This call provides a privilege level 2 IOPL segment to call a privilege level 3 application segment.
DosCallback
Ring3Routine (PFN) - input
This function allows a routine executing with I/O privilege (at privilege level 2) to call a segment that is executing at privilege level 3 and also to have the target routine execute at privilege level 3; for example, not to allow/require the target routine to be privilege level 2 conforming.
The requested routine is given control at privilege level 3 and when it completes execution and returns, return is made to the privilege level 2 calling routine.
All registers except FLAGs are passed intact across this call return sequence and may be used to pass parameters or data. Any addresses passed from privilege level 2 to privilege level 3 must be based on privilege level 3 selectors only as the privilege level 3 code does not have proper addressability to any privilege level 2 data selectors.
The privilege level 2 stack can not be used to pass data to the privilege level 3 routine.
If DosCallback is used in a nested fashion such that a privilege level 3 routine issues a call to a privilege level 2 routine while performing a "Callback" operation (after being invoked by "Callback" but before having issued a Far RET back to the privilege level 2 code), any subsequent DosCallback requests must complete execution and issue their corresponding Far RETs in last-in-first-out (LIFO) order.