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From: Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: DS and ESP
Date: Tue, 19 Feb 2002 12:58:45 CST
Organization: Rice University, Houston TX
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> > If the DPMI provider is done correctly either hardware interrupts or
> > even page faults work fine even when the user ESP is invalid.  All of
> > these should be handled on the locked 4Kb DPMI internal stack and
> > never touch the user stack.  So, it turns out that code isn't broken :-)
> 
> Doesn't the interrupt itself to the stack switching code have to
> store a return address on the user stack? and maybe the flags
> too.  Or is that all in the hardware?

The hardware has a built in feature to swap stacks on a change of ring
(priv).  In the case of a hardware interrupt or exception we change
execution to ring 0 (instead of ring 3) which also swaps to an internal
ring 0 stack.  All values are set on that stack (or one which it also
may swap to).  The appropriate "return" will restore the ring 3 stack
as being current.

This is required for stable operation - what would happen if you need to
expand the stack downward (crossing a page boundary) in the middle of
a page fault?  Or an exception?  This is one of the problems with any
"ring 0" DPMI provider - that there is no way to protect against these
type of double faults - so you must do something special to catch them
in which case they are not recoverable.