Cambridge A Level Computer Science 9608 — 2015 May/June Paper 3 · Variant 2

This document consists of 12 printed pages. DC (FD) 110069 © UCLES 2015 [Turn over Cambridge International Examinations Cambridge International Advanced Level * 4 6 2 2 0 4 8 3 6 1 * COMPUTER SCIENCE 9608/32 Paper 3 Advanced Theory May/June 2015 1 hour 30 minutes Candidates answer on the Question Paper. No Additional Materials are required. No calculators allowed. READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. No marks will be awarded for using brand names of software packages or hardware. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. The maximum number of marks is 75.

2 9608/32/M/J/15 © UCLES 2015 1 The following syntax diagrams, for a particular programming language, show the syntax of: • an assignment statement • a variable • a letter • an operator Variable Assignment statement Variable Letter = Variable Operator Variable a b c d ; Letter Letter Letter Operator + – ÷ (a) The following assignment statements are invalid. Give the reason in each case. (i) a = b + c Reason … …[1] (ii) a = b – 2; Reason … …[1] (iii) a = dd * cce; Reason … …[1]

3 9608/32/M/J/15 © UCLES 2015 [Turn over (b) Write the Backus-Naur Form (BNF) for the syntax diagrams shown on the opposite page. <assignmentstatement> ::= … <variable> ::= … <letter> ::= … <operator> ::= …[6] (c) Rewrite the BNF rule for a variable so that it can be any number of letters. <variable> ::= …[2] (d) Programmers working for a software development company use both interpreters and compilers. (i) The programmers prefer to debug their programs using an interpreter. Give one possible reason why. … …[1] (ii) The company sells compiled versions of its programs. Give a reason why this helps to protect the security of the source code. … …[1]

4 9608/32/M/J/15 © UCLES 2015 2 The incomplete table below shows descriptions and terms relating to malware. (a) Complete the table with appropriate descriptions and terms. Description Term A Unsolicited emails containing advertising material sent to a distribution list. … B A standalone piece of malicious software that can reproduce itself automatically. … C … … … … … Pharming D … … … … … Phishing [4] (b) For one of the terms, describe: • a problem that might arise for a user • a possible solution to the problem Choose between the terms: A / B (circle your choice) Problem … … Solution … …[2]

5 9608/32/M/J/15 © UCLES 2015 [Turn over (c) Explain the following terms: Encryption … … … … Public key … … … …[2] (d) A user downloads software from the Internet. (i) State what should be part of the download to provide proof that the software is authentic. …[1] (ii) Describe the process for ensuring that the software is both authentic and has not been altered. … … … … … … … …[4]

6 9608/32/M/J/15 © UCLES 2015 3 (a) A particular programming language allows the programmer to define their own data types. ThisDate is an example of a user-defined structured data type. TYPE ThisDate DECLARE ThisDay : (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31) DECLARE ThisMonth : (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec) DECLARE ThisYear : INTEGER ENDTYPE A variable of this new type is declared as follows: DECLARE DateOfBirth : ThisDate (i) Name the non-composite data type used in the ThisDay and ThisMonth declarations. …[1] (ii) Name the data type of ThisDate. …[1] (iii) The month value of DateOfBirth needs to be assigned to the variable MyMonthOfBirth. Write the required statement. …[1]

7 9608/32/M/J/15 © UCLES 2015 [Turn over (b) Annual rainfall data from a number of locations are to be processed in a program. The following data are to be stored: • location name • height above sea level (to the nearest metre) • total rainfall for each month of the year (centimetres to 1 decimal place) A user-defined, composite data type is needed. The programmer chooses LocationRainfall as the name of this data type. A variable of this type can be used to store all the data for one particular location. (i) Write the definition for the data type LocationRainfall. … … … … … … …[5] (ii) The programmer decides to store all the data in a file. Initially, data from 27 locations will be stored. More rainfall locations will be added over time and will never exceed 100. The programmer has to choose between two types of file organisation. The two types are serial and sequential. Give two reasons for choosing serial file organisation. … … … …[2]

8 9608/32/M/J/15 © UCLES 2015 4 (a) (i) Complete the truth table for this logic circuit: A B X A B Working space X 0 0 0 1 1 0 1 1 [1] (ii) Complete the truth table for this logic circuit: A B X A B Working space X 0 0 0 1 1 0 1 1 [1]

9 9608/32/M/J/15 © UCLES 2015 [Turn over (b) A student decides to write an equation for X to represent the full behaviour of each logic circuit. (i) Write the Boolean expression that will complete the required equation for X for each circuit: Circuit 1: X = … Circuit 2: X = …[2] (ii) Write the De Morgan’s Law which is shown by your answers to part (a) and part (b)(i). …[1] (c) Write the Boolean algebraic expression corresponding to the following logic circuit: A B X …[3] (d) Using De Morgan’s laws and Boolean algebra, simplify your answer to part (c). Show all your working. … … … … … …[3]

10 9608/32/M/J/15 © UCLES 2015 5 A gardener grows vegetables in a greenhouse. For the vegetables to grow well, the temperature needs to always be within a particular range. The gardener is not sure about the actual temperatures in the greenhouse during the growing season. The gardener installs some equipment. This records the temperature every hour during the growing season. (a) Name the type of system described. …[1] (b) Identify three items of hardware that would be needed to acquire and record the temperature data. Justify your choice for each. Item 1 … Justification … … Item 2 … Justification … … Item 3 … Justification … …[6] (c) The equipment records temperatures in the greenhouse. It does this for seven locations. Each recording is stored as two successive bytes. The format is shown below: Greenhouse location Temperature reading 7 6 5 4 3 2 1 0 Byte 1 Byte 2 The location is indicated by the setting of one of the seven bits in byte 1. For example, location 4 is indicated by setting bit 4. Bit 0 of byte 1 acts as a flag: • the initial value is zero • when the reading has been processed it is set to 1 Byte 2 contains the temperature reading (two’s complement integer).

11 9608/32/M/J/15 © UCLES 2015 [Turn over (i) Interpret the data in byte 1 shown below: 7 6 5 4 3 2 1 0 0 0 1 0 0 0 0 1 0 0 0 1 1 0 0 0 Byte 1 Byte 2 … … … …[2] (ii) The system receives a temperature reading of –5 degrees from sensor 6. Complete the boxes below to show the two bytes for this recording. The reading has not yet been processed. 7 6 5 4 3 2 1 0 Byte 1 Byte 2 [2] (d) (i) The accumulator is loaded with the value of byte 1 from location 106. Write the assembly language instruction to check whether the reading in byte 2 came from location 4. LDD 106 // data loaded from address 106 …[4] (ii) Write the assembly language instruction to set the flag (bit 0) of the byte contained in the accumulator to 1. …[2]

12 9608/32/M/J/15 © UCLES 2015 6 (a) Four descriptions and three protocols are shown below. Draw a line to connect each description to the appropriate protocol. Description Protocol used email client downloads an email from an email server HTTP email is transferred from one email server to another email server POP3 email client sends email to email server SMTP browser sends a request for a web page to a web server [4] (b) Downloading a file can use the client-server model. Alternatively, a file can be downloaded using the BitTorrent protocol. Name the model used. …[1] (c) For the BitTorrent protocol, explain the function of each of the following: (i) Tracker … … …[2] (ii) Seed … … …[2] (iii) Swarm … … …[2] To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge International Examinations Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cie.org.uk after the live examination series.

® IGCSE is the registered trademark of Cambridge International Examinations. CAMBRIDGE INTERNATIONAL EXAMINATIONS Cambridge International Advanced Level MARK SCHEME for the May/June 2015 series 9608 COMPUTER SCIENCE 9608/32 Paper 3 (Written paper), maximum raw mark 75 This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge will not enter into discussions about these mark schemes. Cambridge is publishing the mark schemes for the May/June 2015 series for most Cambridge IGCSE®, Cambridge International A and AS Level components and some Cambridge O Level components.

Page 2 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 32 © Cambridge International Examinations 2015 1 (a) (i) ‘;’ missing 1 (ii) ‘2’ is not a variable 1 (iii) ‘e’ is not a valid letter 1 (b) <assignment statement> ::= <variable> = <variable><operator><variable>; <variable> ::= <letter>|<letter><letter> |<letter><letter><letter> <letter> ::= a|b|c|d <operator> :: =+|-|*|÷ 2 2 1 1 (c) <letter> | <letter><variable> // <letter> | <variable><letter> 2 (d) (i) debugging is faster / easier // can debug incomplete code // better diagnostics 1 (ii) compiler produces executable version – not readable / no need for source code // difficult to reverse-engineer 1 Total: 13 2 (a) • Spam • Worm Pharming redirect website to fake website // domain name server compromised // proxy server compromised Phishing through email attempt to obtain somebody’s confidential data / install malware 1 1 1 1 (b) Spam • user’s inbox is filled by large amount of unwanted email • user / email server employs filtering software that can divert / delete spam email Worm • could corrupt user’s computer // delete data // consume bandwidth • run anti-virus software in the background // not connect to the Internet // keep OS up-to-date 1 1 or 1 1

Page 3 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 32 © Cambridge International Examinations 2015 (c) encryption: process of turning plain text into cipher text public key: key widely available that can be used to encrypt message that only owner of private key can decrypt // can be used to decrypt a message thereby confirming originator of message 1 1 (d) (i) digital signature 1 (ii) • software is put through hashing algorithm • hash total is encrypted with private key (digital signature) • software + encrypted hash / digital signature are sent • receiver is in possession of sender’s public key • the received hash total / digital signature is decrypted with public key (SH) • the receiver hashes received software (RH) • If SH matches RH then software is authentic and has not been altered Any four points 1 mark each Total: 13 3 (a) (i) enumerated 1 (ii) record 1 (iii) MyMonthOfBirth
DateOfBirth.ThisMonth 1 (b) (i) TYPE LocationRainfall DECLARE LocationName : STRING DECLARE LocationHeight : INTEGER DECLARE TotalMonthlyRainfall : ARRAY[1..12] OF REAL ENDTYPE 1 1 1 1 +1 (ii) • no need to re-sort data every time new data is added • only a small file so searching will require little processing • new records can easily be appended 1 1 1 [max 2] Total: 10 4 (a) (i) Circuit 1 A B X 0 0 1 0 1 1 1 0 1 1 1 0 1

Page 4 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 32 © Cambridge International Examinations 2015 (ii) Circuit 2 A B X 0 0 1 0 1 1 1 0 1 1 1 0 1 (b) (i) • circuit 1: B A. • circuit 2: A + B 1 1 (ii) ≡ B A. A + B 1 (c) B B A .) ( + Mark as follows: ) ( B A+ B . bar over whole expression 1 1 1 (d) B B A .) ( + = ) ( B A+ + B = B B A + + ) ( = ) ( B B A + + = 1 + A =1 allow f.t. from (c) 1 1 1 1 1 [max 3] Total: 11 5 (a) Monitoring system 1 (b) • temperature sensor … • transmits measured temperature • analogue to digital converter … • converts analogue signal from sensor to digital value that can be stored • storage device // data logger… • for recording readings from sensor • transmission hardware … • to transfer data from sensor to storage device • processor … • to process incoming data 1 1 1 1 1 1 1 1 1 1 [max 6]

Page 5 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 32 © Cambridge International Examinations 2015 (c) (i) temperature reading in location 5 has been processed 1 1 (ii) 0100 0000 1111 1011 1 mark per byte 2 (d) (i) AND #B00010000 // AND #&10 // AND #16 1 mark for AND, 1 mark for address mode, 1 mark for mask, 1 mark for indication of numbering system 1 + 1 + 1 + 1 (ii) OR #B00000001 // OR #&01 // OR #1 1 mark for OR, 1 mark for mask 1 +1 Total: 17 6 (a) Description Protocol used email client downloads an email from an email server HTTP email is transferred from one email server to another email server POP3 email client sends email to email server SMTP browser sends a request for a web page to a web server 1 mark for correct arrow from each description (b) peer-to-peer 1 (c) (i) Tracker: central server that: stores details of other computers that have all / part of file to be downloaded // has data on those peers downloading and uploading file // shares IP addresses with other clients in swarm allowing them to connect 1 1 (ii) Seed: peer computer that has 100% of file // is uploading downloaded content 1 1 (iii) Swarm: all the connected peer computers that have all or part of the file to be downloaded / uploaded // share a torrent 1 1 Total: 11