A hard disk is one of the important I/O devices and is most commonly used as permanent storage device in any processor. Most PCs have hard disk having capacities in the range of 20GB to 80GB. Due to improvement in technology and density of magnetic disk, it has become possible to have disk with larger  capacity and at a  cheaper rate. A hard disk is basically comprises several metal platters coated with magnetic material.  These plates are rotating with high speed(5,000 revolutions per second) along the central shaft. Each surface has a read/write head mounted on arm. The head can possibly move in radial direction. Usually , both sides of a platter are used for storing data. Each surface is divided into concentric circles called tracks. There are 300 tracks per platter. Each track is divided into about 30 sectors, separated by gaps. To access data in a specific location, the arm has to be positioned over the proper track. This takes about 3-20 minutes. Once the sector is found, the data can be read at a rate of 4 MB/s. The disk controller is requires for buffering  the data to avoid bus contention and for bookkeeping.