The Fetch-Execute cycle

The purpose of the CPU: The Fetch-Execute cycle

1.1.1 Systems Architecture

Systems Architecture — Note 1 of 3

What you need to know

The Central Processing Unit (CPU) is the main component of a computer that executes instructions in order to run programs. It continuously carries out the fetch-execute cycle (also called the fetch-decode-execute cycle) to process instructions one at a time.

What to know

Fetch
- The address of the next instruction is copied from the Program Counter (PC) to the Memory Address Register (MAR).
- The instruction is fetched from memory and copied into the Memory Data Register (MDR).
- The Program Counter is incremented to point to the next instruction.
Decode: The instruction in the MDR is decoded by the Control Unit (CU).
Execute: The decoded instruction is executed — this may involve the ALU performing a calculation.

Key terms

CPU: The central processing unit is the part of the computer that carries out program instructions by processing data and controlling other components using the fetch, decode and execute cycle.
Fetch-Execute Cycle: The repeated process where the CPU retrieves an instruction from memory, decodes it, executes it, and then moves on to the next instruction.

Exam technique

Tip:Be precise when describing the fetch–decode–execute cycle. Explain how an instruction is fetched from memory, decoded by the Control Unit and executed by the CPU in the fetch-execute cycle. You should also know the roles of the Program Counter (PC), Memory Address Register (MAR) and Memory Data Register (MDR), as OCR may ask specifically about these registers. Using accurate register names can strengthen your answer, but whether they are needed for full marks depends on the question.
Mistake:Forgetting that the Program Counter increments DURING the fetch stage, not after execution.
Tip:Use the word 'stores' when describing registers (e.g. 'the MAR stores the address'). Do NOT use action verbs like 'fetches' or 'retrieves' — registers STORE data, they don't perform actions.

Example:Common 2-mark question: 'Identify two events that take place during the fetch-execute cycle.' Use specific events like 'the address is copied from the PC to the MAR' and 'the PC is incremented'.

Ready to test yourself?

Practise 1.1.1 questions

Next note →ALU, CU, Cache, Registers1.1.1 Systems Architecture

Back to Systems Architecture

Revise another OCR GCSE topic: CPU performance, Embedded Systems