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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 8,192 Cylinder (3.94 GiB / 4.22 GB) Barrier

After the discovery of the 504 MiB BIOS barrier, the normal way of getting around that problem was to make use of BIOS geometry translation. (This continued until hard drives exceeded about 8 GB in size and the whole IDE/ATA geometry scheme had to be abandoned altogether.) In a nutshell, this translation works by dividing the hard disk's number of cylinders by a binary number such as 2, 4, 8 or 16, and multiplying the number of heads by the same number. This lets the number of cylinders that the BIOS sees fall below the Int13h limit of 1,024. This translation however causes a problem in some systems when using a hard disk over about 4 GB in size.

Note: To understand how translation causes this particular new barrier to arise, you need to understand how BIOS translation works. See here for an explanation if you are not familiar with this.

When the number of cylinders on the drive is between 8,192 and 16,383, the number typically used for translation is 16. Here's an example of how this might work with a theoretical 6.4 GB hard disk if it used the normal way that IDE/ATA drives are specified, with 16 heads and 63 sectors per track:

 

Cylinders

Heads

Sectors

Capacity

IDE/ATA Limits

65,536

16

256

137 GB

Hard Disk Logical Geometry

12,496

16

63

6.45 GB

BIOS Translation Factor

divide by 16

multiply by 16

--

--

BIOS Translated Geometry

781

256

63

6..45 GB

BIOS Int 13h Limits

1,024

256

63

7.88 GB

This should actually work just fine; it overcomes the BIOS issues and results in geometry that falls within acceptable limits. However, there's an unfortunate gotcha that was discovered when drives first exceeded 8,192 cylinders in around 1997: MS-DOS and early versions of Windows choked when presented with a drive that had (apparently) 256 heads! Thus, this is actually a barrier that is due to both the operating system and the system BIOS: the operating system should have been able to handle 256 heads, but the BIOS was creating the problem due to its translation.

It was decided that the easiest way to deal with this problem was to change the way the BIOS did translation. As a result, BIOSes stopped creating translated geometries that had 256 heads. One common way that this was done was to use 15 as the translation factor instead of 16, resulting in this sort of conversion:

 

Cylinders

Heads

Sectors

Capacity

IDE/ATA Limits

65,536

16

256

137 GB

Hard Disk Logical Geometry

12,496

16

63

6.45 GB

BIOS Translation Factor

divide by 15

multiply by 15

--

--

BIOS Translated Geometry

833

240

63

6..45 GB

BIOS Int 13h Limits

1,024

256

63

7.88 GB

Of course, if you have a BIOS that doesn't know about the 256 head problem, you will need to address this with either a hardware or software solution. To help avoid some of these problems, many hard disk manufacturers also changed their specified geometries to use only 15 heads instead of 16. So instead of the example drive above being specified with 12,496 cylinders, 16 heads and 63 sectors, it might have been 13,329 cylinders, 15 heads and 63 sectors. With these parameters, even if the BIOS uses a translation factor of 16, the resulting number of heads will be only 240.

Next: The 240 Head Int 13 Interface (7.38 GiB / 7.93 GB) Barrier


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