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IBM Journal of Research and Development  
Volume 48, Number 3/4, Page 323 (2004)
IBM eServer z990
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Processor subsystem interconnect architecture for a large symmetric multiprocessing system

by P. Mak, G. E. Strait, M. A. Blake, K. W. Kark, V. K. Papazova, A. E. Seigler, G. A. VanHuben, L. Wang, G. C. Wellwood
Integral to the significant capacity growth of the IBM eServer™ z990 (the eighth-generation zSeries® CMOS-based server) from its predecessor z900 system is the interconnect architecture, which tightly couples 48 customer CPUs in the system. A major attribute of this architecture is a new “hot swap” feature which improves zSeries system availability for customers by permitting the substitution or addition of a field-replaceable unit (FRU) in the processor subsystem without requiring the system to be powered down. The novel two-level interconnect architecture contains a distributed switch which connects up to four processor-memory nodes in book packages. The book packages, which are also FRUs, are connected in a dual concentric ring topology at the second-level (L2) interconnect. This architecture also contains an integrated 32-MB L2 cache and central switch connecting up to eight dual-core processor chips in a star topology at the first-level interconnect inside one of these nodes. This paper describes the bus protocol on the second-level interconnect, the cache coherency management throughout the storage hierarchy, and the ring topology reconfiguration for hot swap. Also described is a memory power management scheme to support the power demand from the 48 CPUs and up to 256 GB of memory.
Related Subjects: Computer architecture; Computer organization and design; IBM eServer z990; Interconnection technology