When Xerox established PARC, they asked the assembled scientists what computer they wanted. The majority view was a DEC PDP-10 KA10, with the BBN memory management unit that let it run Tenex (the ancestor of DEC TOPS-20). Xerox couldn't really buy a competitor's mainframe, so they built MAXC (maximum access computer), which was a complete emulation of the Tenex machines.
>MAXC (maximum access computer), which was a complete emulation of the Tenex machines.
Compuserve also had a software dependency on the PDP-10 and Decsystem 20, and when those machines were no longer available they bought a company making clones so they could continue to manufacture them for themselves. The company they bought, Microsolutions, was Mark Cuban's first startup.
making clones of mainframes (IBM's) had been a big area of intellectual property litigation, but also facing monopolization investigations, IBM had to allow them. They were referred to as "plug compatibles".
very apt that xerox parc was full of tenex engineers!