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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk BIOS and Capacity Factors | Hard Disk Size Barriers ]

The 65,536 Cylinder (31.5 GiB / 33.8 GB) Barrier

This is a relatively new hard disk barrier that showed up in early 1999. It is yet another in a long series of limits caused by the inability of a BIOS version or type to handle a particular number of cylinders, much like several smaller barriers have been. It is often called the "32 GB size barrier", which is approximately correct anyway. :^)

In this particular case, some versions of Award BIOS cannot handle drives that have more than 65,535 cylinders. Since hard disk parameters usually use 16 heads and 63 sectors, this works out to a capacity of about 33.8 GB or 31.5 GiB before trouble occurs. As of about June 1999, this problem had been corrected, so it is most likely to show up on systems purchased before that month.

I must say that I find this to be a rather strange hard disk barrier, because hard disks above about 8 GB in size no longer really use discrete geometry for access; they are instead addressed using LBA and a flat sector number from 0 to one less than the number of sectors on the drive. This 65,536 cylinder problem must be a remnant of some older code, or something related to compatibility with older hard drives. Regardless of its origin, many system owners will have to deal with it.

Next: The ATA Interface Limit (128 GiB / 137 GB) Barrier


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