Computer Science 9618/33 October/November 2025: question paper and mark scheme
Cambridge AS & A Level Computer Science (9618), Paper 3 Advanced Theory, October/November 2025. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.
(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]
* 0000800000003 * DFD
3
Mark scheme
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
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).
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]
* 0000800000004 * DFD
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
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]
(a) State the purpose of multi‑tasking.
…… [1]
(b) Explain how an operating system ensures that multi‑tasking operates efficiently.
…… [3]
* 0000800000005 * DFD
5
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.
(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]
* 0000800000006 * DFD
6
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)
(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]
* 0000800000007 * DFD
7
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
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]
* 0000800000008 * DFD
8
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
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.
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]
* 0000800000009 * DFD
9
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
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.
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]
* 0000800000010 * DFD
10
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.
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
* 0000800000011 * DFD
11
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]
* 0000800000012 * DFD
12
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
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On this page, free: the mark scheme for AS & A Level Computer Science 9618/33, October/November 2025 (9618_w25_ms_33), and the question paper (9618_w25_qp_33), to read or download, with each question's marking points written out underneath.
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Computer Science 9618/33 for October/November 2025 is out of 75 marks, with 1 hour 30 minutes to answer it.
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Yes. Upload your answers to 9618/33/O/N/25 as a PDF or photos, and CIE Buddy marks every question against this paper's official mark scheme, using Cambridge's marking rules. You get your score, each lost mark explained on your own script, and the topics to revise. It's a guide, not an official Cambridge grade.