Fetch Execute Cycle

I was asked for more detail about the fetch execute cycle. So I thought I would look for something new and a simpler way to explain it.  So if you have a look at this BBC page,  you should get a basic overview of what is happening. You can then read what I found on this page, this will help you to understand it a little better.

Then you can read my blog post about it, which also includes a link to an animation.

When I was learning the cycle I had to imagine it in my head as a warehouse with people, I used to work in a warehouse so it worked for me.

  1. At the start of the cycle the boss (processor) gives a runner (address bus) a piece of paper with the name (address) of the thing he wants.
  2. The runner hangs around till the boss shouts at him (read line activated).
  3. The runner the goes and finds the box his boss is looking (memory location) for in the warehouse (RAM) and uses a hand truck (Data bus) as it is heavy to place it on the bosses desk (processors register).
  4. The boss them opens the box (decode) and uses (execute) the photocopier inside.

The main steps are.

Fetch Execute Cycle
  1. Processor sets up address bus with correct address
  2. Processor activates read line on control bus
  3. Instruction from memory location is placed on to data bus and transferred to processor’s register
  4. Instruction is decoded and executed