A computer is an electronic machine that takes in data, processes it following instructions, and gives out useful information. This chapter looks under the hood — the parts that make a computer work (CBSE 165, Unit 1).
1Characteristics of a computer
Computers are everywhere because of a few standout qualities:
| Characteristic | Means |
|---|---|
| Speed | Performs millions of operations per second |
| Accuracy | Gives error-free results (if the input & instructions are correct) |
| Diligence | Works tirelessly without getting bored or tired |
| Storage | Holds huge amounts of data and recalls it instantly |
| Versatility | Does many different tasks — study, design, games, banking |
| Automation | Carries out a task on its own once instructed |
GIGO — Garbage In, Garbage Out. A computer is accurate only if its input and instructions are correct; wrong input gives wrong output. A computer also has no intelligence of its own — it only follows instructions.
Key points
- Key characteristics: speed, accuracy, diligence, storage, versatility and automation.
- GIGO — wrong input gives wrong output; the computer itself doesn't 'think'.
- A computer needs instructions (a program) to do anything.
2Components & the CPU
A computer system has four kinds of part working together:
- Input devices — take data in (keyboard, mouse).
- CPU — the "brain" that processes data.
- Memory & storage — hold data and programs.
- Output devices — give results out (monitor, printer).
Inside the CPU
The Central Processing Unit (CPU) has three parts:
| Part | Job |
|---|---|
| Control Unit (CU) | Directs all operations — fetches and decodes instructions |
| ALU (Arithmetic Logic Unit) | Does the calculations and comparisons |
| Registers | Tiny, very fast storage the CPU uses while working |
Watch how data flows through the parts — click Next step:
Input
CPU
Control Unit
ALU
Registers
Memory
Output
Key points
- A computer system = input devices + CPU + memory/storage + output devices.
- The CPU is the brain; it has the Control Unit (CU), the ALU and registers.
- CU directs operations, the ALU does calculations/comparisons, registers hold data briefly.
3Memory: primary vs secondary
Computers have two kinds of memory:
| Primary memory | Secondary memory | |
|---|---|---|
| Also called | Main memory | Auxiliary/backup storage |
| Speed | Very fast | Slower |
| Holds | Data/programs being used right now | Data stored permanently |
| Examples | RAM, ROM | Hard disk, pen drive, DVD |
RAM vs ROM
| RAM | ROM | |
|---|---|---|
| Full form | Random Access Memory | Read Only Memory |
| Volatile? | Yes — erased when power is off | No — keeps its contents |
| Used for | Temporary working memory | Permanent startup instructions (BIOS) |
RAM is volatile — switch off the computer and whatever is in RAM is lost. That's why you must save your work to secondary storage.
Key points
- Primary memory (RAM, ROM) is fast and holds what's in use; secondary memory (hard disk, pen drive) stores data permanently.
- RAM is volatile (lost when power is off) and is the working memory; ROM is non-volatile and holds startup instructions.
- Save your work to secondary storage because RAM's contents vanish when the power goes off.
4Storage, input & output devices
| Category | Examples |
|---|---|
| Secondary storage | Hard Disk, CD-ROM, DVD, Pen/Flash drive, Memory stick |
| Input devices (data IN) | Keyboard, Mouse, Scanner, Web camera, Microphone |
| Output devices (results OUT) | Monitor, Printer, Speakers |
A handy rule: input devices let you put information into the computer; output devices bring information out. Some devices do both (a touchscreen takes touch input and shows output).
Storage sizes step up in powers of 1024: 1 KB < 1 MB < 1 GB < 1 TB (kilobyte → megabyte → gigabyte → terabyte).
Key points
- Storage devices: hard disk, CD-ROM, DVD, pen/flash drive, memory stick.
- Input devices put data in (keyboard, mouse, scanner, web camera); output devices give results out (monitor, printer, speakers).
- Storage sizes: 1 KB → 1 MB → 1 GB → 1 TB (each step ×1024).
★ Practical: label your computer
Looking at a computer you use:
- List two input devices and two output devices on it.
- Name the secondary storage it uses (hard disk / SSD / pen drive).
- Explain why your unsaved work is lost if the power suddenly goes off.
- State which CPU part does the calculations.
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