📚 Knowledge Library — Topic 3.1 — Hardware

The Role of the CPU

Understand what the CPU does, how its main components work together, and why the Fetch–Decode–Execute cycle sits at the heart of every computer.

Start LearningWorked Example
1. Invitation

The CPU is the computer's worker.

The CPU processes data and executes instructions.

Every program you open is made from instructions. The CPU fetches them, understands them, and carries them out.

💡 Remember: the CPU follows instructions. It does not store all your files.
Figure 1.1
CPU Job
Instruction
↓
CPU processes
↓
Action happens
2. Big Idea

The CPU repeats one simple cycle.

The CPU uses the Fetch–Decode–Execute cycle.

It fetches the next instruction from memory, decodes what it means, then executes it. This happens billions of times per second.

💡 FDE = Fetch, Decode, Execute.
Figure 2.1
FDE Cycle
Fetch
↓
Decode
↓
Execute
↺
3. FutureLogic Bridge

Think of a chef in a kitchen.

The CPU is like a chef following a recipe.

The chef gets the next recipe step, understands it, then does it. The kitchen has small workspaces for temporary notes and results.

💡 Bridge: CPU = chef, RAM = recipe book, registers = small notes, cache = nearby ingredients.
Figure 3.1
Chef Model
Recipe book
↓
Chef reads step
↓
Chef does task
4. Register Introduction

The CPU uses tiny storage spaces called registers.

Registers are very small, very fast storage locations inside the CPU.

You only need to recognise their names here. The next lessons explain how they work during the Fetch–Decode–Execute cycle.

Main CPU registers

Short NameFull Name
PCProgram Counter
MARMemory Address Register
MDRMemory Data Register
CIRCurrent Instruction Register
ACCAccumulator
You will learn each register's job in the next FDE lessons.
Figure 4.1
Register Names
PC
MAR
MDR
CIR
ACC
Names first. Details later.
5. Control Unit and ALU

One controls. One calculates.

The Control Unit manages the CPU and sends control signals.

The ALU performs arithmetic and logical operations. The result is stored in the accumulator.

🎯 Exam Tip: ALU performs calculations. ACC stores the result.
Figure 5.1
CU vs ALU
CU
= controls signals

ALU
= performs maths/logic

ACC
= stores result
6. RAM and ROM

Primary storage supports the CPU.

RAM stores programs and data currently in use. It is volatile, so data is lost when power is off.

ROM stores start-up instructions such as BIOS or firmware. It is non-volatile.

💡 RAM = working memory. ROM = start-up instructions.
Figure 6.1
RAM and ROM
RAM
current work
volatile

ROM
start-up data
non-volatile
7. Exam Tip

Use exact wording for registers.

Cambridge often tests whether students know the difference between addresses, data and instructions.

🎯 Exam Tip: MAR stores addresses, MDR stores data, and CIR stores the current instruction.
Figure 7.1
Do Not Mix Them Up
MAR
= address

MDR
= data

CIR
= instruction
8. Common Mistake

The accumulator does not calculate.

Students often say the accumulator carries out calculations.

That is incorrect. The ALU carries out calculations. The accumulator stores the result.

⚠️ Common Mistake: ACC stores results. ALU performs calculations.
Figure 8.1
ALU and ACC
ALU
↓ calculates
Result
↓ stored in
ACC
9. Summary

The CPU in one screen.

The CPU processes data and executes instructions using the FDE cycle.

Registers hold temporary values. The CU controls. The ALU performs calculations. RAM and ROM support the system.

💡 Key idea: the CPU is organised around small, specific jobs working together very quickly.
Figure 9.1
CPU Summary
CPU
↓
FDE cycle
+
Registers
+
CU + ALU