VM (operating System) - Hypervisor Interface

Hypervisor Interface

At one time, CMS was capable of running on a bare machine, as a true operating system (though such a configuration would be unusual). It now only runs as a guest OS under VM. This is because CMS relies on a hypervisor interface to VM-CP, to perform file system operations and request other VM services. This paravirtualization interface:

• Provides a fast path to VM-CP, to avoid the overhead of full simulation.
• Was first developed as a performance improvement for CP/CMS release 2.1, an important early milestone in CP's efficiency.
• Uses a non-virtualized, model-dependent machine instruction as a signal between CMS and CP: DIAG ("diagnose").

The term "hypervisor" was probably coined during IBM's implementation of VM/370, when it was used to refer to the virtual DIAG handler.