Xenix
Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually replaced it with SCO UNIX (now known as Xinuos OpenServer).In the mid-to-late 1980s, Xenix was the most common Unix variant, measured according to the number of machines on which it was installed. Microsoft chairman Bill Gates said at Unix Expo in 1996 that, for a long time, Microsoft had the highest-volume AT&T Unix license. Provided by Wikipedia