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Increased energy consumption in processors caused by performance enhancement has recently become a critical problem. Many current processors employ dynamic voltage and frequency scaling (DVFS) which dynamically lowers the supply voltage and clock frequency in order to reduce energy consumption. However, it is difficult to deliver fine-grain energy optimization by using DVFS. Since a voltage regulator takes a long time for scaling the voltage and charging/discharging a power line has a large energy overhead, the useful interval of DVFS is limited to coarse-grain. To optimize energy consumption at fine-grain interval, we have proposed a variable stages pipeline (VSP) processor. VSP reduces energy consumption by dynamically varying the pipeline depth to suitable pipeline depth according to behavior of a running program. VSP can obtain finer-grained energy reduction than DVFS because pipeline scaling only requires a small overhead. In this paper, we fabricated a VSP processor chip using 180 nm technology and evaluated energy consumption of the chip. We present that the fabricated VSP chip dynamically varies the pipeline depth while a program is running and reduces the energy consumption at shorter interval than DVFS. We also analyze how to optimize the energy consumption according to system demand. Our analysis result shows that the VSP can adjust the energy consumption in the same manner for diverse program phases.

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