Computer & IT Basics: Notes for Railway, SSC & UPSC Exams

Inside a computer

• The CPU (Central Processing Unit) is the brain. Its Arithmetic Logic Unit (ALU) does calculations and comparisons; its Control Unit (CU) directs the flow of instructions; registers are tiny, fastest storage inside it.
• Primary memory: RAM (Random Access Memory) holds running programs and is volatile — it is wiped when power goes. ROM (Read Only Memory) is non-volatile and stores firmware such as the BIOS that starts the computer.
• Cache memory is a small, very fast memory between the CPU and RAM. Speed order: registers > cache > RAM > secondary storage.
• Secondary storage keeps data permanently: hard disk (magnetic), SSD and pen drive (flash), CD, DVD and Blu-ray (optical).
• The motherboard connects everything; an SMPS supplies power; a GPU handles graphics.

Memory units

Number systems

Computers work in binary (base 2: digits 0 and 1). Other systems you meet are octal (base 8), decimal (base 10) and hexadecimal (base 16, using 0–9 and A–F, where A = 10 and F = 15).

1. Decimal to binary: write the number as a sum of powers of 2 (16, 8, 4, 2, 1). Example: 11 = 8 + 2 + 1, so 11 = 1011.
2. Another example: 25 = 16 + 8 + 1, so 25 = 11001.
3. Binary to decimal: add the place values of the 1s. Example: 10010 = 16 + 2 = 18.
4. Check: a binary number ending in 1 is always odd; ending in 0 is always even.

Input and output devices

OMR (Optical Mark Recognition) reads filled bubbles on answer sheets; OCR (Optical Character Recognition) turns printed text into editable text; MICR (Magnetic Ink Character Recognition) reads the magnetic-ink numbers at the bottom of bank cheques.

Software and language translators

• System software runs the machine — the operating system (Windows, Linux, macOS, Android) manages memory, files and devices.
• Application software does user tasks — word processors, browsers, spreadsheets.
• Machine language (binary) is the only language a CPU understands directly. Assembly language uses short codes (mnemonics); high-level languages (C, Java, Python) read like English.
• Compiler: translates the whole program before running and lists all errors together — fast to run afterwards.
• Interpreter: translates and runs one line at a time and stops at the first error.
• Assembler: converts assembly language into machine code.

History in brief

Charles Babbage designed the Analytical Engine and is called the father of the computer; Ada Lovelace wrote what is considered the first program for it. India's first supercomputer, PARAM 8000, was built by C-DAC in 1991 under Vijay Bhatkar. Supercomputer speed is measured in FLOPS (floating-point operations per second).

Networks and the internet

• By size: PAN (personal, e.g. Bluetooth) < LAN (a building) < MAN (a city) < WAN (countries). The internet is the largest WAN.
• Modem = modulator-demodulator (converts digital signals for phone or cable lines). A router forwards data between networks; a switch connects devices within a LAN.
• Topologies: bus, star, ring, mesh, tree. In a star, every device connects to a central hub or switch.
• The internet grew out of ARPANET (USA, 1969). TCP/IP, its core protocols, were designed by Vint Cerf and Bob Kahn.
• The World Wide Web — pages linked by hyperlinks — was invented by Tim Berners-Lee at CERN in 1989. The web runs on the internet; they are not the same thing.
• Every device online has an IP address. IPv4 addresses are 32 bits long; IPv6 addresses are 128 bits. DNS (Domain Name System) translates names like example.com into IP addresses.
• Ray Tomlinson sent the first network email (1971) and chose the @ symbol.

Cyber safety

• Virus: attaches to files and spreads when they are run. Worm: spreads on its own across networks. Trojan: malware disguised as useful software. Ransomware: locks files and demands payment. Spyware: secretly collects data.
• Phishing: fake emails, SMS or sites posing as banks or companies to steal passwords, card numbers or OTPs. Banks never ask for OTPs or PINs.
• Defences: firewall (filters network traffic), antivirus, regular updates, strong passwords and two-factor authentication.