>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".