magnetic disk
<storage>
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.Last updated: 2007-06-14
Magnetic Ink Character Recognition
(MICR) A character recognition system using special ink and characters which can be magnetised and read automatically.
MICR is used almost exclusively in the banking industry where it is used to print details on cheques to enable automatic processing.Last updated: 1995-04-13
magnetic stripe
<storage>
A black stripe, printed on the back of a credit card or similar, that stores a machine-readable copy of the information on the card. The stripe contains iron particles about 500 nanometers long that can be magnetised like magnetic tape. The data can be read by swiping the card through a card reader.
Last updated: 2007-06-04
magnetic tape
<storage>
(Or "magtape", "tape" - paper tape is now obsolete) A data storage medium consisting of a magnetisable oxide coating on a thin plastic strip, commonly used for backup and archiving.
Early industry-standard magnetic tape was half an inch wide and wound on removable reels 10.5 inches in diameter. Different lengths were available with 2400 feet and 4800 feet being common. DECtape was a variation on this "round tape". In modern magnetic tape systems the reels are much smaller and are fixed inside a cartridge to protect the tape and for ease of handling ("square tape" - though it's really rectangular). Cartridge formats include QIC, DAT, and Exabyte. Tape is read and written on a tape drive (or "deck") which winds the tape from one reel to the other causing it to move past a read/write head. Early tape had seven parallel tracks of data along the length of the tape allowing six bit characters plus parity written across the tape. A typical recording density was 556 characters per inch. The tape had reflective marks near its end which signaled beginning of tape (BOT) and end of tape (EOT) to the hardware. Data is written to tape in blocks with inter-block gaps between them. Each block is typically written in a single operation with the tape running continuously during the write. The larger the block the larger the data buffer required in order to supply or receive the data written to or read from the tape. The smaller the block the more tape is wasted as inter-block gaps. Several logical records may be combined into one physical block to reduce wastage ("blocked records"). Finding a certain block on the tape generally involved reading sequentially from the beginning, in contrast to magnetic disks. Tape is not suitable for random access. The exception to this is that some systems allow tape marks to be written which can be detected while winding the tape forward or rewinding it at high speed. These are typically used to separate logical files on a tape. Most tape drives now include some kind of data compression. There are several algorithms which provide similar results: LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and DLZ1 (DLT). See also cut a tape, flap, Group Code Recording, spool, macrotape, microtape, Non Return to Zero Inverted, Phase Encoded.Last updated: 1997-04-05
magnetic tape drive
<storage>
(Or "tape drive") A peripheral device that reads and writes magnetic tape.
Last updated: 1996-05-25
magneto-optical disk
(MO) A plastic or glass disk coated with a compound (often TbFeCo) with special optical, magnetic and thermal properties. The disk is read by bouncing a low-intensity laser off the disk. Originally the laser was infrared, but frequencies up to blue may be possible giving higher storage density. The polarisation of the reflected light depends on the polarity of the stored magnetic field.
To write, a higher intensity laser heats the coating up to its Curie point, allowing its magnetisation to be altered in a way that is retained when it has cooled. Although optical, they appear as hard drives to the operating system and do not require a special filesystem (they can be formatted as FAT, HPFS, NTFS, etc.). The initial 5.25" MO drives, introduced at the end of the 1980s, were the size of a full-height 5.25" hard drive (like in IBM PC XT) and the disks looked like a CD-ROM enclosed in an old-style cartridge In 2006, a 3.5" drive has the size of 1.44 megabyte diskette drive with disks about the size of a regular 1.44MB floppy disc but twice the thickness. Storage FAQ.Last updated: 2006-07-25
magneto-optical drive
magneto-optical diskmagnetostrictive delay line
An early storage device that used tensioned wires of nickel alloy carrying longitudinal waves produced and detected electromagnetically.
They had better storage behaviour than mercury delay lines. [H. Epstein and O.B. Stram, "A High Performance Magnetostriction-Sonic Delay Line," Transactions, Institute of Radio Engineers, Professional Group on Ultrasonic Engineering, 1957, pp. 1-24].Last updated: 2002-11-08
Nearby terms:
MagmaLISP ♦ magnetic disk ♦ Magnetic Ink Character Recognition ♦ magnetic stripe
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