Abstract: An operating system having a multitude of subsystems is provided where each subsystem performs a distinct function and may register with other subsystems to receive notification (or callbacks) upon the ocurrence of certain events. Moreover, the subsystems are programmed in such a manner where in the event of a fault (or exception) they can be instructed to cease the calling of other subsystems, thereby allowing each of the subsystems to complete its distinctive task without commencing further additional activities. When all of the activities have ceased, the state of the system as indicated by the parameters at the various registers and addresses are saved. At this time, the system can initiate a service call for an attendant through one of the provided mechanisms or routine calls.