Fetch-Execute Cycle

Executing a computer program involves a series of activities, which are carried out by the central processor. This mechanism can be illustrated by considering the execution of a single instruction. The way the central processor executes an instruction can be initially examined as follows.

Central Control Unit gets an instruction, examines it and figures out what needs to be done and then passes instructions to the Arithmetic unit to proceed with the execution. This process is called Instruction Time or I-time. Then the Arithmetic unit executes the instruction and stores the result in the memory. This process is called Execution Time or E-time. The combination of Instruction Time and Execution Time is called Machine Cycle or Fetch-Execute Cycle.

The machine cycle illustrates the instruction execution mechanism of a single instruction. The earlier generations of computers could execute only one instruction at instruction. The earlier generations of computers could execute only one instruction at a time. But with the advancements of the technology, the computers available today can handle more than one instruction simultaneously. Nevertheless, the fundamental aspects of executing an instruction can still be described based on one single instruction; and the mechanism will be adapted accordingly when multiple instructions are handled.

The input device(s) will read the data and instructions and place them in the memory.The central control unit then starts executing the instructions by taking one at a time.Execution of an instruction is handled in four main steps.

Fetch

The control unit gets (fetches) the instruction from the memory. Very often it will be read from the high-speed register. 

Decode

The control unit identifies what needs to be done, and recognizes and loads the data that will be required by the arithmetic unit to perform the operation.

Execute

The control unit instructs the Arithmetic unit to execute the instructions. Arithmetic unit performs the required operation on data that is already made available by the Control unit.

Store

The Arithmetic unit will then store the results in the memory, as instructed by the control unit.