Computing Dictionary
magnetic disk storage
A flat rotating disc covered on one or both sides with magnetisable material. The two main types are the hard disk and the floppy disk.
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. To reduce rotational latency it is possible, though expensive, to have multiple heads at different angles.
The head writes binary data by magnetising small areas or "zones" of the disk in one of two opposing orientations. It reads data by detecting current pulses induced in a coil as zones with different magnetic alignment pass underneath it.
In theory, bits could be read back as a time sequence of pulse (one) or no pulse (zero). However, a run of zeros would give a prolonged absence of signal, making it hard to accurately divide the signal into individual bits due to the variability of motor speed. Run Length Limited is one common solution to this clock recovery problem.
High speed disks have an access time of 28 milliseconds or less, and low-speed disks, 65 milliseconds or more. The higher speed disks also transfer their data faster than the slower speed units.
The disks are usually aluminium with a magnetic coating. The heads "float" just above the disk's surface on a current of air, sometimes at lower than atmospheric pressure in an air-tight enclosure. The head has an aerodynamic shape so the current pushes it away from the disk. A small spring pushes the head towards the disk at the same time keeping the head at a constant distance from the disk (about two microns).
Disk drives are commonly characterised by the kind of interface used to connect to the computer, e.g. ATA, IDE, SCSI.
See also winchester. Compare magnetic drum, compact disc, optical disk, magneto-optical disk.
Suchanka's PC-DISK library.
(2007-06-14)
