DOCUMENT:Q123688  04-MAY-2001  [masm]
TITLE   :Turning a 16-bit Protected Segment into a 32-bit Segment
PRODUCT :Microsoft Macro Assembler
PROD/VER::6.0,6.1,6.11
OPER/SYS:
KEYWORDS:

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-------------------------------------------------------------------------------
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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Copyright Microsoft Corporation 2001.