Magnetic Disk

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.

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magnetic disk [mag′ned·ik ′disk]

(astronomy)

disk

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(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.

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Magnetic Disk 

a digital computer memory in which the data carrier is a thin aluminum or plastic disk coated with a layer of magnetic material.

Magnetic disks are 180-1,200 mm in diameter and 2.5-5.0 mm thick; Ni-Co-P or Co-W alloys are used for the magnetic coating. Data are recorded magnetically on the disks in concentric tracks on the working surface and are coded by an address, which indicates the number of the disk and the number of the track. There may be a fixed magnetic head for recording or readout on each track or a single movable head that is common to several tracks and sometimes to several disks. The pickup lever of the selection mechanism, together with the magnetic heads mounted on it, is moved by an electric or pneumatic operating mechanism that moves the heads to any disk and also to any track on a disk. The most common design has “floating” heads.

A magnetic-disk memory usually contains several dozen disks mounted on a common axle, which is turned by an electric motor. One or more disks (a packet) may be replaced, creating disk index files. There may be as many as 100 disks in a memory and 64-5,000 data tracks on each operating surface of a disk; the recording density is 20-130 impulses per millimeter. The data capacity of magnetic disk memories ranges from several tens of thousands up to several billion bits, and the average access time is 10-100 millisec.

Magnetic disks appeared during the mid-1950’s and immediately became widely used because of their excellent technical characteristics. In speed of response they are between immediate-access memories and external storages. They can store an adequate volume of data, the cost per unit of stored information (bit) is low, and their service reliability is excellent.

REFERENCE

Kagan, B. M., V. I. Adas’ko, and R. R. Pure. Zapominaiushchie ustroistva boVshoi emkosti. Moscow, 1968.

D. P. BRUNSHTEIN V. P. ISAEV