Endorsed for learner supportCambridgeand 0 LevelComputerScienceComputer SystemsHODDEREDUCATION25YEARSEndorsed for learner supportCambridgeand O LevelComputerScienceComputer SystemsDavid WatsonHelen WilliamsHODDEREDUCATIONEvery effort has been made to trace all copyright holders,but if any have been inadvertentlyoverlooked,the Publishers will be pleased to make the necessary arrangements at the firstAlthough every effort has been made to ensure that website addresses are correct at time ofgoing to press,Hodder Education cannot be held responsible for the content of any websitementioned in this book.It is sometimes possible to find a relocated web page by typing in theaddress of the home page for a website in the URL window of your browser.Hachette UK's policy is to use papers that are natural,renewable and recyclable products andmade from wood grown in well-managed forests and other controlled sources.The logging andmanufacturing processes are expected to conform to the environmental regulations of thecountry of origin.Orders:please contact Hachette UK Distribution,Hely Hutchinson Centre,Milton Road,Didcot,Oxfordshire,0X11 7HH.Telephone:+44 (0)1235 827827.Email education@hachette.co.uk Linesare open from 9 a.m.to 5 p.m.,Monday to Friday.You can also order through our website:www.hoddereducation.comISBW:9781398318496David Watson and Helen Williams 2021First published in 2021 byHodder EducationAn Hachette UK CompanyCarmelite House50 Victoria EmbankmentLondon EC4Y ODZwww.hoddereducation.comImpression number 10 9 8 7 6 5 4 3 2 1Year20252024202320222021All rights reserved.Apart from any use permitted under UK copyright law,no part of thispublication may be reproduced or transmitted in any form or by any means,electronic ormechanical,including photocopying and recording,or held within any information storageand retrieval system,without permission in writing from the publisher or under licence fromthe Copyright Licensing Agency Limited.Further details of such licences(for reprographicreproduction)may be obtained from the Copyright Licensing Agency Limited,www.cla.co.ukCover phonlamaiphoto -stock.adobe.comPrinted in the UKA catalogue record for this title is available from the British Library.