Topics / Hardware and Software / Fetch Execute Cycle

Fetch Execute Cycle

The fetch execute cycle is the basic operation (instruction) cycle of a computer (also known as the fetch decode execute cycle).

During the fetch execute cycle, the computer retrieves a program instruction from its memory.  It then establishes and carries out the actions that are required for that instruction.

The cycle of fetching, decoding, and executing an instruction is continually repeated by the CPU whilst the computer is turned on.

StepFetch execute cycle stepsSimplified description
1The PC contains the address of the memory location that has the next instruction which has to be fetchedPC has address of next instruction
2This address is then copied from the PC to the MAR via the address busPC copied to the MAR
3The contents (instruction) at the memory location (address) contained in MAR are then copied into the MDRLookup MAR and get contents. Copy contents into the MDR
4The contents (instruction) in the MDR is then copied and placed into the CIRCopy MDR contents into the CIR
5The value in the PC is then incremented by 1 so that it now points to the next instruction which has to be fetchedPC is then incremented by 1
6The instruction is finally decoded and then executed by sending out signals (via control bus) to the various components of the computerThe instruction is decoded and then executed
7Repeat

 

Topics / Hardware and Software / Fetch Execute Cycle