The computer processor fetches, decodes, and executes program instructions. A computer program consists of a series of steps called instructions which tell the computer what to do. Each instruction can be a basic arithmetic calculation or a logic operation. Before the program can be executed it is loaded into the main storage (memory).
It is the job of the microprocessor, which is controlling the computer to fetch a program instruction from the memory, decode the instruction and then carry out any action that might be needed which is the execution process. It is the responsibility of the processor inside the computer to carry out the fetch-decode-execute cycle over and over again operating from the instructions it obtains from the main memory.
This fetch – decode – execute cycle is often referred to as the fetch – execute cycle.
The CPU uses a timing signal to be able to fetch and execute instructions. The timing signal is provided by the system clock. The clock speed is measured in Hz (cycles per second). In early processors speed was measured in Megahertz (MHz) is one million hertz (1 million cycles per second). Most of the computers we have today operate in the GHZ (Gigahertz) range. The clock speed varies from one computer processor to another.