What is an algorithm?

An algorithm is a precise, step-by-step set of instructions for solving a problem or completing a task. It must be unambiguous, finite (it stops eventually) and produce a correct result for all valid inputs. Algorithms can be shown as pseudocode, flowcharts or written descriptions.

Computational thinking skills

AQA expects four core skills.

• Abstraction: removing unnecessary detail and keeping only what matters to the problem.
• Decomposition: breaking a large problem into smaller, manageable sub-problems.
• Algorithmic thinking: designing the logical steps needed to reach a solution.
• Pattern recognition: spotting similarities between problems so existing solutions can be reused.

Searching algorithms

• Linear search: checks each item in turn from the start. Works on unsorted or sorted lists. Worst case checks every item, so it is O(n).
• Binary search: repeatedly halves a sorted list, comparing the middle item to the target. Much faster than linear search on large lists, O(log n), but the data MUST be sorted first.

Sorting algorithms

• Bubble sort: repeatedly swaps adjacent items that are in the wrong order, passing through the list until no swaps are needed. Simple but slow, O(n squared).
• Merge sort: splits the list in half repeatedly until single items remain, then merges them back together in order. Faster than bubble sort on large data, O(n log n).
• Insertion sort: builds a sorted section one item at a time, inserting each new item into its correct place.

Expressing algorithms

You need to be able to read, trace and write algorithms in AQA pseudocode and flowcharts, using sequence, selection (if/else) and iteration (for, while, do until). Trace tables are used to track variable values line by line as an algorithm runs, and are the standard way to check your understanding and find errors (bugs).

Common mistakes

• Confusing linear and binary search conditions, or trying binary search on unsorted data.
• Miscounting comparisons/swaps in bubble sort trace tables.
• Forgetting that arrays in AQA pseudocode are usually zero-indexed.
• Mixing up efficiency (Big O) with simply 'which one is faster to write'.
• Not decomposing a problem before attempting to code or design it, leading to messy logic.

What programming fundamentals covers

This topic is about how programs actually work under the hood: sequence, selection and iteration, the three core data types, arrays, string handling, file handling, SQL basics, and validation. AQA expects you to trace code by hand and to write short programs in pseudocode or your chosen language (Python is most common).

The three constructs

• Sequence: instructions run in order, one after another.
• Selection: IF, ELSE IF, ELSE (and CASE/switch) branch the flow based on a condition.
• Iteration: count-controlled loops (FOR) run a set number of times; condition-controlled loops (WHILE, DO UNTIL/REPEAT) run until a condition is met. WHILE checks before the loop body runs; DO UNTIL checks after, so it always runs at least once.

Data types and casting

• Integer: whole numbers. Real/float: numbers with a decimal point. Boolean: True or False only. Character: a single symbol. String: a sequence of characters.
• Casting converts one type to another, eg str(), int(), float() in Python. A common exam mistake is forgetting that input() always returns a string, so numbers typed in must be cast before doing maths on them.
• Concatenation joins strings with +; you cannot concatenate a string and an integer directly without casting first.

Arrays

• An array holds multiple values of the same type under one identifier. 1D arrays are lists; 2D arrays are grids (rows and columns), accessed with two indices like array[row][column].
• Indexing starts at 0 in most exam pseudocode and in Python, so an array of 5 items runs from index 0 to 4.

String handling operations

• Know length (len), substring/slicing, concatenation, converting case (upper/lower), and converting between string and number types. These crop up constantly in exam trace tables.

File handling

• Standard operations are open, read, write and close a file. Always close a file after use to save changes and free the resource.

SQL basics

• SELECT chooses columns, FROM names the table, WHERE filters rows. Example: SELECT name FROM students WHERE grade = 9.
• Also know INSERT INTO (add a record), UPDATE (change a record), DELETE FROM (remove records).

Validation vs authentication

• Validation checks data is sensible (eg range check, length check, type check, presence check). It does not check the data is true, only that it is usable.
• Authentication checks a user's identity, eg a username and password.

Common mistakes

• Mixing up validation (checking format) with verification (checking accuracy, eg double-entry or proofreading).
• Off-by-one errors when looping through arrays because indexing starts at 0.
• Forgetting WHILE loops can run zero times if the condition is false immediately.

Why binary?

Computers are built from transistors that are either on or off, so they can only reliably store two states: 1 and 0. This is called binary (base 2). Everything a computer stores, text, numbers, images, sound, is ultimately just patterns of binary digits (bits).

Bits, nibbles, bytes

• A bit is a single 0 or 1.
• A nibble is 4 bits.
• A byte is 8 bits, and can represent 256 different values (0 to 255).
• Storage units scale up in multiples of 1024 for the exam: 1 KB = 1024 bytes, 1 MB = 1024 KB, 1 GB = 1024 MB, 1 TB = 1024 GB.

Binary to denary (and back)

Each bit position in an 8-bit binary number is worth a power of 2, reading left to right: 128, 64, 32, 16, 8, 4, 2, 1. To convert binary to denary, add up the place values where there is a 1. To convert denary to binary, subtract the largest place value that fits, repeatedly, marking 1s and 0s as you go.

Binary addition and overflow

Add binary numbers column by column like normal addition, carrying when a column totals 2 or more (1+1=10, carry the 1). If an 8-bit register cannot hold the result of an addition, this is called overflow, and it causes an error because the number is too big to store.

Hexadecimal

Hexadecimal (base 16) uses digits 0 to 9 then A to F (A=10 up to F=15). It is used because it is a much shorter way of representing binary, one hex digit exactly represents one nibble (4 bits), so a byte is always 2 hex digits. Hex is used for MAC addresses, colour codes (like #FF0000), and memory addresses because it is easier for humans to read than long strings of 1s and 0s.

Character encoding

Characters are stored using a code that maps each character to a binary number. ASCII uses 7 or 8 bits, giving 128 or 256 possible characters, enough for English letters, numbers and symbols. Unicode uses more bits (commonly 16), allowing over 100,000 characters, so it can represent every language and symbol in the world, including emoji.

Images

A bitmap image is stored as a grid of pixels. Each pixel's colour is stored as a binary number. Colour depth (bits per pixel) determines how many colours are possible, more bits means more colours but a bigger file. Resolution (the number of pixels) also affects file size and image quality, higher resolution means more pixels and a larger file.

Sound

Sound is analogue in the real world, so it must be sampled to be stored digitally. Sample rate is how many samples are taken per second (measured in Hz). Bit depth is how many bits are used to store each sample. Higher sample rate and higher bit depth both improve sound quality but increase file size.

Common mistakes

• Forgetting binary place values start at 128 for 8 bits, not 256.
• Mixing up bits and bytes (8 bits = 1 byte).
• Forgetting each hex digit maps to exactly a nibble.
• Confusing sample rate (samples per second) with bit depth (bits per sample).

The CPU and the fetch-execute cycle

The CPU (Central Processing Unit) carries out instructions from programs, following the fetch-decode-execute cycle non-stop while the computer is on.

• Fetch: the next instruction address is held in the Program Counter (PC); it is copied into the Memory Address Register (MAR), fetched from memory and placed in the Memory Data Register (MDR).
• Decode: the Control Unit works out what the instruction means.
• Execute: the instruction is carried out, often using the Arithmetic Logic Unit (ALU) for maths/logic, with results held briefly in registers like the Accumulator.

Common mistake: mixing up MAR (holds an address) and MDR (holds the actual data/instruction).

Von Neumann architecture

Most computers use Von Neumann architecture: one memory stores both data and instructions, and they travel along shared buses (address bus, data bus, control bus). The system clock generates pulses that pace the fetch-execute cycle.

CPU performance

Three main factors affect CPU performance:

• Clock speed: measured in GHz (billions of cycles per second); higher means more fetch-execute cycles per second.
• Number of cores: more cores can process more instructions in parallel, but only if software is written to use them.
• Cache size: a small, very fast memory on the CPU itself that stores frequently used data/instructions, cutting down slow trips to RAM.

Common mistake: assuming clock speed alone determines speed — cache and cores matter just as much, and software must be optimised to benefit.

Embedded systems

An embedded system is a computer built into a larger device to perform one specific, dedicated task (e.g. a washing machine controller, a satnav, a microwave). They are usually cheap, small, and have limited processing power compared to general-purpose computers.

Memory: RAM vs ROM

• RAM (Random Access Memory): volatile (loses contents when powered off), stores currently running programs and data, can be read from and written to.
• ROM (Read Only Memory): non-volatile, stores the BIOS/bootstrap program that starts the computer, cannot normally be written to.

Common mistake: saying RAM is 'permanent storage' — RAM is temporary and volatile; only secondary storage and ROM keep data without power.

Secondary storage

Secondary storage is non-volatile and holds data/software long-term when the computer is off. Three types with trade-offs on capacity, speed, portability and durability:

• Magnetic (e.g. hard disk drives): large capacity, cheap per GB, but slower and has moving parts that can fail.
• Optical (e.g. DVD, Blu-ray): cheap, portable, but lower capacity and can scratch.
• Solid state (e.g. SSD, USB flash): fast, durable (no moving parts), more expensive per GB.

Units and data storage

Data is stored in bits (0 or 1); 8 bits = 1 byte. Storage scales in these approximate steps: 1000 bytes = 1 kilobyte, 1000 KB = 1 megabyte, 1000 MB = 1 gigabyte, 1000 GB = 1 terabyte. Know how to compare file sizes and storage capacity, and calculate how many files fit in a given space.

Networks: types and hardware

A LAN (Local Area Network) covers one site, like a school or office, and is owned by one organisation.

A WAN (Wide Area Network) covers a large geographical area and links multiple LANs, often using infrastructure owned by third parties (the internet is the biggest WAN).

Key hardware: a switch connects devices within a LAN and sends data only to the correct device using MAC addresses; a router connects different networks together and directs data between them using IP addresses; a wireless access point (WAP) lets devices connect without cables; a network interface card (NIC) lets a device connect to a network at all, wired or wireless.

Network topologies

Star topology: every device connects to a central switch. If one cable fails only that device drops off, but if the switch fails the whole network goes down. This is the standard setup in schools and offices.

Mesh topology: devices connect directly to many other devices, giving strong reliability with no single point of failure, but it is expensive and complex to set up.

Wired vs wireless and protocols

Ethernet is the standard for wired connections; Wi-Fi is the standard for wireless. Wired is generally faster and more secure but less flexible.

TCP/IP is the protocol suite that governs how data is split into packets, addressed and reassembled across the internet.

Common application protocols: HTTP (unencrypted web browsing), HTTPS (encrypted web browsing, uses TLS), FTP (file transfer), SMTP/IMAP/POP3 (email sending and receiving).

The key common mistake is mixing up HTTP and HTTPS, or forgetting that HTTPS uses encryption via TLS, not just 's' for 'secure' with no mechanism behind it.

Network security threats

Malware includes viruses (attach to files, need a host to spread), worms (self-replicate across networks without a host), trojans (disguised as legitimate software) and ransomware (encrypts files and demands payment).

Social engineering exploits human weakness rather than technical flaws: phishing (fake emails/links to steal data) and shoulder surfing are the classic exam examples.

Brute-force attacks try every possible password combination; SQL injection inserts malicious SQL code into input fields to access or damage a database; DoS (Denial of Service) attacks flood a server with traffic to take it offline.

Prevention methods

Firewalls monitor and filter incoming and outgoing traffic based on rules.

Encryption scrambles data so it is unreadable without a key, protecting data in transit.

Strong passwords, user access levels and biometric authentication reduce unauthorised access.

Physical security (locked server rooms) and regular software updates/patches close known vulnerabilities.

A common exam mistake is calling a firewall 'anti-virus' -- they are different: anti-virus detects and removes malware already on a device, a firewall controls network traffic.

Why this topic matters

Computing has huge impact on individuals, society and the environment. AQA expects you to discuss issues sensibly, giving both benefits and drawbacks, and to know the key laws that regulate computer use in the UK.

Ethical and legal issues

• Ethical issues are about right and wrong even when something is legal, eg using AI to generate fake images, monitoring employees, or web scraping without permission.
• Legal issues are about breaking actual laws.
• Key UK laws to name exactly:
• The Data Protection Act 2018 (and UK GDPR) controls how personal data is collected, stored and used. Data must be accurate, kept no longer than necessary, and kept secure.
• The Computer Misuse Act 1990 makes it illegal to gain unauthorised access to computer systems or data, to gain unauthorised access with intent to commit further crime, and to make unauthorised modifications to data (this covers spreading malware).
• The Copyright, Designs and Patents Act 1988 protects original work such as software, music and images from being copied or distributed without permission.
• The Regulation of Investigatory Powers Act 2000 (RIPA) governs surveillance and interception of communications by public bodies.

Common mistakes

• Mixing up the three sections of the Computer Misuse Act 1990 — learn all three offences separately.
• Saying 'hacking is illegal' without naming the Act.
• Forgetting that copying software counts as breaking copyright law, not just copying music or films.

Environmental issues

• E-waste: old devices contain toxic materials (lead, mercury) and need proper recycling, not landfill.
• Energy use: data centres consume large amounts of electricity and water for cooling; this drives interest in energy-efficient hardware and renewable power.
• Manufacturing: mining rare earth metals for chips and batteries damages ecosystems and often happens in developing countries.
• Planned obsolescence, where devices are designed to become outdated quickly, increases waste. Some manufacturers are moving to modular, repairable designs to reduce this.

Cultural and social issues

• The digital divide: unequal access to technology and the internet based on income, location or age can exclude people from services, education and jobs.
• Automation and AI can remove jobs but also create new roles; exam answers should weigh both sides.
• Online communication changes culture (eg social media, remote working) with both positive (connection) and negative (cyberbullying, misinformation) effects.

Exam tip

When asked to 'discuss', always give a balanced answer with at least one benefit and one drawback, and use precise legal or technical terms rather than vague statements.