Instruction Cycle (Fetch-Decode-Execute)

The instruction cycle is the basic loop a CPU repeats for every instruction in a program. In the fetch step, the control unit reads the next instruction from memory at the address in the program counter. In the decode step, it works out what the instruction means and which data it needs. In the execute step, the ALU or other units carry it out and results are stored in registers or memory.

The program counter then moves to the next instruction and the cycle repeats billions of times per second. Modern CPUs overlap these steps for several instructions at once, a technique called pipelining.