The MMU is implemented in hardware on the CPU board. The main logic unit is located on the M2 CPU board. To support the new R10000s, a new CPU board, the "IP25" was introduced. The P112 is a stand-alone 8-bit CPU board. The replaceable CPU board allows future upgrades of CPU and RAM. Some systems which used UltraSPARC III processors could accept UltraSPARC IV CPU board upgrades. The CPU board included a fully compatible Multibus (IEEE 796 bus). A PC/104-Express CPU board may be used with PCI-104 and PC/104-"Plus" peripheral modules. However, a PC/104-"Plus" CPU board may be used with a PC/104 peripheral module. In this case it used a "CPU board" and a "Video Board". Domain granularity is one CPU board (single system may have 1-16 of them).