Storage : Magnetic Disks

Magnetic disks are the most common backing storage device. Data is stored using magnetised spots called domains on the disk. Each domain can store one bit of data (a 0 or a 1). A 3.5" 1.44Mb floppy disk used on a PC contains 16,777,216 such spots. A 3.5" 120Gb hard disk drive contains over 120 billion spots!

Data stored on disks is arranged along a series of concentric rings called tracks. Each track is divided up into a number of sectors. Data is read to and written from a disk one sector at a time. A sector usually contains 512 or 1024 bytes of data.

The process of dividing a disk up into tracks and sectors so it can be used on a computer is known as formatting. You must format a new disk before you can use it.

Data is read from the disk using a disk head which moves mechanically about the disk (rather like a record player tone arm). The disk head can move directly to any sector on the disk. Because of this a computer system can load a file or a record from a file very quickly. The system can move directly to the location of the record/file and read it without having to read any other data from the disk. This is known as direct access. For most applications using a direct access medium is much faster than using a serial access medium.

The two main types of magnetic disks are floppy disks and hard disks.

GCSE ICT Companion 04 - (C) P Meakin 2004