WJEC GCSE Computer Science

Inside a computer system

A desktop computer is built from a small number of components, each with one main job. The motherboard is the main printed circuit board and everything else connects to it, either by a socket, a slot or a cable. It carries the buses (the sets of wires that move data, addresses and control signals between components) and lets the CPU, the memory, the storage and the peripherals communicate.

The central processing unit (CPU) carries out the instructions of programs and does the calculations. It repeats the fetch-decode-execute cycle billions of times a second, and the way it does so is covered in its own note on the CPU.

The power supply unit (PSU) takes the mains supply from the wall, which is alternating current at a high voltage, and converts it into the low-voltage direct current that the components need. It has to deliver enough power for everything in the machine at once, so a computer with a powerful graphics card needs a PSU with a higher power rating than an office machine.

Input devices send data into the computer and output devices present results to the user. Together with storage they make up the peripheral devices described below.

Expansion cards

An expansion card is a circuit board that plugs into an expansion slot on the motherboard to add a capability the motherboard does not provide, or to provide it at higher quality.

A wired NIC has a socket for an Ethernet cable, and a wireless NIC has an aerial and connects by Wi-Fi. Most motherboards now have a NIC and basic sound built in, and a card is added when the built-in one is not good enough or a different type is wanted.

The graphics processing unit

A graphics processing unit (GPU) is a processor designed for the calculations that build images: working out the colour of millions of pixels many times a second. It has a large number of simple cores that do the same kind of calculation on different pixels at the same time, which suits graphics far better than the small number of general-purpose cores in a CPU. GPUs are used for games, video editing and 3D modelling.

There are two kinds. An integrated GPU is built into the same chip as the CPU and shares the main memory with it, so it is cheap and uses little power, but it is slower. A dedicated GPU sits on a graphics card with its own memory, called video RAM (VRAM), so it does not compete with the CPU for memory and is much faster, but it costs more, uses more power and gives off more heat.

Cooling systems

The CPU and GPU turn most of the electrical power they use into heat. If they overheat, they slow themselves down to protect themselves, and sustained high temperatures can damage them, so every computer needs a cooling system.

Air cooling uses a heat sink, a block of metal with fins that draws heat away from the chip and spreads it over a large surface, and a fan that blows air across it. It is cheap and simple, but the fan is noisy and the cooling is limited. Liquid cooling circulates a liquid through a block on the chip and then through a radiator where fans remove the heat. It moves heat more effectively, so it is used in high-performance machines, but it costs more and is more complicated to fit.

Connectivity ports

Ports are the sockets through which the computer connects to other devices.

Peripheral devices

A peripheral device is hardware outside the main processing system that is connected to it. Choosing the right one depends on what has to be entered or presented, and on who is using it.

Input devices

Output devices

The same hardware can suit one user and not another. A Braille display is the right output for a blind user and pointless for a sighted one, while a microphone with voice control lets someone with limited use of their hands operate a computer. Naming the user and the task is what makes a choice of device appropriate.