The first step is the CPU carries out is to fetch some data and instructions from the main memory then store them in temporary memory areas. These memory areas are called registers. For this to happen the CPU makes use of the address bus. The CPU places the address of the next item to be fetched on to the address bus. Data from this address then moves from main memory into the CPU.
The next step is for the CPU to make sense of the instruction it has just fetched. This process is called decode. The CPU is designed to understand a specific set of commands. These are called the 'instruction set' of the CPU. The CPU decodes the instruction and prepares for the next step
Exceute: This is the part of the cycle when data processing actually takes place. The result of this processing is stored in another register. Once the execute stage is complete, the CPU sets itself up to begin another cycle.