On Jun 22, 2019, at 22:37, Rudi Blom <rudi.j.blom(a)gmail.com> wrote: Probably already known, but to be sure Interesting options: MX11 - Memory Extension Option: this enabled the usage of 128 KW memory (18-bit addressing range); KS11: this option provided hardware memory protection, which the plain /20 lacked. Both options were developed by the Digital CSS (Computer Special Systems). http://hampage.hu/pdp-11/1120.html PS the page listed below has a very nice picture of the 'two fathers of UNIX" working on a PDP-11/20 http://hampage.hu/unix/unix1.html