Central Processing Unit

The Central Processing Unit (CPU) is the part of a computer that processes data and instructions. It is also called the processor. Its purpose is to carry out the instructions of the programs held in memory, which is how a computer runs software.

The CPU does this by repeatedly carrying out the fetch-execute cycle, once for each instruction. After it has dealt with one instruction, it moves on to the next one in the program. For example, a program for a school bell might contain an instruction to read the clock and another to switch on the bell: the CPU deals with the first instruction, then the second, and so on until the program ends.

Inside the CPU are the components that make all this happen: the Control Unit, the Arithmetic Logic Unit (ALU), registers and cache. Each one has its own job to do while the CPU works through a program. They are all built into the processor itself rather than sitting out in RAM, so the CPU can reach the values it needs without the delay of a fetch from memory.