I hope this is an appropriate place to pass along that I've noticed the AVGCart has a problem with Ultima IV, when using the game's ATR images via the SIO functionality.
When you enter a location like a town, changes to the world map aren't saved. For instance, if you have a ship on the map or a chest or whatever and you enter a town, normally all that stuff would still be there when you exit the town. I'm pretty sure the game is supposed to re-load the world map from your last saved game and then update it with recent changes, but I could be wrong. With the AVGCart you get a pristine world map, devoid of recent items, each time you exit a town. Tested with the same images on Altirra, Atari800, and the Atari 800 MiSTer core, and on those the game works as it should. Also works as it should running the game from an SDDrive Max on real hardware. Put the AVGCart into my 130XE (my main machine is an 800XL), loaded the game there, and the problem is back once again. An example of the behavior would be to take a ship to an island with a town, exit your ship and enter the town. When you leave the town back to the world map, your ship will be gone and you will be stuck on the island.
Bonus: one weird side effect of my testing and copying these images back and forth is that one time while playing on the MiSTer, I acquired a ship and subsequently saved the game. Upon copying that image back onto the AVGCart SDCard and playing around a bit, I noticed I now have a permanent ship on the world map. I can take my ship somewhere and lose it and it will always return back to the position where I saved the game while testing on the MiSTer as soon as I enter then leave a location. I sorta chuckled over that one.
Anyhow, all in all, still loving the cartridge, beyond this one little sniggle that I've noticed.