8bit roms to 16bit databus
I've been wrecking my brain over something.
I'm trying to build a genesis homebrew cartridge out of a none working sonic 1 I have here (the maskrom is toast)
So the easy part is when I just have 2 8bit roms (27c801) which I just hook up together and have an ODD and EVEN rom so I can use roms up to 2 MB.
Now the fun part is when I want to add another 2 roms, making the total space 4MB.
I know I have to use a 74LS139, tie the comined OE's of roms U1-HI and U1-LO (I'll call the first set of combined eproms U1 and the other combined set U2) to one output, the OE combined output of U2-HI and U2-LO to another output and the OE on the cart to the enable pin of the 139.
But then comes the tricky part of what address lines need to be connected to the 139.
I think it needs A20 (which is connected to also connected to A19 of my eproms) to select 1 of my 139 and address line A21 from the cartridge connector to select 2 of the 139.
And also, what is a good too for splitting the rom to 4 seperate files so I can put the on my roms?
I found some tools but man where are the good tools if you need them