Instruction Cycle

Every time the CPU executes an instruction, it takes a series of steps called the machine cycle. |The machine cycle consists of two sub-cycles- the instruction cycle and the execution cycle. The machine cycle consists of four steps-fetching, decoding, executing and storing.

1. Fetching - Before the CPU can execute an instruction, the control unit must retrieve or fetch the command or data from the computer's memory. The fetching process fetches the instruction from memory. This step brings the instruction into the instruction register, a circuit that hold the instruction so that it can be decoded and executed.
2. Decoding - Before the command can be executed, the control unit must breakdown or decode the command into instructions that corresponds to those in the CPU's instruction set.
3. Executing - When the command is executed, the CPU carries out the instructions in order by converting them into microcode.
4. Storing - The CPU may be required to store the results of an instruction in memory.