G4: a fault-tolerant CMOS mainframe

G4 is IBM's fourth generation CMOS microprocessor-based S/390 mainframe but the first to achieve fault tolerant equivalence-or superiority-with its predecessor ECL mainframes. CMOS technology provides much greater density and integration, assuring superior fault avoidance characteristics. The reduced power of CMOS makes bulk power redundancy and battery backup practical. However, the high density and circuit properties of CMOS pose new challenges for detection, recovery, and online repair. G4 implements an innovative design for a high performance, fault tolerant, single-chip microprocessor. Microprocessor sparing is used as a concurrent repair mechanism. Increased memory density requires new (76,64) S4EC/DED Error Correction Codes so that all single chip failures are correctable. As many as four I/O interfaces are packaged on an individual card, requiring both configuration management and automated maintenance procedures to assure all devices maintain connectivity during online repair.