The Fetch-Decode-Execute cycle:

FETCH:

1. Copy the memory address from the program counter to the MAR.
2. Copy the instruction stored in the MAR address to the MDR.
3. Increment (increase) the program counter to the point to the address of the next instruction, ready for the next cycle.

DECODE:

The instructions in the MDR is decided by the CU. The CU may then prepare for the next step, e.g. By loading values into the MAR or the MDR.

EXECUTE:

The instruction is performed. This could be:

•  Load data from memory.
•  Write data to memory.
•  Do a calculation or logic operation(using the ALU).
•  Change the PC.
•  Halt the program.
• Cycle is then repeated.

By Anshjeet Singh