This DPMI 0.9 Specification is hosted by Tenberry Software, Inc.

11. Translation Services


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These services are provided so that protected mode programs can call real mode software that DPMI does not support directly. The protected mode program sets up a data structure that contains the values for every register. The data structure is defined as:


                  Offset      Register

                    00h         EDI

                    04h         ESI

                    08h         EBP

                    0Ch  Reserved by system

                    10h         EBX

                    14h         EDX

                    18h         ECX

                    1Ch         EAX


                    20h    Flags


                    22h      ES


                    24h      DS


                    26h      FS


                    28h      GS


                    2Ah      IP


                    2Ch      CS


                    2Eh      SP


                    30h      SS

You will notice that all of the fields are dwords so that 32 bit registers can be passed to real mode. Most real mode software will ignore the high word of the extended registers. However, you can write a real mode procedure that uses 32-bit registers if you desire. Note that 16-bit DPMI implementations may not pass the high word of 32-bit registers or the FS and GS segment registers to real mode even when running on an 80386 machine.

Any interrupt handler or procedure called must return with the stack in the same state as when it was called. This means that the real mode code may switch stacks while it is running but it must return on the same stack that it was called on and it must pop off the entire far return/iret structure.

After the call or interrupt is complete, all real mode registers and flags except SS, SP, CS, and IP will be copied back to the real mode call structure so that the caller can examine the real mode return values.

Remember that the values in the segment registers should be real mode segments, not protected mode selectors.

The translation services will provide a real mode stack if the SS:SP fields are zero. However, the stack provided is relatively small. If the real mode procedure/interrupt routine uses more than 30 words of stack space then you should provide your own real mode stack.

It is possible to pass parameters to real mode software on the stack. The following code will call a real mode procedure with 3 word parameters:

     Protected_Mode_Code:
             push    Param1
             push    Param2
             push    Param3
             (Set ES:DI to point to call structure)
             mov     cx, 3           ; Copy 3 words
             mov     ax, 0301h       ; Call real mode proc
             int     31h             ; Call the procedure
             add     sp, 6           ; Clean up stack

The real mode procedure would be called with the following data on the real mode stack:


                           Param1

                           Param2

                           Param3

                           Return
                             CS


                           Return
                             IP

                                     <-- Real mode SS:SP

If your program needs to perform a series of calls to a real mode API it is sometimes more convenient to use the translation services to call a real mode procedure in your own program. That procedure can then issue the API calls in real mode and then return to protected mode. This also avoids the overhead of a mode switch for each API call.

There is also a mechanism for protected mode software to gain control from real mode via a real mode call-back address. Real mode call-backs can be used to hook real mode interrupts or to be called in protected mode by a real mode driver. For example, many mouse drivers will call a specified address whenever the mouse is moved. This service allows the call-back to be handled by software running in protected mode.



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This HTML edition of the DPMI 0.9 Specification is hosted by Tenberry Software, Inc., makers of the DOS/16M and the DOS/4G family of DOS extenders. Page last modified 2003.1.28, <webmaster@tenberry.com>