Computer Organization and Architecture Notes for High School






Computer Organisation and Architecture


1.2 Computer Organisation and Architecture

1. Computer Architecture

Computer architecture refers to the overall design and logical structure of a computer system. It defines how a computer system is designed to function and how its major components interact to execute programs.

Computer architecture focuses on what the computer can do from a user’s or programmer’s point of view, rather than how the hardware components are physically built. It describes the capabilities of the system that are visible to software.

Key aspects include:

In simple terms: “What is this computer capable of doing?”

Importance of Computer Architecture

Good computer architecture leads to faster, more reliable, and more flexible computer systems.

2. Computer Organisation

Computer organisation refers to the internal operational structure of a computer system. It explains how the hardware components are arranged and how they work together to implement the architectural design.

Focuses on:

Key components:

In simple terms: “How is the computer built to perform its tasks?”

Difference Between Computer Architecture and Computer Organisation

3. John von Neumann

John von Neumann was a Hungarian-American mathematician, physicist, and computer scientist
who made major contributions to the development of modern computers.

In 1945, he introduced the stored-program concept, which became the foundation of most
modern computer systems. Before this idea, computers had to be physically rewired
to change programs, which was slow and inefficient.

The stored-program concept allows:

This concept made computers more flexible, faster, and easier to use,
and it laid the groundwork for today’s digital computers.

4. Von Neumann Architecture

Von Neumann architecture is a computer design model based on the stored-program concept
introduced by John von Neumann.

In this architecture:

The main components of the Von Neumann architecture include:

The CPU follows a cycle known as the fetch–decode–execute cycle,
where each instruction is processed sequentially.

Advantages of Von Neumann Architecture

Because of its simplicity and practicality, the Von Neumann architecture
is still widely used in many computer systems today.

5. Functional Units of a Computer

The functional units of a computer are the main parts that work together
to perform data processing tasks. Each unit has a specific role, and together
they ensure that input is received, processed, stored, and output is produced.

The main functional units include:

5.1 Input Unit

The Input Unit is one of the main components of a computer system.
It is responsible for receiving data and instructions from the user
or external sources and transmitting them to the computer for processing.

Since computers operate using binary language (0s and 1s), the input unit
converts human-readable data (such as text, numbers, images, and sound)
into a machine-readable format that the computer can understand and process.

Without the input unit, users would not be able to interact with the computer.

Functions of the Input Unit

The input unit performs the following functions:

Common Input Devices (Most Widely Used)

5.2 Central Processing Unit (CPU)

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

It is called the brain of the computer.

It controls all activities of the computer.

It processes all data and instructions given to the computer.

Every task performed by the computer passes through the CPU.

Functions of the CPU

The CPU:

Main Parts of the CPU

The CPU has three main parts:

1. Control Unit (CU)

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

It tells other parts of the computer what to do and when to do it.

Functions of the Control Unit

⚠ The Control Unit does not perform calculations. It only controls operations.

2. Arithmetic and Logic Unit (ALU)

The ALU performs all calculations and comparisons.

It helps the computer solve problems and make decisions.

Arithmetic Operations (Mathematical Calculations)

Logical Operations (Comparisons and Decisions)

The ALU allows the computer to calculate results and compare values.

3. Registers

Registers are very small memory spaces inside the CPU.

They are very fast.

Functions of Registers

Because registers are very fast, they help the CPU work quickly.

How the CPU Works (Fetch–Decode–Execute Cycle)

The CPU works in three main steps:

This cycle happens very fast, millions or billions of times per second.

5.3 Memory Unit

The Memory Unit is a part of the computer that stores data and instructions.

It keeps information that the CPU needs to perform tasks.

Without memory, the computer cannot function properly.

The memory unit allows the computer to work smoothly and quickly by
providing the CPU with the information it needs at the right time.

Functions of the Memory Unit

The memory unit:

Types of Computer Memory

Computer memory is mainly divided into two categories:

1. Primary Memory (Main Memory)

Examples of Primary Memory

(a) Random Access Memory (RAM)

(b) Read Only Memory (ROM)

2. Secondary Memory (Secondary Storage)

Examples of Secondary Memory

Importance of the Memory Unit

The memory unit:

5.4 Output Unit

The Output Unit is the part of the computer that presents processed information to the user.

It shows the results after the CPU has finished processing data.

It converts binary results (0s and 1s) into a form that humans can understand.

The output can be in the form of text, images, sound, or printed documents.

Functions of the Output Unit

The output unit:

Types of Output

Computer output can be grouped into:

Common Output Devices

1. Monitor

2. Printer

3. Speakers

4. Headphones

5. Projector

6. Plotter

Importance of the Output Unit

The output unit:

6. System Bus

The system bus is a communication pathway that connects the CPU, memory unit,
and input/output devices. It allows all parts of the computer system to
exchange data and control signals.

Without the system bus, the components of a computer would not be able
to communicate with each other.

The system bus consists of three main types:

Data Bus

Address Bus

Control Bus

7. Fetch–Execute Cycle

The Fetch–Execute Cycle is the process the computer follows to carry out instructions.

It is the basic working process of the CPU.

The cycle repeats continuously while the computer is turned on.

It happens very fast — millions or even billions of times per second.

Every program you run (for example, opening a browser or typing a document)
works through this cycle.

Why the Fetch–Execute Cycle is Important

Stages of the Fetch–Execute Cycle

The cycle has three main stages:

1. Fetch Stage

The CPU retrieves (fetches) the next instruction from main memory (RAM).

The instruction is stored in a special register inside the CPU.

The Program Counter (PC) keeps track of the address of the next instruction.

👉 In simple terms:
The computer gets the instruction it needs to work on.

2. Decode Stage

The Control Unit (CU) interprets (decodes) the instruction.

It determines what action is required.

It identifies which data is needed and where it is located.

👉 In simple terms:
The computer understands what the instruction means.

3. Execute Stage

The instruction is carried out.

If it is a calculation, the ALU performs it.

If it involves data movement, the CPU transfers data between memory and registers.

The result may be stored in memory or sent to an output device.

👉 In simple terms:
The computer performs the task.

After Execution

This process continues until the program is completed or the computer is turned off.

8. RISC and CISC Architectures

RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer)
are two types of CPU designs.

They are designed to improve how efficiently a CPU processes instructions.

RISC Architecture

Characteristics of RISC

Examples of RISC Processors

CISC Architecture

Characteristics of CISC

Examples of CISC Processors

RISC vs CISC – Simple Comparison Table

Feature RISC CISC
Instruction Set Small and simple Large and complex
Execution Speed Fast per instruction Slower per instruction
Program Length Longer (more instructions) Shorter (fewer instructions)
CPU Design Simple Complex
Common Use Mobile devices, embedded systems Desktop computers, servers
Examples ARM, MIPS Intel x86, AMD

Summary

RISC = small, simple, fast instructions (good for devices with low power)

CISC = complex, powerful instructions (good for desktop and server tasks)








Computer Architecture Interactive Quiz


🧠 Computer Organisation & Architecture Quiz

1. What does computer architecture describe?


It describes what the computer can do and its capabilities visible to software developers.

2. What is the main function of the Control Unit?


It directs and coordinates all activities inside the computer.

3. Name the three main parts of the CPU.


Control Unit (CU), Arithmetic and Logic Unit (ALU), and Registers.

4. Which memory is volatile?


RAM (Random Access Memory).

5. What are the three stages of the Fetch–Execute Cycle?


Fetch, Decode, Execute.

6. Which bus carries memory addresses?


Address Bus.

7. What did John von Neumann introduce?


The stored-program concept.

8. Which architecture uses simple instructions?


RISC.

9. Which architecture is commonly used in desktop computers?


CISC.

10. What does the Program Counter do?


It keeps track of the address of the next instruction.

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