Academic Operating System (AOS) was IBM's version of 4.3BSD Unix for the IBM RT. It was offered to academic institutions as an alternative to AIX, the usual RT operating system. AOS had a few extra features, compared to standard 4.3BSD, notably NFS, and an almost ANSI C-compliant C compiler. A later version of AOS existed that was derived from 4.3BSD-Reno, but it never was widely distributed.