Computer Science 9618/12 — October/November 2025

Background Concept

This part is about software ownership and licensing. When software is created, it is not automatically free for anyone to copy or change. The creator or company that owns it has legal rights over it.

Copyright is the legal protection for original work, including software. It gives the owner control over copying, distribution and use.

A software licence is the set of rules that tells the user what they are allowed to do with the software. For example, it may limit the number of devices, forbid redistribution, or allow modification.

Different distribution models sit under licensing:

• Open source software makes the source code available and allows users to inspect, modify and often redistribute it, but still under licence conditions.
• Shareware is usually given out freely at first, but only as a trial, limited version or time-limited version.

So, copyright is about ownership rights, while a licence is about permission to use.

Understanding the Question

The question gives four terms and asks for a description of each. That means each answer should be short, precise and clearly different from the others.

A common challenge here is that some of these terms overlap. For example, open source software is still copyrighted, and it still has a licence. So the key is to define each one by its main idea:

• copyright = legal ownership/protection
• open source = source code available with permission to modify/use
• shareware = trial/limited-use distribution
• software licence = rules/conditions of use

Approach

For each row, identify the essential feature that makes that term unique:

1. State what the term is.
2. State its purpose or effect.
3. Avoid mixing it up with another term.

A good exam answer here is one sentence per term, focused on the defining point.

Step-by-Step Reasoning

• Copyright: This is not a type of software. It is the legal protection attached to the software. The key idea is that the owner has rights and others cannot legally copy or use it without permission.

• Open Source (Initiative): The defining feature is access to the source code. Users are allowed to inspect it and usually modify and redistribute it, but only according to the stated open-source licence.

• Shareware: The important idea is that it is not fully free software. It is usually provided so people can try it first, often with restricted features or a time limit, and then pay if they want full use.

• Software Licence: This is the legal agreement or terms of use. It explains what the user can and cannot do, such as install, copy, share or modify the software.

The reason these definitions score well is that each one highlights a different role:

• ownership right
• source-code availability and modification rights
• trial distribution model
• usage agreement

Key Takeaways

• Copyright protects software legally.
• A software licence sets the rules for use.
• Open source does not mean "no copyright"; it means licensed access to source code and modification rights.
• Shareware is trial or limited-use software, not fully unrestricted software.

Common Mistakes

• Saying open source means free of charge. Open source refers mainly to source-code access and modification rights, not necessarily price.
• Saying shareware is freeware. Shareware is normally trial software, while freeware is fully free to use.
• Defining copyright as "a licence". Copyright is ownership protection; the licence is the permission granted.
• Forgetting that open source still has a licence. It is not public domain by default.

Things to Be Careful About

• Use legally accurate wording such as "permission", "rights", "conditions" and "owner".
• Do not claim users can do absolutely anything with open-source software; they must still follow the licence.
• Make each definition distinct. If two rows sound the same, one of them is probably too vague.
• Keep descriptions short and direct, because this is a definition question, not an essay.

Background Concept

A code of conduct is a set of rules or principles that members of a profession are expected to follow. In computing, professional bodies such as the BCS or IEEE publish codes of conduct or codes of ethics.

These codes exist because people working in computing can affect privacy, security, safety, fairness and access to information. Their decisions may affect large numbers of users, so technical ability alone is not enough; professionals also need ethical guidance.

A code of conduct typically covers ideas such as:

• acting honestly
• protecting confidential information
• respecting the law and intellectual property
• avoiding harm to users
• behaving professionally and responsibly

Understanding the Question

The question asks for the purpose of a code of conduct, not a list of rules from one. So the answer should explain why such a code is used.

The important points are:

1. it sets expected standards of behaviour
2. it guides ethical actions and protects the public and the profession

Because it is a 2-mark question, two clear points are enough.

Approach

Think in terms of function:

• What does the code do for the professional?
• What does it do for other people affected by the professional's work?

That leads naturally to two strong ideas:

• guidance/standards for behaviour
• protection of clients, users and the profession

Step-by-Step Reasoning

First, a code of conduct gives members a clear standard to follow. Without this, different people may make inconsistent or poor decisions. So one purpose is to define acceptable professional behaviour.

Second, a code of conduct helps with ethical decision-making. In computing, professionals may face issues involving personal data, software safety, bias, system misuse or copyright. The code provides principles to guide them.

Third, this guidance does not only help the professional. It also protects:

• clients and users, because they are less likely to be harmed by careless or unethical behaviour
• the profession, because common standards improve trust and reputation

So a good concise answer mentions both standards and protection/guidance.

Key Takeaways

• A code of conduct sets expected professional standards.
• It helps professionals choose ethical actions in difficult situations.
• It protects users, clients and public trust in the profession.

Common Mistakes

• Writing what a code of conduct contains instead of its purpose. The question asks why it exists.
• Giving only one vague point such as "to make people behave well". That is too general unless linked to standards or ethics.
• Confusing a code of conduct with a legal document. It may support lawful behaviour, but it is mainly professional and ethical guidance.

Things to Be Careful About

• Use the idea of professional behaviour rather than personal opinion.
• Make the answer purposeful: words like "provide", "guide", "protect" and "maintain standards" are useful.
• Do not overcomplicate it with details of a specific body unless asked.
• For a short-mark question, give two distinct points rather than repeating the same idea in different words.

Background Concept

This question is about addressing modes and tracing what happens when instructions are executed.

An addressing mode tells the CPU how to obtain the value needed by an instruction.

For this instruction set:

• LDM #n uses immediate addressing: the value is written directly in the instruction.
• LDD <address> uses direct addressing: go to that address and load the value found there.
• LDI <address> uses indirect addressing: first go to the given address, treat the value found there as a second address, then load the value from that second address.
• LDX <address> uses indexed addressing: add the given address to the contents of the index register IX, then load the value from the resulting address.

Tracing means following these rules carefully using the memory table provided.

Understanding the Question

You are given:

• a small instruction set
• the contents of memory locations 98 to 102
• the value of the index register IX = 2

You must work out what value ends up in the accumulator after each instruction is executed. The important point is that each instruction uses a different addressing rule, so you must not treat them all the same way.

The memory contents are:

• address 98 contains 8
• address 99 contains 16
• address 100 contains 3
• address 101 contains 98
• address 102 contains 32

Approach

Take each instruction separately.

1. Identify its addressing mode from the opcode.
2. Apply the correct rule for that mode.
3. Use the memory table only as many times as needed.
4. Write the final value loaded into ACC.

A very common exam skill here is distinguishing between:

• immediate: use the number itself
• direct: use the contents at the address
• indirect: use an address stored at an address
• indexed: calculate a new address using IX

Step-by-Step Reasoning

First instruction: LDM #98

LDM means immediate addressing. The # symbol tells you the operand is the actual number to load, not a memory address.

So the CPU loads 98 directly into ACC.

Result: ACC = 98

Second instruction: LDI 101

LDI means indirect addressing. This takes two memory lookups.

• Look at address 101.
• The contents of address 101 are 98.
• That 98 is not the final value; it is the second address to use.
• Now look at address 98.
• The contents of address 98 are 8.

So the accumulator receives 8.

Result: ACC = 8

Third instruction: LDX 100

LDX means indexed addressing.

• Start with the given address: 100
• Add the contents of IX: 2
• Calculated address = 102
• Look at address 102
• The contents of address 102 are 32

So the accumulator receives 32.

Result: ACC = 32

That gives the final answers:

• LDM #98 -> 98
• LDI 101 -> 8
• LDX 100 -> 32

Key Takeaways

• Immediate addressing loads the value written in the instruction.
• Indirect addressing needs two memory accesses.
• Indexed addressing requires an address calculation using IX.
• In trace questions, always follow the rule for the specific opcode before reading the table.

Common Mistakes

• Treating LDM #98 as if 98 were a memory address and answering 8. The # means immediate value, so the answer is 98.
• Stopping too early with LDI 101 and answering 98. In indirect addressing, 98 is the second address, not the final value.
• Forgetting to add IX in LDX 100. Indexed addressing must use address + IX.
• Adding incorrectly and using the wrong memory location.

Things to Be Careful About

• Read the opcode carefully because one letter changes the whole method.
• Use the memory table exactly as given; the left column is the address and the right column is the value stored there.
• For indirect addressing, make sure you perform both stages.
• For indexed addressing, use the current IX value given in the question, which here is 2.

Background Concept

Computer performance depends on several factors, including processor clock speed and cache size.

Clock speed is the rate at which the CPU clock ticks, usually measured in GHz. Each tick helps coordinate operations in the fetch-execute cycle. In general, a higher clock speed allows the processor to carry out more cycles per second.

Cache memory is a small, very fast memory located close to or inside the CPU. It stores frequently used instructions and data. Because cache is much faster than RAM, the CPU can get what it needs more quickly if it is already in cache.

Understanding the Question

The question compares an old computer and a new computer. The new one has:

• a higher clock speed: 2.3 GHz instead of 1.8 GHz
• a larger cache: 32 MB instead of 16 MB

You are asked to explain why both of these changes improve performance. This means you should not just say "it is faster". You need to connect each specification to the reason the CPU can do its work more efficiently.

Approach

Treat the two headings separately:

1. For clock speed, explain that more cycles happen each second, so processing can happen faster.
2. For cache memory, explain that more frequently used data/instructions can be kept in fast memory near the CPU, reducing accesses to slower main memory.

The key to scoring well is giving cause and effect, not just repeating the specification.

Step-by-Step Reasoning

Clock speed:

A processor works through the fetch-execute cycle repeatedly. The clock controls the timing of these operations. If the clock speed increases from 1.8 GHz to 2.3 GHz, the CPU can perform more clock cycles in one second.

That means:

• more processing steps can be completed per second
• instructions can be fetched, decoded and executed more quickly
• overall processing is faster

So increasing clock speed improves performance because the CPU can carry out more work in the same amount of time.

Cache memory:

The CPU often needs the same instructions and data repeatedly. If these are stored in cache, they can be accessed much faster than if the CPU has to fetch them from RAM.

When the cache is larger:

• more frequently used data and instructions can be stored there
• the chance of finding needed items in cache is higher
• the CPU has to go to slower RAM less often
• the CPU spends less time waiting for data

So increasing cache improves performance because memory access becomes faster on average.

The question says increasing clock speed and increasing cache memory, so a full answer should explain both separately.

Key Takeaways

• Higher clock speed usually means more CPU cycles per second.
• More cycles per second usually means faster instruction processing.
• Cache is fast memory close to the CPU.
• Larger cache reduces the number of accesses to slower main memory.

Common Mistakes

• Saying only "clock speed is faster" without explaining that it means more cycles per second.
• Saying only "cache stores data" without explaining that it reduces access to slower RAM.
• Confusing cache with secondary storage such as an SSD or hard disk.
• Repeating the figures from the table without linking them to performance.

Things to Be Careful About

• The question asks why performance improves, so include the mechanism, not just the result.
• Cache is about speed of access, not permanent storage capacity.
• A larger cache does not guarantee every task doubles in speed; it improves performance because needed items are more likely to be available quickly.
• Keep the two explanations separate: clock speed affects cycle rate, while cache affects memory access time.

Background Concept

In a relational database, a primary key uniquely identifies each record in a table. A foreign key is a field in one table that stores values matching the primary key of another table, so that the tables can be linked.

The placement of the foreign key tells you the relationship:

• if table A's primary key appears as a foreign key in table B,
• then one record in A can be related to many records in B.

That is a one-to-many relationship.

Understanding the Question

The question gives two tables:

• SHIP(ShipID, Type, Capacity, ShipName)
• CONTAINER(ContainerID, Type, Weight, OwnerName, ShipID)

You are asked to describe the relationship between them and specifically mention the primary key and foreign key.

So this is not just asking for "one-to-many" on its own. You must also say which field is the primary key and which field is the foreign key.

Approach

First, identify the unique identifier in each table:

• ShipID identifies each ship.
• ContainerID identifies each container.

Then look for the repeated linking field. ShipID appears in both tables, so in CONTAINER it must be the foreign key linking each container to a ship.

From that, decide the cardinality:

• one ship can have many containers
• each container is assigned to one ship

Step-by-Step Reasoning

In SHIP, the field ShipID is the field that uniquely identifies a ship, so it is the primary key.

In CONTAINER, the field ContainerID uniquely identifies a container, so it is the primary key for that table.

The field ShipID also appears in CONTAINER. Since it matches the primary key in SHIP, it is acting as a foreign key.

That means each container record stores the ShipID of the ship carrying it.

Because many different container records can contain the same ShipID, several containers can be linked to the same ship. But each container record contains only one ShipID, so each container is linked to one ship.

Therefore the relationship is:

• SHIP to CONTAINER = one-to-many

Key Takeaways

• A primary key uniquely identifies a record in its own table.
• A foreign key links one table to another by storing the primary key value from the related table.
• If the primary key of one table appears many times in another table, the relationship is one-to-many.

Common Mistakes

• Saying only "one-to-many" without naming the key fields.
• Calling ShipID the primary key in both tables. In CONTAINER, ShipID is a foreign key, not the primary key.
• Reversing the meaning and saying one container has many ships.
• Forgetting that ContainerID is the primary key for the CONTAINER table.

Things to Be Careful About

• Use the exact field names from the question: ShipID and ContainerID.
• Be clear which table each key belongs to.
• State the relationship from the data structure, not from guesswork about real ships and containers.
• Make sure your answer includes both the relationship and the primary/foreign key references, because the question explicitly asks for both.

Background Concept

SQL DDL stands for Data Definition Language. These are SQL commands used to create or change the structure of a database, such as:

• CREATE TABLE
• ALTER TABLE
• DROP TABLE

When a table already exists and you want to add a new field, the normal command is ALTER TABLE ... ADD.

A field storing a calendar date should use a suitable date data type, typically DATE.

Understanding the Question

The table CONTAINER already exists. The question says it needs an additional field to store the last inspection date.

So you are not creating the whole table again. You are changing the existing table by adding one new field.

The example date 08/07/2019 shows the field must store a date value, so the data type should be DATE.

Approach

Because the table already exists:

1. use ALTER TABLE CONTAINER
2. use ADD to insert a new field definition
3. choose a sensible field name such as LastInspectionDate
4. assign the type DATE

That is enough for full credit.

Step-by-Step Reasoning

ALTER TABLE CONTAINER tells the DBMS that the existing table called CONTAINER is being changed.

ADD LastInspectionDate DATE says to add one new field called LastInspectionDate, and that field will store date values.

So the complete SQL statement is:

ALTER TABLE CONTAINER
ADD LastInspectionDate DATE;

This matches the requirement exactly:

• table altered: CONTAINER
• one field added
• field stores dates

Key Takeaways

• Use ALTER TABLE when modifying an existing table.
• Use ADD to add a new field.
• Choose a data type that matches the kind of data stored.

Common Mistakes

• Writing CREATE TABLE instead of ALTER TABLE.
• Using a text type instead of DATE for a date field.
• Forgetting to name the new field.
• Adding more than one field when the question asks for only one.

Things to Be Careful About

• The question asks for an SQL script, but for this task a single correct SQL statement is enough.
• Keep the table name exactly as given: CONTAINER.
• Use an appropriate field name; different sensible names may be accepted, but it must clearly represent the last inspection date.
• Do not include unnecessary clauses or recreate the table structure.

Background Concept

A SELECT query is used to retrieve data from one or more tables. When the data you need depends on fields stored in different tables, you must join those tables.

Here the link between the tables is:

• SHIP.ShipID as the primary key
• CONTAINER.ShipID as the foreign key

To count how many rows match a condition, SQL uses the aggregate function COUNT(*).

So the overall pattern is:

• join the tables
• filter the correct records
• count the remaining rows

Understanding the Question

You are asked to return the number of containers for the ship with the name Caledonia.

Important detail:

• the table CONTAINER has the containers
• the field ShipName is not in CONTAINER; it is in SHIP

So you cannot solve this using only CONTAINER. You must connect CONTAINER to SHIP through ShipID, then select the ship whose name is Caledonia, and count the matching container rows.

Approach

The required strategy is:

1. start from CONTAINER because those are the rows being counted
2. join SHIP using CONTAINER.ShipID = SHIP.ShipID
3. use WHERE ShipName = 'Caledonia'
4. use COUNT(*) to return the number of matching containers

This is the cleanest standard SQL solution.

Step-by-Step Reasoning

SELECT COUNT(*) means the query will return one number: the number of rows that match.

FROM CONTAINER means the counting is based on container records.

JOIN SHIP ON CONTAINER.ShipID = SHIP.ShipID connects each container to the ship that carries it. This works because ShipID is the linking field between the tables.

WHERE ShipName = 'Caledonia' filters the joined data so only containers on the ship called Caledonia are considered.

After the filtering, COUNT(*) counts those container rows.

So the final query is:

SELECT COUNT(*)
FROM CONTAINER
JOIN SHIP ON CONTAINER.ShipID = SHIP.ShipID
WHERE ShipName = 'Caledonia';

This returns exactly one value: the number of containers linked to the ship named Caledonia.

Key Takeaways

• Use a join when the filter field is in a different table from the data being counted.
• Join parent and child tables using the primary key and foreign key.
• Use COUNT(*) to return the number of matching rows.

Common Mistakes

• Trying to query only CONTAINER and using ShipName there, even though ShipName is in SHIP.
• Forgetting the join condition, which can produce incorrect results.
• Counting Ship rows instead of CONTAINER rows.
• Misspelling 'Caledonia' or forgetting the quotes around the text value.

Things to Be Careful About

• Use the exact linking fields: CONTAINER.ShipID = SHIP.ShipID.
• Use COUNT(*) or another valid count expression; the aim is to return a total number.
• The condition must be on ShipName, not Type, ShipID, or any container field.
• Text values in SQL should be enclosed in single quotes.
• If field names are repeated across tables, qualifying them with table names avoids ambiguity.

Background Concept

A DBMS usually provides different interfaces for different users. For example:

• an end-user interface for entering or viewing data
• a developer interface for building and changing the database system

A developer interface is aimed at the person designing or maintaining the database. It provides tools to create the structure of the database and other database objects.

These objects can include:

• tables
• relationships
• queries
• forms
• reports
• validation rules or other application components

Understanding the Question

The question asks for the purpose of a developer interface in a DBMS. That means it wants what the interface is used for, not just a vague statement like "it helps the developer".

A strong answer should mention both:

• who it is for: the developer
• what it is used to create or edit: the database structure and related objects

Approach

To answer clearly:

1. state that the interface is for the database developer or application developer
2. explain that it is used to create, amend and manage database objects
3. give examples of those objects

That gives a complete purpose-based description.

Step-by-Step Reasoning

A developer interface is not mainly for everyday data entry. Its role is to support the building of the database system.

That means it lets the developer:

• define tables and fields
• set up relationships between tables
• create queries to retrieve or update data
• design forms for input screens
• generate reports for output

So the purpose is to provide tools to develop and maintain the database and its front-end components.

Key Takeaways

• Different DBMS interfaces exist for different types of users.
• A developer interface supports the creation and maintenance of database objects.
• Good answers usually mention examples such as tables, queries, forms and reports.

Common Mistakes

• Describing a user interface for entering data instead of a developer interface.
• Saying only "to access the database" without mentioning development tasks.
• Giving examples of hardware or networking tools, which are not relevant here.

Things to Be Careful About

• Focus on the purpose of the interface, not just its name.
• Mention development or maintenance of the database structure or objects.
• Keep the answer in DBMS terms such as tables, relationships, queries, forms and reports.
• Do not confuse the developer interface with DBMS security features or end-user data-entry screens.

Background Concept

A local area network (LAN) is a network that connects devices over a limited geographical area. Typical examples are a school, office, home, or one campus site. Because the network is local, it is usually under the control of one person or organisation rather than a telecommunications company.

Common LAN characteristics include:

• small geographical coverage
• high data transfer speeds compared with a WAN
• private ownership and management
• connecting devices such as computers, printers, switches, and servers within one site

Understanding the Question

The question asks for the characteristics of a LAN, not an example of a LAN and not a diagram. For 2 marks, the examiner is usually expecting two clear properties.

The best answer is to give the core textbook features of a LAN:

• it covers a small area
• it is normally owned or managed by one organisation

Approach

Pick the most standard identifying features of a LAN. In exam questions like this, short precise statements score better than vague comments such as "used in offices".

Step-by-Step Reasoning

A LAN is defined mainly by its scale and control:

1. Small geographical area: a LAN does not stretch across cities or countries. It is confined to somewhere local, such as a building or site.
2. Single organisation control: the hardware, cabling, switch, and server are usually installed and managed by the same organisation.

These two points are enough for full marks here.

Key Takeaways

• A LAN connects devices in a limited local area.
• A LAN is usually privately owned or controlled by one organisation.
• When asked for characteristics, give defining features rather than examples.

Common Mistakes

• Saying only that a LAN "connects computers". That is too general because many networks do that.
• Describing a WAN instead, for example saying it covers a large geographical area.
• Giving topology names such as star or bus instead of LAN characteristics.

Things to Be Careful About

• Use the word small or local area clearly.
• Make sure your second point is about ownership/management or another accepted LAN feature, not just a repeat of the first point.
• Do not confuse the type of network (LAN) with the arrangement of devices (topology).

Background Concept

In a star topology, every device has its own separate connection to one central device, usually a switch or hub. The central device is the point through which all communication passes.

Important features of a star topology are:

• one central device
• each computer or server connects directly to that central device
• end devices are not connected directly to each other

Understanding the Question

The diagram already shows five computers, one server, and one switch. The task is to complete it as a star topology.

That means the switch must be the centre, and every other device must have exactly one direct line to the switch.

Approach

Find the central networking device first. In this question, that is the switch. Then draw one separate connection from each of the five computers to the switch, and one connection from the server to the switch. Do not draw lines between computers.

Step-by-Step Reasoning

A correct star network here has:

• Computer 1 connected to the switch
• Computer 2 connected to the switch
• Computer 3 connected to the switch
• Computer 4 connected to the switch
• Computer 5 connected to the switch
• Server connected to the switch

There should be no other connections.

This is what makes the topology a star: all devices radiate out from the switch like spokes from the centre.

Key Takeaways

• In a star topology, the switch is the centre.
• Every node has its own direct link to the switch.
• There are no direct device-to-device links in the basic star layout.

Common Mistakes

• Connecting computers to each other as well as to the switch.
• Leaving the server unconnected.
• Drawing one long shared cable, which would represent a bus topology instead.

Things to Be Careful About

• Every device except the switch must connect directly to the switch.
• The server is also a node on the network, so it needs its own connection.
• Do not add extra lines not required by the topology.

Background Concept

In a star topology, devices do not send data directly to each other over a shared cable. Instead, each device connects to a central switch. When one host wants to communicate with another, the packet first goes to the switch.

A packet contains data plus control information, including addressing information. A switch uses this addressing information to decide where the packet should go.

Understanding the Question

The question asks what happens when one host sends data to another in a star topology. Since the network in the stem includes a switch, the expected explanation is about the switch receiving the packet and then sending it to the correct destination.

Approach

Describe the journey in three stages:

1. source host sends to switch
2. switch checks the destination address
3. switch sends the packet on the correct link to the destination host

This matches the network layout in a star topology.

Step-by-Step Reasoning

1. Source transmits: the sending host places the packet on its own connection to the switch.
2. Switch receives: because the switch is the centre of the star, the packet always reaches the switch first.
3. Address checked: the switch examines the destination address in the packet.
4. Correct forwarding: the switch sends the packet out through the port connected to the destination host.
5. Destination receives: the intended host receives the packet.

The important idea is that the packet is not broadcast blindly to every device in the simplified exam description; the switch uses the destination information to forward it appropriately.

Key Takeaways

• In a star topology, traffic passes through the central switch.
• The switch reads the destination address.
• The switch forwards the packet to the correct device.

Common Mistakes

• Saying the source host sends directly to the destination host without involving the switch.
• Describing a bus network instead, where devices share one main cable.
• Forgetting to mention the destination address check.

Things to Be Careful About

• Use the term switch, not just "central computer".
• Make clear that the switch decides where to send the packet.
• Do not mix this up with routing across the internet; this question is about a LAN star topology.

Background Concept

A bus network uses one shared communication line. Because all devices use the same cable, two devices may try to transmit at the same time. If this happens, the signals interfere with each other. This is called a collision.

Ethernet on a shared medium uses CSMA/CD:

• Carrier Sense: listen first to see whether the line is busy
• Multiple Access: many devices share the same medium
• Collision Detection: detect when two transmissions overlap

Once a collision happens, devices must recover and try again later.

Understanding the Question

The question is specifically about a bus network using Ethernet. It asks both how collisions are detected and how they are managed.

So a complete answer needs both parts:

• devices check the line and detect a collision
• they stop and retransmit after a delay

Approach

Use the standard CSMA/CD sequence:

1. listen before sending
2. detect if a collision happens
3. stop transmission
4. wait a random backoff time
5. retransmit

Step-by-Step Reasoning

In a bus network, all devices share one cable, so only one should transmit successfully at a time.

1. Before sending, a device listens to the network to check whether the cable is currently in use.
2. If the cable seems free, it starts transmitting.
3. Sometimes two devices both decide the line is free and start transmitting almost together.
4. Their signals interfere. This is the collision.
5. The devices detect that a collision has occurred.
6. They stop transmitting because the current data has been corrupted.
7. Each waits for a random period of time so they are less likely to clash again.
8. After waiting, they attempt to retransmit.

The random wait matters: if both retried immediately, another collision could happen again.

Key Takeaways

• Bus networks use a shared medium, so collisions are possible.
• CSMA/CD is the standard method for dealing with collisions on shared Ethernet.
• The recovery step is to stop, wait a random time, and retransmit.

Common Mistakes

• Saying collisions cannot happen on a bus network. They can, because the cable is shared.
• Mentioning only detection but not what happens next.
• Forgetting the random delay and simply saying "send again immediately".

Things to Be Careful About

• This explanation applies to shared-medium Ethernet, such as the exam's bus network context.
• The key word is random waiting time or backoff period.
• Do not describe switch-based star behaviour here; this part is specifically about a bus network.

Background Concept

A network or internet threat is something that can harm computer systems, steal data, or trick users into giving away private information. Common examples include malware, hacking, phishing, and pharming.

A good security answer usually has three parts:

• the name of the threat
• what the threat does
• how to reduce or prevent it

Understanding the Question

The question asks for one threat only. That means you do not need several examples. You just need one complete row with:

• the threat name
• a description of the threat
• a prevention method

Approach

Choose a threat that is easy to explain clearly. Phishing is a strong choice because it naturally leads to a clear prevention method.

Then write:

1. what phishing is
2. what information it tries to steal
3. how users can avoid it

Step-by-Step Reasoning

The chosen threat is phishing.

• Threat: phishing
• Description: the attacker sends fake emails or creates fake websites that look genuine. The aim is to trick the victim into typing in confidential information such as usernames, passwords, or bank details.
• Prevention: user education helps people recognise suspicious emails and fake links. A user should verify links before clicking and avoid entering details on untrusted pages.

Any one valid threat with a correct description and matching prevention could score, but the answer must include all three parts.

Key Takeaways

• Security threats should be named precisely.
• A complete answer includes both the danger and a realistic prevention.
• Prevention must match the chosen threat.

Common Mistakes

• Giving only the threat name with no description.
• Giving a prevention method that does not fit the threat.
• Naming something too vague, such as just "internet", instead of a specific threat.

Things to Be Careful About

• The question asks for one threat, so one well-developed example is enough.
• Make sure the description explains the mechanism of the threat, not just that it is "bad".
• The prevention should be practical and specific, such as training, anti-malware, firewall use, authentication, or careful link checking depending on the threat chosen.

Background Concept

Sound is stored digitally by taking samples of the original analogue waveform at regular intervals. The sampling rate is how many samples are taken per second. A higher sampling rate keeps more detail from the original sound; a lower sampling rate keeps less detail.

Compression can be either lossless or lossy. Lossless compression reduces file size without throwing away information, so the original can be recreated exactly. Lossy compression reduces file size by permanently removing some data, which means the reconstructed file is not identical to the original.

Understanding the Question

The question says the sound file is compressed by reducing the sampling rate. That is the key clue. We are not being asked about a named compression method such as RLE; we are being asked to decide whether lowering the sampling rate keeps all the original data or discards some of it.

Approach

Ask one simple question: after reducing the sampling rate, can the exact original sound file be recovered? If the answer is no, then the compression is lossy. Then justify it by referring to the missing samples.

Step-by-Step Reasoning

Reducing the sampling rate means fewer sound samples are stored each second.

Those missing samples were part of the original sound data.

Because some original data has been removed, the sound cannot be rebuilt exactly as it was before compression.

That matches the definition of lossy compression.

A good justification must mention loss of data, not just "the file gets smaller". Both lossy and lossless methods reduce file size, so file-size reduction alone is not enough.

Key Takeaways

• Lowering sampling rate reduces sound quality because less of the waveform is captured.
• If original data is discarded and cannot be recovered exactly, the method is lossy.
• The definition of lossless is exact reconstruction of the original.

Common Mistakes

• Saying "lossless" because the file is still usable. A file can still play correctly and yet be lossy.
• Saying only "the file size is reduced" as the justification. That does not distinguish lossy from lossless.
• Confusing sampling rate with bit depth. Both affect sound quality and file size, but the question specifically mentions sampling rate.

Things to Be Careful About

Use the exact idea of irreversible data loss in the justification. In exam questions, the mark is usually for recognising that reducing the sampling rate removes samples, so the original sound cannot be recreated exactly.

Background Concept

Binary is base 2, so each bit position has a place value that is a power of 2. In an 8-bit number, the place values from left to right are 128, 64, 32, 16, 8, 4, 2, 1. To convert a binary value to denary, add the place values where there is a 1.

Once the denary number is known, this question uses a lookup table to map that value to a word.

Understanding the Question

You are given three 8-bit binary values and a table that pairs words with denary values from 55 to 62. The task is to work out the denary value of each binary byte, then find the matching word from the table.

Approach

Convert each binary value into denary. Then read across the provided word table and select the word with the same denary value.

Step-by-Step Reasoning

For 00111000:

• The 1s are in the 32, 16 and 8 positions.
• So the value is $32 + 16 + 8 = 56$.
• Denary 56 corresponds to Science.

For 00111100:

• The 1s are in the 32, 16, 8 and 4 positions.
• So the value is $32 + 16 + 8 + 4 = 60$.
• Denary 60 corresponds to is.

For 00111110:

• The 1s are in the 32, 16, 8, 4 and 2 positions.
• So the value is $32 + 16 + 8 + 4 + 2 = 62$.
• Denary 62 corresponds to Amazing!

So the completed row of words is Science, is, Amazing!

Key Takeaways

• Convert binary to denary by summing the place values of the 1 bits.
• In lookup questions, conversion is only the first step; then match to the given table.
• 8-bit unsigned binary values like these are read using place values from 128 down to 1.

Common Mistakes

• Reading the binary number as decimal digits instead of base 2 place values.
• Missing one of the middle place values, especially 8 or 4.
• Choosing the wrong word because of copying the denary value incorrectly from the table.

Things to Be Careful About

Keep the bit positions in the correct order from left to right: 128, 64, 32, 16, 8, 4, 2, 1. Also, use the table exactly as given; this is a lookup, not a guess from the meaning of the words.

Background Concept

For an uncompressed bitmap image, file size depends mainly on two things:

• resolution, which is the number of pixels
• colour depth, which is the number of bits used for each pixel

The basic calculation is:

$$\text{file size in bits} = \text{width} \times \text{height} \times \text{colour depth}$$

Then convert bits to bytes by dividing by 8. If the question asks for an estimate in megabytes, exam questions often use $1\text{ MB} = 1\,000\,000$ bytes for a simple estimate.

Understanding the Question

The image is 1000 pixels wide and 2000 pixels high, with a colour depth of 16 bits. You need to estimate the file size in megabytes and show the working.

The word "estimate" is important because it signals that a straightforward conversion to MB is acceptable.

Approach

Find the number of pixels first. Then multiply by 16 bits per pixel. Then divide by 8 to get bytes. Finally convert to megabytes.

Step-by-Step Reasoning

First calculate the number of pixels:

$$1000 \times 2000 = 2\,000\,000 \text{ pixels}$$

Each pixel uses 16 bits, so total bits are:

$$2\,000\,000 \times 16 = 32\,000\,000 \text{ bits}$$

Convert bits to bytes:

$$\frac{32\,000\,000}{8} = 4\,000\,000 \text{ bytes}$$

Convert bytes to megabytes:

$$\frac{4\,000\,000}{1\,000\,000} = 4 \text{ MB}$$

So the estimated file size is 4 MB.

Key Takeaways

• Bitmap file size depends on pixel count and colour depth.
• Multiply width by height to get the number of pixels.
• Multiply pixels by bits per pixel, then divide by 8 for bytes.
• For an estimate, use a simple MB conversion unless told otherwise.

Common Mistakes

• Adding width and height instead of multiplying them.
• Forgetting to multiply by colour depth.
• Giving the answer in bits or bytes when the question asks for megabytes.
• Forgetting to divide by 8 when converting bits to bytes.

Things to Be Careful About

Read whether the image is bitmap and whether compression is mentioned. This question does not mention compression, so use the raw bitmap calculation. Also watch the unit: "megabytes" means convert from bits properly before giving the final answer.

Background Concept

Binary addition works in base 2, so each column can only total to 0, 1, 10 or 11 in binary. The key rules are:

• 0 + 0 = 0
• 0 + 1 = 1
• 1 + 1 = 10 so write 0 and carry 1
• 1 + 1 + 1 = 11 so write 1 and carry 1

When a question says the addition is done using 8-bit registers, it means the result should be written using 8 bits. Any carry beyond the leftmost bit would indicate overflow, but here the sum still fits into 8 bits.

Understanding the Question

You are given two 8-bit binary numbers:

• 10000111
• 00111001

You must add them exactly as binary numbers, column by column from right to left, and write the 8-bit result.

Approach

The correct method is standard binary column addition:

1. Start at the least significant bit on the right.
2. Add the two bits, including any carry from the previous column.
3. Write the result bit for that column.
4. Carry 1 to the next column whenever the total is 10 or 11 in binary.
5. Continue until the leftmost bit is reached.

Step-by-Step Reasoning

Add the bits from right to left:

• Rightmost column: 1 + 1 = 10 so write 0, carry 1
• Next column: 1 + 0 + carry 1 = 10 so write 0, carry 1
• Next column: 1 + 0 + carry 1 = 10 so write 0, carry 1
• Next column: 0 + 1 + carry 1 = 10 so write 0, carry 1
• Next column: 0 + 1 + carry 1 = 10 so write 0, carry 1
• Next column: 0 + 1 + carry 1 = 10 so write 0, carry 1
• Next column: 0 + 0 + carry 1 = 1 so write 1
• Leftmost column: 1 + 0 = 1

So the final 8-bit result is:

11000000

A quick denary check confirms it:

• 10000111 = 135
• 00111001 = 57
• 135 + 57 = 192
• 192 in binary is 11000000

Key Takeaways

• Binary addition follows fixed bit rules and uses carries just like denary column addition.
• Always add from right to left.
• In an 8-bit question, make sure the final answer is written as 8 bits.

Common Mistakes

• Forgetting to include a carry in the next column.
• Writing a 9-bit answer when the result actually fits in 8 bits.
• Mixing up 1 + 1 = 10 with 1 + 1 = 2; in binary you must write 0 and carry 1.

Things to Be Careful About

• Keep the bits aligned by column before adding.
• Do not drop leading bits from the answer if the question expects an 8-bit result.
• If a carry comes out of the leftmost bit, check whether the question expects you to mention overflow; here it is not needed because the correct 8-bit sum is 11000000.

Background Concept

A character set is a system for representing characters as binary codes inside a computer. Different character sets use different numbers of bits and can represent different numbers of symbols.

In this syllabus, ASCII is the smaller character set and Unicode is the larger one. Unicode is taught as using 16 bits per character, allowing far more characters from many languages and symbol sets to be represented.

Understanding the Question

The question does not ask you to calculate anything. It tells you the computer uses Unicode and asks for the number of bits needed to store one character.

So this is a recall question about character representation.

Approach

Recognise the keyword Unicode and recall the standard syllabus fact:

• ASCII uses fewer bits.
• Unicode uses 16 bits per character.

Then state that value clearly with the unit bits.

Step-by-Step Reasoning

Because the question is specifically about the Unicode character set, the required storage per character is:

• 16 bits

That is the whole answer.

Key Takeaways

• ASCII and Unicode are both character sets.
• Unicode can store many more characters than ASCII.
• For this exam, Unicode is taken as 16 bits per character.

Common Mistakes

• Writing 8 bits, which is usually associated with ASCII or extended ASCII, not Unicode.
• Giving the number of characters possible instead of the number of bits.
• Omitting the unit and writing only 16.

Things to Be Careful About

• Answer the exact question asked: it wants the number of bits for one character, not a comparison with ASCII.
• Use the syllabus convention expected by the mark scheme: 16 bits.

Background Concept

A logic circuit can be turned into a logic expression by following the signal from the inputs through each gate, one stage at a time. Each gate has a standard meaning:

• XOR gives true when the two inputs are different.
• NOT inverts a single input.
• AND gives true only when both inputs are true.
• OR gives true when at least one input is true.
• NAND means NOT of AND.

When writing the expression, brackets are important because they show which sub-expression is formed first. In questions like this, "do not simplify" means you should copy the structure of the circuit exactly, even if part of it could be reduced using Boolean algebra.

Understanding the Question

You are given a completed circuit with inputs A, B and C, and you must write the expression for output X. The key task is not to simplify it, but to mirror the gates exactly as they appear.

From the circuit:

• A and B go into an XOR gate.
• That result goes into a NOT gate.
• B and C go into an AND gate.
• That AND output and B go into an OR gate.
• The two results then go into a NAND gate to produce X.

So the answer must show the two separate branches first, then combine them with NAND.

Approach

Start by writing the output of each small gate:

1. Write the XOR result from A and B.
2. Apply NOT to that result.
3. Write the AND result from B and C.
4. OR that result with B.
5. Combine the two branch results using NAND.

This stage-by-stage method prevents missing brackets or using the wrong operator.

Step-by-Step Reasoning

First branch:

• Inputs A and B enter an XOR gate.
• This gives (A XOR B).
• That output goes through a NOT gate.
• So the first branch becomes NOT (A XOR B).

Second branch:

• Inputs B and C enter an AND gate.
• This gives (B AND C).
• That result and input B enter an OR gate.
• So the second branch becomes ((B AND C) OR B).

Final gate:

• The outputs of those two branches go into a NAND gate.
• Therefore the complete expression is:

X = (NOT (A XOR B)) NAND ((B AND C) OR B)

That is unsimplified, which is exactly what the question asks for. A simplified version is not needed and could lose marks if it no longer reflects the circuit structure clearly.

Key Takeaways

• Build a logic expression one gate at a time.
• Use brackets to preserve the order of operations.
• Keep repeated inputs if the question says not to simplify.
• A NAND gate can be written directly as NAND rather than rewriting it as NOT of an AND, unless you prefer that form.

Common Mistakes

• Writing NOT A XOR B instead of NOT (A XOR B). The NOT acts on the whole XOR output, not just A.
• Simplifying ((B AND C) OR B) to B. The question specifically says not to simplify.
• Using AND instead of NAND for the final gate. The small bubble on the output means inversion, so it is NAND.
• Missing one set of brackets, which changes the meaning of the expression.

Things to Be Careful About

• Follow the circuit in the correct order from left to right.
• Treat each gate output as a complete sub-expression before using it in the next gate.
• Make sure the final operator is NAND, not AND.
• Keep the exact branch structure of the diagram, because this is a transcription question rather than a simplification question.

Background Concept

A truth table lists every possible combination of input values and the corresponding output of a logic expression. For three inputs A, B and C, there are 8 rows because there are 2 possible values for each input and 2 × 2 × 2 = 8.

For a compound logic expression, the safest method is to evaluate it in stages using intermediate results. Here the expression is:

X = NOT ((A NAND B) XOR (NOT A OR NOT C))

Useful gate rules:

• A NAND B means NOT (A AND B).
• NOT A OR NOT C is true unless both A and C are 1.
• P XOR Q is true when P and Q are different.
• NOT (P XOR Q) is true when P and Q are the same.

So this question is really checking whether you can evaluate a nested expression accurately across all rows.

Understanding the Question

You are not being asked to create a truth table from scratch. A truth table is already given, but the question says there are three errors in the X column. Your job is to find exactly which row numbers are wrong.

That means you should:

1. Work out the correct value of X for each row.
2. Compare your result with the value shown in the table.
3. Record the row numbers where the two values differ.

Because there are only three mistakes, the final answer is just three row numbers.

Approach

The best approach is to create intermediate values for:

• A NAND B
• NOT A OR NOT C
• the XOR of those two results
• then the final NOT

This is much less error-prone than trying to do the whole expression mentally in one step. Once the correct X values are known, compare them row by row with the given table.

Step-by-Step Reasoning

Let:

• P = A NAND B
• Q = NOT A OR NOT C
• X = NOT (P XOR Q)

Now evaluate each row.

Now identify the mismatches:

• Row 2 is wrong.
• Row 4 is wrong.
• Row 7 is wrong.

So the three errors are rows 2, 4 and 7.

A quick pattern check also helps: since NOT (P XOR Q) is true when P and Q match, rows where both intermediate values are the same should produce 1. That confirms rows 2 and 4 should both be 1, not 0.

Key Takeaways

• For a complex logic expression, calculate intermediate columns first.
• NAND means do AND first, then invert the result.
• NOT (XOR) is effectively an XNOR pattern: the result is 1 when the two inputs are equal.
• When checking a given truth table, compare your computed output row by row rather than guessing from patterns.

Common Mistakes

• Treating A NAND B as the same as A AND B. This loses the inversion and changes several rows.
• Misreading NOT A OR NOT C as NOT (A OR C). Those are different expressions.
• Applying NOT to only one part of the expression instead of the entire XOR result.
• Stopping after finding one or two wrong rows, even though the question says there are three.

Things to Be Careful About

• Keep the row order exactly as given: 000, 001, 010, 011, 100, 101, 110, 111.
• Use brackets carefully. The final NOT applies to the whole (A NAND B) XOR (NOT A OR NOT C) expression.
• XOR is 1 only when the two inputs differ; if they are both 0 or both 1, XOR is 0.
• Since the question asks for row numbers, your final response should be the row numbers only, not rewritten truth-table values.

Background Concept

An interpreter and a compiler are both translators, but they work differently.

An interpreter translates source code one statement at a time and usually executes each translated line immediately. This is useful while developing programs because the programmer can test small sections and see errors as they occur.

A compiler translates the whole program before execution. If the compilation is successful, it produces object code or an executable file. This is more suitable for the finished version of the program because the end user can run the compiled program directly.

The key idea in this question is not just to define each translator, but to explain why a programmer would use them in that order during development.

Understanding the Question

The question says the programmer can first use an interpreter and then a compiler. So it is asking for a sequence:

1. What the interpreter is used for early in development.
2. What the compiler is used for later, after testing/debugging.

The wording "develop the computer program" is important. It means the answer should focus on writing, testing, correcting and then releasing the program, not just giving textbook definitions.

Approach

A good approach is:

• For the interpreter, link it to testing and debugging.
• For the compiler, link it to translating the finished program for release.

That gives the clear development flow the examiner wants:

• write code
• test small parts quickly with the interpreter
• fix errors as they appear
• once correct, compile the full program for distribution/use

Step-by-Step Reasoning

The first stage is development. During development, programmers usually need quick feedback. An interpreter helps because it does not require the whole program to be translated before anything can be tested. It works line by line.

That means:

• the programmer can run part of the program
• if an error is reached, it is reported at that point
• the programmer can correct the mistake and test again quickly

This is why the interpreter is used first.

After the program has been tested and corrected, the programmer no longer needs line-by-line translation for debugging. At that point, the program is ready to be turned into a form that users can run more easily.

So the programmer then uses a compiler to:

• translate the entire source program
• produce object code or an executable file
• release that compiled version to users

This fits the phrase "then a compiler" because compilation is typically the later stage, once the program is working properly.

A strong exam answer should therefore mention both the method and the purpose:

• interpreter: line by line, quick testing, immediate error detection
• compiler: whole program, final translation, release/distribution

Key Takeaways

• An interpreter is best associated with testing and debugging during development.
• A compiler is best associated with producing the final translated version of a program.
• In scenario questions, always explain why the programmer chooses each tool, not just what it is.

Common Mistakes

• Saying both simply "translate the program" without explaining the difference in how and when they are used.
• Saying the interpreter translates the whole program first. It does not; it works line by line.
• Forgetting the development order. The question specifically wants interpreter first, compiler second.
• Giving only advantages of compiled code such as "runs faster" without linking it to the program being finished and released.

Things to Be Careful About

• Do not confuse an interpreter with an IDE. The IDE may contain tools for editing and debugging, but the translator itself is different.
• Do not say a compiler is used to find errors one line at a time; that is the interpreter-style idea.
• Use the correct development context: interpreter for debugging, compiler for final translation/release.
• If marks are limited, make each point distinct rather than repeating "translates code" in different words.

Background Concept

Free Software is about freedom, not simply about price. In computing, it means software released under a licence that allows users to do things such as:

• run the program
• copy it
• share it
• study how it works
• modify it

To make studying and modifying possible, the source code must be available.

This is different from freeware, which only means the software is available at no cost. Freeware may still forbid copying, changing or redistributing the program.

Understanding the Question

The question says the programmer releases the program as Free Software and asks what that means.

So the answer must describe the licence freedoms given to the user. The strongest points are:

• users are allowed to use/copy/distribute it
• users can access the source code and modify it

The question is only 2 marks, so the answer should be short and precise.

Approach

To answer well, focus on the two most creditworthy ideas:

1. Freedom to use/share the software.
2. Access to source code so it can be studied and changed.

That avoids the very common mistake of talking only about the software being free of charge.

Step-by-Step Reasoning

The phrase Free Software refers to the permissions granted by the licence.

First, the user is not restricted to just running one purchased copy. The licence allows the software to be copied and distributed.

Second, the user is not locked out of the program's internals. The source code is available, which means the user can study how the program works and modify it.

Those two ideas together capture what the examiner is looking for.

If you only wrote "it costs nothing," that would not be enough, because software can be free to download but still not be free software.

Key Takeaways

• Free Software means freedom of use, copying, sharing and modification.
• Source code availability is central because it makes study and modification possible.
• Do not confuse free software with freeware.

Common Mistakes

• Saying only "it is free to download" or "it costs no money." That describes price, not software freedom.
• Confusing Free Software with open source in a vague way without mentioning the actual freedoms.
• Forgetting to mention source code when talking about modification.

Things to Be Careful About

• The word free here means liberty, not necessarily zero cost.
• A 2-mark answer should include two distinct points, not the same idea repeated.
• If you mention modification, make it clear that this is possible because the source code is available.

Background Concept

When software is downloaded from the internet, a user may want to know whether it is really from the claimed publisher and has not been altered. This is the idea of authenticity.

A common way to support authenticity is a digital certificate. A digital certificate links an identity, such as a software publisher, to a public key and is issued by a trusted authority. It can be used to verify that the software really comes from that source.

In practice, authenticity may also involve a digital signature, but in many exam questions the expected named item is a digital certificate.

Understanding the Question

The question asks what should be included with the download to make sure the program is authentic.

The key word is authentic, meaning genuine or really from the claimed source. This is not mainly about virus removal or downloading speed; it is about verifying identity.

So the answer should name the security item that helps prove the software's origin.

Approach

Look for the security method connected to authentication rather than general protection.

• firewall: blocks network traffic
• anti-malware: detects malicious software
• encryption: protects confidentiality
• digital certificate: helps verify identity/authenticity

Therefore the correct response is the certificate.

Step-by-Step Reasoning

If a user downloads a program, there is a risk it could be fake or altered.

To address that, the download should include a digital certificate. The certificate allows the user's system to check that the software publisher is genuine and trusted.

That directly matches the requirement to make sure the program is authentic.

Because this is a 1-mark question, the examiner is likely just looking for the named term.

Key Takeaways

• Authenticity means confirming that something is genuine and from the claimed source.
• A digital certificate is used to support authenticity.
• In security questions, match the method to the exact goal being asked about.

Common Mistakes

• Writing anti-virus or anti-malware. That protects against malicious files but does not prove who created the program.
• Writing encryption on its own. Encryption protects data from being read, but does not necessarily prove the sender's identity.
• Giving password or username, which are about user login rather than verifying downloaded software.

Things to Be Careful About

• Focus on the exact term authentic. That points to identity verification.
• For a 1-mark state question, give one precise item rather than a long explanation.
• If your course or teacher uses the paired idea of digital certificate/digital signature, remember that the certificate is the clearest syllabus-aligned answer here.