magnetic disk
    n 1: (computer science) a memory device consisting of a flat
         disk covered with a magnetic coating on which information
         is stored [syn: magnetic disk, magnetic disc, disk,
         disc]

magnetic disc \magnetic disc\, magnetic disk \magnetic disk\n.
   A ditical memory device consisting of a flat disk covered
   with a magnetic coating on which information is stored; a
   hard disk, floppy disk, and diskette are typically
   magnetic disks.

   Syn: disk, disc.
        [WordNet 1.5]

magnetic disk

    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)