Cambridge A Level Computer Science 9608 — 2019 May/June Paper 3 · Variant 1

1 In a computer system, real numbers are stored using normalised floating-point representation with: • twelve bits for the mantissa • four bits for the exponent. The mantissa and exponent are both in two’s complement form. (a) Calculate the denary value for the following binary floating-point number. Show your working. Mantissa Exponent 1 0 0 1 0 1 1 1 0 0 1 1 0 1 1 1 Working ..................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Answer ...................................................................................................................................... [3] (b) Calculate the normalised floating-point representation of +1.5625 in this system. Show your working. Working ..................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Mantissa Exponent [3] (c) (i) Write the largest positive number that can be stored as a normalised floating-point number using this format. Mantissa Exponent [2] (ii) Write the smallest non-zero positive number that can be stored as a normalised floating-point number using this format. Mantissa Exponent [2] (d) The developer of a new programming language decides that all real numbers will now be stored using 20-bit normalised floating-point representation. She must decide how many bits to use for the mantissa and how many bits for the exponent. Explain the trade-off between using either a large number of bits for the mantissa, or a large number of bits for the exponent. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: Question Answer Marks 1(a) 2 marks for working shown 3 1 mark for the correct answer Working: • Correct calculation of negative value (any method) (= – 0.11010001101) • Correctly moving the binary point 7 places (= – 01101000.1101) // Exponent 7 Answer: 13 • −104.8125 // –104 16 1(b) 2 marks for working shown 3 1 mark for the correct answer Working: • Correct conversion to binary (01.1001) • Correct calculation of exponent (1) Answer: • (Mantissa) 0110 0100 0000 (Exponent) 0001 1(c)(i) 1 mark per bullet point 2 • Mantissa = 0111 1111 1111 • Exponent = 0111 1(c)(ii) 1 mark per bullet point 2 • Mantissa = 0100 0000 0000 • Exponent = 1000 1(d) 1 mark per bullet point to max 3 3 • The trade-off is between range and precision • Any increase in the number of bits for the mantissa, means fewer bits available for the exponent // Any decrease in the number of bits for the mantissa, means more bits available for the exponent • More bits used for the mantissa will result in better precision • More bits used for the exponent will result in a larger range of numbers • Fewer bits used for the mantissa will result in worse precision • Fewer bits used for the exponent will result in a smaller range of numbers

2 Cables connect the computers in a university admissions department in a star topology. The server room contains the server and printer for the employees to use. The department has three employees. Each employee has a computer connected to the star network. (a) (i) Draw a diagram to show this topology. [3] (ii) Explain the benefits to the admissions department of using a star topology. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [4] (b) Each department of the university has its own network. All the department networks connect to the university’s main Local Area Network (LAN). The LAN has a bus topology and uses the CSMA/CD protocol. Describe the CSMA/CD protocol. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (c) Explain how the following devices are used to support the university LAN. (i) Router ............................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Network Interface Card (NIC) ............................................................................................ ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) Wireless Access Point ....................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 2(a)(i) 1 mark per bullet point 3 Diagram shows: z All four devices labelled and connected directly to the server • And no other device • Two-way flow of data between each device and the server Computer Printer 1 Server Computer Computer 2(a)(ii) 1 mark per benefit and 1 mark for a further explanation in context to max 2 4 (x2) For example: • Personal data (used by admissions department) is kept secure • « transmissions only go between server and destination • A new device/employee can be easily added to the network • «only one connection direct to server needs setting up • If one node or link fails • « the other employees can continue working / the rest of network is unaffected • If the department has a range of different devices • « they can all operate at different speeds or with different protocols 2(b) 1 mark per bullet point (max 3) 3 • Carrier Sense Multiple Access (with) Collision Detection • Before transmitting a device checks if the channel is busy • If it is busy the device waits // if channel free data is sent • When transmission begins the device listens for other devices also beginning transmission • If there is a collision, transmission is aborted / transmitting a jam signal • Both devices wait a (different) random time, then try again 2(c)(i) 1 mark per bullet point (max 2) 2 • Allows (internal) connections between the university LANs • Allows external connection from the main LAN 2(c)(ii) 1 mark per bullet point (max 2) 2 • Provides device with a MAC address • «to uniquely identify it on the network • Allows each individual device to connect to the network 2(c)(iii) 1 mark per bullet (max 2) 2 • Allowing devices to connect to the LAN via radio communication • « instead of using a cable • « easy to move a device to as different location

3 (a) The following logic circuit can be simplified to use only one gate. A X B Give the name of this single gate. ............................................................................................................................................. [1] (b) (i) Complete the truth table for the logic circuit. A X B Y Working space A B X Y 0 0 0 1 1 0 1 1 [2] (ii) Give the name of the logic circuit that has this truth table. ..................................................................................................................................... [1] (iii) Give the uses for outputs X and Y. X ........................................................................................................................................ Y ........................................................................................................................................ [2] (c) Consider the following Boolean algebraic expression: A . B . C . D + A . B . C . D + A . B . C . D + A . B . C . D + A . B . C . D Use Boolean algebra to simplify the expression. Show your working. Working .................................................................................................................................. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Simplified expression ....................................................................................................... [5]

Mark scheme: 3(a) NOR 1 3(b)(i) 1 mark for X column, 1 mark for Y column 2 A B Working Space X Y 0 0 0 0 0 1 0 1 1 0 0 1 1 1 1 0 3(b)(ii) Half adder 1 3(b)(iii) 1 mark per bullet 2 • X is (used for) carry • Y is (used for) sum 3(c) 1 mark per bullet for working (max 4) 5 A.B.C .D + A.BC. .D + A.BC. .D + A.BC. .D + A.BC. .D = A.B.C .D + A.BC. .D + A.BC. .D + A.BC. .D + A.BC. .D + A.B.C .D • Adding in a second copy of the first term (Use of Idempotent Law) = A.B. C .D + C .D + C.D + C.D + AC. .D. B + B ( ) ( ) • Taking A.B and AC. .D outside brackets (Associative Law) = A.B. C . D + D + C. D + D + AC. .D. B + B ( ( ) ( ) ) ( ) • Grouping C . D + D + C. D + D (Associative Law and Commutative ( ) ( ) Law) = A.B. + AC. .D. (1) ( C . (1) + C . (1) ) = A.B. C + C + AC. .D. (1) ( ) = A.B . (1) + AC. .D. (1) • Replacing D + D with 1 and replacing C + C with 1 (Use of ( ) ( ) Complement Law) = A.B + AC. .D • Reducing first four terms to A.B and reducing last two terms to AC. .D (Use of Identity Law) 1 mark for correct answer = A.( B + C.D )

4 A compiler uses a keyword table and a symbol table. Part of the keyword table is shown. • Tokens for keywords are shown in hexadecimal. • All of the keyword tokens are in the range 00 – 5F. Keyword Token 01 + 02 = 03 <> 04 IF 4A THEN 4B ENDIF 4C ELSE 4D REPEAT 4E UNTIL 4F TO 50 INPUT 51 OUTPUT 52 ENDFOR 53 Entries in the symbol table are allocated tokens. These values start from 60 (hexadecimal). Study the following piece of pseudocode. Counter 0 INPUT Password REPEAT IF Password <> "Cambridge" THEN INPUT Password ENDIF Counter Counter + 1 UNTIL Password = "Cambridge" OUTPUT Counter (a) Complete the symbol table to show its contents after the lexical analysis stage. Token Symbol Value Type Counter 60 Variable [3] (b) The output from the lexical analysis stage is stored in the following table. Each cell stores one byte of the output. Complete the output from the lexical analysis using the keyword table and your answer to part (a). 60 01 [2] (c) The following table shows assembly language instructions for a processor which has one general purpose register, the Accumulator (ACC). Instruction Explanation Op code Operand Direct addressing. Load the contents of the location at the given LDD <address> address to ACC ADD <address> Add the contents of the given address to the ACC STO <address> Store the contents of ACC at the given address After the syntax analysis is completed successfully, the compiler generates object code. The following lines of high level language code are compiled. X = X + Y Z = Z + X The compilation produces the assembly language code as follows: LDD 236 ADD 237 STO 236 LDD 238 ADD 236 STO 238 (i) The final stage in the compilation process that follows this code generation stage is code optimisation. Rewrite the equivalent code after optimisation. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (ii) Explain why code optimisation is necessary. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 4(a) 1 mark for 2 correct rows, 2 marks for 3 correct rows, 3 marks for 4 correct 3 rows Token Symbol Value Type Counter 60 Variable 0 61 Constant Password 62 Variable "Cambridge" 63 Constant 1 64 Constant 4(b) 2 60 01 First two cells given in question. 1 mark for next 3 cells 61 51 62 1 mark for the remainder 4E 4A 62 04 63 4B 51 62 4C 60 .... 01 60 02 64 4F 62 03 63 52 60 .... 4(c)(i) 1 mark per bullet point 3 • Removing the fourth line (LDD 238) « • Changing operand for second ADD from 236 to 238 « • « First three lines and last line unchanged LDD 236 ADD 237 STO 236 ADD 238 STO 238 4(c)((ii) 1 mark per bullet point (max 2) 2 • Optimisation means that the code will have fewer instructions • Optimised code occupies less space in memory • Fewer instructions reduces the execution time of the program

5 (a) Wiktor is an employee of a travel agent. He uses asymmetric encryption to send confidential information to his manager. Fill in the spaces with an appropriate term to complete the descriptions. Asymmetric encryption uses different ……………………………. for encrypting and decrypting data. When Wiktor sends a message to his manager, the message is encrypted into ……………………………. using his manager’s ……………………………. key. When the manager receives the message, it is decrypted using her ……………………………. key. When the manager replies, the message is encrypted using Wiktor’s ……………………………. key, and when Wiktor receives the message, it is decrypted into ……………………………. using his ……………………………. key. [5] (b) When customers pay for their travel booking online, a secure connection is established using Secure Socket Layer (SSL). Explain how the customer’s browser and the server used to collect the payment will establish a secure connection. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [6] (c) The manager is concerned about the threat of malware to the company computer systems. Name two types of malware. State what the company should do to help prevent the effect of the malware. The two methods of prevention must be different. Malware type 1 ......................................................................................................................... Prevention ................................................................................................................................ ................................................................................................................................................... Malware type 2 ......................................................................................................................... Prevention ................................................................................................................................ ................................................................................................................................................... [4]

Mark scheme: 5(a) 1 mark per bullet point 5 • Keys • Cipher text • Manager’s public and private keys in correct spaces • Wiktor’s public and private keys in correct spaces • Plain text Asymmetric encryption uses different keys for encrypting and decrypting data. When Wiktor sends a message to his manager, the message is encrypted into cipher text using his manager’s public key. When the manager receives the message, it is decrypted using her private key. When the manager replies, the message is encrypted using Wiktor’s public key, and when Wiktor receives the message, it is decrypted into plain text using his private key. 5(b) 1 mark per bullet point (max 6) 6 • Browser requests that the server identifies itself • Server sends a copy of its (Digital) Certificate • « containing its public key • Browser checks the certificate • «against a list of trusted Certificate Authorities • If the browser trusts the certificate • « a symmetric session key is created • «this is (by the browser) encrypted using the server’s public key and sent to the server • Server decrypts the symmetric session key • « using its private key • Server and browser now encrypt all transmitted data with the session key 5(c) 1 mark for type of malware and 1 mark for prevention, max 2 (x 2) 4 For example: • Virus • Have company policies to ensure that anti-virus software is installed, regularly updated and run • Spyware • Have company policies to ensure that anti-spyware software is installed, regularly updated and run • Phishing • Have network policies to ensure that the firewall criteria include SPAM filters, whitelist, blacklist etc.

6 Monitoring and control systems have many different applications. (a) Explain the importance of feedback in a control system. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) An indoor swimming pool is to be kept at a constant temperature of 28 degrees. Describe the use of feedback in this control system. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4] (c) Give one example of a monitoring system. Explain why this is a monitoring system. Monitoring system .................................................................................................................... ................................................................................................................................................... Explanation ............................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [3]

Mark scheme: 6(a) 1 mark per bullet point (max 3) 3 • To ensure that the system operates within the given criteria • By enabling system output to affect subsequent system inputs • Thus enabling the system to automatically adjust conditions • Suitable example of feedback 6(b) 1 mark per bullet point 4 • Sensors continually measure the temperature of the water in the swimming pool • The (stream of) readings are sent to a processor and compared with 28 degrees • If the reading is out of range (by a system set amount) then actuators turn the heater/cooler on or off as necessary • Feedback ensures that the water temperature remains close to 28 degrees 6(c) 1 mark for example of monitoring system, max 2 for explanation 3 • Suitable example identified • Use of data captured • No feedback as there is no output that could change the system environment For example: • Monitoring the rainfall • The amount of rainfall collected over a specific time is measured • There is no output to change the level of rainfall or • Security camera • Sending pictures to control room • No changes made to environment by system