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Chapter 5. Secondary Storage > 5.2 CLASSIFICATION OF SECONDARY STORAGE DEVICES

5.2 CLASSIFICATION OF SECONDARY STORAGE DEVICES

The secondary storage devices allow permanent storage of data and instructions. The data stored on a secondary storage device can be accessed, depending upon how it is stored on the device. Primarily, there are two methods of accessing data from the secondary storage devices:

  • Sequential: Sequential access means that the computer system must search the storage device from the beginning until the desired data are found. The most common sequential access storage device is magnetic tape, where data are stored and processed sequentially. Suppose, a tape contains information regarding employees of an organization. Now, to look for employee number 100's information, the computer will have to start with employee number 1 and then go past 2, 3 and so on, until it finally comes to 100. The sequential access method is quite simple than other methods but searching for data is slow.

  • Direct: Direct access, also known as random access, means that the computer can go directly to the location, where the data that the user wants are stored. The most common direct access storage devices are magnetic disk and optical disk. In these devices, the data are stored as a numbered sequence of blocks. Thus, one can access block 12, then block 78 and then block 2. The direct access method is ideal for applications like airline reservation systems or computer-based directory assistance operations as in these applications, there is no fixed pattern of the requests for the data (Figure 5.2).


  

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