Tru64 UNIX is a 64-bit UNIX operating system for the DEC Alpha microprocessor architecture, currently owned by Hewlett-Packard. Previously, Tru64 UNIX was a product of Compaq, and before that, Digital Equipment Corporation (DEC), where it was known as Digital UNIX (formerly DEC OSF/1 AXP).
As its original name suggests, Tru64 UNIX is based on the OSF/1 operating system. DEC's previous UNIX product was known as Ultrix and was based on BSD UNIX.
It is unusual among common commercial UNIX implementations because it is built on top of the Mach kernel. (Other commercial UNIX implementations built on top of the Mach kernel are NeXTSTEP, MkLinux, and Mac OS X.)
Tru64 and the Emulator