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Because of the complex architecture of the zSeries® processors, an internal code, called millicode, is used to implement many of the functions provided by these systems. While the hardware can execute many of the logically less complex and high-performance instructions, millicode is required to implement the more complex instructions, as well as to provide additional support functions related primarily to the central processor. This paper is a review of millicode on previous zSeries CMOS systems and also describes enhancements made to the z990 system for processing of the millicode. It specifically discusses the flexibility millicode provides to the z990 system.
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