The 240 Head DOS Limitation


Summary: Using 15 physical drive Heads (P-CHS), to work-around the 8192 cylinder DOS & BIOS limitation, restricts the maximum number of Logical Heads (L-CHS) to 240. Thus for anything using CHS addressing instead of LBA, access could be limited to the first 7.38GB of each physical drive. And there are several circumstances or methods by which this 15 drive head work-around could be invoked, on a drive by drive basis.


    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.

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

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

    This should be used only as a last resort. As a BIOS upgrade, that fully supports the 256 heads issue, is the strongly preferred solution.
  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."

Micro Firmware: Issues With Hard Drives Over 4GB
Harddisk Upgrade: The BIOS IDE Harddisk Limitations
Back UP To: Specific Drive Capacity Limits
Next Page: 255 Heads DOS & BIOS barrier
Previous: Windows NT FAT16 Size limits


Monitor this Webpage

E-mail the Webmaster
Page Content Updated: 10 June 2005