Compaq Tru64 Unix. Before Compaq bought Digital it was called Digital Unix (DU). Before that it was called OSF1 or DEC OSF-1 AXP. Earlier Unix product from Digital (DEC) was Ultrix. OSF1 was a new system, not derived from Ultrix. OSF1 Unix Operating Systems Software Computers.
Tru64 UNIX is a discontinued 64-bit UNIX operating system for the Alpha instruction set architecture (ISA), currently owned by Hewlett-Packard (HP). (wikipedia)