How data is organised on a magnetic disk (Reading/Listening) (Pr./Eng)

          How data is organised on a magnetic disk

Before the computer can use a diskette to store data, the disks surface must be

magnetically mapped so that the computer can go directly to a specific point without searching through all the data. This process of mapping a disk is called formatting or initialising. When you purchase a new disk, they should be formatted for either PC or Mac.

It may be helpful to reformat disks from time to time as this deletes all the data on

disk. During the formatting you process you can also determine whether the disk has any faulty spots and you can copy important system files onto the disk.

Hard disks must also be formatted so that the computer can locate data on them.

When you buy a computer, the hard disk has already been formatted correctly and

probably contains some programs and data. You can format your hard disk if

necessary but the process is different to that for a diskette.

Modern diskettes store data on both sides of the disk (numbered side 0 and side 1) and each side has its own read/write head. When formatting a disk, the disk creates a set of magnetic concentric circles called tracks, on each side of the disk. The number of tracks required. Most high -density diskettes have 80 tracks on each side. A hard disk may have several hundred tracks on each side of each platter. Each track is a separate circle. These are numbered from the outermost circle to the innermost, starting with zero.

Each track on a disk is also split into smaller parts. Imagine slicing a disk as you

would a pie. Each slice cuts across all the tracks resulting in short segments or

sectors. A sector can contain up to 512 bytes. All the sectors are numbered in one

long sequence so the computer can access each small area on the disk with a unique number. This scheme simplifies a 2 dimensional set of co-ordinates into a single numeric address.

The diskettes outermost track is longer than the innermost one but each track is still divided into the same number of sectors. Regardless of physical size all the sectors hold the same amount of data.

A disks allocation of sectors per track is somewhat wasteful because the outer longer tracks could theoretically hold more data than the shorter inner ones. For this reason hard disks allocate more sectors to the longer tracks on the disks surface. As you move towards the disks centre, each subsequent track has fewer sectors. This arrangement takes advantage of the hard disks potential capacity and enables a typical hard disk to store data more efficiently than a floppy disk.