P-UX (Hewlett-Packard UniX) is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V (initially System III) and first released in 1984. Recent versions support the HP 9000 series of computer systems, based on the PA-RISC processor architecture, and HP Integrity systems, based on Intel's Itanium architecture. Earlier versions of HP-UX supported the HP Integral PC and HP 9000 Series 200, 300, and 400 computer systems based on the Motorola 68000 series of processors, as well as the HP 9000 Series 500 computers based on HP's proprietary FOCUS processor architecture. HP-UX was the first Unix to offer access control lists for file access permissions as an alternative to the standard Unix permissions system. HP-UX was also among the first Unix systems to include a built-in logical volume manager. HP has had a long partnership with Veritas Software, and integrates VxFS as the primary file system. HP-UX 11i is currently credited with leadership in integrated mission-critical virtualization, observed performance, high availability and manageability.