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        Interrupts


            Protected mode  programs can hook both hardware and software
            interrupts  using  the  DPMI  get  and  set  protected  mode
            interrupt vector functions. All interrupts
            from  hardware   devices  such  as  the  timer  or  keyboard
            controller will  always be  reflected to  the protected mode
            interrupt handler  first.   If the  protected  mode  handler
            jumps to  or calls  the previous  interrupt handler then the
            interrupt will be reflected to real mode.

            As in real mode, interrupt procedures can either service the
            interrupt and  iret or they can chain to the next handler in
            the interrupt chain by executing pushf/call or by jumping to
            the next  handler.  The final handler for all protected mode
            interrupts will reflect the interrupt to real mode.

            When an  interrupt is  reflected to real mode, the EAX, EBX,
            ECX, EDX,  ESI, EDI,  EBP registers,  and flags  will all be
            passed from  protected to  real mode unaltered.  The segment
            registers  will  contain  undefined  values  unless  an  API
            translator (such  as a  DOS or  BIOS translator)  explicitly
            sets a  real mode segment register.  DPMI will automatically
            provide a  real mode stack for interrupts that are reflected
            to real mode.


        Hardware Interrupts

            The interrupt controllers are mapped to the system's default
            interrupts.   On an  IBM AT-compatible  system, for example,
            the master  interrupt controller  is programmed  with a base
            interrupt of  8 and  the slave controller has a base of 70h.
            The virtualized  interrupt controllers  can be reprogrammed;
            the base  setting may be examined in protected mode with Int
            31h function 0400h.

            Hardware interrupt  procedures and  all of  their data  must
            reside in  locked memory.   All  memory that  is touched  by
            hardware interrupt  hooks must  be locked.  The handler will
            always be  called on  a locked  stack.

            As in real mode, hardware interrupt handlers are called with
            interrupts disabled.    Since  iret  will  not  restore  the
            interrupt flag, hardware interrupt hooks must execute an sti
            before  executing   iret  or  else  interrupts  will  remain
            disabled.

            Protected mode  hardware interrupt  handlers will  always be
            called even  for interrupts  that occur  in real  mode.  The
            last hook on the protected mode interrupt chain will reflect
            the interrupt to real mode.

            Protected mode hardware interrupt handlers that need to call
            software running  in real  mode must either be sure that the
            real mode  software that  they are  calling will  not modify
            segment registers  or they  must use  the state save service
            to  save and  restore the  real mode  segment
            registers.   However, any  interrupt handler  that  executes
            completely in  protected mode,  or uses translation services
            0300h, 0301h,  or 0302h  does not need to save the real mode
            register state.   Therefore,  this is  not an issue for most
            interrupt handlers.



            For compatibility  with older  systems, computers  with  two
            interrupt controllers  have the  BIOS redirect  one  of  the
            interrupts from  the slave  controller into the range of the
            master controller.   For example, devices jumpered for IRQ 2
            on IBM  AT-compatible computers  actually interrupt on IRQ 9
            (interrupt 71h).   In  real mode,  the BIOS on these systems
            will convert  interrupt 71h  to Int  0Ah and  EOI the  slave
            controller.   A protected  mode program that needs access to
            the redirected  interrupt may  use variations  on either  of
            these techniques:

                 1.   Hook the  target interrupt  in real  mode.    This
                      takes advantage of the built in redirection.  This
                      is robust  on systems  where  other  software  has
                      reprogrammed the  interrupt controllers,  or where
                      the slave interrupt controller may be absent.

                 2.   Hook  the   actual  interrupt  in  both  real  and
                      protected mode.   In  this case,  the program must
                      EOI  both   the   slave   and   master   interrupt
                      controllers since  the BIOS  will not get control.
                      This is  more efficient  in that there will not be
                      any unnecessary switches to real mode.

        Software Interrupts

            Most software  interrupts executed  in real mode will not be
            reflected to  the protected  mode interrupt hooks.  However,
            some software  interrupts are  also reflected  to  protected
            mode programs when they are called in real mode.  These are:


                       INT            DESCRIPTION

                       1Ch    BIOS timer tick interrupt
                       23h    DOS Ctrl+C interrupt
                       24h    DOS critical error interrupt


            Programs should  not terminate  during interrupts  that were
            reflected from  real mode.   Terminating the program at this
            point may prevent the DPMI host from cleaning up properly.

            Of all software interrupts, only Ints 00h-07h will be called
            with virtual interrupts disabled.  For these interrupts, the
            handler should  return with  interrupts enabled.   All other
            interrupts will not modify the interrupt flag state.

            Since most  software interrupts  that are  executed in  real
            mode are  not reflected  to protected  mode interrupt hooks,
            programs would  be required to install a real mode interrupt
            hook to monitor these interrupts.