Hard disks are nonvolatile random access secondary data storage devices, i.e. the desired data item can be accessed directly without actually going through or referring to other data items. They store the data on the magnetic surface of hard disk platters.

Platters are made of aluminium alloy or a mixture of glass and ceramic covered with a magnetic coating. Figure 15.32 shows the internal structure of a typical hard disk. As can be seen from the figure, there are a few (two or more) platters stacked on top of each other on a common shaft.

The shaft rotates these platters at speeds of several thousand rpm. Each platter is organized into tracks and sectors (Fig. 15.33), both having a physical address used by the operating system to look for the stored data.

Tracks are concentric circles used to store data. Each track is further subdivided into sectors so that the total numbers of sectors per side of the magnetic disk are the product of the number of tracks per side and the number of sectors per track.

And if it is a double-sided disk, the total number of sectors gets further multiplied by 2. From known values of the total number of sectors and the number of bytes stored per sector, the storage capacity of the disk in bytes can then be computed.

There is a read/write head on one or both sides of the disk, depending upon whether it is a single sided or a double-sided disk. The head does not physically touch the disk surface; it floats over the surface and is close enough to detect the magnetized data.

The direction or polarization of the magnetic domains on the disk surface is controlled by the direction of the magnetic field produced by the write head according to the direction of the current pulse in the winding. This magnetizes a small spot on the disk surface in the direction of the magnetic field.

A magnetized spot of one polarity represents a binary ‘1’, and that of the other polarity represents a binary ‘0’. One of the most important parameters defining the performance of the hard disk is the size of the disk.

Disks are available in various sizes ranging from 20 GB to as large as 80 GB. Other parameters defining the hard disk performance include seek time and latency time. Seek time is defined as the average time required by the read/write head to move to the desired track.

Latency time is defined as the time taken by the desired sector to spin under the head once the head is positioned over the desired track.

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