1. A method of extending the functionality of channel subsystems, said method comprising: configuring a channel subsystem as a plurality of channel subsystem images, said channel subsystem directing a flow of information between one or more input/output devices and main storage coupled to the channel subsystem, and said channel subsystem comprising at least: a first channel subsystem image identified by a first channel subsystem image identifier, said first channel subsystem image appearing to a program as an independent channel subsystem and comprising a first set of one or more channel paths and a first multiple image facility (MIF) having one or more MIF images, each of said one or more MIF images being identified by a MIF image identifier (IID); and a second channel subsystem image identified by a second channel subsystem image identifier, said second channel subsystem image appearing to a program as an independent channel subsystem and comprising a second set of one or more channel paths and a second multiple image facility having one or more MIF images, each of said one or more MIF images of the second MIF being identified by a MIF image identifier (IID); identifying a channel path of the first set of channel paths by a concatenation of the first channel subsystem image identifier and a first channel path identifier, and a channel path of the second set of channel paths by a concatenation of the second channel subsystem image identifier and a second channel path identifier; including within a subsystem identification word at least one channel subsystem image identifier of the first channel subsystem image identifier and the second channel subsystem image identifier, the subsystem identification word being included within a general register that is an implied operand for at least one input/output (I/O) instruction; and performing validity checking for the at least one channel subsystem image identifier.