hard disks are on-line storage devices.The term on-line means that the device (hard disks) is permanently connected to the computer system.when computer is on,the device (hard disks) is available to store information or to give information.Hard disks are used as secondary memory for mass (bulk) storage of information permanently.They store program,data,operating system,compilers,assemblers,application programs,database,etc.
A hard disk is made of aluminum( or other metal or metal alloy) with a thin coating of magnetic material (iron oxide) over it,standard size of hard disks is 3.5 inch.The 2.5 inch disk are used in portable computers.Hard disks and read/write heads are kept in a sealed,air filtered enclosure.This technique is known as winchester technique.Hard disks together with read/write heads,access mechanism and driving motor constitute an unit called hard disk drive platter.It cannot be removed or inserted into a hard disk drive unit.Hard disk drive unit under discussion is of large capacity,and is used as secondary memory.Recently removable hard disks and external hard disks of smaller capacity for backup memory have also been developed.
To increase the storage capacity several hard disk(platters) are mounted on common drive to constitute a disk pack,as shown in fig.The set of all tracks at the same distance from the spindle on the record able surface of the disk pack will lie on the surface of an imaginary cylinder.All the tracks which lie on a cylinder have same number.for example,track-10 of each recordable surface of the pack will lie on the cylinder which is numbered as cylinder-10.The disks are addressed by drive number(because more than one drives are used in a computer),cylinder number (i.e. track number),surface number and sector number.
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