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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 4,096 Cylinder (1.97 GiB / 2.11 GB) Barrier

As discussed in great detail in the section discussing the 504 MiB barrier, the basic problem with BIOS-related capacity barriers is that the normal system BIOS interface on older PCs is not designed to handle hard disks that employ over 1,024 cylinders. Every hard disk made today uses more than 1,024, which causes a drastic reduction in visible capacity due to this limitation. See that section if you need to get an understanding of this basic matter.

Systems that use an enhanced BIOS are able to employ translation to get around the 1,024 cylinder limitation. However, some BIOSes, despite supporting translation, will again choke if the number of cylinders exceeds 4,095, causing the same problems with the 504 MiB barrier to occur all over again. 2^12 is 4,096, which means that if you go beyond 4,095 cylinders on a drive, the number requires a 13th bit to properly represent it (for example, 4,097 in decimal is 1000000000001 in binary). This should be no problem, but due to poor BIOS code writing on some systems, only 12 bits of the cylinder number are recognized. The actual limitation is 4,096 * 16 * 63 * 512 bytes, which is about 1.97 binary gigabytes or 2.11 decimal gigabytes. Some BIOSes with this problem will show a disk with more than 4,096 cylinders as being 1.97 GB, while others will show it as substantially less. For example, 4,097 cylinders may show up as only 1 cylinder if the 13th bit is just ignored and the lower-order 12 bits are used by themselves!

Note: Do not confuse this capacity barrier with the other capacity barrier which is exactly 2 binary gigabytes. That one is a file system issue and is unrelated to the BIOS matter we are discussing here.

This particular size barrier began to show up on systems in 1996. The options for solving it are similar to those for dealing with the 504 MiB limitation.

Next: The FAT16 Partition Size (2.00 GiB / 2.15 GB) Barrier

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