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Jan's Computer Basics:

Processing: Machine Cycle

The computer can only do one thing at a time. Each action must be broken down into the most basic steps. One round of steps from getting an instruction back to getting the next instruction is called the Machine Cycle.

The Machine Cycle

Fetch - get an instruction from Main Memory
Decode - translate it into computer commands
Execute - actually process the command
Store - write the result to Main Memory

machine cycle gif

For example, to add the numbers 5 and 6 and show the answer on the screen requires the following steps:

1. Fetch instruction: "Get number at address 123456" 
2. Decode instruction.
3. Execute: ALU finds the number. (which happens to be 5)
4. Store: The number 5 is stored in a temporary spot in Main Memory.
5 - 8  Repeat steps for another number (= 6)
9. Fetch instruction: "Add those two numbers"
10. Decode instruction.
11. Execute: ALU adds the numbers.
12. Store: The answer is stored in a temporary spot.
13. Fetch instruction: "Display answer on screen."
14. Decode instruction.
15. Execute: Display answer on screen.


The immense speed of the computer enables it to do millions of such steps in a second.
In fact, MIPS, standing for millions of instructions per second, is one way to measure computer speeds.