Key Stage 3 computing · Computer systems

KS3 computer hardware: CPU, memory and storage explained

Hardware is the physical parts of a computer system that you can touch. In Years 7 to 9, children learn what the main components do, how they work together, how the processor runs stored instructions, and how to explain why a computer is fast or slow in terms of its parts, not just its price.

Computer hardware components, explained

At the centre of a computer is the central processing unit, or CPU. Its job is to run instructions, billions of tiny steps every second. Two figures are often quoted for a CPU. Clock speed, measured in gigahertz, says roughly how many cycles it completes each second. The number of cores says how many instruction streams it can work on at once. Cache, a very small, very fast memory inside the CPU, keeps recently used data close to hand. Children learn that all three matter, so a bigger clock speed on its own does not tell you how fast a computer will feel.

Computers use two main kinds of memory, and the distinction trips up many adults. RAM (random access memory) is the working space where programs and data are held while they are in use. It is fast but volatile, which means it is emptied when the power goes off. Storage, such as a solid-state drive, a hard disk or a memory card, keeps files for good, even when the power is off, but is slower. A useful picture is a desk and a filing cabinet: the desk (RAM) holds what you are working on now, and the cabinet (storage) holds everything else.

The curriculum also expects pupils to understand how instructions are stored and executed. A program is a long list of instructions in binary. When you open it, it is copied from storage into RAM. The CPU then repeats a cycle: it fetches the next instruction from memory, decodes it to work out what it means, and executes it. Then it fetches the next, again and again. Because instructions and data are both stored as binary numbers in memory, the same hardware can run any program. This idea underlies every general-purpose computer.

Around the CPU and memory sit the motherboard that connects everything together, the power supply, and often a graphics processor for pictures and video. Input devices send data in, such as a keyboard, mouse, microphone, camera or touch sensor. Output devices send data out, such as a screen, speaker or printer. A touchscreen is both. Components communicate through connections that carry data between them, and with other systems through network connections, which the guide on software and system communication covers further. Phones, tablets, games consoles and the computers inside cars and washing machines all use the same ideas in a smaller form.

Words your child will hear

CPU: The central processing unit, which carries out the instructions of a program.
RAM: Fast working memory that holds running programs and data and is emptied when the power goes off.
Secondary storage: Memory that keeps files when the power is off, such as a solid-state drive, hard disk or memory card.
Fetch-decode-execute: The cycle the CPU repeats: fetch an instruction, work out what it means, then carry it out.
Input device: Hardware that sends data into a computer, such as a keyboard or microphone.
Output device: Hardware that gets data out of a computer, such as a screen or speaker.
Motherboard: The main circuit board that the CPU, memory and other parts plug into and communicate through.

What the National Curriculum says

In Key Stage 3, pupils should be taught to:

Understand the hardware and software components that make up computer systems, and how they communicate with one another and with other systems
Understand how instructions are stored and executed within a computer system; understand how data of various types (including text, sounds and pictures) can be represented and manipulated digitally, in the form of binary digits

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: explaining why a computer slows down

A laptop becomes very slow when a lot of programs are open at once. Use what you know about hardware to explain why, and suggest a sensible upgrade.

Describe what happens when each program is opened. It is copied from storage into RAM, and the CPU fetches its instructions from there. Every open program and every open browser tab uses some RAM.
Explain what happens when RAM fills up. The operating system moves some of the data from RAM into a reserved area on the storage drive. This is slower than RAM, so whenever that data is needed again, the computer has to wait.
Add the effect on the CPU. It shares its time between all the running programs, so more programs mean each one gets less attention. If a program is also doing a heavy task, the CPU may be working at its limit.
Match the symptom to the part. Slowing down when many programs are open points to too little RAM, or storage that is slow to swap data. A faster screen or a bigger storage drive would not help, because neither is the bottleneck.
Suggest fixes and say what each does: close programs that are not needed, add more RAM if the machine allows it, or switch to a solid-state drive, which swaps data faster than a hard disk.

The computer slows because RAM is full, so data is swapped to slower storage and the CPU is shared across many programs. Adding RAM, or closing programs, is the most direct fix.

Common mistakes and how to help

Calling the whole box the CPU

In everyday speech people point at a desktop tower and call it the “CPU” or “hard drive”. The CPU is a chip a few centimetres across inside it.

What helps: Ask your child to name the parts of a computer from memory and say what each does. The box is the case, and the CPU is one chip inside it.

Mixing up memory and storage

Shops and phone makers use the word “memory” for storage. A phone with “128 GB of memory” has 128 GB of storage and far less RAM.

What helps: Use the desk and filing cabinet picture. Ask what happens to an unsaved document in a power cut: it is in RAM, so it is lost.

Believing a higher clock speed always means a faster computer

Clock speed is the figure most often advertised, and it is easy to compare one number.

What helps: Explain that speed depends on cores, cache, RAM and storage as well. A computer with a fast CPU but little RAM still slows down when memory is short.

Thinking a device is either an input or an output

Textbook lists put keyboards on one side and screens on the other, so children assume every device fits one list.

What helps: Ask about a touchscreen, a headset with a microphone, or a games controller that vibrates. Each sends data in and takes data out.

How to help at home

Find the specification of a family device. Look in the settings or “about” page of a phone, tablet or laptop and find the processor, the amount of RAM and the amount of storage (some devices do not show every figure). Ask your child to say which is which, and why the RAM figure is much smaller than the storage figure.
Sort household devices. Pick six devices around the house, such as a smart speaker, a games controller, a printer and a phone, and sort their parts into input, output and both. Then ask which ones contain a processor and memory: many everyday machines, including most modern washing machines, do.
Act out fetch, decode, execute. Write a few simple instructions on numbered cards, such as “clap twice” or “stand up”, and lay them out in a row. One person plays the CPU and fetches the cards in order, says what each one means, and then does it. Add a card that says “go back to card 2” to show jumping.
Use the desk and cabinet picture. When your child is working on something, ask what is on the desk and what is in the cabinet. A document open on the screen but not saved is only on the desk. This is a gentle reminder to save work as well.
Leave the opening up to a professional. Do not open a computer or phone to look inside. Use photographs or a diagram of a motherboard instead and ask your child to point out the CPU, RAM and storage. Opening devices is unsafe, especially if they contain batteries.

Leads on to

Key Stage 4: GCSE computer science