There were not a lot of PC hard drive configurations in use at that time. The original AT BIOS identified disk characteristics using a table of preestablished disk characteristics. For example, type 1 is a 10.5 megabyte disk with 306 cylinders, 4 heads, and 17 sectors per track. Write precompensation is 128 and the landing zone is set to 305. In general, these types correspond to actual disk configurations. For example, types 26 and 31 correspond to the hard drives in the PS/2 Model 30 and PS/2 Model 60 respectively. The great advantage of the disk type table was a simple user interface. The user need only enter one or two digits to define a disk which actually had five obscure (and easily misentered) variable parameters. Autotyping--The capability to read recommended disk configuration information from the hard drive ROM was many years in the future when the PC-AT was introduced.
Because ATA (IDE) does not demand an exact match between declared disk characteristics and actual characteristics, it was possible to use drives not included in the type table by declaring them to be some disk of the same capacity or smaller that was in the type table. That allowed the type table to hang on longer than it might otherwise have done, but eventually, the size of the table became a problem. A halt was called by BIOS mongers at about 45 to 47 entries. Code was incorporated in the BIOS to allow a user to store "user defined" characteristics. These are usually Type 47, occasionally Type 48. Rarely both (in order to accommodate dual drive PCs). In modern BIOSes, the user parameters do not have user visible type number and are just called USER.
Type tables have been retained because some software still in use still depends on them. They provide a solution for software that needs to apply an arbitrary geometry to the disk and have the user set up software based on that geometry. For example, some Disk Dive Overlay software requires the disk to be declared as Type 17 notwithstanding that the actual disk may have 1000 times the 83 megabyte capacity of Type 17.
Type 15 is generally reserved/unused.
A representative set of type tables can be viewed at:
Return To Index Copyright 1994-2002 by Donald Kenney.