The fetch–decode–execute cycle is one of the most fundamental processes in IB Computer Science. It explains how the CPU runs every single instruction, from simple calculations to complex programs. Despite its importance, many students memorise the steps without understanding how they connect, which leads to weak exam answers.
In IB Computer Science, students are expected to explain this cycle clearly, logically, and in the correct order, using correct terminology.
What Is the Fetch–Decode–Execute Cycle?
The fetch–decode–execute cycle is the repeating process the CPU uses to run program instructions.
Every program, regardless of size or complexity, is broken into instructions. The CPU processes these instructions one at a time, using the same three-stage cycle:
- Fetch
- Decode
- Execute
This cycle continues until the program ends.
Step 1: Fetch
During the fetch stage, the CPU:
- Uses the Program Counter (PC) to find the address of the next instruction
- Sends this address to memory
- Retrieves the instruction from RAM
- Stores it in the Instruction Register (IR)
After fetching the instruction, the Program Counter is updated to point to the next instruction.
Key IB point:
Fetching does not involve understanding the instruction — it only retrieves it.
Step 2: Decode
In the decode stage, the CPU:
- Interprets the fetched instruction
- Determines what operation is required
