Well, the page does say they had two -11/20's, apparently one with and one without the KS11. Also, ISTR that the source for the -11/20 system has been recovered from a listing and run, and IIRC that didn't have the KS11 stuff in it (but it might be worth checking). Next to nothing is known of the KS11. Dennis' page "Odd Comments and Strange Doings in Unix": https://www.bell-labs.com/usr/dmr/www/odd.html has a story involving it (at the end), and that is all I've ever been able to find out about it (if anyone has anything more, please let me know). My original guess as to its functionality, from that, was that it's not part of the CPU, but a UNIBUS device, which perhaps maps addresses around (and definitely limits user access to I/O page addresses). It might also have mapped part of the UNIBUS space which the -11/20 CPU _can_ see (i.e. in the 0-56KB range) up to higher addresses, where 'extra' memory is configured. However, on re-reading that page, I see it apparently supported some sort of user/kernel mode distinction, which might have require a tie-in to the CPU. (But not necessarily; if there was a flop in the KS11 which stored the 'CPU mode' bit, it might be automatically cleared on all interrupts. Not sure how it would have handled traps, though. I'll have to enquire on the TUHS list. Noel