The execution of a machine-level instruction involves two phases: the instruction and the execution phase.

In the instruction phase (I-time):

Step 1: Fetch instruction – The control unit accesses the instruction from memory
Step 2: Decode instruction – The instruction is decoded and relevant data is moved from memory to the register storage area, and the location of the next instruction is identified

In the execution phase (E-time):

Step 3: Execute the Instruction- The ALU does what is instructed to do (arithmetic operation or comparison)
Step 4: Store results- The results are stored in registers or memory


Microseconds - 1 millionth
Nanoseconds - 1 billionth
Picoseconds - 1 trillionth
MIPS - 1 million instructions per second


An instruction cycle (also called fetch-and-execute cycle, fetch-decode-execute cycle, and FDX) is the time period during which a computer processes a machine language instruction from its memory or the sequence of actions that the central processing unit (CPU) performs to execute each machine code instruction in a program.