Computer Knowledge Notes

These Computer Knowledge Notes cover the complete computer awareness syllabus asked in SSC CGL, CHSL, MTS, GD Constable, CPO and other competitive exams. They start from the basics and move step by step to hardware, software, number systems, MS Office, internet, networking, cyber security and emerging technologies. Every section ends with the facts that are repeated most often in exams, and a practice set with answers is given at the end.

Read the notes once from the top to understand the subject, and then use the tables, full forms and shortcut lists for quick revision in the last few days before the exam.

Computer Basics

What Is a Computer?

A computer is an electronic device that accepts data as input, processes it according to a set of instructions called a program, stores the result and gives the output. The word "computer" comes from the Latin word computare, which means to calculate. Charles Babbage is called the father of the computer for his Analytical Engine, and Ada Lovelace is regarded as the first programmer.

Characteristics of a Computer

• Speed: performs millions of operations per second.
• Accuracy: results are correct as long as the input and the program are correct (GIGO: Garbage In, Garbage Out).
• Diligence: can work for long hours without tiredness.
• Storage: can store huge amounts of data and retrieve it quickly.
• Versatility: can perform many different tasks.
• No IQ: a computer cannot think on its own and works only on instructions.

Data and Information

Data is raw, unprocessed facts and figures. Information is processed, meaningful data. A computer follows the IPO cycle: Input, Processing, Output (with Storage as a supporting step).

Generations of Computers

Generation	Period	Technology	Examples / Features
First	1940 to 1956	Vacuum tubes	ENIAC, UNIVAC; very large, slow and generated heat
Second	1956 to 1963	Transistors	IBM 1401, IBM 7094; smaller and more reliable
Third	1964 to 1971	Integrated Circuits (IC)	IBM 360; use of keyboard, monitor and operating system
Fourth	1971 to present	Microprocessor (VLSI)	Intel 4004 (1971), personal computers, laptops
Fifth	Present and future	Artificial Intelligence (ULSI)	Robotics, expert systems, natural language processing

Types of Computers

• By size and power: supercomputer (fastest, used in weather forecasting and research; PARAM is India's supercomputer series), mainframe (used by banks and large organisations), minicomputer, and microcomputer (PC, laptop, tablet, smartphone).
• By data handled: analog computers work on continuous data (thermometer, speedometer), digital computers work on binary data (0 and 1), and hybrid computers combine both (used in hospitals, for example ICU monitors).
• By purpose: general-purpose computers do many tasks, while special-purpose computers are made for a single task, for example a washing machine controller.

Computer Hardware

Central Processing Unit (CPU)

The CPU is the brain of the computer. It has three parts: the Arithmetic Logic Unit (ALU) for calculations and comparisons, the Control Unit (CU) which directs all operations, and registers, which are very fast temporary storage inside the CPU. The speed of a CPU is measured in hertz (MHz, GHz).

Input Devices

Keyboard, mouse, trackball, joystick, light pen, touch screen, scanner, OCR (Optical Character Reader), OMR (Optical Mark Reader used in exam answer sheets), MICR (Magnetic Ink Character Reader used for bank cheques), barcode reader, microphone and webcam.

Output Devices

Monitor (CRT, LCD, LED), printers (impact printers like dot matrix and line printers; non-impact printers like inkjet and laser), plotter (for large drawings and maps), speaker and projector.

Memory and Storage

Type	Details
Registers	Smallest and fastest memory, inside the CPU
Cache memory	Very fast memory between CPU and RAM; stores frequently used data
RAM	Primary, volatile, read and write memory; data is lost when power is off
ROM	Primary, non-volatile memory; stores the start-up (boot) instructions. Types: PROM, EPROM, EEPROM
Secondary storage	Hard disk (HDD), SSD, CD, DVD, Blu-ray, pen drive, memory card; permanent storage
Virtual memory	Part of the hard disk used as RAM when the RAM is full

Memory hierarchy from fastest to slowest: Registers, Cache, RAM, Secondary storage. Speed and cost per byte decrease as we move down the hierarchy while capacity increases.

Units of Memory

Unit	Value
1 bit	Smallest unit (0 or 1)
1 nibble	4 bits
1 byte	8 bits
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

Ports and Connectors

USB (Universal Serial Bus) is used for pen drives, keyboards and mice. HDMI carries high-definition audio and video. VGA connects older monitors. Ethernet (RJ-45) is used for wired network connection. The motherboard is the main circuit board that connects all components, and SMPS (Switched Mode Power Supply) provides power to them.

Computer Software

• System software: controls the hardware and runs the computer. It includes the operating system, device drivers, language translators and utility programs.
• Application software: performs tasks for the user, such as MS Word, Excel, Tally, Photoshop, browsers and media players. It can be general-purpose or customised.
• Utility software: helps in maintenance, for example antivirus, disk cleanup, file compression (WinZip) and backup tools.
• Firmware: permanent software stored in ROM, such as BIOS.
• Freeware, shareware and open source: freeware is free to use, shareware is free for a limited time, and open-source software (Linux, Firefox) has source code that anyone can modify.

Operating System (OS)

The operating system is the interface between the user and the hardware. Its functions are process management, memory management, file management, device management, security and providing a user interface. Examples: Windows, Linux, macOS, Android (based on Linux) and iOS.

• Batch OS: jobs are collected and processed in groups.
• Multiprogramming and multitasking OS: many programs run at the same time.
• Multi-user OS: many users use the system together (Unix, Linux).
• Real-time OS: responds within a fixed time, used in missiles and medical systems.
• GUI uses icons, windows and menus, and CLI uses typed commands (MS-DOS).

Booting

Booting is the process of starting the computer. Cold boot means starting from the power-off state, and warm boot (restart) means restarting without switching off the power, for example Ctrl + Alt + Delete. During start-up, the BIOS runs the POST (Power On Self Test) and then loads the operating system.

Programming Languages and Translators

• Machine language: binary code (0 and 1), directly understood by the computer.
• Assembly language: uses short codes called mnemonics. It is converted to machine code by an assembler.
• High-level languages: C, C++, Java, Python, COBOL, FORTRAN and BASIC are close to human language.
• Compiler: translates the whole program at once and shows all the errors together.
• Interpreter: translates and runs the program line by line.
• An algorithm is a step-by-step solution to a problem, and a flowchart is its diagram.

Number Systems and Codes

System	Base	Digits used
Binary	2	0, 1
Octal	8	0 to 7
Decimal	10	0 to 9
Hexadecimal	16	0 to 9 and A to F (A = 10, F = 15)

• Decimal to binary: divide the number by 2 repeatedly and read the remainders from bottom to top. 13 becomes 1101.
• Binary to decimal: multiply each bit by powers of 2. 1101 = 8 + 4 + 0 + 1 = 13.
• Binary to octal: group the bits in threes from the right. Binary to hexadecimal: group the bits in fours.
• Hexadecimal 1F = 1 x 16 + 15 = 31 and octal 17 = 1 x 8 + 7 = 15.
• ASCII is a 7-bit code with 128 characters (A = 65, a = 97). Unicode supports characters of all languages.

Logic Gates

AND gives 1 only if all inputs are 1. OR gives 1 if at least one input is 1. NOT reverses the input. NAND and NOR are called universal gates because any circuit can be built from them. XOR gives 1 when the inputs are different.

MS Windows and MS Office

Common File Extensions

Extension	File type	Extension	File type
.docx	MS Word document	.xlsx	MS Excel workbook
.pptx	PowerPoint presentation	.accdb	MS Access database
.txt	Plain text	.pdf	Portable Document Format
.jpg / .png	Image	.mp3 / .mp4	Audio / Video
.exe	Executable program	.zip	Compressed file
.html	Web page	.csv	Comma separated values

Important Keyboard Shortcuts

Shortcut	Function	Shortcut	Function
Ctrl + C	Copy	Ctrl + X	Cut
Ctrl + V	Paste	Ctrl + Z	Undo
Ctrl + Y	Redo	Ctrl + S	Save
F12	Save As	Ctrl + P	Print
Ctrl + A	Select all	Ctrl + F	Find
Ctrl + H	Replace	Ctrl + B / I / U	Bold / Italic / Underline
Ctrl + E	Centre align	Ctrl + J	Justify
Ctrl + Home	Start of document	Ctrl + End	End of document
F7	Spelling check	F5	Slide show in PowerPoint / Refresh
Alt + Tab	Switch windows	Alt + F4	Close window
Windows + D	Show desktop	Windows + L	Lock computer

MS Word, Excel and PowerPoint

• MS Word is a word processor. Its main tabs are File, Home, Insert, Design, Layout, References, Mailings, Review and View. Mail Merge is used to send the same letter to many persons.
• MS Excel is a spreadsheet program. A workbook contains worksheets, and each worksheet has rows (numbers) and columns (letters). A formula starts with the equal sign. Common functions: SUM, AVERAGE, MAX, MIN, COUNT and IF. Cell references can be relative (A1), absolute ($A$1) or mixed ($A1). Alt + = inserts AutoSum.
• MS PowerPoint is a presentation program. Transitions control how slides change, and animations control how objects move on a slide.
• MS Access is a database program and MS Outlook is an email and calendar program.

Internet and Networking

Internet Basics

• The internet is a global network of networks. Its origin is ARPANET (1969). Tim Berners-Lee invented the World Wide Web (WWW) around 1989 to 1991.
• An IP address identifies a device on a network. IPv4 has 32 bits and IPv6 has 128 bits. DNS converts a domain name into an IP address.
• A URL (Uniform Resource Locator) is the address of a web page, for example https://www.example.com/page. A browser (Chrome, Firefox, Edge, Safari) is used to view web pages and a search engine (Google, Bing) is used to find information.
• ISP (Internet Service Provider) gives internet connection. Cloud computing offers computing services over the internet: IaaS (infrastructure), PaaS (platform) and SaaS (software).

Email

An email address has the format username@domain. Fields: To, Cc (carbon copy), Bcc (blind carbon copy) and Subject. SMTP is used to send emails, while POP3 and IMAP are used to receive them.

Network Types

Type	Full form	Range / Example
PAN	Personal Area Network	Few metres; Bluetooth
LAN	Local Area Network	A building or campus
MAN	Metropolitan Area Network	A city; cable TV network
WAN	Wide Area Network	Country or world; the internet is the largest WAN

Network Topologies

• Bus: one common cable; cheap, but a cable break stops the network.
• Star: all devices connect to a central hub or switch; the most common topology, and failure of one device does not affect the others.
• Ring: devices form a closed loop; data travels in one direction.
• Mesh: every device is connected to every other device; very reliable but costly.
• Tree: a combination of star and bus topologies.

Network Devices

Modem converts digital signals to analog and back. Hub broadcasts data to all devices. Switch sends data only to the intended device. Router connects different networks and chooses the best path. Bridge connects two LANs. Gateway connects networks that use different protocols. A repeater regenerates weak signals.

Common Protocols

Protocol	Use
HTTP / HTTPS	Transfer of web pages (HTTPS is the secure version)
FTP	File transfer
SMTP, POP3, IMAP	Sending and receiving email
TCP/IP	Basic communication rules of the internet
DHCP	Gives IP addresses automatically
Telnet / SSH	Remote login (SSH is secure)

OSI Model

The OSI model has seven layers, from bottom to top: Physical, Data Link, Network, Transport, Session, Presentation and Application. A common memory aid is "Please Do Not Throw Sausage Pizza Away".

Cyber Security

• Virus: a program that attaches to files and spreads when they are run. Worm: spreads by itself through the network. Trojan horse: looks like useful software but contains harmful code.
• Malware is the general name for harmful software. Spyware secretly collects information, adware shows unwanted ads, and ransomware locks files and demands money.
• Phishing: fake emails or sites to steal passwords and bank details. Hacking: unauthorised access to a system. Spoofing: pretending to be a trusted source.
• Protection: antivirus, firewall, regular updates, strong passwords, two-factor authentication and backups.
• The Information Technology Act, 2000 is India's main law on cyber crime and electronic commerce. CERT-In is the national agency for cyber security incidents.

Database Basics

• A database is an organised collection of data, and a DBMS is the software used to create and manage it (MySQL, Oracle, MS Access).
• In a relational database, data is stored in tables made of rows (records or tuples) and columns (fields or attributes).
• A primary key uniquely identifies each record, and a foreign key links one table to another.
• SQL (Structured Query Language) is used to work with the database. Basic commands: SELECT, INSERT, UPDATE and DELETE.

Emerging Technologies

• Artificial Intelligence (AI) and Machine Learning (ML): machines that learn from data.
• Internet of Things (IoT): everyday devices connected through the internet.
• Blockchain: a secure, shared digital ledger used for cryptocurrencies.
• Big Data, Cloud Computing, 5G, Augmented Reality (AR) and Virtual Reality (VR).
• Digital India was launched in 2015 to make government services available digitally, and UPI (Unified Payments Interface) enables instant mobile payments.

Important Full Forms

Short form	Full form	Short form	Full form
CPU	Central Processing Unit	ALU	Arithmetic Logic Unit
RAM	Random Access Memory	ROM	Read Only Memory
BIOS	Basic Input Output System	POST	Power On Self Test
USB	Universal Serial Bus	HDMI	High Definition Multimedia Interface
URL	Uniform Resource Locator	HTTP	HyperText Transfer Protocol
HTML	HyperText Markup Language	FTP	File Transfer Protocol
ISP	Internet Service Provider	DNS	Domain Name System
LAN	Local Area Network	WAN	Wide Area Network
GUI	Graphical User Interface	CLI	Command Line Interface
DBMS	Database Management System	SQL	Structured Query Language
PDF	Portable Document Format	JPEG	Joint Photographic Experts Group
ASCII	American Standard Code for Information Interchange	GIGO	Garbage In Garbage Out
SMPS	Switched Mode Power Supply	OCR	Optical Character Reader
MICR	Magnetic Ink Character Reader	OMR	Optical Mark Reader

Quick Revision Points

• Father of the computer: Charles Babbage. First programmer: Ada Lovelace.
• Brain of the computer: CPU. Fastest memory: registers. Volatile memory: RAM.
• Smallest unit of data: bit. 1 byte = 8 bits.
• The first generation used vacuum tubes, the second transistors, the third ICs and the fourth microprocessors.
• Compiler translates the whole program, interpreter line by line.
• NAND and NOR are universal gates.
• IPv4 is 32-bit and IPv6 is 128-bit.
• Star topology is the most commonly used topology.
• Router connects different networks, switch connects devices inside a network.
• Ctrl + Z is undo, Ctrl + Y is redo, F12 is Save As, F7 is spell check.

Practice Questions With Answers

1. Which part of the CPU performs arithmetic and logical operations? Answer: ALU
2. Which generation of computers used integrated circuits? Answer: Third generation
3. Which memory is volatile? Answer: RAM
4. How many bits are there in one byte? Answer: 8
5. Which device is used to read answer sheets marked with pencil? Answer: OMR
6. Which software translates the whole program at once? Answer: Compiler
7. What is the decimal value of binary 1101? Answer: 13
8. Which gate is called a universal gate? Answer: NAND (and NOR)
9. Which shortcut key is used for Save As in MS Word? Answer: F12
10. Which device connects two different networks and finds the best path? Answer: Router
11. The length of an IPv6 address is: Answer: 128 bits
12. Which protocol is used to send email? Answer: SMTP
13. Which program looks useful but contains harmful code? Answer: Trojan horse
14. Who invented the World Wide Web? Answer: Tim Berners-Lee
15. Which topology is the most reliable but costly? Answer: Mesh

How to Use These Computer Notes

1. Read each section once and highlight the facts that are new to you.
2. Write the full forms and shortcut tables on a single sheet and revise them every few days.
3. Practise the shortcuts on a real computer to remember them permanently.
4. Solve the computer questions from our previous year question papers and try the practice papers to test yourself.
5. Continue with the General Awareness and General Science notes to complete the GK section.