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        Virtual DOS Environments


            Many  DPMI   implementations  are  simulated  "virtual  DOS"
            sessions.  In other words, the DOS interface and environment
            presented  to  the  program  are  not  actually  the  native
            interface of  the operating  system.   Hardware  interrupts,
            I/O, and  processor exceptions  will be  virtualized by  the
            operating system.   This  means, for  example, that  a  DPMI
            program may  receive a simulated keyboard interrupt and read
            simulated I/O from the keyboard controller ports.

            In these  environments, actual  hardware interrupts  will be
            handled by  the operating  system.   The physical interrupts
            will be  invisible to  the DPMI application program.  If the
            operating system  so  chooses,  it  may  reflect  a  virtual
            interrupt to  the DPMI  program.   The DPMI program does not
            need to know, nor should it care, if this is the case.  From
            the program's  point of  view, the  interrupt looks  exactly
            like a  "real" interrupt.   The  operating system  will also
            virtualize I/O  to the  interrupt controller  ports and  any
            other simulated devices.

            There are basically three levels of virtualization that DPMI
            implementations can provide:

        No Virtualization

            In general,  stand-alone single tasking DPMI implementations
            will not  virtualize  any  hardware  devices.    These  hose
            extension programs  will execute  as standard  DOS real mode
            drivers or  programs.   Extenders  which  use  the  services
            provided by these DPMI host drivers will translate protected
            mode DOS  calls to  real mode  DOS calls.    Normally  these
            extenders will  invoke DPMI services to return the processor
            to real  mode (instead  of virtual  8086 mode)  when calling
            DOS.

        Partial Virtualization

            Some environments  that execute  under DOS  will  virtualize
            hardware devices,  provide virtual  memory, or provide other
            services  that   require  virtualization  of  some  hardware
            devices.   Under these  environments, DPMI applications will
            always run  at a non-privileged ring (usually ring 3).  Some
            or all  hardware interrupts will be virtualized, some or all
            I/O  will   be  virtualized,   and  virtual  memory  may  be
            supported.     Under  these  implementations,  page  locking
            services  usually   must  be  used  to  lock  interrupt  and
            exception handling code.


        Complete Virtualization

            These  environments   provide  a  completely  simulated  DOS
            environment.  The native operating system is something other
            than MS-DOS.   Under  these  implementations  of  DPMI,  all
            devices will  be virtualized to some extent.  Normally, page
            locking services  will be  ignored by  these implementations
            since all physical device interrupt and I/O handling will be
            performed by the operating system.  Programs will always run
            at a non-privileged ring.