PDP-10. That's what Ivan & Bert Sutherland said (it was Bert, I think) in a Computer History Museum video on google I watched the other day. --- google for "Dan Murphy" and "TENEX" for the history. Zane will probably pop up about this at some point as well. TENEX was developed at BBN to develop a more modern operating system for the original PDP-10 (KA-10) processor. It required special hardware to support demand paging (the BBN Pager Box) TOPS-10 was the original PDP-10 operating system. DEC hired Murphy later to modify TENEX for the third generation (KL-10) processor. This became TOPS-20. A couple of other variants of TENEX were developed as well; the one used on the PARC-devoped MAXC and the Foonly (called FOONEX) Docs for the BBN pager are scanned under bbn on bitsavers.