The Fetch-Execute Cycle (sometimes called the Fetch-Decode-Execute Cycle) describes how a CPU carries out instructions.

The cycle consists of three stages:

• The Fetch Stage
  The next instruction is loaded from main memory into the Instruction Register in the CPU. The Program Counter (the register that stores what memory address the next instruction is at) is incremented to point to the next instruction.
• The Decode Stage
  The CPU works out what needs to be done in order to carry out the instruction.
• The Execute Stage
  The operation is carried out by the CPU. If necessary, data is sent between the CU and the ALU. Data may be written to or read from main memory. Any relevant registers are updated.

Then, the cycle starts again. Each stage takes one* clock tick.

Note: it is better not to use the words "Fetch", "Decode", or "Execute" if you are asked to describe the stages of the cycle in an exam. Instead, use synonyms like in the description above.

_{*In reality, stages of the cycle can take more than one clock tick, but it will always be a whole number of ticks. However, for GCSE, you will need to write that each stage takes exactly one clock tick.}

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How will the clock speed of a CPU affect its performance?

If the clock speed is higher, the Fetch-Execute Cycle takes less time and therefore the CPU performs better. However, increasing the clock speed above the recommended level is called "overclocking" - which could be dangerous for your computer.

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