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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 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
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.
- Spring '14