Making Tracks

Making Tracks
On a hard disk, data is stored in thin, concentric bands. A drive head, while in one position can
read or write a circular ring, or band called a track. There can be more than a thousand tracks on a
3.5-inch hard disk. Sections within each track are called sectors. A sector is the smallest physical
storage unit on a disk, and is almost always 512 bytes (0.5 kB) in size.
The figure below shows a hard disk with two platters.
Figure 3-1 Parts of a Hard Drive
The structure of older hard drives (i.e. prior to Windows 95) will refer to a cylinder/ head/ sector
notation. A cylinder is formed while all drive heads are in the same position on the disk. The
tracks, stacked on top of each other form a cylinder. This scheme is slowly being eliminated with
modern hard drives. All new disks use a translation factor to make their actual hardware layout
appear continuous, as this is the way that operating systems from Windows 95 onward like to
work..
To the operating system of a computer, tracks are logical rather than physical in structure, and are
established when the disk is low-level formatted. Tracks are numbered, starting at 0 (the outermost
edge of the disk), and going up to the highest numbered track, typically 1023, (close to the center).
Similarly, there are 1,024 cylinders (numbered from 0 to 1023) on a hard disk.
The stack of platters rotate at a constant speed. The drive head, while positioned close to the center
of the disk reads from a surface that is passing by more slowly than the surface at the outer edges
of the disk. To compensate for this physical difference, tracks near the outside of the disk are
less-densely populated with data than the tracks near the center of the disk. The result of the
different data density is that the same amount of data can be read over the same period of time,
from any drive head position.
The disk space is filled with data according to a standard plan. One side of one platter contains
space reserved for hardware track-positioning information and is not available to the operating
system. Thus, a disk assembly containing two platters has three sides available for data.
Track-positioning data is written to the disk during assembly at the factory. The system disk
controller reads this data to place the drive heads in the correct sector position.