Chapter 1: Computer System Organization

Introduction

A computer is an electronic device that accepts a set of instructions in the form of a program, executes it and displays the output to the user.

Basic Components of a Computer System

• Input Unit
• Central Processing Unit (CPU)
• Memory Unit
• Output Unit
• Software Concepts

Input Unit

An input unit fetches the input and converts it into binary form which is directly understood by the computer. The term Binary means two states ON/OFF, HIGH/LOW, 0/1.

1. Keyboard

Definition: A keyboard is an input device used to enter data and instructions into a computer.

Layout: Most commonly used layout is the QWERTY layout (named after the first six letters on the top row).

Types of Keys:

• Alphanumeric Keys – Letters (A-Z), numbers (0-9)
• Function Keys – F1 to F12, perform special functions
• Control Keys – Ctrl, Alt, Windows key, etc.
• Navigation Keys – Arrow keys, Home, End, Page Up, Page Down
• Numeric Keypad – Found on the right side, used for quick numeric entry
• Special Keys – Enter, Spacebar, Backspace, Shift, Tab, Caps Lock, Esc

Functionality: When a key is pressed, it sends a digital signal to the computer which is then interpreted as input.

Connection Types:

• Wired keyboards – Connect using USB or PS/2 port
• Wireless keyboards – Use Bluetooth or RF (radio frequency) technology

2. Mouse

Definition: A mouse is a pointing input device used to interact with items on a computer screen.

Basic Functions:

• Pointing – Move the cursor on the screen
• Clicking – Select or open files and folders
• Dragging – Move items by holding and moving the mouse
• Scrolling – Move through long documents using the scroll wheel

Types of Mouse:

• Mechanical Mouse – Uses a rubber ball for movement (old model)
• Optical Mouse – Uses LED light to detect motion (modern)
• Laser Mouse – More accurate, uses a laser for tracking
• Wireless Mouse – Connects through Bluetooth or RF technology

Mouse Buttons:

• Left Button – For selecting and clicking
• Right Button – For opening context menus
• Scroll Wheel – For scrolling and sometimes used as a third button

3. Light Pen

Definition: A light pen is a pointing input device that allows the user to draw or select objects directly on the screen.

Working Principle: It detects light from the computer screen (usually a CRT screen). When the pen touches the screen, it identifies the exact location using the screen's brightness.

Main Functions:

• Pointing – Select icons or menu items
• Drawing – Used in CAD (Computer-Aided Design) and graphic design applications
• Writing – Input handwriting directly on the screen

4. Optical Mark Reader (OMR)

Definition: OMR is an input device used to read marks made on paper using a pencil or pen.

Where It Is Used:

• Entrance exams like NEET, JEE, etc.
• Surveys and polls
• Attendance sheets

5. Smart Card Reader

Smart Card Reader is used to read data from a smart card. A smart card is like an ATM card with a chip inside. The chip stores important information like ID, bank, or health details.

Used in:

• ATM machines
• Office ID cards
• Metro/train cards
• Health and government cards

6. Barcode Reader

A Barcode Reader is used to read barcodes printed on products. A barcode is a pattern of black and white lines that stores product information. The reader uses laser light to scan the barcode.

Used in:

• Supermarkets
• Libraries
• Shopping malls
• Inventory systems

7. QR Code Reader

A QR Code Reader is used to scan QR codes. QR code means Quick Response code – it looks like a square box with black and white patterns. It stores more data than a barcode.

Used in:

• Online payments (like Google Pay, PhonePe)
• E-tickets and ID cards
• Product info and advertisements

8. Biometric Sensor

A Biometric Sensor is used to identify a person using body features. It reads fingerprints, face, eye (iris), or voice.

Types:

• Fingerprint scanner
• Face recognition sensor
• Iris scanner
• Voice recognition

9. Touch Sensor

A Touch Sensor detects when you touch a surface. It is used in touch screens – like in mobile phones, tablets, and ATMs.

10. Microphone

A microphone is an input device that captures sound (like your voice). It converts sound waves into electrical signals that the computer can understand.

11. Webcam

A webcam is a small camera used to capture live video and images. It sends the video data to the computer in real-time.

12. Magnetic Ink Character Reader (MICR)

MICR is used to read special characters printed with magnetic ink on documents. Mainly used to read cheques in banks.

13. Optical Character Reader (OCR)

OCR is used to read printed or typed text from paper documents. The OCR scans the document and converts the text into digital form.

Central Processing Unit (CPU)

1. Arithmetic Logic Unit (ALU)

The Arithmetic Logic Unit (ALU) is a part of the CPU. It performs all arithmetic operations like addition, subtraction, multiplication, and division. It also performs logical operations like comparing numbers (greater than, less than, equal to).

2. Control Unit (CU)

The Control Unit (CU) is a part of the CPU. It directs and controls all the operations inside the computer. CU fetches instructions from memory and decodes them. Think of CU as the "manager" or "traffic controller" inside the CPU.

3. Registers

Registers are small, very fast storage locations inside the CPU. They temporarily hold data, instructions, or addresses during processing.

Different types of registers:

• Program Counter (PC): holds address of next instruction
• Instruction Register (IR): holds current instruction
• Accumulator (ACC): holds intermediate arithmetic results

Memory Unit

The Memory Unit stores data and instructions needed by the CPU. It holds both program instructions and data temporarily or permanently.

Primary Memory (Main Memory)

RAM (Random Access Memory)

• Temporary or volatile memory
• Stores data and instructions while the computer is ON
• Data is lost when power is turned off
• Allows both read and write operations

Types of RAM:

• Static RAM (SRAM)
  • Stores data using flip-flops
  • Faster and more expensive than DRAM
  • Used in cache memory
• Dynamic RAM (DRAM)
  • Stores data using capacitors
  • Needs to be refreshed to keep the data
  • Slower and cheaper than SRAM
  • Used as main memory (RAM) in computers

ROM (Read-Only Memory)

• Permanent or non-volatile memory
• Stores essential instructions needed to start the computer (like BIOS)
• Data cannot be modified or deleted easily
• Retains data even when power is off

Types of ROM:

• PROM (Programmable ROM)
  • Can be programmed only once after manufacturing
  • Once written, data cannot be changed
• EPROM (Erasable Programmable ROM)
  • Data can be erased using UV light
  • After erasing, it can be reprogrammed
• EEPROM (Electrically Erasable Programmable ROM)
  • Data can be erased and rewritten using electric signals
  • Used in BIOS chips and smart cards

Units of Memory

Secondary Memory

Secondary Memory (also called external memory or non-volatile memory) is used to store data and programs permanently. It retains data even when the computer is turned off.

1. Hard Disk Drive (HDD)

Main secondary storage device in most computers. Consists of magnetic disks that rotate to read/write data.

• Track: Circular paths on the surface of a disk platter
• Sector: Each track is divided into small segments called sectors

2. Optical Storage Devices

• Compact Disc (CD): Stores up to 700 MB of data
• Digital Versatile Disc (DVD): Stores 4.7 GB to 17 GB
• Blu-ray Disc (BD): Stores 25 GB to 128 GB

3. Flash Storage

• USB Pen Drives: Portable flash memory devices
• Memory Cards: Used in smartphones, cameras, tablets

4. Magnetic Tapes

Long strips of plastic film coated with magnetic material. Used for sequential data storage and backups.

Output Unit

The Output Unit is responsible for presenting processed data (output) to the user. It converts digital data into human-readable form.

1. Visual Display Unit (VDU) / Monitor

• LCD Screen: Uses liquid crystals and backlight
• LED Screen: Uses LED backlighting, better efficiency
• OLED Screen: Each pixel emits its own light, better contrast

2. Printers

• Dot Matrix Printer: Impact printer, noisy but reliable
• Inkjet Printer: Sprays ink droplets, good for color prints
• Laser Printer: Uses laser technology, fast and high-quality

3. Speakers

Convert digital audio signals into sound.

4. Plotters

Specialized devices for printing vector graphics and engineering drawings.

Software Concepts

Software is a set of instructions or programs that tell the computer what to do.

Types of Software

• System Software
• Application Software
• Utility Software
• Programming Tools

System Software

Manages and controls hardware components and enables other software to run.

• Operating System (Windows, Linux, macOS)
• Device Drivers
• Language Translators (Compilers, Interpreters)

Operating System

An Operating System (OS) manages computer hardware and software resources and provides services for programs.

Functions of Operating System:

• Resource Management
• Process Management
• Memory Management
• File Management
• Device Management

OS User Interfaces:

• Command Line Interface (CLI): Text-based commands
• Graphical User Interface (GUI): Icons, windows, menus
• Touch User Interface: Touch screen interaction
• Voice Based Interface: Voice commands
• Gesture Based Interface: Hand/body movements

Language Translators

• Assembler: Translates assembly language to machine code
• Compiler: Translates high-level language to machine code all at once
• Interpreter: Translates and executes code line by line

Application Software

Designed to perform specific tasks for users.

• General Purpose: MS Word, Excel, Web browsers
• Specific Purpose/Customized: School Management System, Banking Software

Utility Software

Performs maintenance tasks to improve computer performance.

• Antivirus Software
• Disk Defragmentation
• Backup Utility
• Compression Utility
• Disk Cleaner
• Device Drivers

Programming Tools

Used by developers to write, test, and debug programs.

• Compilers and Interpreters
• Text Editors (Notepad++, VS Code)
• Debuggers
• IDEs (Eclipse, PyCharm)

Classification of Programming Languages

• Low-Level Languages: Machine Language, Assembly Language
• High-Level Languages: Python, C++, Java
• Generations:
  • 1st Gen: Machine Language
  • 2nd Gen: Assembly Language
  • 3rd Gen: High-Level Languages (C, Java)
  • 4th Gen: Very high-level (SQL, MATLAB)
  • 5th Gen: AI-focused (Prolog, LISP)