State Saving


            Because DPMI  hosts switch  stacks automatically across mode
            switches,  it  is  sometimes  necessary  to  use  the  state
            save/restore functions  while  using  the  raw  mode  switch
            services.  The host will maintain information on the "other"
            mode's current  state.   This information  will include  the
            CS:(E)IP, SS:(E)SP,  and segment register values.  Since the
            DPMI client  has no  way to directly access these values, it
            will need to call the state saving functions when performing
            nested mode switches.

            For example,  during hardware interrupts, the DPMI host will
            preserve the  real mode's  segment  registers,  CS:EIP,  and
            SS:ESP on the ring 0 stack.  However, they are not pushed on
            any stack  in the  VM --  They are  only visible  at ring 0.
            When the  raw mode  switch functions  are called  they  will
            overwrite the information saved by the host.  At this point,
            the program  would return  to the  wrong  address  when  the
            interrupt returned.