Hello, Folks,
I finally managed to get here thanks to Vlad's hints 😉
First of all, there is a different behaviour with Nvidia Cards between Windows 2000 and Windows XP. I haven't researched Win2000 for that reason. The real issue for us was WinXP only.
What I don't get at this point is the crashes respective disabling of the full screen modes in WinXP as reported here. I have to admit, that I actually didn't yet modify the VGA.SYS driver but manipulate the memory image only at this point (waiting for MS to respond for a DDK - to enable a proper and easy go-around the file protection). With that fix I didn't encounter any problem whatsoever, but a full working VESA, that enables proper pixel clock and timing for all modes up to 1280*1024. That's why I approached Vlad with the idea to have that hack perhaps incorporated into NTVDM (which isn't feasable for technical reasons as Vlad told me).
I shall certainly try to get to the point, where I can trick out the SFC either via DDK or other hacks and then see, what is happening on my machine. To answer someone's questions: My debugging system has been a WinXP professional with an Nvidia MX440. I look forward to test the hack on my WinXP home machine with considerable interest, too.
As I indicated, I don't get why the modified VGA.SYS causes problems. I can only imagine, that the driver isn't loaded because of a check sum problem or the like, which clearly results in lack of full screen support altogether - and may perhaps even cause crashes.
I hope, that helps to clarify some of the issues and provide some thoughts of how some of the details can be worked around.
Simon