The disks used by large computers- mainframes, super-minis, and supercomputers- are commonly 14-inch-diameter aluminium platters that are coated on both sides with a magnetizable substance such as ferrous oxide.
Records are stored in concentric rings, or tracks. Characters are represented by binary bits, which appear as magnetic fields on the tracks. Each track may consist of several records.
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On most disk systems that serve large computers, each track is designed to carry the same total amount of data even though the tracks near the outer edge are much longer than those nearer the center and move past the read/write heads faster.
This design constraint keeps the data transfer rate constant throughout the system, independent of the location of the track being accessed. The number of tracks per disk varies among manufacturers, but it is typically several hundred.
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Data are read or written by a read/write head, which moves above or below the spinning disk to access the disk tracks.
Disks on disk systems designed for large computers often are assembled into groups of six, eight, ten, twelve, or some other number, depending on the manufacturer, and mounted on a shaft that spins all the disks at the same rate of speed.
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The disks are spaced far enough apart to permit the read/write heads to move in and out between the disks. Such an assembly is called a disk pack. Depending upon the disk system in use, packs may be removable or no removable.
The removable disk pack is encased in a plastic shell, similar to a cake cover, to protect the recording surfaces from foreign objects. On removable-pack systems, the top and bottom surfaces of the pack aren’t used, because they are the ones most exposed to dust.
Nonremovable-pack disk systems use hermetically sealed packs and are much less vulnerable to exposure to foreign matter. Disk packs function on a device called a disk unit. These units are plugged into the computer, enabling it t access any of the data recorded on the disks.
