Ah, OK, I guess it all depends on exactly what one means by 'directly'... :-) I was using it in a high-level architectural sense: there's a 1:1 correspondence between UNIBUS addresses and Fastbus memory addresses; a UNIBUS read/write cycle completes immediately with the contents of that cell; etc - all quite different from the RP0x example. IOW, at a very high level, it looks like other 'memory on the UNIBUS'. I seem to recall reading (don't remember where, it was a long time ago) of some place that actually made use of the dual UNIBUS thing; they hung some other PDP-11 (don't recall what kind) off UNIBUS B, and ran a primitive multi-processor. It was some sort of high-speed data acquisition, or perhaps a real-time simulator - something like that. Anyone else know of any place that used the dual UNIBUS capability? Noel