Tilegen background colour

[Bart]

Well I think I found out where the white is coming from. I *think* it's coming from jtag, it seems to get written at exactly the point where the screen flashes white in harley, and the same for the intro in lost world.

I believe there are some tilegen registers (and almost certainly other video-related registers) that are not exposed in the memory map but which are documented by Charles MacDonald's System 24 docs. They must be set through JTAG. I wonder if the JTAG routines themselves are structured in a logical way (e.g., write this 32-bit value to this register, with the JTAG bit pattern being constructed on the fly) or whether everything is pre-encoded as a bitstream. If the former, then it might be possible to make sense of what is going on by simply reverse engineering the top-level routines that write to JTAG.

[Jiterdomer]

Are you guys still working on this one?

[Ian]

Not currently
But I wonder if the jtag (assuming that is what is changing the colour) is just setting an xor flag. Something like that would invert the bits being written. OpenGL can do something like this to invert the colours written to the colour buffers.