The DPMI API Layer has a small, well defined interface with the kernel. At system initialization time, the DPMI API Layer is registered with the kernel through a VDH call and reports which version of DPMI it supports. If the VDD supports only DPMI Version 0.9, the kernel (which supports DPMI Version 1.0) adjusts the way in which it handles certain DPMI tasks.
Kernel services that may be useful to VDDs other than the DPMI API layer are exported as VDH services. Services that should have use restricted only to the DPMI API layer are made available through structures exchanged when the DPMI API Layer virtual device driver is registered.