PRODUCT  :  C++                                    NUMBER  :  865
  VERSION  :  All
       OS  :  WIN
     DATE  :  October 19, 1993                         PAGE  :  1/3

    TITLE  :  Using Windows Exported Selectors




  To allow access to various physical memory locations containing
  information related to the system setup or hardware devices,
  versions 3.x of Microsoft Windows exports several selectors (the
  selectors are more specifically exported by the KERNEL module of
  Windows).  The exported selectors include the following:

       Selector   Base   Description
       ========  ======  ==========================================
        __0000H  00000H  Real Mode Interrupt Vector Table
        __0040H  00400H  BIOS DATA AREA
        __A000H  A0000H  EGA/VGA Graphics RAM
        __B000H  B0000H  MDA RAM - Hercules Graphics RAM
        __B800H  B8000H  CGA/EGA/VGA RAM
        __C000H  C0000H  EGA/VGA BIOS - HD Adapt BIOS ROM
        __D000H  D0000H  Miscellaneous
        __E000H  E0000H  Miscellaneous
        __F000H  F0000H  System/BIOS ROM

  To access the physical memory using the KERNEL Exported Selectors
  from your Borland/Turbo C/C++ Windows Applications, the following
  approach may be used:

       (1)  Declare an extern far variable ( of type WORD )
            corresponding to one or more of the selectors listed
            above.  For example:

                 #include <windows.h>
                 extern WORD far _B000H;  // KERNEL's MDA Selector

            NOTE:  Since the Borland C/C++ Compiler automatically
                   inserts a leading underscore the extern variable
                   has only one underscore.

       (2)  Declare a far pointer whose segment is initialized with
            the LOWORD of the address of the external variable
            mentioned in section (1) above.  For example:

                 #include <windows.h>
                 #include <dos.h>

                 extern WORD far _B000H;

                 void far * GetMDARamPtr()













  PRODUCT  :  C++                                    NUMBER  :  865
  VERSION  :  All
       OS  :  WIN
     DATE  :  October 19, 1993                         PAGE  :  2/3

    TITLE  :  Using Windows Exported Selectors




                 {
                      void *lpMDA = MK_FP(LOWORD(&_B000H), 0x0000);
                      return(lpMDA);
                 }

  The far pointer obtained from section (2) can be used to access
  the physical memory from a Windows Application.  Since the
  selectors are exported by Windows KERNEL module, the IMPORT
  library (IMPORT.LIB) provided with Borland C++/Turbo C++ for
  Windows contains a reference to the selectors hence enabling the
  linker to resolve the external variable defined in section (1).

  The selectors (and far pointers) must be used with caution since
  the Windows 3.0 does not enable any protection attributes on the
  selectors exported by Kernel.

  The following code provides an example of the technique described
  above.  The current "Video Mode" is read from the BIOS Data Area
  (@0040:0049) using the _0040H selector and displayed using the
  Windows API function MessageBox.

  /* *********************************************************** */
  /*   Using Windows Exported Selectors Example                  */
  /* *********************************************************** */

  #include <windows.h>
  #include <dos.h>

  extern WORD far _0040H;        // Kernel Selector: BIOS Data Area
  extern WORD far _B000H;        // Kernel Selector: MDA Video RAM

  int PASCAL WinMain(HANDLE hInst, HANDLE hPrev, LPSTR CmdLine,
       int nCmdShow)
  {
       char szBuff[80], cVideoMode;
       char far *lpBDARam = (char far *)MK_FP(LOWORD(&_0040H), 0);

       cVideoMode = *(lpBDARam + 0x49);    // Video Mode: 0040:0049

       wsprintf(szBuff, "Video Mode ( _0040H:0049H): 0%Xh (%d)\n"
            "Use __B000H to write mode to Mono Screen?",
            cVideoMode, cVideoMode);














  PRODUCT  :  C++                                    NUMBER  :  865
  VERSION  :  All
       OS  :  WIN
     DATE  :  October 19, 1993                         PAGE  :  3/3

    TITLE  :  Using Windows Exported Selectors




       MessageBox(NULL, szBuff, "Video Mode",
            MB_TASKMODAL | MB_YESNO) == IDYES)

      return(TRUE);
  }

  DISCLAIMER: You have the right to use this technical information
  subject to the terms of the No-Nonsense License Statement that
  you received with the Borland product to which this information
  pertains.