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ENDORSED BY

CAMBRIDGE

\t.~ lnt~mational F.11aminarmns

IGCSE®

Computer

Science

,.,,.,.__ ,ff David Watson

Helen Williams

HODDER

1 EDUCATION

Cambridge

IGCSE®

Computer

Science

This page intentionally left blank

Computer

Science

David Watson

Helen Williams

i.7 HODDER

EDUCATION

AN HACHETTE UK COMPANY

Acknowledgements

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Photo credits

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Contents

Introduction

Section 1 Theory of computer science

Chapter 1 Binary systems and hexadecimal

1.1 Introduction

1.2 111c binary system

1.3 Measurement of the size of computer memories

1.4 Example use ofbinary

1.5 The hexadecimal

Chapter 2 Communication and internet technologies

2.1 Introduction

2.2 Data transmission

2.3 Error-checking methods

2.4 lntcrnettechnologies

Chapter 3 Logic gates and logic circuits

3.1 Introduction

3.2 Logic gates

3.3 Truth tables

3.4 The fimction ofthe

Chapter 4 Operating systems and computer architecture

4.1 Introduction

4.2 Operating systems

4.3 Interrupts

4.4 Computer architecture

4.5 The fetch-execute

Chapter 5 Input and output devices

5.1 Introduction

5 .2 lnputdeviccs

5.3 Outputdevices

Chapter 6 Memory and data storage

6.1 Introduction

6.2 File formats

6.3 Lossless and lossy file compression

6.4 Memory and storage

6.5 H ow to estimate the size ofa file

Chapter 7 High-and low-level languages 90

7.1 Programming languages 90

7.2 Translators 92

7.3 What happens when things go wrong? 94

Chapter 8 Security and ethics 97

8.1 Introduction 97

8.2 Security and data integrity 97

8.3 Cookies 100

8.4

Loss of data and data corruption 101

8.5

Firewallsandproxyservers 102

8.6

Security protocols 103

8.7

Encryption 104

8.8

Applications 109

8.9

Computer ethics 110

8.10

Freesoftware,freewareandshareware 111

Section 2 Practical problem-solving and programming 114

Chapter 9 Problem-solving and design 115

9.1 Introduction 115

9.2 Algorithms 118

9.3 Test data 119

9.4 Validation

Chapter 10 Pseudocode and flowcharts 134

10. l Introduction 134

10.2

Assignment 134

10.3

Conditional statements 135

10.4

Loop structures 137

10.5

Inputandoutputstatements 139

l 0.6

Chapter 11 Programming concepts

11. l Introduction

11.2 Programming

11.3 Declaration and use of

Chapter 12 Data structures: arrays and using pre-release material

12.1 Introduction

12.2

Chapter 13 Databases

13.1 Introduction

13.2 What

171
178

I ntrod ucti on

Aims l11is textbook

Cambridge

and GCE O Level courses. The textbook is part of a package which includes a student CD-ROM. A teacher's CD-ROM is also available separately which includes additional guidance and other useful information ( see later in this introduction). This

Cambridge

International papers

Cambridge

syllabus in mind, it can still be used as especially at AS level.

Using the book

111c textbook contains 13 chapters. Although it is possible for some elements

of the practical problem-solving chapters to be examined in Paper I (Theory of Computer Science),

1 (TI1eory of computer science) and Section 2 (Practical

problem-solving and programming) to follow the Cambridge syllabus as closely as possible. TI1e 0 additional teacher's CD-ROM is available to accompany this textbook. TI1is

CD-ROM includes the following material:

Cambridge reference

• relevant page David

This page intentionally left blank

G) Binary systems and hexadecimal

1.1 Introduction

As you progress through this book you will begin to realise how complex computer systems really By the time you reach Chapter 12 you should

A s,.vitch

1.2 The binary system

We arc all familiar with the dcnary (base 10) number

OOO 1000 100 10

1.2.1 Converting from binary to

It is fairly

+ 64 + 32 + 8 + 4 + 2 -238 (dcnary)

The O values arc simply ignored.

0 1.2

Activity 1.1

Convertthelollowingb.narynumber,;

e11111111 f00001111 g10001111 h11110000 i01110000 j11101110

1.2.2 Converting from denary to

by 2. The remainders arc then read from BOTTOM to TOP to give the binary value. Again using 107, we get:

Activity 1.2

readlhe,emainde,lrombottomto 41
b 67 '86 d100 e111 f g144 h189 i200 j255 The

1 BINARY SYSTEMS AND HEXADECIMAL

1.3 Measurement of the size of computer memories

A binary digit is commonly referred to as a

(1MB) e(1GB)

OOO 000000 OOO bytes and so on.

111c !EC convention for computer internal memories (including

in Table 1.1. This also ties up with the

Cambridge International

1.4 Example use of binary

This section

0 1.5

Figur•1.2

only forward and backward

BdirPCtion

If the register contains I O 1 0 1 0 l O this means 'motor

Effecti\·ely,

Activity 1.3

a What be the jf the be the jf the

1.5 The hexadecimal system

The HEXADECIMAL

The

1 BINARY SYSTEMS AND HEXADECIMAL

Because it is a system based on 16 different digits, the numbers Oto 9 and the letters A to F arc used to represent each hexadecimal (hex) digit. (A -10, B -11,C-12,D -13, E-14 and F-15.) Using the same method asdcnary and binary, this (v

Co1wcrting from binary to hexadecimal is

Example 1

I O 1I111000 0 1

First split this up into groups of 4 bits:

101 1 111 0 0 0 0 I

TI1cn, using Table 1.2, find the

Example 2

100 0 0 I 111 111 0 l

0 1.5

First split this up into groups of4 bits:

I O OOO I 1111 1101

The left

Activity 1.4

Convertthelollowingb4narynumbers

e000111100001 f100010011110 g0010011111110 h0111010011100 i 1111111101111101 j00110011110101110 D

Con\'erting

A Using Table 1.2, find the 4-bit code for each digit:

0 100 0 1 0 I 101 0

Put the groups together to form the binary number:

0 100 0 101 l O 1 0

Example 4

Again just

Activity 1.5

a 6C f BA6 b 59

AA h 40AA

d AOO i DA47

J 1AB0

The 0

1 BINARY SYSTEMS AND HEXADECIMAL

1.5.2 Converting from hexadecimal to denary

and from denary to

Example 1

A First

Example 2

First multiply each digit

Activity 1.6

Coovert

6B b9C < 4A dFF e1FF f AOl gBB4 CAB i 12AE j A089 To 0

1.6 Use of the hexadecimal

Method l

Consider the conversion of the dcnary number, 2004, into hexadecimal. This method + 13 + (4 by 16. The remainders

BOTTOM to TOP to

16

Activity 1. 7

1 98
b227 c490 d511 e826 f1000 g2634 h3743 i4007 jSOOO

1.6 Use of the hexadecimal system

This section reviews five uses of the hexadecimal system. 111c information in this chapter gives the reader sufficient grounding in each topic at this

1.6.1 Memory dumps

Since it is much easier to work with: B 5 A 4 I A F C rather than: l O 0

1 BINARY SYSTEMS AND HEXADECIMAL

1-··

009901'77 n 20 12 79

A program developer can look at each of the hexadecimal codes ( as shown in Figure 1.6) and determine where the error lies. The

1.6.2 HyperText Mark-up Language (HTML)

HYPERTEXT

1.6 Use of the hexadecimal

and

Activity 1.8

be changed ,;uch aswww.html.arrltoproduce using available a programming vecy little programmingskillisrequilrotouseHTML A

B3 -4F -25 -FE is the MAC address of a

type of

1 BINARY SYSTEMS AND HEXADECIMAL

• It may be necessary to bypass ,. .... , Chu ,. <:SPACh SS " ;e s "'' ;s . G n .. '"' >A m s, n S> m s, SS SS ,s "" SS n w aA SA m >A SC K so m <:OHETb h d d k (Note: the% sign is used to denote that hexadecin1al is being used.) 1.6

Activity 1.9

Using ASCII rnde

a www.de.org.uk

Use of the hexadecimal

Sometimes the hexadecimal addresses arc used in

It is also

Communication and internet technologies

2.1 Introduction

When data is sent from one device to another, it is important to consider how that data is transmitted. It is also important to ensure that the data hasn't been changed in any way. The internet has now become an integral part of all of our

2.2 Data transmission

Data transmission can be either over a short distance ( for example, from computer to printer) or over longer distances (for example, over a telephone network). Essentially, three factors need to be considered when transmitting data ( each factor has to be agreed by both sender and receiver for this to work without error): • the direction of the data transmission (i.e. in one direction only or in both directions) the method of

2.2.1 Simplex, half-duplex and

SIMPLEX one only (i.e. from sender to receiver).

Example:

data being sent from a computer to both but not at bothdirectiom (i.e. data can be sent from 'A' to 'B' and from 'B' to 'A' along the same line, both

Example: broadband connection on

2.2.2 Serial and parallel data

SERIAL 011e over t1

arc sent one after the other in a

Figure2.1

(Note: 2.2

This method of data transmission works well

PARAILBL is when sevem/ are

sent down several one wire or channel is used to transmit each bit.

Data transmission

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Figur•2.2 ;;::=,,p

(Note: bits can be transmitted as simplex, half-duplex or full-duplex.) •

This method of data transmission

Figure 2.3

Activity 2.1

Describewhatisme,mtby:

a serial, b parallel,full-duplexdatatransmiOOn

2 COMMUNICATION AND INTERNET TECHNOLOGIES

1,tartbit I O IOI O I O I O IOI siopbit I

cootrolbit type of data transmission.

2.2.4 Universal Serial Bus (USB)

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