CHS addressing for hard drives

What is CHS addressing

A long time ago, when the capacity of the hard disk was very small, people used a similar structure to produce hard disks. That is, each track of a hard disk disc has the same number of sectors. This results in the so-called csh 3D parameter (Disk Geometry). That is, the number of heads (Heads), the number of cylinders (cylinders), the number of sectors (sectors), and the corresponding CHS addressing method.

CHS addressing mode divides the hard disk into heads (Heads), Cylinders (Cylinder), sectors (Sector).

• Head (Heads): Each of the two sides of the magnetic plate has a head, a magnetic head corresponding to a piece of a face. Therefore, the first magnetic head can be used to represent the surface of the data.

• Cylinder (Cylinder): Concentric tracks of the same radius in all the magnets constitute "cylinders", meaning that the series of tracks are stacked vertically together, forming a cylindrical shape. Simply understood, the number of cylinders = number of tracks.

• sector (Sector): Divides the track into a number of small segments, which is the sector. Although very small, but actually a fan shape, it is called sector. The capacity of each sector is 512 bytes.

The following figure may help understand better:

Maximum capacity for CHS addressing

The capacity of CHS addressing mode is determined by the CHS three parameters:

• The maximum number of heads is 255 (with 8 bits storage). Numbering starts from 0.
• The maximum number of cylinders is 1023 (with 10 bits storage). Numbering starts from 0.
• Number of sectors maximum 63 (with 6 bits storage). Numbering starts from 1.

So the maximum addressing range for CHS addressing is:
255 * 1023 * 63 * 512 / 1048576 = 7.837 GB ( 1M =1048576 Bytes )

or hard disk manufacturers commonly used units:
255 * 1023 * 63 * 512 / 1000000 = 8.414 GB ( 1M =1000000 Bytes )

Disadvantages of CHS addressing

Obviously, because the number of sectors required for each track is equal, and the perimeter of the heretics is greater than the inner lane, so the heretics record density is much lower than the internal channel, not only caused by the waste of hard disk space, but also limit the capacity of the hard disk. In order to solve this problem, further increase the capacity of hard disk, people use the density structure to produce hard disk. That is, the sector of the outer ring track is more than the inner ring track, after this structure, the hard disk no longer has the actual CHS parameters, the addressing method is also changed to linear addressing, that is, the sector is addressed in units.
However, some old software still uses CHS addressing methods (such as software using the Biosint13h interface), in order to be compatible with such a program, an address translator is installed inside the hard disk controller to translate the old CHS parameters into new linear parameters.

From for notes (Wiz)

CHS addressing for hard drives