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

The Int13h interface limit normally restricts some systems to 7.88 GiB or 8.46 GiB as a result of the limits of the BIOS Int13h interface: 1,024 cylinders, 256 heads and 63 sectors of 512 bytes. (I discuss this very important size barrier in some detail here, and you may want to read that section before you read this one.) However, in some systems the Int13h interface restriction results in a smaller limit: only 7.38 GiB (7.93 GB).

The reason why this occurs is related to a different size barrier problem (sigh, can't these engineers get their acts together? :^) ) As I described in the discussion of the 8,192 cylinder limit, DOS and some Windows versions cannot handle translated geometry that specifies 256 heads. To get around this, some BIOSes change their translation method so that only 240 heads are presented to the operating system. This fixes the "256 head problem" but shaves some capacity off the Int13h limit. The 1,024 cylinder and 63 sector restrictions remain, but with only 240 heads the maximum drive capacity becomes 1024 * 240 * 63 = 15,482,880 sectors of 512 bytes, or 7,927,234,560 bytes.

In practical terms, there isn't any difference in how this barrier is handled than the standard Int13h problem is tackled. You still need to use Int13h extensions; see the discussion of the Int13h interface barrier for more.

Next: The Int 13 Interface (7.88 GiB / 8.46 GB) Barrier


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