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PostPosted: Fri Jan 03, 2014 1:27 pm 
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Joined: Sun Feb 26, 2006 4:16 pm
Posts: 60
Hi,

I happened to read the handwritten notes I made when figuring out the Cheetahmen II board back in 1999, and compared with the mapper 228 info on the nesdev wiki. The wiki page mentions "It is claimed that there are four 4-bit RAM locations at $4020-$4023, mirrored throughout $4020-$5FFF."

Does anyone know where that claim originated? It's definitely not true for the Cheetahmen II board I examined, and it's almost certainly not true for Action 52 either. (I say almost because I don't have the notes I made about the Action 52 board layout handy. But there's nothing on the Action 52 board which could implement that. And the Action 52 board schematic shown here confirms that.)

Anyway. A couple of notes if anyone wants to add this info to the wiki entry...

On the Cheetahmen II cart I examined (PCB 023-N509 REV 0) the PRG ROM can't be disabled (unlike with Action 52). However on the board there was a cut trace. Had that trace not been cut, it would lead to the /CE pin of the PRG ROM. Would Cheetahmen II fail/crash on an emulator which always implements PRG ROM selection in its mapper 228 code?

Also, though of course no game uses it, I think the most likely use for the latched A4 address value would have been as the high CHR bit. E.g. if a board contained two 4Mbit CHR ROMs, the latched A4 could have selected which CHR ROM chip to use.


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PostPosted: Fri Jan 03, 2014 2:31 pm 
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Joined: Sun Apr 13, 2008 11:12 am
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Location: Seattle
mark_k wrote:
Would Cheetahmen II fail/crash on an emulator which always implements PRG ROM selection in its mapper 228 code?
Pretty easy to throw cheetahmen into a debugger and wait for mapper writes. It looks like it writes to $8800 often, which would have selected the "second" ROM of the four A52 supports.

In any case, I don't think A52 relies on the "third" ROM being absent, so this becomes a question of "how do you map the input N bytes over the 2MiB address space" rather than "can a game deselect itself"...


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