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Q123688: Turning a 16-bit Protected Segment into a 32-bit Segment

Article: Q123688
Product(s): Microsoft Macro Assembler
Version(s): 6.0,6.1,6.11
Operating System(s): 
Keyword(s): 
Last Modified: 04-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 6.0, 6.1, 6.11 
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SUMMARY
=======

By default, 32-bit code created in MASM exists in 16-bit segments. This article
shows by example how to use DPMI calls to change a 16-bit segment into a 32-bit
segment.

NOTE: On a 386, almost no improvement in performance results from using 32-bit
code in a 32-bit segment over using 32-bit code in a 16-bit segment. There may
be slight performance improvement on a 486 due to the effects of the 66h prefix
byte on instruction pipelining.

MORE INFORMATION
================

The following code fragment uses DPMI calls to turn a 16-bit protected mode
segment into a 32-bit protected mode segment. A number of the instructions are
encoded using the DB directive because you need to generate USE16 instructions
inside of a USE32 segment. The sample includes an interface function so that you
can call the 32-bit segment from a 16-bit C, C++, or FORTRAN module.

NOTE: Your debugger may still report these segments to be 16-bit segments. It all
depends on what the debugger expects.

Sample Code
-----------

     .MODEL large
     .386

     .CODE
     SwitchInterface PROC FAR16
        call FAR32 PTR Switch16bitTo32bit
        ret
     SwitchInterface ENDP

     CODE32 SEGMENT PARA PUBLIC USE32 'CODE'
     Switch16bitTo32bit PROC FAR32
        xor eax, eax                       ;zero set
        xor eax, DWORD PTR 0C08B0000h      ;zero clear for 32-bit
        jz Still16bit                      ;zero set for 16-bit
        ret

     Still16bit:
        DB 083h, 0ECh, 008h        ;sub sp, 8
        DB 08Ch, 0D0h              ;mov ax, ss
        DB 08Eh, 0C0h              ;mov es, ax
        DB 08Bh, 0FCh              ;mov di, sp

        DB 08Ch, 0CBh              ;mov bx, cs
        DB 0B8h, 00Bh, 0000h       ;mov ax, 000Bh
        int 31h

        DB 026h, 080h, 04Dh, 006h, 040h    ;or BYTE PTR es:[di+6], 40h
        DB 0B8h, 00Ch, 0000h       ;mov ax, 000Ch
        int 31h

     Now32bit:
        add sp, 8
        ret
     Switch16bitTo32bit ENDP
     CODE32 ENDS

     END

Additional query words: kbinf 6.00 6.10

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611
Version           : :6.0,6.1,6.11

=============================================================================

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