PRB: Problems with Using the Digital Output of a VGA Adapter

MS-DOS                 | WINDOWS

The information in this article applies to:

• The C Run-time (CRT), included with:

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

When running programs such as GRDEMO that call the graphics function _setvideomode(), a border may be displayed around the monitor. Also, the functions _remappalette() and _remapallpalette() do not seem to work correctly.

CAUSE

Some VGA display adapters have analog- and digital-output capabilities. The above behavior occurs when the digital output of a VGA display adapter is connected to the monitor.

When the digital output is connected to an ECD (EGA monitor), a green border is displayed. When the digital output is connected to a multisync class monitor, a gray border is displayed. In either case, remapping the palette does not function. When the analog output is connected to a multisync or VGA monitor, it displays correctly.

The problem is that the graphics adapter is rightfully determined to be VGA class, although for practical purposes it is really EGA class. At this point, the C graphics library treats the display incorrectly as VGA. Connecting a digital monitor to the digital output of a VGA adapter is a nonstandard hardware configuration (the digital output of a VGA graphics adapter is nonstandard). The behavior of a nonstandard hardware configuration is unpredictable.