The increasingly constrained power budget of today's microprocessor has resulted in a situation where power savings of all components in a system have to be taken into consideration. The Operating System (OS), which manages both software and hardware resources, dissipates a significant portion of power in the execution of many modern applications. This paper profiles run-time OS power/performance characteristics and advocates a routine based OS-aware microprocessor resource adaptation mechanism to save run-time OS power. This approach permits precise hardware reconfigurations for the OS with low overhead and allows fine-grained performance/power tuning at the microarchitectural level. Simulation results show that compared to existing sampling-based adaptation schemes, this novel methodology yields a more attractive power and performance trade-off on OS execution. To our knowledge, this work is the first to address the power saving issue of the OS itself, an increasingly important...