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

Int 13h Extensions

The only solution to the 8GB hard disk size barrier is to do away with using the standard Int13h disk access routines. This new method of access requires changes to the BIOS to support what are called Int 13h Extensions. These extensions are discussed in general terms here.

Using Int13h extensions in fact requires changes to everything that is associated with accessing the disk: the disk itself, the BIOS, and the operating system. Newer operating systems all support this important change to how hard disks are addressed, including all versions of Windows from Windows 95 on. Modern drives also support this method of being addressed. This means that getting past the 8 GB hard disk barrier is generally a matter of ensuring that the system BIOS or hard disk controller supports Int13h extensions.

Tip: Some hard disk manufacturers make available drive utilities that can test your system BIOS and determine if it will natively support Int13h extensions. Check the download section of the drive manufacturer you are considering.

Another implication of going away from Int13h is that the tired old "geometry method" of specifying hard disk size finally must be put to rest. In fact, due to multiple levels of translation both within the BIOS and the hard disk itself, the logical geometry parameters of IDE/ATA hard disks stopped having any real relationship to the actual drive characteristics long ago. However with drives now above 8.4 GB in size, they cannot even be expressed using traditional geometry terms, without going over the cylinder, head and sector limits mentioned above. Instead, the total number of sectors on the drive is now the key parameter, and the drive is accessed using logical block addressing. For example, a 45 GB IBM Desktop 75GXP drive would, if expressed in conventional geometry, have to be said to have 89,355 cylinders, 16 heads and 63 sectors. Instead, it is just said to have 90,069,840 data sectors. By convention, and for compatibility, all drives over 8.4 GB have logical geometry parameters of 16,383 cylinders, 16 heads and 63 sectors, which is why these drives show up as being about 8.4 GB in size if Int13h extensions have not been implemented.

Next: Hard Disk Interfaces and Configuration

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