Memory Units

Memory Units:-

There are two kinds of computer memory: primary and secondary. Primary memory is accessible directly by the Central Processing Unit. RAM is an example of primary memory. You will know more about RAM later in this lesson. As soon as the computer is switched off, the primary memory loses its contents. But data can be stored and retrieved at much faster rate with primary memory as compared to secondary memory. Examples of secondary memory storage devices are floppy disk, magnetic disk, hard disk, CD, pen drive etc. The secondary memory devices may be inside or located outside the computer. Primary memory is more expensive than secondary memory.

When the computer is doing any task, the data that have to be processed are stored in the primary memory. This data may come from an input device like keyboard or from a secondary storage device like a hard disk. As program or the set of instructions is kept in primary memory, the computer is able to follow them instantly. For example, when you book ticket from railway reservation counter, the computer has to follow some steps, viz., take the request, check the availability of seats, calculate fare, wait for money to be paid, save the reservation and print the ticket. The program containing these steps is kept in primary memory of the computer.

 But inside the computer, the steps followed are quite different from what we see on the monitor or screen. In computer’s memory both programs and data are stored in the binary form. You have already been introduced with decimal number system that is the numbers 0 to 9. The binary system has only two digits, 0 and 1. These are called bits. A bit is an acronym for binary digit, which stands for one binary piece of information. As human beings we all understand decimal system but the computer can only understand binary system. Consider that a computer consists of numerous switches. If a switch is ON then it is considered as 1 otherwise it is considered as 0. A number of switches in different states will give you a message like this: 110101......10. So the computer takes input in the form of 0 and 1 and gives output in the same form, i.e., 0 and 1 only. But we do not receive output in the form of 0’s & 1’s only. This is because every number stored in binary form can be converted to decimal system and vice versa. For example, 1010 in binary is equivalent to 10 in decimal. Therefore, it is the computer that takes information or data in decimal form from you, converts it into binary form, processes it, produces output in binary form and finally again converts the output to decimal form.

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Measuring Memory

The primary or internal storage unit is made up of several small storage locations called cells. Each of these cells can store a fixed number of bits called word length. Each cell has a unique number assigned to it called the address of the cell and it is used to identify the cells. Hence we can say that memory is like a large cabinet containing as many drawers as there are addresses on memory. Each drawer contains a word and the address is written on outside of the drawer. You know that data in computer is stored in the form of 0s and 1s. Each of these digits is known as a bit. A collection of 8 bits constitutes a byte. Each cell of memory contains one character or 1 byte of data. So the capacity is defined in terms of bytes or words. However higher units of memory are Kilobytes, Megabytes, Gigabytes etc. 1 Kilobyte is equal to 1024 bytes. Thus 64 Kilobyte (KB) memory is capable of storing 64 × 1024 = 32,768 bytes.

Memory units are used to measure and represent data. Some of the commonly used memory units are:

1) Bit: The computer memory units start from bit. A bit is the smallest memory unit to measure data stored in main memory and storage devices.  A bit can have only one binary value out of 0 and 1.

2) Byte: It is the fundamental unit to measure data. It contains 8 bits or is equal to 8 bits. Thus a byte can represent 2*8 or 256 values.

3) Kilobyte: A kilobyte contains 1024 bytes.

4) Megabyte: A megabyte contains 1024 kilobytes.

5) Gigabyte: A gigabyte contains 1024 megabyte.

6) Terabyte: A terabyte contains 1024 gigabytes.

Other units of memory are listed in the table below:

Table  1. shows different measurement units for digital data stored in storage devices.

A computer system uses binary numbers to store and process data. The binary digits 0 and 1, which are the basic units of memory, are called bits. Further, these bits are grouped together to form words. A 4-bit word is called a Nibble. Examples of nibble are 1001, 1010, 0010, etc. A two nibble word, i.e., 8-bit word is called a byte, for example, 01000110, 01111100, 10000001, etc. Like any other standard unit, bytes are grouped together to make bigger chunks or units of memory.