Fetch, Decode, Execute Cycle
Computer Science: OCR A Level, topic 1.1.4
The Fetch, Decode, Execute Cycle
To process instructions and data, the processor needs to retrieve them from memory. To do this, it follows the fetch, decode, execute (FDE) cycle.
Fetch
- The address of the next instruction to be processed is copied from the Program Counter (PC) to the Memory Address Register (MAR).
- The PC is incremented to point to the next instruction that will be needed when the cycle starts again.
- The instruction stored at the location held by the MAR is copied to the MDR.
Decode
- The Control Unit decodes the instruction and sends control signals to the component within or outside the CPU that needs to act.
Execute
- The operation indicated by the instruction is performed by the appropriate component. For example:
- The Arithmetic Logic Unit (ALU) performs the operation given by the Control Unit.
- The value stored by the Program Counter or Memory Address Register might be changed.
Test yourself on Fetch, Decode, Execute Cycle
Toggle until all are correct: Which registers are used in the fetch stage?
What happens during the fetch stage of the fetch, decode, execute cycle?
During a CPU cycle, the Arithmetic Logic Unit has been activated. During which stage of the FDE cycle was it activated?
The Fetch Stage of the Fetch, Decode, Execute Cycle