16 Background Colors in Text Mode with MCGA, EGA, or VGA

• The C Run-Time (CRT), included with:
  • Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
  • Microsoft C/C++ for MS-DOS, version 7.0
  • Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5

SUMMARY

The attribute byte for video display in text mode is

     7  6  5  4   3   2  1  0
     -------------------------
    | |         |   |         |
     ------------------------- 

   Bits  Control
   ----------------------------------
   0-2   Foreground color
     3   Foreground intensity
   4-6   Background color
     7   Blinking for foreground
          - or -
         Intensity for background 

MORE INFORMATION

The BIOS call that toggles the interpretation of bit 7 is Interrupt 10h, function 10h, subfunction 03h. This call is available only with an MCGA, EGA, or VGA. A zero (0) in BL enables intensity, a one (1) in BL enables blinking text.

The following sample program illustrates the effect of this toggle. First, bit 7 is left as the blink bit; therefore, there are eight background colors and 32 foreground colors. Each of the 32 foreground colors is displayed once and each background color is repeated four times.

Next, bit 7 is toggled to intensity; therefore, there are 16 background colors and no blinking text. Still, you have to go through foreground colors 0-31 to view all 16 background colors. This is because bit 7 is still controlled in the setting of the foreground color; however, it is now interpreted to affect the display of the background color rather than the foreground color. Viewing the output of the sample program should clarify this concept.

Sample Code

/* Compile options needed: none
*/ 
#include <graph.h>
#include <dos.h>
#include <conio.h>
#include <stdio.h>
#define TRUE  1
#define FALSE 0
#define VIDEO_IO        0x10
#define BIOS_CALL       0x10
#define INTENSITY_TGL   0x03
void toggle_intensity( int );
void main( )
{
   long i;
   int row;
   int color;
   char buf[80];
   _settextrows( 43 );
   _clearscreen( _GCLEARSCREEN );
   toggle_intensity( 1 );
/*  Bit 7 = blink; there are 8 background colors and blinking text */ 
   for (i = 0, row = 1, color = 31; i <= 31; i++, row++, color--)
   {
      _setbkcolor( i );
      _settextcolor( color );
      _settextposition( row, 10 );
      sprintf( buf, "  Background %2ld  Foreground %2d\n", i,color );
      _outtext( buf );
   }
   _setbkcolor( _BLACK );
   _settextcolor( 7 );
   _outtext( "\n\nSetting 8 background colors, blinking text." );
   _outtext( "\n\nStrike any key to continue..." );
   getch( );
   _clearscreen( _GCLEARSCREEN );
   toggle_intensity( 0 );
/*  Bit 7 = intensity; there are 16 background colors */ 
   for (i = 0, row = 1, color = 31; i <= 31; i++, row++, color--)
   {
      _setbkcolor( i );
      _settextcolor( color );
      _settextposition( row, 10 );
      sprintf( buf, "  Background %2ld  Foreground %2d\n", i,color );
      _outtext (buf);
   }
   _setbkcolor( _BLACK );
   _settextcolor( 7 );
   _outtext( "\n\nSetting 16 background colors, no blinking text." );
   _outtext( "\n\nStrike any key to quit..." );
   getch( );
   _setvideomode( _DEFAULTMODE );
   _clearscreen( _GCLEARSCREEN );
}
void toggle_intensity( int onoff )
{
   union REGS regs;
   regs.h.ah = VIDEO_IO;
   regs.h.al = INTENSITY_TGL;
   if ( !onoff )
      regs.h.bl = 0x0;
   else
      regs.h.bl = 0x1;
   int86( BIOS_CALL, &regs, &regs );
} 

Additional query words: kbinf 1.00 1.50 5.10 6.00 6.00a 6.00ax 7.00

Keywords          : kb16bitonly 
Version           : 
Platform          : 
Issue type        : 

Last Reviewed: July 21, 1999