Q64875: C’s _getvideoconfig Returns Incorrect Mode If Set from BASIC

Article: Q64875
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom S_C S_QuickC SR# S900808-148 | mspl13_basic
Last Modified: 4-SEP-1990

The C graphics function _getvideoconfig will not correctly return the
current video mode set by a BASIC program. To work around this
problem, the BIOS interrupt 10 (hex) function 0F (hex) can be used in
place of the _getvideoconfig function. This is shown in the sample
programs below.

This information applies to QuickBASIC versions 4.00, 4.00b, and 4.50;
to Microsoft BASIC Compiler versions 6.00 and 6.00b; and to Microsoft
BASIC Professional Development System (PDS) versions 7.00 and 7.10 for
MS-DOS and MS OS/2.

The following BASIC program, TEST.BAS, sets the video to screen mode 9
(EGA) and then calls a C function to return the current graphics mode:

   DECLARE FUNCTION CProc% CDECL ()
   SCREEN 9
   CLS
   PRINT "Video mode: "; CProc%
   while inkey$="" : wend
   END

The following C program, CVID.C, uses the C graphics function
_getvideoconfig, which returns an incorrect value when the video mode
has been set by BASIC.

#include <graph.h>

int CProc(void) {
   struct videoconfig vc;
   _getvideoconfig(&vc);
   return(vc.mode);
}

Compile and link as follows:

   BC TEST.BAS ;
   CL /c /AL CVID.C ;
   LINK /NOE TEST CVID ;

Normally, the returned video mode will be 3, which is incorrect. The
following C routine replaces CVID.C. When this is compiled and linked
to TEST.BAS, the correct video mode, 16, is returned.

#include <dos.h>

int CProc(void) {
   union REGS inregs, outregs;
   inregs.h.ah = 0xF;
   int86(0x10, &inregs, &outregs);
   return(outregs.h.al);
}

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