The 8192 Cylinder DOS Limitation

 

    Due to a bug or design flaw with how DOS and other older operating systems access a hard drive, they cannot handle a Logical CHS (L-CHS) with 256 Heads. So for any hard drive that is configured as a 16 Head device, the next smaller drive geometry corresponds to a Physical CHS (P-CHS) with at most 8192 Cylinders. As more than 8192 drive Cylinders causes the translation to create a Logical CHS using 256 Heads. Thus the next smaller geometry-- which has 128 Logical Heads-- results in a drive capacity limitation of 3.94GB (4.22 billion Bytes).

    There are two ways to provide a "work-around" for this limitation. The first method is based on manipulating the P-CHS to L-CHS translation to use 15 x 16, instead of 16 x 16. Which then limits the L-CHS to a maximum of 240 heads. (See the DOS 240 Heads limit page for details.) The second method is to just limit any L-CHS and LBA translation to using 255 instead of 256 for the Heads.