Wednesday, April 3, 2013

Magnetic disks

     Magnetic disks of the computer serve for long storage of information (it isn't erased at COMPUTER switching off). Thus in the course of work data can be removed, and others to register.

     Allocate hard and floppy disks. However flexible disks are used now already very seldom. Flexible disks were especially popular in the 80-90th years of last century.

     The flexible disks (diskettes) called sometimes by floppy disks (Floppy Disk), represent the magnetic disks concluded in square plastic cartridges of 5,25 inches in size (133 mm) or 3,5 inches (89 mm). Flexible disks allow to transfer documents and programs from one computer to another, to store information, to do archival copies of information containing on a hard disk.

    Information on a magnetic disk registers and read out by magnetic heads along concentric paths. At record or reading information the magnetic disk rotates round its pivot-center, and the head by means of the special mechanism is brought to the necessary path.

    Diskettes of 3,5 inches in size have the volume of 1,44 Mb. This type of diskettes is most widespread now.

    Unlike flexible disks the hard disk allows to store large volumes of information. Capacity of hard disks of modern computers can make terabytes.

     The first hard disk was created by IBM firm in 1973. He allowed to store to 16 Mb of information. As this disk had 30 cylinders broken into 30 sectors, it was designated as 30/30. On analogies to the automatic rifles having caliber 30/30, this disk I received the nickname "winchester".

     The hard disk represents a tight iron box in which is one or several magnetic disks together with the block of heads of reading/record and the electric motor. At turning on of the computer the electric motor untwists a magnetic disk to the high speed (some thousand revolutions per minute) and the disk continues to rotate all the time while the computer is switched on. Over a disk special magnetic heads which write down "soar" and read out information as well as on flexible disks. Heads soar over a disk owing to its high speed of rotation. If heads concerned a disk, because of friction force the disk quickly would leave out of operation.

     During the work with magnetic disks the following concepts are used.

Path – a concentric circle on a magnetic disk which is a basis for a data recording.

The cylinder is a set of the magnetic paths, located the friend over the friend on all working surfaces of disks of the winchester.

Sector – a site of a magnetic path which is one of the main units of a data recording. Each sector has own number.

Cluster - the minimum element of a magnetic disk with which the operating system operates during the work with disks. Each cluster consists of several sectors.

     Any magnetic disk has logical structure which includes the following elements:

  • loading sector;
  • tables of placement of files;
  • area of data.

    The loading sector (Boot Record) occupies sector with number 0. It contains the small IPL2 program (Initial Program Loading 2) by which the computer defines opportunity to load an operating system from this disk.

     Feature of the winchester is existence besides loading sector of one more area - the main loading sector (Master Boot Record). The matter is that the uniform hard disk can be broken into some logical disks. For the main loading sector on a hard disk physical sector 1 is always allocated. This sector contains the IPL1 program (Initial Program Loading 1) which at the performance defines a loading disk.

     The table of placement of files is used for storage of data on placement of files on a disk. For magnetic disks two copies of tables which one follows another are usually used, and their contents completely coincide. It becomes if on a disk there were any failures, the disk always can be "repaired", using the second copy of the table. If both copies are damaged, all information on a disk will be lost.

     The area of data (Data Area) occupies the main part of disk space and serves directly for data storage.

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