The purpose of the CPU
Introduction
The central processing unit (or CPU for short) is the brain of the computer. It is sometimes referred to as simply the 'processor'.
Fetch - Decode - Execute cycle
Word, Firefox, Messenger, RealPlayer and so on are all 'programs' made up of 'instructions'. These are stored on the hard drive until you need them, when they are moved into Random Access Memory (RAM), ready for the processor to access and run. The job of the processor is to fetch the first instruction that makes up a program from the computer's Random Access Memory. It sends the address of the instruction it wants along an address bus and receives the instruction and any data back along the data bus. Once it has fetched an instruction, it then has to decide what to do with it (called 'decoding' an instruction). Finally, it carries out the instruction (known as 'executing' an instruction). It sends out signals along the control bus in the computer to make the hardware 'execute' the instruction. Once the first instruction has been fetched, decoded and executed, it then does the same to the next instruction, and the next, and the next, until the program is finished. Unsurprisingly, this process is known as the 'Fetch - Decode - Execute cycle'.
There are a range of instructions that might be executed. Some of them do arithmetic jobs like adding and subtracting numbers. Others instructions might use something called Boolean logic (AND, OR and NOT, for example). There are also instructions for running different parts of a program, for searching through data and storing data, to name but a few.
The jobs that the CPU does (or the purpose of a CPU) can be summarised as FETCHING, DECODING and EXECUTING instructions.