kyuusaku wrote:
the PPU doesn't have way to capture digital pixels
Fortunately, available tools make it easy to develop and run such a test ROM as long as it doesn't rely on the power-up state. Then in your test ROM, stick to monochrome video ($0F, $30) and sync to the master clock, and it might be possible to make PPU output "look digital" to a logic analyzer. Or are the levels all wrong for this?
But what people are eager to learn are how the addresses for the garbage nametable fetches are constructed.