15KHz Singlescan on Voodoo3

[SailorSat]

Hi Folks.

So I managed to add various Low-Res Modes to my Voodoo3's Timings via the Windows Registry.

So now I got a simple problem which I believe is either a bug in the driver, or a bug related to some registry keys.


So I added a Key
"HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\3dfxvs\Device0\TIMINGS\392,240\60Hz"

So I can get the 392x240 Resolution with 60Hz.

Also...
This Key has the value of
"504,408,448,265,243,246,12,8000000,5990,8,8924" which tells the driver about the monitor scanrate, pixelclock and negativ syncs etc.

As far as my knowledge is correct the value of "12" is the scan options field, and so, its bit encoded.

+1 = Doublescan
+4 = negative H-Sync
+8 = negative V-Sync

As you see, I don't use doublescan on this mode hence i get a nice and clear 15KHz Sync.

However... If I use this new Mode in Windows (as Desktop Resolution), Direct3D or even Glide, I only see the top half of the screen, because it gets doublescanned. The mouse cursor placement although is correct, looks quite funny if you draw a box using the mouse.
DirectDraw indeed works as it should, and gives a nice singlescanned image.



Now the 1-Million Dollar Question...
Why does Windows AND Direct3D AND Glide get Doublescanned although it shouldn't.
Why is DirectDraw correct?

Another Side-Question is...
The v4 and v5 can produce interlaced images, at least with a recent linux driver. How about the v3?
I would suspect that, in the syncoptions mentioned above, the value of +2 would indicate interlace, however, it doesn't do anything, and I've read on quite a few sites that a voodoo3/4/5 can't output interlace.