Cambridge A Level Computer Science 9618 — 2025 Oct/Nov Paper 3 · Variant 3

1 (a) An enumerated data type, Spectrum, is required to hold the names of colours. (i) Write the pseudocode statement for the type declaration of Spectrum to hold the names of the colours available: Red, Orange, Yellow, Green, Blue, Indigo, Violet. ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Identify two characteristics of the list of values given in an enumerated data type declaration. 1 ......................................................................................................................................... ........................................................................................................................................... 2 ......................................................................................................................................... ........................................................................................................................................... [2] (b) ColourData is a composite data type to store definitions of colours. Write pseudocode statements to declare ColourData to hold the following fields: Field Example data ColourCode XYZ12345 Colour Red Wavelength 650 Frequency 4.62 PrimaryColour Yes Use the most appropriate data type in each case, including the enumerated data type Spectrum from part a(i). ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4]

Mark scheme: Question Answer Marks Guidance 1(a)(i) One mark per mark point (Max 2) 2 MP1 TYPE Spectrum = MP2 (Red, Orange, Yellow, Green, Blue, Indigo, Violet) Example answer TYPE Spectrum = (Red, Orange, Yellow, Green, Blue, Indigo, Violet) 1(a)(ii) One mark per mark point (Max 2) 2 MP1 the list is ordered/ordinal MP2 the list contains all possible values MP3 no duplicate values in list MP4 all values are the same data type 1(b) One mark for TYPE ColourData and ENDTYPE correct 4 One mark for correct use of DECLARE in all declarations One mark for correct use of Spectrum in declaration One mark for remaining four declarations correct (STRING, INTEGER, REAL, BOOLEAN) Example answer TYPE ColourData DECLARE ColourCode : STRING DECLARE Colour : Spectrum DECLARE Wavelength : INTEGER DECLARE Frequency : REAL DECLARE PrimaryColour : BOOLEAN ENDTYPE

2 Numbers are stored in a computer system using binary floating‑point representation with: • 12 bits for the mantissa • 4 bits for the exponent • two’s complement form for both the mantissa and the exponent • 2 bytes in total. (a) Outline the effect of changing the number of bits used to store the mantissa to 10 bits, in this system. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) Describe one example of a situation that could cause an overflow to occur and the consequences of such an overflow. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 2(a) One mark per mark point (Max 3) 3 MP1 the precision of the number stored in the mantissa will be reduced MP2 the number of bits available for the exponent will increase to 6 MP3 … this will increase the range of numbers that can be stored. 2(b) One mark per mark point (Max 3) 3 MP1 a process involving a calculation / the multiplication of two large numbers could take place MP2 the result might be outside of the range of values possible to store in the given system MP3 … leading to the most significant bits of the mantissa/exponent being lost (which is an overflow).

3 Describe how packet switching is used to pass messages across a network. Do not include checking for completeness and resending packets in your answer. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [4]

Mark scheme: 3 One mark per mark point (Max 4) 4 MP1 the message is divided into small chunks called packets MP2 each packet is given a header with important data including source and destination IP addresses MP3 each packet is sent independently // packets can be sent through different routes MP4 packets are sent through the optimum route // packets can be re- routed if a route is unavailable MP5 packets are reassembled at the destination into the whole message

4 Identify and describe three protocols that are used in the Application Layer of the TCP/IP protocol suite. Protocol 1 ......................................................................................................................................... Description ....................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... Protocol 2 ......................................................................................................................................... Description ....................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... Protocol 3 ......................................................................................................................................... Description ....................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... [6]

Mark scheme: 4 One mark for a correct protocol and one mark for a correct corresponding 6 description (Max 6) Three from: HTTP/S [1] a protocol/secure protocol used to transfer hypermedia documents / web pages / (data) files between networked devices [1] FTP [1] a protocol to download, upload and transfer files from one location to another on a network [1] POP3 [1] a protocol used to retrieve/receive emails from a mail server to a computer [1] IMAP [1] allows users to access/read their emails from any device without removing the message from the mail server // synchronises emails on any device [1] SMTP [1] a protocol used to send emails between mail servers // a protocol used to send emails from a computer to a mail server [1] BitTorrent [1] a communication protocol used for peer-to-peer file sharing [1]

5 (a) State the purpose of multi‑tasking. ................................................................................................................................................... ............................................................................................................................................. [1] (b) Explain how an operating system ensures that multi‑tasking operates efficiently. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 5(a) To enable multiple programs/processes to be executed at the same time 1 5(b) One mark per mark point (Max 3) 3 MP1 the operating system monitors the state of each task/process MP2 using scheduling to ensure hardware resources are used efficiently MP3 … and making sure that tasks/processes do not clash.

6 (a) The Karnaugh map (K‑map) represents a logic circuit with four inputs. AB CD 00 01 11 10 00 0 0 0 0 01 1 1 1 0 11 0 1 1 1 10 0 0 1 1 (i) Draw loop(s) around appropriate group(s) in the K‑map to produce an optimal sum‑of‑products. [3] (ii) Write the Boolean expression from your answer to part a(i) as a simplified sum‑of‑products. Do not carry out any further simplification. ........................................................................................................................................... ..................................................................................................................................... [2] (b) Simplify the following expression using De Morgan’s laws and Boolean algebra. Show all the stages in your simplification. X = A+B+C+B+C ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 6(a)(i) One mark for each correct loop (Max 3) 3 AB CD 00 01 11 10 00 0 0 0 0 01 1 1 1 0 11 0 1 1 1 10 0 0 1 1 6(a)(ii) One mark per mark point (Max 2) 2 MP1 One correct Boolean term with an OR sign MP2 All three correct Boolean terms connected by OR signs and no other terms present. A.C.D + B.D + A.C 6(b) One mark for correct use of De Morgan’s laws 3 One mark for correct use of any other Boolean algebra law X = A+B+C + B+C (X=) A.B.C + B.C ………………………………….(De Morgan’s) (X=) A.B.C + B.C ………………………………….(Double negation) (X=) C.(A.B + B) …………………………………..(Distributive) One mark for correct answer (X=) C.(A + B) ……………………………………..(Redundancy)

7 (a) Outline the process of optimisation during the compilation of a program. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) Write the Reverse Polish Notation (RPN) for the given infix expression: ((6 + 12) / (16 – 10)) * 18 ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (c) The RPN expression c a – b d + * b c + / is to be evaluated, where: a = 4, b = 12, c = 24 and d = 6. Show the changing contents of the stack as the RPN expression is evaluated. [4]

Mark scheme: 7(a) One mark per mark point (Max 2) 2 MP1 To identify and remove redundant code / To simplify expressions / To reorder the code MP2 … so that storage size/memory use/power consumption/program execution time/CPU time is minimized. 7(b) One mark per mark point (Max 2) 3 MP1 6 12 + MP2 16 10 – / MP3 18 * Final correct expression 6 12 + 16 10 – / 18 * 7(c) One mark per ring (Max 3) 4 One mark for the interim total (360) and the final total (10). 6 24 4 12 12 18 12 12 36 24 24 20 20 20 20 360 360 360 360 10

8 Calculate the shortest distance between the Start node and each of the nodes in the graph using Dijkstra’s algorithm. Show your working on the graph or in the working space. Write your answers in the table provided. V 4 9 T 3 X 5 W 3 6 10 Y 22 8 Start Z Working ............................................................................................................................................ .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... Answers: T V W X Y Z [5]

Mark scheme: 8 One mark per mark point - working (Max 3) 5 May be seen on diagram or in working section MP1 Initialisation – setting Start to 0 MP2 … and the rest of the towns to  MP3 Evidence to show values at nodes being updated MP4 Evidence to show ‘visited node(s)’ MP5 Evidence to show a correct calculation of at least one route MP6 Evidence to show more than one route has been calculated for at least one town Correct Answers (Max 2) Two marks for all six correct values One mark for four or five correct values. T V W X Y Z 6 10 13 18 21 28

9 A company requires a digital certificate to ensure the authenticity of its online communications. Outline the process followed to acquire a digital certificate. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .................................................................................................................................................... [4]

Mark scheme: 9 One mark per mark point (Max 4) 4 MP1 the company generates a public and private key pair locally using a web browser and an RSA key generator tool, which may be part of the CA’s web page MP2 the company requests a digital certificate from a Certificate Authority (CA) MP3 the CA responds with its public key and digital certificate, MP4 … signed with its private key MP5 the company gathers authentication information e.g. its public key MP6 … and sends it to the CA signed with the company’s private key and encrypted with the CA’s public key MP7 the CA verifies the received information and generates/issues the digital certificate.

10 A stack, StackArray, is to be implemented using pseudocode, to store a maximum of 100 string items in an appropriate array. Declarations are required so that the stack has a beginning, an end and a maximum size. An array is also required to store the data. (a) Write pseudocode to declare the variables, constant and array required to implement the stack. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) Write pseudocode for a procedure to initialise the top and bottom pointers of the stack to appropriate values. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 10(a) One mark per mark point (Max 3) 3 MP1 Correct declaration of constant Maximum MP2 Both correctly declared integers MP3 Correct array declaration Example answer CONSTANT Maximum = 100 DECLARE Base : INTEGER DECLARE Top : INTEGER DECLARE StackArray : ARRAY[1:100] OF STRING 10(b) One mark per mark point (Max 3) 3 MP1 Correct procedure definition structure MP2 Base pointer with appropriate value (0 or 1) MP3 Top pointer with appropriate value (–1, 0, 1) Example answer PROCEDURE InitialiseStack() Base  0 Top  0 ENDPROCEDURE

11 Describe when the use of recursion would be beneficial and give an example. Description ....................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... Example ........................................................................................................................................... .......................................................................................................................................................... [3]

Mark scheme: 11 One mark per mark point (Max 3) 3 MP1 Recursion is beneficial for problems that can be broken down into smaller, repetitive problems MP2 … especially for problems that have many possible branches / are too complex for an iterative approach MP3 An example e.g. solving mathematical series, sorting a pile of documents, etc.

12 An exception can occur when running a program. (a) Explain what is meant by an exception. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) Identify one example of an exception and give one reason why the exception may cause a problem. Example .................................................................................................................................... ................................................................................................................................................... Reason ...................................................................................................................................... ................................................................................................................................................... [2]

Mark scheme: 12(a) One mark per mark point (Max 3) 3 MP1 An exception is an unplanned / unexpected event that occurs when a program is running MP2 … due to an error in programming/logic that wasn’t detected during program construction/compilation MP3 The effect of an exception can be that the program halts unexpectedly. 12(b) One mark per mark point (Max 2) 2 MP1 One mark for an example of an exception (see list in guidance) MP2 One mark for the reason for the given exception Example answer Division by zero The processor will not be able to evaluate this answer because a number divided by zero is infinity, so the program will crash.

13 The table shows assembly language instructions for a processor that has one register, the Accumulator (ACC). Instruction Label Opcode Operand Explanation LDM #n Load the number n to the ACC Load the contents of the location at the given LDD <address> address to the ACC The address to be used is at the given address. LDI <address> Load the contents of this second address to the ACC Add the contents of the given address to the ADD <address> ACC ADD #n Add the number n to the ACC Subtract the contents of the given address SUB <address> from the ACC SUB #n Subtract the number n from the ACC Store the contents of the ACC at the given STO <address> address Gives a symbolic address <label> to the <label>: <data> memory location with the contents <data> # denotes a denary number, e.g. #123 <label> can be used in place of <address> The current contents of memory are: Address Contents 563 125 899 63 Write assembly language code, using only the given instruction set to: • store the denary value 250 as labelled variable X • store the value stored in location 563 as labelled variable Y • add the value stored in variable X to the value stored in variable Y • subtract the value stored in location 899 from the current value in the Accumulator • store the result in variable Total. Show the initialisation and values of the variables X, Y and Total in the table provided. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... Label Content [7]

Mark scheme: 13 One mark per mark point (Max 7) 7 MP1 LDM #250 seen MP2 Correct use of LDD 563 MP3 Correct use of ADD with labelled address X MP4 Correct use of SUB with address 899 MP5 At least one correct use of STO Opcode Operand LDM #250 STO X LDD 563 STO Y ADD X SUB 899 STO Total 13 MP6 Correct setting up of at least one labelled address and its value MP7 Correct setting up of remaining two labelled addresses and their values Label Contents X: 250 Y: 125 Total: 312