Chapter 5. Managing Storage

59

5.2. Storage Addressing Concepts

The configuration of disk platters, heads, and access arms make it possible to position the head over

any part of any surface of any platter in the mass storage device. However, this is not sufficient; in

order to use this storage capacity, we must have some method of giving addresses to uniform sized

parts of the available storage.

There is one final aspect to this process that is required. Consider all the tracks in the many cylinders

present in a typical mass storage device. Because the tracks have varying diameters, their circumfer 

ence also varies. Therefore, if storage was addressed only to the track level, each track would have

different amounts of data   track 0 (being near the center of the platter) might hold 10,827 bytes,

while track 1,258 (near the outside edge of the platter) might hold 15,382 bytes.

The solution is to divide each track into multiple sectors or blocks; consistently sized (often 512 bytes)

segments of storage. The result is that each track contains a set number

3

of sectors.

A side effect of this is that every track contains unused space   the space between the sectors. Because

of the constant number of sectors in each track, the amount of unused space varies   relatively little

unused space in the inner tracks, and a great deal more unused space in the outer tracks. In either case,

this unused space is wasted, as data cannot be stored on it.

However, the advantage offsetting this wasted space is that effectively addressing the storage on a

mass storage device is now possible. In fact, there are two methods of addressing   geometry based

addressing, and block based addressing.

5.2.1. Geometry Based Addressing

The term geometry based addressing refers to the fact that mass storage devices actually store data at

a specific physical spot on the storage medium. In the case of the devices being described here, this

refers to three specific items that define a specific point on the device's disk platters:

Cylinder

Head

Sector

The following sections describe how a hypothetical address can describe a specific physical location

on the storage medium.

5.2.1.1. Cylinder

As stated earlier, the cylinder denotes a specific position of the access arm (and therefore, the

read/write heads). By specifying a particular cylinder, we are eliminating all other cylinders,

reducing our search to only one track for each surface in the mass storage device.

Cylinder

Head

Sector

1014

X

X

Table 5 1. Storage Addressing

In Table 5 1, the first part of a geometry based address has been filled in. Two more components to

this address   the head and sector   remain undefined.

3. While early mass storage devices used the same number of sectors for every track, later devices divided

the range of cylinders into different "zones," with each zone having a different number of sectors per track. The

reason for this is to take advantage of the additional space between sectors in the outer cylinders, where there is

more unused space between sectors.