magnetic disk
The primary computer storage device. Like tape, it is magnetically recorded and can be re-recorded over and over. Disks are rotating platters with a mechanical arm that moves a read/write head between the outer and inner edges of the platter's surface. It can take as long as one second to find a location on a floppy disk to as little as a couple of milliseconds on a fast hard disk. See hard disk for more details.
Tracks and Spots
The disk surface is divided into concentric tracks (circles within circles). The thinner the tracks, the more storage. The data bits are recorded as tiny magnetic spots on the tracks. The smaller the spot, the more bits per inch and the greater the storage.
Sectors
Tracks are further divided into sectors, which hold a block of data that is read or written at one time; for example, READ SECTOR 782, WRITE SECTOR 5448. In order to update the disk, one or more sectors are read into the computer, changed and written back to disk. The operating system figures out how to fit data into these fixed spaces.
Modern disks have more sectors in the outer tracks than the inner ones because the outer radius of the platter is greater than the inner radius (see CAV). See magnetic tape and optical disc.
 |
| Tracks and Sectors |
|---|
| Tracks are concentric circles on the disk, broken up into storage units called "sectors." The sector, which is typically 512 bytes, is the smallest unit that can be read or written. |
Magnetic Disk Summary
The following magnetic disk technologies are summarized below. Several have been discontinued, but are often still used long after their official demise. Media tend to be made for many years thereafter.
DISCONTINUED TECHNOLOGIES
 |
| The Early 1990s |
|---|
| This RAID II prototype in 1992, which embodies principles of high performance and fault tolerance, was designed and built by University of Berkeley graduate students. Housing 36 320MB disk drives, its total storage was less than the disk drive in the cheapest PC only six years later. (Image courtesy of The Computer History Museum, www.computerhistory.org) See RAID. |
| (storage) | 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. | |
