Hi Paolo,
First of all - you'll need to supply some more info because otherwise it can be a bit difficult to answer your questions!

- Did you install NVDI (and in such case - which version)
- Does NVDI start AFTER SV_XBIOS.PRG
- Did you copy the SuperVidel NVDI drivers (SVSCREEN.SYS, SV_BPSxx.SYS) to your GEMSYS folder?
- Do you use the ASSIGN.SYS file provided with the SuperVidel drivers, or did your modify your own? In case of the latter - did you make sure that the device entries in this file are written *exactly* they way they're written in the supplied ASSIGN.SYS file? Did you put this file in the correct place (usually C:\)?
- Which version of the CT60-TOS are you using?
Basically, a faulty installation step can appear to "work", but there can be annoying side effects.
Paolo wrote:By reading the SV.inf file, you see that the 8bit chunky and 24 bit modes are set to false, with the comment that NVDI takes care of them.When I open the videomode dialog, however, the 8bit mode is enabled and the 24 and 32 bit modes are disabled, no matter if I use a low resolution, double line or whatever.
The 8bpp chunky and 32bpp modes are disabled by default since Atari didn't 100% finish the TOS4 VDI for these modes. The NVDI drivers enabled support for them since NVDI has full support for these modes. If you like you can enable them in SV.INF too, but it's not very useful since some applications/icons etc can look a bit wrong. The "Operating Mode" setting in the desktop resolution dialog *must* be set to "SUPER" or these modes won't be available. If you still have trouble setting a 32bpp mode, we'll have to investigate it (because it's supposed to work; it works on my machine here using the same driver version).
I can't understand why NVDI lets me select the 8bitc mode and not the 24 bit mode.
The SuperVidel doesn't actually have a 24-bit mode; it "only" has 16-bit and 32-bit truecolor modes. This is because 32-bit modes are actually much more efficient. 24-bit mode was planned, but really it's a 150% useless feature because it's slower than 32-bit modes, and it doesn't support alpha blending etc. So take a deep breath and just forget about 24-bit graphics, because 32-bit graphics is way much cooler

I couldn't find any reference to this in the manual.
To be honest - the SV.INF file is a late invention and isn't documented because of that. The contents of it *will* change somewhat the next few driver versions. You can try changing the options in it, some of them are attached to a function and some are not. Most notably the file is used to select boot resolution and to select a resolution for the AES (default VDI workstation resolution) etc. The other stuff may change, and there will be new options for dual screen configuration etc. So don't worry too much about it right now, and if you have questions about settings, post them here and I'll answer then for you!