[ovale]

Hello,

I was wondering about how Agnus uses the bitplanes modulo registers to increment the bitplanes pointers.

When, during a scanline, the modulo is added instead of 2?

If it would possible to use the copper to trick Agnus and cause the use of the modulo at a precise place of the scanline the it would be an effective way to implement horizontal split screen.

For example implement a 160+160 pixels split screen requires a 10 words modulo and the two bitplanes rows interleaved in RAM.

By changing the modulo just before it is added it would be possible to fine scroll vertically and coarsely horizontally the 2 sides indipendently.

So, this is the idea and a possible use. Some reverse engineering and tests are required to understand how to trigger the modulo use.

Does anyone already knows how it works (emulators authors and FPGA developers maybe know this) or someone has time to try it out?

Thanks,
Ovale