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
- Identifies which data or memory locations are needed
This stage is controlled by the Control Unit (CU).
For example, the CPU may decode an instruction as:
- An arithmetic operation
- A logical comparison
- A data transfer
- A jump or branch
No calculations happen here — the CPU is preparing to act.
Step 3: Execute
In the execute stage, the instruction is carried out.
This may involve:
- The ALU performing a calculation
- Data being moved between registers and memory
- A comparison being made
- The Program Counter being updated (for loops or conditions)
The result of the operation is stored in a register or written back to memory.
Why the Cycle Repeats Continuously
Once execution is complete, the CPU immediately returns to the fetch stage for the next instruction.
This continuous repetition is what allows:
- Programs to run smoothly
- Loops to function
- Conditional statements to work
The speed of this cycle is influenced by factors such as clock speed, cache, and instruction complexity.
Why This Topic Is Important in IB Exams
IB exam questions often ask students to:
- Describe the cycle in order
- Explain the role of registers
- Link the cycle to CPU components
- Apply the cycle to a simple scenario
Answers that list steps without explanation usually score poorly.
Common Student Mistakes
Students often:
- Confuse fetch and decode
- Forget the role of the Control Unit
- Skip registers entirely
- Write vague explanations
Precision matters.
Final Thoughts
The fetch–decode–execute cycle explains how software becomes action. Understanding it properly helps students connect programming, CPU components, and system performance. It is not just a process to memorise — it is the foundation of how computers work.
This is why RevisionDojo emphasises system understanding over rote learning — exactly what the IB rewards.
