CGA Graphics Modes Support Only One Video Page

MS-DOS                      | WINDOWS

The information in this article applies to:

• 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, version 1.0
  

In Microsoft C versions 5.0, 5.1, 6.0, 6.0a, 6.0ax, C/C++ version 7.0, and Visual C++ version 1.0 the functions _setactivepage() and _setvisualpage() from GRAPHICS.LIB can be used to change which portion of the video memory is being used. These two functions can be used in both text and graphics video modes. However, in a CGA graphics mode you are limited to one video page because the design of the original CGA adapter supports only one page in graphics mode.

This holds true even if your system has a newer graphics adapter, such as an EGA or VGA, which contains more video memory. Calling the function _getvideoconfig() correctly indicates that only one video page is available when set to any of the CGA graphics modes, because emulating a CGA graphics mode includes the 16K limit to video memory.