Hard Disk BIOS Translation Modes

One of the most important techniques used to break the 504 MiB disk size barrier that results from the combination of BIOS and IDE/ATA hard disk restrictions is the use of BIOS translation. This can be a rather confusing subject and so this section discusses in detail the various types of BIOS translation that are used in PCs. A BIOS that supports the extended CHS and/or LBA modes is often said to be an enhanced BIOS.

Note: For completeness, and to assist those working with older hardware, I maintain the descriptions of translation modes in some detail here. Realize however that on most modern systems they are no longer of much relevance. Drives over 8.4 GB in size, which is what modern PCs use, are generally accessed by setting the BIOS to dynamically autodetect their size at boot time, and then using logical block addressing. Most of the issues that PC users once had to deal with in terms of tweaking BIOS translation modes are no longer necessary for modern drives over 8.4 GB in size, because these drives are no longer address using cylinder, head and sector numbers.

Note: Little of this section has any relevance when using only SCSI hard disks. They are not generally subject to the BIOS limitations that are overcome through translation.

Next: Normal / Standard CHS Mode