On Jan 9, 11:36, Frank Arnold wrote: Almost. They are ROMs, but EIS/FIS is for the 11/03 KD11 processor, not the F-11 family. CIS is a set of microcode ROMs for the F-11. There's quite a lot of information in the KDF11 CPU Module User Guide, and probably more in a technical manual. Basically, it seems that the the microcode is stored in a device with two bidirectional 16-bit busses (MIB and CDAL). MIB is a multiplexed bus. Part of the time it outputs the current microinstruction and the rest of the time it receives control signals from the CPU. The CDAL bus (which is actually a 22-bit bus, but only 16 bits are presented to the option sockets) is what eventually connects to the external BDAL bus, but it's also used internally for control signals. -- Pete Peter Turnbull Network Manager University of York