Cambridge International AS and A Level Computing: Coursebook

LEADBETTER, BLACKFORD AND PIPER: COMPUTING (AS AND A LEVEL) CVR C M Y K

Coursebook

Chris Leadbetter, Roger Blackford and Tony Piper Based on the new Cambridge International Examinations AS and A Level Computing syllabus (9691), this coursebook is written by experienced authors. It follows the syllabus in a chronological fashion with four Modules to match the four Sections of the syllabus and each Module divided into chapters to match the syllabus subsections. Material that is not required for the exam is highlighted as ‘Extension’ material allowing students to investigate topics of particular interest in more depth, whilst reassuring other students that it is detail they can ignore if they want to.

The coursebook contains: - narrative in plain English suitable for International students throughout the world - clear explanations of concepts using diagrams, screenshots and photos - self-assessment questions with hints at the end of each chapter, and answers at the back of the book, to give students the opportunity to assess their own development and/or revision needs - exam questions from real past papers at the end of each unit to give the students plenty of exam practice - Syllabus codes throughout the text to allow easy crossreferencing to the syllabus - Activity and Discussion boxes throughout the book that can be used for classroom work or independent investigation - a chapter devoted to exam preparation to help students get ready for the exam - a glossary to allow quick access to key and/or difficult terms.

Completely Cambridge – Cambridge resources for Cambridge qualifications Cambridge University Press works closely with Cambridge International Examinations as parts of the University of Cambridge. We enable thousands of students to pass their Cambridge exams by providing comprehensive, high-quality, endorsed resources.

To find out more about Cambridge International Examinations visit www.cie.org.uk Visit education.cambridge.org/cie for information on our full range of Cambridge International A Level titles including e-book versions and mobile apps.

Leadbetter, Blackford and Piper

9780521186629

Computing

Cambridge International AS and A Level Computing Coursebook

Cambridge International AS and A Level

Chris Leadbetter, Roger Blackford and Tony Piper

Cambridge International AS and A Level

Computing Coursebook

Chris Leadbetter, Roger Blackford and Tony Piper

Cambridge International

AS and A Level Computing Coursebook Completely Cambridge – Cambridge resources for Cambridge qualifications Cambridge University Press works closely with University of Cambridge International Examinations (CIE) as parts of the University of Cambridge. We enable thousands of students to pass their CIE Exams by providing comprehensive, high-quality, endorsed resources.

To find out more about CIE visit www.cie.org.uk To find out more about Cambridge University Press visit www.cambridge.org/cie

cambridge university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Mexico City Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK www.cambridge.org Information on this title: www.cambridge.org/9780521186629 Š Cambridge University Press 2012 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press.

First published 2012 2nd printing 2013 Printed in the United Kingdom by Latimer Trend

A catalogue record for this publication is available from the British Library ISBN 978-0-521-18662-9 Paperback Cover image: Gusto images/Science Photo Library Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Information regarding prices, travel timetables and other factual information given in this work is correct at the time of first printing but Cambridge University Press does not guarantee the accuracy of such information thereafter.

Contents Inroduction

v

Acknowledgements

vi

1

Computer systems, communications and software

1

1.1

Components of a computer system and modes of use

2

1.2

System software

4

1.3

Data: Its representation, structure and management

15

1.4

Hardware

30

1.5

Data transmission and networking

44

1.6

Systems development life cycle

54

1.7

Choosing appropriate applications software

66

1.8

Handling of data in information systems

76

1.9

Designing the user interface

87

1.10

Logic gates

91

Module 1 Exam questions

98

2

Practical programming techniques

107

2.1

Designing solutions to problems

108

2.2

The structure of procedural programs

122

2.3

Data types and data structures

142

2.4

Common facilities of procedural languages

155

2.5

Writing maintainable programs

163

2.6

Testing and running a solution

171

Module 2 Exam questions

177

Contents

iii

3

Systems software mechanisms, machine architecture, database theory, programming paradigms and integrated information systems

181

3.1

The functions of operating systems

182

3.2

The functions and purposes of translators

195

3.3

Computer architectures and the fetch–execute cycle

206

3.4

Data representation, data structures and data manipulation

212

3.5

Programming paradigms

233

3.6

Databases

262

3.7

Simulation and real-time processing

274

3.8

Networking

281

Module 3 Exam questions

292

4

iv

Computing project

299

General description of the computing project

300

4.1

Quality of report

304

4.2

Definition, investigation and analysis

305

4.3

Design

310

4.4

Software development, programming, testing and installation

315

4.5

Documentation

319

4.6

Evaluation

322

Revision guidelines

324

Answers to Test yourself questions

329

Answers to Exam questions

360

Glossary

415

Index

430

Contents

Introduction This full-colour, highly-illustrated textbook has been written by experienced authors specifically for the University of Cambridge International Examinations AS and A Level Computing syllabus (9691). The four modules of the book follow the four sections of the syllabus, and there are syllabus codes <insert fig 1 here> throughout to provide an easy reference to the syllabus.

This enables students and teachers alike to make sure they are on track to cover the complete course.

The narrative provides clear explanations that are supported by diagrams, screenshots and photographs, bringing the topics to life. At the end of each chapter, there are self-assessment questions with answers at the back of the book. These relate to the material covered in the chapter, and offer an opportunity to revise concepts just learned, or to test understanding. Whilst these are not examination questions, each comes with a mark allocation to get the students into the practice of always thinking carefully about each point they make. The questions often include useful ‘Hints’ from the authors, particularly with respect to mark allocations. The answers also include ‘Comments’ from the authors in places where there are common pitfalls or typical errors.

Throughout each chapter there are ‘Activity’ and ‘Discussion’ boxes (see images below). These are questions or points for individual or class discussion.

These allow students to explore the topics in more detail and really test their understanding.

Discussion

At the end of each Module, there are examination questions from real past papers. These will cover content from all the chapters in the Module in a random order.

Each has the real mark allocations indicated and gives the students targeted exam practice for the given topics of that unit. The answers to these questions are provided at the back of the book, complete with ‘Comments’ from the authors in places where, in their experience, students have struggled in the examinations. There is also a chapter at the end of the book devoted to exam preparation that offers student general advice for coping with the examinations.

However, learning is about more than passing an examination; it is about increasing knowledge.

Therefore, there are numerous places where the book goes beyond the scope of the syllabus. This ‘Extension’ material (indicated by the boxes shown below) is there for those that want to carry out independent study or further research into the given topic. Please note that this material is not in the syllabus and therefore will not be in the examination.

Extension

Activity Finally, there is the glossary of key terms. Here, you should find an easily accessible means to remind yourselves of the important terms you have learned as you progress through the course. We hope you enjoy the textbook and the course.

The author team.

Introduction

v

Acknowledgements We would like to thank the following for permission to reproduce images: Cover: Gustoimages/Science Photo Library 1.2.7 Reproduced with the permission of Winzip Computing; 1.4.3 Shutterstock / Joel Dietle; 1.4.5 Shutterstock/ Kostia; 1.4.6 Shutterstock / Stephen Coburn; 1.4.7 Shutterstock / Vixit; 1.4.8 Shutterstock / Iakov Filimonov; 1.4.9 Shutterstock / OtnaYdur; 1.4.10 Shutterstock / shutswis; 1.4.11a Shutterstock / Daboos; 1.4.11b Shutterstock/ 3d brained; 1.4.12 Doug Steley A / Alamy; 1.4.13a Ted Foxx / Alamy; 1.4.13b Shutterstock Dja65; 1.4.13c Shutterstock / Robert Milek; 1.4.14 Courtesy of mucad.com; 1.5.1 Shutterstock / Deymos; 1.7.1 Chris Fertnig / Alamy; 1.7.2 Shutterstock

vi

Acknowledgements

/ Yuri Arcurs; 1.7.3 Shutterstock / Chuck Rausin; 1.7.4 Shutterstock / SFC; 1.7.8 aberCPC / Alamy; 1.8.1 Courtesy of Tom Dalton, Assistive Technologies; 1.8.2 focus mankind / Alamy; 1.8.4 Shutterstock / Jiri Hera; 1.8.5 Len Holsborg / Alamy; 1.9.1 Reproduced with the permission of Articulate; 3.7.1 Reproduced with the permission of LEGO速 MINDSTORMS速; 3.7.2a JHP Public Safety / Alamy; 3.7.2b Shutterstock / lenetstan; 3.7.2c Art Directors & TRIP / Alamy; 3.7.3 Shutterstock / John Kasawa; 3.7.4 Francisco Cruz / Superstock; 3.8.1a Wendy White / Alamy; 3.8.1b Sergey Galushko / Alamy; 3.8.1c: Hugh Threlfall / Alamy.

Examination questions are reproduced by permission of University of Cambridge International Examinations.

1

Computer systems, communications and software

1.1

Components of a computer system and modes of use

A “computer system” is made up of hardware and

flow of data and information. The storage device is both

software. In this chapter we explain the terms relating

an input device and an output device.

to a computer system that will be used throughout the rest of the coursebook.

1.1 a, b

main memory (hard disk drive)

Definitions and purposes of hardware devices

input device

The hardware is the physical electronic and electrical T components that make up the computer system. For

processor (CPU)

output device

storage device

example, these include the motherboard, a keyboard and a monitor. In this section, we explain terms relating

Figure 1.1.1 Fi 1 1 1 The Th typical i lh hardware d off a computer system.

to hardware devices that we use throughout the rest of the text.

In Chapter 1.4, we look at various types of hardware

Peripheral devices are hardware devices that

in more detail.

are outside the central processing unit (CPU). They are normally connected to the computer by internal

1.1 a, c

Definitions and purposes of software types

wiring (buses), cables or wireless technology. A

Software is the set of instructions that make the

printer is obviously a peripheral as it is separate from

computer hardware usable. These sets of instructions

the computer. A hard disk drive is also a peripheral

are known as “programs”. So, a computer program is a

because it is not part of the CPU. This is less obvious

piece of software. It is important to distinguish between

because it is in the same box as the computer.

operating system software and generic applications

Most peripheral devices are one of three types: ●

the computer processor. A computer is only of

●

Operating system software is the set of instructions that make the hardware of the computer available for

value if it is possible to give it data and processing

use. It includes programs:

instructions.

●

An output device conveys the results of processing from the computer processor to the user.

●

software.

An input device transmits data from the user to

computer ●

A storage device can store data outside the processor. When a computer system is switched off, data in the processor are lost. Data that are needed for future use must be saved to a storage device

that allow the user to communicate with the that control the movement of data around the computer

●

that turn instructions that people can understand into instructions that a computer can recognise.

Applications software is a set of instructions designed

Figure 1.1.1 shows a typical set of hardware in a

to make the computer do something for the user. For

computer system. The arrows show the direction of

example, the user might want to write a letter, create a report or create a slide show presentation.

2

Module 1

Software that can be used for many tasks is

can process payroll data, print payslips and

called generic applications software or general purpose

generate BACS payments. It cannot process electricity

applications software. Microsoft Office is a suite of

billing data.

generic applications, including Microsoft Word (a

In Chapter 1.2, we discuss types of system software

word processing application), Microsoft Excel

in more detail. You will notice that applications

(a spreadsheet application) and Microsoft PowerPoint

software is covered throughout much of the rest

(a presentation application).

of the syllabus, which should give you an idea of

Other applications software carries out a specific sequence of tasks. For example, a payroll application

its importance. Chapter 1.7 looks specifically at applications software.

Summary ●

Hardware is the physical components that make up the computer system.

●

Software is the set of instructions (programs) that make the computer hardware usable.

●

An input device is a physical component that transmits data to the computer.

●

A storage device is a physical component that stores data for use by the computer.

●

An output device is a physical component displays data transmitted from the computer.

●

Operating system software comprises the programs that make the computer hardware available for use, often unseen by the user.

●

Generic/general purpose applications software comprises programs that can be used for many tasks, such as a word processor, spreadsheet software and database software.

Test yourself 1. Explain the difference between hardware and software.

[2]

Hint The question mentions two items (“hardware” and “software”) and two marks are available.

Therefore, you should make two points. Make sure that the second point is not just the opposite of the first. The word “explain” indicates that the two points need to say how the items differ.

2. Give two reasons why a computer system would need to have some type of external storage device.

[2]

Hint Notice that the question asks for reasons why the system needs external storage. Do not say what is stored or what sort of device might be used. There are two indicators in the question that you should make two points: the word in bold and the marks awarded.

3. Describe the difference between system software and application software.

[2]

Hint This question is similar in style to Question 1. Remember that you need to state two distinct points that are related to each other.

1.1 Components of a computer system and modes of use

3

1.2 1.2 a

System software

The purpose of an operating system T

●

An operating system (OS) is the main item of system A

The OS provides a platform on which the applications software can run.

software used by a computer system. The OS is a set

●

of programs designed to run in the background on a

The OS hides the complexity of the hardware from the user.

computer system and provide an environment in which

●

application software can be executed.

The OS controls access to the computer system so that you can put passwords on your files to stop

Most operating systems comprise a large set

other people seeing them. It controls the access

of programs, only some of which are stored in the

rights of different users.

computer’s memory all the time. Many of the routines

Whether or not a certain feature is classified as

available in the OS are stored on the hard drive so that

part of the operating system or as a utility program

they can be accessed when required. This saves space

varies considerably across operating systems. We

in the computer’s main memory .

consider utility software in more detail at the end of

When you are using applications software, you

this chapter.

are not communicating directly with the computer hardware. Your applications software communicates

1.2 b

Types of operating system T

with OS program modules that communicate with the

Batch

computer hardware on its behalf. Without an operating

When computing was still a new science, there

system, a computer is useless no matter how many

were not enough machines to satisfy the demand

programs you have.

for processor time. There was a “speed mismatch”

There are many different types of operating system.

between the user sitting at the keyboard, who was very

Each type makes the computer behave differently

slow, and the processing by the computer, which was

and is appropriate to a given computer system and its

very fast. One solution to this problem is to buy more

applications. We look at some of the different types of OS

machines. Another solution is to make the machines

in this chapter and again in other parts of the syllabus.

work more effectively by taking away the slowest part

All operating systems have some common purposes:

of the system – the human being. Nowadays we might

The OS must manage the physical resources of the

well opt to buy more machines! When computers were

computer. Some resources are limited and must be

very expensive, an aim for efficiency improvements

managed to maximise the use of the computer system:

gave rise to batch processing.

●

− A simple system has only one processor.

for interaction between the user and the processor

− Some input/output devices (e.g. printers) are

during the execution of the program. Lots of programs

shared. ●

4

A batch processing operating system does not allow

− Secondary storage is of a fixed size.

(or lots of data to be run through the same program)

The OS provides a means of communication (the

are collected together (to form a batch). The batch

user interface) between the human user, or the

operating system then controls their passage through

outside world, and the computer.

the computer.

Module 1

on the floor. The processor makes decisions about

Nowadays, batch processing is used for applications

steering to keep the trolley on the black line. The

where: ●

there are large amounts of data to be processed

●

the data are very similar in nature and require similar processing

●

●

trolley would very soon lose its direction if it was not steering quickly enough. ●

A catalogue shop processes orders. The code for a

the computer system has identifiable times

product is input and the system compares it with

when it is not being used and so has available

information in its files. When it finds the correct

processor time

code, it can report to the user the quantity of that

the application does not require interaction by

product in stock. It is necessary to record a purchase

a user.

before the next shopper’s request is processed

Typical examples of applications which use batch

otherwise the second person might be sold the

processing include:

same item.

●

the processing of payroll information

●

the production of bank statements from customer files at periodic intervals; e.g. all accounts with surnames starting A–E could be processed on the fifth of each month, surnames F–J on the tenth of each month, and so on

●

the production of utility (gas, electricity and telephone) bills from customer records.

Real-time A real-time operating system can react quickly enough to affect the next input or process that needs

Single-user A single-user operating system controls a system which has only one user, and their programs, at any one time. A perfect example of a single-user system is the PC or laptop that you may have at home. Only one person uses the computer system at any one time. Note that it does not mean a system that only ever has one user! This means that security measures are important so that the system only allows access to files that belong to the appropriate user.

to be carried out. There is a continuous cycle of input– processing–output.

Most real-time systems are based on the control of

Multi-user Again, as the name implies, this type of operating

some process in the real world (where the processing

system allows the computer to service more than one

takes place on a timescale of milliseconds). Some

user simultaneously. A multi-user operating system

information processing applications require the

has a single (normally powerful) computer which is

processing to be done within seconds; it still fits the

connected to a number of terminals. These terminals

description of the continuous cycle and is known as a

are not computers, although they may have a very

pseudo real-time operating system.

limited amount of processing power. They rely on the

The following examples of real-time applications show why immediate response can be vital: ●

The computer sends a message to each of the

A chemical plant has a reaction vessel in which

terminals in turn, asking if it wants any processing

the temperature is critical to the result of the

to be carried out. This process is called a time-share

process. The temperature is monitored by a

system or a round robin system. Each of the small

computer that accepts input from a sensor. The

amounts of processor time is called a “time slice”. The

computer uses the sensor data to make decisions

length of a time slice varies according to the system but

about adjusting the heating elements in the vessel.

is typically about a hundredth of a second.

A delay in the decision-making process might ●

main computer to carry out any processing.

A configuration in which a central computer

corrupt the reaction.

(a server) processes data from several terminals

A robot trolley is controlled by a processor that

(see Figure 1.2.1 on page 6) is called a “thin-client

takes input from a sensor following a black line

network”.

1.2 System software

5

terminal 1

terminal 2

terminal 3 server stores: - application programs - users’ data files

network cable server terminals do not have any processing capability

server processes each terminal’s application program

Figure 1.2.1 A multi-user computer system. Since terminals do no processing this is a thin client network.

There is a limit to the number of terminals that can

Network

be serviced. The system needs to be fast enough that

A network comprises a number of computers linked

a user at a terminal does not have to wait for too long

together for the purposes of communication and the

before being given processing time. The system can be

sharing of resources. Networks are important because

speeded up if the computer ignores terminals that are

they allow hardware and software to be shared. They

not active. Terminals can send a signal to the computer

require the computers to run a network operating

when they need processor time. Such a system is

system. Often, one of the machines is used to control

only satisfactory if the response time for all users is

the rest of the system; this machine is called the server.

acceptable; no user should have to wait for a long time

A network OS must carry out tasks such as:

for a program to respond.

Multi-tasking

●

control of access to the network

●

management of the filing system

●

management of all applications programs available

A multi-tasking operating system allows several

from the server

applications to be available simultaneously. On a

●

single-user system, you are probably used to having

A common misunderstanding is to confuse network

a number of processes loaded in main memory

systems with multi-user systems. A multi-user system

at the same time. One window may show a spreadsheet

has many users using one computer at the same time;

while another shows a word processing application;

a network system has many computers each using

you may also be connected to the Internet through an

shared hardware and software.

email client; and you may be playing music while you

management of all shared peripherals.

In Chapter 3.8, we consider networking in detail.

work. It appears that more than one task is running simultaneously but they aren’t, they just appear to be.

1.2 c

Application requirements for operating systems A

The OS can switch between tasks so quickly that it

IIn an examination, you will be asked to identify

seems to the user as if they are all being done at once.

applications that require batch processing and

The method used to do this is very similar to the multi-

applications that require real-time processing.

user OS. The tasks are allocated time slices and the

Applications that require batch processing include:

computer goes from one to the other servicing them.

payroll, the production of bills and the production

Microsoft Windows is a multi-tasking operating

of bank statements. Be very careful in choosing any

system. You can view a list of the running processes

other application – it must be clear that there are

in the Task Manager operating system utility

good reasons for using batch processing. For example,

(Figure 1.2.2). In Chapter 3.1, we further consider

simply to say that an ATM machine uses batch

multi-tasking in Windows.

processing is not enough. Requests for bank statements

6

Module 1

Figure 1.2.2 The Windows Task Manager utility.

are batched for later processing but the PIN for a

sensible to try to memorise a list of applications for

customer must be checked in real time. You must be

each type. You must learn to identify the characteristics

specific and justify your choice. For example, you could

of each of the types of processing for a given

write “Because statements must be sent out by post,

application.

they can be processed while the computer is not doing other things. It makes sense to process a large number of statements at the same time as a batch.”

1.2 d

Types of user interface T

Control applications, in which the results of a

The user interacts with a computer system through its T

process are used to inform the next input, use real-

user interface. The user gives input (e.g. the click of a

time processing. A good example would be the

mouse) and receives responses from the computer. A

control of a robotic machine on a production line.

user interface (also called a human–computer interface

Information systems also require real-time processing.

(HCI)) consists of both hardware and software.

It is necessary to update the file of information before

When a user types instructions into a computer and

the next enquiry is dealt with. The classic example is

the computer responds by displaying details on a screen,

the airline (or theatre) booking system. If a customer

then that is an interface. The keyboard and the screen

decides to buy a ticket for a flight, the number of tickets

are the hardware components; the software components

available must be updated before the next person

of the interface allow the computer to understand the

makes an enquiry, otherwise another person may be

typed instructions. In the early days of computing,

sold the same seat.

people could use a teleprinter instead of a monitor.

In general terms, an examination question will ask

The teleprinter was similar to a typewriter. As the user

you to decide which of these two types of processing

typed commands on it, the computer would print the

is most appropriate for a given application. It is not

commands and its response on paper. The hardware

1.2 System software

7

The interface:

and software components of the interface need to be appropriate to the use. For example, a keyboard and

●

screen are needed for an interface that enables the user to make enquiries about theatre tickets; a printer would

in turn ●

enable the user to print a booked ticket.

There are many different types of interface. Their

prompts the operator to ask each of the questions makes the operator input the information in the correct order and ensures that nothing is missed out

●

features vary depending on the application, the

ensures that the information is input in the correct format

conditions in which it is to be used and the knowledge

●

and skills of the user. From the many types of HCI, you are

A form-based interface (see Figure 1.2.3) has a specific

expected to be able to describe the five software interfaces

area for each piece of data. For example, there may be:

discussed below and give a typical application for each

●

of them. In Chapter 1.9, we discuss the hardware that is

makes the checking of the information easier.

a drop-down list for restricted input (such as the person’s title or a date)

necessary to put the software interface into operation.

●

Form-based interface

●

boxes for textual information (such as the name or email address)

If the majority of the input to a system is standardised,

check boxes for yes–no information (such as a box to indicate acceptance of conditions).

a typical interface is an on-screen form to be filled in.

The form uses standard widget controls – text boxes,

This type of interface is used by an operator inputting

radio buttons, check boxes and drop-down lists.

information while talking to a customer over the

automatically to the next box to be filled in. Sometimes a

telephone.

Figure 1.2.3 Form-based interface.

8

When the user enters data, the cursor often moves

Module 1

box is highlighted to make it clear to the operator where

of types of accommodation (hotels, guest houses, etc.).

the data are to be inserted. Some of the data are more

The third screen may offer different price bands. Finally,

important than others and the cursor does not move on

the user may see a list of all the available properties

until data have been supplied. The interface checks that

that match the choices of type and price band. Such an

the input is sensible for that box before moving on.

information system often uses a touch screen because

When a person orders something from a supplier

other peripherals are less appropriate. For example, a

on the Internet, a form-based interface is used. The

mouse connected to an information system in a train

customer fills in a number of standard personal and

station might be vandalised or damaged by the weather.

payment details.

Graphical user interface Menu-based interface

A graphical user interface (GUI) uses windows, icons,

A menu-based interface is used in situations where

menus, and a pointer (giving rise to the acronym

the user does not know what options are available.

WIMP). The user of a GUI expects to be able to view

Examples of this are information systems for tourists

different applications or files on the screen at the

and the on-screen menus for digital television. A list

same time. This is done by putting each into its own

of choices is made available. When the user selects an

bordered area known as a window. The user can select

option, a further set of choices is displayed, and so on

a command by clicking on a small picture (an icon)

until the result is obtained.

that represents it. The user selects further options from

Imagine an information system at a train station in a popular holiday location. The first screen may ask for the general area of interest (accommodation,

menus. Icons and menu options are selected by using some sort of pointing device such as a mouse.

In Figure 1.2.4, the main menu is displayed

trips, shopping or entertainment). If the user selects

horizontally as text options (File, Edit, etc.). Selecting

â&#x20AC;&#x153;accommodationâ&#x20AC;?, the next screen may offer a choice

Shape from the menu opens a drop-down menu of

Figure 1.2.4 Graphical user interface.

1.2 System software

9

selections. Selecting Rotate or Flip gives access to a

simple responses which the computer can understand.

final menu from which the user can select the final

If the user does not give one of the expected responses,

command – Rotate Right. The user can save a file in one

a message is produced which makes it clear that the

step by clicking the “Save” icon (it represents a floppy

user must try again.

disk); in Figure 1.2.4 you can see this button directly under the “Edit” button.

The whole principle of a GUI is to make using the

It is often assumed that a natural language interface is very close to a spoken language. This may be true but it is likely to be typed into the machine rather than actually

system as simple as possible by hiding all the complicated

being spoken. For example, some Internet search engines

bits! For example, when the user clicks an icon to bring

or large information systems can process natural language

a piece of software onto the screen, it involves only one

queries. This is the next step up from a simple keyword

action. In the background, the computer is executing a

search, where the software picks out the keywords and

complicated process involving a lot of instructions. When

then searches for matching documents or files.

the icon is chosen, the computer is simply told to run

Computerised telephone systems may use voice

those instructions so the software will appear. The icon

input to select menu choices. A true spoken interface

has hidden the complexity from the user.

might be used by a disabled person to communicate with a computer.

Natural language interface A natural language interface is sometimes referred to

Command line interface

as a “conversational interface”. The computer displays

In a command line interface, or “command-based

or speaks questions that give the user the impression

interface”, the user types a series of commands at

that the computer is talking to them. However, the

the keyboard and the computer displays appropriate

system restricts itself to questions that will provoke very

output (see Figure 1.2.5). The computer does not

Figure 1.2.5 Command line interface.

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prompt the user to enter any particular information. If

The disk surface is divided into a number of

the user enters a command incorrectly, the computer

tracks and each track is divided into smaller blocks

will give an error message.

called sectors.

To use a command line interface, the user needs:

The amount of information that can be stored is

●

to know what commands are available

enormous. One of the tracks is used as an index, to

●

to understand the commands

hold data about where the other data are stored. The

●

to understand how material is stored in the

formatting process removes all data from the disk so

computer system.

you should save the contents of a disk elsewhere before

A command line interface, has two very important

re-formatting it.

characteristics: ●

The computer system is very much more open than

Hardware drivers

in the other types of interface. Other interfaces

Any piece of hardware needs to be controlled and

restrict the options that the user has available to

set up for communication with the processor.

them. A command line interface allows anyone with

The hardware drivers are programs that control

a knowledge of the commands access to the entire

the communication between the device and the

operating system. Hence, they have access to the

operating system.

workings of the entire computer. ●

Command line interfaces can only be used by people who are computer literate. Users need to understand the commands and their uses. They also need to understand something about how the computer operates and how information is stored.

1.2 e

File handling Data stored on a computer system are organised as files. A set of utility programs is needed to handle the storage and use of these files. There are programs to store and retrieve the files and programs that allow files

Utility software

to be altered or deleted. Sometimes the contents of two

Programs that carry out tasks necessary to the operation P

files need to be combined (merged) or the contents of a

of the computer are known as utility software.

file may need to be put into a particular order (sorted).

Disk formatter When a disk is first produced the surface is blank.

It cannot be used to store data until it has been formatted by disk formatter software. The formatting process divides the disk into smaller areas, each of which can be searched more easily (see Figure 1.2.6).

A file handling utility normally does all these tasks.

If an examination question asks for two examples of utility software, you should not describe two file handling routines.

Automatic backup or archive Files on a computer system need to be protected from being damaged. A backup or archiving utility is a simple routine that copies the contents of files to

track

another location. If the original file is damaged, the copy can be used to replace the damaged file.

index track

If an examination question asks for an example of a utility, simply saying “backup” is not a satisfactory response. You can make a backup manually as a simple file copy process. A good example of a utility program

sector

would be one that automatically backs up the changes to files since the previous backup copy was made (incremental backups).

Email client software encourages the user to

Figure 1.2.6 A formatted hard disk.

regularly archive files to reduce storage space.

1.2 System software

11

Anti-virus software

File compression software

A computer virus is a small program that can reproduce

Files containing sound and video information are

itself. If it “infects” a computer system, a virus can

usually large in size. File compression software

cause a range of damage including deleting files.

reduces the size of a file by cutting out much of the

An anti-virus software utility program continually

duplication of data in the file.

compares a dictionary of known viruses against the

If a file is to be sent electronically (e.g. as an

files accessed by the computer. It alerts the user if any

email attachment), you can use a utility (such as

of the files matches a virus “signature”. It is crucial that

WinZip, shown in Figure 1.2.7) to compress it. Files

the virus dictionary is kept up-to-date on a daily basis

may also be compressed to save space on a secondary

as new viruses are constantly appearing.

storage device.

Figure 1.2.7 WinZip file compression software.

Summary ●

An operating system: – controls the hardware – provides a platform for application software to run on – provides an HCI – manages the resources of the computer system.

●

A batch operating system controls the passage of jobs through the computer without user interaction.

●

A real-time operating system reacts quickly enough to affect the next input or process that needs to be carried out. It operates a continuous cycle of input–processing–output.

12

●

A single-user operating system enables only one user at a time to access the system.

●

A multi-user operating system enables more than one user to access the system at the same time.

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Summary continued … ●

A multi-tasking operating system gives the user the impression that they can carry out more than one task at the same time.

●

A network operating system links a number of computers together and enables them to share peripherals.

●

Applications that require batch processing include: – payroll – bank statements – utility bills

●

Applications that require a real-time response include: – industrial control systems – robots – ticket-booking systems.

●

A form-based user interface provides boxes into which the user can type data. It provides the user with help (on-screen prompts and validation) in completing the data.

●

A menu-based user interface provides the user with a set of options that restrict the available information. An information system for tourists and on-screen menus for digital television are easy for users to operate.

●

A graphical user interface (GUI) provides windows, icons, menus and a pointer to enable the user to interact with the computer in complex ways.

●

A natural language user interface enables the user to use a natural language (such as English) to interact with the computer. It may be spoken or typed input.

●

A command line user interface requires the user to type commands to give specific instructions to the computer. It enables a technician to get close to the workings of the computer.

●

Disk formatting software prepares a disk for use by the operating system.

●

File handling software enables the user to move, copy and delete files.

●

Hardware drivers enable successful communication between devices and the operating system.

●

File compression software allows data to be stored in a smaller amount of storage space.

●

Virus-checking software monitors input and stored data to ensure that it does not contain malicious software.

Test yourself There are some areas of the specification where the topic is so narrow that only one question can possibly be asked.

It may look different from one exam paper to the next but really it is testing the same knowledge. Question 1 is from one of those sections.

1. State three purposes of an operating system as part of a computer system.

[3]

Hint There are many possible responses to this question. It asks for three purposes and awards three marks, one for each purpose. The word “state” indicates that you simply need to list the purposes.

1.2 System software

13

2. a. Distinguish between a multi-tasking and a multi-access operating system.

[2]

Hint When a question asks you to “distinguish” two things, you must choose facts that compare the things. b. State what is meant by a network system and give an advantage of this type of system over a set of stand-alone computers.

[2]

Hint When a question asks you to “give an advantage”, it usually tells you what to compare. In this instance, you must compare a network system with stand-alone computers.

3. A company payroll system uses a personnel file. a. Explain the difference between batch processing and real-time processing.

[2]

b. Explain how both batch processing and real-time processing can be used sensibly in the context of the payroll being calculated using the personnel file.

[4]

Hint Notice that you are not asked for separate uses of batch processing and real-time processing.

You must consider how batch processing and real-time processing apply to the given application.

4. A computer operator takes phone calls from the public who ring up asking whether a particular item in a catalogue is available. The operator needs to type in a series of responses to questions put to the caller, so that the computer can check the file and determine whether that item is available. Suggest what type of interface is appropriate and its possible contents in terms of options presented to the user.

[4]

Hint Notice that four marks are available. You are asked to suggest an interface design and its contents.

Assume that one mark will be allocated to the interface type and give three or four contents of the interface you suggest. The contents must be based on the scenario described in the question.

5. The technician responsible for maintaining the system in Question 4 uses a command line interface. a. Explain what is meant by a command line interface.

[2]

Hint Two marks are available, so you should make two points. b. Give two advantages and one disadvantage to the technician of using a command line interface rather than a menu-based interface.

6. a. Explain the need to have driver software installed for a printer.

[3] [2]

b. Give three different utility programs which would be part of a single-user operating system and state what each program would be used for.

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Module 1

[6]