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        INTRODUCTION


            The DOS Protected Mode Interface (DPMI) was defined to allow
            DOS  programs   to  access   the  extended   memory  of   PC
            architecture computers  while maintaining system protection.
            DPMI defines  a specific  subset of  DOS and BIOS calls that
            can be made by protected mode DOS programs.  It also defines
            a new  interface via  software interrupt  31h that protected
            mode programs  use to  allocate memory,  modify descriptors,
            call real  mode software,  etc.   Any operating  system that
            currently supports virtual DOS sessions should be capable of
            supporting DPMI without affecting system security.

            Some DPMI  implementations can  execute  multiple  protected
            mode programs  in independent  virtual machines.  Thus, DPMI
            applications can  behave exactly like any other standard DOS
            program and  can, for example, run in the background or in a
            window  (if   the  environment   supports  these  features).
            Programs that  run in  protected  mode  also  gain  all  the
            benefits of  virtual memory and can run in 32-bit flat model
            if desired.

            Throughout this  document, the  term "real mode" software is
            used to  refer to  code that  runs in  the  low  1  megabyte
            address space  and uses  segment:offset addressing.    Under
            many implementations  of DPMI,  so called real mode software
            is actually  executed in  virtual 8086 mode.  However, since
            virtual 8086  mode is  a very  close approximation  of  real
            mode, we will refer to it as real mode in this document.

            DPMI services are only available to protected mode programs.
            Programs running  in real  mode can  not use these services.
            Protected mode  programs must  use  the service described to
            enter  protected mode  before  calling  Int  31h services.

            All Int 31h functions will modify flags and the AX register.
            All other  registers  will  be  preserved  unless  they  are
            specified as  return values.   Unsupported calls will return
            with the  carry flag set.  Since Int 31h is set up as a trap
            gate, the interrupt flag will not be modified by any Int 31h
            calls  except  for  memory  management  and  interrupt  flag
            management calls.   All  memory management  calls may enable
            interrupts.  Interrupt flag management calls will modify the
            interrupt flag  as specified  by the  call.    All  Int  31h
            services are reentrant.

            Some implementations  of DPMI  can run 32-bit 80386 specific
            programs.   DPMI functions  that take pointers as parameters
            will use  the extended  32-bit registers  for  offsets  (for
            example, ES:EDI  instead of  ES:DI) when running 32-bit mode
            programs.   The high  word of  the 32-bit  registers will be
            ignored when running 16-bit protected mode programs.

            DPMI services  are provided  by what  will be referred to as
            the DPMI  host  program.    The  program(s)  that  use  DPMI
            services are  called DPMI  clients.  Generally, DPMI clients
            are two categories:

                 o    Extended Applications
                 o    Applications that use DPMI directly

            It is  believed that most DPMI applications will be extended
            applications.   Extended  applications  are  bound  with  an
            extender that  is the actual DPMI client and the application
            calls extender  services that  then are  translated  by  the
            client into  DPMI calls.    The  advantage  of  an  extended
            application over  one that  calls DPMI  services directly is
            that generally an extender will support more than just DPMI.
            In fact  it is recommended that extenders look for extension
            services in the following order:

                 o    DPMI
                 o    VCPI/EMS
                 o    XMS
                 o    Top-down (Int 15h)

            An extender  can provide  a single set of APIs to the actual
            application and then translate them to the services that are
            provided.   Where the  host extension services are "lacking"
            in a  particular function  the extender  must  provide  that
            function for the application.

            Figure 1 shows a picture of how this works. The
            application code  sits on  top of  a set  of  base  extender
            functions and  APIs.  The extender then has separate modules
            for each  type of extension service and code to "fill in the
            slack" where  services are lacking.  An example of a typical
            extender service  is protected  mode program  loading.   The
            actual shipped  application is the application code bound in
            with the extender and all of its styles of client support.

            The host  support is  generally an  extension of the base OS
            functions or  a device  driver used  to extend  the base  OS
            functions.

            This document  is intended  to provide  a definition  of the
            DPMI  services  that  a  DPMI  host  would  be  required  to
            implement and that a DPMI client would use.




            Figure 1.  Application/Extender/Client/Host/OS structure


            +----------------------------------------------------------+
            |                                                          |
            |  +----------------------------------------------------+  |
            |  |                                                    |  |
            |  |                  Application Code                  |  |
            |  |                                                    |  |
            |  +----------------------------------------------------+  |
            |                                                          |
            |  +----------------------------------------------------+  |
            |  |           Extender Base (including APIs)           |  |
            |  | -------------------------------------------------- |  |
            |  |    DPMI                                            |  |
            |  |   client                                           |  |
            |  +------------+                                       |  |
            |               |    VCPI                               |  |
            |               |   client                              |  |
            |               +------------+                          |  |
            |                            |    XMS                   |  |
            |                            |   client                 |  |
            |                            +------------+             |  |
            |                                         |  Top-down   |  |
            |                                         |   client    |  |
            |                                         +-------------+  |
            |                                                          |
            +----------------------------------------------------------+

               +------------+
               |            |
               |            |
               |            |------------+
               |            |            |
               |    DPMI    |            |
               |    host    |    VCPI    |------------+
               |            |            |            |
               |            |            |            |
               |            |------------|    XMS     |-------------+
               |            |    EMS     |            |  Top-down   |
               |            |            |            |  (Int 15h)  |
               +----------------------------------------------------+

               +----------------------------------------------------+
               |                                                    |
               |             Operating System (e.g. DOS)            |
               |                                                    |
               +----------------------------------------------------+