet is queued until the link is available, when it will
be transmitted to node 2. Node 2 will forward the packet to node 3, which will forward
it to node6 and then to the recipient. Note that other packets take different routes.

The nodes within a packet switched system are different from the nodes used in circuit switching.
Packet switching nodes have the ability to store packets until they are ready to be retransmitted.
Packets are queued at nodes and then transmitted as rapidly as possible over the link to the next
node. In this way, packets from different messages can be multiplexed together at nodes as they
pass through the network so pathways are kept operating at full capacity to avoid wasting
bandwidth. Time slots are not pre-allocated as in circuit switching and two computers of different
transmission data rates can exchange packets, since each connects to the network at its own data
rate.
The ability of nodes to store packets means that when traffic on links is heavy, packets can continue
to be accepted unlike calls in a circuit switched network. Some calls are refused in the latter’s case
when traffic becomes heavy. Delivery delay just increases in the case of a busy packet switched
network.

ADDITIONAL NOTES:

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Also note that packet switching systems allow priorities to be used. Thus, if a node has a number of
packets queued for transmission, it can transmit the higher-priority packets first. These packets will
experience less delay than lower priority packets.

Comparison of circuit switching and packet switching

Circuit switching Packet switching
A path is set up between the sender and the The message is divided into packets which
receiver for the duration of the transmission.. may use different routes to get to the receiver.
The message is received in order and does not The packets will be received in no particular
have to be reassembled. order and will need to be reassembled before
the message can be read.
If the circuit develops a fault, then the If one part of the network develops a fault then
communication collapses. the packets are routed through a different part.
Message can be intercepted at any point within Not possible to intercept message – the
the circuit. packets do not all go through the same point.
Ties up a route for the duration of the Does not tie up part of the network.
message.

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Protocols

50. Define the term protocol and explain the importance of a
protocol to the transmission of data.
51. Describe the need for communication between devices and
between computers, and explain the need for protocols to
establish communication links.
52. Explain the need for both physical and logical protocols and
the need for layering in an interface.

Definition
Protocols are the rules that define the ways that different computers can be connected, and send
signals to each other, in order that ensure that communications will be successful.

Different networks use different protocols for establishing communication between their connected
computers. To allow manufacturers to produce hardware and software to work with different
networks, protocols have been divided into seven layers (groups) each of which covers a different
aspect of the communications process. This ‘OSI model’ is outlined below:

Type of protocol Functions
Physical The types of connectors to use; the types of cables; the signal voltages
Data Link The size of ‘Frame’ that is transmitted and frame addressing (ie using MAC
numbers)
Network Use of numbers or names to address sender and receiver; Whether
numbering is structured or random; how a particular computer is found
Transport Are messages/files divided into packets or sent complete; the procedure to
follow when more than one computer wants to transmit a message at the
same time; The methods of detecting and correcting errors
Session How the connection is established and later terminated; The idle time allowed
before being disconnected; The number of different sessions allowed on
each computer at the same time; Is encryption being used
Presentation Whether text is encoded as ASCII or ANSI or Unicode
Application HTTP, FTP, POP, IMAP, Telnet, …

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ADDITIONAL NOTES:

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1.11 IMPLICATIONS OF COMPUTER USE

53. Discuss changing trends in computer use and their economic,
social, legal and ethical effects on society.
54. Explain changes to society brought about by the use of
computer systems, e.g. in changing leisure patterns and work
expectations.

Computers are used more and more at home, at work and in education. Their increased use is due to
the tremendous benefits that they offer, but computers are not without their drawbacks.

Home and Leisure
New gadgets such as automatic washing machines, dishwashers and microwaves have made our
lives easier.
Computers are used to control heating, ventilation, hot water and security devices such as alarms
and closed circuit TV cameras.
We use the Internet to check our bank account, pay utility bills, purchase goods and book tickets for
holidays or shows.
VHS video recorders have very few of the features of modern systems. Now the BBC iPlayer lets
us watch programs up to a week after they have been broadcast. Sky+ allows viewers to pause a
‘live’ broadcast to continue watching later and also allows the viewer to rewind!
Home computers and games consoles offer a new form of entertainment – some of the most recent
developments allow many users to play games against each other over the Internet.

Some of the developments in home computer uses:
• Broadband Internet – e-mail, instant messaging, social networking, on-line video, music
downloads, gaming, gambling;
• Entertainment systems – DAB radio, digital TV, mp3 music players; DVD and HD video;
Sky+ (and V+) systems, games’ consoles.
• Household appliances – washing machine, dishwasher, fridge all with built-in
microprocessors;
• Cars – Automatic Breaking (ABS) prevents skidding, fuel control, Sat-Nav;
• Gadgets – e.g children’s toys;
• Mobile/Smart phones;
• Wireless communications (including Bluetooth) – built into many devices;
• RFID – Radio frequency ID chips are a modern form of bar coding, but can be read from a
distance.

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Education
There are many uses of computers in education for both students and staff. Schools (and other
educational institutions) use computers in the classroom and for administrative tasks.

Administrative uses
These include:
• Computerised attendance systems;
• Timetabling;
• Exam entry;
• Report writing;
• Progress monitoring;
• Budgeting and accounting;

Classroom uses
Computers have made significant impact in the classroom for both students and the teachers – many
teachers prepare PowerPoint presentations or use other interactive software tools and students
research topics on the Internet then type up their findings in a word processor. Computers have also
made distance learning possible where students access resources from a college web site and use
email to communicate with their tutors.
Some of the uses in teaching and learning are listed below:
• presentation of work (word processor);
• Internet research (Web browser);
• interactive/computer-based learning (simulations and on-line quizzes);
• multimedia demonstrations (including PowerPoint presentations);
• distance learning/online courses (Open University and Learn Direct)

Benefits and drawbacks of online learning

Benefits Drawbacks
Students can learn at their own pace, The computer does not have the extra
repeating units when they want. motivational ability of a good teacher;
The student is given information of a The computer is limited in the number of
guaranteed quality that is independent of the different ways that it will present the
skills of the teacher. information.
Learning can be presented in an interactive Some students do not find learning at a
way. It can also include sound and video. computer enjoyable.
The computer is not impatient and will not
become frustrated if a student continuously
makes mistakes.
Fewer teachers are needed and so the running
costs are cheaper after the initial investment.

ADDITIONAL NOTES:

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Employment
Ever since the industrial revolution, people have feared that machinery will displace workers, and
information technology is no exception.
Note, though, there is no evidence that computers have led to mass unemployment – in fact, many
people argue, more jobs have been created by computers than have been displaced by them.
In some areas, however, computers have substantially replaced the workforce. In the 1980s,
thousands of factory workers were made redundant by the introduction of robots on the factory
floor making everything from biscuits to cars. In the 1990s, thousands of clerical and white-collar
workers have seen their jobs disappear with the introduction of company databases, desktop
publishing, computerised accounting systems and increased automation in banks, building societies
and organisations of all kinds, large and small.

The changing nature of employment
In today’s society, there are far fewer people who work in farms or factories than in the past. Most
jobs, nowadays, are found in the ‘service’ industries such as sales, education, health care, banks,
insurance and law. Also in catering, computer support, or advertising and delivering goods. These
jobs primarily involve working with, creating or distributing new knowledge or information.

In many cases, workers displaced by computers are retrained to perform computer-related jobs that
may be more satisfying than their original jobs.
• a secretary may find it more satisfying to use a word processor to produce high quality
output which can be saved and amended, rather than having to retype whole pages because a
minor error was made.
• an engineer or draughtsman may find it more satisfying to create designs using a computer-
aided design system with complete accuracy, than drawing by hand;
• an accounts clerk may prefer to use an Accounts software package rather than to do the
accounts manually.

Changing locations of work
Not only are the type of jobs we do changing, the location of work is changing too. When Britain
changed in the 19th century from an agricultural to an industrial society, more and more workers
were forced to move from farm work into towns and large industrial centres. The advent of
communications technology is now starting to reverse this trend. There is no need for much of the
work of an organisation to be done at a Head Office in a city; it is often more economical for it to
be done in a more remote area where office rates and housing are cheaper, and employees can be
paid less. Results of data processing can be transferred to wherever they are needed via a
telecommunications line – at the moment much UK work is performed by workers who are actually
located in India (particularly ‘help-desk’ work).

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Benefits and drawbacks of computers at work
Computer in the workplace have many benefits, but they also have drawbacks:

Benefits Drawbacks
One computer can do the work of many Fears of redundancy – worries that computers
humans and can work without tea-breaks. will replace human workers.
Easier to communicate with workers at Less face-to-face social interaction
different branches.
Work-rate of employees can be better Older employees may fear new technology and
monitored – e.g. an EPOS can monitor the find it difficult to adapt.
number of customers served each hour.
Employees fear that they will be spied upon

Teleworking
Teleworking is when an employee works away from the office and communicates with their
colleagues through the use of a computer and telecommunications systems (i.e. the Internet).

Often teleworkers are based at home, but they can also work from satellite offices or even be on the
move.

Benefits and drawbacks of teleworking

Benefits Drawbacks
It may be easier to concentrate on work in a Some teleworkers may find it difficult to
quiet environment at home than in a noisy separate home from work, and find work
office. encroaching on their leisure or family time.
Workers save on commuting time and costs, Management may fear difficulties in controlling
and there is the additional environmental a workforce that is not in the office.
benefits of keeping cars off the roads.
Workers enjoy greater flexibility, and can There is a problem in ensuring that remote
arrange working hours around other activities staff understand corporate goals and retain a
such as taking children to school. sense of loyalty to the organisation.
Employers save on the costs of office space Employees may feel isolated and miss the
and overheads such as heat and light. social environment of an office.
People can be recruited from a much wider Employees may find it difficult to work in
geographical area. teams, or to get help when they need it.
People who are not able to take employment in
standard office hours can be recruited.

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The digital divide
The ‘digital divide’ is the term used to describe the gap that exists between people who have access
to computers and computer-based information, and those who do not.

The digital divide usually refers to the differences between the poor and the relatively rich.
Unemployed people, for example, may not be able to afford to buy computers, pay a subscription
for Internet access or have the skills to be able to get certain types of job.
Another group of people who may find themselves disadvantaged are pensioners. Their opportunity
to access modern technology may be limited by their income, their background and the perception
that computers are for younger people.
There are many examples of organisations trying to reduce the effect of the digital divide. Schemes
exist to give people better opportunities of access. For example:
• there is subsidised or free access to computers and the Internet in many libraries;
• recycled computers are given to disadvantaged groups.
• pensioners and other groups are having training plans designed especially for them and
delivered at local schools and colleges.

Third world countries, already economically disadvantaged, are also affected by the digital divide.
Their citizens generally do not have the money to buy expensive computers and there may not be
the same opportunity of education that you and I take for granted – the priority is often simply to
find enough food for that day!

Environmental issues
The manufacture of computers requires energy. The majority of energy sources contribute in
various degrees to polluting the atmosphere and the environment – either by producing toxins or
greenhouse gases such as carbon dioxide.

Computers also require a variety of raw materials. Some of these raw materials are scarce and
expensive to obtain, both in terms of money and the energy that is required to extract them from the
ground.

It has been estimated that to build just one computer requires approximately ten times its weight in
chemicals and energy! Scientists at the UN University in Tokyo, have estimated that the
manufacture of just one computer requires:
• about 250 kg of fossil fuel.
• over 20 kg of chemicals.
• over 1000 different chemicals, including lead, arsenic, cadmium (all toxic).
• over 1500 kg of water.

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Around the world, computers continue to be made in huge quantities. Approximately 150 million
will be made in the next 12 months! If you then multiply this number by the amount of energy,
chemicals and water required, it is easy to see how computers contribute to pollution.
If you also consider that a typical PC will be in use for about 3-5 years (compared to 10-15 years
for a car and about 15 for a fridge), this is a further indication of how PCs contribute to the overall
pollution/green house gas levels.

European legislation has been introduced that demands 70% of a PC is recycled when it is scrapped.
Whilst governments have encouraged recycling, some people have argued that this is the wrong
approach. It takes a lot of energy, they argue, to recycle computers! It can also in itself be the cause
of environmental damage. For example, PCs are being transported to developing countries such as
China for recycling.
The process to recover metals such as copper, lead and gold from computer hardware involves the
use of acid baths – and this is not good for the environment. It is also not good for the people
directly involved in the recovery process. This is because health and safety standards are well below
those of the developed countries and the appropriate protective equipment is rarely provided.
A better plan of attack would be to reduce the number of computers being built in the first place.
This could be done, for example, by:
• Encouraging people to buy second-hand PCs – not many people know about the existence of
companies who buy and refurbish computers.
• Giving tax breaks to companies who buy second-hand PCs, rather than giving companies
tax breaks to buy new PCs.
• Refurbishing PCs and sending them to developing countries for use there.
• Encouraging people to upgrade existing PCs with new innovations, rather than replacing
their PC.
• Encouraging PC manufacturers to invest in making products last longer.
• Encouraging a paperless society. A lot of hard copies are still produced. The more paper
used, the more trees have to be cut down. Even if these are from renewable sources, turning
trees into paper involves energy as well as involves making parts of forests an eyesore.

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Privacy

55. Discuss the effects on privacy and confidentiality of data held
in computer systems, and steps that can be taken to protect
confidentiality.

Privacy is the principle of not allowing unauthorised people view certain data – particularly
personal data.

The right to privacy is a fundamental human right and one that we take for granted. Most of us
would not want our medical records freely circulated, and many people are sensitive about
revealing their age, religious beliefs, family circumstances or academic qualifications. In the UK
even the use of name and address files for mail shots is often felt to be an invasion of privacy.
When data/information is stored in a computer system such as a database, it is easy for
organisations such as credit card companies, banks, prospective employers, supermarkets that
operate loyalty card schemes, insurance companies and the security services to access and share
sensitive personal information.
Individuals give away personal information to computer systems sometimes without realising it.
For example, the loyalty card scheme operated by some major supermarket chains can represent an
invasion of privacy. A shopper using his/her loyalty card at the time of a purchase is inadvertently
giving away personal information such as
• spending power
• purchasing habits
• daily/ weekly movements
• whether married or not by correlating cards linked to one address

This information once in the store’s computer system could be passed easily to a third party without
the knowledge or permission of the shopper.

Protecting confidentiality
Protection is by:
• Passwords;
• Encryption;
• The Data Protection Act (DPA)

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Encryption

Encryption is the process of ‘scrambling’ data so that it cannot be interpreted by an unauthorised
person.

Data is encrypted using a special ‘encryption key’. In simple encryption systems, the same key is
used to decrypt the data, but in the most secure systems two different keys are used. In either
system, the data is kept secure by making sure that the decryption key is kept secret from anyone
other than those authorised to view the data.
The following encryption algorithm uses a 5-digit encryption key. To encrypt the message, each
character is converted into its ASCII code and the digits of the encryption key are then added to
each resulting code in turn. The encryption key is reused as many times as necessary.

This message can decrypted by applying the algorithm in reverse – i.e. subtract one from every
single character. Note that encryption done in this way would be fairly easy to crack.

Use of encryption
Encryption is used when transmitting data over a network and also when storing files securely on a
hard disk drive.

Benefits and drawbacks of encryption

Benefits Drawbacks
Private e-mails cannot be read if they are Employees within an organisation could send
intercepted. messages and let out company secretes
without the knowledge of their managers.
Credit cards can be used to shop on the Spies, criminal gangs and terrorists could send
Internet messages to each other without the authorities
knowing.
Files stored on a CD/DVD are still safe, even if The data is lost to everyone if the encryption
the CD is stolen. key is lost.
Secure bill payments using Internet banking.

ADDITIONAL NOTES:

166



Legal issues

56. Understand the need for legislation governing computer use.

There are four laws that are directly concerned with the protection of programs and data that are
held within a computer system. These are:

• The Computer Misuse Act (1990)
• The Copyright Designs and Patents Act – (Computer Programs) Regulations (1992)
• The Data Protection Act (1998)
• The Regulation of Investigatory Powers Act (2000)

There is also the Health and Safety at Work Act, which protects the health of employees who are
required to use computers.

The Computer Misuse Act (1990)
The Computer Misuse Act legislates against people accessing and modifying computer data without
permission.

The CMA defines three specific offences:
• unauthorised access to computer material (programs or data);
• unauthorised access to computer material with a further criminal intent;
• unauthorised modification of computer material (programs or data).

The first offence is aimed to stop hackers breaking into a computer system just to browse or simply
to show that they can.
The second offence is aimed at the hacker who has some definite and criminal objective in mind
when accessing a computer system – this would include the hacker who accesses credit card details
so that they can use them for the even more serious offence of fraud. This is a more serious crime
than the previous one because of the criminal intent behind the attempted access.
The third offence is intended to prevent unauthorised changes to the data or software stored on a
computer system – this includes changing exam grades or details of bank account transactions, and
also includes the deliberate spreading of a virus (since a virus changes data).

Note that there have been relatively few prosecutions under this law – probably because most
organisations are reluctant to admit hackers have overcome their security precautions.

ADDITIONAL NOTES:

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Copyright Designs and Patents Act (1988)
The original Copyright Designs and Patents Act covered intellectual property such as music and
literature, but in 1992 the Copyright (Computer Programs) Regulations 1992 was added to make it
illegal to:
• copy software without a licence;
• transmit software over a telecommunications line – thereby creating a copy
• sell illegally copied (pirated) software
• run pirated software;
• reverse engineer software – decompile or unassembled the program code so that sections
of it can be copied;
• lend software without a licence
• illegally modify software

These regulations were necessary because it is very easy to copy software and sell it on illegally. It
is also possible to ‘reverse engineer’ machine code to recreate the specific programming code that
has been used.

To help deter copyright infringements, some software manufacturers put ‘fingerprints’ into their
code – these are usually lines of program code that do not actually do anything. If these lines (i.e.
fingerprints) are found in a competitor’s program, then it is proof that it was copied illegally.

Software licensing
Software companies do not actually sell software – they sell a licence to use their software.
Normally there are limits on the way in which the software can be used. Typical types of software
licence are:
• Single user – here the user is allowed to install the software on just one computer.
Sometimes this will be extended to allow the user to install the software on both a desktop
and a laptop as long as only one of the copies is in use at any one time.
• Multiple user – organisations often buy multiple user licences. For example, an
organisation may buy a licence to allow 10 ‘consecutive’ users to operate software. With
these licenses, an organisation can often install the software on all their computers as long as
they can ensure that never more than 10 of them are running the software at any one time.
• Network – a network licence allows the software to be installed on all the computers on a
local area network. There are sometimes different prices for network licenses depending
upon whether a network is large or small.
• Site – a site licence allows the user to install the software on all machines – whether or not
they are networked – on a particular site. It is common for different prices to apply for
different site sizes (usually measured by the number of employees).

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The Data Protection Act (1998)

The purpose of the Data Protection Act is to prevent the misuse of ‘personal data’, which is stored
within a computer database or another system. It also to gives rights to individuals to whom the
information relates.

The Data Protection Act 1998 replaces the original Act of 1984 – mainly because the use of
computers makes it far easier to pass on and process data in large quantities.
Personal data is any data that relates to an individual where that
Personal information is worth
individual can be identified in some way (e.g. if their name or money
national insurance number is also stored with the data) A company that holds personal
The right to privacy is a fundamental human right and one that data could sell the details to
we take for granted. Most of us, for instance, would not want our other similar companies.
medical records freely circulated, and many people are sensitive A company that has personal
about revealing their age, religious beliefs, family circumstances data can contact an individual
and attempt to sell other goods
or academic qualifications. In the UK even the use of name and or services to them.
address files for mail shots is often felt to be an invasion of An organisation that holds
privacy. personal data will save
Nowadays, with so many large computerised databases it is quite advertising costs by being able
feasible for sensitive personal information to be stored without to target potential customers
with appropriate offers.
an individual’s knowledge and accessed by, say, a prospective
employer, credit card company or insurance company to assess
the individuals suitability for employment, credit or insurance.

Definitions

Personal data any data relating to a living person who can be identified from
it (for example, by name or National Insurance number).
Data subject an individual on whom data is being held.
The data controller the person within an organisation that has the responsibility of
controlling the way in which the organisation stores and
processes personal data.
The Information Commissioner the person responsible for enforcing the Act and making the
public aware of the act.

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The eight principles

The DPA consists of eight principles, summarised as follows:

Personal data shall be obtained and This means that either the data subject gives their
processed fairly and lawfully permission or the processing is necessary for one
of a number of specific reasons (e.g. for the
administration of justice).
Each time personal data is collected, the form,
which is completed, must contain a DPA statement
that informs the data subject that the data is going
to be stored and the purpose for which it will be
used.
Personal data shall be obtained for only This is the use given within the DPA statement – a
one or more specified purposes change of use is not allowed.

Personal data shall be adequate, relevant This avoids the temptation of collecting data that is
and not excessive not needed.

Personal data must be accurate and,
where necessary, kept up-to-date

Personal data must not be kept longer
than necessary

Personal data shall be processed in This means that the data subject has the right to:
accordance with the rights of the data • see all data that is stored about them and
subjects have incorrect data corrected or deleted;
• prevent processing that is likely to cause
damage or distress;
• prevent processing for the purpose of direct
marketing;
• compensation for unauthorised access,
loss or destruction of data.
Appropriate technical and organisational This must prevent the unlawful processing of
measures shall be taken to ensure the personal data and the accidental loss or
security of the personal data destruction of personal data.
Personal data shall not be transferred to a Unless the country in question ensures an
country or territory outside the European adequate level of protection for the rights and
Economic Area freedom of the data subjects

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170



Note that the DPA requires that an organisation that wants to store and process data must:
• nominate a data controller, who will be responsible for ensuring that the organisation
adheres to the principles of the DPA;
• register their intention to store personal data with the Information commissioner;
• inform the potential data subjects that their intention is to store data and get the permission
to do so;
• inform the data subjects of the reasons for gathering the data;
• gather only information that is truly needed and gather it opening and lawfully.
• obtain the data subject’s permission to pass on data that relates to them;

So far as keeping the data after collection the organisation must ensure that the data is kept up to
date; not held longer than is necessary and allow individuals to view data held on them and make
corrections if necessary.

The type of information covered by the DPA
The type of information that is covered by the act is ‘personal data’ that is held in a form that is
structured – e.g. arranged alphabetically. This includes all personal data that is held within
computer system and any personal data that is stored in a filing cabinet where the files are arranged
according to name or a similar field.
Note that some data is considered to be sensitive personal data. This is data concerning racial or
ethnic origin; political opinion; religious beliefs; physical and mental health of the data subject; etc.
There are additional conditions that apply to the processing of such sensitive data.

Exemptions
For practical reasons not all data is covered by the DPA. For example if the data:
• are for personal, family, household or recreational use;
• relates to payroll, pensions and accounts;
• is being held for the prevention or detection of crime.

ADDITIONAL NOTES:

171



The Regulation of Investigatory Powers Act

The Regulation of Investigatory Powers Act 2000 replaces the Interception of Communications Act
1985 to take account of technological advances in communications and to accommodate the
expanding use of e-mail and the internet.
The Act regulates the power of government security services and law enforcement authorities by
allowing the interception, surveillance and investigation of electronic data in specified situations
such as when preventing and detecting crime. Powers include being able to demand the disclosure
of data encryption keys.
In addition, the Act empowers the Secretary of State to make regulations which allow businesses to
intercept communications in the course of lawful business practice and in specific circumstances
without the express consent of either the sender or the recipient. Under the regulations, businesses
are required to make all reasonable efforts to inform users of their own systems that such
interceptions might take place.
The Regulations allow businesses to intercept communications without consent in the following
circumstances:
• to establish the existence of facts – for example to obtain evidence of a business transaction;
• to ascertain compliance with regulatory or self regulatory practices or procedures relevant to
the business – to ascertain whether the business is abiding by its own policies;
• to ascertain or demonstrate standards which are achieved or ought to be achieved by persons
using the system – for example staff training or quality control, but not for market research;
• to prevent or detect crime – including crimes such as fraud as well as infringement of IT
related legislation such as the Computer Misuse Act 1990 or the Data Protection Act 1998;
• to investigate or detect unauthorised use of the systems – e.g. to check whether the user is
breaking regulations;
• to ensure the effective operation of the system – e.g. to protect against viruses or other
threats such as hacking or denial of service attacks, to monitor traffic levels, to forward e-
mails to correct destinations;
• to check whether or not communications are relevant to the business – e.g. checking email
accounts when staff are absent on holiday or sick leave to access business communications;
• to monitor (but not record) calls to confidential, counselling help-lines run free of charge by
the business, provided that users are able to remain anonymous if they so choose.

ADDITIONAL NOTES:

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