nesdev.com

Technically speaking, NES games consist of software and hardware, so anything that fits inside a cartridge and works without modifications to the console should be considered a proper NES game.

In the days before emulation, I doubt anyone would even argue over this, like nobody ever did with SuperFX SNES games, it was only after ROM files became common that people started expecting the software alone to be enough for running games, but that's only true if the emulator takes care of simulating all the hardware that cartridges can include.

The Super Gameboy obviously doesn't count as an SNES game because it doesn't work like other SNES cartridges, as it takes cartridges labeled with the name of another game console, so the most reasonable thing to do is call it an adapter.

It'd be really cool if ROM files could contain mapper implementations that emulators would somehow interpret. This could simplify emulators and solve compatibility issues, but I don't know how feasible it'd be to simulate the actual electronic signals between the emulated console and the emulated mapper... Emulators could still ignore these mapper definitions and do things the old way, though.

Exactly! The NES emulator would only really need to be programmed to deal with mapper 0 (and properly the NES's own internal workings), which is what the NES hardware does. It "thinks" every cartridge is a mapper 0 game, even this raspberry on a pi project. Which is why I don't see why it wouldn't be considered a NES game.

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Super FX and S-DD1 games: The game logic runs on the S-CPU, not the GSU or S-DD1.
SA1 games: Both processors have the same ISA.

Yoshi's Island definitely puts game logic on the GSU, but I don't know how much of it. I remember reading that it put most of the sprite/enemy logic on there, but I can't seem to find a source for that. That Data Crystal link has routines that handle things like object edibility, that I feel wouldn't be on the GSU side unless a big chunk of the game engine were already on the GSU side.

The RAM map confirms this too.

If anyone wants to talk more specifically about how it works, we seem to have a second thread going for that:
viewtopic.php?f=5&t=17409

It exists a way to create something like this?
I do not finish to understand by that it does not publish the diagrams and the software

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tom7's hack is ... well, a hack. I wouldn't use it, because the RasPi is basically wholly unsuitable for real-time uses.

(Beyond all the other complaints I've enumerated just about latency, supposedly whatever the GPU is doing causes an unpreventable pause about twice a second for DRAM refresh.)

We actually already had someone come to the forum to describe doing this The Right Way before, previously linked: viewtopic.php?t=16807

I remember that when I was a kid me and some friends misunderstood an magazine ad and were anxiously waiting for an adapter to use SNES games on our NES clones.
A friend of mine, who also misunderstood another ad, also said that a MegaDrive adapter for the Master System already existed.

Now I know that technically that's possible, altough not practical neither cost efective, be it now or back in the day.