MS-DOS Extenders Used in Microsoft Development Tools

ID: Q123667

The information in this article applies to:

SUMMARY

A number of Microsoft compilers, assemblers, and linkers use a DOS extender to gain access to memory beyond conventional memory. The LINK.EXE, BSCNAME.EXE, and CV.EXE programs that ship with Microsoft C/C++ version 7.0 and MASM version 6.1 have a built-in 16-bit DOS extender that can work with DPMI, VCPI, and XMS memory. The compiler that comes with Microsoft C/C++ version 7.0 uses a 32-bit DOS extender that only works with DPMI memory. Other compilers, assemblers, and utilities use a 32-bit DOS extender that will work with DPMI, VCPI, and XMS memory. This article describes these DOS extenders and memory managers.


MORE INFORMATION

When a DOS extender accesses memory, it goes through an extended memory manager to ask for extended memory. This prevents the DOS extender from conflicting with another program that is using extended memory.

Windows and OS/2 version 2.0 have extended memory managers built into their virtual machines (VM). If you are not using Windows or OS/2 version 2.0, you can use extended memory managers such as HIMEM, EMM386, 386MAX, and QEMM. These memory managers provide three different types of extended memory. The three types are: XMS (provided by HIMEM), VCPI (provided by EMM386 and older versions of 386MAX and QEMM), and DPMI (provided by everything else).

You can have your 16-bit program run in conventional memory and access data in extended memory through the memory manager, but this is tedious because the memory has to be swapped back and forth between conventional and extended memory, and only data can be stored in the extended memory. In addition, the program has to be written to do this swapping of conventional and extended memory. You can picture a program that uses extended memory, but does not use a DOS extender, to work like this: 


          -------------
         |   program   |
          -------------
            V       V
       --------   ----------------
      | MS-DOS | | memory manager |
       --------   ----------------
NOTE: The program accesses MS-DOS and the memory manager directly, but the program doesn't run in extended memory.

The DOS extender is what allows a program to run in extended memory and use extended memory. However, instead of accessing MS-DOS or the memory manager, the program does everything through the DOS extender. Using a DOS extender typically just requires the program to be linked with the DOS extender or linked with code that makes the program DOS extender ready. No special code has to be written to handle the extended memory. You can picture a program that uses a DOS extender to work like this: 

          -------------
         |   program   |
          -------------
                V
         -------------------
        |  MS-DOS extender  |
         -------------------
            V       V
       --------   ----------------
      | MS-DOS | | memory manager |
       --------   ----------------
NOTE: The program goes through the DOS extender for everything, and the program runs in extended memory.

The MASM version 6.0 assembler and LINK version 5.2 linker can use a 16-bit DOS extender that they load from MLX.EXE or LINKX.EXE respectively. This DOS extender is referred to as DOSX16. The DOSX16 DOS extender will work with a DPMI, VCPI, or XMS memory manager, in that order. If DOSX16 doesn't find DPMI memory, it checks for VCPI memory. If DOSX16 doesn't find VCPI memory, it checks for XMS memory. Running MLX or LINKX loads DOSX16 and allows the DOS-extender-ready ML or LINK to run using extended memory. You can picture the interaction between the program, DOSX16, the memory manager, and MS-DOS like this: 

          -------------
         |   program   |
          -------------
                V
     --------------------------
    |          DOSX16          |
     --------------------------
         V                    V
         V               <<< ------
         V             V    | DPMI |
         V       <<--------  ------
         V      V   | VCPI |
   --------   -----  ------
  | MS-DOS | | XMS |
   --------   -----
The C/C++ version 7.0 compiler uses a 32-bit DOS extender that it loads from MS32KRNL.DLL. This MS-DOS extender is referred to as DOSX32. Because 32-bit programs can't run on MS-DOS directly, you have to load the MS-DOS extender to run the compiler. The DOSX32 DOS extender will only work with a DPMI memory manager, so you have to load a DPMI memory manager. You could picture the interaction between the program, DOSX32, memory manager, and MS-DOS like this: 

          -------------
         |   program   |
          -------------
                V
      ---------------------
     |        DOSX32       |
      ---------------------
          V          V
     --------     ------
    | MS-DOS |   | DPMI |
     --------     ------
The compilers included with Visual C++ for Windows and Fortran Powerstation, the assemblers included with MASM version 6.1 and later, and some of the other utilities in these products use a 32-bit DOS extender that it loads from DOSXNT.EXE. This DOS extender is referred to as DOSXNT. Because 32-bit programs can't run on MS-DOS directly, you have to load the DOS extender to run the assembler. Unlike DOSX32, the DOSXNT DOS extender will work with a DPMI, VCPI, or XMS memory manager. Also unlike DOSX16, in cases where a VCPI memory manager is present but there is no page frame available, DOSXNT is still capable of accessing extended memory. You could picture the interaction between the program, DOSXNT, memory manager, and MS- DOS like this: 

          -------------
         |   program   |
          -------------
                V
     -------------------------
    |         DOSXNT          |
     -------------------------
       V                    V
       V              <<< ------
       V             V   | DPMI |
       V        <<-------------
       V       V  | VCPI |
   --------   ------------
  | MS-DOS | | XMS |
   --------   -----

Additional query words: kbinf 6.00 6.10 7.00 6.11 1.00 1.00a 1.50 


Keywords          : kb16bitonly 
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Last Reviewed: July 29, 1999