Chapter 1: Computer System

CBSE Unit: Unit 1, Computer Systems and Organisation (10 marks) Marks Weightage: ~5-6 marks (shared with Ch 2) Priority: HIGH, foundational chapter, definitions heavily tested

Key Concepts

1.1 Components of a Computer System

A computer is an electronic device that accepts data (input), processes it, and generates results (output). A computer system comprises:

Central Processing Unit (CPU) - the brain of the computer
Primary Memory (RAM and ROM)
Input Devices (keyboard, mouse, scanner, touch screen)
Output Devices (monitor, printer, speaker)
Secondary Storage Devices (HDD, SSD, pen drive, CD/DVD)

1.2 Central Processing Unit (CPU), Also called processor or microprocessor

Placed on microchips called Integrated Circuits (IC) made of semiconductor materials, Stores data and instructions temporarily in registers (limited in size and number)

Two main components of CPU:

ALU (Arithmetic Logic Unit): Performs arithmetic and logic operations
CU (Control Unit): Controls sequential instruction execution, interprets instructions, guides data flow

1.3 Input and Output Devices

Input Devices: Convert input data into digital form acceptable by computer, Keyboard, mouse, scanner, touch screen, barcode reader, microphone, Braille keyboards for visually impaired

Output Devices: Convert digital information into human-understandable form, Monitor, printer (inkjet/laserjet/dot matrix), speaker, projector, 3D printers for physical replicas of digital designs, Braille display monitors for visually challenged

1.4 Computer Memory

Units of Memory

Bit: Binary digit (0 or 1), basic unit
Nibble: 4 bits
Byte: 8 bits (2 nibbles)

Unit	Size
1 KB (Kilobyte)	1024 Bytes
1 MB (Megabyte)	1024 KB
1 GB (Gigabyte)	1024 MB
1 TB (Terabyte)	1024 GB
1 PB (Petabyte)	1024 TB
1 EB (Exabyte)	1024 PB
1 ZB (Zettabyte)	1024 EB
1 YB (Yottabyte)	1024 ZB

Types of Memory

(A) Primary Memory

RAM (Random Access Memory): Volatile (data lost when power off); temporary storage during processing; faster than secondary storage
ROM (Read Only Memory): Non-volatile (data retained without power); permanent; stores boot loader (startup program)

(B) Cache Memory

Very high-speed memory placed between CPU and primary memory
Stores copies of frequently accessed data, Reduces average access time, CPU checks cache first, then primary memory

(C) Secondary Memory

Non-volatile, larger capacity, slower, cheaper than primary memory, Cannot be accessed directly by CPU (must be loaded into RAM first), Examples: HDD, SSD, CD/DVD, pen drive, memory card

Feature	RAM	ROM	Cache	Secondary
Volatile	Yes	No	Yes	No
Speed	Fast	Fast	Fastest	Slowest
Size	Medium	Small	Very small	Largest
CPU Access	Direct	Direct	Direct	Indirect (via RAM)

1.5 Data Transfer, System Bus

Physical wires called bus transfer data between components.

Bus Type	Function	Direction
Data Bus	Transfers data between components	Bidirectional
Address Bus	Transfers memory addresses from CPU to memory	Unidirectional
Control Bus	Communicates control signals	Unidirectional

All three together form the System Bus.

Memory Controller: Dedicated hardware managing data flow into/out of main memory.

1.6 Evolution of Computing

Key milestones:

500 BC: Abacus (mechanical arithmetic)
1642: Pascaline by Blaise Pascal (addition/subtraction)
1834: Analytical Engine by Charles Babbage (basis of modern computers)
1890: Tabulating Machine by Herman Hollerith (punched cards)
1937: Turing Machine (general purpose programmable machine)
1945: EDVAC/ENIAC, Von Neumann's stored program concept (data + program in memory)
1947: Transistors replaced vacuum tubes (Bell Labs)
1970: Integrated Circuits (ICs), entire circuit on small silicon chip
1981: IBM PC; 1984: Apple Macintosh
1990s: WWW, GUI-based OS, widespread computer use

Von Neumann Architecture: CPU + Memory + Input/Output + Communication channels

Moore's Law: Number of transistors on a chip doubles every two years while cost halves.

1.7 Microprocessors, A processor implemented on a single microchip

Contains millions of components (resistors, transistors, diodes)

Generations:

Gen	Era	Chip	Word Size	Max Memory	Clock Speed	Cores
1st	1971-73	LSI	4/8 bit	1 KB	108-200 KHz	Single
2nd	1974-78	LSI	8 bit	1 MB	Up to 2 MHz	Single
3rd	1979-80	VLSI	16 bit	16 MB	4-6 MHz	Single
4th	1981-95	VLSI	32 bit	4 GB	Up to 133 MHz	Single
5th	1995+	SLSI	64 bit	64 GB	533 MHz, 34 GHz	Multicore

Specifications:

Word Size: Max bits processed at a time (currently 64 bits)
Clock Speed: Pulses per second; measured in GHz
Cores: Basic computation units; dual-core (2), quad-core (4), octa-core (8)

1.8 Microcontrollers vs Microprocessors

Feature	Microprocessor	Microcontroller
Contains	CPU only on chip	CPU + RAM + ROM + peripherals on single chip
Purpose	General purpose computing	Specific tasks
Examples	Intel Pentium, Core i7	Washing machine controller, remote control, mouse
Size/Cost	Larger/More expensive	Smaller/Cheaper

1.9 Data and Information

Data: Raw, unorganised facts (input)
Information: Processed, meaningful data (output)

Types of Data:

Structured: Organised in row/column format (tables, spreadsheets, ATM transactions)
Unstructured: No predefined format (audio, video, images, social media posts)
Semi-structured: Internal tags/markings but no strict format (email, HTML, CSV)

Data Operations: Capturing --> Storage --> Retrieval --> Processing

Data Recovery: Possible if deleted space not overwritten, Deleted data address marked as free but data not actually erased immediately

1.10 Software

Set of instructions that make hardware functional.

Categories:

System Software: Operates computer, interacts with hardware

Operating System (Windows, Linux, Android, macOS, Ubuntu)
System Utilities (disk defragmenter, antivirus, disk cleaner)
Device Drivers (interface between device and OS)

Programming Tools: For writing/translating instructions

Low-level languages: Machine language (0s/1s), Assembly language (mnemonics)
High-level languages: C++, Java, Python (machine independent)
Translators: Assembler (assembly to machine), Compiler (entire source to object code), Interpreter (line by line)
IDE: Integrated Development Environment (editor + compiler + debugger)

Application Software: For specific user tasks

General Purpose: Spreadsheet, word processor, browser
Customised: School management software, accounting software

FOSS vs Proprietary:

FOSS (Free and Open Source): Source code freely available (Python, Ubuntu, LibreOffice, Firefox)
Freeware: Free to use but source code not available (Skype, Adobe Reader)
Proprietary: Must be purchased (Windows, Tally, QuickHeal)

1.11 Operating System

Resource manager managing CPU, RAM, disk, network, I/O devices, Most important system software

Functions:

Process Management: Managing multiple processes, CPU allocation
Memory Management: Allocating/freeing RAM for running processes
File Management: Creation, updating, deletion, protection of files in secondary storage
Device Management: Managing I/O devices and hardware

User Interfaces:

Command-based (MS-DOS, Unix), keyboard input, text commands
GUI (Windows, Ubuntu, macOS), icons, menus, windows
Touch-based (Android, iOS, Windows 10)
Voice-based (Siri, Google Now, Cortana)
Gesture-based (waving, tilting, eye motion)

Important Definitions

CPU: Electronic circuitry that carries out processing; brain of computer
ALU: Performs arithmetic and logic operations
CU: Controls instruction execution and data flow
Register: Small, fast memory within CPU for temporary storage
RAM: Volatile primary memory for temporary data storage
ROM: Non-volatile primary memory storing permanent data (boot loader)
Cache: High-speed memory between CPU and RAM
Bus: Physical wires for data transfer between components
Microprocessor: CPU on a single microchip
Microcontroller: CPU + memory + peripherals on a single chip
Software: Set of instructions for computer to perform tasks
Operating System: System software that manages all computer resources
Compiler: Translates entire source code to machine code at once
Interpreter: Translates source code line by line

Common Board Exam Question Patterns

Draw and explain block diagram of computer system (3 marks)
RAM vs ROM differences (2 marks)
Compiler vs Interpreter (2 marks)
Microprocessor vs Microcontroller (2 marks)
System software vs Application software (2 marks)
FOSS vs Proprietary software with examples (2 marks)
Types of buses and their functions (2 marks)
Functions of OS (2-3 marks)
Identify input/output device for given tasks (1-2 marks)
Classify software as system/application/programming tool (1-2 marks)

Key Points Students Miss

Data bus is bidirectional; address and control buses are unidirectional
Cache is between CPU and RAM (not between RAM and secondary storage)
CPU cannot directly access secondary storage, data must be in RAM first
Compiler translates entire program at once; Interpreter translates line by line
Compiler is not needed after translation; Interpreter is always needed
ROM stores boot loader (program that loads OS into RAM)
Freeware is NOT the same as FOSS, freeware does not provide source code
Word size determines max bits processed at once (not memory size)
Volatile = data lost on power off (RAM); Non-volatile = data retained (ROM, secondary)
Moore's Law: Transistor count doubles every 2 years (not clock speed)