Question: how do you access the RAM? I see you've use the top bit of the address to toggle between ROM and RAM, but can you input that bit in the code?
If the instruction takes the top four bits, then the address can only be the bottom four. That makes only 16 memory locations total.
I'm running into the same issue on my system and I'm considering adding control line to select either RAM or ROM. (though it would still be limited to 16 locations each).