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Unformatted text preview: ess devices are also called random-access devices because the information is literally available at random, i.e., it is available in any order. Thus a random-access storage device is one in which any location in the device may be selected at random, access to the information stored is direct, and approximately equal access time is required for each location. This is analogous to a music CD. If 10 songs are recorded on a music CD and if you want to listen to (or access) the 6 th song, you need not listen to or fast forward the 5 songs that come before your favorite song. All you have to do is select track number 6 on the player and the player directly moves the pickup arm across the CD to the groove where the 6 th song begins and starts playing. Magnetic, optical and magneto-optical disks of various types are typical examples of direct-access storage devices. MAGNETIC TAPE Magnetic tape is one of the oldest forms of secondary storage device. It is one of the most popular storage medium for large data that are sequentially accessed and processed or that are not used very often and need to be stored off-line (not directly accessible to the computer system). It is also used as a backup storage (additional copy of data kept away from the computer system) for data stored on on-line storage devices such as a hard disk. The magnetic tape medium is a plastic ribbon usually XA inch or % inch wide and 50 to 2400 feet long. It is coated with a magnetizable recording material such as iron-oxide or chromium dioxide. Data are recorded on the tape in the form of tiny invisible magnetized and non-magnetized spots (representing Is and 0s) on the coated surface of the tape. The tape ribbon is itself stored in reels or a small cartridge or cassette. Basic Principles of Operation Storage Organization The tape of a magnetic tape storage is divided into vertical columns called frames and horizontal rows called channels or tracks. As shown in Figure 8.2, older tapes had 7 tracks and they used the 6-bit BCD code format f...
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This document was uploaded on 04/07/2014.

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