BIOS Geometry Translation

The use of logical hard disk geometry gets around the problem that physical hard disk geometries cannot be properly expressed using simple BIOS settings. Logical geometries don’t go far enough, however. In most cases, higher levels of translation are needed as well, because other problems relating to old design decisions make it impossible for even the logical geometry to be used with modern large hard disks. These are the infamous BIOS capacity barriers such as the 504 MB limit on standard IDE/ATA hard disks, the 8.4 GB limit for standard Int 13h BIOS addressing, and other similar issues.

In order to get around these barriers, another layer of translation is often applied on top of the geometry translation that occurs inside the hard disk. This translation is performed by the BIOS (or sometimes, third-party overlay software.)

There are three BIOS Translation Modes: Standard CHS, Extended CHS (ECHS) / Large and Logical Block Addressing.