VAX 6000 Error Handling: A Pragmatic Approach

The VMS operating system's CPU-dependent support of the VAX 6000 family of computers implements a complex and sophisticated set of error-handling routines. At the start of a VMS session, these routines help construct the necessary framework to support the I/O subsystem as the system begins to emerge. For much of a VMS session, these routines then lay dormant within the SYSLOA image. Periodically, when aroused, they peer into hardware registers looking for signs of trouble. Often, all is well, and the routines return to hibernation. On those occasions when the hardware requires assistance, error handling takes complete control of the system. It has but one mission: identify the error, recover if possible, but at all costs ensure that the integrity of the system remains intact and that data is preserved.