There are the thing we do because we have to, and the things we do because we can. Climbing Mount Everest. Installing Doom on everything. Programmer Tom Murphy, who goes by Tom7, wants to see just what the original NES is capable of. Over the course of the below video, he gets it to do some amazing stuff, from running some very lo-fi full-motion video to running software it definitely shouldn't be able to run.
A lot of this goes a little bit over my head, but here's the short version: While games these days are simple executable files stored on discs and drives, games back then were on cartridges with microchips inside them. Game developers were able to add extra chips and circuitry into these cartridges to do things like add extra sound channels and even improve animation, squeezing more power out of the system than it shipped with. Konami's VRC6 chip made Castlevania III sound completely different to Japanese gamers; the American version of the system didn't support this particular chip.
What Tom's doing is sort of a modern, cranked-up version of that. here, he cannibalizes an existing Nintendo cartridge and wires in a Raspberry Pi 3 mini-computer that runs a game console emulator. The work includes a lot of manual wiring and some serious programming on Murphy's part.
He's not modding the NES itself at all. The way electronics were regulated by the FCC back then, Murphy notes, is what allows him to do this. These systems had to accept signal interference even if it would make them operate incorrectly, and he's able to use that to his advantage.
When compared to the way modern consoles work, this is kind of wild to imagine. An Xbox is an Xbox and a PlayStation is a PlayStation. No matter what game you run on the system, it's still the same system. Putting a cartridge into an NES or SNES essentially meant you were changing the hardware itself. It would be like slotting in a new card for each game you install on your PC because the game requires specialized hardware. It's unimaginable today, but it was commonplace back then.
Now, Murphy uses it to rickroll us. The whole video is a little long, but if you have any fondness for Nintendo hardware, it's worth a watch. And if you dig it, he also put together a 43-minute "Making Of" video to go with it.