Storage Devices

Any storage unit of a computer system is ranked according to the following criteria :

1. Access time : This is the time required to locate and retrieve stored data from the storage unit in response to a program instruction. A fast access time is preferred.

2. Storage capacity : It is the amount of data that can be stored in the storage unit. A large capacity is desired.

3. Cost per bit of storage. An obvious goal is to minimize this cost.

There are two types of storage devices :

1. Primary storage device. 

2. Secondary storage device.

1. Primary Storage Device :

Primary storage is usually referred to as Random Access Memory (RAM) because it is possible to randomly select and use any location of this memory to directly store and retrieve data and instructions. It is also referred to as read/write memory because information can be 'read' from a RAM chip and can also be 'written' into it. The Random Access Memory requires some time for accessing data and is considered to be the fastest memory. Each bit in a RAM stores information by means of electric charge, where the presence of an electric charge indicates '1' and the absence as '0'. RAM is called as Temporary storage, because data persists until the machine is on.

2. Secondary Storage Device :

The storage capacity of the primary storage of today's computers is not sufficient to store the large volume of data handled by most data processing centres. As a result, additional memory, called auxiliary memory or secondary storage, is used with most computer systems. This section of the computer's memory is characterised by low cost per bit stored, but it generally has an operating speed far slower than of the primary storage. This section of the memory is also referred to as backup storage because it is used to store large volume of data on a permanent basis which can be partially transferred to the primary storage as and when required for processing.