MMC5 Hacking binge, found a few things.

I took a long break from this place. We just had a big snowstorm here, and in preparation, I made a new test setup that allows me to run an MMC5A in an NES with easily reprogrammable ROM and easily probed on all signals. All of my previous MMC5 testing was bench-testing and not in a real NES, which was insightful in some ways, but not really realistic in other ways. So this let me finally dig into a few things that I was curious about, but were really too complicated to replicate with a bench test. The setup allows for relatively easy swapping between MMC5 and MMC5A, though I have only used MMC5A so far. The setup can also be relatively easily swapped to a PAL NES, which I do have available, though I have only used an NTSC NES so far. The setup does not have any consideration for expansion audio.

I found a few new things, which I have already updated in the wiki:

• Found the function of pin 92; it disables all of the extended graphics modes when driven low.
• We knew that $2001 disabled 8x16 independent bank sprite mode. I found that it also similarly disables Extended Attribute Mode and Vertical Split mode.
• Found that extended attribute mode can be realistically used in conjunction with vertical split mode.
• Found that extended attributes are applied for fill mode, thus disabling the global palette index in $5107. A multi-tile, animated pattern/tesselation covering the screen could be employed this way.
• Confirmed that "in-frame" status does not reflect H-Blank (pretty sure we knew this already.)
• Corrected error I made with $5207. Output/input selection has no effect when set in $5800 peripheral mode (always output in this mode).

I would like some feedback/challenges on these things, and any additional ideas. When I found pin 92, it lead me to test $2001 and a number of other things, so please give your thoughts if these spark any new ideas.

Also, I was wondering if anyone has heard from org about decapping the MMC5. I had sent an MMC5A to krzysiobal in hopes to forward it to org in Russia, but I think that global tensions may have understandably prevented it making it there. I am wondering if there is anything any of us can do to help make progress with that.

Statistics: Posted by Ben Boldt — Wed Mar 27, 2024 11:13 am — Replies 0 — Views 115

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