AQA GCSE Computer Science: Computer Systems and Data Representation

Computer systems and data representation form two of the largest topic areas on AQA GCSE Computer Science Paper 2. Together, they account for a significant proportion of the marks available, and the examiners expect you to go beyond surface-level definitions. You need to understand how the hardware works, why design decisions involve trade-offs, and how all data -- whether numbers, text, images, or sound -- is ultimately stored and processed as binary.

This guide covers every sub-topic you need for these areas of the specification, from CPU architecture through to compression algorithms. It is designed to be used alongside targeted practice questions so that you can test your understanding after revising each section.

Computer Systems: The Big Picture

A computer system is made up of hardware and software working together to process data. Hardware refers to the physical components -- the processor, memory, storage devices, and peripherals. Software refers to the programs and instructions that tell the hardware what to do.

AQA expects you to understand the relationship between these layers: how the processor fetches and executes instructions stored in memory, how the operating system manages resources, and how data moves between different parts of the system.

The Central Processing Unit (CPU)

The CPU is the brain of the computer. It fetches instructions from memory, decodes them to work out what needs to happen, and then executes them. Every task the computer performs -- from opening a file to running a game -- is ultimately carried out by the CPU processing a sequence of simple instructions.

Key Components of the CPU

Arithmetic Logic Unit (ALU): The ALU performs all arithmetic calculations (addition, subtraction, multiplication, division) and logical comparisons (greater than, less than, equal to). Whenever the processor needs to do any kind of maths or make a decision based on a comparison, the ALU handles it.

Control Unit (CU): The control unit manages and coordinates the operations of the CPU. It sends control signals to other components, directing them to carry out the correct operations at the correct time. The CU decodes instructions and ensures that data flows to the right places within the processor and across the system.

Cache: Cache is a small amount of very fast memory located inside (or very close to) the CPU. It stores frequently used data and instructions so that the processor can access them more quickly than fetching them from main memory (RAM). Modern processors typically have multiple levels of cache -- L1 (smallest and fastest), L2, and L3 (larger but slightly slower).

Registers: Registers are tiny, extremely fast storage locations within the CPU itself. They hold the data currently being processed, the address of the next instruction, and the result of the most recent calculation. Key registers include:

• Program Counter (PC): Holds the address of the next instruction to be fetched.
• Memory Address Register (MAR): Holds the address of the memory location currently being read from or written to.
• Memory Data Register (MDR): Holds the data that has just been fetched from memory or is about to be written to memory.
• Accumulator: Stores the result of calculations performed by the ALU.

Von Neumann Architecture

The Von Neumann architecture is the fundamental design model for most modern computers. Its defining feature is that both program instructions and data are stored together in the same main memory. The processor fetches instructions from memory one at a time, decodes them, and executes them sequentially.

Key characteristics of Von Neumann architecture:

• A single shared memory holds both data and instructions.
• The CPU fetches instructions from memory using the address held in the program counter.
• Instructions are processed sequentially unless a branch instruction redirects the flow.
• There is a single bus system connecting the CPU to memory.

This design is simple and flexible -- any program can be loaded into memory and executed -- but it can create a bottleneck because data and instructions share the same pathway.

The Fetch-Decode-Execute Cycle

The fetch-decode-execute cycle is the fundamental process by which the CPU processes every instruction. It repeats continuously while the computer is running.

Fetch:

1. The program counter holds the address of the next instruction.
2. This address is copied to the Memory Address Register (MAR).
3. The instruction at that address is fetched from RAM and placed in the Memory Data Register (MDR).
4. The program counter is incremented to point to the next instruction.

Decode: 5. The control unit decodes the instruction in the MDR to determine what operation needs to be carried out and what data (if any) is needed.

Execute: 6. The instruction is carried out. This might involve the ALU performing a calculation, data being read from or written to memory, or a result being stored in a register.

This cycle then repeats. The speed at which it runs is determined by the clock speed of the processor -- a higher clock speed means more cycles per second, which generally means faster performance.

Primary Storage

Primary storage refers to memory that is directly accessible by the CPU. It is fast but generally limited in capacity compared to secondary storage.

RAM (Random Access Memory)

RAM is the computer's main working memory. It holds the data and instructions for programs that are currently running. RAM is volatile -- it loses all its contents when the power is switched off. When you open an application, it is loaded from secondary storage into RAM so the CPU can access it quickly.

More RAM allows a computer to run more programs simultaneously without slowing down. If RAM fills up, the operating system may use part of the hard drive as virtual memory, which is significantly slower.

ROM (Read Only Memory)

ROM is non-volatile -- it retains its contents even when the power is off. It typically stores the boot-up instructions (the BIOS or UEFI firmware) that the computer needs to start. ROM is generally not written to during normal operation; its contents are set during manufacture or through a special process.

Key Differences

Feature	RAM	ROM
Volatile?	Yes	No
Read/Write?	Read and write	Read only (in normal use)
Purpose	Holds running programs and data	Holds start-up instructions
Speed	Fast	Fast (but smaller capacity)

Secondary Storage

Secondary storage provides long-term, non-volatile storage for data and programs when they are not in use. The three main types you need to know are magnetic, solid-state, and optical.

Magnetic Storage (e.g. Hard Disk Drives -- HDDs)

HDDs store data on spinning magnetic platters. A read/write head moves across the surface to access data.

• Capacity: Very high -- commonly 1 TB to several TB.
• Speed: Slower than SSDs because data access depends on the physical movement of the platters and read/write head.
• Cost: Low cost per gigabyte, making them the cheapest option for large storage.
• Durability: Vulnerable to damage from physical shocks because of the moving parts.
• Portability: Heavier and bulkier than SSDs; less suitable for portable devices.

Solid-State Storage (e.g. SSDs, USB Flash Drives)

SSDs use electronic circuits (flash memory) to store data with no moving parts.

• Capacity: Available in high capacities, though typically lower than the largest HDDs.
• Speed: Much faster than HDDs for both reading and writing data.
• Cost: Higher cost per gigabyte than magnetic storage.
• Durability: Very durable -- resistant to physical shocks because there are no moving parts.
• Portability: Lightweight and compact, ideal for laptops and portable devices.

Optical Storage (e.g. CDs, DVDs, Blu-rays)

Optical discs store data using tiny pits burned into the surface of the disc, which are read by a laser.

• Capacity: Relatively low -- CDs hold around 700 MB, DVDs around 4.7 GB, Blu-rays around 25-50 GB.
• Speed: Slowest of the three types for data access.
• Cost: Very low cost per unit for the discs themselves, but low capacity limits usefulness.
• Durability: Can be scratched or damaged by sunlight; not suitable for frequent rewriting.
• Portability: Very lightweight and easy to transport.

Comparison Summary

When choosing a storage device, the decision always involves trade-offs. HDDs are best for cheap, high-capacity storage where speed is not critical. SSDs are best for performance and portability. Optical media is rarely used for everyday storage today but is still relevant for distributing software or archiving small amounts of data.

System Software

System software manages the computer's hardware and provides a platform for application software to run. The two main types are operating systems and utility software.

Operating Systems

The operating system (OS) is the most important piece of system software. It acts as an intermediary between the user, application software, and the hardware. Key functions include:

Memory Management: The OS allocates RAM to programs when they need it and deallocates it when they finish. It ensures that programs do not interfere with each other's memory space. When RAM is full, the OS may use virtual memory -- a section of the hard drive that temporarily acts as extra RAM, though at much slower speeds.

Multitasking: Modern operating systems allow multiple programs to appear to run simultaneously. The OS rapidly switches between tasks, allocating small time slices of CPU time to each program. This gives the illusion of parallel execution, even on a single-core processor.

User Interface: The OS provides the interface through which users interact with the computer -- whether that is a graphical user interface (GUI) with windows and icons, or a command-line interface (CLI) where users type text commands. The choice of interface depends on the user and the task.

Peripheral Management and Drivers: The OS manages communication between the computer and peripheral devices such as printers, keyboards, and mice. It uses device drivers -- small programs that translate generic OS instructions into specific commands that the hardware device understands. Each peripheral requires its own driver.

File Management: The OS organises files into a directory structure, controls access permissions, and manages how data is stored on and retrieved from secondary storage.

Utility Software

Utility software performs specific maintenance or housekeeping tasks to keep the system running efficiently. Examples include:

• Antivirus software: Scans for and removes malware.
• Disk defragmentation tools: Reorganise fragmented files on magnetic drives to improve access speed.
• Encryption software: Encodes data so that it cannot be read without the correct key.
• Data compression tools: Reduce file sizes for storage or transmission.
• Backup software: Creates copies of data to protect against loss.

Data Representation

Computers can only process binary -- sequences of 0s and 1s. Every piece of data, whether it is a number, a letter, an image, or a sound, must be represented in binary before a computer can store or process it.

Number Systems

Binary (Base 2): Uses only the digits 0 and 1. Each digit is a bit. Eight bits make a byte.

Denary (Base 10): The standard number system we use in everyday life, using digits 0-9.

Hexadecimal (Base 16): Uses digits 0-9 and letters A-F (where A=10, B=11, C=12, D=13, E=14, F=15). Hexadecimal is used as a shorthand for binary because it is more compact and easier for humans to read. Each hexadecimal digit represents exactly four binary digits (a nibble).

Conversions

Binary to Denary: Write out the binary place values (128, 64, 32, 16, 8, 4, 2, 1 for an 8-bit number) and add up the values where there is a 1. Example: 11001010 = 128 + 64 + 0 + 0 + 8 + 0 + 2 + 0 = 202

Denary to Binary: Repeatedly divide by 2 and record the remainders, or use the place value method -- start from the largest place value and subtract where possible.

Binary to Hexadecimal: Split the binary number into groups of four bits (nibbles), then convert each group to its hexadecimal equivalent. Example: 11001010 = 1100 1010 = C A = CA

Hexadecimal to Binary: Convert each hexadecimal digit to its 4-bit binary equivalent.

Hexadecimal to Denary: Multiply each digit by its place value (16^1, 16^0 for a two-digit number) and add the results.

Binary Arithmetic

Binary Addition: The rules are straightforward:

• 0 + 0 = 0
• 0 + 1 = 1
• 1 + 0 = 1
• 1 + 1 = 10 (0, carry 1)
• 1 + 1 + 1 = 11 (1, carry 1)

If the result exceeds the number of bits available (for example, a 9-bit result in an 8-bit system), this is called an overflow error.

Binary Shifts: A left shift moves all bits one or more places to the left, filling the vacated positions with 0s. Each left shift by one position multiplies the number by 2. Example: 00001100 (12) shifted left by 1 = 00011000 (24)

A right shift moves all bits one or more places to the right. Each right shift by one position performs an integer division by 2 (the result is rounded down, and any bits shifted off the right end are lost). Example: 00001100 (12) shifted right by 1 = 00000110 (6)

Character Encoding

Computers store text by assigning a unique binary number to each character. The two main encoding standards are ASCII and Unicode.

ASCII

ASCII (American Standard Code for Information Interchange) uses 7 bits per character, giving 128 possible characters (0-127). This is enough to represent uppercase and lowercase English letters, digits 0-9, punctuation marks, and control characters (such as newline and tab). Extended ASCII uses 8 bits, providing 256 characters.

Unicode

Unicode was developed to represent characters from every writing system in the world -- including Chinese, Arabic, Hindi, emojis, and mathematical symbols. It can represent over 100,000 characters. Unicode uses more bits per character than ASCII (commonly 8, 16, or 32 bits depending on the encoding form -- UTF-8, UTF-16, or UTF-32).

Key Differences and Trade-offs

Feature	ASCII	Unicode
Characters supported	128 (7-bit) or 256 (8-bit extended)	Over 100,000
Bits per character	7 or 8	Variable (8, 16, or 32)
File size	Smaller	Larger (more bits per character)
Language support	English and basic symbols only	All major world languages

ASCII is a subset of Unicode -- the first 128 Unicode characters are identical to ASCII. The trade-off is straightforward: Unicode supports far more characters but uses more storage space per character, resulting in larger file sizes for the same amount of text.

Image Representation

Digital images are made up of tiny dots called pixels (picture elements). Each pixel is assigned a binary value that represents its colour.

Key Concepts

Resolution: The number of pixels in an image, usually expressed as width x height (e.g. 1920 x 1080). Higher resolution means more pixels, which produces a sharper, more detailed image but also increases file size.

Colour Depth: The number of bits used to represent the colour of each pixel. A colour depth of 1 bit gives 2 colours (black and white). A colour depth of 8 bits gives 256 colours. A colour depth of 24 bits gives over 16 million colours (true colour). Greater colour depth produces more realistic images but increases file size.

Metadata: Image files also contain metadata -- additional information stored alongside the pixel data. This can include the image dimensions, colour depth, file format, date taken, and camera settings. Metadata adds to the overall file size but does not affect the pixel data itself.

Calculating Image File Size

The formula is:

File size (bits) = width (pixels) x height (pixels) x colour depth (bits per pixel)

To convert to bytes, divide by 8. To convert to kilobytes, divide by 1,024. To convert to megabytes, divide by 1,024 again.

Example: An image that is 800 x 600 pixels with a 24-bit colour depth: 800 x 600 x 24 = 11,520,000 bits = 1,440,000 bytes = approximately 1,406 KB = approximately 1.37 MB

Note that this calculation gives the uncompressed size. The actual file size may be smaller if compression is applied, and slightly larger when metadata is included.

Sound Representation

Sound is an analogue signal -- it varies continuously over time. To store sound digitally, a computer must convert it to binary through a process called sampling. At regular intervals, the computer measures the amplitude (volume) of the sound wave and records it as a binary value.

Key Concepts

Sample Rate: The number of samples taken per second, measured in Hertz (Hz). A sample rate of 44,100 Hz (CD quality) means 44,100 measurements are taken every second. A higher sample rate captures more detail and produces a more accurate representation of the original sound, but increases file size.

Bit Depth: The number of bits used to represent each sample. A higher bit depth means each sample can be recorded with greater precision -- more possible amplitude levels -- resulting in higher audio quality but a larger file.

Calculating Sound File Size

File size (bits) = sample rate (Hz) x bit depth (bits) x duration (seconds)

For stereo sound, multiply by 2 (because there are two channels).

Example: A 30-second mono recording at 44,100 Hz with a 16-bit depth: 44,100 x 16 x 30 = 21,168,000 bits = 2,646,000 bytes = approximately 2,584 KB = approximately 2.52 MB

Data Compression

Files are often compressed to reduce their size. This makes them faster to transmit over a network and means they take up less storage space. There are two types of compression.

Lossy Compression

Lossy compression permanently removes some data from the file to reduce its size. The removed data is chosen to be the least noticeable to human perception -- for example, sounds outside the range of human hearing or subtle colour variations in an image. The original file cannot be perfectly reconstructed from the compressed version.

• Advantages: Achieves much smaller file sizes than lossless compression. Ideal for media files (images, audio, video) where a small loss in quality is acceptable.
• Disadvantages: Quality is permanently reduced. Repeatedly compressing and decompressing a file causes progressive quality loss. Not suitable for text files or program code where every bit matters.
• Examples: JPEG (images), MP3 (audio), MP4 (video).

Lossless Compression

Lossless compression reduces file size without losing any data. The original file can be perfectly reconstructed from the compressed version. It works by identifying and encoding patterns and redundancies in the data more efficiently.

• Advantages: No loss of quality. The original data is perfectly preserved. Suitable for all file types, including text and program code.
• Disadvantages: Achieves smaller compression ratios than lossy compression -- files will still be larger than their lossy equivalents.
• Examples: PNG (images), FLAC (audio), ZIP (general files).

Why Compression Matters

Compression is essential in modern computing. Without it, streaming music and video would require far more bandwidth, websites would load slowly, and storage devices would fill up much faster. The choice between lossy and lossless depends on the use case: if perfect accuracy is needed, use lossless; if smaller file size is the priority and some quality loss is acceptable, use lossy.

How These Topics Appear on Paper 2

Computer systems and data representation are examined on Paper 2 (Computing Concepts). Questions in these areas typically fall into several patterns:

Short-answer recall questions: You may be asked to define terms such as volatile, cache, or sample rate. Be precise -- vague definitions will not earn full marks.

Comparison questions: Expect questions asking you to compare storage types (e.g. "Compare HDD and SSD storage"), or to explain differences between RAM and ROM, or ASCII and Unicode. Structure your answer around specific comparison points such as speed, capacity, cost, and durability.

Calculation questions: These are common in data representation. You may need to convert between binary, denary, and hexadecimal, perform binary addition, apply binary shifts, or calculate the file size of an image or sound file. Show your working clearly -- method marks are available even if your final answer is wrong.

Extended response questions: Some questions worth 4-6 marks require you to explain a concept in depth or discuss trade-offs. For example, you might be asked to explain why a company would choose SSD storage over HDD for a laptop, or to discuss the advantages and disadvantages of lossy and lossless compression for a specific scenario. Use technical vocabulary, give specific examples, and structure your answer logically.

Application questions: AQA frequently sets questions in an unfamiliar context. You might be given a scenario about a music streaming service and asked to explain how sample rate and bit depth affect the quality and file size of streamed audio. The key is to apply your knowledge to the specific scenario rather than writing a generic answer.

For more detailed exam technique advice, including how to approach pseudo-code questions and trace tables on Paper 1, see our AQA GCSE Computer Science Exam Guide.

Prepare with LearningBro

These topics require both understanding and practice. Use our targeted courses to test yourself with questions that mirror the style and difficulty of the real exam:

• AQA GCSE Computer Science: Computer Systems -- covers CPU architecture, memory, storage, and system software.
• AQA GCSE Computer Science: Data Representation -- covers binary, hexadecimal, character encoding, image and sound representation, and compression.
• AQA GCSE Computer Science Exam Guide -- detailed exam technique for both Paper 1 and Paper 2.

Revise the theory, then put it into practice. The more questions you answer under timed conditions, the more confident you will be on exam day.