The Purpose of the CPU

This lesson introduces the Central Processing Unit (CPU) and its role within a computer system, as specified in OCR J277 Section 1.1.1. You will learn what the CPU does, why it is essential, and how it fits into the wider architecture of a modern computer.

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What Is the CPU?

The CPU (Central Processing Unit) is the main processing component of a computer. It is responsible for:

• Fetching instructions from main memory
• Decoding those instructions to understand what action is required
• Executing the instructions by carrying out the required operation

The CPU is often referred to as the "brain" of the computer, although this is a simplification. The CPU does not think or reason — it rapidly processes binary instructions according to a fixed set of rules.

Modern CPUs are fabricated on a single silicon chip, often smaller than a postage stamp, yet they contain billions of transistors — tiny electronic switches that can be either on (1) or off (0). This is the physical basis of all digital computation.

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Where the CPU Sits in a Computer System

A computer system consists of several key components that work together:

Component	Role
CPU	Processes instructions and data
Main memory (RAM/ROM)	Stores data and instructions currently in use
Secondary storage	Stores data permanently (e.g. HDD, SSD)
Input devices	Allow users to send data into the system
Output devices	Present processed data to the user
System bus	Connects all components so they can communicate

The CPU sits at the centre of this system. It communicates with all other components through the system bus — a set of wires that carry data, addresses, and control signals.

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The CPU's Three Main Activities

At the most fundamental level, the CPU repeats three activities in a continuous loop known as the fetch-decode-execute (FDE) cycle:

1. Fetch

The CPU retrieves (fetches) the next instruction from main memory (RAM). The Program Counter (PC) register tells the CPU which memory address to fetch from.

2. Decode

The Control Unit (CU) examines the fetched instruction and works out what operation needs to be performed. This involves splitting the instruction into its opcode (the operation) and operand (the data or address the operation acts on).

3. Execute

The CPU carries out the instruction. This might involve:

• Performing a calculation in the ALU (Arithmetic Logic Unit)
• Moving data between registers or memory
• Branching to a different instruction address

This cycle repeats billions of times per second in a modern computer.

OCR Exam Tip: You must be able to describe the purpose of the CPU in simple terms. A good one-sentence answer is: "The CPU fetches, decodes, and executes program instructions stored in main memory."

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Why the CPU Matters

Without the CPU, a computer cannot process any instructions. Every action a computer performs — from booting up the operating system, to displaying a webpage, to running a game — requires the CPU to process instructions.

The speed and capability of the CPU directly affects the overall performance of the computer. A faster CPU can process more instructions per second, which generally means programs run more quickly and the system feels more responsive.

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CPUs in Different Devices

CPUs are not only found in desktop computers and laptops. They are present in a wide range of devices:

• Smartphones and tablets — use mobile CPUs (e.g. ARM-based processors)
• Games consoles — use specialised CPUs optimised for gaming workloads
• Embedded systems — washing machines, traffic lights, and smart thermostats all contain small, dedicated CPUs
• Servers — use powerful multi-core CPUs to handle thousands of requests simultaneously

The key difference between these CPUs is their power, size, and purpose, but they all perform the same fundamental task: fetching, decoding, and executing instructions.

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Key Terminology

Term	Definition
CPU	Central Processing Unit — the main processor in a computer
Instruction	A single operation that the CPU can carry out
FDE cycle	Fetch-Decode-Execute — the continuous cycle the CPU follows
Transistor	A microscopic electronic switch that represents a binary digit
System bus	The communication pathway connecting the CPU to other components

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Summary

• The CPU is the main processing component of a computer (OCR J277 1.1.1).
• It fetches instructions from memory, decodes them, and then executes them.
• This happens in a continuous loop called the fetch-decode-execute cycle.
• The CPU communicates with other components via the system bus.
• CPUs are found in many types of devices, not just traditional computers.

OCR Exam Tip: When asked about the purpose of the CPU, always mention fetching, decoding, and executing instructions. This demonstrates that you understand the FDE cycle, which is a key concept in Section 1.1.