We describe the design, analysis, and performance of an on-line algorithm to dynamically control the frequency/voltage of a Multiple Clock Domain (MCD) microarchitecture. The MCD microarchitecture allows the frequency/voltage of micrprocessor regions to be adjusted independently and dynamically, allowing enery savings when the frequency of some regions can be reduced without significantly impacting performance. <BR><BR> Our algorithm achieves on average a 19.0% reduction in Energy Per Instriction (EPI), a 3.2% increase in Cycles Per Instruction (CPI), a 16.7% improvement in Energy–Delay Product, and a Power Savings to Performance Degradation ratio of 4.6. Traditional frequency/voltage scaling techniques which apply reductions globally to a fully synchronous processor achieve a Power Savings to Performance Degradation ratio of only 2–3. Our Energy–Delay Product improvement is 85.5% of what has been achieved using an off–line algorithm. These results were achieved using a broad range of applications from the MediaBench, Olden, and Spec2000 benchmark suites using an algorithm we show to require minimal hardware resources.