Last reviewed 16 Sept 2026 · 9 min read
What is a computer?
A computer is an electronic device that accepts data (input), processes it according to a set of instructions (program), stores data and results, and produces information (output).
The word comes from "compute" (to calculate). The basic cycle is Input → Process → Output, with Storage (IPO-S cycle).
Characteristics
| Characteristic | Meaning |
|---|---|
| Speed | Performs millions/billions of operations per second (measured in MIPS, FLOPS) |
| Accuracy | Results are accurate if input and program are correct — GIGO ("garbage in, garbage out") |
| Diligence | No fatigue or loss of concentration |
| Versatility | Can perform very different tasks |
| Storage (memory) | Stores large volumes of data for retrieval |
| Automation | Works automatically once instructed |
| Reliability | Consistent performance |
Limitations
No intelligence or common sense of its own (acts only as programmed, though AI systems emulate some capabilities), no feelings, dependence on power and human instructions, cannot make independent judgements outside programmed logic.
History of computing (milestones)
| Period / year | Development | Contributor |
|---|---|---|
| Ancient | Abacus — counting frame | China/Mesopotamia |
| 1617 | Napier's bones | John Napier |
| 1642 | Pascaline — mechanical adding machine | Blaise Pascal |
| 1670s | Stepped reckoner (multiplication) | Gottfried Leibniz |
| 1801 | Punched-card loom | Joseph Marie Jacquard |
| 1822 | Difference Engine | Charles Babbage |
| 1830s | Analytical Engine — design with input, "store" (memory), "mill" (processor), output — basis of modern computers | Charles Babbage — "Father of the Computer" |
| 1840s | Notes containing the first algorithm intended for a machine | Ada Lovelace — often called the first programmer |
| 1890 | Punched-card tabulating machine for the US census (later IBM) | Herman Hollerith |
| 1936 | Turing machine concept | Alan Turing |
| 1940s | Early electronic computers: Atanasoff–Berry Computer, Colossus, Harvard Mark I (electromechanical) | — |
| 1946 | ENIAC — general-purpose electronic digital computer using vacuum tubes | Eckert and Mauchly |
| 1945 onwards | Stored-program concept — programs and data stored in the same memory (EDVAC, EDSAC) | John von Neumann and others |
| 1951 | UNIVAC I — early commercial computer | Eckert and Mauchly |
Generations of computers
(Year ranges are approximate and vary between sources.)
| Generation | Period (approx.) | Technology | Features / examples |
|---|---|---|---|
| First | 1940s–1950s | Vacuum tubes | Huge, high power and heat, unreliable; machine language; punched cards — ENIAC, UNIVAC |
| Second | 1950s–1960s | Transistors | Smaller, faster, more reliable; assembly and early high-level languages (FORTRAN, COBOL); magnetic core memory — IBM 1401 |
| Third | 1960s–1970s | Integrated circuits (ICs) | Smaller, cheaper; operating systems, multiprogramming; keyboards and monitors — IBM System/360 |
| Fourth | 1970s–present | Microprocessors (VLSI) | Personal computers, GUIs, networks, internet — Intel 4004 (1971), IBM PC, Apple Macintosh |
| Fifth | Present and future | ULSI, parallel processing, artificial intelligence | Natural language processing, machine learning, robotics, quantum computing research |
Classification of computers
By working principle
| Type | Data handled | Examples |
|---|---|---|
| Analog | Continuous physical quantities (voltage, pressure, temperature) | Speedometer, thermometer, old slide rules, analog flight simulators |
| Digital | Discrete values (binary 0 and 1) | Personal computers, calculators, smartphones |
| Hybrid | Both analog and digital | ICU patient monitoring systems, petrol pump dispensers, industrial process control |
By purpose
- General purpose — many kinds of tasks (PCs).
- Special purpose — dedicated tasks (ATMs, traffic signal controllers, washing machine controllers, weather forecasting systems).
By size and capability
| Type | Features | Uses |
|---|---|---|
| Supercomputers | Fastest, massively parallel processors; speed in FLOPS | Weather forecasting, climate modelling, nuclear research, aerodynamic simulation, earthquake modelling |
| Mainframes | Large, many simultaneous users, high reliability | Banks, railways reservation, insurance, government databases |
| Minicomputers (mid-range) | Smaller than mainframes, multi-user | Departments, small businesses (historically) |
| Microcomputers (personal computers) | Single-user, microprocessor-based | Desktops, laptops, tablets, smartphones |
| Workstations | High-performance single-user machines | CAD, graphics, engineering analysis |
| Embedded computers | Built into other devices | Cars, appliances, medical devices, IoT sensors |
| Servers | Provide services to other computers over a network | Web, email, file, database servers |
Supercomputers in India
- PARAM 8000 (1991), developed by C-DAC (Centre for Development of Advanced Computing), Pune, is regarded as India's first supercomputer.
- Later systems include the PARAM series and high-performance computers at IITM (Pune) and NCMRWF (Noida) for weather and climate, many under the National Supercomputing Mission.
Block diagram of a computer (functional units)
Input unit → Central Processing Unit (CPU) ↔ Memory unit → Output unit, all connected by buses.
| Unit | Function |
|---|---|
| Input unit | Accepts data and instructions; converts them into binary form — keyboard, mouse, scanner |
| Central Processing Unit (CPU) — "brain of the computer" | Executes instructions and controls all operations |
| — Arithmetic Logic Unit (ALU) | Performs arithmetic (+, −, ×, ÷) and logical (comparisons AND, OR, NOT, >, <, =) operations |
| — Control Unit (CU) | Directs and coordinates all operations — fetches instructions, decodes and signals other units; does not process data itself |
| — Registers | Very small, very fast storage inside the CPU (program counter, accumulator, instruction register) |
| Memory unit | Primary memory (RAM, ROM, cache) holds data and programs in use; secondary storage holds data permanently |
| Output unit | Converts results into human-readable form — monitor, printer, speakers |
Machine cycle
Fetch → Decode → Execute → Store (instruction cycle), repeated millions of times per second, timed by the system clock (clock speed in hertz, e.g. GHz).
Buses
- Address bus (unidirectional — location), data bus (bidirectional — data), control bus (control signals).
von Neumann vs Harvard architecture
| von Neumann | Harvard |
|---|---|
| Same memory and bus for data and instructions (stored program) | Separate memory and buses for data and instructions |
| Simpler; "von Neumann bottleneck" | Faster in some applications — microcontrollers, DSPs |
Data and information
- Data — raw, unprocessed facts and figures (e.g. marks of students).
- Information — processed, meaningful data (e.g. class average, rank list).
- Data processing cycle: collection → preparation → input → processing → output → storage.
Units of data
| Unit | Equivalent |
|---|---|
| Bit (binary digit) | 0 or 1 — smallest unit |
| Nibble | 4 bits |
| Byte | 8 bits — typically one character |
| Kilobyte (KB) | 1024 bytes (binary convention; 1000 bytes in decimal/SI usage) |
| Megabyte (MB) | 1024 KB |
| Gigabyte (GB) | 1024 MB |
| Terabyte (TB) | 1024 GB |
| Petabyte (PB) | 1024 TB |
| Exabyte (EB) → Zettabyte (ZB) → Yottabyte (YB) | Each 1024 times the previous |
(The IEC binary prefixes KiB, MiB, GiB denote powers of 1024 explicitly; storage manufacturers often use powers of 1000.)
Word — number of bits processed by a CPU at a time (e.g. 32-bit, 64-bit).
Number systems (summary)
| System | Base | Digits |
|---|---|---|
| Binary | 2 | 0, 1 |
| Octal | 8 | 0–7 |
| Decimal | 10 | 0–9 |
| Hexadecimal | 16 | 0–9, A–F |
- Decimal → binary: repeated division by 2 (read remainders upward).
- Binary → octal/hexadecimal: group bits in 3s/4s.
- Character codes: ASCII (7-bit, 128 characters; extended 8-bit), EBCDIC (IBM mainframes), Unicode (UTF-8, UTF-16 — supports world scripts including Devanagari and Gurmukhi).
(See Digital Electronics for arithmetic and codes.)
Worked examples
Worked ExampleExample 1 — number conversion
Convert decimal 156 to binary, octal and hexadecimal.
Solution. → 10011100₂; octal: 010 011 100 → 234₈; hexadecimal: 1001 1100 → 9C₁₆
Worked ExampleExample 2 — binary to decimal
Convert 110101₂ to decimal.
Solution. 53
Worked ExampleExample 3 — storage units
How many bytes are in 2 MB (binary convention)? How many bits?
Solution. 2 097 152 bytes = 16 777 216 bits
Worked ExampleExample 4 — text storage
Approximately how much storage does a 5000-character plain ASCII text file need?
Solution. 1 byte per character → 5000 bytes ≈ 4.88 KB
Worked ExampleExample 5 — classification
Classify: (a) a digital wristwatch, (b) a car speedometer with a needle, (c) an ICU monitor that converts heartbeat signals into numbers.
Solution. (a) Digital; (b) analog; (c) hybrid
Frequently tested points
- Computer: input → process → output with storage; GIGO.
- Charles Babbage — Father of the Computer (Difference Engine, Analytical Engine); Ada Lovelace — first programmer; Pascal — Pascaline; Hollerith — punched cards.
- ENIAC (1946) — vacuum tubes; stored-program concept — von Neumann; UNIVAC I early commercial computer.
- Generations: 1st vacuum tubes, 2nd transistors, 3rd ICs, 4th microprocessors (VLSI), 5th AI/ULSI.
- Analog (continuous), digital (discrete), hybrid (both).
- Supercomputer fastest (FLOPS); mainframe — many users (banks, railways); microcomputer — PC.
- PARAM 8000 (1991, C-DAC Pune) — India's first supercomputer.
- CPU = ALU + CU (+ registers); CU directs, ALU calculates and compares; CPU called the brain.
- Machine cycle: fetch, decode, execute, store.
- Bit < nibble (4) < byte (8) < KB < MB < GB < TB < PB < EB < ZB < YB (×1024).
- ASCII 7-bit; Unicode for all scripts.
Common MistakeCommon mistakes
- Saying the control unit performs calculations (the ALU does).
- Treating 1 KB as exactly 1000 bytes in questions that use the binary convention.
- Confusing ENIAC (first general-purpose electronic) with UNIVAC (early commercial).
Revision SummaryChapter summary
- A computer accepts, processes, stores and outputs data, offering speed, accuracy, diligence and versatility but no independent judgement.
- Computing evolved from the abacus and Babbage's engines through ENIAC and the stored-program concept to modern microprocessor-based systems across five generations.
- Computers are classified as analog, digital or hybrid; general or special purpose; and super, mainframe, mini, micro, workstation, embedded or server.
- The functional units — input, CPU (ALU, CU, registers), memory and output — are linked by buses and operate through the fetch–decode–execute cycle.
- Data become information through processing and are measured in bits and bytes, represented using binary, octal, hexadecimal and character codes.