When DEC became Compaq, Digital UNIX became Tru64 Unix. It's a 64-bit UNIX OS designed for use on the Alpha processor. Digital Unix 4.0, also called OSF/4, became Tru64 4.0. Thus the new version, Tru64 5.0, is also known as OSF/5.

This seemingly trivial bit of info causes a lot of GNU-ish software to fail to configure and build on it, because the software doesn't realize that osf/5 works just like osf/4, and instead complains about an unknown system type and dies. Caveat Programmor.