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

This document consists of 14 printed pages and 2 blank pages. DC (ST/SW) 95542/2 © UCLES 2015 [Turn over Cambridge International Examinations Cambridge International Advanced Level * 4 4 4 9 7 9 0 3 3 7 * COMPUTER SCIENCE 9608/33 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 9608/33/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 number • a letter • a digit • an operator Variable Assignment statement Variable Digit := Variable Operator Number 1 2 3 5 4 Letter Number Number Digit Letter A B C Operator + – /

3 9608/33/M/J/15 © UCLES 2015 [Turn over (a) The following assignment statements are invalid. Give a reason in each case. (i) A2 = B3 + 123 Reason … …[1] (ii) B3 := B3 – 203 Reason … …[1] (iii) A2414 := A3 * B Reason … …[1] (b) Complete the Backus-Naur Form (BNF) for the syntax diagrams shown on the opposite page. <letter> has been done for you. <assignmentstatement> ::= … <variable> ::= … <number> ::= … <letter> ::= A | B | C <digit> ::= … <operator> ::= … [6]

4 9608/33/M/J/15 © UCLES 2015 (c) A company develops software. It provides virtual machines for its software developers. The company has a large number of clients who use a wide range of hardware and software. (i) Explain the term virtual machine. Ensure that your answer includes the terms hardware and software. … … … …[2] (ii) Give one benefit to the company of using virtual machines. … …[1] (iii) Give one drawback to the company of using virtual machines. … …[1]

5 9608/33/M/J/15 © UCLES 2015 [Turn over 2 (a) Four descriptions and three types of local area network (LAN) are shown below. Draw a line to connect each description to the type of LAN it applies to. Description Type of LAN Any packet the listening computer receives may be part of a message for itself Bus with terminators at each end Connection provided through an access point Star A process for handling collisions has to be implemented Wireless Listening computer only receives packets that are addressed to itself [4] (b) A user downloads a file using the FTP protocol. Explain the function played by each of the following: (i) Server … … …[2] (ii) Command … … …[2] (iii) Anonymous … … …[2]

6 9608/33/M/J/15 © UCLES 2015 3 The incomplete table below shows descriptions and terms relating to malware. (a) Complete the table with appropriate descriptions and terms. Description Term A Sending emails which contain a link to a website that attempts to trick users into giving confidential personal data. … B It replicates by inserting itself into another piece of software. … C … … … Worm D … … … Spam [4] (b) Choose term A or term B and describe: • a problem that might arise for a user • a possible solution to the problem Term … Problem … … Solution … …[2]

7 9608/33/M/J/15 © UCLES 2015 [Turn over (c) Explain the following terms: Cipher text … … … Private key … … …[2] (d) Bill, a manager of a company, sent an email with very sensitive information to a work colleague, Alison. However, Bill also accidentally sent it to everybody in the company. Describe the method used that ensured only Alison was able to read the original contents of the email. … … … … … …[4]

8 9608/33/M/J/15 © UCLES 2015 4 (a) A particular programming language allows the programmer to define their own data types. An example of a user-defined data type for an address is: TYPE ThisAddress DECLARE ThisHouseNo : INTEGER DECLARE ThisStreet : STRING DECLARE ThisTown : STRING ENDTYPE A variable of this new type is declared as follows: DECLARE HomeAddress : ThisAddress (i) Write the statement that assigns the house number 34 to HomeAddress. …[1] (ii) The type definition for ThisAddress is to be changed. Rewrite one line from the definition for each of the following changes. House numbers are in the range from 1 to 10. DECLARE … The possible towns are limited to: Brightown, Arunde and Shoram. DECLARE …[2]

9 9608/33/M/J/15 © UCLES 2015 [Turn over (b) Temperature data from a number of weather stations are to be processed by a program. The following data are to be stored: • weather station ID (a unique four-letter code) • latitude (to 2 decimal places) • average temperature (to the nearest whole number) for each year from 2001 to 2015 inclusive A programmer designs a composite data type WeatherStation. A variable of this type can be used to store all the data for one particular station. (i) Write the definition for the user-defined data type WeatherStation. … … … … … …[5] (ii) The programmer decides to store all the data in a file. The number of weather stations could grow to reach 20000, but not all stations will be present at first. The programmer decides on random organisation for the file. Describe three steps which show how a new weather station record is added to the file. 1 … … 2 … … 3 … …[3]

10 9608/33/M/J/15 © UCLES 2015 5 (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]

11 9608/33/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]

12 9608/33/M/J/15 © UCLES 2015 6 A company grows vegetables in a number of large greenhouses. For the vegetables to grow well, the temperature, light level and soil moisture need to always be within certain ranges. The company installs a computerised system to keep these three growing conditions within the best ranges. Sensors are used for collecting data about the temperature, light level, and moisture content of the soil. (a) Name the type of system described. …[1] (b) Give three items of hardware that would be needed for this system. Justify your choice. Do not include sensors in your answer. Item 1 … Justification … … Item 2 … Justification … … Item 3 … Justification … …[6] (c) (i) Describe what is meant by feedback in the above system. … … … … … …[3] (ii) When the system was designed, various parameters for temperature were set. Name one of these parameters. … …[1]

13 9608/33/M/J/15 © UCLES 2015 [Turn over (iii) Explain how this parameter value is used by the feedback system. … … … …[2] Question 6 continues on page 14.

14 9608/33/M/J/15 © UCLES 2015 Each greenhouse has eight sensors (numbered 1–8). • The byte at address 150 is used to store eight 1-bit flags. • A flag is set to indicate whether its associated sensor reading is waiting to be processed. • More than one sensor reading may be waiting to be processed at any particular moment. • Data received from the sensors is stored in a block of eight consecutive bytes (addresses 201–208). • The data from sensor 1 is at address 201, the data from sensor 2 is at address 202, and so on. Sensor number 1 2 3 4 5 6 7 8 150 0 1 0 0 0 1 0 1 201 0 0 0 0 0 0 0 0 202 0 0 0 0 0 1 0 0 203 0 0 0 0 0 0 0 0 204 0 0 0 1 0 0 0 0 205 0 0 0 0 0 0 1 0 206 0 0 0 1 0 1 0 0 207 0 0 0 1 0 0 1 0 208 0 0 0 1 0 0 1 0 (d) (i) Interpret the current reading for sensor 2. … …[2] (ii) The accumulator is loaded with the data from location 150. Write the assembly language instruction to check whether there is a value waiting to be processed for sensor 6. LDD 150 // data loaded from address 150 …[3]

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® 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 9608/33 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 33 © Cambridge International Examinations 2015 1 (a) (i) Wrong assignment operator (should be ‘:=’ not ‘=’) 1 (ii) 0 is not a digit 1 (iii) ‘B’ is not a number 1 (b) <assignmentstatement> ::= <variable> := <variable><operator><number> <variable> ::= <letter><number> <number> ::= <digit>| <digit><number> <letter> ::= A|B|C <digit> ::= 1|2|3|4|5 <operator> ::= +|-|*|/ 1 + 1 1 1 + 1 1 (c) (i) Use of software … (idea of using) to implement a hardware set-up (idea of implementing / simulating / emulating) 1 1 (ii) e.g. no need to acquire client hardware for testing / reduces set-up time for test system / common development system for all developers 1 (iii) e.g. software emulation runs slower than real hardware / not possible to emulate some hardware 1 Total: 13 2 (a) Description Type of LAN any packet the listening computer receives may be part of a message for that computer bus with terminators at each end connection provided through an access point star a process for handling collisions has to be implemented wireless listening computer only receives packets that are addressed to this computer 1 mark for correct arrow(s) from each description 1 1 1 1 (b) (i) Server: central computer stores files that are to be downloaded 1 1

Page 3 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 33 © Cambridge International Examinations 2015 (ii) Command: user can send action/instruction (or by example, e.g. change directory) that are carried out on server 1 1 (iii) Anonymous: allows user to access files user does not need to identify themselves to server 1 1 Total: 10 3 (a) A Phishing B Virus C a standalone piece of software which can reproduce itself automatically D sending unsolicited emails to a distribution list 1 1 1 1 (b) e.g. phishing problem: identity fraud / misuse of financial data solution: ignore email / don’t respond to email e.g. virus problem: computer may stop working // lost files solution: running anti-virus software 1 1 or 1 1 (c) cipher text: encrypted text which is not understandable private key: key only known to owner that can be used to encrypt message to confirm author of message // can be used by owner to decrypt a message thereby ensuring only owner can read message 1 1 (d) • Manager encrypts email • using public key of colleague • colleague decrypts email • using his/her private key 1 1 1 1 Total: 12 4 (a) (i) HomeAddress.ThisHouseNo ← 34 1 (ii) DECLARE ThisHouseNo: 1..10 1 DECLARE ThisTown: [Brightown, Arunde, Shoram] 1 (b) (i) TYPE WeatherStation DECLARE StationID : STRING DECLARE Latitude : REAL DECLARE Temperature : ARRAY[1..15] OF INTEGER ENDTYPE 1 1 1 + 1 1 (ii) StationID is hashed to produce home location If home location is free insert record Else use overflow method to find free location 1 1 1 Total: 11

Page 4 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 33 © Cambridge International Examinations 2015 5 (a) (i) Circuit 1 A B X 0 0 1 0 1 0 1 0 0 1 1 0 1 (ii) Circuit 2 A B X 0 0 1 0 1 0 1 0 0 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 : ) . ( B A B + bar over whole expression 1 1 1 (d) B B A + ) . ( = ) . ( B A . B = B B A ). . ( = ) . .( B B A = 0. A =0 allow f.t. from (c) 1 1 1 1 1 max 3 Total: 11

Page 5 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9608 33 © Cambridge International Examinations 2015 6 (a) Control system 1 (b) Any three different items – max 6 marks heater / water pump / blinds pump … for altering temperature / watering / light level actuator for… fan for distributing air / water pump / blinds motor analogue to digital converter / digital to analogue convertor … converts analogue signal from sensor to digital value for processing / converts digital signal to analogue signal for controlling actuator microprocessor … executes control software warning device (speaker/buzzer/light)… to give warning if conditions out of range / hardware malfunction 1 1 1 1 1 1 1 1 1 1 max 6 (c) (i) output of system (alter temperature / light level / soil moisture) affects input from sensors continuous 3 (ii) min / max / ideal / mean / extreme temperature // sampling rate // tolerance interval 1 (iii) reading from sensor is compared with parameter appropriate action is taken (by example) 2 (d) (i) reading available for processing reading value is 4 1 1 (ii) AND #B00000100 // AND #&04 // AND #4 1 mark for AND, 1 mark for address mode, 1 mark for mask, 1 mark for indication of numbering system max 3 Total: 18