Is `Tru64 UNIX` open source?

It is HP's commercial 64-bit UNIX operating system (based on the Mach kernel - which Mac OS X is also based on)

It is for the Alpha instruction set architecture only - although there was a short lived Intel port.

Broadly speaking, usage is similar to many other *nixes, but the location of various config files, executables etc may be not where you expect.

Tru64 is a commercial Unix system. It wass originally called OSF/1, OSF standing for Open Software Foundation. It is original in that it is neither based on AT&T source code (System V) nor on BSD, the two major Unix flavor. The OSF was led by DEC as an attempt to compete against System V which they felt excluded companies other than AT&T and Sun.

Only DEC made significant use of OSF/1. DEC's OSF/1 was known as Digital Unix (from Digital Equipment Corporation, and it was the operating system shipped with most of DEC's workstations (there was aslo a version of Windows NT for some low-to-mid-end Alphas). first based on MIPS processors (32-bit), then later on Alpha. Because Alphas were at the time the only affordable 64-bit platforms (not as cheap as a PC, but cheaper than “big iron”), and as part of the rebranding of DEC products after Compaq bought DEC in 1998.

In 2002, Hewlett-Packard bought Compaq. HP had its own 64-bit platforms (the traditional PA-RISC, the new high-end Itanium and more recently the cheaper AMD64) and its own Unix brand (HP-UX) plus Linux, so it had no use for either Alpha or OSF/1.

While OSF/1 did have innovative features at the time it was developped, none were licensed for reuse in other systems, as far as I know. Nowadays I think any worthwhile feature has been imitated by others already. HP has not released the OSF/1 intellectual property, except for a version of the AdvFS filesystem.

As per the above Wikipedia article Tru64 UNIX is a commercial system and it works only on the DEC Alpha platform.