What you'll learn
Computer hardware is the set of physical parts that make a computer work: the processor that carries out instructions, the memory that holds what it is working on, the boards and buses that connect everything, and the devices that cool and power it. This guide explains what each component does, how they work together, and — most importantly for the exam — why particular components affect performance. By the end you should be able to name and describe the parts of the CPU, explain the difference between RAM, ROM, cache and virtual memory, compare types of computer (desktop, laptop, tablet, smartphone, server, embedded system, wearable), and reason about which upgrade will actually make a slow computer faster.
Questions on hardware appear in every paper. Short questions ask you to define a component; longer ones give a scenario — a gamer, a video editor, a school buying devices — and ask you to choose and justify. Those scenario questions are where most marks are lost, because students name a component without explaining why it helps in that situation.
Key terms and definitions
Hardware — The physical components of a computer system that you can touch.
CPU (central processing unit) — The component that fetches, decodes and executes program instructions.
ALU (arithmetic logic unit) — The part of the CPU that performs calculations and logical comparisons.
Control unit — The part of the CPU that decodes instructions and coordinates the flow of data through the processor.
Clock speed — The number of cycles the CPU's clock completes each second, measured in gigahertz (GHz).
Core — An independent processing unit within a CPU. A quad-core processor has four.
Cache — A small amount of very fast memory on or very near the CPU that holds frequently used data and instructions.
RAM (random access memory) — Volatile working memory that holds the programs and data currently in use.
ROM (read-only memory) — Non-volatile memory that holds instructions such as the start-up (boot) program.
Volatile — Loses its contents when the power is switched off.
Virtual memory — Part of secondary storage used as an extension of RAM when RAM is full.
Motherboard — The main circuit board that connects the CPU, memory, storage and expansion cards.
GPU (graphics processing unit) — A processor specialised for producing images and video.
PSU (power supply unit) — Converts mains electricity into the low-voltage direct current the components need.
Embedded system — A computer built into a larger device to perform one dedicated function.
Peripheral — A device connected to a computer that is not part of its core, such as a printer or external drive.
Core concepts
The central processing unit
The CPU is often called the brain of the computer, but it is more useful to think of it as a very fast, very obedient worker. It does exactly one thing over and over: it fetches an instruction from memory, decodes it to work out what is being asked, and executes it. This is the fetch–decode–execute cycle, and it happens billions of times a second.
Inside the CPU are two main parts you must know:
- The control unit manages the cycle. It decodes each instruction and sends control signals that tell other components what to do and when.
- The arithmetic logic unit (ALU) does the actual work of calculating (addition, subtraction) and comparing (is this value greater than that one? are they equal?).
The CPU also contains tiny, extremely fast storage locations called registers, which hold the instruction and data being worked on at that moment.
What affects CPU performance
Three factors come up again and again.
Clock speed. Each tick of the clock allows part of the cycle to happen. A 3.5 GHz processor ticks 3.5 billion times a second, so a higher clock speed means more instructions processed per second. Higher speeds also produce more heat.
Number of cores. Each core can process its own stream of instructions. A quad-core processor can, in principle, work on four tasks at once. In practice the gain is smaller than "four times faster", because many programs cannot split their work into independent parts, and cores still share memory and cache.
Cache size. If the data the CPU needs is already in cache, it does not have to wait for slower RAM. A larger cache means more "hits" and less waiting. For some tasks, reducing waiting matters more than calculating faster.
Memory: RAM, ROM, cache and virtual memory
These four are the most commonly confused terms in the topic, so learn them as a set.
RAM is the computer's working memory. When you open a program, it is copied from storage into RAM because RAM is far faster to access. RAM is volatile: switch off, and everything in it is lost. That is why unsaved work disappears in a power cut.
ROM is non-volatile. Its main job is to hold the start-up instructions (often called the BIOS or firmware) that run as soon as the power comes on. They cannot be in RAM because RAM is empty at switch-on.
Cache sits between the CPU and RAM. It is much smaller than RAM and much more expensive per byte, but it is the fastest memory after the registers.
Virtual memory is a fallback. When RAM fills up, the operating system moves some data out to secondary storage (a hard disk or SSD) and brings it back when needed. Because storage is far slower than RAM, heavy use of virtual memory — called swapping — makes the computer feel sluggish. Adding more RAM is the usual cure.
The motherboard and buses
The motherboard is the main printed circuit board. The CPU sits in a socket on it, RAM plugs into slots on it, and storage and expansion cards (such as graphics cards) connect to it. The motherboard carries the buses — sets of parallel connections that move information between components. You may meet three:
- the data bus, which carries the data itself;
- the address bus, which carries the memory location being read or written;
- the control bus, which carries signals such as "read" and "write".
Graphics, power and cooling
A GPU has many simple cores working in parallel, which suits the repetitive calculations needed to draw images. Gamers, video editors and designers benefit most from a powerful graphics card.
The PSU converts high-voltage alternating current from the wall into the low-voltage direct current components use. A failing power supply can cause random crashes.
Processors produce heat. A heat sink (a block of metal with fins) and a fan carry that heat away. If cooling is inadequate, the processor protects itself by slowing down — this is called thermal throttling — and severe overheating can damage components.
Types of computer
Different devices make different trade-offs between power, portability and cost.
- Desktop — powerful, easy to upgrade, good cooling, but not portable.
- Laptop — portable with a built-in screen, keyboard and battery; parts are smaller and run cooler, so performance per pound is usually lower than a desktop.
- Tablet — light, touchscreen, long battery life; good for reading, presenting and apps, weaker for long typing and specialist software.
- Smartphone — a pocket computer with sensors, camera and mobile data; processors are designed for low power use to protect battery life.
- Server — a powerful computer that provides services (files, email, websites) to many users over a network; often has redundant parts so it keeps running if one fails.
- Embedded system — a computer inside another device, such as a washing machine, car or microwave. It does one job, so it needs little power or memory and is cheap to make, but it is hard to upgrade.
- Wearable — a device worn on the body, such as a smartwatch or fitness tracker, using sensors to collect data like heart rate and steps.
Bottlenecks
A computer is only as fast as its slowest relevant part. If a video editor's computer keeps using virtual memory, buying a faster processor barely helps — the processor just waits faster. Identifying the bottleneck is the key skill in scenario questions: ask what the task actually needs (processing, memory, graphics, or storage speed) and which component is currently holding it back.
Worked examples
Example 1: Choosing an upgrade
A student's laptop has 4 GB of RAM. With a web browser, a word processor and a music app open, it becomes very slow. Which upgrade would help most, and why?
Answer: Adding more RAM. With several programs open, 4 GB is likely to be full, so the operating system is moving data to virtual memory on the storage drive, which is much slower than RAM. More RAM means the programs fit in memory and less swapping is needed. A faster processor would not fix this, because the processor is waiting for data rather than struggling to calculate.
Example 2: Explaining ROM at start-up
Explain why the instructions that start a computer are stored in ROM rather than RAM. (2 marks)
Answer: RAM is volatile, so it is empty when the computer is switched on (1). ROM is non-volatile and keeps its contents without power, so the start-up instructions are available immediately to load the operating system (1).
Example 3: Cores versus clock speed
A video-rendering program can split its work into many independent parts. A writer mainly uses a word processor. Which user benefits more from an eight-core processor?
Answer: The video renderer. Their software can divide the work between cores, so eight cores can work in parallel. A word processor does little that can be split up, so extra cores would sit mostly idle; the writer would notice little difference.
Example 4: Selecting a device
A school is buying devices for pupils to do research and write coursework. Compare tablets and laptops.
Answer: Tablets are cheaper, lighter and have long battery life, which suits research and apps. However, writing long coursework on a touchscreen is slow, and some specialist software may not run. Laptops have keyboards and run full software, making coursework easier, but they cost more and are heavier. If extended writing is the main task, laptops are the better choice; if pupils mainly read and research, tablets may be enough.
Common mistakes and how to avoid them
- Confusing memory with storage. RAM is memory (temporary, in use now). A hard disk or SSD is storage (permanent). "My computer has 512 GB of memory" usually means storage.
- Saying ROM "can't be changed at all". At this level, describe ROM as non-volatile and holding start-up instructions; firmware in flash memory can be updated, so avoid absolute claims.
- Assuming more cores means proportionally faster. Doubling cores rarely doubles speed. Say that it helps if the software can split the work.
- Naming a component without linking it to the scenario. "Get a better GPU" earns little; "a better GPU would increase frame rates because game graphics are processed by the GPU" earns the marks.
- Mixing up cache and virtual memory. Cache is small and fast, near the CPU. Virtual memory is slow, on the storage drive. They solve opposite problems.
- Forgetting heat. Questions about a laptop slowing down during long tasks often expect thermal throttling.
Exam technique for "Components of Computer Systems: Hardware"
- For "describe" questions, give what the component is and what it does. One sentence each is often enough for two marks.
- For "explain" questions, add a because: the effect and the reason. Explaining why cache improves performance needs "the CPU spends less time waiting for data from RAM".
- For scenario questions, identify the bottleneck first. Read what the user is doing and ask which component that task stresses.
- For comparison questions (tablet vs laptop, desktop vs laptop), give at least one advantage and one disadvantage of each, then make a choice linked to the user's needs. Unsupported conclusions lose marks.
- Use correct units: clock speed in GHz, memory and storage in GB or TB.
Quick revision summary
- The CPU fetches, decodes and executes instructions. The control unit coordinates; the ALU calculates and compares.
- Performance depends on clock speed (instructions per second), cores (parallel work, if software allows) and cache (less waiting for RAM).
- RAM is volatile working memory; ROM is non-volatile and holds start-up instructions; cache is small and very fast; virtual memory is slow overflow space on storage.
- The motherboard connects components; buses carry data, addresses and control signals.
- GPUs handle graphics; PSUs convert mains power; heat sinks and fans prevent overheating and throttling.
- Device types trade power against portability: desktop, laptop, tablet, smartphone, server, embedded system, wearable.
- In scenario questions, find the bottleneck and explain why the upgrade addresses it.