Cambridge A Level Computer Science 9608 — 2019 Oct/Nov Paper 3 · Variant 2

1 (a) The following incomplete table shows descriptions relating to the security of data transmission. Complete the table with the appropriate terms. Description Term A The original data to be transmitted as a message ......................................... An electronic document from a trusted authority that B ......................................... ensures authentication An encryption method produced by a trusted authority C ......................................... that can be used by anyone [3] (b) (i) Explain the purpose of a digital signature. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Describe how a digital signature is produced for transmission with the message. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3]

Mark scheme: Question Answer Marks 1(a) 1 mark per correct row 3 Description Term A The original data to be transmitted as a message Plain text B An electronic document from a trusted authority that Digital ensures authentication certificate C An encryption method produced by a trusted authority Public key that can be used by anyone 1(b)(i) 1 mark per bullet point to max 2 2 • To ensure a document is authentic // came from a trusted source • To ensure a document has not been altered during transmission • Non repudiation 1(b)(ii) 1 mark per bullet point to max 3 3 • The message is hashed with the agreed hashing algorithm « • « to produce a message digest • The message digest is encrypted with the sender’s private key« • « so the digital signature can be decrypted with sender’s public key

2 (a) A Boolean 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) Write the Boolean expression for the truth table by applying the sum-of-products. X = ..................................................................................................................................... ..................................................................................................................................... [3] (ii) Complete the Karnaugh Map (K-map) for the truth table in part (a). 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 groups in the table in part (a)(ii), to produce an optimal sum-of-products. [2] (iv) Write, using your answer to part (a)(iii), a simplified Boolean expression for your Karnaugh map. X = ............................................................................................................................... [2] (b) Simplify the following expression using De Morgan’s laws. Show your working. (W + X) • (Y + Z) ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 2(a)(i) 1 mark for each 2 correct products, i.e. 3 marks for 6, 2 marks for 4 or 5, 1 mark 3 for 2 or 3 ܆ ൌ ۯ . ࡮. ࡯ ൅ ࡭ . ۰ . ۱ ൅ ۯ . ۰ . ࡯ ൅ ࡭ . ۰ . ۱ ൅ ۯ . ࡮ . ࡯ ൅ ࡭ . ࡮ . ۱ 2(a)(ii) 1 mark for the correct K-map 1 AB 00 01 11 10 0 1 1 0 1 C 1 1 1 0 1 2(a)(iii) 1 mark for each correct loop 2 AB 00 01 11 10 0 1 1 0 1 C 1 1 1 0 1 2(a)(iv) 1 mark per bullet point: 2 • ۯ • + ۰ ܆ ൌ ۯ ൅ ۰ // ܆ ൌ ۰ ൅ ۯ 2(b) 3 ܆ ൌቀ൫܅ ൅ ܆൯. ሺ ܇ ൅ ܈ ሻቁ One mark for correct use of De Morgan’s law to + • ܆ ൌ൫ ܅ ൅ ܆൯൅൫܇ ൅ ܈൯ One mark for correct use of De Morgan’s law + to • ܆ ൌ ܅ . ܆ + ܇ . ܈ One mark for correct answer • ܆ ൌ ܅ . ܆ + ܇ . ܈

3 A computing department in a school has a Local Area Network (LAN) with a bus topology. (a) A description of sending a message on a bus network is given. Complete the following description by inserting an appropriate term in each space. Computer 1 and Computer 2 are on the same bus network. Computer 1 sends a message to Computer 2. Before the message is sent, it is split into ................................................ . Computer 1 needs to check that the ................................................ is free, before sending the message, otherwise a ................................................ will occur that will be managed by the ................................................ protocol. [4] (b) The computing department’s LAN needs to connect to the Internet. Explain how each device is used in the operation of the bus network. Router ....................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Network Interface Card (NIC) ................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [4] (c) The computing department’s network is being adapted to allow students to connect wireless devices. (i) Identify two types of hardware components the computing department will need to allow wireless connection. 1 ......................................................................................................................................... 2 ......................................................................................................................................... [2] (ii) Describe how the wireless connection sends and receives data. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [4]

Mark scheme: 3(a) 1 mark per appropriate term: 4 Computer 1 and Computer 2 are on the same bus network. Computer 1 sends a message to Computer 2. Before the message is sent, it is split into packets. Computer 1 needs to check that the line / path / bus / channel is free before sending the message, otherwise a collision will occur that will be managed by the CSMA/CD protocol. 3(b) 1 mark per bullet point, max 2 marks per device: 4 Router • Contains a routing table • « holds the (private) IP / MAC addresses of the devices on the bus • provides the public IP address for the network • forwards packets to the correct IP / MAC address • may contain a firewall / proxy server NIC • Has a (unique) MAC address • « to identify a device • Provides a hardware interface // physical connection between computer and network // connection to Ethernet cable 3(c)(i) 1 mark per bullet point to max 2 2 • Wireless Access Points // WAP • Wireless Network Interface Card // WNIC // Wireless adapter • Wireless Router • (Wireless) Repeater 3(c)(ii) 1 mark per bullet point to max 4 4 Send (max 3 marks) • Encrypts the data (using TKIP or AES) • « encodes the data into radio / analogue signal • Listens for other radio signals • Transmits / broadcasts when quiet • « using an antenna / WNIC / WAP / aerial Receive (max 3 marks) • (Constantly) checking / tuning into / detecting signals • Receives radio / analogue signal • « using an antenna / WNIC / WAP / aerial • Checks if the signal is for that device • If so, decodes the signal • « and decrypts the data

4 Physical memory is managed using virtual memory and paging. (a) Describe what is meant by virtual memory. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) (i) Explain how paging is used to manage virtual memory. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [4] (ii) Give a suitable page replacement algorithm for this process. ..................................................................................................................................... [1] (iii) One drawback of using virtual memory is disk thrashing. Describe what is meant by the term disk thrashing. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 4(a) 1 mark per bullet point to max 2 2 • Disk / secondary storage is used to extend the RAM / memory available • « so CPU can access more memory space than available RAM • Only part of program / data in use needs to be in RAM • Data is swapped between RAM and disk 4(b)(i) 1 mark per bullet point to max 4 4 • Divide memory / RAM into frames • Divide virtual memory into blocks of same size called pages • Frames / pages are a fixed size • Set up a page table to translate logical to physical addresses • Keep track of all free frames • Swap pages in memory with new pages from disk when needed 4(b)(ii) First-in-first-out // least-recently-used page // least-used-page 1 4(b)(iii) 1 mark per bullet point to max 2 2 • Pages are required back in RAM as soon as they are moved to disk • There is continuous swapping (of the same pages) • No useful processing happens // deadlock • « (because) pages that are in RAM and on disk are inter-dependent • « (nearly) all processing time is used for swapping pages

5 A weather station uses monitoring and control systems. (a) Describe the difference between a monitoring system and a control system. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) (i) The weather station records how the outside temperature changes over a period of time. The system will read the temperature once every hour, over a period of 100 days. The temperature readings are automatically stored in a file. No other data are stored. Explain why the weather station has decided to use serial organisation for the file. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Serial files can be accessed using sequential access. Explain how sequential access could be used for the temperature readings file. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) Name and describe a method of file organisation other than serial or sequential. Method .............................................................................................................................. Description ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [4]

Mark scheme: 5(a) 1 mark per bullet point to max 2 2 • Monitoring only gathers information but control systems also perform actions • Output in a control system can affect the input • There is no feedback in a monitoring system // a control system relies on feedback • Example of monitoring applied to the weather station 5(b)(i) 1 mark per bullet point to max 2 2 • So the readings are stored in chronological order • Easy to add / append each new reading to the end of the file // no further processing is required • Allows the readings to be read in the order that they were taken • Readings do not need to be given further identification as to date / time // no key field needs to be added 5(b)(ii) 1 mark per bullet point max 2 2 • Earliest temperature reading is accessed first • and each successive temperature reading is read (in date / time order) • until the final reading has been accessed 5(b)(iii) 1 mark for Random 4 1 mark per bullet point for description to max 3 • Record locations are calculated • « using a hashing algorithm on a key field • If a record cannot be stored / found at that location • « then subsequent locations are searched // closed hash • ... or an overflow area is searched // open hash

6 (a) State what is meant by a user-defined data type. ................................................................................................................................................... ............................................................................................................................................. [2] (b) A pseudocode declaration for a user-defined data type for the months of the year is as follows: TYPE DECLARE Months: ( January, February, March, April, May, June, July, August, September, October, November, December) ENDTYPE (i) Identify this type of user-defined data type. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Write a pseudocode statement to declare a variable CurrentMonth of data type Months. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) Write a pseudocode statement to assign the value August to the variable CurrentMonth. ........................................................................................................................................... ..................................................................................................................................... [1]

Mark scheme: 6(a) 1 mark per bullet point to max 2 2 • Derived from one or more existing data types • Used to extend the built-in data types • Creates data-types specific to applications // programmer’s requirements 6(b)(i) Enumerated (data type) 1 6(b)(ii) DECLARE CurrentMonth : Months 1 6(b)(iii) CurrentMonth ← August 1

7 The following are the first few lines of a source code program written in a high-level language. The source code program is to be translated by the language compiler. // program written on 15 June 2019 DECLARE IsFound : Boolean; DECLARE NoOfChildren : Integer; DECLARE Count : Integer; Constant TaxRate = 15; // start of main program For Count = 1 to 50 … … … (a) During the lexical analysis stage, the compiler will use a keyword table and a symbol table. (i) Identify two types of data in the keyword table. Type 1 ................................................................................................................................ Type 2 ................................................................................................................................ [2] (ii) Identify two types of data in the symbol table. Type 1 ................................................................................................................................ Type 2 ................................................................................................................................ [2] (iii) Explain how the contents of the keyword and symbol tables are used to translate the source code program. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iv) State one additional task completed at the lexical analysis stage that does not involve the use of a keyword or a symbol table. ........................................................................................................................................... ..................................................................................................................................... [1] (b) The final stage of compilation can be code optimisation. Explain why code is optimised. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2]

Mark scheme: 7(a)(i) 1 mark for each bullet point to max 2 2 Keyword table: • The reserved words used • The operators used • Their matching tokens 7(a)(ii) 1 mark for each bullet point to max 2 2 Symbol table: • Identifier name used • « the (data) type • « role, e.g. variable, constant, array, procedure / scope • Location (marker) // value of constant 7(a)(iii) 1 mark per bullet point to max 2 2 • Keywords / operators are looked up (in the keyword table) • Keywords / operators are represented by tokens • Identifiers are looked up in (the symbol table) • Identifiers are converted to locations / addresses • Used to create a sequence of tokens (for the program) 7(a)(iv) The white space removed // redundant characters are removed // removal of 1 comments // identification of errors 7(b) 1 mark per bullet point to max 2 2 • Redundant code removed // fewer instructions required • Program requires less memory / storage space • Code reorganised to make it more efficient • Program will complete task in a shorter time

8 (a) The following 16-bit binary pattern represents a floating-point number stored in two’s complement form. The twelve most significant bits are used for the mantissa and the four least significant bits are used for the exponent. Most Least significant bit significant bit 0 1 1 1 0 0 0 0 0 0 0 0 1 1 0 1 (i) Identify the binary value of the exponent. ..................................................................................................................................... [1] (ii) Identify the binary value of the mantissa. ..................................................................................................................................... [1] (iii) State whether the number stored is positive or negative. Justify your choice. Positive or negative ........................................................................................................... Justification ....................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (iv) Convert the binary floating-point number in part (a) into denary. Show your working. Working ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Denary value ..................................................................................................................... [3] (b) The number of bits used for the exponent is increased to eight, and the number of bits used for the mantissa is decreased to eight. State the effects of this change. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2]

Mark scheme: 8(a)(i) 1101 1 8(a)(ii) 011100000000 1 8(a)(iii) 1 mark for positive, 1 for justification 2 • Positive « • « the most significant / first bit in the mantissa is 0 8(a)(iv) 1 mark per bullet point 3 • Exponent = 1011 = − 3 // binary point moved 3 places left • Mantissa 0.111 becomes 0.000111 // ⅞ // ½ + ¼ + ⅛ // 2-1 + 2-2 + 2-3 • Answer: 7 / 64 // 0.109375 8(b) 1 mark per bullet point 2 • Increases the range • Decreases the precision