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OpenVMS

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Originally developed by Digital Equipment Corporation OpenVMS in currently owned by Hewlet Packard.

In contrast to what its name implies OpenVMS is not open source or free.
It is a mainframe oriented, clustering, multiuser, multiprocessing, timesharing, proprietary operating system.
It runs on VAX, Alpha and Itanium based computers. It has a GUI (graphical user interface) and full graphics support including OpenGL.

HP OpenVMS Systems
OpenVMS Community Portal