Computer Fundamentals: Generations, Types, Hardware, Software, Memory and Storage Units · কম্পিউটারের মৌলিক ধারণা: প্রজন্ম, প্রকারভেদ, হার্ডওয়্যার, সফটওয়্যার, মেমোরি ও স্টোরেজের একক

1. What a computer is: input, processing, output, storage

A computer is an electronic, programmable machine that accepts data (raw facts), processes it according to a stored set of instructions (a program) and produces information (processed, meaningful results). Its four basic functions, often called the IPOS cycle, are input, processing, output and storage.

The classic functional units (a frequent written question, "describe the functional units with a diagram"):

Unit	Job	Examples
Input unit	Accepts data and instructions and converts them to binary	Keyboard, mouse, scanner
Memory (storage) unit	Holds data, programs and results; primary memory for the work in progress, secondary storage for keeping it	RAM, ROM, SSD, hard disk
Arithmetic Logic Unit (ALU)	Calculations and comparisons	Part of the CPU
Control Unit (CU)	Fetches and decodes instructions and directs all other units	Part of the CPU
Output unit	Converts results into a form people or other machines can use	Monitor, printer, speaker

The ALU and the control unit together form the CPU (see the CPU Organization page). A computer is the machine; a computer system is the whole working arrangement: hardware, software, data and the people who use it (called liveware or humanware).

2. The generations of computers

A generation is named after the main electronic component used to build the processor. Each step made computers smaller, faster, cheaper, more reliable and less power-hungry. Sources give different year ranges, and both common sets appear as MCQ options: the international set (1940s–1956, 1956–1963, 1964–1971, 1971–present) and the set used in many Bangladeshi texts (1946–1959, 1959–1965, 1965–1971, 1971–present). The table shows both. The main component, not the year, decides the generation.

Generation	Period: international (Bangladeshi texts)	Main component	Main memory	Programming	Examples
First	1940s – 1956 (1946 – 1959)	Vacuum tube	Magnetic drum	Machine language	ENIAC, UNIVAC I, IBM 650
Second	1956 – 1963 (1959 – 1965)	Transistor (invented 1947 at Bell Labs)	Magnetic core	Assembly; first high-level languages (FORTRAN, COBOL)	IBM 1401, IBM 7090
Third	1964 – 1971 (1965 – 1971)	Integrated circuit (IC)	Magnetic core, then semiconductor	High-level languages; operating systems, multiprogramming	IBM System/360, PDP-11
Fourth	1971 – present (same)	Microprocessor (VLSI: a whole CPU on one chip)	Semiconductor (RAM)	GUIs, C and later languages, networks and the Internet	Intel 4004 based systems, Apple II, IBM PC (1981)
Fifth	1980s – present / future	ULSI, parallel processing, artificial intelligence	Semiconductor	Natural language, AI, knowledge-based systems	AI systems, supercomputers

Points exams pick out:

• The base of the fifth generation is artificial intelligence (with ULSI chips and parallel processing). The name comes from Japan's Fifth Generation Computer Systems project, started in 1982.
• Chips (integrated circuits) were first used in computers in the third generation, from about 1964 (the usual keyed year). The IC itself was invented in 1958–59 (Jack Kilby at Texas Instruments, Robert Noyce at Fairchild).
• The microprocessor defines the fourth generation; the first commercial one was the Intel 4004 (1971).

3. Types of computers

Basis	Types	What distinguishes them	Examples
Working principle	Analog	Works on continuously varying quantities (voltage, pressure, speed)	Analog speedometer, slide rule
	Digital	Works on discrete values (binary 0 and 1)	PC, laptop, smartphone
	Hybrid	Combines both: measures in analog, processes in digital	ICU patient monitors, some industrial controllers
Size and power	Supercomputer	Fastest; thousands of processors working in parallel	Weather forecasting, nuclear and climate research (Frontier was the first exascale system, 2022)
	Mainframe	Very reliable; serves thousands of users and transactions at once	Banks, airlines, census
	Minicomputer	Mid-sized, for departments (now largely replaced by servers)	PDP-8, PDP-11
	Microcomputer	Built around one microprocessor; personal use	Desktop, laptop, tablet, smartphone
Purpose	General-purpose	Runs many kinds of programs	PC
	Special-purpose	Built for one job	ATM controller, washing-machine controller

4. Hardware and software

Hardware is every physical part you can touch: CPU, memory, motherboard, keyboard, disk. Software is the set of programs that tell the hardware what to do ("programs consist of instructions that control the computer"). Firmware is software stored permanently in a chip on the hardware, such as the BIOS or UEFI that starts the computer.

Point	System software	Application software
Purpose	Runs and manages the computer and provides a platform for other programs	Does a specific task for the user
Who uses it directly	Mostly the computer itself; the user rarely works "inside" it	The user
Needed for the computer to run?	Yes (at least an operating system)	No
Examples	Operating systems (Windows, Linux such as Red Hat or Ubuntu, macOS, Android); device drivers; utilities (antivirus, disk cleanup, backup); language translators (compiler, interpreter, assembler)	Word processors (MS Word), spreadsheets (MS Excel), presentation software (PowerPoint), web browsers, banking software, graphics software (Photoshop, Illustrator, CorelDRAW, GIMP)

Compiler vs interpreter (often asked alongside): a compiler translates the whole program into machine code before it runs and reports all errors together (C, C++); an interpreter translates and runs one statement at a time and stops at the first error (classic BASIC; Python's standard implementation compiles to bytecode and then interprets it). An assembler translates assembly language into machine code.

5. Input and output devices

Input devices	Output devices	Both input and output
Keyboard, mouse, touchpad, joystick, light pen	Monitor (display), projector	Touch screen
Scanner, OCR, OMR, MICR, barcode reader	Printer, plotter	Modem, network card
Microphone, webcam, digital camera	Speaker, headphones	Headset (microphone and speaker)
Biometric readers (fingerprint, iris, face)		Storage drives (read and write)

Optical input devices (they read using light): scanner, OCR reader, OMR reader, barcode reader, webcam.

Device	Full form	What it reads	Typical use
OCR	Optical Character Recognition	Printed or handwritten characters, turned into editable text	Converting scanned documents to text
OMR	Optical Mark Recognition (Reader)	The presence of marks (filled bubbles) at fixed positions	MCQ answer sheets, surveys
MICR	Magnetic Ink Character Recognition	Characters printed in magnetic ink	The code line on bank cheques

Printers: an impact printer strikes the paper through a ribbon (dot-matrix, daisy-wheel, line printers; can print carbon copies, noisy); a non-impact printer forms the image without striking (laser, inkjet, thermal; quiet, better quality). Printer resolution is measured in dots per inch (DPI); screen resolution in pixels (for example 1920 × 1080). When buying a monitor consider size, resolution, panel type, refresh rate, response time, brightness and the ports it offers.

6. The memory hierarchy, RAM and ROM

No single memory is fast, large and cheap at the same time, so computers use a hierarchy: small, fast, expensive memory close to the CPU and large, slow, cheap storage further away.

Level	Technology	Typical access time	Typical size	Cost per byte	Volatile?
Registers	Flip-flops inside the CPU	Under 1 ns	Bytes (a few dozen registers)	Highest	Yes
Cache (L1, L2, L3)	SRAM	About 1 – 20 ns	KB to tens of MB	Very high	Yes
Main memory (RAM)	DRAM	About 50 – 100 ns	GB	Moderate	Yes
ROM / flash firmware	Mask ROM, EEPROM, flash	Similar to or slower than RAM	KB to MB	Moderate	No
Secondary storage	SSD (flash) / hard disk (magnetic)	Tens of µs (SSD) / several ms (HDD)	Hundreds of GB to TB	Low	No
Offline / tertiary	Magnetic tape, optical disc	Seconds or more	TB and beyond	Lowest	No

Fastest to slowest (a written question): register → cache → main memory → magnetic disk → magnetic tape.

Primary memory (registers, cache, RAM, ROM) is accessed directly by the CPU; secondary memory (SSD, HDD, optical disc, USB drive) is not, and data must be loaded into RAM before it is used.

Point	RAM	ROM
Full form	Random Access Memory	Read Only Memory
Read/write	Read and write	Read (written at the factory or by special means)
Volatile?	Yes: contents lost when power goes off	No: contents kept without power
Holds	Programs and data in use	Firmware: BIOS/UEFI start-up code
Types	SRAM, DRAM (SDRAM, DDR4, DDR5)	Mask ROM, PROM, EPROM, EEPROM, flash

Type	Full form	How it is written or erased	Main use
SRAM	Static RAM	Each bit in a flip-flop (about 6 transistors); no refresh needed; fast, costly	Cache
DRAM	Dynamic RAM	Each bit as charge in a capacitor (1 transistor + 1 capacitor); must be refreshed every few milliseconds (every row at least once per 64 ms in DDR standards); cheaper, denser	Main memory
PROM	Programmable ROM	Written once by the user; cannot be erased	One-time programming
EPROM	Erasable PROM	Erased by ultraviolet light (the whole chip), then rewritten	Older firmware chips
EEPROM	Electrically Erasable PROM	Erased and rewritten electrically, byte by byte	Settings memory
Flash	(a kind of EEPROM)	Erased electrically in blocks; fast	SSDs, USB drives, memory cards, BIOS chips

An SSD (Solid State Drive) stores data in NAND flash chips managed by a controller; with no moving parts it is faster, quieter and tougher than a hard disk (HDD), which reads magnetic platters with a moving head. A CD/DVD is read and written with a laser (optical storage).

7. Storage units: bit, byte and the 1024 vs 1000 question

• Bit = one binary digit (0 or 1), the smallest unit.
• Nibble = 4 bits. Byte = 8 bits (enough for one ASCII character).

Two conventions exist for the bigger units:

Binary (IEC) name	Value	Decimal (SI) name	Value	Difference
1 KiB (kibibyte)	2¹⁰ = 1,024 bytes	1 kB (kilobyte)	10³ = 1,000 bytes	2.4%
1 MiB (mebibyte)	2²⁰ = 1,048,576 bytes	1 MB	10⁶ bytes	4.9%
1 GiB (gibibyte)	2³⁰ = 1,073,741,824 bytes	1 GB	10⁹ bytes	7.4%
1 TiB (tebibyte)	2⁴⁰ = 1,099,511,627,776 bytes	1 TB	10¹² bytes	10.0%
1 PiB (pebibyte)	2⁵⁰ bytes	1 PB	10¹⁵ bytes	12.6%
1 EiB (exbibyte)	2⁶⁰ bytes	1 EB	10¹⁸ bytes	15.3%

• Binary convention: each step is × 1,024. Memory (RAM) sizes are always powers of two, and the traditional computing usage, still used by JEDEC memory standards and by Windows, calls 1,024 bytes "1 KB". To remove the ambiguity the IEC introduced the names KiB, MiB, GiB in 1998 (now in the IEC 80000-13 standard).
• Decimal (SI) convention: each step is × 1,000. Disk and pen-drive makers, and network speeds (1 Mbps = 10⁶ bits per second), use it.
• Which one exams mean: Bangladeshi job exams use 1 KB = 1,024 bytes, 1 MB = 1,024 KB and so on, unless the question says otherwise. Order: bit < nibble < byte < KB < MB < GB < TB < PB < EB < ZB < YB.

Worked example 1: 5 GB in bytes and bits (exam convention). 1 GB = 1,024 × 1,024 × 1,024 bytes = 2³⁰ = 1,073,741,824 bytes. So 5 GB = 5 × 1,073,741,824 = 5,368,709,120 bytes, and × 8 = 42,949,672,960 bits.

Worked example 2: why a "500 GB" disk shows about 465 GB. The maker means 500 × 10⁹ bytes. The operating system divides by 2³⁰: 500,000,000,000 ÷ 1,073,741,824 ≈ 465.66 GiB, which Windows labels "465 GB". No space is missing; only the unit differs.

Worked example 3: songs on a pen drive. A 16 GB pen drive holds songs of 5 MB each. 16 GB = 16 × 1,024 = 16,384 MB; 16,384 ÷ 5 = 3,276.8, so 3,276 whole songs (ignoring file-system overhead).

8. Famous firsts and acronyms

Fact	Answer
Father of the computer	Charles Babbage: designed the Difference Engine (1822) and the programmable Analytical Engine (1837)
First computer programmer	Ada Lovelace: her 1843 notes on the Analytical Engine include an algorithm to compute Bernoulli numbers
First general-purpose electronic digital computer	ENIAC (University of Pennsylvania, completed 1945, unveiled 1946). The earlier Atanasoff–Berry Computer (1942) was electronic but not general-purpose; exam keys answer ENIAC. Britain's Colossus (1943–44) was electronic and programmable but built only to break German ciphers, so it is not the answer either
First commercially produced computer in the USA	UNIVAC I (1951)
First high-level programming language in wide use	FORTRAN (IBM, 1957)
First commercial microprocessor	Intel 4004 (1971)
First computer in Bangladesh	An IBM 1620 at the Atomic Energy Centre, Dhaka (1964), then run by the Pakistan Atomic Energy Commission
Chess computer that beat world champion Garry Kasparov	IBM's Deep Blue (1997)
Moore's law	Gordon Moore (1965, revised 1975): the number of transistors on a chip doubles about every two years (his 1965 paper said every year)

Common full forms: SSD Solid State Drive; DOS Disk Operating System; BIOS Basic Input/Output System; URL Uniform Resource Locator; FTP File Transfer Protocol; ASCII American Standard Code for Information Interchange; SCSI Small Computer System Interface (pronounced "scuzzy"); MOOC Massive Open Online Course; MIME Multipurpose Internet Mail Extensions; PNG Portable Network Graphics; JPEG Joint Photographic Experts Group. VIRUS is popularly expanded as "Vital Information Resources Under Siege"; that is a made-up backronym, and a virus is simply a program that copies itself into other files.

9. Tricky exam cases

• "Which is not application software?" An operating system such as Red Hat Linux is system software, even though it comes with applications.
• RAM is volatile, ROM is not. "Random access" does not mean "temporary": ROM is also randomly accessible.
• EPROM is erased with ultraviolet light, EEPROM electrically. Flash is a block-erased form of EEPROM.
• Units: 1 KB = 1,024 bytes in exams, but a "1 TB" disk holds 10¹² bytes (≈ 931 GiB). Watch bits (b) against bytes (B): 16 Mbps is megabits.
• Keyboard facts asked in MCQs: F1 opens Help in almost every program; F5 refreshes a browser page and starts a PowerPoint slide show; Ctrl + W closes a browser tab (Ctrl + T opens one); Shift, Ctrl and Alt are modifier keys. In MS Word, F8 turns on extend-selection mode: pressing it twice selects a word, three times a sentence, four times a paragraph and five times the whole document.
• USB generations: there are four major families: USB 1.x, 2.0, 3.x and USB4.