One of the goals of 11M was to get something that could run on a really small PDP-11 without an MMU, which 11M can. M can run without an MMU?? RSX-11S can, but I don't think 11M can. OK, 11S is just a stripped down 11M, but it was still a separate product. Can you actually gen an unmapped system with a 11M distribution? Also, 11S could run without any kind of disk or mass storage (e.g. a complete 11S image could be downloaded via MOP) but I don't think 11M ever could. RSX-11S was the "embedded system OS" of the PDP-11 world (at least as far as DEC's offerings went). In an unrelated question - was RSX-20F (the CFE for the KL10) based on 11S? In ran on an unmapped 11/40, so I assume it must have been. Bob