Page 226 - Electrical Engineering Dictionary

change data, control, and status signals with disk platter metal disks covered with a
an input/output module, as required for the magnetic material for recording information.
proper reading or writing of data to or from
a disk. The head disk assembly plus all the disk scheduling algorithms used to re-
associated electronics. duce the total mechanical delays in disk ac-
cesses, as seen by a queue of simultane-
disk format the (system-dependent) man- ous I/O requests. E.g., if a “shortest-seek-
ner in which a track of a disk is partitioned time-first” scheduling algorithm is used, seek
so as to indicate for each sector: the identity, times can be reduced. That is, among the
the start and end, synchronization informa- queue of pending I/O requests, the one next
tion, error-checking information, etc. A disk serviced is the one requiring the shortest
must be formatted before any initial writing seek time from the current location of the
can take place. read/write head. The disk scheduling algo-
rithm is run by the computer operating sys-
disk head read/write head used in a disk tem.
drive. Such a head may be fixed-gap, in
which the head is positioned at a fixed dis- disk sector the smallest unit that can be
tance from the disk surface; contact-head, in read or written on a track; adjacent sectors
which the head is always in contact with the are separated by a gap. A typical track has
surface; and aerodynamic, in which the head 10–100 sectors.
rests lightly on the surface when the disk is
motionless but floats a small distance above
disk spindle a stack of disk platters.
when the tape is rotating. Typically, contact
heads are used in floppy disks, and aerody-
disk striping the notion of interleaving
namic head are used in Winchester disks.
data across multiple disks at a fine grain, so
In earlier systems, “fixed-head disks” hav-
that needed data can be accessed from all the
ing one read/write head per track were used
disks simultaneously, thus providing much
in some disk systems, so the seek time was
higher effective bandwidth.
eliminated. However, since modern disks
have hundreds of tracks per surface, placing
disk track connectric circle over which
a head at every track is no longer considered
a read/write head moves; adjacent tracks are
an economical solution.
separated by a gap. A typical disk has hun-
dreds to thousands of tracks.
disk latency time between positioning a
read/write head over a track of data and when
the beginning of the track of data passes un- diskette a floppy disk is a flexible plastic
diskette coated with magnetic material. It is
der the head.
a smaller, simpler, and cheaper form of disk
storage media than the hard disk, and also
disk operating system (DOS) a set of
procedures, services, and commands that can easily removed for transportation.
be used by the computer user for managing
its resources with a special attention to disks disocclusion the uncovering of an object.
managing. The most famous DOS for per- See occlusion.
sonal computers is the MS-DOS (Microsoft-
DOS). It is a mono-task and mono-user op- disparity in binocular vision, the relative
erating system. difference of position of an object with re-
spect to a background between the left and
disk pack a stack of disk platters that can right images. It is usually measured in min-
be removed for off-line storage. utes of arc.

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2000 by CRC Press LLC