Summary: The S/390 G5, the latest generation IBM mainframe system, more than doubles the performance of its predecessor. The microprocessor used in the system is a sophisticated CISC processor that features a large cache and a very high clock frequency to achieve excellent performance. We discuss the microarchitecture of the processor followed by a description of how this processor is integrated into the system. Finally, we consider the state-of-the-art reliability and availability features that make this mainframe system unique.