Pariksha SaathiLesson 3 of 10 · Computer Knowledge
Storage Devices, Input Devices and Output Devices
Drafted with AI assistance and fact-checked on 28 October 2026; not yet read line by line by a human editor. Spotted a mistake? Tell us. Editorial standards & corrections
Secondary storage
Primary memory is fast but small and mostly volatile, so data is kept permanently on secondary storage, which is non-volatile, larger, cheaper per byte and slower. It is of three kinds: magnetic (hard disk, magnetic tape, the old floppy disk), optical (CD, DVD, Blu-ray) and solid-state or flash (SSD, pen drive, memory card). Magnetic tape allows only sequential access, reading data in order from the start, so it is used for backups and archives; disks allow direct (random) access.
Hard disk and SSD
A hard disk drive (HDD) stores data magnetically on spinning metal platters, read and written by heads on a moving arm. Each surface is divided into circular tracks and pie-shaped sectors, and the same track on all platters forms a cylinder; formatting a disk creates this structure. Disk speed is given in revolutions per minute, such as 5400 or 7200 rpm. IBM's RAMAC of 1956 was the first computer sold with a hard disk. A solid-state drive (SSD) has no moving parts and stores data in NAND flash chips, so it is faster, quieter and more shock-resistant than an HDD but costs more per gigabyte. The old 3.5-inch floppy disk held 1.44 MB.
Optical and flash storage
Optical discs are read by a laser that detects tiny pits and flat lands on the surface. A CD (Compact Disc) holds about 700 MB, a single-layer DVD (Digital Versatile Disc) 4.7 GB, a dual-layer DVD 8.5 GB and a single-layer Blu-ray disc 25 GB; Blu-ray uses a blue-violet laser of shorter wavelength, which lets it pack data more tightly. ROM discs (CD-ROM) can only be read, R discs can be written once and RW discs can be erased and rewritten. A pen drive is flash memory with a USB (Universal Serial Bus) plug, and memory cards such as SD (Secure Digital) and microSD are used in phones and cameras. Their capacities keep growing, so remember the technology rather than the largest size on sale.
Input devices
The keyboard is the main input device; its standard layout is QWERTY, named after the first six letters on the top row of letter keys, and its keys are grouped into alphanumeric keys, function keys F1 to F12, the numeric keypad, cursor keys and modifier keys (Shift, Ctrl, Alt). The mouse, invented by Douglas Engelbart in the 1960s, is a pointing device; others are the trackball, touchpad, joystick (for games), light pen and stylus. A scanner converts paper pages and photographs into digital images. A touch screen works as both an input and an output device. Microphones, webcams and biometric readers (fingerprint, iris) are also input devices.
Special readers
OMR (Optical Mark Recognition) detects pencil or pen marks in fixed positions and is used to evaluate objective answer sheets. OCR (Optical Character Recognition) turns an image of printed or handwritten text into editable text. MICR (Magnetic Ink Character Recognition) reads characters printed with magnetic iron-oxide ink at the bottom of cheques, which banks use to process cheques quickly; the 9-digit MICR code on an Indian cheque has 3 digits for the city, 3 for the bank and 3 for the branch. A barcode reader scans the black and white bars on products at shop counters, and the QR (Quick Response) code is a two-dimensional barcode developed in Japan in 1994.
Output devices
A monitor or VDU (visual display unit) shows soft copy. Old monitors used a CRT (cathode ray tube); flat screens use LCD (liquid crystal display), LED-backlit LCD or OLED panels. A screen is made of pixels (picture elements); resolution is the number of pixels across and down, and screen size is measured diagonally in inches. Speakers and headphones give sound output, and a projector throws the screen image onto a wall or screen.
Printers and plotters
Printers give hard copy. Impact printers strike an inked ribbon against the paper and can make carbon copies: the dot matrix printer (pins forming dots, speed in characters per second), the daisy wheel printer (letter quality but slow) and the line printer (a whole line at a time, speed in lines per minute). Non-impact printers do not strike the paper and are quieter: the inkjet sprays tiny ink droplets, the laser printer uses a laser, a charged drum and toner powder (speed in pages per minute), and the thermal printer uses heat-sensitive paper, as in ATM slips and billing receipts. Print quality is measured in dpi (dots per inch). A plotter draws large, precise drawings such as maps, building plans and engineering designs.
Key facts
- Magnetic tape allows only sequential access and is used for backup; hard disks, SSDs and optical discs allow direct (random) access.
- HDD: spinning platters divided into tracks and sectors; SSD: NAND flash chips, no moving parts, faster and more shock-resistant.
- Standard capacities: 3.5-inch floppy 1.44 MB; CD about 700 MB; single-layer DVD 4.7 GB; dual-layer DVD 8.5 GB; single-layer Blu-ray 25 GB.
- CD-ROM is read-only, CD-R can be written once, CD-RW can be erased and rewritten.
- OMR reads marked bubbles on answer sheets; OCR converts images of text into editable text; MICR reads magnetic-ink characters on cheques.
- The 9-digit MICR code on an Indian cheque: first 3 digits city, next 3 bank, last 3 branch.
- Douglas Engelbart invented the mouse; the QWERTY keyboard is named after the first six letters of its top letter row.
- A touch screen is both input and output; scanner, microphone, webcam and barcode reader are input; monitor, printer, plotter, speaker and projector are output.
- Impact printers (dot matrix, daisy wheel, line) strike a ribbon and can make carbon copies; non-impact printers (inkjet, laser, thermal) are quieter and faster.
- Dot matrix speed is in characters per second, line printer speed in lines per minute, laser printer speed in pages per minute; print resolution is in dpi.
Quick revision
Secondary storage is non-volatile: magnetic (HDD, tape - sequential access), optical (CD about 700 MB, DVD 4.7 GB, Blu-ray 25 GB), flash (SSD, pen drive, SD card); HDD tracks, sectors, rpm; input: keyboard (QWERTY), mouse (Engelbart), scanner, OMR (answer sheets), OCR (text), MICR (cheques, 9-digit code), barcode, QR code; output: monitor (pixels; CRT, LCD, LED), impact printers (dot matrix, daisy wheel, line), non-impact (inkjet, laser, thermal), plotter.