Mega Man 2 Sprite Lag Reduction Patch
v1.0

By Justin Olbrantz (Quantam)

This patch eliminates a gross inefficiency in MM2/RM2's sprite buffer clearing method, reducing overall CPU usage by about 8% and lag by at least as much.

This patch has both Mega Man 2 and Rockman 2 versions: mm2spritelag.ips and rm2spritelag.ips, respectively.

Mega Man 2 (USA):
PRG-ROM CRC32 0FCFC04D / MD5 0527A0EE512F69E08B8DB6DC97964632
File CRC32 5E268761 / MD5 8E4BC5B03FFBD4EF91400E92E50DD294
File CRC32 80E08660 / MD5 302761A666AC89C21F185052D02127D3
File CRC32 A9BD44BC / MD5 CAAEB9EE3B52839DE261FD16F93103E6

Rockman 2:
PRG-ROM CRC32 6150517C / MD5 770D55A19AE91DCAA9560D6AA7321737
File CRC32 30B91650 / MD5 055FB8DC626FB1FBADC0A193010A3E3F

Others with the same PRG-ROM checksums will likely work just as well.

COMPATIBILITY

The clear OAM buffer function uses memory address $c7 as an out-of-interrupt temporary. It should be safe to use this address for other out-of-interrupt temporaries so long as they don't overlap with the clear OAM buffer function.

This patch uses the ROM range (file offsets) 2bb46-2bb87 that was previously free.

INCORPORATION INTO OTHER HACKS

As everybody needs less lag, it's perfectly fine to incorporate this patch into other hacks (especially content hacks). In such cases, please include a line in the readme (or hack description/credits if there is no readme) indicating that the derivative hack uses this and link to this patch on RHDN.


Finally, thanks to Kasumi for pointing out this inefficiency in MM2 on the NesDev Discord.