The Message Facility, which is an optional part of the channel subsystem, is the hardware that connects a processor complex to the coupling facility. CP simulates the message facility for the benefit of CF Service Machines and coupled guests. The simulated Message Facility allows a coupled guest to exchange information with a CF Service Machine, and when the hardware and z/VM® are so capable, it allows CF Service Machines to exchange information with each other. From the coupled guest's point of view, the Message Facility appears as a set of virtual message devices. The CF Service Machine's view of the Message Facility is proprietary.