In computer architecture, an instruction is a command given to computer system to perform a specific task or operation on given data. The microprocessor requires steps to fetch an instruction and necessary data from the memory, and to execute it, this whole process constitute an Instruction cycle.
The time required to fetch the instruction (FC) is fixed slot of time while the time required to execute an instruction (EC) is variable, depending on the type of instruction to be executed. Therefore the total time required to execute an instruction (IC) is:
IC=FC + EC