A number representing a standard configuration of physical parameters (cylinders, heads, and sectors) of a particular type of disc drive. Each at system bios contains a list of drive types that the system considers standard types. These types are not necessarily the same from one type of BIOS to the next. For example, drive type 25 on one bios may represent a drive that has 615 cylinders, 4 data heads, and 17 sectors per track, while type 25 on another bios could be totally different.