RS64 IV is a multithreaded PowerPC processor. It is based on prior versions, which were used in the pSeries and iSeries servers. The RS64 IV is a map to CMOS 8S technology with a few microarchitectural changes and a few cycle-time optimizations. These have increased the clock frequency, improved the performance, and shrunk the chip size. Because these processors are used only in iSeries and pSeries commercial servers and not in any workstations, they are optimized solely for commercial server workloads.Multithreading is the most unique feature of this processor, and the orientation to server applications has led to a unique approach. Because the threads look like multiple processors, no application changes are needed. By keeping the number of threads small, the addition to the chip area is kept small, and impacts on software scalability are minimized. By keeping chip area small, implementing coarse-grained rather than fine-grained multithreading, and implementing a priority mechanism, single-thread execution performance and chip cost are kept competitive with non-multithreaded processors. In conclusion, when compared with other performance-improvement techniques, multithreading yields an excellent ratio of performance gain to implementation cost.