Hardware and software
Hardware is every physical part of a computer: CPU, keyboard, monitor, RAM, hard disk. You can touch it.
Software is a set of programs (instructions) that tell the hardware what to do: Windows, Android, Python, a web browser. You cannot touch it.
Hardware without software is a dead box. Software without hardware has nowhere to run. A computer system needs both, plus data and a user.
Input and output devices
An input device sends data or commands into the computer: keyboard, mouse, microphone, scanner, webcam, touch screen, barcode reader, biometric (fingerprint) sensor.
An output device gives the result to the user: monitor, printer, speaker, projector, headphones, plotter, 3D printer.
Some devices do both, such as a touch screen (you touch = input, it shows = output).
All devices send and receive data as binary (0s and 1s).
The CPU (Central Processing Unit)
The CPU is the brain of the computer. It is also called the processor. It has:
- ALU (Arithmetic Logic Unit): does +, −, ×, ÷ and comparisons like >, <, =.
- Control Unit (CU): fetches each instruction, understands it and tells the other parts what to do.
- Registers: very small, very fast storage places inside the CPU for data being used this very moment.
A CPU with many cores can work on many tasks at the same time. Its speed is measured in GHz.
Primary, cache and secondary memory
Primary memory is the main memory the CPU reads directly.
- RAM (Random Access Memory): holds the programs and data in use now. It is volatile: it empties when power goes.
- ROM (Read Only Memory): holds start-up instructions (like BIOS). It is non-volatile.
Cache memory sits between the CPU and RAM. It keeps copies of the most used data, so the CPU does not wait for slower RAM.
Secondary memory stores data permanently: hard disk (HDD), SSD, pen drive, memory card, CD/DVD. It is big and cheap but slower. The CPU cannot use it directly; data is first loaded into RAM.
Speed order (fastest first): registers → cache → RAM → secondary storage.
Memory units from bit to petabyte
A bit (binary digit) is 0 or 1, the smallest unit.
| Unit | Equals |
|---|---|
| 1 nibble | 4 bits |
| 1 byte | 8 bits |
| 1 KB (kilobyte) | 1024 bytes |
| 1 MB (megabyte) | 1024 KB |
| 1 GB (gigabyte) | 1024 MB |
| 1 TB (terabyte) | 1024 GB |
| 1 PB (petabyte) | 1024 TB |
Going to a bigger unit: divide by 1024. Going to a smaller unit: multiply by 1024. 1024 = 210.
Board exam focus
Usual questions (1–3 marks): difference between RAM and ROM, primary vs secondary memory, why cache is needed, parts of the CPU, and unit conversions like "how many bits in 2 KB?". Always show the ×1024 steps.
Key formulas and definitions
- 1 nibble = 4 bits; 1 byte = 8 bits
- 1 KB = 1024 B; 1 MB = 1024 KB; 1 GB = 1024 MB
- 1 TB = 1024 GB; 1 PB = 1024 TB
- Speed: registers > cache > RAM > secondary storage
- CPU = ALU + Control Unit + registers
Worked examples
1. How many bits are there in 2 KB?
2 KB = 2 × 1024 = 2048 bytes. Bits = 2048 × 8 = 16384 bits.
2. Convert 3 GB to MB and to KB.
3 GB = 3 × 1024 = 3072 MB. 3072 MB = 3072 × 1024 = 3,145,728 KB.
3. A file is 5120 KB. How many MB is it?
Going to a bigger unit, divide: 5120 ÷ 1024 = 5 MB.
4. Classify: scanner, plotter, touch screen, SSD, webcam, projector.
Input: scanner, webcam. Output: plotter, projector. Both: touch screen. Storage (secondary memory): SSD.
5. Why is cache memory placed between the CPU and RAM?
The CPU is much faster than RAM. Cache keeps a copy of the most used data right next to the CPU, so the CPU gets it quickly instead of waiting for RAM. This makes the whole computer faster.
6. How many 4 GB videos can fit on a 1 TB disk?
1 TB = 1024 GB. 1024 ÷ 4 = 256 videos.
Common mistakes
- Using 1000 instead of 1024 in CBSE unit conversions. In this syllabus 1 KB = 1024 bytes.
- Saying RAM keeps data after the computer is switched off. RAM is volatile; data is lost.
- Calling the hard disk "memory of the CPU". The CPU cannot read the hard disk directly; data is first loaded into RAM.
- Forgetting that a nibble is 4 bits (half a byte), not 4 bytes.