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. | Card | What it does | |---|---| | Graphics card | Produces the image sent to the…