Tru64 UNIX 5.1B

Tru64, formerly Digital UNIX, formerly OSF/1 was DEC's UNIX product for their line of Alpha based systems. After DEC's acquisition by Compaq, it was renamed to Tru64. DEC previously had sold a BSD derivative named Ultrix for Vax and MIPS based systems. Their new UNIX on Alpha was meant to counter AT&T and Sun's SVR4 Unix. Tru64 / DIGITAL UNIX / OSF/1 is somewhat interesting in that it used portions of the Mach microkernel and BSD kernel and userland in a manner not entirely unlike NeXTSTEP or Mac OS X. Like OS X, Tru64 itself is not a microkernel system but uses Mach code in its kernel to implement threading and scheduling and possibly other features.