Fetch - Execute Cycle

Syllabus Detail

  • Concept of fetch-execute cycle. 11 ATAR, 12 GEN
  • Purpose and stages (fetch, decode, execute, store) of fetch-execute cycle. 12 ATAR

 

 

What is the fetch-execute cycle and why do we need it?

  • Also known as the instruction cycle, the fetch-execute cycle is the basic operation cycle of a computer
  • It's the process in which a computer receives information (data) and determines what to do with said data
  • This cycle is run continuously by the central processing unit (CPU)
  • The process follows four phases: fetch, decode, execute and store
  • Read The Computing Teacher's article on the Central Processing Unit here

 

Which parts in the CPU complete the fetch-execute cycle?

  • Three main parts inside the CPU that do this are; the registers, the control unit and the Arithmetic Logic Unit (ALU)

1. Fetch

  • The CPU collects an instruction and prepares it for decoding
    • The instruction is collected from a memory location.

 

2. Decode

  • The decoder in the control unit works out (decodes) what to do with  the instruction
    • Each CPU is designed to understand a specific set of commands
    • These commands are referred to as the instruction set
    • These instruction sets work by providing the CPU with machine code (hexadecimal or binary instructions)

 

3. Execute

  • The Arithmetic Logic Unit (ALU)  then executes the decoded instructions
    • These instructions can be arithmetic (manipulation of numbers) or logical processes
    • The data from the fetch phase of the cycle is manipulated according to the instructions received from the program
    • All the steps above can be seen as prerequisites for the execute phase

 

4. Store

  • Send and write the results back in main memory.
    • The newly processed data is then passed back in to memory fresh from its processing via the data bus. The data can be re-submitted for further processing or be used for a practical use, such as reporting and other outputs. (See: The Computing Teacher's article on the Flow of Data).

 

More explanation (see attached)

Further Research

  1. Wikipedia has a very good article here.
  2. The Fetch Execute Cycle learning and test at BBC Bitesize
  3. The CPU and the Fetch Execute Cycle by BBC here

Worksheet and Practice (yet to be added)

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