The 255 Head DOS Limitation

 

    Any modern BIOS will use 255 heads, when translating a Logical CHS (Cylinder/Head/Sector) address to LBA. So the implications of this limitation are discussed as part of the BIOS Int 13h CHS barrier.

 

More detail on translating Logical CHS using LBA

"This method basically just reduces 256 heads to 255 heads. There's a little more to it - the example below shows more detail. Depending on the CHS values used by a particular model of drive, there may or may not be a very small loss in the overall drive capacity due to rounding odd numbers down at any step in the translation algorithm."

"Here is an example using the Maxtor 85120A 5GB IDE drive:

"The drive should autotype in CMOS setup to its factory (C/H/S) parameters of 9924 x 16 x 63. The BIOS will then multiply these 3 numbers together to get the total number of sectors, which would be 10,003,392. This total is then divided by 63 which would be 158,784. This number is then divided by 1024 to determine the value to use for heads, which must be 16, 32, 64, 128, or 255. The result here is 155, which is then adjusted up to 255. 158,784 is then divided by the number of heads, in this case 255, which gives us 622.682... which would round down to 622. So we end up with a translated geometry of 622 x 255 x 63."

An important issue with this translation algorithm is the fact that the value for sectors per track is now always 63. This differs from the translation method previously used by many BIOSes-- which were still using 256 heads instead of 255. This is an important consideration because some drives may use a value other than 63 for sectors per track; and if that drive was set up under a different BIOS, it will now be subject to a different type of translation. Where the use of a different translated geometry also means the drive will need to be repartitioned and reformatted! This will be an issue when upgrading, if any existing drives have other than 63 sectors per track. Note that there is not an inherent problem with setting up a drive with less than 63 sectors per track under either type of BIOS. The problem is just that different BIOSes may treat such a drive differently.