Storing data in SWCHB

[sa666666]

SWCHB can actually be used to store a small amount of data, as illustrated in the following:
http://atariage.com/forums/topic/167678-strange-color-problem-with-stella/page-2#entry2075671

This is already emulated in Stella some time ago. In my testing, only MAME emulates this; z26, javatari and Stellerator don't. I think it's actually used in Combat too, so it's not just an idiosyncrasy of the hardware that nobody uses.

You'll know if it works when you see the display cycling through the contents of ZP RAM, starting from $FF and going downward. When not implemented, the ZP RAM address stays at $FF (which makes sense, as the counter used the ROM is stored in SWCHB).

[DirtyHairy]

Thank you for bringing this up. Actually, DDRA / DDRB and the O in GPIO are currently unimplemented. I have read about ROMs storing data in SWCHB before, but I forgot where and which ones, and so I have ignored this aspect. The test ROM above should be enough fodder for testing, though 😏

The implementation should include a refactoring that separates RIOT GPIO from the controls (joysticks and switches) --- those are hardwired atm. This is a necessary prerequisite for implementing more esoteric controllers like the Atari keypads. It might even be worthwhile to communicate between the workers on GPIO level rather than on controller level.