This is how an Atari 2600 would run Doom or try to anyway. I think Youtube exaggerates the flicker, which would be less noticeable on a CRT TV. It’s still not pretty.
Lol, I started that video in-thread and it was so irritating that I backed out in 3, 4 seconds.
That doesn’t look like the same sort of thing we’re talking about here, though, where there’s a real computer in the cartridge doing all the heavy lifting. That looks like someone attempted to dumb down the Doom executable to the point that it could at least approximately be run on an Atari. Note, for instance, that the view direction is always parallel and perpendicular to the walls: We never see a wall at an angle.
The previous video, though, about de-emulating an NES, is on point. And note that even though the NES is a much more capable device than the 2600, the end result still has some serious failings (most notably in the limited palette).
As I mentioned above, there’s no need to even perform the “load A”. The 6507 chip data bus drivers pull the outputs high, but they can be driven low by something else on the bus (like the cartridge). So the CPU just needs to store 0xFF over and over, and the cartridge can pull the appropriate pins low to get whatever data value is required.
I think this is true of the address bus as well, since it’s all the same NMOS technology, but I haven’t found explicit confirmation of that.
Some have mentioned the Starpath adapter, but there was also this adapted which was much less known about. Online Gaming On Demand for the Atari 2600! - CVC Gameline - YouTube. I don’t think it goes to answer it’s ultimate capability but it is interesting in allowing modem function.