Stacks and Stack Switching


            Every  DPMI   task  runs  on  four  different  stacks:    An
            application ring  protected mode  stack, a  locked protected
            mode stack, a real mode stack, and a DPMI host ring 0 stack.

            The protected  mode stack  is the  one the  DPMI client  was
            running on  when it  switched into protected mode by calling
            the protected  mode entry  point (although  the  client  can
            switch to  another protected  mode stack  if desired).   The
            locked protected  mode stack  is provided by the DPMI server
            and  is   used  for   simulating  hardware   interrupts  and
            processing real mode call-backs.  The DPMI host provides the
            real mode  stack, which  is usually located in the data area
            provided by the client.  The ring 0 stack is only accessible
            by the  DPMI host.   However,  this stack  may contain state
            information about the currently running program.


        Protected Mode Stack

            This is  the stack that the client uses for normal execution
            in protected  mode.   The protected  mode stack  of  a  DPMI
            client can  be unlocked  if desired.    Software  interrupts
            executed in protected mode will be reflected on this stack.

        Locked Protected Mode Stack

            During hardware  interrupts, Int  1Ch,  Int  23h,  Int  24h,
            exceptions, and  real mode  call-back handling  in protected
            mode, the  DPMI will  host automatically  switch to a locked
            protected mode  stack.   When the interrupt or call returns,
            the host  will return  to the original protected mode stack.
            Note that  there is  only one,  4K, locked stack provided by
            the host.  The stack will be switched onto the first time an
            interrupt or  call is  reflected to protected mode, and will
            be switched  away from  when the client returns.  Subsequent
            nested interrupts  or calls  will not  cause a stack switch.
            Software interrupts do not automatically switch stacks.

        Real Mode Stack

            The DPMI host will provide the client with a real mode stack
            that is  at least  200h bytes  in size  and will  always  be
            locked.   Interrupts that  are reflected  into real mode, as
            well as  calls made  using the translation services, will be
            reflected on  this stack.  DPMI hosts will not automatically
            switch stacks for hardware interrupt processing in real mode
            since DOS performs this function automatically.

        DPMI Host Ring 0 Stack

            DPMI hosts  will normally  have a stack associated with each
            DPMI task.   The DPMI client will not be able to access this
            stack in  any way -- it is used by the host for execution at
            ring 0 to handle interrupts and exceptions.  This stack will
            sometimes be used to store state information while switching
            modes.   For example,  the original  SS:ESP of the protected
            mode program  could be  saved on  the ring 0 stack while the
            DPMI host switches onto the locked protected mode stack.