Instruction

The two principal characteristics of a computer are: It responds to a specific instruction set in a well defined manner and it can execute a prerecorded list of instructions call a program.

Hardware is directed by the software to execute any command or instruction.

They consist of an arithmetic logic unit and processor registers, a control unit containing an instruction register and program counter, a memory to store both data and instructions, external mass storage, and input and output mechanisms.

The meaning of the term has evolved to mean a stored-program computer in which an instruction fetch and a data operation cannot occur at the same time because they share a common bus.