The Fetch-Decode-Execute Cycle (Cambridge (CIE) IGCSE Computer Science)

Revision Note

James Woodhouse

Written by: James Woodhouse

Reviewed by: Lucy Kirkham

Fetch-Decode-Execute Cycle (FDE)

What is the purpose of the CPU?

  • The purpose of the Central Processing Unit (CPU) is to fetch, decode and execute instructions

  • The CPU is the brain of the computer and its job is to take an input, process data and produce an output

What is the Fetch-Decode-Execute cycle?

  • The Fetch-Decode-Execute Cycle is the cycle that the central processing unit (CPU) runs through billions of times per second to make a computer work

  • A computer takes an input, processes the input and then delivers an output for the user

    • Input: Clicking a button on the gamepad 

    • Process: The CPU inside the console follows a set of instructions to carry out the task

    • Output: The player moving on screen

the-cpu

The Fetch-Decode-Execute cycle stages

Fetch stage

  • During the fetch stage of the cycle, the program counter holds the address of the next instruction to be fetched from memory

  • The address of the next instruction or data to be fetched is copied into the memory address register (MAR)

  • The address of the instruction or data is then sent along the address bus and awaits a signal from the control bus

  • The signal sent along the control bus is sent from the control unit (CU) to the main memory

  • The data or instructions received from main memory is fetched to the memory data register (MDR) via the data bus

  • A copy of the instruction or data is stored in the current instruction register (CIR)

  • The program counter (PC) increments by 1 so it is pointing to the next instruction to be executed

Decode stage

  • During the decode stage of the cycle, the CPU needs to work out what is required from the instruction 

  • This is done as the instruction is split into two parts:

    • Opcode - what the instruction is

    • Operand - what to do it to

      • This could be either data or an address where the data is stored

Execute stage

  • During the execute stage of the cycle, the CPU will carry out the instruction that was fetched

  • Some examples that would take place at this stage are

    • Performing a calculation

    • Storing a result or data back in main memory (RAM)

    • Going to main memory to fetch data from a different location

The important things to remember are:

  • An instruction or data is fetched from memory

  • The instruction is decoded 

  • The instruction is executed

  • The cycle repeats billions of times per second

Examiner Tips and Tricks

Make sure you read the question carefully and look at the number of marks allocated to judge the level of detail required. Often questions on the fetch-decode-execute cycle only require you to describe the steps rather than explain how the registers and buses are used during each step

Worked Example

Explain how an instruction is fetched using Von Neumann architecture

[6]

Answer

  • The Program Counter (PC) holds the address/location of the next instruction to be fetched [1]

  • The address held in the PC is sent to the Memory Address Register (MAR) [1]

  • The memory address is sent using the address bus [1]

  • The Program Counter is incremented [1]

  • The instruction is sent from the address in memory to the Memory Data Register (MDR) [1]

  • The instruction is transferred using the data bus [1]

  • The instruction is sent to the Current Instruction Register (CIR) [1]

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James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.

Lucy Kirkham

Author: Lucy Kirkham

Expertise: Head of STEM

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.