CBSE Class 5 Computer Science Storage and Memory Devices Notes Set 01

Why a computer needs memory

A school library may hold thousands of books and fill a whole room. A computer can keep the same amount of information on a device small enough to fit in a pocket. To do this, a computer saves data and information in its memory or in storage devices, and brings them back whenever we need them. Saving things on a computer also saves paper and so helps to protect trees and other natural resources.

Data and information

Data is a collection of raw facts, such as numbers, names or marks, that has no clear meaning on its own. When data is organised and processed, it becomes information, which has a meaning we can use. For example, the numbers 45, 38 and 50 are data. When we say 'the class average is 44.3', that is information. Information for one person can be data for another, because the next person may process it further.

How memory is measured

A computer understands only two digits, 0 and 1. Each digit is called a bit. A group of 8 bits is one byte, and a byte can store one letter or number. Larger units are formed from bytes.

1. 8 bits = 1 byte
2. 1 KB (kilobyte) = 1,024 bytes
3. 1 MB (megabyte) = 1,024 KB
4. 1 GB (gigabyte) = 1,024 MB
5. 1 TB (terabyte) = 1,024 GB

Types of memory

Computer memory is divided into two main kinds: primary memory and secondary memory.

Primary memory

Primary memory is the internal storage inside the computer that the CPU uses directly. It is also called main memory. It is fast, so the computer can find the data and instructions it needs quickly. It has two parts.

1. RAM (Random Access Memory): holds the data and programs we are using right now. It is volatile, which means everything in it disappears when the computer is switched off. RAM comes in sizes such as 4 GB, 8 GB and 16 GB.
2. ROM (Read Only Memory): holds the instructions that the computer needs to start itself. It is non-volatile, so the data stays even when the power is off. We can only read it, not change it in normal use.

Secondary memory (storage)

Secondary memory is where we keep our work permanently. It is non-volatile and has a much larger capacity than primary memory. The files stay there until we delete them.

1. Hard disk: a high-capacity disk inside the computer. The operating system, programs and our saved files are kept on it. Capacities are 500 GB, 1 TB, 2 TB and more.
2. Pen drive: a small, portable device used to carry files from one computer to another. It plugs into the USB port.
3. CD (Compact Disc): a round optical disc that stores up to about 700 MB.
4. DVD (Digital Versatile Disc): looks like a CD but stores more, about 4.7 GB on a single layer.
5. Blu-ray disc: an optical disc that stores 25 GB, or 50 GB on a two-layer disc, and is used for high-quality films.
6. Memory card: a tiny card used in cameras, phones and music players to store photos, videos and songs.

Primary and secondary memory compared

Primary memory is fast, small in size and holds data only while the work is going on (RAM) or holds start-up instructions (ROM). Secondary memory is slower, much bigger, and keeps data permanently. When we open a file, the computer copies it from the secondary memory to the RAM so the CPU can work on it.

Worked examples

Example 1. How many bytes are there in 2 KB?
1 KB = 1,024 bytes, so 2 KB = 2 × 1,024 = 2,048 bytes.

Example 2. A pen drive has 16 GB of space. How many MB is this?
1 GB = 1,024 MB, so 16 GB = 16 × 1,024 = 16,384 MB.

Example 3. A DVD stores 4.7 GB and a CD stores 700 MB. Which holds more?
Change 4.7 GB to MB: 4.7 × 1,024 = 4,812.8 MB. This is far more than 700 MB, so the DVD holds more, almost 7 times as much.

Remember

1. Data is raw facts; information is processed data.
2. 1 byte = 8 bits and the other units go up by 1,024 each time.
3. RAM is temporary (volatile); ROM and secondary memory are permanent.
4. Always save your work to the secondary memory before you switch off the computer.

Quick Check Questions

Answer in one or two sentences.