General Certificate of Secondary Education (GCSE) Computer Science Syllabus

What is the result of the binary addition $0101\,1100_2 + 0011\,0101_2$ in 8 bits?

$1001\,0001_2$ (which is $92 + 53 = 145$ in denary).

What happens, and what is the term, when a binary addition produces a result too large for the available number of bits?

An overflow occurs - the carry out of the most significant bit is lost, giving an incorrect result. The condition is called overflow.

Performing a binary shift left by 2 places on an unsigned number has what arithmetic effect?

It multiplies the number by $2^{2} = 4$. Each left shift multiplies by 2; vacated positions are filled with 0s.

What is a character set, and name two common examples.

A character set is a defined collection of characters that a computer can represent, each mapped to a unique binary code. Examples: ASCII and Unicode.

How many bits does standard ASCII use per character, and how many characters can it represent?

Standard ASCII uses 7 bits per character, allowing $2^{7} = 128$ different characters.

Why was Unicode developed to replace/extend ASCII?

ASCII could only represent 128 (or 256 for extended) characters, insufficient for the world's languages and symbols. Unicode uses more bits per character to represent characters from many languages and emoji, covering over a million possible code points.

The ASCII code for 'A' is 65. What is the ASCII code for 'C', and why?

It is 67. Letters are stored in sequential order, so 'C' is two positions after 'A' ($65 + 2 = 67$).

Define resolution and colour depth in the context of a bitmap image.

Resolution is the number of pixels in the image (e.g. width times height). Colour depth is the number of bits used to represent each pixel's colour.

Give the formula for the file size of a bitmap image in bits.

$\text{file size (bits)} = \text{width} \times \text{height} \times \text{colour depth}$, where width and height are in pixels.

A bitmap image is 100 by 50 pixels with a colour depth of 4 bits. Calculate its size in bytes.

$100 \times 50 \times 4 = 20000$ bits; $\frac{20000}{8} = 2500$ bytes.

With a colour depth of $n$ bits, how many different colours can each pixel represent?

$2^{n}$ different colours.

Define sample rate and bit depth (sample resolution) for digital sound.

Sample rate is the number of samples taken per second (measured in Hz). Bit depth is the number of bits used to store each sample (amplitude).

Give the formula for the file size of a sound recording.

$\text{file size (bits)} = \text{sample rate} \times \text{bit depth} \times \text{duration (seconds)}$ (multiply by number of channels if more than one).

How does increasing the sample rate affect a digital sound recording?

It produces a more accurate representation of the original analogue sound (higher quality) but increases the file size.

What is the purpose of data compression?

To reduce the file size so files take up less storage space and can be transmitted faster over a network.

What is the key difference between lossy and lossless compression?

Lossy compression permanently removes some data to reduce size, so the original cannot be perfectly restored (e.g. JPEG, MP3). Lossless compression reduces size with no loss of data, so the original is restored exactly (e.g. PNG, FLAC).

Explain how Run Length Encoding (RLE) compresses data.

RLE replaces sequences (runs) of repeated identical values with a single value and a count of how many times it repeats, e.g. AAAA becomes 4A. It is lossless and works best on data with many repeated values.

How does Huffman coding reduce file size?

It assigns shorter binary codes to the most frequently occurring characters and longer codes to less frequent ones, reducing the overall number of bits needed (a form of lossless compression).

What is the difference between a LAN and a WAN?

A LAN (Local Area Network) covers a small geographic area such as one building/site and is usually owned by one organisation. A WAN (Wide Area Network) covers a large geographic area (e.g. cities/countries) and often uses third-party infrastructure, e.g. the internet.

Name four factors that affect the performance of a network.

Bandwidth available, number of devices/users on the network, the transmission medium (e.g. fibre vs copper), and interference or the network topology used.

Describe the star network topology and one advantage.

In a star topology every device is connected to a central switch/hub. Advantage: if one cable or device fails the rest of the network keeps working, and it handles high traffic well with few collisions.

What is the purpose of a network protocol? Give two examples.

A protocol is a set of rules governing how devices communicate so they can understand each other. Examples: TCP/IP, HTTP, HTTPS, FTP, SMTP, IMAP.

What is the difference between a MAC address and an IP address?

A MAC address is a permanent, unique physical address assigned to a device's network hardware. An IP address is a logical address assigned to a device on a network and can change (it identifies the device's location on the network).

Why is layering used in network protocol stacks (e.g. TCP/IP)?

It breaks communication into self-contained layers, each with a specific role. This makes protocols easier to develop, maintain and standardise, and allows one layer to be changed without affecting the others.

Name four common forms of network/cyber attack.

Malware, phishing, brute-force attacks, denial of service (DoS) attacks, data interception (and SQL injection / social engineering).

What is a brute-force attack and one way to defend against it?

A brute-force attack tries many password combinations systematically until it finds the correct one. Defences include strong passwords, account lockout after failed attempts, and limiting login rate.

Explain how phishing works and one method of prevention.

Phishing sends fake emails/messages pretending to be from a trusted source to trick users into revealing personal details (e.g. passwords). Prevention: user education, spam filters, and checking sender addresses/links.

What is penetration testing and why is it carried out?

Penetration testing is authorised testing of a system to find security weaknesses before attackers do, by simulating attacks. It helps an organisation identify and fix vulnerabilities.

List four methods used to prevent network security threats.

Firewalls, encryption, strong passwords/authentication, anti-malware software, user access levels, and physical security.

Compare wired and wireless connectivity in terms of speed, reliability and mobility.

Wired (e.g. Ethernet/fibre) is generally faster, more reliable and more secure but lacks mobility. Wireless (e.g. Wi-Fi) offers mobility and easy setup but can be slower, suffer interference and is potentially less secure.

What is encryption and why is it important when transmitting data wirelessly?

Encryption scrambles data using a key so that only authorised parties with the key can read it. It is important wirelessly because radio signals can be intercepted, so encryption keeps intercepted data unreadable.

What do the three pillars of computational thinking - decomposition, abstraction and algorithmic thinking - mean?

Decomposition: breaking a problem into smaller, manageable sub-problems. Abstraction: removing unnecessary detail to focus on the essentials. Algorithmic thinking: defining clear step-by-step rules/instructions to solve the problem.

Describe how a linear search works.

It checks each item in the list one at a time from the start until the target is found or the end of the list is reached. The list does not need to be sorted.

Describe how a binary search works and state its key requirement.

It repeatedly checks the middle item of a sorted list; if the target is smaller it searches the lower half, if larger the upper half, halving the search each time. Key requirement: the list must be sorted.

Why is binary search generally faster than linear search on large lists?

Binary search halves the number of remaining items each step (roughly $\log_{2}n$ checks), whereas linear search may check up to all $n$ items, so binary search needs far fewer comparisons on large sorted lists.

Explain how a bubble sort orders a list.

It repeatedly passes through the list comparing each adjacent pair of items and swapping them if they are in the wrong order. Passes repeat until a complete pass occurs with no swaps, meaning the list is sorted.

Explain how a merge sort works.

It uses divide and conquer: the list is repeatedly split in half until each sublist has one item, then the sublists are merged back together in the correct order to produce a single sorted list.

Explain how an insertion sort works.

It builds a sorted section one item at a time: each new item is taken from the unsorted part and inserted into its correct position within the already-sorted part of the list.

What are the three main ways to represent algorithms in GCSE Computer Science?

Pseudocode, flowcharts, and program code (in a high-level language).

In a flowchart, what do the following shapes represent: rectangle, diamond (rhombus), and parallelogram?

Rectangle = a process/instruction; diamond = a decision (yes/no condition); parallelogram = input or output.

Name the three basic programming constructs.

Sequence (instructions in order), selection (choosing between paths, e.g. IF/ELSE), and iteration (repetition, e.g. loops).

What is the difference between a count-controlled loop and a condition-controlled loop?

A count-controlled loop (e.g. FOR) repeats a set number of times. A condition-controlled loop (e.g. WHILE/REPEAT-UNTIL) repeats while or until a condition is true, with the number of repetitions not fixed in advance.

List the common comparison (relational) operators and what each means.

$=$ equal to, $\neq$ not equal to, $<$ less than, $>$ greater than, $\leq$ less than or equal to, $\geq$ greater than or equal to.

What is the difference between the DIV and MOD operators? Evaluate $17 \bmod 5$ and $17 \;\text{DIV}\; 5$.

DIV gives the integer (whole-number) result of a division; MOD gives the remainder. $17 \;\text{DIV}\; 5 = 3$ and $17 \bmod 5 = 2$.

What is the difference between a constant and a variable?

A variable's value can change while the program runs. A constant is assigned a value that stays the same throughout the program's execution.

Name five common data types used in programming and give an example value of each.

Integer ($42$), real/float ($3.14$), Boolean (True/False), character ('A'), and string ("Hello").

What is casting in programming?

Casting is converting data from one data type to another, e.g. converting the string "42" into the integer $42$, or an integer into a string for output.

What is a string, and what does string concatenation mean?

A string is a sequence of characters. Concatenation joins two or more strings together to make one, e.g. "Hello" + "World" gives "HelloWorld".

What is a one-dimensional array and how does it differ from a two-dimensional array?

A 1D array is an ordered list of items of the same data type accessed by a single index. A 2D array stores data in rows and columns (like a table/grid) and items are accessed using two indices (row and column).

What is a record, and how does it differ from an array?

A record is a data structure that groups together related fields which may be of different data types (e.g. name, age, grade). An array stores multiple items that are all the same data type.

What is the difference between a procedure and a function (subprogram)?

Both are named blocks of reusable code. A function returns a value to the part of the program that called it; a procedure performs a task but does not return a value.

Give two advantages of using subprograms (procedures/functions).

They make code easier to read, write and maintain by breaking it into smaller parts, and they allow code to be reused without rewriting it (reducing duplication and errors).

What is the difference between a local variable and a global variable?

A local variable exists only inside the subprogram where it is declared and cannot be accessed elsewhere. A global variable is declared so it can be accessed and changed throughout the whole program.

What is a parameter (argument) in the context of a subprogram?

A parameter is a value passed into a subprogram when it is called, allowing the subprogram to work with different data each time it runs.

What does it mean to write robust code, and name two defensive design techniques.

Robust code anticipates and handles errors and misuse without crashing. Techniques: input validation, anticipating misuse, sanitising/authentication, and clear maintainable code with comments.

Distinguish between validation and verification of input.

Validation checks that input is sensible/of the correct format before it is accepted (e.g. range check). Verification checks that input matches an expected value or that data has been entered/copied correctly (e.g. double entry, confirmation).

Name four common validation checks.

Range check, presence check, length check, type check, and format/lookup check.

What is the difference between a syntax error and a logic error?

A syntax error breaks the rules/grammar of the programming language so the program will not run/compile. A logic error lets the program run but produces incorrect or unexpected results because the algorithm is flawed.

What is the difference between iterative testing and final/terminal testing?

Iterative testing is carried out repeatedly during development as the program is built, fixing errors as you go. Final (terminal) testing is done once on the completed program to check it meets all requirements.

What are the three types of test data, with an example for a field accepting values 1-10?

Normal data (valid, e.g. 5), boundary data (on the edge of valid, e.g. 1 and 10), and erroneous/invalid data (should be rejected, e.g. 0, 11 or "x").

What does the Computer Misuse Act 1990 make illegal?

Unauthorised access to computer material (hacking), unauthorised access with intent to commit further offences, and unauthorised modification of computer material (e.g. spreading malware).

What does the Data Protection Act protect, and state two of its principles.

It protects individuals' personal data held by organisations. Principles include: data must be used fairly and lawfully, kept secure, accurate and up to date, used only for the stated purpose, and kept no longer than necessary.

What does copyright legislation (e.g. the Copyright, Designs and Patents Act) protect?

It protects the intellectual property of creators, making it illegal to copy, use or distribute someone's work (software, music, text, images) without permission.

What is the difference between open source and proprietary (closed source) software?

Open source software is distributed with its source code, which users can view, modify and redistribute, often free. Proprietary software is sold under licence with the source code hidden; users cannot legally modify or share it.

Give two examples of how computer technology has environmental impact.

Manufacturing and running devices consume energy and natural resources, and discarded electronics create e-waste containing toxic materials. Devices also consume large amounts of electricity (e.g. data centres).

What is the digital divide and why is it a cultural/ethical concern?

The digital divide is the gap between people who have access to digital technology and the internet and those who do not. It is a concern because lack of access can disadvantage people in education, employment and access to services.

What is artificial intelligence (AI) in computing?

AI is the development of computer systems able to perform tasks that normally require human intelligence, such as recognising patterns, making decisions, understanding language and learning from data.

What is machine learning and how does it relate to AI?

Machine learning is a branch of AI in which systems learn and improve from data/experience rather than being explicitly programmed for every rule, finding patterns in large datasets to make predictions or decisions.

State two benefits and two risks of using AI and emerging technologies.

Benefits: automation of tedious/dangerous tasks and faster, data-driven decision making. Risks: job losses through automation, biased decisions from biased training data, and privacy/ethical concerns over data use.