Since initiating the information technology industry-wide transition from bipolar to CMOS technology with the first generation of S/390@ processors in 1994, IBM reached another major milestone with the introduction of the third generation in September 1996. The balanced system and cache structure and the modularity of the components of Generation 3 support a wide performance range from a uniprocessor to a high-performance multiprocessing system. Because of this modularity, Generation 4 is also based on this structure.

This paper describes the migration of the hardwareimplemented run-control functions from a single-book structure with one flexible service processor (FSP) and one service element (SE) per system to a multibook structure with one FSP per book and one SE per system. The new system structure required two new interfaces between the clock chips on the different books. The first interface is required for dynamic configuration data exchange between books. The alternative path via the SE would not meet the performance requirements. This interface is available in the initial millicode load flow before the L2 caches with their ring structure are operational. Another requirement is the necessity of starting and stopping all books synchronously. The second additional interface between the clock chips on different books enables this function. Nevertheless, the hardware implementation is so flexible that each book may operate independently of the other books. The clock chips are connected as a peer-to-peer network, so no special master is necessary in the system.

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