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Chapter 5. Managing Storage

5.1.2. Data reading/writing device

The data reading/writing device is the component that takes the bits and bytes on which a computer

system operates and turns them into the magnetic or optical variations necessary to interact with the

materials coating the surface of the disk platters.

Sometimes the conditions under which these devices must operate are challenging. For instance, in

magnetically based mass storage the read/write devices (known as heads) must be very close to the

surface of the platter. However, if the head and the surface of the disk platter were to touch, the

resulting friction would do severe damage to both the head and the platter. Therefore, the surfaces of

both the head and the platter are carefully polished, and the head uses air pressure developed by the

spinning platters to float over the platter's surface, "flying" at an altitude less than the thickness of a

human hair. This is why magnetic disk drives are sensitive to shock, sudden temperature changes, and

any airborne contamination.

The challenges faced by optical heads are somewhat different than for magnetic heads   here, the

head assembly must remain at a relatively constant distance from the surface of the platter. Otherwise,

the lenses used to focus on the platter will not produce a sufficiently sharp image.

In either case, the heads use a very small amount of the platter's surface area for data storage. As the

platter spins below the heads, this surface area takes the form of a very thin circular line.

If this was how mass storage devices worked, it would mean that over 99% of the platter's surface area

would be wasted. Additional heads could be mounted over the platter, but to fully utilize the platter's

surface area over a thousand heads would be necessary. What is required is some method of moving

the head over the surface of the platter.

5.1.3. Access Arms

By using a head attached an arm that is capable of sweeping over the platter's entire surface, it is

possible to fully utilize the platter for data storage. However, the access arm must be capable of two

things:

Moving very quickly

Moving very precisely

The access arm must move as quickly as possible, because the time spent moving the head from one

position to another is wasted time. That is because no data can be read or written until the access arm

stops moving

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The access arm must be able to move with great precision because, as stated earlier, the surface area

used by the heads is very small. Therefore, in order to efficiently use the platter's storage capacity, it

is necessary to move the heads only enough to ensure that any data written in the new position will not

overwrite data written at a previous position. This has the affect of conceptually dividing the platter's

surface into a thousand or more concentric "rings" or tracks. Movement of the access arm from one

track to another is often referred to as seeking, and the time it takes the access arms to move from one

track to another is known as the seek time.

Where there are multiple platters (or one platter with both surfaces used for data storage), the arms

for each surface are stacked, allowing the same track on each surface to be accessed simultaneously.

If the tracks for each surface could be visualized with the access stationary over a given track, they

would appear to be stacked one on top of another, making up a cylindrical shape; therefore, the set of

tracks accessible at a certain postion of the access arms are known as a cylinder.

2. In some optical devices (such as CD ROM drives) the access arm is continually moving, causing the head

assembly to describe a spiral path over the surface of the platter. This is a fundamental difference in how the

storage medium is used and reflects the CD ROM's origins as a medium for music storage, where continuous

data retrieval is a more common operation than searching for a specific data point.