Since the Nubus uses only 32-bit wide accesses, 8-bit wide peripherals such as the IP-Octal Serial appear in the host address space every fourth byte.
To calculate the RM1260 Springboard register addresses from the VMEbus address (given in the previous section in Figures 4 and 5), multiply by two and subtract one. To convert VME addresses to RM1270 SupportBoard addresses multiply by two and add one.