SSC CGL · Computer Knowledge
Hardware & I/O Devices
Input, output and processing hardware including CPU components like ALU and control unit.
Seven concepts. SSC and railway papers ask which part of the CPU computes, which reader banks use on cheques, and whether a plotter is input or output — classifications and named devices, not arithmetic. Tables and pegs carry the load; nothing here earns a worked ledger.
- SSC CGL
- Easy level
- 7 concepts
- 40 practice questions
1CPU: ALU, Control Unit and registers
The CPU is the processing core of the machine. Three named parts share the work: the Arithmetic Logic Unit (ALU) performs arithmetic and logic operations such as addition and comparison; the Control Unit (CU) directs and coordinates data flow without itself computing; and registers provide very fast temporary storage inside the CPU.
The exam cue is short: CU controls, ALU calculates. A stem that asks which component actually carries out addition is pointing at the ALU, not the Control Unit.
Figure. Inside the CPU: Control Unit directs, ALU computes, registers hold working values.
| Part | Job | Exam cue |
|---|---|---|
| ALU | Arithmetic and logic operations | Adds, compares, calculates |
| Control Unit (CU) | Directs data flow and coordinates | Controls — does not compute |
| Registers | Fast temporary storage inside the CPU | Smallest, fastest working store |
A question asks which CPU component actually carries out addition and comparison on two numbers. The Control Unit is offered as a distractor because it "runs" the machine. Which part is correct, and why is the CU wrong?
- Registers — they store the answer, so they perform the arithmetic
- ALU — it executes arithmetic and logic; the CU only directs and coordinates
- Control Unit — directing the machine includes doing the addition
Addition and comparison are ALU work. The CU coordinates data flow but does not compute. Registers hold values temporarily; holding is not performing the operation.
2Input devices feed data in
Input devices feed data into the computer. Everyday examples are the keyboard, mouse, scanner, microphone, joystick and barcode reader. A light pen, touch screen and joystick are input devices — a common distractor is to call them output because the user "sees" a result on a touch screen, but the touch itself is input.
Three specialised readers dominate exam lists: OMR, OCR and MICR. Match each acronym to its use-case rather than inventing a new expansion.
Figure. Input devices only feed data in — keyboard, mouse, scanner toward the machine.
| Device | What it feeds in | Trap to avoid |
|---|---|---|
| Keyboard / mouse | Text and pointer commands | — |
| Scanner / mic / joystick | Images, sound, game control | Joystick is input, not output |
| Barcode reader | Product codes | — |
| Light pen / touch screen | Positional / touch input | Not classified as output devices |
A coaching booklet lists light pen, touch screen and joystick under "output devices" because the user interacts with a display. How should those three be classed for banking and SSC hardware questions?
- Output, because each input ends as something visible
- Input — they feed data or commands into the system
- Neither input nor output; only the CPU counts as a device
Light pen, touch screen and joystick are classic input-device examples in the notes. Visibility of a screen does not move them into the output class.
3OMR, OCR and MICR readers
Three acronyms are exam favourites because each maps to one clear use-case. MICR — Magnetic Ink Character Recognition — is used by banks to read cheque numbers printed in magnetic ink. OMR — Optical Mark Recognition — reads shaded bubbles on OMR answer sheets. OCR — Optical Character Recognition — turns scanned printed text into machine-readable characters.
The night-before match is: bank cheques → MICR, answer sheets → OMR, scanning printed text → OCR.
Figure. OMR reads marks, OCR reads printed text, MICR reads magnetic cheque characters.
| Acronym | Expansion | Typical use |
|---|---|---|
| MICR | Magnetic Ink Character Recognition | Bank cheque numbers / codes |
| OMR | Optical Mark Recognition | Shaded bubbles on OMR sheets |
| OCR | Optical Character Recognition | Scanning printed text |
A clearing house must read the account and cheque numbers printed along the bottom of a large stack of bank cheques. Which recognition technology is the stem aiming at?
- OMR, because cheques have printed marks like answer sheets
- OCR, because any printed character is OCR by definition
- MICR — banks use magnetic-ink character recognition on cheques
Cheque processing is the MICR use-case in the notes. OMR is for shaded bubbles; OCR scans general printed text but is not the banking-cheque cue these papers test.
4Output devices: soft copy and hard copy
Output devices present results leaving the computer — monitor, printer, speaker and plotter are the usual list. Soft copy is on-screen output (the monitor); hard copy is physical output on paper or similar media (printer or plotter).
A plotter produces a physical drawing on paper, so it is an output (hard-copy) device, used especially for large engineering or architectural drawings and graphs. If a device produces a result the user can take away on paper, class it as hard-copy output.
Figure. Soft copy stays on a temporary display; hard copy is a lasting printed artefact.
| Device | Copy type | What it delivers |
|---|---|---|
| Monitor | Soft copy | On-screen display |
| Printer | Hard copy | Printed page |
| Plotter | Hard copy | Large drawings / graphs on paper |
| Speaker | — (audio out) | Sound |
An architect's office uses a machine that draws large building plans in ink on paper rolls. Is that machine input or output, and what copy type is it?
- Input — plans are "fed into" the project file
- Output (hard copy) — a plotter produces physical drawings on paper
- Soft-copy output only — anything with ink is still called soft copy
A plotter delivers results out of the computer onto paper, so it is hard-copy output. Soft copy is on-screen; calling a paper plotter input confuses direction of data flow.
5Impact printers, non-impact printers and DPI
Printers split into two mechanical classes. Impact printers strike the paper — the classic exam example is the dot-matrix printer. Non-impact printers do not strike the paper; inkjet and laser printers sit in that class.
Printer resolution is measured in DPI (dots per inch). Higher DPI means a sharper print. Questions that name "strike the paper" are asking for impact; questions that name laser or inkjet are asking for non-impact.
Figure. Impact printers strike a ribbon; non-impact (inkjet/laser) do not. DPI measures print resolution.
| Class / measure | Meaning | Examples / cue |
|---|---|---|
| Impact | Strikes the paper | Dot-matrix |
| Non-impact | Does not strike the paper | Inkjet, laser |
| DPI | Dots per inch | Higher DPI → sharper print |
A branch still uses a noisy printer that physically hammers pins against an ink ribbon to form characters on multipart carbon paper. Which printer class is that, and which modern office printers sit in the other class?
- Non-impact; the other class is only plotters
- Impact (dot-matrix style); inkjet and laser are non-impact
- Neither — DPI classifies impact versus non-impact
Striking the paper through a ribbon is impact (dot-matrix). Inkjet and laser are non-impact. DPI measures resolution; it does not name the mechanical class.
6Ports, buses and the motherboard
Ports and buses connect components and peripherals. USB, HDMI and VGA are common ports that connect external devices; the system bus carries data, address and control signals between internal components.
The motherboard hosts the CPU, RAM and chipset — it is the board that holds the main processing and memory parts together. Port questions name the connector; bus questions name the internal signal path; motherboard questions name the host board.
Figure. The motherboard hosts CPU and RAM; buses carry signals; ports are the external openings.
| Item | Role | Exam cue |
|---|---|---|
| USB / HDMI / VGA | Ports for peripherals / displays | External connectors |
| System bus | Data, address and control signals | Internal pathways between components |
| Motherboard | Hosts CPU, RAM, chipset | Main board of the machine |
A technician says two chips on the board exchange information on paths that carry data bits, memory addresses and control lines together. Which named pathway is that description aiming at?
- HDMI — because it carries video "signals"
- The system bus — data, address and control between components
- MICR — because it reads magnetic "signals" on cheques
Data, address and control signals together name the system bus. HDMI is an external display port; MICR is a cheque-reading technology, not an internal bus.
7Devices that do both input and output
Some hardware sits in both classes because it both sends data to the computer and receives data from it. Exam lists that do both include the touchscreen, modem, network card and hard disk — often labelled I/O devices in that sense.
The trap is forcing every device into exactly one of input or output. When a stem describes a device that both reads and writes (or sends and receives), allow the both / I/O answer.
Figure. Some devices are both input and output: touchscreen, modem, network interface.
| Device | Why it is both |
|---|---|
| Touchscreen | Touch is input; display is output |
| Modem | Sends and receives signals |
| Network card | Transmits and receives packets |
| Hard disk | Reads and writes stored data |
A desktop's network interface card sends frames out to a switch and also accepts frames coming back. Relative to pure input or pure output devices, how should that card be classed?
- Input only — frames arrive from the network
- Output only — frames leave toward the network
- Both input and output (I/O) — it transmits and receives
A network card is on the source list of devices that do both. Treating it as input-only or output-only ignores half of its traffic.
Notes
- CPU parts: The ALU performs arithmetic and logic operations, the Control Unit (CU) directs data flow, and registers provide fast temporary storage.
- Input devices feed data in — keyboard, mouse, scanner, microphone, joystick, barcode reader; the OMR/OCR/MICR readers are exam favourites.
- Output devices present results — monitor, printer, speaker, plotter; the monitor is a soft-copy device while the printer gives a hard copy.
- Ports/buses: USB, HDMI and VGA connect peripherals; the system bus carries data, address and control signals between components.
- Motherboard hosts the CPU, RAM, and chipset; MICR is used by banks to read cheque numbers and OMR reads shaded bubbles on OMR sheets.
Formulas
- CPU = ALU + Control Unit + Registers (the processing core of the machine).
- MICR = Magnetic Ink Character Recognition (cheque processing); OMR = Optical Mark Recognition (OMR sheets); OCR = Optical Character Recognition (text scanning).
- Soft copy = on-screen output (monitor); Hard copy = physical output (printer/plotter).
- Impact printers strike the paper (dot-matrix); non-impact printers do not (inkjet, laser).
- Printer resolution is measured in DPI (dots per inch); higher DPI = sharper print.
Exam traps & shortcuts
- Devices that do BOTH input and output: touchscreen, modem, network card, and hard disk (I/O devices).
- Bank cheques -> MICR, Answer sheets -> OMR, Scanning printed text -> OCR: match the acronym to the use-case.
- A light pen, touch screen and joystick are input devices, not output — a common distractor.
Reference tables
The same facts as the concept tables, gathered for revision: CPU parts, reader acronyms, copy types, printer classes and I/O-both devices.
| Topic | Hold this |
|---|---|
| CPU | ALU calculates; CU controls; registers = fast temp store |
| Readers | MICR → cheques; OMR → bubble sheets; OCR → printed text |
| Copy type | Monitor = soft copy; printer/plotter = hard copy |
| Printers | Impact strikes (dot-matrix); non-impact = inkjet/laser; DPI = sharpness |
| Bus / board | System bus = data + address + control; motherboard hosts CPU/RAM/chipset |
| Both I/O | Touchscreen, modem, network card, hard disk |
Recap
If you keep only pegs, keep these.
- CPU core
- CPU = ALU + Control Unit + Registers
- ALU vs CU
- ALU calculates; CU controls (does not compute)
- MICR
- Magnetic Ink Character Recognition — bank cheques
- OMR / OCR
- OMR = shaded bubbles; OCR = scanned printed text
- Soft / hard copy
- Monitor = soft copy; printer/plotter = hard copy
- Plotter
- Output (hard copy) — large drawings and graphs
- Printers
- Impact strikes paper (dot-matrix); non-impact = inkjet/laser
- DPI
- Dots per inch — higher DPI → sharper print
- System bus
- Carries data, address and control signals
- Both I/O
- Touchscreen, modem, network card, hard disk
Practise Hardware & I/O Devices
Reading is free and needs no account. Practice, mocks and progress live in the app.
- 40 exam-style questions on this topic, with explanations
- A 5-question practice set that ends the chapter
- Timed mocks scored with the real marking scheme
- Readiness tracked per topic, kept on your device