disk drive definition
<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. ST506 used to be common in microcomputers
(in the 1980s?).
(1997-04-15)
Nearby terms:
Disjunctive Normal Form « disk « disk controller « disk drive » disk duplexing » diskette » disk farm
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