Cambridge A Level Computer Science 9608 — 2016 May/June Paper 3 · Variant 2

This document consists of 15 printed pages and 1 blank page. DC (ST/AR) 126657 © UCLES 2016 [Turn over * 6 9 4 8 8 4 1 8 3 2 * COMPUTER SCIENCE 9608/32 Paper 3 Advanced Theory May/June 2016 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. Cambridge International Examinations Cambridge International Advanced Level

2 9608/32/M/J/16 © UCLES 2016 1 A Local Area Network (LAN) consists of four computers and one server. The LAN uses a bus topology. (a) Complete the diagram below to show how the computers and the File server could be connected. Computer D File server Computer B Computer A Computer C [2] (b) Computer C sends a data packet to Computer A. Three statements are given below. Tick (✓) to show whether each statement is true or false. Statement True False Computer C uses the IP address of Computer A to indicate that the packet is for Computer A. Computer B can read the packet sent from Computer C to Computer A. The File server routes the packet to Computer A. [3]

3 9608/32/M/J/16 © UCLES 2016 [Turn over (c) Computer A starts transmitting a packet to Computer C. At exactly the same time, the File server starts transmitting a packet to Computer D. This causes a problem. (i) State the name given to this problem. … …[1] (ii) Give three steps taken by both Computer A and the File server to allow them to transmit their packets successfully. Step 1 … … Step 2 … … Step 3 … …[3] (d) Adding a switch to the LAN changes its topology. Explain how the use of a switch removes the problem identified in part (c)(i). … … … … … … … …[4]

4 9608/32/M/J/16 © UCLES 2016 2 Digital certificates are used in Internet communications. A Certificate Authority (CA) is responsible for issuing digital certificates. (a) Name three data items present in a digital certificate. 1 … 2 … 3 …[3] (b) The method of issuing a digital certificate is as follows: 1 A user starts an application for a digital certificate using their computer. On this computer a key pair is generated. This key pair consists of a public key and an associated private key. 2 The user submits the application to the CA. The generated … (i) … key and other application data are sent. The key and data are encrypted using the CA’s … (ii) … key. 3 The CA creates a digital document containing all necessary data items and signs it using the CA’s … (iii) … key. 4 The CA sends the digital certificate to the individual. In the above method there are three missing words. Each missing word is either ‘public’ or ‘private’. State the correct word. Justify your choice. (i) … Justification … …[2] (ii) … Justification … …[2] (iii) … Justification … …[2]

5 9608/32/M/J/16 © UCLES 2016 [Turn over (c) Alexa sends an email to Beena. Alexa’s email program: • produces a message digest (hash) • uses Alexa’s private key to encrypt the message digest • adds the encrypted message digest to the plain text of her message • encrypts the whole message with Beena’s public key • sends the encrypted message with a copy of Alexa’s digital certificate Beena’s email program decrypts the encrypted message using her private key. (i) State the name given to the encrypted message digest. …[1] (ii) Explain how Beena can be sure that she has received a message that is authentic (not corrupted or tampered with) and that it came from Alexa. … … … …[2] (iii) Name two uses where encrypted message digests are advisable. 1 … 2 …[2]

6 9608/32/M/J/16 © UCLES 2016 3 (a) The following diagram shows how applications X, Y and Z can run on a virtual machine system. Application X Application Y Application Z Guest operating system 1 Guest operating system 2 Virtual machine software / virtual machine monitor Host operating system Hardware (i) The virtual machine software undertakes many tasks. Describe two of these tasks. Task 1 … … Task 2 … …[2] (ii) Explain the difference between a guest operating system and a host operating system. … … … …[2]

7 9608/32/M/J/16 © UCLES 2016 [Turn over (b) A company uses a computer as a web server. The manufacturer will no longer support the computer’s operating system (OS) in six months’ time. The company will then need to decide on a replacement OS. The company is also considering changing the web server software when the OS is changed. Whenever any changes are made, it is important that the web server service is not disrupted. In developing these changes, the company could use virtual machines. (i) Describe two possible uses of virtual machines by the company. Use 1 … … … … Use 2 … … … …[4] The web server often has to handle many simultaneous requests. (ii) The company uses a virtual machine to test possible solutions to the changes that they will need to make. Explain one limitation of this approach. … … … …[2]

8 9608/32/M/J/16 © UCLES 2016 4 (a) Three file organisation methods and two file access methods are shown below. Draw lines to link each file organisation method to its appropriate file access method or methods. serial sequential random File organisation method direct sequential File access method [4]

9 9608/32/M/J/16 © UCLES 2016 [Turn over (b) A bank has a very large number of customers. The bank stores data for each customer. This includes: • unique customer number • personal data (name, address, telephone number) • transactions The bank computer system makes use of three files: • A – a file that stores customer personal data. This file is used at the end of each month for the production of the monthly statement. • B – a file that stores encrypted personal identification numbers (PINs) for customer bank cards. This file is accessed when the customer attempts to withdraw cash at a cash machine (ATM). • C – a file that stores all customer transaction records for the current month. Every time the customer makes a transaction, a new record is created. For each of the files A, B and C, state an appropriate method of organisation. Justify your choice. (i) File A organisation … Justification … … … …[3] (ii) File B organisation … Justification … … … …[3] (iii) File C organisation … Justification … … … …[3]

10 9608/32/M/J/16 © UCLES 2016 5 (a) Complete the truth table for this NAND gate: A B X A B X 0 0 0 1 1 0 1 1 [1] A SR flip-flop is constructed using two NAND gates. S Q R Q (b) (i) Complete the truth table for the SR flip-flop. S R Q Q– Initially 1 0 0 1 R changed to 1 1 1 S changed to 0 0 1 S changed to 1 1 1 S and R changed to 0 0 0 [4] (ii) One of the combinations in the truth table should not be allowed to occur. State the values of S and R that should not be allowed. Justify your choice. S = … R = … … … … …[3]

11 9608/32/M/J/16 © UCLES 2016 [Turn over Another type of flip-flop is the JK flip-flop. (c) (i) Give one extra input present in the JK flip-flop. … …[1] (ii) Give one advantage of the JK flip-flop. … …[1] (d) Describe the role of flip-flops in a computer. … … … …[2]

12 9608/32/M/J/16 © UCLES 2016 6 An intruder detection system for a large house has four sensors. An 8-bit memory location stores the output from each sensor in its own bit position. The bit value for each sensor shows: • 1 – the sensor has been triggered • 0 – the sensor has not been triggered The bit positions are used as follows: Not used Sensor 4 Sensor 3 Sensor 2 Sensor 1 The output from the intruder detection system is a loud alarm. (a) (i) State the name of the type of system to which intruder detection systems belong. …[1] (ii) Justify your answer to part (i). … …[1] (b) Name two sensors that could be used in this intruder detection system. Give a reason for your choice. Sensor 1 … Reason … … Sensor 2 … Reason … …[4]

13 9608/32/M/J/16 © UCLES 2016 [Turn over The intruder system is set up so that the alarm will only sound if two or more sensors have been triggered. An assembly language program has been written to process the contents of the memory location. The table shows part of the instruction set for the processor used. Instruction Explanation Op code Operand LDD <address> Direct addressing. Load the contents of the given address to ACC STO <address> Store the contents of ACC at the given address INC <register> Add 1 to the contents of the register (ACC or IX) ADD <address> Add the contents of the given address to the contents of ACC AND <address> Bitwise AND operation of the contents of ACC with the contents of <address> CMP #n Compare the contents of ACC with the number n JMP <address> Jump to the given address JPE <address> Following a compare instruction, jump to <address> if the compare was True JGT <address> Following a compare instruction, jump to <address> if the content of ACC is greater than the number used in the compare instruction END End the program and return to the operating system

14 9608/32/M/J/16 © UCLES 2016 (c) Part of the assembly code is: Op code Operand SENSORS: B00001010 COUNT: 0 VALUE: 1 LOOP: LDD SENSORS AND VALUE CMP #0 JPE ZERO LDD COUNT INC ACC STO COUNT ZERO: LDD VALUE CMP #8 JPE EXIT ADD VALUE STO VALUE JMP LOOP EXIT: LDD COUNT TEST: CMP … JGT ALARM

15 9608/32/M/J/16 © UCLES 2016 (i) Dry run the assembly language code. Start at LOOP and finish when EXIT is reached. BITREG COUNT VALUE ACC B00001010 0 1 [4] (ii) The operand for the instruction labelled TEST is missing. State the missing operand. …[1] (iii) The intruder detection system is improved and now has eight sensors. One instruction in the assembly language code will need to be amended. Identify this instruction … Write the amended instruction …[2]

16 9608/32/M/J/16 © UCLES 2016 BLANK PAGE Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. 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. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.

® IGCSE is the registered trademark of Cambridge International Examinations. This document consists of 6 printed pages. © UCLES 2016 [Turn over Cambridge International Examinations Cambridge International Advanced Level COMPUTER SCIENCE 9608/32 Paper 3 Written Paper May/June 2016 MARK SCHEME Maximum Mark: 75 Published 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 2016 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 2016 9608 32 © Cambridge International Examinations 2016 Question Answer Marks 1 (a) Single line joining all four computers and file server One “terminator” at each end 1 1 (b) Statement True False Computer C uses the IP address of Computer A to indicate that the packet is for Computer A.
Computer B can read the packet sent from Computer C to Computer A.
The File server routes the packet to Computer A.
1 1 1 (c) (i) Collision 1 (ii) Both stop transmitting Each uses a random time Wait for time period Check for bus status Attempt to re-transmit 1 1 1 1 1 Max 3 (d) Star topology created A switch has a number of ports Each connects to a single device (using a dedicated cable) Switch provides direct transmission/path from device to device Collisions are no longer possible There are dedicated links from Computer A to Computer C AND from the Server to Computer D 1 1 1 1 1 1 Max 4 2 (a) Examples: Serial number Certificate Authority that issued certificate CA digital signature Name of company/organisation/individual/subject/owner owning Certificate ‘Subject’ public key Period during which Certificate is valid // some relevant date A mark for each correct data item – Max 3 (b) (i) Public The individual keeps their private key private // the public key can be known by others (the public) 1 1 (ii) Public The individual does not know the private key of the CA // the individual only knows the public key of the CA // only the CA can decrypt the packaged information 1 1

Page 3 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2016 9608 32 © Cambridge International Examinations 2016 (iii) Private ‘Only’ the CA’s public key will allow decryption of the Certificate // proving the certificate was issued by the CA 1 1 (c) (i) Digital signature 1 (ii) Alexa’s digital certificate (Includes) Alexa’s public key Used to hash message received // produce message digest Generated hash compared to digital signature 1 1 1 1 Max 2 (iii) Examples: Financial transaction Legal document Software distribution 1 1 1 Max 2 3 (a) (i) Examples: Create / delete virtual machine Existing hardware made available to guest OS // hardware emulation Ensures each virtual machine is protected from actions of another virtual machine 1 1 1 Max 2 (ii) Guest operating system: An operating system running in a virtual machine // Controls virtual hardware // OS is being emulated Host operating system: The operating system that is actually controlling the physical hardware // the operating system for the physical machine// the OS running the VM software Guest OS is running under the Host OS software 1 1 1 Max 2 (b) (i) Examples: Trial/use alternative replacement operating system(s) … Test to identify possible problems Much easier to create VM with a new OS than create new computer system Trial/use alternative replacement web server software … Test to identify possible problems Easier to try alternative new software and new OS combinations To provide some additional service(s) Trial/test its use - description e.g. a print server General description point – to provide a safe environment during testing (which does not disrupt the web server service) Two marks for each use Maximum two uses Max 4

Page 4 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2016 9608 32 © Cambridge International Examinations 2016 (ii) Examples: Using virtual machine means execution of extra code // emulation of some hardware … Non-VM installation may not perform in the same way Execution speed slower than non-VM system Problems in judging actual response times at time of maximum traffic needs fastest possible speed Particular hardware may be difficult to emulate 1 1 1 1 1 1 Max 2 4 (a) File organisation method File access method serial direct sequential sequential random 1 2 1 (b) (i) Sequential As all customers get statement … // high hit rate Suitable for batch processing of the records // the records will be processed one after the other File organised using customer’s unique ID (as primary key field) // Serial As all customers get statement … // high hit rate Suitable for batch processing of the records // the records will be processed one after the other Order not important 1 1 1 1 1 1 1 1 Max 3 (ii) Random Real-time transaction processing Requires fastest access to data No need to search through records 1 1 1 1 Max 3

Page 5 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2016 9608 32 © Cambridge International Examinations 2016 (iii) Serial Each new record is appended Transactions are recorded in chronological order File re-organisation not required for each new record // no need for the records to be sorted 1 1 1 Max 3 5 (a) A B X 0 0 1 0 1 1 1 0 1 1 1 0 1 (b) (i) S R Q Q 1 0 0 1 1 1 0 1 0 1 1 0 1 1 1 0 0 0 1 1 1 1 1 1 (ii) S = 0 R = 0 Produces Q = 1 , Q =1 // Q and Q have same value But Q and Q should be complements of each other Becomes unstable 1 1 1 1 Max 3 (c) (i) Clock (pulse) 1 (ii) All four possibilities are valid The 1-1 combination changes output to logical complement Unstable state avoided Invalid state cannot occur // the flip-flop is stable 1 1 1 1 Max 1 (d) Memory // data storage Stores a single bit 1 1 6 (a) (i) Monitoring system 1 (ii) This is not a ‘feedback’ system // There is no ‘control’ taking place/use of actuators // No output other than from alarm 1

Page 6 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2016 9608 32 © Cambridge International Examinations 2016 (b) Examples: Pressure … If intruder steps on sensor Infra-red … If beam cut by intruder Motion / ultrasonic… Detects any movement in an area Contact / magnetic … If door / window opened 1 – sensor 1 – justification Maximum 2 sensors Max 4 (c) (i) BITREG COUNT VALUE ACC B00001010 0 1 B00001010 B00000000 1 2 2 B00001010 B00000010 0 1 1 2 4 4 B00001010 B00000000 4 8 8 B00001010 B00001000 1 2 2 8 Mark as follows: 1 mark for: COUNT column VALUE column First two values in ACC column Rest of ACC column Max 4 (ii) #1 1 (iii) CMP #8 CMP #128 1 1