I am not aware of any ROM restrictions in this regard. The banks are dynamically sized. I'm not specifically familiar with the 605, since it is slightly different than the other djmemc controllers, but... There are 2 banks allocated for the soldered on memory. If you have the 605 with 4MB soldered, that's all in one bank. If you have the 605 with 8MB soldered, it's 4MB in each of the two banks, which means the banks are interleaved, resulting in about 10% or so faster memory accesses. So, if you're really trying to get the most out of your 605, make sure both banks are populated with the same memory.