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190 CHAPTER 6 / EXTERNAL MEMORY

Read–write head (1 per surface) Direction of
arm motion

Surface 9
Platter
Surface 8
Surface 7

Surface 6
Surface 5

Surface 4
Surface 3

Surface 2
Surface 1

Surface 0
Spindle Boom
Figure 6.5 Components of a Disk Drive

Some disk drives accommodate multiple platters stacked vertically a fraction
of an inch apart. Multiple arms are provided (Figure 6.5). Multiple–platter disks em-
ploy a movable head, with one read-write head per platter surface. All of the heads
are mechanically fixed so that all are at the same distance from the center of the
disk and move together.Thus, at any time, all of the heads are positioned over tracks
that are of equal distance from the center of the disk.The set of all the tracks in the
same relative position on the platter is referred to as a cylinder. For example, all of
the shaded tracks in Figure 6.6 are part of one cylinder.
Finally, the head mechanism provides a classification of disks into three types.
Traditionally, the read-write head has been positioned a fixed distance above the

Figure 6.6 Tracks and Cylinders