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Q43587: Why _setvideomode() May Fail to Set Some Modes

Article: Q43587
Product(s): See article
Version(s): 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_QUICKC setvideomode _setvideomode | mspl13_c
Last Modified: 25-MAY-1989

The following are possible reasons that _setvideomode() might fail
when attempting to set a video mode:

1. Some of the older IBM EGA (enhanced graphics adapter) cards only
   have 64K of video memory. As such, they cannot display the
   640 x 350 16 (_ERES16COLOR) color EGA mode. However,
   lower-resolution modes may work correctly because they don't
   require that much memory.

2. There are EGA cards that have dip-switch settings to set the card
   into high- or low-resolution modes. Some BIOS ignore these dip
   switches and set the mode to high resolution. However, the
   _setvideomode() function does its own checking and may honor the
   dip-switch settings depending on the particular card. Setting the
   dip switches properly for the high-resolution modes should resolve
   the problem.

3. The video card may not support the requested mode. If the card does
   not support the mode, then _setvideomode() cannot set it into that
   mode.

4. Many VGA cards have non-standard extended modes that are unique to
   those cards. _setvideomode() supports only standard video modes
   and, thus, will not recognize the extended modes.

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