Basic Computer Organisation

This assignment explores the basic organisation of computers, including its major components such as the Central Processing Unit (CPU), Memory Unit, Input and Output Units, and Storage Systems. It also explains how data flows between these components and how instructions are processed within the computer system.

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2. Meaning of Computer Organisation

Computer organisation is the study of how a computer system’s hardware components are linked together to form a complete system. It describes how the computer operates internally, how instructions are executed, and how different parts communicate.

It is different from computer architecture, which refers to the design principles and specifications visible to the programmer (such as instruction sets, data formats, and addressing modes). Organisation, on the other hand, focuses on the implementation of these architectural concepts in hardware.

For example:

• Architecture specifies what operations a computer can perform.
• Organisation defines how these operations are carried out.

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3. Functional Units of a Computer System

A computer can be divided into five main functional units:

1. Input Unit
2. Memory Unit
3. Arithmetic Logic Unit (ALU)
4. Control Unit (CU)
5. Output Unit

All these components work together under the coordination of the Control Unit to process information and generate results.

3.1 Input Unit

The input unit is responsible for providing data and instructions to the computer system for processing.
Common input devices include:

• Keyboard
• Mouse
• Scanner
• Microphone
• Barcode Reader
• Touchscreen

Functions of Input Unit:

1. Accepts data and instructions from the user.
2. Converts them into a binary format (0s and 1s) understandable by the computer.
3. Transfers converted data to the memory or processor for further processing.

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3.2 Memory Unit

The memory unit is used to store data, instructions, and results temporarily or permanently. It can be divided into two main types:

Primary Memory

This is the internal memory directly accessible by the CPU. It includes:

• RAM (Random Access Memory):
  Used to store data temporarily while the computer is running. It is volatile, meaning data is lost when power is turned off.
• ROM (Read Only Memory):
  Contains permanent instructions required for system startup. It is non-volatile.

Secondary Memory

Used for long-term data storage. Examples include:

• Hard Disk Drives (HDDs)
• Solid State Drives (SSDs)
• CDs, DVDs, and USB Drives

Cache Memory

A small, high-speed memory located between the CPU and RAM. It stores frequently accessed data and instructions to speed up processing.

Registers

These are small storage units within the CPU used for immediate data processing.

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3.3 Arithmetic Logic Unit (ALU)

The ALU performs all arithmetic and logical operations. It is sometimes called the mathematical brain of the computer.

Functions of ALU:

1. Performs arithmetic operations like addition, subtraction, multiplication, and division.
2. Performs logical operations such as comparison, AND, OR, and NOT.
3. Transfers results to registers or memory.

For example, when a user performs a calculation in a calculator app, the ALU processes the numbers and logic behind it.

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3.4 Control Unit (CU)

The Control Unit acts as the central nervous system of the computer. It directs the operation of all other units by sending control signals.

Functions of CU:

1. Fetches instructions from memory.
2. Decodes instructions to determine the type of operation.
3. Controls the sequence of operations.
4. Coordinates data movement between CPU, memory, and I/O devices.

In summary, the Control Unit manages the execution of instructions and ensures the computer operates correctly.

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3.5 Output Unit

The output unit displays or provides the results of processed data to the user in a readable form.

Common output devices include:

• Monitor (Visual Display Unit)
• Printer
• Speaker
• Plotter
• Projector

Functions of Output Unit:

1. Converts digital information into human-readable form.
2. Displays results or stores them for future use.

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4. Central Processing Unit (CPU)

The CPU is the brain of the computer. It performs all the calculations and coordinates other components. It is composed of three main parts:

1. Arithmetic Logic Unit (ALU)
2. Control Unit (CU)
3. Registers

Functions of CPU:

• Executes instructions stored in memory.
• Performs data processing operations.
• Controls input, output, and storage operations.

The CPU’s performance depends on its clock speed, number of cores, and cache memory. Modern processors like Intel Core i9 or AMD Ryzen have multiple cores that allow parallel processing.

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5. The Concept of Bus Organisation

To transfer data between components, computers use a communication system known as a bus.
A bus is a set of parallel wires or lines that carry data, control signals, and addresses.

Types of Buses:

1. Data Bus – Carries actual data between CPU, memory, and I/O devices.
2. Address Bus – Carries the address of data (location in memory).
3. Control Bus – Carries control signals like read/write commands and interrupts.

Bus organisation ensures coordinated and efficient communication among system components.

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6. Instruction Cycle

The instruction cycle refers to the sequence of steps the CPU performs to execute an instruction. It has four main phases:

1. Fetch – The control unit fetches the instruction from memory.
2. Decode – The instruction is decoded to determine the operation.
3. Execute – The CPU performs the operation.
4. Store – The result is stored in memory or a register.

This continuous cycle allows the computer to execute millions of instructions per second.

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7. Input–Output Organisation

Input and output devices communicate with the CPU using I/O interfaces. These interfaces act as translators between the CPU and peripheral devices.

There are two major methods for I/O data transfer:

1. Programmed I/O – CPU controls all data transfer operations.
2. Interrupt-driven I/O – Devices interrupt the CPU when they are ready for data transfer.
3. Direct Memory Access (DMA) – Allows data transfer between memory and devices without CPU involvement.

DMA is preferred in modern systems as it improves efficiency and speed.

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8. Memory Hierarchy

The computer memory is organised in a hierarchy to balance speed, cost, and capacity.

Level	Type of Memory	Speed	Cost	Capacity
1	Registers	Fastest	Highest	Very Low
2	Cache	Very Fast	High	Low
3	Main Memory (RAM)	Fast	Moderate	Moderate
4	Secondary Storage	Slow	Low	High
5	Tertiary Storage (Cloud, Tape)	Slowest	Lowest	Very High

This hierarchy ensures that frequently used data remains in faster storage for quick access.

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9. Storage Devices

Storage devices are essential for saving data permanently.

Primary Storage:

• RAM, ROM, Cache

Secondary Storage:

• Hard Disk, SSDs, Optical Discs

Tertiary Storage:

• Cloud Storage, Magnetic Tape, External Drives

Each device is characterised by storage capacity, data access time, and portability.

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10. Performance Factors in Computer Organisation

The performance of a computer depends on several organisational factors:

1. Processor Speed – Measured in GHz, defines how fast the CPU executes instructions.
2. Memory Access Time – Faster memory access improves performance.
3. Bus Width – A wider bus allows more data transfer per clock cycle.
4. Cache Efficiency – Reduces the time to access frequently used data.
5. Parallel Processing – Multi-core processors increase computation speed.

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11. Von Neumann Architecture

The Von Neumann Architecture, proposed by John von Neumann in 1945, forms the foundation of most modern computers.

Key Characteristics:

1. Uses a single memory to store both data and instructions.
2. Executes instructions sequentially.
3. Employs a control unit, ALU, memory, and input/output devices.

Advantages:

• Simplicity in design and implementation.
• Suitable for general-purpose computing.

Limitation:

• The “Von Neumann Bottleneck” – since both data and instructions share the same bus, it limits processing speed.

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12. Flynn’s Classification of Computer Architecture

Michael Flynn classified computers based on the number of instruction and data streams:

1. SISD (Single Instruction, Single Data): Traditional single-processor systems.
2. SIMD (Single Instruction, Multiple Data): Used in vector processors.
3. MISD (Multiple Instruction, Single Data): Rare in practical use.
4. MIMD (Multiple Instruction, Multiple Data): Used in modern multi-core processors and parallel systems.

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13. Microprocessor and Microcontroller Overview

A microprocessor is the CPU on a single chip, while a microcontroller integrates CPU, memory, and I/O ports on one chip.

Feature	Microprocessor	Microcontroller
Purpose	General-purpose	Specific control applications
Components	CPU only	CPU + Memory + I/O
Example	Intel Core i7	Arduino, 8051

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14. Modern Developments in Computer Organisation

1. Parallel Processing: Use of multiple processors for simultaneous computation.
2. Pipelining: Overlapping instruction execution to improve efficiency.
3. RISC and CISC Architectures:
   • RISC (Reduced Instruction Set Computer) simplifies instruction sets.
   • CISC (Complex Instruction Set Computer) uses complex instructions.
4. Multicore Processors: Multiple cores within a single CPU chip.
5. Cloud-based Architecture: Distributed systems storing and processing data online.

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15. Applications of Computer Organisation Knowledge

Understanding computer organisation is crucial for:

1. Designing efficient software and hardware.
2. Developing embedded systems.
3. Enhancing system performance.
4. Optimising algorithms for real hardware.
5. Troubleshooting and system maintenance.

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16. Conclusion

Computer organisation provides the foundation for understanding how computers work internally. It explains the structure, components, and operations that enable computers to perform complex tasks efficiently.

By studying basic computer organisation, students gain valuable insights into data processing, memory systems, and hardware-software interaction. This knowledge is essential not only for computer science students but also for professionals working in electronics, programming, and information technology.

In essence, a computer is not just a machine but an intelligently organised system — a collaboration of hardware, control logic, and memory that together make our digital world function seamlessly.