3.4 Computer Systems (2026)

Card 1.1: What does an AND gate do? Give its truth table.

Output is 1 only when BOTH inputs are 1.

Card 1.2: What does an OR gate do? Give its truth table.

Output is 1 when AT LEAST ONE input is 1.

Card 1.3: What does a NOT gate do? Give its truth table.

It reverses the input.

Card 1.4: What does an XOR gate do? Give its truth table.

Output is 1 when the two inputs are DIFFERENT.

Card 1.5: Which symbols are used in AQA Boolean expressions?

AQA Boolean symbols:

Card 1.6: What must you be able to do with Boolean logic circuits?

• Create, change and interpret simple logic circuit diagrams. • Create and interpret truth tables with up to 3 inputs. • Create Boolean expressions from circuits and circuits from Boolean expressions.

Card 2: Define hardware and software. How are they related?

• Hardware = the physical parts of a computer system. • Software = programs/instructions that run on the hardware. • Software tells the hardware what tasks to carry out.

Card 2.1: Give examples of internal hardware and external hardware.

• Internal: CPU, RAM, motherboard, SSD/HDD. • External (peripherals): keyboard, mouse, monitor, printer. • Hardware means physical equipment you can touch.

Card 3: What is the difference between system software and application software?

• System software manages computer resources and provides a platform for applications. • Application software carries out tasks for the end user. • Examples: OS = system software; word processor = application software.

Card 4: What is the purpose of an operating system (OS)?

• It manages the computer's hardware and software resources. • It provides the platform needed to run application software. • Examples include Windows, Linux, Android and iOS.

Card 5: What five areas does an operating system manage?

• 1. Processor(s) • 2. Memory • 3. Input/output (I/O) devices • 4. Applications • 5. Security

Card 6: What is utility software? Give examples.

• Utility software helps maintain, protect or manage a computer system. • Examples include antivirus, backup and file-compression utilities. • Utilities are a type of system software.

Card 7: What is an embedded system?

• A computer system built into a larger device. • It is designed to carry out a specific job or small set of jobs. • Examples: washing machine controller, car engine-control system.

Card 7.1: How does an embedded system differ from a non-embedded system?

• Embedded: part of a larger device and usually designed for a specific purpose. • Non-embedded: general-purpose computer that can run many different applications. • Example non-embedded system: desktop PC or laptop.

Card 8: What is a low-level programming language?

• A language close to the computer's hardware. • Machine code and assembly language are low-level languages. • Low-level languages are more processor-specific than high-level languages.

Card 8.1: What is machine code?

• Instructions written in binary that the processor can execute directly. • Each processor or processor family has its own machine-code instruction set. • Machine code does not need translating before execution.

Card 8.2: What is assembly language?

• A low-level language that uses short instruction names called mnemonics. • Each assembly instruction has a 1:1 match with a machine-code instruction. • It must be translated by an assembler.

Card 9: What is a high-level programming language?

• A language designed to be easier for humans to read, write and understand. • It is less closely tied to a particular processor. • Examples include Python, C# and VB.NET.

Card 9.1: Why are most programs written in high-level languages?

• They are easier and quicker for programmers to write and understand. • They are easier to debug and maintain. • They are usually more portable between different processors/systems.

Card 10.1: What does a compiler do?

• Translates a whole high-level program into machine code before it runs. • The translated program can then be run without translating the source again. • Useful when a finished program will be run many times.

Card 10.2: What does an interpreter do?

• Works through high-level code while the program is running. • It does not produce a separate machine-code program directly. • Useful during development because errors can be found as code is run.

Card 10.3: What does an assembler do?

• Translates assembly language into machine code. • Each assembly instruction is translated into one machine-code instruction. • Assembly language must be assembled before the CPU can execute it.

Card 10.4: When might assembly language be used?

• For software that needs close control of hardware. • It is often used for embedded systems and specific hardware components. • It can be efficient, but is harder to write and is processor-specific.

Card 12: What are the main CPU components AQA requires you to know?

• Arithmetic Logic Unit (ALU) • Control Unit (CU) • Clock • Registers • Bus • You must also understand the role of main memory.

Card 12.1: What is the role of the ALU?

• ALU = Arithmetic Logic Unit. • It carries out arithmetic calculations. • It also carries out logical operations and comparisons.

Card 12.2: What is the role of the Control Unit (CU)?

• It controls and coordinates the CPU. • It decodes instructions and sends control signals to other components. • It helps manage the fetch-decode-execute cycle.

Card 12.3: What is the role of the CPU clock?

• It sends regular timing signals to coordinate CPU operations. • Clock speed is measured in hertz (Hz), usually GHz for modern CPUs. • A higher clock speed means more cycles can happen each second.

Card 12.4: What are CPU registers?

• Very small, very fast memory locations inside the CPU. • They temporarily hold data or instructions currently being used. • AQA does NOT require the names of specific registers for 2027 exams.

Card 12.5: What is a bus in a computer system?

• A collection of wires/connections used to carry data or signals between components. • For the 2027 specification, you do not need to learn separate address/data/control buses.

Card 13.1: How does clock speed affect CPU performance?

• Clock speed = number of clock cycles per second. • Higher clock speed can allow the CPU to process more instructions in a given time. • Performance also depends on other factors, not clock speed alone.

Card 13.2: How does the number of processor cores affect CPU performance?

• Each core can process instructions. • More cores can allow more tasks/instructions to be processed at the same time. • The benefit depends on whether the software can make use of multiple cores.

Card 13.3: What is cache and how can cache size affect CPU performance?

• Cache is small, very fast memory used by the CPU for frequently/recently needed data and instructions. • A larger cache can reduce how often the CPU must access slower main memory. • This can improve performance.

Card 16: Explain the fetch-decode-execute cycle.

• FETCH: the next instruction is fetched from main memory to the CPU. • DECODE: the CPU works out what the instruction means. • EXECUTE: the instruction is carried out. • The cycle then repeats.

Card 17: What is RAM and what is it used for?

• RAM = Random Access Memory. • It stores programs and data currently in use. • It is volatile: its contents are lost when the power is switched off.

Card 18: What is ROM and what is it used for?

• ROM = Read Only Memory. • It stores instructions/data that need to remain when power is off, such as start-up instructions. • It is non-volatile.

Card 19: What is the difference between main memory and secondary storage?

• Main memory is directly accessible by the CPU and holds data/instructions needed while working. • Secondary storage keeps data long term and is not directly accessible by the CPU. • Secondary storage is non-volatile.

Card 19.1: What do volatile and non-volatile mean?

• Volatile: contents are lost when power is switched off. • Non-volatile: contents remain when power is switched off. • RAM is volatile; ROM, SSDs and HDDs are non-volatile.

Card 20: How does solid-state storage work? Give advantages and disadvantages.

• Stores data persistently using electrical circuits; no moving parts. • Advantages: fast, quiet, robust, low power use. • Disadvantages: usually more expensive per GB than magnetic storage. • AQA does not require precise NAND-gate details.

Card 21: How does magnetic storage work? Give advantages and disadvantages.

• Stores data by magnetising areas of a surface, e.g. a hard disk drive (HDD). • Advantages: high capacity and low cost per GB. • Disadvantages: slower and less robust than SSDs because HDDs have moving parts.

Card 23: What factors can be used to compare secondary storage?

• Capacity: how much data it stores. • Speed: how quickly data can be read/written. • Cost: price per device or per GB. • Durability/robustness, portability and power use may also matter.

Card 24: What is cloud storage?

• Data is stored on remote storage that is accessed over a network/Internet. • The remote storage itself uses magnetic and/or solid-state storage. • The user does not need the storage device to be physically local.

Card 24.1: What are advantages and disadvantages of cloud storage compared with local storage?

• Advantages: access from different locations/devices; easy sharing; provider may manage backups. • Disadvantages: needs a network connection; upload/download speed can limit access; privacy/security and ongoing cost may be concerns.