Preview lessons, content and tests

Computer Science & Programming solved. All in one platform.

1. To trial the platform and take tests, please take a few seconds to SIGN UP and SET UP FREE.

2. Searching for something specific? See our text overview of all tests. Scroll right for levels, and lists.

3. Student and Teacher User Guides |  Schemes of Work |   Real Teacher use Videos |


Join 30000+ teachers and students using TTIO.

Fetch Execute Decode Cycle

The main job of the CPU is to execute programs using the fetch-decode-execute cycle (also known as the instruction cycle). This cycle begins as soon as you turn on a computer. To execute a program, the program code is copied from secondary storage into the main memory.

Learning Video

What happens in the cycle?

The fetch-execute cycle (also known as fetch-decode-execute cycle) is followed by a processor to process an instruction. The cycle consists of several stages.

  • The memory address held in the program counter is copied into the MAR.
  • The address in the program counter is then incremented - increased - by one. The program counter now holds the address of the next instruction to be fetched.
  • The processor sends a signal along the address bus to the memory address held in the MAR.
  • The instruction/data held in that memory address is sent along the data bus to the MBR/MDR.
  • The instruction/data held in the MBR/MDR is copied into the CIR.
  • The instruction/data held in the CIR is decoded and then executed. Results of processing are stored in the ACC.
  • The cycle then returns to step one.

 

Additional Resources

https://www.bbc.co.uk/bitesize/guides/z2342hv/revision/5

https://en.wikipedia.org/wiki/Instruction_cycle

https://www.desktopclass.com/computer-it/explain-the-fetch-decode-execute-cycle-of-cpu.html

www.teachyourselfpython.com