An OS/2 Version 2.0 DASD or SCSI device driver is no longer one large module as with previous versions of OS/2 but a layered device driver. The higher layer that interfaces with the OS/2 file systems and the OS/2 kernel is the Device Manager. The lower layer that interfaces with the device adapter is the adapter device driver (ADD). In addition, a filter ADD can exist in the middle between the Device Manager and the ADD. A filter ADD can be used to provide special services such as data compression or data encryption.
The new Adapter Device Driver interface for an ADD module defines direct call commands that are issued by the Device Manager or filter ADDs down to the registered entry point of an ADD.
It is now possible to add new device support to OS/2 V2.0 merely by writing the device support portion of the device driver. The higher level routines, required to support disk and SCSI devices, are only written once.