English Literature For BCS. pdf

Lesson 7 : Hard Disk Drive

7.1. Learning Objectives
On completion of this lesson you will be able to describe:
 Components of a hard disk drive
 Working principle of hard disk drive.

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7.2. Hard Disk Drive

Hard disk drive is used to store data and programs. It is one kind of memory. But the information stored in it doesn’t vanish when the computer is off.

7.3. Structure of Hard Disk
 A hard disk consists of many round shape disks called platters.
 A platter consists of many concentric circles called tracks.
 A track is divided in many semicircles called sectors.
 A sector has many bytes.
 A byte has eight bits.
 Data is stored in Bits.

The outer most track is numbered as track-0. On an 80 track platter the inner most track is numbered as track-79. In the figure each of the three platters has a track-0. If we combine all the track-0 of three platters in parallel then it forms a cylinder. Thecylinder formed by combining all track-0s is called cylinder-0. Similarly the cylinder formed by combining all track-1s is called cylinder-1.Heads are used to store and read data from disks. If data is stored on both sides of a platter then two heads are needed for a platter. If there are 3 disks then 6 platters and so on. Heads move from track to track and read data. But all the heads in a hard disk move jointly. That means, if head of first platter is at track-0, then all the other heads are also on track-0 on other platters. Thus all the heads are on cylindar-0. In such a way heads move from cylinder to cylinder.