Mega Man 2 + Rockman 2 Scrolling Fix

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Mega Man 2 + Rockman 2 Scrolling Fix

Hack of Mega Man 2

Description

This patch rewrites the method used by MM2/RM2 for vertical scrolling so that it no longer shows visual artifacts on the top and bottom of the screen during scrolling.

This patch began as part of the MM2 FamiTracker support patch, before being split into its own patch as it could be useful even without FamiTracker support. The buggy implementation of DPCM sound in the NES can corrupt reads from other hardware such as the screen buffer and controllers, and requires special care when reading from this hardware to work around this corruption. One of the two places MM2 reads from the screen buffer is in performing vertical scrolls, as it does not take advantage of NES screen mirroring in this scroll direction (seemingly a holdover from MM1, which lacked the ability to switch between screen mirroring directions). This design also results in a visible scrolling seam at the top and bottom of the screen as new level data is loaded in real time.

This patch implements mirroring for vertical scrolling, allowing smooth, artifact-free vertical scrolling. In doing so, it also eliminates one of the ways MM2 reads from the screen buffer.

Patches for both Mega Man 2 (USA) and Rockman 2 (Japan) are included.

This patch incorporates my MM2 MMC3 patch from https://www.romhacking.net/hacks/7478/.

Title

Mega Man 2 + Rockman 2 Scrolling Fix

Game

Mega Man 2

Released By

Justin Olbrantz (Quantam)