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BIOS Handling of "Oversized" Hard Disks

When you put a hard disk into a machine that has a BIOS unable to handle its size, the system can react in a number of different ways. How it responds depends on the system, how old the BIOS is, and how well tested and debugged the BIOS routines are. These issues normally are a result of the hard disk having a number of cylinders larger than the maximum the BIOS supports.

These are the four most common ways that a machine with an older BIOS will handle a hard disk larger than it supports:

• Truncation: Many BIOSes, when presented with a logical geometry containing more cylinders than they can handle, will simply truncate the total to the maximum they support. This is usually seen in older BIOSes that don't support more than 1,024 cylinders, and also in some cases in a BIOS that maxes out at 4,096 cylinders. It is also commonly found in systems that do not support Int13h extensions--these units will typically see a drive larger than 8.4 GB as being just 8.4 GB in size. Truncation of course wastes some space on the drive, but is still far preferable to the other possibilities described below.
• Wrap-Around: Some very old BIOSes, assuming that the number of cylinders will always be 1,024 or below, only look at the bottom 10 bits of the cylinder number coming from the hard disk (2^10 = 1,024). As a result, when used with numbers over 1,023, they do the equivalent of counting up to 1,024 and then "wrapping around" to zero again and starting over. (This is equivalent to N modulo 1024, where N is the number of real cylinders, for those who know what that means). As an example, if you tried to use a drive with 3,500 cylinders, the BIOS would see 428 cylinders, because it would count up 1,024 three times (to yield 3,072), wrap around three times, and then end up with 428 cylinders (3,500 minus 3,072).
  The same exact thing can happen to a BIOS that support only 4,096 cylinders: it may only look at the bottom 12 bits. This means that in some cases you can put a 2.5 GB hard disk into your system and end up with only about 400 MB of usable space showing up. This is unfortunately a common failure mode with BIOSes that don't support more than 4,096 cylinders.

Note: Some BIOSes that support translation will do this wrapping around if you disable translation. When you turn it on again, the problem may go away.

• "Ignorance": Some BIOSes will report the true number of logical cylinders that the drive has, making you think your system supports the full size of the hard disk. Really, the BIOS just has no clue what it is seeing. When you go to partition and format the hard disk, you will be stuck with the same limit (which can be both confusing and frustrating). This is usually seen with older BIOSes and the 1,024 cylinder limitation.
• Failure: Some BIOSes will totally lock up if you try to use them with a disk larger than they can support. These are actually pretty uncommon, fortunately. They are also more common with some of the larger hard disk barriers and also with some of the more obscure ones.

Next: BIOS Translation Modes

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