gdjacobs wrote:I understand that some early GFX modes (certain CGA modes, for instance) weren't carried forward to be backwards compatible in the VGA standard.
To be exact, EGA and VGA are BIOS-compatible with CGA, but not register-compatible.
This means that as long as software sticks to BIOS-routines only, CGA software should run as-is on EGA and VGA.
In practice however, most games access the registers directly, at least to set the CGA palette, and as a result, CGA games running on VGA will generally not show the correct palette, but stay in the default cyan/magenta/white/black palette mode.
More advanced trickery, such as in 8088 MPH, is also doomed to fail, since it exploits the raster timing of CGA, and also assumes that you have composite output.
Some VGA clones can be switched to a CGA-compatibility mode, which will give it some level of CGA-compatibility. This usually fixes the palette issues, but other register-tricks or timings will still not work, because VGA uses different resolutions (CGA uses 200-line modes at 59.94 Hz refresh-rate, compatible with NTSC. VGA uses a 400-line mode at 70 Hz, and 200-line modes are simulated by line-doubling).
There already is the CGA compatibility tester made by Trixter, which covers these issues to a certain extent:
http://www.oldskool.org/pc/cgacomp
CGA Compatibility Tester vs. VGA cards
Re: CGA Compatibility Tester
http://www.vcfed.org/forum/showthread.php?137 … ard-in-CGA-mode
The short version however, is that VGA is too different from CGA for any clone to hope to get anywhere near 100% compatibility.