Cambridge A Level Computer Science 9608 — 2019 May/June Paper 3 · Variant 2

1 (a) A computer stores real numbers using floating-point representation. The floating-point numbers have: • eight bits for the mantissa • four bits for the exponent. The mantissa and exponent are both stored in two’s complement format. (i) Calculate the denary value of the following floating-point number. Show your working. Mantissa Exponent 0 0 1 1 0 1 1 1 0 1 0 1 Working ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Answer .............................................................................................................................. [3] (ii) State why the floating-point number in part (a)(i) is not normalised. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Give the floating-point number in part (a)(i) in normalised two’s complement format. Mantissa Exponent [2] (b) (i) Convert the denary number +11.625 into a normalised floating-point number. Show your working. Working ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Mantissa Exponent [3] (ii) Convert the denary number −11.625 into a normalised floating-point number. Show your working. Working ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Mantissa Exponent [3] (c) A student enters the following into an interpreter: OUTPUT(0.2 * 0.4) The student is surprised to see that the interpreter outputs the following: 0.08000000000000002 Explain why the interpreter outputs this value. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: Question Answer Marks 1(a)(i) 2 marks for working 3 1 mark for correct answer Working: • = 0. 0110111 x 2^5 places // exponent = 5 • = 1101.11 (moving bp 5) Answer: • = 13.75 // 13 ¾ 1(a)(ii) The first two bits of the mantissa are 0 / the same / not different / are not 01 1 1(a)(iii) 1 mark per bullet point 2 • Mantissa = 01101110 • Exponent = 0100 1(b)(i) 2 marks for working 3 1 mark for correct answer Working: • 01011.101 • 0.1011101 × 2^4 // showing calculation of exponent = 4 Answer: • 01011101 0100 1(b)(ii) 2 marks for working 3 1 mark for correct answer Working: • 10100.011 // 10100011 correct use of two’s complement or other method • Exponent = 4 Answer: • 10100011 0100 1(c) 1 mark per bullet point (max 3) 3 • 0.2/0.4 cannot be represented exactly in binary / rounding error • 0.2 has been represented by a value just greater than 0.2 // 0.4 has been represented by a value just greater than 0.4 • Therefore multiplying these two representations together increases the difference • difference after the calculation is significant enough to be seen (given the number of positions after the decimal place)

2 Packet switching can be used to transmit data across the Internet. Packet switching is not always the most appropriate method of transferring data. (a) Name an alternative method of transferring data across the Internet. ............................................................................................................................................. [1] (b) Give an example of a situation where the method you identified in part (a) is more appropriate. Justify your choice. Example .................................................................................................................................... ................................................................................................................................................... Justification ............................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [3]

Mark scheme: 2(a) Circuit switching 1 2(b) 1 mark 3 Any real-time application e.g. video conferencing // live streaming of a concert Justification 1 mark per bullet to max 2 • reduced latency • there are little/no delays in sending/receiving data once the circuit is set up • because (stringent) error checking (as used in packet switching) is not required • circuit made available is dedicated to this communication stream

3 (a) A Boolean algebraic expression produces the following truth table. INPUT OUTPUT A B C X 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 0 1 1 1 0 (i) Complete the Karnaugh Map (K-map) for the truth table. AB 00 01 11 10 0 C 1 [1] The K-map can be used to simplify the expression that produced the truth table in part (a). (ii) Draw loops around appropriate groups of 1s in the K-map to produce an optimal sum-of- products. [2] (iii) Write the simplified sum-of-products Boolean expression for the truth table. X = ............................................................................................................................... [2] (b) A logic circuit with four inputs produces the following truth table. INPUT OUTPUT A B C D X 0 0 0 0 0 0 0 0 1 0 0 0 1 0 1 0 0 1 1 1 0 1 0 0 0 0 1 0 1 0 0 1 1 0 1 0 1 1 1 1 1 0 0 0 1 1 0 0 1 1 1 0 1 0 0 1 0 1 1 0 1 1 0 0 1 1 1 0 1 1 1 1 1 0 0 1 1 1 1 0 (i) Complete the K-map for the truth table. AB CD [4] (ii) Draw loops around appropriate groups of 1s in the K-map to produce an optimal sum-of-products. [2] (iii) Write the simplified sum-of-products Boolean algebraic expression for the truth table. X = ............................................................................................................................... [2]

Mark scheme: 3(a)(i) AB 1 00 01 11 10 0 1 1 0 1 C 1 1 1 0 1 3(a)(ii) 1 mark for each correct loop 2 AB 00 01 11 10 C 0 1 1 0 1 1 1 1 0 1 3(a)(iii) 1 mark per bullet point 2 • A • + B X = A + B 3(b)(i) 1 mark correct values and order of row and column headings 4 3 marks fully correct table entries (based on headings) or 2 marks table entries contain one error (based on headings) or 1 mark table entries contain two errors (based on headings) AB 00 01 11 10 00 0 0 1 1 01 0 0 1 1 CD 11 1 1 0 0 10 1 1 0 0 3(b)(ii) 1 mark per loop 2 AB 00 01 11 10 00 0 0 1 1 01 0 0 1 1 CD 11 1 1 0 0 10 1 1 0 0 3(b)(iii) 1 mark for each bullet point 2 • A.C • + A.C X = A.C + A.C

4 (a) Describe the main steps in the evaluation of a Reverse Polish Notation (RPN) expression using a stack. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4] (b) The infix expression 8 * (5 – 2) – 30 / (2 * 3) converts to: 8 5 2 – * 30 2 3 * / – in Reverse Polish Notation (RPN). Show the changing contents of the stack as this RPN expression is evaluated. [4]

Mark scheme: 4(a) 1 mark per bullet point (max 4) 4 • Working from left to right in the expression • If element is a number PUSH that number onto the stack • If element is an operator then POP the first two numbers from stack « • « perform that operation on those numbers • PUSH result back onto stack • End once the last item in the expression has been dealt with 4(b) 1 mark per ring (not all stacks are shown) 4 Do not allow operators in stacks Accept intermediate correct stack values 3 2 2 6 5 3 30 30 5 8 8 24 24 24 24 19

5 Sanjeet is a member of the public, and he wants to send a private message to a government department. (a) Explain how asymmetric encryption is used to ensure that the message remains private. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) When the government department replies to Sanjeet, it needs to send a verified message. Explain how asymmetric encryption can be used to ensure that it is a verified message. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (c) The government’s computer systems are vulnerable to malware. (i) Describe two vulnerabilities that malware can exploit in computer systems. 1 ........................................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... 2 ......................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [4] (ii) Identify one method that can be used to restrict the effect of malware. ........................................................................................................................................... ..................................................................................................................................... [1]

Mark scheme: 5(a) 1 mark per bullet point 2 • Sanjeet’s computer/software encrypts the message with the government department’s public key • The government department’s computer/software decrypts the message with their private key 5(b) 1 mark per bullet point (max 2) 2 • The government department’s computer/software creates the message digest • Sanjeet’s computer/software recreates this message digest • If both copies of the message digest match the message has been verified 5(c)(i) For each of the two: 4 1 mark for the identification of the vulnerability and 1 mark for further description, effect or example e.g. not updating virus definitions (1 mark) which would allow for recently developed viruses to attack the computer system (1 mark) opening email from unknown sources (1 mark) download a virus (1 mark) 5(c)(ii) 1 mark from: 1 • Anti-malware software running in the background • up-to-date anti-virus (definitions) • logging off when not using computer • ensuring firewall is enabled • strong password • not sharing passwords, etc.

6 A company sells plant watering systems that automatically turn on water sprinklers when the soil becomes too dry. The plant watering system has a processor and connecting cables. Identify two other hardware devices that are required in this system. State the purpose of each device. Device 1 ........................................................................................................................................... Purpose ............................................................................................................................................ .......................................................................................................................................................... Device 2 ........................................................................................................................................... Purpose ............................................................................................................................................ .......................................................................................................................................................... [4]

Mark scheme: 6 1 mark for identifying hardware, 1 mark for the purpose to max 2 hardware 4 devices. • Moisture sensor « • « to measure the level of moisture in the soil • Humidity sensor « • «to measure the level of moisture in the air • Pressure/Precipitation sensor « • «to measure the amount of rainfall • Actuator « • «to turn the sprinklers on/off • Analogue to Digital Converter/ADC « • «to convert analogue signal from a sensor to a digital value that can be stored / recorded

7 (a) RISC (Reduced Instruction Set Computing) and CISC (Complex Instruction Set Computing) are two types of processor. Tick (3) one box in each row to show if the statement applies to RISC or CISC processors. Statement RISC CISC Larger instruction set Variable length instructions Smaller number of instruction formats Pipelining is easier Microprogrammed control unit Multi-cycle instructions [3] (b) In parallel processing, a computer can have multiple processors running in parallel. (i) State the four basic computer architectures used in parallel processing. 1 ........................................................................................................................................ 2 ........................................................................................................................................ 3 ........................................................................................................................................ 4 ........................................................................................................................................ [4] (ii) Describe what is meant by a massively parallel computer. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3]

Mark scheme: 7(a) 1 mark for 2/3 rows correct 3 2 marks for 4/5 rows correct 3 marks for 6 correct rows Statement RISC CISC Larger instruction set 9 Variable length instructions 9 Smaller number of instruction formats 9 Pipelining is easier 9 Microprogrammed control unit 9 Multi-cycle instructions 9 7(b)(i) 1 mark per bullet point 4 • SISD // Single instruction single data • SIMD // Single instruction multiple data • MISD // Multiple instruction single data • MIMD // Multiple instruction multiple data 7(b)(ii) 1 mark per bullet point (max 3) 3 • Large number of processors • « working collaboratively on the same program • « working together simultaneously on the same program • « communicating via a messaging interface

8 (a) A computer process can be in one of three states. Identify and describe two of these states. State 1 ...................................................................................................................................... Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... State 2 ...................................................................................................................................... Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... [6] (b) One of the main tasks of an operating system is resource management. Describe how an operating system can maximise the use of resources. Primary memory ....................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Disk ........................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [6]

Mark scheme: 8(a) 1 mark per bullet point 6 1 mark for identifying the state, max 2 for description Max 3 marks for each state • Ready • The process is not being executed • The process is in the queue« • « waiting for the processor’s attention / time slice • Running • The process is being executed by the processor • The process is currently using its allocated processor time / time slice • Blocked • The process is waiting for an event « • « so it cannot be executed at the moment • «e.g. input/output 8(b) For up to 2 maximisation techniques for each of memory and disk 6 Max 2 for Memory, Max 2 for disk if no descriptions are given 1 mark for identification of maximisation technique, 1 mark for description, 1 mark for further description or information about improvement to max 4 for memory Memory • Moving frequently accessed instructions to cache (1) for faster recall (1) as SRAM is used rather than DRAM for cache (1) • Making use of virtual memory (1) with paging or segmentation (1) to swap memory to and from a disk (1) • Partitioning memory (1) dividing main memory into static/dynamic partitions (1) to allow for more than one program/task to be available //multiprogramming (1) • Removing unused items/tasks from RAM (1) by marking a partition as available (1) as soon as the process using it has terminated (1) 1 mark for identification of maximisation technique, 1 mark for description, 1 mark for further description or information about improvement to max 4 for disk Disk • Disk caching (1) a disk cache holds data that is frequently transferred to/from the disk (1) the cache can be held on disk or in RAM (1) • Compression utility (1) decreasing the size of a file stored on disk (1) in order fit more / larger files on the disk (1) • Defragmentation utility (1) files are rearranged to occupy contiguous disk space (1) this reduces the time taken to access files// decreases latency (1)