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Cambridge

International AS & A Level

Computer

Science

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457591_FM_CI_AS & A_Level_CS_i-xiv.indd 2 4/30/19 7:42 AM

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Cambridge

International

AS & A Level

David Watson

Helen Williams

Computer

Science

457591_FM_CI_AS & A_Level_CS_i-xiv.indd 3 4/30/19 7:42 AM

Unless otherwise acknowledged, the questions, example answers and comments that appear in this book were

written by the authors. In examinations, the way marks are awarded may be different. Questions from the

Cambridge International AS & A Level Computer Science papers are reproduced by permission of Cambridge

Assessment International Education. Cambridge Assessment International Education bears no responsibility for

the example answers to questions taken from its past question papers which are contained in this publication.

The publishers would like to thank the following who have given permission to reproduce the following material

in this book:

Page 181 Extract from IEEE Code of Ethics. Reprinted with permission of IEEE with the copyright notice ©Copyright

Association for Computing Machinery, Inc.; Page 187 eBay software pirates stump up $100,000 – https://www.

theregister.co.uk/2006/11/24/ebay_pirates_payup/. Reprinted with permission of Out-Law.com, the news service of

Photo credits

Figures 1.1 and 1.2 © David Watson; Figure 1.3 ©Sébastien Delaunay/stock.adobe.com; Figure 2.18 ©Forgem/

Shutterstock.com; Figure 3.1 tl ©studio306fotolia/stock.adobe.com; tr ©Chavim/stock.adobe.com; bl ©pozdeevvs/

stock.adobe.com; br © Sergey Yarochkin/stock.adobe.com; Figure 3.4 ©Mau Horng/stock.adobe.com; Figure 3.5

©science photo/stock.adobe.com; Figure 3.9 ©Maksym Dykha/Shutterstock.com; Figure 3.10 ©Hurst Photo/

Shutterstock.com; Figure 3.11 ©philipus/stock.adobe.com; Figure 3.12 ©belekekin/ Shutterstock.com;

Figure 4.4 l ©cybertrone/stock.adobe.com, c ©Tungphoto/Shutterstock.com, r ©Luminis/Shutterstock.com;

Figure 5.1 l ©Stuart Brady (Public Domain) via Wikipedia Commons; r ©Jiri Hera/stock.adobe.com; Figure 6.3

©Andrey Burmakin/stock.adobe.com; Figure 6.4 ©bkilzer/stock.adobe.com; Figure 7.2 l ©Pres Panayotov/

Shutterstock.com; c ©James Balog/Getty Images; r ©caluian/stock.adobe.com; Figure 18.19 ©seewhatmitchsee/

123rf.com; Figure 18.21 b ©Garmon/stock.adobe.com; t ©Christian Musat/stock.adobe.com; ct ©Ammit/stock.

adobe.com; cb ©Martina Berg/stock.adobe.com; Figure 18.24 Harshal/stock.adobe.com; Figures 18.27 and 18.28

all © David Watson.

l = left, c = centre, b = bottom, t = top, r = right

Every effort has been made to trace and acknowledge ownership of copyright. The publishers will be glad to

make suitable arrangements with any copyright holders whom it has not been possible to contact. Computer

hardware and software brand names mentioned in this book are protected by their respective trademarks and

are acknowledged.

Although every effort has been made to ensure that website addresses are correct at time of going to press,

Hodder Education cannot be held responsible for the content of any website mentioned in this book.

Hachette UK’s policy is to use papers that are natural, renewable and recyclable products and made from wood

grown in well-managed forests and other controlled sources. The logging and manufacturing processes are

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First published 2019 by

Hodder Education,

An Hachette UK Company

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London EC4Y 0DZ

www.hoddereducation.com

Impression number 10 9 8 7 6 5 4 3 2 1

Year 2023 2022 2021 2020 2019

reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and

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publisher or under licence from the Copyright Licensing Agency Limited. Further details of such licences (for

reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited, www.cla.co.uk

Cover photo © Terrance Emerson – stock.adobe.com

Illustrations by Aptara Inc. and Hodder Education

Typeset by Aptara Inc.

Printed by Bell & Bain Ltd, Glasgow

A catalogue record for this title is available from the British Library.

ISBN: 9781510457591

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Conte nts

Introduction viii

AS LEVEL

1 Information representation and multimedia 1

1.1 Data representation 2

1.2 Multimedia 15

1.3 File compression 21

2 Communication 27

2.1 Networking 28

2.2 The internet 54

3 Hardware 68

3.1 Computers and their components 68

3.2 Logic gates and logic circuits 89

4 Processor fundamentals 107

4.1 Central processing unit (CPU) architecture 107

4.2 Assembly language 121

4.3 Bit manipulation 130

5 System software 136

5.1 Operating systems 136

5.2 Language translators 149

6 Security, privacy and data integrity 159

6.1 Data security 159

6.2 Data integrity 169

7 Ethics and ownership 178

7.1 Legal, moral, ethical and cultural implications 179

7.2 Copyright issues 186

7.3 Artificial intelligence (AI) 189

8 Databases 196

8.1 Database concepts 196

8.2 Database management systems (DBMSs) 208

8.3 Data definition language (DDL) and data manipulation

language (DML) 211

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Contents

9 Algorithm design and problem solving 217

9.1 Computational thinking skills 217

9.2 Algorithms 219

10 Data types and structures 238

10.1 Data types and records 238

10.2 Arrays 241

10.3 Files 249

10.4 Abstract data types (ADTs) 250

11 Programming 264

11.1 Programming basics 264

11.2 Programming constructs 271

11.3 Structured programming 275

12 Software development 283

12.1 Program development lifecycle 283

12.2 Program design 287

12.3 Program testing and maintenance 293

A LEVEL

13 Data representation 304

13.1 User-defined data types 304

13.2 File organisation and access 308

13.3 Floating-point numbers, representation and manipulation 312

14 Communication and internet technologies 328

14.1 Protocols 328

14.2 Circuit switching and packet switching 337

15 Hardware 346

15.1 Processors and parallel processing 346

15.2 Boolean algebra and logic circuits 354

16 System software and virtual machines 372

16.1 Purposes of an operating system (OS) 372

16.2 Virtual machines (VMs) 392

16.3 Translation software 394

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vii

Conte nts

17 Security 410

17.1 Encryption 410

17.2 Quantum cryptography 414

17.3 Protocols 416

17.4 Digital signatures and digital certificates 418

18 Artificial intelligence (AI) 425

18.1 Shortest path algorithms 425

18.2 Artificial intelligence, machine learning and deep learning 434

19 Computational thinking and problem solving 450

19.1 Algorithms 450

19.2 Recursion 490

20 Further programming 498

20.1 Programming paradigms 498

20.2 File processing and exception handling 525

Glossary 541

Index 553

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viii

Introduction

This textbook provides the knowledge, understanding and practical skills

to support those studying Cambridge International AS & A Level Computer

Science. This textbook is part of a suite of resources which include a

Programming Skills Workbook and an Online Teacher’s Guide.

The syllabus content has been covered comprehensively and is presented in two

sections: Chapters 1 to 12 cover the AS Level, Chapters 13 to 20 cover the extra

content required for the full A Level.

How to use this book

To make your study of Computer Science as rewarding and successful as possible,

this textbook, endorsed by Cambridge Assessment International Education, offers

the following important features.

Organisation

The content is presented in the same order as in the syllabus, and the chapter

titles match those in the syllabus.

Features to help you learn

Each chapter is broken down into several sections, so that the content is

accessible.

At the start of each chapter, there is a blue box that gives a summary of the

syllabus points to be covered in that chapter, to show you what you are going

to learn.

In this chapter, you will learn about

★ binary magnitudes, binary prefixes and

decimal prefixes

★ binary, denary and hexadecimal number

systems

★ how to carry out binary addition and

subtraction

★ the use of hexadecimal and binary coded

decimal (BCD) number systems

★ the representation of character sets (such as

ASCII and Unicode)

★ how data for a bit-mapped image is encoded

★ how to estimate the file size for a bit-map image

★ image resolution and colour depth

★ encoding of vector graphics

★ the representation of sound in a computer

★ the effects of changing sampling rate and

resolution on sound quality

★ the need for file compression methods (such

as lossy and lossless formats)

★ how to compress common file formats

(such as text files, bit-map images, vector

graphics, sound files and video files).

The grey-blue What you should already know boxes at the beginning of each

chapter or section help you to check you have the right level of knowledge

before you begin. You may have already studied Computer Science at IGCSE,

O Level or equivalent, or you may not have. These boxes contain questions to

find out how much you remember, or to gauge your previous learning. If you are

unable to answer the questions, you will need to refresh your memory, or make

sure you are familiar with the the relevant ideas, before continuing.

IntRoDUCtIon

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IntRoDUCtIon

Key terms for each chapter or section are listed, with definitions. When you are

reading through the chapter and you come across a term you don’t understand,

go back and see if it has been explained here.

There are Activities throughout, so that you can apply what you have learned.

Some of these take the form of questions, to allow you to test your knowledge;

others aim to give you experience of practical work. Some of these will also

give you opportunities to work collaboratively with other students.

WHAT YOU SHOULD ALREADY KNOW

Try these four questions before you read this

chapter.

1 What are the column weightings for the

binary number system?

2 Carry out these binary additions. Convert

your answers to denary.

a) 0 0 1 1 0 1 0 1 + 0 1 0 0 1 0 0 0

b) 0 1 0 0 1 1 0 1 + 0 1 1 0 1 1 1 0

c) 0 1 0 1 1 1 1 1 + 0 0 0 1 1 1 1 0

d) 0 1 0 0 0 1 1 1 + 0 1 1 0 1 1 1 1

e) 1 0 0 0 0 0 0 1 + 0 1 1 1 0 1 1 1

f) 1 0 1 0 1 0 1 0 + 1 0 1 0 1 0 1 0

3 What are the column weightings for the

hexadecimal (base 16) number system?

4 Carry out these hexadecimal additions.

Convert your answers to denary.

a) 1 0 7 + 2 5 7

b) 2 0 8 + A 1 7

c) A A A + 7 7 7

d) 1 F F + 7 F 7

e) 1 4 9 + F 0 F

f) 1 2 5 1 + 2 5 6 7

g) 3 4 A B + C 0 0 A

h) A 0 0 1 + D 7 7 F

i) 1 0 0 9 + 9 F F 1

j) 2 7 7 7 + A C F 1

Key terms

Logic gates – electronic circuits which rely

on ‘on/off’ logic; the most common ones

are NOT, AND, OR, NAND, NOR and XOR.

Logic circuit – formed from a

combination of logic gates and designed

to carry out a particular task; the output

from a logic circuit will be 0 or 1.

Truth table – a method of checking the

output from a logic circuit; they use all

the possible binary input combinations

depending on the number of inputs; for

example, two inputs have 2

(4) possible

binary combinations, three inputs will

have 2

(8) possible binary combinations,

and so on.

Boolean algebra – a form of algebra

linked to logic circuits and based on

TRUE and FALSE.

ACTIVITY 3B

Produce truth tables for each of the following logic circuits. You are advised

to split them up into intermediate parts to help eliminate errors.

a) b)

d) e)

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IntRoDUCtIon

There are also some Extension activities. These go beyond the requirements of

the syllabus, but it is good to see if you know the answers. We hope they will

be of interest to you.

The End of chapter questions are practice exam-style questions; these provide

a more formal way to check your progress. Some questions from Cambridge

International AS & A Level Computer Science past papers are included.

EXTENSION ACTIVITY 3E

1 Look at this simplified diagram of a keyboard; the letter H has been

pressed. Explain:

a) how pressing the letter H has been recognised by the computer

b) how the computer manages the very slow process of inputting data

from a keyboard.

2 a) Describe how these types of pointing devices work.

i) Mechanical mouse

ii) Optical mouse

b) Connectivity between mouse and computer can be through USB cable

or wireless. Explain these two types of connectivity.

conductive layers

letter H has been pressed

and now makes contact with

bottom conductive layer

letter H

interpreted

by computer

insulating layer

1 a) The following bytes represent binary integers using the two’s complement

form. State the equivalent denary values.

i) 0 1 0 0 1 1 1 1 [1]

ii) 1 0 0 1 1 0 1 0 [1]

iii) Write the integer −53 in two’s complement form. [1]

iv) Write the maximum possible range of numbers using the two’s

complement form of an 8-bit binary number.

Give your answers in denary. [2]

b) i) Write the denary integer 798 in binary-coded decimal (BCD)

format. [1]

ii) Write the denary number that is represented by the following BCD number.

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

[2]

c) Give one use of binary-coded decimal system. [1]

End of chapter

questions

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IntRoDUCtIon

Assessment

If you are following the AS Level course, you will take two examination papers:

» Paper 1 Theory Fundamentals (1 hour 30 minutes)

» Paper 2 Fundamental Problem-solving and Programming Skills (2 hours)

If you are studying the A Level course, you will take four examination papers,

Papers 1 and 2 and also:

» Paper 3 Advanced Theory (1 hour 30 minutes)

» Paper 4 Practical (2 hours 30 minutes)

Note that calculators must not be used in any paper.

Command words

The table below includes command words used in the assessment for this

syllabus. The use of the command word will relate to the subject context.

Make sure you are familiar with these.

Command

word

What it means

Analyse examine in detail to show meaning, identify elements and the

relationship between them

Assess make an informed judgement

Calculate work out from given facts, figures or information

Comment give an informed opinion

Compare identify/comment on similarities and/or differences

Complete add information to an incomplete diagram or table

Consider review and respond to given information

Contrast identify/comment on differences

Define give precise meaning

Demonstrate show how or give an example

Describe state the points of a topic/give characteristics and main features

Develop take forward to a more advanced stage or build upon given information

Discuss write about issue(s) or topic(s) in depth in a structured way

Draw draw a line to match a term with a description

Evaluate judge or calculate the quality, importance, amount, or value of something

Examine investigate closely, in detail

Explain set out purposes or reasons/make the relationships between things

evident/provide why and/or how and support with relevant evidence

Give produce an answer from a given source or recall/memory

Identify name/select/recognise

Justify support a case with evidence/argument

Outline set out main points

Predict suggest what may happen based on available information

Sketch make a simple freehand drawing showing the key features, taking care

over proportions

➔

457591_FM_CI_AS & A_Level_CS_i-xiv.indd 11 4/30/19 7:42 AM

xii

Command

word

What it means

State express in clear terms

Suggest apply knowledge and understanding to situations where there are a

range of valid responses in order to make proposals

Summarise select and present the main points, without detail

Write write an answer in a specific way

From the authors

We hope you enjoy this book. It encourages you to develop your computational

thinking while broadening your understanding of computer science. This

should prove helpful when you go on to further study, where topics such as

artificial intelligence, quantum cryptography and imperative and declarative

programming will be studied; all of these are covered in the later chapters of

the book. In order to handle such topics confidently, you will need to be a

competent programmer who uses computational thinking to solve problems and

has a good understanding of computer architecture. All chapters are designed

to build on your previous experience in a way that develops essential skills and

at the same time expands the techniques you are able to use.

David Watson

Helen Williams

Notes for teachers

Key concepts

These are the essential ideas that help learners to develop a deep

understanding of the subject and to make links between the different topics.

Although teachers are likely to have these in mind at all times when they are

teaching the syllabus, the following icons are included in the textbook at

points where the key concepts relate to the text:

Computational thinking

Computational thinking is a set of fundamental skills that help produce

a solution to a problem. Skills such as abstraction, decomposition and

algorithmic thinking are used to study a problem and design a solution that

can be implemented. This may involve using a range of technologies and

programming languages.

Programming paradigms

A programming paradigm is a way of thinking about or approaching problems.

There are many different programming styles that can be used, which are

suited to unique functions, tools and specific situations. An understanding of

programming paradigms is essential to ensure they are used appropriately, when

designing and building programs.

IntRoDUCtIon

457591_FM_CI_AS & A_Level_CS_i-xiv.indd 12 4/30/19 7:42 AM

xiii

IntRoDUCtIon

Communication

Communication is a core requirement of computer systems. It includes the

ability to transfer data from one device or component to another and an

understanding of the rules and methods that are used in this data transfer.

Communication could range from the internal transfer of data within a

computer system, to the transfer of a video across the internet.

Computer architecture and hardware

Computer architecture is the design of the internal operation of a computer

system. It includes the rules that dictate how components and data are

organised, how data are communicated between components, to allow hardware

to function. There is a range of architectures – with different components and

rules – that are appropriate for different scenarios. All computers comprise

a combination of hardware components, ranging from internal components,

such as the central processing unit (CPU) and main memory, to peripherals. To

produce effective and efficient programs to run on hardware, it is important to

understand how the components work independently and together to produce

a system that can be used. Hardware needs software to be able to perform a

task. Software allows hardware to become functional. This enables the user to

communicate with the hardware to perform tasks.

Data representation and structures

Computers use binary and understanding how a binary number can be

interpreted in many different ways is important. Programming requires an

understanding of how data can be organised for efficient access and/or transfer.

Additional support

The Programming Skills Workbook provides practice for the programming papers

and includes exercises designed to give students the necessary experience

of working in one of the three prescribed high-level programming languages:

Java (Console mode), Visual Basic and Python (Console mode). It is a write-in

workbook designed to be used throughout the course.

Answers to questions are available in the Online Teacher’s Guide.

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