<
hardware, storage> (Or "hard
disk drive", "hard
drive",
"floppy
disk drive", "floppy
drive") A
peripheral device
that reads and writes
hard disks or
floppy disks. The
drive contains a motor to rotate the
disk at a constant rate
and one or more read/write heads which are positioned over the
desired
track by a servo mechanism. It also contains the
electronics to amplify the signals from the heads to normal
digital logic levels and vice versa.
In order for a
disk drive to start to read or write a given
location a read/write head must be positioned radially over
the right track and rotationally over the start of the right
sector.
Radial motion is known as "
seeking" and it is this which
causes most of the intermittent noise heard during
disk
activity. There is usually one head for each
disk surface and
all heads move together. The set of locations which are
accessible with the heads in a given radial position are known
as a "
cylinder". The "
seek time" is the time taken to
seek to a different cylinder.
The
disk is constantly rotating (except for some
floppy disk
drives where the motor is switched off between accesses to
reduce wear and power consumption) so positioning the heads
over the right sector is simply a matter of waiting until it
arrives under the head. With a single set of heads this
"
rotational latency" will be on average half a revolution
but some big drives have multiple sets of heads spaced at
equal angles around the
disk.
If seeking and rotation are independent, access time is seek
time + rotational latency. When accessing multiple tracks
sequentially, data is sometimes arranged so that by the time
the seek from one track to the next has finished, the
disk has
rotated just enough to begin accessing the next track.
See also
sector interleave.
The disks may be
removable disks; floppy disks always are,
removable hard disks were common on
mainframes and
minicomputers but less so on
microcomputers until the mid
1990s(?) with products like the
Zip Drive.
A
CD-ROM drive is not usually referred to as a
disk drive.
Two common interfaces for
disk drives (and other devices) are
SCSI and
IDE.
ST-506 used to be common in
microcomputers (in the 1980s?).
(1997-04-15)