An Instruction cycle (sometimes called a fetch–decode–execute cycle) is the basic operational process of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction dictates, and carries out those actions. This cycle is repeated continuously by a computer's central processing unit (CPU), from boot-up to when the computer is shut down.

Component :

Program counter (PC)
Memory address register (MAR)
Memory data register (MDR)
Instruction register (IR)
Control unit (CU)
Arithmetic logic unit (ALU)
Floating-point unit (FPU)

Steps of the An Instruction cycle

Fetching the Instruction :

Step 1 of the Instruction Cycle is called the Fetch Cycle. This step is the same for each instruction:
The CPU sends PC to the MAR and sends a READ command on the control bus
In response to the read command (with address equal to PC), the memory returns the data stored at the memory location indicated by PC on the databus
The CPU copies the data from the databus into its MDR (also known as MBR, see section Components above)
A fraction of a second later, the CPU copies the data from the MDR to the Instruction Register (IR)
The PC is incremented so that it points to the following instruction in memory. This step prepares the CPU for the next cycle.

Decoding the instruction
Step 2 of the instruction Cycle is called the Decode Cycle. The decoding process allows the CPU to determine what instruction is to be performed, so that the CPU can tell how many operands it needs to fetch in order to perform the instruction.

Reading the effective address
Step 3 is evaluating which operation it is. If this is a Memory operation - in this step the computer checks if it's a direct or indirect memory operation:
Direct memory instruction - Nothing is being done.


Indirect memory instruction - The effective address is being read from the memory.
If this is an I/O or Register instruction - the computer checks its kind and executes the instruction.

Executing the instruction
Step 4 of the Instruction Cycle is the Execute Cycle. Here, the function of the instruction is performed.

If the instruction involves arithmetic or logic, the Arithmetic Logic Unit is utilized.