Establishing and restoring paths in a data processing I/O system

A data processing I/O system including a main storage for storing data and data processing instructions arranged in software programs, and a channel subsystem having hardware for reporting conditions in the channel subsystem to the software. A special Channel Subsystem Call (CHSC) instruction is used to store event information from the channel subsystem into the main memory for use by the software such that new or restored resources such as a channel path, a channel path and partial control unit link address, or a channel path and full control unit link address, may be originally made accessible, or may be restored to the I/O system. A mechanism is also provided to inquire of a device how long it expects to not be accessible to give the device sufficient time to reset itself before it is varied offline, thus preventing a device from being isolated or "boxed" prematurely.