The Fetch-Decode-Execute Cycle

The Fetch-Decode-Execute (FDE) cycle is the fundamental operating process of a processor. Every instruction a CPU runs goes through these three stages. For the OCR H446 exam you must be able to describe each stage in detail, explain the role of each register at every step, and discuss the effect of the clock speed on performance.

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Overview of the FDE Cycle

The FDE cycle is a continuous loop that the processor repeats for every instruction:

  +-------+      +--------+      +---------+
  | FETCH | ---> | DECODE | ---> | EXECUTE |
  +-------+      +--------+      +---------+
       ^                              |
       +------------------------------+

1. Fetch — the next instruction is retrieved from main memory.
2. Decode — the Control Unit interprets the instruction.
3. Execute — the instruction is carried out.

The cycle then repeats with the next instruction.

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The Fetch Stage

During the Fetch stage the processor retrieves the next instruction from memory. Here is what happens step by step: