Hence the reading operation is non destructive of

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Unformatted text preview: rom one computer to another, which are not linked together. 5. Distribution of software by vendors. Originally sold software or software updates are often distributed by vendors on magnetic tapes. MAGNETIC DISK Magnetic disks are the most popular medium for direct-access secondary storage. Because of their random access capability, they are the most popular on-line secondary storage devices. A magnetic disk is a thin, circular plate/platter made of metal or plastic that is usually coated on both sides with a magnetizable recording material sych as iron-oxide. Data are recorded on the disk in the form of tiny invisible magnetized and non-magnetized spots (representing Is and Os) on the coated surfaces of the disk. A standard binary code, usually 8-bit EBCDIC, is used for recording data. The disk itself is stored in a specially designed protective envelope or cartridge, or several of them may be stacked together in a sealed, contamination-free container. Like magnetic tapes, magnetic disks can also be erased and reused indefinitely. Old data on a disk are automatically erased as new data are recorded in the same area. However, the information stored can be read many times without affecting the stored data. Hence the reading operation is non-destructive of stored data, and the data stored on a magnetic disk remains indefinitely until they are erased or over written at a future time. Basic Principles of Operation Storage Organization For data recording, the surface of a disk is divided into a number of invisible concentric circles called tracks. As shown in Figure 8.13, the tracks are numbered consecutively from outermost to innermost starting from zero. The number of tracks varies greatly between disks, from as few as 40 on some small, low-capacity disks to several thousand on large, high-capacity disks. Each track is further subdivided into sectors. For this, in addition to the concentric circles, the disk surface is also divided into invisible pie-shaped segments (see Figure...
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