storage A flat rotating disc covered on one or both sides
with some magnetisable material. The two main types are the
Small areas or "zones" on a magnetic disk are magnetised. The
magnetisation is aligned in one of two opposing orientations
with respect to the recording
head. The
head writes data
by altering the alignment and reads data by detecting current
pulses induced in a coil as zones with different magnetic
alignment pass underneath it.
Data is stored on either or both surfaces of discs in
concentric rings called "
tracks". Each track is divided
into a whole number of "
sectors". Where multiple (rigid)
discs are mounted on the same axle the set of tracks at the
same radius on all their surfaces is known as a "
cylinder".
Data is read and written by a
disk drive which rotates the
discs and positions the read/write "
heads" over the desired
track(s). The latter radial movement is known as "
seeking".
There is usually one head for each surface that stores data
though, for increased performance (from reduced
rotationallatency), it is possible, though expensive, to position
multiple heads at equally spaced angles around the discs.
Therefore there are two states that can be detected for each
zone - a change in alignment, or no change.
Ideally a data bit of one or zero can be recorded in each zone
of magnetisation, however, if a zero represents an absence of
magnetic change, the detection of several consecutive zeros
would imply accurately measuring the length of time of the
absence, i.e., measuring the time between two magnetic
changes. However, the variability of motor speed limits the
number of consecutive zeros which can be read reliably.
The best recording methods accurately follow the
characteristics of the magnetic and rotational aspects in
recording the disk, to be as dense as possible in recording
bits.
(2003-03-10)