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Hack description

This patch converts MM2/RM2 to use the MMC3 mapper, allowing both larger ROMs (up to 512 KB PRG-ROM) and 2 switchable 8-KB banks, offering much greater flexibility for derivative hacks. Additionally, while this patch is primarily intended for hackers making their own MM2 hacks, it does slightly reduce lag, reducing CPU usage in the base game by about 0.6%, as well as fix the delay-scroll bug.

There were several motivations to create yet another MM2 MMC3 conversion.

1. Poor optimization. The previous MMC3 conversion adds an additional +25 cycles to each bank switch. While this doesn't sound like much, bank switches are observed to occur up to 25 times per frame (1500 per second). 25x25 cycles/frame add up to over 2% of the CPU, which directly increases lag. In contrast, this patch is actually slightly faster than the original game (-7 cycles per bank switch).

2. The previous conversion's banking implementation is not preemption-safe. That is, it is not safe to use 8 KB banks outside the NMI because the NMI handler reverts to 16 KB banking each frame. This patch provides preemption-safe bank-switching APIs for use outside the NMI.

3. This patch inherently fixes the delay-scroll bug present in the original game and the previous conversion.

Patches for both Mega Man 2 and Rockman 2 are included.

Contributions

Contributor	Type of contribution	Description
Justin Olbrantz (Quantam)	Hacking