CHS

Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.
Though CHS values no longer have a direct physical relationship to the data stored on disks, pseudo CHS values (which can be translated by disk electronics or software) are still being used by many utility programs.

Definitions:

Heads

Data is written to and read from the surface of a platter by a device called a head. Naturally, a platter has 2 sides and thus 2 surfaces on which data could be manipulated; usually there are 2 heads per platter--one on each side, but not always. (Sometimes the term side is substituted for head, since platters might be separated from their head assemblies; as is definitely the case with the removable media of a floppy drive.)

Tracks

Are the thin concentric circular strips on a floppy medium or platter surface which actually contain the magnetic regions of data written to a disk drive.

Cylinders

A cylinder comprises all the tracks (on every usable platter surface) that can be accessed by the read/write heads while the actuator assembly remains stationary.

Sectors

Each usable side of a platter can also be thought of as a collection of slices called sectors. (See Blocks for an alternate definition of sector.)

Blocks

The intersection of a track and a sector is called a block. Thus, blocks are delineated by specifying a certain cylinder, head and sector. These blocks are the smallest geometrical breakdown of a disk, and represent the smallest amount of data which can be transferred to or from a disk (usually 512 bytes).
However, many PC engineers and technicians use the term sector (instead of block) as if it were also defined as the smallest geometrical breakdown of a disk.[1] The UNIX/Linux communities, however, continue to employ the term block. For example, the Linux fdisk utility normally displays partition table information using 1024-byte blocks while also using sector to help describe a disk's size with its phrase, 63 sectors/track.

CHS Addressing:

Hence, each block of data can be addressed by specifying a cylinder, head, and sector. The following formulas detail the CHS geometry and addressing scheme.
The number of blocks on one side of a platter is:

blocks_Per_Platter_Side = (cylinders_Per_Platter)*(Sectors_Per_Platter)The number of

blocks per platter is:

blocks_Per_Platter = (blocks_Per_Platter_Side)*(sides_Used_Per_Platter)

which is usually written in terms of the number of heads used:

blocks_Per_Platter = (blocks_Per_Platter_Side)*(Heads_Per_Platter)

This is usually expanded to:

blocks_Per_Platter = (cylinders_Per_Platter)*(Sectors_Per_Platter)*(Heads_Per_Platter)

and rearranged:

blocks_Per_Platter = (cylinders_Per_Platter)*(Heads_Per_Platter)*(Sectors_Per_Platter)

Since all the platters are the same size and hard drives usually have more than one platter, the total number of blocks on the drive can be written as:

total_Blocks = (Cylinders)*(Heads_Per_Platter)*(Sectors)*(Number_Of_Platters)

If the number of platters is combined with the number of heads per platter to form the single parameter Heads, the equation can be written in its final form as:

total_Blocks = (Cylinders)*(Heads)*(Sectors)