1.2: The FDE Cycle

Exam Board:

Eduqas / WJEC


2016 + 

The Fetch - Decode - Execute (FDE) cycle is performed by the CPU millions of times every second.

This cycle is how the CPU processes data and instructions for each program or service that requires its attention.

Important Registers

A register is a small storage space for temporary data in the CPU.


Each register has a specific role. There are three essential registers used in the FDE cycle:

Program Counter (PC) 


A register that tracks the RAM address of the next instruction to be fetched.

Memory Address Register (MAR) 


A register that tracks the RAM address of data that is currently being accessed.

Current Instruction Register (CIR)


A register that stores the instruction that has been fetched from RAM, and is about to be decoded or executed.