The magnetic disk recorder/reproducer (RD) ordisk unit controller contains the circuitry to control thereading and writing of data on a disk pack. It alsocontrols the interface with the computer. The disk unitcan control from one to four memory units (disk packs).The memory units (MUs) contain only the logiccircuitry to record data on and read data from their owndisk packs. They do not contain controllers. Theyoperate only as slave units to the disk unit controller.MAGNETIC DISK PACKSThe recording medium for a magnetic disk memoryset is a removable disk pack made up of one to over ten14-inch disks, depending on the type.Disk Pack ConstructionThe disks are coated with magnetic iron oxide. Thetop and bottom platters of some disk packs are used asprotection for the inner disks recording surfaces. Thedisk pack comes with a storage canister consisting of atop and bottom cover as shown in figure 10-10. The topcover is used to install the disk pack in the desired diskor memory unit and to remove the disk pack from a unitfor storage. The bottom cover is removed just beforeinstallation of the disk pack and replaced after the diskpack has been removed from a unit to protect the diskpack from physical damage and contamination.Disk Pack Data SurfacesLooking at a disk pack with five platters, the topand bottom platters are used to protect the six insidesurfaces (fig. 2-10). Five of the six inner disk surfacesare used for data storage. Each recording surfacecontains 823 tracks.Of the 823 tracks, 822 areaddressable and can be used for data storage with theremaining track being used for maintenanceapplications. The tracks occupy a 2-inch band aroundthe circumference of the disk’s recording surface. Theindividual tracks are .0026-inches apart. Each track canstore 6,038 BPI with a storage capacity per disk pack (5recording surfaces) of 640 million bits (megabits).Disk Servo SurfaceThe sixth surface, called the servo surface, containsprerecorded dibits used to control the movement of theread/write heads to the desired position (cylinder) onthe recording surfaces, and to maintain alignment of theread/write heads over the centerline of the track. Dibitis an abbreviation of a dipole bit. It is an analog bit witha positive or negative signal used to indicate odd or eventracks on the disk. As the read-only servo head movesFigure 10-10.—A disk pack and storage canister.10-14
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