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Energy efficiency has become a very important issue to be addressed in today's system-on-a-chip (SoC) designs. One way to lower power consumption is to reduce the supply voltage. Multisupply voltage (MSV) is thus introduced to provide flexibility in controlling the power and performance tradeoff. In region-based MSV, circuits are partitioned into Â¿voltage islandsÂ¿ where each island occupies a contiguous physical space and operates at one voltage level. These tasks of island partitioning and voltage level assignment should be done simultaneously in the floorplanning process in order to take those important physical information into consideration. In this paper, we consider this core-based voltage island driven floorplanning problem including islands with power down mode, and propose a method to solve it. Given a candidate floorplan solution represented by a normalized Polish expression, we are able to obtain optimal voltage assignment and island partitioning (including islands with power down mode) simultaneously to minimize the total power consumption. Simulated annealing is used as the basic searching engine. By using this approach, we can achieve significant power saving (up to 50%) for all datasets, without any significant increase in area and wire length. We compared our approach with the most updated previous work on the same problem, and results show that our approach is much more efficient and is able to save more power in most cases. We have also studied two other approaches to solve the same problem, a simple dynamic programming approach and a lowest possible power consumption approach. Experimental results show that ours can perform the best among these three approaches. Our floorplanner can also be extended to minimize the number of level shifters, to address a minVdd version of the problem and to simplify the power routing step by placing islands close to their corresponding power pins.