VMS by Digital Equipment Corporation in 1977, is a high-reliability multi-user operating system designed for the VAX architecture and later ported to Alpha, Itanium, and AMD64 systems. Developed with strong influences from RSX-11 and lessons learned from earlier DEC operating systems, VMS emphasized security, stability, clustering, and large-scale timesharing from the outset. It introduced features that were highly advanced for its era, including fine-grained privilege controls, virtual memory, record-oriented filesystems, integrated networking, and sophisticated batch and queue management. VMS became widely used in government, finance, industry, universities, and scientific computing, where its reputation for reliability and uptime became legendary.

One of the defining features of VMS was the VMScluster technology, which allowed multiple systems to operate together as a single coherent computing environment with shared storage, distributed locking, and transparent failover capabilities. Combined with DECnet networking and support for large enterprise storage systems, VMS clusters formed the backbone of many mission-critical installations throughout the 1980s and 1990s. The operating system also provided a rich command-line environment through DCL (Digital Command Language), extensive system management tools, and support for a wide range of programming languages and networking protocols. Although its commercial prominence declined after the fall of DEC, OpenVMS remains in active use today in specialist environments and is regarded as one of the most robust and influential operating systems ever produced.