The 240 Head DOS Limitation

    Due to a bug or design flaw with how DOS and other older operating systems access a hard drive, they cannot handle an Logical CHS (L-CHS) with 256 Heads. So when Physical CHS addressing (P-CHS) is used to access a hard drive, the P-CHS to L-CHS translation must be manipulated to use 15 x 16 instead of 16 x 16. Which results in using 240 instead of 256 Heads.
 
1024 Cylinders x 240 Heads x 63 Sectors x 512 Bytes/Sector
 
= 7927234560 Bytes
 
Thus incurring a 7.38GB (7.93 billion Bytes) capacity limitation.
 

    There are three possible ways to manipulate the drive geometry translation, to obtain an L-CHS with 240 Heads.

1. Limit the hard drive itself to 15 instead of 16 Heads.

• For example, IBM shipped a number of hard drives that provided a jumper to select if the CHS provided by the drive used 15 or 16 Heads.
   

2. Have the BIOS do a translation, to change 16x16 to 15x16.

• This requires an upgraded BIOS, which will translate the drive geometry to one with 15 heads, whenever a multiple of 16 would cause an L-CHS geometry that uses 256 heads.
• By translating the drive geometry to 15 heads, instead of just changing the multiple to 15, the very fast "bit shifting" method can still be used to provide L-CHS to P-CHS translation. (Where P-CHS is the new 15 head geometry.)
• This drive geometry translation, is performed using the same method, as the manual 15 head geometry translation, detailed below.
   

3. Manually change the CMOS setup (BIOS) to use a 15 Head geometry

1. Enter the (published) CHS values for the drive into the CMOS setup, or Autotype the drive and do not select a translation mode yet.
2. Manually adjust the number of heads from 16 to 15.
3. Multiply the number of cylinders by 16/15 (rounded down to a whole number). Since 16/15 is 1.06667, the simplest way to multiply by 16/15 would be to multiply by 1.06667 (and then round down to a whole number).
4. Adjust the number of cylinders to this larger amount.
5. Select ECHS or LARGE as the access method, to use CHS addressing instead of LBA.
6. Write down these adjusted values for cylinders, heads and sectors.
7. Save changes to CMOS, partition and format the drive.

"This can be considered a temporary workaround, but there should be no problem with continuing to use a hard drive set up this way. If an updated  BIOS is obtained at a later date, it should not be necessary to repartition and reformat the drive.

"The important thing to keep in mind in using the above workaround is that you must keep a record of the CHS values used so that they can be reentered if the contents of CMOS RAM are lost or if the drive is moved to another system."