Page 59 - UNI 101 Computer Science Handout.

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capacity of the disk by utilizing the space at the outer tracks to pack more bits onto the disk. But this

would result in a different number of bytes per sector or a different number of sectors per track

depending on which track is being accessed.

Figure 2 A Hard Disk Layout.

This would make it more difficult to locate the required sector. Notice, too, that with a constant speed
motor, the time to move the head over a pie-shaped sector at the edge is the same as that near the

center. If there were more bits packed into the outer tracks, the data would be transferred faster at the

edge than at the center. Since the disk controller is designed to expect data at a constant speed, it would

seem to be necessary to design the motor so that it would slow down when the head was accessing the
outer tracks to keep the data transfer speed constant. In this case, the motor speed would be adjusted

such that the speed along the track would be constant regardless of the position of the head. This

approach is called CLV, for constant linear velocity. The capacity of a CLV disk with the same diameter

and bit density is approximately double that of an equivalent CAV disk. Although CLV technology is
commonly used with CDs and DVDs, the design makes it more difficult to access individual blocks of data

rapidly, so it is rarely used for hard disks. As a compromise, modern disk drives divide the disk into a

number of zones, typically sixteen. This approach is shown in Figure below. The cylinders in different

zones have a different number of sectors but the number of sectors within a particular zone is constant.

59 Academic Year 2025/2026