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CPU Explained – Central Processing Unit & Computer Architecture (O Level Computer Science 2210 / IGCSE 0478)

What Is the CPU?

The Central Processing Unit (CPU) is the main processor of a computer.

It is often called the “brain of the computer” because it performs the instructions needed to run programs.

The CPU processes:

  • Input data

  • Program instructions

  • Calculations and logical operations

It then produces the required output.

computer ol

What Is a Microprocessor?

A microprocessor is a CPU built on a single integrated circuit (chip).

Modern computers, smartphones, and tablets all use microprocessors to perform processing tasks.

Microprocessors allow devices to be:

  • Faster

  • Smaller

  • More efficient

Main Components of the CPU

In a computer system using Von Neumann architecture, the CPU contains two main units.

1. Control Unit (CU)

The Control Unit manages and coordinates all activities inside the computer.

Its functions include:

  • Fetching instructions from memory

  • Decoding instructions

  • Sending control signals to other components

The CU ensures that instructions are executed in the correct order.

2. Arithmetic Logic Unit (ALU)

The Arithmetic Logic Unit (ALU) performs:

Arithmetic operations

Examples:

Addition
Subtraction
Multiplication
Division

Logical operations

Examples:

AND
OR
NOT
Comparisons

These operations are essential for program execution and decision-making.

CPU Registers

Registers are very small storage locations inside the CPU used to temporarily hold data.

Important registers include:

RegisterPurposeProgram Counter (PC)Stores address of next instructionMemory Address Register (MAR)Stores memory address being accessedMemory Data Register (MDR)Stores data being transferredCurrent Instruction Register (CIR)Stores current instructionAccumulator (ACC)Stores intermediate results

Registers allow the CPU to process data quickly.

System Buses

Buses are communication pathways used to transfer data between CPU components.

There are three main types of buses.

BusFunctionAddress BusCarries memory addressesData BusCarries data between componentsControl BusCarries control signals

These buses allow different parts of the computer to communicate efficiently.

Why the CPU Is Important

The CPU is responsible for:

  • Running programs

  • Processing data

  • Controlling system operations

Without the CPU, a computer cannot execute instructions or perform tasks.

Exam Tip (2210 / 0478)

Students are commonly asked to:

  • Define CPU

  • Explain the role of the ALU

  • Explain the role of the Control Unit

  • Identify CPU registers

A common exam question is:

Explain the role of the ALU in a computer system.

Practice Question

Explain one role of the Control Unit.

Answer

The Control Unit fetches and decodes instructions from memory and coordinates the activities of the computer system.

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