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Chapter 5. Managing Storage

5.2.1.2. Head

Although in the strictest sense we are selecting a particular disk platter, because each surface has a

read/write head dedicated to it, it is easier to think in terms of interacting with a specific head. In

fact, the device's underlying electronics actually select one head and   deselecting the rest   only

interact with the selected head for the duration of the I/O operation. All other tracks that make up the

current cylinder have now been eliminated.

Cylinder

Head

Sector

1014

2

X

Table 5 2. Storage Addressing

In Table 5 2, the first two parts of a geometry based address have been filled in. One final component

to this address   the sector   remains undefined.

5.2.1.3. Sector

By specifying a particular sector, we have completed the addressing, and have uniquely identified the

desired block of data.

Cylinder

Head

Sector

1014

2

12

Table 5 3. Storage Addressing

In Table 5 3, the complete geometry based address has been filled in. This address identifies the

location of one specific block out of all the other blocks on this device.

5.2.1.4. Problems with Geometry Based Addressing

While geometry based addressing is straightforward, there is an area of abiguity that can cause prob 

lems. The ambiguity is in numbering the cylinders, heads, and sectors.

It is true that each geometry based address uniquely identifies one specific data block, but that only

applies if the numbering scheme for the cylinders, heads, and sectors is not changed. If the numbering

scheme changes (such as when the hardware/software interacting with the storage device changes),

then all bets are off.

Because of this potential for ambiguity, a different approach to addressing was developed. The next

section describes it in more detail.

5.2.2. Block Based Addressing

Block based addressing is much more straightforward than geometry based addressing. With block 

based addressing, every data block is given a unique number. This number is passed from the computer

to the mass storage device, which then internally performs the conversion to the geometry based

address required by the device's control circuitry.

Because the conversion to a geometry based address is always done by the device itself, it will always

be consistent, eliminating the problem inherent with giving the device geometry based addressing.