The Central Processing Unit (CPU) is often described as the “brain of the computer,” but this description can feel vague to IB Computer Science students. To score well in exams, students must understand how the CPU actually processes data, not just memorise definitions.
In IB Computer Science, the CPU is studied as a system made up of key components, each with a specific role. Understanding how these components work together is essential for questions on computer architecture, performance, and the fetch–decode–execute cycle.
What Is the CPU?
The CPU is responsible for:
- Executing program instructions
- Performing calculations
- Controlling data flow within the computer
Every instruction a program runs — from adding numbers to loading a file — passes through the CPU.
In IB Computer Science, the CPU is usually explained using three core components:
- The Arithmetic Logic Unit (ALU)
- The Control Unit (CU)
- Registers
The Arithmetic Logic Unit (ALU)
The ALU is the part of the CPU that performs calculations and logical operations.
It is responsible for:
- Arithmetic operations (addition, subtraction, multiplication, division)
- Logical operations (AND, OR, NOT, comparisons)
For example, when a program checks whether one value is greater than another, the ALU performs that comparison.
IB exam questions often test whether students understand that:
- The ALU does not control program flow
- The ALU only performs operations requested by instructions
The Control Unit (CU)
The Control Unit manages and coordinates everything the CPU does.
Its role includes:
- Fetching instructions from memory
- Decoding instructions
- Sending control signals to the ALU, registers, and memory
The Control Unit does not perform calculations itself. Instead, it acts like a manager, ensuring that each part of the CPU works at the right time and in the correct order.
In exam questions, students often lose marks by confusing the Control Unit with the ALU. Remember: the CU controls, the ALU calculates.
Registers: Fast Temporary Storage
Registers are very small, very fast memory locations inside the CPU.
They are used to:
- Store data being processed
- Hold instructions currently being executed
- Store addresses of memory locations
Registers are much faster than RAM, which is why the CPU relies on them heavily.
Common registers IB students should recognise include:
- Instruction registers
- Memory address registers
- Accumulators
How These Components Work Together
When a program runs:
- The Control Unit fetches an instruction
- The instruction is decoded
- Data is loaded into registers
- The ALU performs the operation
- The result is stored or sent to memory
This process forms the basis of the fetch–decode–execute cycle, which is assessed separately but depends on CPU component knowledge.
Common Student Mistakes
Students often:
- Mix up the roles of the ALU and CU
- Forget the purpose of registers
- Describe the CPU too generally
Precise explanations are required for full marks.
Final Thoughts
Understanding how the CPU works means understanding how its components interact, not just naming them. The ALU calculates, the Control Unit controls, and registers provide fast temporary storage. Together, they allow programs to run efficiently and correctly.
This system-level understanding is essential for success in IB Computer Science — and it makes later topics like performance and optimisation much clearer.
