When I first got my Atari 800 in the 80's, I used it to develop some games + Utility software. I realised there was a 4K hole in the system that could be used to put debugging software etc.   I rarely played Computer Chess, so that cartridge got the chop. Also nothing used the right side cartridge slot.   I found a 4K RAM chip with the the same pinout as the ROM, did a little rewire of the cartridge and the 800 motherboard cartridge slot to enable writing to the RAM.   It worked a treat