It is desirable that operating speed of primary storage of a computer system be as fast as possible since most of the data transfer to and from processing unit is via main memory. For this reason storage devices with fast access times like semiconductors are normally used for design of primary storage. These high-speed storage devices are costly and therefore cost per bit of storage is also high for a primary storage. However primary memory has following limitations:

a)  Limited capacity: Storage capacity of primary storage of today's computers is not sufficient to store large volume of data handled by most of data processing organisations.

b)  Volatile: Primary storage is volatile and data stored in it is lost when electric power is turned off. But computer systems need to store data on a permanent basis for different days and months or even several years.