Ice Man wrote:
I tried wiring PRG A15-A18 ONLY to the '161 as well as MMC3 as well. Neither worked.
Well, that's definitely a major requirement, too, although evidently insufficient. (Only one output should be connected to any given wire, unless there's a way to make only one drive the wire at any given time)
My understanding is that mapper 189 actually comprises several major variations:
1- Several don't enable the MMC3 WRAM register before expecting to be able to select the 32K PRG bank. (Master Fighter 2 does appear to write $80 to $A001, however)
2- Several write to the $4020-$5FFF region instead of $6000-$7FFF region. (Master Fighter 2 does appear to write to $6132, however)
3- Several specify the bank in the lower nybble instead of the upper nybble. (Master Fighter 2 does appear to use the upper nybble, however)
4- Several use N108 clones instead of MMC3 clones
Interestingly, Mesen's and Nintendulator's implementations of mapper 189 causes Master Fighter 2 to crash, but Nestopia's doesn't. And there's nothing obviously different between their implementations... And NintendulatorDX says that it's not relying on uninitialized memory either...