Many thanks. I had thought of XXDP as the "ugly duckling" of PDP-11 O/S's because of its ungainly UI, however the code turned out to be incredibly subtle, forced by fixed space constraints and permitted by that elegant instruction set. It was so much fun unraveling its secrets. IJH.
Not with the current release... XXDP has a total memory footprint of 1.5k 16-bit words and allows programs to overwrite the low .5k (which it restores on image exit like a skink regrowing its tail). But its diagnostics can rattle every micro state of every peripheral attached to a machine.
Reading that source brings a rush of emotional memory of that incredible instruction set. So simple, so rational, so orthogonal. Ah bliss(-11 ;-).