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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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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