I'll post a more detailed blog when it's working I guess...

After fiddling with so many options, I decided to go ahead with my original plan, just this time with two ROMs to get the 4MB.

For my first test, I burned the first 2MB to contain the menu and Thunder Force 2, and plugged it into the Genesis raw. To my surprise, TF2 came up instead of the menu. I theorized that the donor cart was only 512k, and it tied A19 high... this turned out to be the case. Tying the ROM's A19 to the Genesis A20 (which is the correct mapping because of the word instead of byte size), made it come up with the menu.

I then found my first issue -- I forgot to debounce the inputs. As a result, pressing up or down is as likely to jump two steps as one. That kind of sucks, but, the ROM is already burned and soldered, so heck with it, I thought.

Next, I added the flipflop. I used a 74LS174, which is a hex flip flop containing only a set and non-inverted output. There is a single clock and single clear. The idea being that two bits can be captured to use as address bits.

This was I realized my second problem. With two ROM chips, I need an inverted select in order to differentiate between the two chips. I could fake it with an extra line on the flipflop and a change to the menu, but as I said, that's already said and done.

I'm going to attempt to proceed as is, I should be to add an inverter to one of the lines to act as chip select. Worst comes to worst, I can fix the menu debounce.

Anyway, so I added the flipflop, and set it up for just the single chip for now. Unfortunately, it doesn't work, and that's why this entry has no real tech details either. The menu comes up, suggesting the flipflop is resetting correctly, but it never selects TF2, which it should, suggesting that the Flipflop isn't latching. I will put the logic analyzer on it tomorrow and try to see why not.