Cambridge A Level Computer Science 9608 — 2021 May/June Paper 3 · Variant 2

1 In a computer system, two real numbers, A and B, are stored using floating-point representation with: • 12 bits for the mantissa • 4 bits for the exponent • two’s complement form for both mantissa and exponent. Number A Mantissa Exponent 0 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 Number B Mantissa Exponent 1 1 1 0 0 0 0 0 0 0 0 0 0 1 0 0 (a) (i) Convert the binary values of the mantissa and the exponent for each number to their separate denary values. A mantissa ....................................................................................................................... ........................................................................................................................................... A exponent ....................................................................................................................... ........................................................................................................................................... B mantissa ....................................................................................................................... ........................................................................................................................................... B exponent ....................................................................................................................... ........................................................................................................................................... [4] (ii) Calculate the denary value of each floating-point number using your values from part (a)(i). Number A ......................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Number B ......................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [2] (b) State which number, A or B, is stored in normalised floating-point form. Justify your answer. Number ..................................................................................................................................... Justification ............................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [3]

Mark scheme: Question Answer Marks 1(a)(i) A mantissa: 0.75 // 3/4 4 A exponent: −1 B mantissa: −0.25 // –1/4 B exponent: 4 1(a)(ii) A: 0.375 // 3/8 2 B: −4 1(b) Number A 3 Justification Using the mantissa The first two bits are different // first bit 0 second bit 1

2 The TCP/IP protocol suite can be viewed as a stack with four layers. (a) Write the correct descriptions for the two layers and the correct layers for the two descriptions given in the following table. Layer Description Application Transport Handles transmission of data Handles how data is physically sent [4] (b) Identify and state the purpose of two communication protocols other than TCP/IP. Protocol 1 ................................................................................................................................. Purpose .................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Protocol 2 ................................................................................................................................. Purpose .................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [4]

Mark scheme: 2(a) One mark for each correct row 4 Layer Description Handles access to services // Application manages data exchange // defines protocols used Transport Handles packets Internet // Network Handles transmission of data Network Access // Interface // Handles how data is physically sent (Data) Link // Physical 2(b) One mark for identification, one mark for purpose 4 Max four for two protocols e.g. Protocol: FTP// File Transfer Protocol Purpose: To directly transfer data between two computers over a network Protocol: POP3 //Post Office Protocol 3 // IMAP // Internet Message Access Protocol Purpose: To receive e-mail // download emails from a server Protocol: SMTP // Simple Mail Transfer Protocol Purpose: To send e-mail Protocol: Ethernet Purpose: To send/receive data along a cable Protocol: BitTorrent Purpose: used for peer-to-peer file sharing Protocol: HTTP(S)//Hypertext Transfer Protocol (Secure) Purpose: transfer of web pages/hypertext

3 (a) Describe, with the aid of a diagram, a bus topology network. Description ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [3] (b) Describe the way in which a bus network uses Ethernet technology for communication. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4]

Mark scheme: 3(a) Any three from: 3 Max 2: • Diagram showing bus topology structure … • … correct labels (e.g. terminator, device/node/workstation, backbone/main cable) Max 2: • All nodes connected to a single cable … • … with a terminator at each end • Uses half duplex 3(b) Any four from: 4 • Ethernet operates over cable // Ethernet does not operate over Wi-Fi • Destination and source nodes are identified by MAC addresses • Data to be transmitted is divided into frames … • … containing, e.g. source and destination addresses/data/error checking data • All data is transmitted over the same cable • Using CSMA/CD

4 (a) The truth table for a logic circuit with four inputs is shown. INPUT OUTPUT P Q R S X 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 0 0 1 0 0 1 0 1 0 1 0 0 1 1 0 0 0 1 1 1 0 1 0 0 0 0 1 0 0 1 0 1 0 1 0 0 1 0 1 1 1 1 1 0 0 0 1 1 0 1 0 1 1 1 0 0 1 1 1 1 1 (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. PQ 00 01 11 10 00 01 RS 11 10 [2] (iii) The K-map can be used to simplify the expression in part (a)(i). Draw loops around appropriate groups of 1s in the table in part (a)(ii) to produce an optimal sum-of-products. [2] (iv) Write the simplified sum-of-products expression for your answer to part (a)(iii). X = ............................................................................................................................... [2] (b) Simplify your expression for X in part (a)(i) using Boolean algebra. Show your working. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2]

Mark scheme: 4(a)(i) One mark for 2 correct terms or two marks for 4 correct terms and no other 2 terms X = P.Q.R.S + P.Q.R.S + P.Q.R.S + P.Q.R.S 4(a)(ii) Two marks for fully correct K-map 2 One mark for a K-map with one error Zero marks for a K-map with two or more errors PQ 00 01 11 10 00 1 1 0 0 01 0 0 0 0 RS 11 0 0 1 1 10 0 0 0 0 4(a)(iii) One mark for each correct loop max 2 2 PQ 00 01 11 10 00 1 1 0 0 01 0 0 0 0 RS 11 0 0 1 1 10 0 0 0 0 4(a)(iv) One mark per point 2 • P.R.S • +P.R.S X = P.R.S + P.R.S 4(b) One mark for correct use of distributive law 2 One mark for correct use of complementary law One mark for correct use of redundancy law Max two e.g. X = P.Q.R.S + P.Q.R.S + P.Q.R.S + P.Q.R.S X = P.R .S ( Q + Q ) + P.R.S ( Q + Q ) X = P.R.S + P.R.S // P.R .S (1) + P.R.S (1)

5 Flip-flops are used in computer construction. (a) Describe the role of flip-flops in a computer. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) Describe the difference between an SR flip-flop and a JK flip-flop. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2]

Mark scheme: 5(a) One mark per bullet point to max 3 3 • Circuit / electronic components (construction) • With two states • Used for data storage elements // memory • …to store 1 bit of data 5(b) One mark per bullet point to max 2 2 • SR flip-flop has undefined / invalid / indeterminate state // JK flip-flop is stable • Description of undefined / invalid / indeterminate state for SR // Description of why JK flip flop is stable • JK flip flop has a clock pulse

6 The syntax diagrams for a programming language show the syntax of: • an assignment statement • a variable • an unsigned integer • a letter • an operator • a digit assignment statement unsigned variable = variable operator integer variable letter digit unsigned integer digit digit letter operator digit X + 1 Y 2 – Z * 3 (a) Give reasons why each of these statements is invalid. X1 = Y - 21 ................................................................................................................................................... Y3 := Y3 + 1 ................................................................................................................................................... X1 = X2 * 7 ................................................................................................................................................... [3] (b) Complete the Backus-Naur Form (BNF) for the syntax diagrams shown. <letter> has been completed for you. <letter> ::= X|Y|Z <assignment_statement> ::= ................................................................................................................................................... <variable> ::= ................................................................................................................................................... <unsigned_integer> ::= ................................................................................................................................................... <operator> ::= ................................................................................................................................................... <digit> ::= ................................................................................................................................................... [5] (c) The syntax of an assignment statement is changed to allow for a variable or an unsigned integer before and after the operator. (i) Draw an updated syntax diagram for the assignment statement. [2] (ii) Give the BNF for the revised assignment statement syntax. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 6(a) • Y is not a variable 3 • := should be = for an assignment statement • 7 is not a valid digit 6(b) <assignment_statement> ::= <variable> = <variable> 1 5 <operator> <unsigned_integer> 1 <variable> ::= <letter> <digit> 1 <unsigned_integer> ::= <digit> | <digit><digit> 1 <operator> ::= + | - | * <digit>::= 1 | 2 | 3 1 6(c)(i) assignment statement 2 Syntax diagram shows variable and unsigned integer in correct places 1 with correct arrows throughout 1 6(c)(ii) Two marks fully correct 2 Or One mark <assignment statement>::= and any 2 correct alternatives Or One mark missing <assignment statement> ::= and rest correct <assignment statement> ::= <variable> = <variable><operator><variable> |<variable> = <unsigned integer><operator><variable> |<variable> = <variable><operator><unsigned integer> |<variable> = <unsigned integer><operator> <unsigned integer>

7 (a) A digital certificate and a digital signature are used to ensure that a message is not changed during transmission. Write an appropriate term in each space to complete the descriptions. A digital certificate contains the ...................................... key of the owner. A digital certificate is obtained from the ...................................... . Before a private message is sent to the owner of the digital certificate, this key is used to ...................................... the message. A digital signature is also sent. The message is hashed to produce a ...................................... , which is then encrypted with the sender’s ...................................... key to obtain the digital signature. [5] (b) State two encryption protocols used in data transmission. 1 ................................................................................................................................................ 2 ................................................................................................................................................ [2] (c) Malware can harm computer systems. Describe two methods that can be used to restrict the effect of malware. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4]

Mark scheme: 7(a) Public 5 Certificate Authority // CA Encrypt (Message) Digest Private 7(b) Any two from: 2 • TLS // Transport Layer Security • SSL// Secure Socket Layer • HTTPS 7(c) One mark method, one mark description 4 Any two from: e.g. Use of firewall (1) to enforce rules for downloading data (1) Use of antivirus software (1) to quarantine viruses(1) Not clicking on links in emails from unknown source (1) to redirection to a bogus website (1)

8 Four greenhouses are used to grow tomatoes. The temperature inside each greenhouse should be kept between 10 and 20 degrees Celsius inclusive. Each greenhouse has a temperature sensor. A computer system is programmed to control each greenhouse’s temperature by: • turning on the heater and closing the ventilation when the temperature falls below 10 degrees • turning off the heater and opening the ventilation when the temperature rises above 20 degrees. (a) (i) State the name given to the type of system described. ..................................................................................................................................... [1] (ii) Justify your answer to part (i). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) The computer system stores the temperature readings for the four sensors in two’s complement form and in four eight-bit memory locations with addresses 701 to 704. 701 0 0 0 0 1 0 1 0 Greenhouse 1 702 0 0 0 1 0 1 1 1 Greenhouse 2 703 0 0 0 0 1 1 1 0 Greenhouse 3 704 1 1 1 1 1 1 1 1 Greenhouse 4 State the greenhouse number(s) where the temperature is out of range and give the value(s) of these temperature(s) in denary. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (c) The status of the heaters and the ventilation is shown at location 700. • A value of 1 means that the heater is on. • A value of 0 (zero) means that the heater is off. • A value of 1 means that the ventilation is open. • A value of 0 (zero) means that the ventilation is closed. The status of the heaters is shown in the most significant four bits; the status of the ventilation is shown in the least significant four bits. The pattern of bits at location 700 shows that the heater for greenhouse 3 is on and the ventilation for greenhouse 1 is open. Greenhouse number 1 2 3 4 1 2 3 4 700 0 0 1 0 1 0 0 0 Heater Ventilation Show the pattern of bits when the heater is on for greenhouses 1 and 2 only and no ventilation is open. 700 [1] (d) The table shows assembly language instructions for the greenhouse computer system that has one general purpose register, the accumulator (ACC). Instruction Explanation Label Op code Operand LDM &n Load the hexadecimal number n to ACC Load the contents of the location at the given address LDD <address> to ACC STO <address> Store the contents of ACC at the given address Bitwise AND operation of the contents of ACC with the AND &n hexadecimal number n Bits in ACC are shifted denary number n places to the LSL #n left. Zeros are introduced at the right hand end Compare the contents of ACC with the hexadecimal CMP &n number n Following a compare instruction, jump to <address> JPE <address> or <label> if the compare was True <label>: <op code> <operand> Labels an instruction If the bit for a greenhouse’s heater and the bit for the same greenhouse’s ventilation are both set to 1, a routine at label ERROR is executed. This routine has not been provided. (i) These assembly language instructions check for an error in the greenhouse 1 system. LDD 700 AND &88 CMP &88 JPE ERROR Explain the purpose of each instruction. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [4] (ii) Write the assembly language instructions to check for an error in the greenhouse 2 system. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3]

Mark scheme: 8(a)(i) Control (system) 1 8(a)(ii) Any two from: 2 • Automatically controls devices / heaters / vents • … using actuators • With the use of feedback // output affects the values that are input • To maintain the required temperature range 8(b) (Greenhouse) 2: 23 degrees 2 (Greenhouse) 4: −1 degree 8(c) 1 700 1 1 0 0 0 0 0 0 8(d)(i) One mark each from: 4 • Load the accumulator with status of heaters and vents // Load the accumulator with the contents of address 700 • Mask out the bits for greenhouse 1 // Mask out 4th and 8th bit • See whether both heater is on and vent is open // Compare the contents of the accumulator/previous result with 10001000 / (&88) • IF the heater is on and the vent is open go / jump to ERROR routine // go / jump to ERROR routine if bit patterns are equal 8(d)(ii) One mark keeping instructions 1 and 4 the same 3 One mark AND &44 as second instruction One mark CMP &44 as third instruction LDD 700 AND &44 // AND B01000100 CMP &44 // CMP B01000100 JPE ERROR