CBSE Class 7 Computer Science Computer Hardware Input Devices, Memory and Storage Notes Set 02

Hardware and Software

Computer hardware means all the parts of a computer that you can touch, such as the keyboard, mouse, monitor and storage disk. Software is the set of instructions that tells the hardware what to do. A computer needs both to work, just as a body needs both organs and a brain giving directions.

Input Devices

An input device sends data or instructions from the user into the computer. Common input devices are given below.

1. Keyboard: The most common input device, used to type text, numbers and commands. A standard keyboard has about 101 to 106 keys. The most widely used key arrangement is QWERTY, named after the first six letters of the top row of letters. This layout was designed by Christopher Latham Sholes for typewriters. Other layouts include AZERTY, QWERTZ and Dvorak.

2. Mouse: A hand-held pointing device moved on a flat surface. It moves a pointer on the screen, and its buttons and scroll wheel are used to click and scroll. It was invented by Douglas Engelbart.

3. Joystick: A pointing device in which the pointer keeps moving in the direction the stick is tilted, until the stick is returned to its upright position. It is common in games.

4. Light pen: A pen-shaped device with a light-sensitive tip used to select or draw on the screen.

5. Scanner: Converts printed pictures and text into digital form.

6. Optical Mark Reader (OMR): Detects dark marks on paper. It is used to check objective answer sheets.

7. Optical Character Reader (OCR): Recognises printed characters from a document and converts them into editable text.

8. Magnetic Ink Character Reader (MICR): Used by banks to read the special magnetic ink printed on cheques.

9. Bar code reader: Reads the pattern of vertical bars of different widths printed on products.

10. Microphone: Takes in sound and changes it into digital form.

11. Digital camera and web camera: A digital camera stores pictures as digital files, and its picture quality is measured in pixels. A web camera sends live video to a computer or network.

Memory

Memory is where a computer keeps data and instructions. It is of two kinds, primary and secondary.

Primary memory is the main memory used directly by the processor and has limited capacity. It has two parts. RAM (Random Access Memory) holds the programs and data being used at the moment. It is volatile, which means its contents are lost when the power is switched off. ROM (Read Only Memory) holds fixed start-up instructions, such as the BIOS, and keeps them even without power, so it is non-volatile. Cache memory is a very small, very fast memory placed close to the processor to speed up work.

Secondary memory stores data permanently. Its examples are the hard disk, CD, DVD, Blu-ray disc, pen drive and memory card.

1. Hard disk: Spinning metal platters coated with magnetic material, with read/write heads. It has large capacity, measured in hundreds of GB or in TB.

2. Optical discs read and written by laser: CD (about 700 MB), DVD (about 4.7 GB on a single layer) and Blu-ray (25 GB on a single layer, 50 GB on a double layer).

3. Flash memory: Pen drives and memory cards have no moving parts. A pen drive is small, light and plugs into a USB port. Memory cards such as the micro SD are used in cameras and phones.

Units of Storage

A computer works with two digits, 0 and 1, called bits. 4 bits make a nibble and 8 bits make a byte. Larger units go up in steps of 1024:

1. 1 KB (kilobyte) = 1024 bytes

2. 1 MB (megabyte) = 1024 KB

3. 1 GB (gigabyte) = 1024 MB

4. 1 TB (terabyte) = 1024 GB

Worked Examples

Example 1. How many bytes are in 2 KB? Answer: 2 × 1024 = 2048 bytes.

Example 2. How many MB are in 4 GB? Answer: 4 × 1024 = 4096 MB.

Example 3. A teacher wants to check 200 objective answer sheets quickly. Which device should she use? Answer: An OMR, because it detects the marked circles automatically.

Quick Check Questions