With ASIC9 machines (3a, 3c, 3mx, Siena, Workabout), RAM above 384K is paged in in 64K banks. The newly added 2MB is in banks 0x20-0x3F.
There are definite similarities in what's stored in 0x20-0x23, with some bytes just having one bit flipped. It then changes again for 0x24-0x27, 0x28-0x2B, 0x2C-0x2F. For all of those, the 4 high bits are predominantly on, and the 4 low bits predominantly off.
