When starting a game that was made for the original DS, the DSi removes access to all new features and cuts down the RAM to 4MB so it acts like a regular DS. Your flashcart will be detected as old DS software and run in this mode.
It would help with porting other N64 games that use the Fast3D microcode, as what this port mainly consists of is a layer that "translates" the Fast3D commands to something the DS GPU can use. However, as the other person said we need a fully decompilation of the game before this can be done.
Thanks a lot for the clarification!
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also, at this point someone needs to make a list of every port of sm64 made
So far I know about these ports:
-3DS
-Android
-Dreamcast
-MS-DOS
-Playstation 2
-Playstation Portable
-Playstation Vita
-Switch
-Wii U
Don't know if there have been any "new" port added, but would love to see a Wii port, don't know if its possible though.