Cambridge A Level Computer Science 9608 — 2018 May/June Paper 3 · Variant 1

1 In a computer system, real numbers are stored using normalised floating-point representation with: • 12 bits for the mantissa • 4 bits for the exponent • Two’s complement form for both mantissa and exponent. (a) Find the denary value for the following binary floating-point number. Mantissa Exponent 1 0 1 1 1 0 0 1 1 0 1 0 0 1 0 1 Show your working. Working ..................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Answer ...................................................................................................................................... [3] (b) Calculate the normalised floating-point representation of 5.25 in this system. Show your working. Working ..................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Mantissa Exponent [3] (c) The size of the mantissa is decreased and the size of the exponent is increased. State how this affects the range and precision of the numbers that the computer system can represent. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...............................................................................................................................................[2]

Mark scheme: Question Answer Marks 1(a) 1 mark per bullet max 2 3 • 0101 = 5 (conversion of exponent to denary) • 1.01110011010 = –0.10001100110 (conversion of mantissa to negative binary number) • –10001.100110 (binary value)// –0.54980469 (denary value of mantissa) // –563/1024 Or • Use exponent to denormalise mantissa 1 mark for correct answer • = –17 19/32 // –17.59375 1(b) 1 mark per bullet 3 • 5.25 = 101.01 (conversion to binary) • = 0.10101 × 23 (evidence of shifting binary point appropriately) • 010101000000 0011 (stored as mantissa and exponent) 1(c) 1 mark per bullet 2 • (Size of mantissa decreased means that) precision is reduced • (Size of exponent is increased means that) range is increased

2 A programmer uses non-composite and composite data types to create a program. (a) Define the term non-composite data type. ................................................................................................................................................... ...............................................................................................................................................[1] (b) Describe two different non-composite data types. Data type 1 ............................................................................................................................... Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... Data type 2 ............................................................................................................................... Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... [4] (c) Define the term composite data type. ................................................................................................................................................... ...............................................................................................................................................[1] (d) Describe two different composite data types. Data type 1 ............................................................................................................................... Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... Data type 2 ............................................................................................................................... Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... [4]

Mark scheme: 2(a) single data type that does not involve a reference to another type/usually 1 built in to a programming language 2(b) 1 mark for data type, 1 for definition, max 4, 2 data types 4 • Integer • Stores a whole number • Boolean • Stores true or false/1 or 0/on or off • Real/Single/Double/Float/Decimal • Stores decimal numbers • String • Stores zero or more characters • Char • Stores a single character • Pointer • Whole number used to reference a memory location 2(c) data type constructed from other data types 1 2(d) 1 mark for naming, 1 for description, max 4, 2 data types 4 • Record • collection of related items which may have different data types • Array • (Indexed) collection of items with the same data type • List • (Indexed) collection of items that can have different data types • Set • stores a finite number of different values that have no order // supports mathematical operations • Class/Structure • Gives the properties and methods for an object

3 Star and bus are two types of topology that can be used in a Local Area Network (LAN). Bus topology Star topology (a) (i) State one benefit and one drawback of the star topology. Benefit ............................................................................................................................... ........................................................................................................................................... Drawback .......................................................................................................................... ........................................................................................................................................... [2] (ii) State one benefit and one drawback of the bus topology. Benefit ............................................................................................................................... ........................................................................................................................................... Drawback .......................................................................................................................... ........................................................................................................................................... [2] (b) The sequence of steps 1 to 7 describes what happens when the LAN transmits data from Computer X to Computer Y using circuit switching. Four statements (4 to 7) are missing from the sequence. A Computer X sends the data. B The sender signals node to deallocate resources. C Computer Y sends a receipt signal. D If available, Computer X sets up path between nodes. Write one letter (A to D) in the appropriate space to complete the sequence. 1 Computer X sends a connection request to Computer Y. 2 Computer Y sends ready or busy signal. 3 If busy, Computer X waits and then resends the connection request to Computer Y. 4 …………… 5 …………… 6 …………… 7 …………… [3] (c) (i) Protocols are essential for successful transmission of data over a network. The TCP/IP protocol suite operates on many layers. State the appropriate layer for each protocol in the following table. Protocol Layer TCP IP SMTP [3] (ii) Peer-to-peer (P2P) file sharing uses the BitTorrent protocol. Explain how the BitTorrent protocol allows files to be shared. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... .......................................................................................................................................[3] Question 4 begins on the next page.

Mark scheme: 3(a)(i) 1 mark per bullet, max 1 benefit, max 1 drawback 2 Benefits • Signals only go to destination//secure • Easy to connect/remove nodes or devices/trouble shoot. • Centralised management helps in monitoring the network. • Failure of one node or link doesn’t affect the rest of network. • Performance does not degenerate under load • Connections may use different protocols • Fewer collisions Drawbacks • If central device fails then whole network goes down. • Performance is dependent on capacity of central device. 3(a)(ii) 1 mark per bullet, max 1 benefit, max 1 drawback 2 Benefits • Easier to set-up/extend. • Less cable required Drawbacks • If the main cable breaks, network performance badly degraded. • Difficult to detect and troubleshoot fault at an individual station. • Efficiency reduces as the number of devices connected to it increases. • Collisions // not suitable for networks with heavy traffic. • Security is lower (because several computers receive the sent signal from the source.) 3(b) 1 mark for each correct pair of letters in the right order max 3 3 1 Computer X sends a connection request to Computer Y. 2 Computer Y sends ready or busy signal. 3 If busy, Computer X waits and then resends the connection request to Computer Y. 4 D 5 A 6 C 7 B 3(c)(i) 1 mark for each layer 3 Protocol Layer TCP Transport IP Internet/Network SMTP Application 3(c)(ii) Any three points from: 3 • BitTorrent client software made available • One computer must keep a complete copy of the torrent/file to be shared • Torrent/file is split into small pieces • A computer joins (a swarm) by using the BitTorrent software to load a torrent descriptor file • The computer can now download a piece of the file • Once a computer has a piece it can become a seed and upload (to other members of the swarm) • Pieces of the torrent are both downloaded and uploaded (by each member of the of the swarm) • A server called a tracker keeps records of all the computers in the swarm • The tracker shares their IP addresses allowing them to connect to each other

4 (a) A Boolean expression produces the following truth table. INPUT OUTPUT A B C X 0 0 0 0 0 0 1 0 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) Write the Boolean expression for the truth table as a sum-of-products. X = .................................................................................................................................[2] (ii) Complete the Karnaugh Map (K-map) for the truth table in part (a)(i). AB 00 01 11 10 0 C 1 [1] The K-map can be used to simplify the function in part (a)(i). (iii) Draw loop(s) around appropriate group(s) of 1s to produce an optimal sum-of-products for the table in part (a)(ii). [2] (iv) Write the simplified sum-of-products expression for your answer to part (a)(iii). 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 0 0 0 1 1 0 0 1 0 0 1 0 1 0 1 1 0 1 1 0 1 0 1 1 1 1 1 0 0 0 0 1 0 0 1 0 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 that corresponds to the truth table. AB CD [4] (ii) Draw loop(s) around appropriate group(s) of 1s to produce an optimal sum-of-products for the table in part (b)(i). [2] (iii) Write the simplified sum-of-products expression for your answer to part (b)(ii). X = .................................................................................................................................[2]

Mark scheme: 4(a)(i) 2 marks all products correct, 1 mark 2 or 3 products correct 2 X = A.B.C + A.B.C + A.B.C + A.B.C 4(a)(ii) 1 mark for all correct bits 1 AB 00 01 11 10 0 0 1 0 1 C 1 0 1 0 1 4(a)(iii) 1 mark for each correct loop 2 AB 00 01 11 10 0 0 1 0 1 C 1 0 1 0 1 4(a)(iv) 1 mark per bullet – allow follow through from 4(a)(iii) 2 • A.B • + A.B X = A.B + A.B 4(b)(i) 1 mark per bullet max 2 4 • Correct column headings and row headings – values only • Correct column headings and row headings – order 1 mark for 2 correct rows/columns, 2 marks for 4 correct rows/columns (based on headings) max 2 AB 00 01 11 10 00 0 1 1 0 01 0 1 1 0 CD 11 0 1 0 0 10 0 1 0 0 4(b)(ii) 1 mark for each correct loop 2 00 01 11 10 00 0 1 1 0 01 0 1 1 0 CD 11 0 1 0 0 10 0 1 0 0 4(b)(iii) 1 mark per bullet 2 A.B +B.C X = A.B + B.C

5 The following syntax diagrams show the syntax of: • an assignment statement • a variable • a signed integer • a letter • a digit • an operator assignment statement variable = variable operator signed integer variable letter letter signed integer + digit - letter x y z digit operator 1 ˄ 2 \ 3 4 5 (a) The following assignment statements are invalid. Give the reason in each case. (i) xy = xy ^ c4 Reason .............................................................................................................................. .......................................................................................................................................[1] (ii) zy = zy \ 10 Reason .............................................................................................................................. .......................................................................................................................................[1] (iii) yy := xz ^ − 6 Reason .............................................................................................................................. .......................................................................................................................................[1] (b) Complete the Backus-Naur Form (BNF) for the syntax diagrams on the opposite page. <assignment statement> ::= ................................................................................................................................................... <variable> ::= ................................................................................................................................................... <signed integer> ::= ................................................................................................................................................... <operator> ::= ................................................................................................................................................... [4] (c) Rewrite the BNF rule for a variable so that it can be any number of letters. <variable> ::= ...............................................................................................................................................[2]

Mark scheme: 5(a)(i) c4 is not a signed integer 1 5(a)(ii) 10 is not a valid signed integer // 0 is not a valid digit/signed integer 1 // only one digit allowed 5(a)(iii) wrong assignment operator // should be = not := 1 // 6 is not a valid digit/signed integer 5(b) 1 mark per bullet 4 assignment • <variable>=<variable><operator><signed integer> variable • <letter><letter> signed integer • +<digit> | -<digit> operator • ^ | \ <assignment statement> ::= <variable> = <variable><operator><signed integer> <variable> ::= <letter><letter> <signed integer> ::= +<digit> | -<digit> <operator> ::= ^ | \ 5(c) 1 mark per bullet 2 • <letter>| • <letter><variable> For example: <letter>|<letter><variable> <letter>|<variable><letter>

6 A company specialises in educational software. (a) The company is concerned that malware might disrupt their business. (i) Add appropriate descriptions and terms in the table. Description Term Redirection to a bogus website that appears to be A legitimate to gain confidential data. ................................. B Use email to attempt to gain a user’s confidential data. ................................. ......................................................................................... C Spyware ......................................................................................... ......................................................................................... ......................................................................................... D Worm ......................................................................................... ......................................................................................... [4] (ii) A member of staff is using the Internet to carry out research. They are worried about the threat from terms A and B. Identify one solution to the each of the threats. Term A ............................................................................................................................... ........................................................................................................................................... Term B ............................................................................................................................... ........................................................................................................................................... [2] (b) A customer downloads a new educational software package from the company. Explain how the customer’s and the company’s computers use a hashing algorithm to assure the customer that: • the software has come from the company (is authentic) and • no one has altered it. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...............................................................................................................................................[4]

Mark scheme: 6(a)(i) 1 mark for each term or description 4 Description Term Redirection to a bogus website that appears to be Pharming legitimate to gain confidential data Use email to attempt to gain a user’s confidential Phishing data A piece of software that records/stores user Spyware actions/keystrokes without the user’s knowledge and sends them to a third party for analysis A standalone piece of malicious software that Worm replicates itself 6(a)(ii) 1 mark for pharming solution, 1 for phishing 2 Allow follow through from (a)(i) Pharming • Use a reliable ISP //check URL is spelt correctly // check that http has changed to https //security software installed and kept updated // only accept valid public key certificates // check that links are genuine Phishing • ignore email // delete email // don’t click on links in emails 6(b) 1 mark per bullet to max 4 4 • software is put through a hashing algorithm by the company • hash total is encrypted with the company’s private key • company sends software and encrypted hash • customer is in possession of company’s public key (from the digital certificate) • customer decrypts the received hash with public key • customer hashes the received software with the hash algorithm (from the digital certificate) • if decrypted hash and the software hash match, the software has come from the company/is authentic and has not been altered.

7 A museum stores antique items that need to be kept at constant temperature. The museum is not sure about the actual temperatures. The museum installs some equipment. This records the temperatures every hour and ensures the temperature stays within a set range. (a) Identify the type of system described. ...............................................................................................................................................[1] (b) The system has a temperature sensor. Identify two other items of hardware that the museum can use for the type of system identified. Describe the purpose of each item. Item 1 ........................................................................................................................................ Purpose .................................................................................................................................... ................................................................................................................................................... Item 2 ........................................................................................................................................ Purpose .................................................................................................................................... ................................................................................................................................................... [4] (c) The equipment records the temperature in all seven rooms in the museum. Each recording is stored as two successive bytes in memory. The format is as shown. Temperature Room 7 6 5 4 3 2 1 0 Byte 1 Byte 2 The room is indicated by the setting of one of the bits in Byte 2 to 1. For example, room 7 is indicated by setting bit 7 to 1. Bit 0 of Byte 2 is a flag: • The flag’s initial value is zero. • When the reading has been processed, the flag’s value is set to 1. Byte 1 contains the temperature reading as an unsigned integer. One reading returns the following binary data. Temperature Room 7 6 5 4 3 2 1 0 1 0 1 1 0 0 1 1 0 0 1 0 0 0 0 1 Byte 1 Byte 2 (i) Analyse the data contained in the two bytes. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... .......................................................................................................................................[3] (ii) The system receives a temperature reading of 238 from room number 4. Complete the bytes 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]

Mark scheme: 7(a) control system // monitoring and control system 1 7(b) 1 mark for identifying hardware, 1 for purpose to max 4 for 2 hardware 4 devices For example: • actuator/relay/switch (1) to turn a heater/fan on or off (1) • heater (1) to heat the museum (1) • fan (1) to cool the museum (1) • analogue to digital converter (1) to convert analogue signal from sensor to a digital value that can be stored/manipulated (1) • transmission hardware//cable (1) to transfer data/signals (1) • processor (1) to manage the temperature control (1) • Visible/audible warning device (1) to give warning to a human if temperature is at a dangerous level (1) 7(c)(i) 1 mark per bullet 3 • Temperature reading is 179 • reading in room 5 • has been processed 7(c)(ii) 1 mark for each 8 bits 2 7 6 5 4 3 2 1 0 1 1 1 0 1 1 1 0 0 0 0 1 0 0 0 0 Byte 1 Byte 2