When trn says that I/O is handled by the FNP, he actually means terminal I/O and communications I/O. Actual disk, tape, and other peripheral I/O is performed by the Input Output Multiplexer (IOM) — yet another hardware component in the DPS8 hardware suite. The CPUs. IOMs, SCUs, and FNPs are emulated by the DPS8M simulator. And yes, the DPS8M simulator supports multiple of these components for a single Multics system.