What you'll learn
This overview brings together the hardware unit of CIE IGCSE Computer Science (0478): the architecture of the CPU and the von Neumann model, the fetch–decode–execute cycle, the factors affecting CPU performance, instruction sets and embedded systems, input and output devices including sensors, primary memory (RAM and ROM), secondary storage (magnetic, optical and solid state), virtual memory and cloud storage, and network hardware — NICs, MAC addresses, IP addresses and routers. Each section summarises what you need to know, with examples; the sub-topic guides on individual parts of hardware go into more detail.
Hardware questions often ask you to describe a process in order (the fetch–decode–execute cycle), to explain how a device works (a touchscreen, a laser printer, an SSD), or to choose and justify hardware for a scenario. Precise terms and clear sequences are essential.
Key terms and definitions
CPU — The central processing unit, which processes instructions and data.
Von Neumann architecture — A design where data and instructions are stored together in memory and processed by a CPU with registers, an ALU and a control unit.
ALU — Arithmetic logic unit, which performs calculations and logical operations.
CU — Control unit, which decodes instructions and sends control signals.
Registers — Small, fast storage locations in the CPU: PC, MAR, MDR, CIR and ACC.
Buses — The address bus, data bus and control bus, which carry addresses, data and signals.
Cache — Fast memory in the CPU storing frequently used data and instructions.
Instruction set — The list of all the commands a CPU can process.
Embedded system — A computer system that performs a dedicated function within a larger device.
Sensor — An input device that measures a physical property.
Actuator — An output device that produces movement or action.
RAM / ROM — Volatile working memory / non-volatile memory holding start-up instructions.
Virtual memory — Secondary storage used as extra RAM when RAM is full.
MAC address — A unique identifier for a device's network interface card.
IP address — An address assigned to a device on a network, which can change.
Core concepts
The CPU and von Neumann architecture
The CPU processes instructions and data that are input into the computer so that the result can be output. In the von Neumann model, instructions and data are stored together in main memory. The CPU contains:
- the ALU, which carries out calculations and logic operations;
- the CU, which decodes instructions and sends control signals to coordinate components;
- registers: the program counter (PC), which holds the address of the next instruction; the memory address register (MAR), which holds the address being accessed; the memory data register (MDR), which holds data or an instruction being transferred; the current instruction register (CIR), which holds the instruction being decoded and executed; and the accumulator (ACC), which holds the result of calculations.
The CPU connects to memory via the address bus (one-directional, carrying addresses), the data bus (two-directional, carrying data and instructions) and the control bus (two-directional, carrying control signals such as read and write).
The fetch–decode–execute cycle
- The address in the PC is copied to the MAR via the address bus.
- The instruction at that address is fetched into the MDR via the data bus.
- The instruction is copied from the MDR into the CIR.
- The PC is incremented to point to the next instruction.
- The CU decodes the instruction in the CIR.
- The instruction is executed, with the ALU used for any calculations and results stored in the ACC.
The cycle then repeats.
CPU performance
- Clock speed — the number of cycles per second; higher speed means more instructions processed per second.
- Cache size — more cache means more data can be accessed quickly instead of fetching from slower RAM.
- Number of cores — more cores allow more instructions to be processed simultaneously, though not all software can use them and cores must communicate.
Instruction sets and embedded systems
An instruction set is the complete list of commands, as machine code operations, that a particular CPU can process. Programs must be translated into the instruction set of the CPU that runs them.
Embedded systems perform a dedicated function within a larger device — washing machines, security systems, central heating, vending machines, car engine management and lighting controls. They are small, cheap to produce, use little power and are reliable, but are hard to upgrade and often depend on firmware.
Input devices
You should be able to describe how each works and give uses:
- Barcode scanner — shines light on the bars; the dark bars reflect less light; the pattern is converted into a code.
- QR code scanner — reads a 2D pattern, often using a phone camera; stores more data than a barcode, such as web addresses.
- Digital camera, keyboard, microphone, optical mouse.
- 2D and 3D scanners — capture images or the shape of objects.
- Touchscreens: resistive (two layers pressed together complete a circuit — cheap, work with gloves), capacitive (the finger changes the electrostatic field — sharper, supports multi-touch) and infrared (a grid of infrared beams is broken by touch).
Output devices
- Inkjet printers spray tiny droplets of ink — cheap to buy, good for photos.
- Laser printers use a laser to create a charged image on a drum, which attracts toner that is fused to the paper by heat — fast and good for large volumes.
- 3D printers build objects layer by layer.
- LCD and LED screens, projectors (DLP and LCD), speakers and actuators.
Sensors
Sensors measure physical properties and are used in monitoring and control systems: temperature, moisture, humidity, light, infrared (motion), pressure, acoustic, gas, pH, level, flow and proximity sensors. Sensor readings are usually analogue and are converted to digital by an analogue-to-digital converter before the microprocessor compares them with stored values and, in a control system, signals actuators.
Primary memory
RAM is volatile and stores the data, programs and parts of the operating system currently in use; it can be read from and written to. ROM is non-volatile and stores the start-up instructions (the bootstrap); in normal use it is read-only.
Secondary storage
- Magnetic (hard disk drives): platters with magnetised areas read by heads — high capacity, low cost per gigabyte, but moving parts.
- Optical (CD, DVD, Blu-ray): a laser reads pits and lands — cheap and portable, but lower capacity and easily scratched.
- Solid state (SSD, USB flash drives, SD cards): data is stored using transistors and floating gates that trap charge in flash memory — no moving parts, fast, durable and low power, but higher cost per gigabyte.
Virtual memory and cloud storage
When RAM is full, the operating system moves data not currently in use into virtual memory on secondary storage, swapping it back when needed. This prevents the computer running out of memory but slows performance because secondary storage is slower.
Cloud storage stores data on remote servers accessed over the internet. It can be accessed from anywhere and does not require local hardware, but depends on an internet connection, involves ongoing costs and places data security in the provider's hands.
Network hardware
A NIC (network interface card) allows a device to connect to a network. Each NIC has a MAC address set by the manufacturer — a 48-bit identifier shown as six pairs of hexadecimal digits — which identifies the device and does not usually change.
An IP address identifies a device on a network and can change. IPv4 uses 32 bits written as four denary numbers; IPv6 uses 128 bits written in hexadecimal, providing far more addresses. Addresses can be static (fixed) or dynamic (assigned each time the device connects).
A router sends data packets between networks, using IP addresses to forward packets towards their destination, and can assign IP addresses to devices on a home network.
Monitoring and control systems compared
A monitoring system uses sensors to collect data and alerts people or records readings, but does not change the conditions itself — for example, a hospital system that sounds an alarm if a patient's heart rate falls outside safe limits, or a weather station logging temperature. A control system uses the readings to make changes automatically through actuators — for example, a central heating system turning the boiler on when the temperature drops below the set value. Both use sensors, an ADC and a microprocessor that compares readings with stored values; only control systems use actuators to act on the result.
Worked examples
Example 1: The fetch stage
Describe how an instruction is fetched in the fetch–decode–execute cycle. (4 marks)
Answer: The address in the PC is copied to the MAR (1). The instruction at that address is sent along the data bus to the MDR (1). The instruction is copied into the CIR (1). The PC is incremented to point to the next instruction (1).
Example 2: Touchscreens
Explain why a capacitive touchscreen is used on smartphones rather than a resistive one. (2 marks)
Answer: Capacitive screens support multi-touch gestures such as pinch to zoom (1) and give a clearer, more responsive display because there are no flexible layers (1).
Example 3: Choosing storage
A photographer working outdoors needs storage for a camera. Recommend a type and justify.
Answer: Solid state storage such as an SD card. It has no moving parts, so it is durable if the camera is knocked, it is small and light, and it uses little power.
Example 4: Virtual memory
Explain what virtual memory is and why it is needed. (3 marks)
Answer: Virtual memory is an area of secondary storage used as if it were RAM (1). It is used when RAM is full (1), so that data not currently in use can be moved out, allowing more programs to run without the computer crashing (1).
Common mistakes and how to avoid them
- Forgetting the CIR in CIE's fetch–decode–execute cycle.
- Mixing up MAR and MDR. MAR holds an address; MDR holds data or an instruction.
- Saying the address bus is two-way. It is one-directional.
- Describing SSDs as having moving parts.
- Confusing MAC and IP addresses. MAC is fixed by the manufacturer; IP addresses can change.
Exam technique for "Hardware"
- Learn the five registers and the three buses with their directions.
- For "how does it work" questions on devices, give the physical process in order.
- For choosing hardware, link a feature to the scenario: "durable, because it is used outdoors".
- Use the correct terms: volatile, non-volatile, embedded, actuator.
Quick revision summary
- CPU: ALU, CU, registers (PC, MAR, MDR, CIR, ACC), buses (address, data, control).
- FDE cycle: PC → MAR → memory → MDR → CIR; PC incremented; decode; execute.
- Performance: clock speed, cache, cores. Instruction sets; embedded systems.
- Input devices including touchscreens (resistive, capacitive, infrared); output devices including laser, inkjet and 3D printers.
- Sensors feed monitoring and control systems via ADCs and actuators.
- RAM volatile, ROM non-volatile; magnetic, optical and solid state storage; virtual memory; cloud storage.
- NIC, MAC address (fixed), IP address (IPv4/IPv6, static/dynamic), router.