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Deep-submicron CMOS designs maintain high transistor switching speeds by scaling down the supply voltage and proportionately, reducing the transistor threshold voltage. Lowering the threshold voltage increases leakage energy dissipation due to subthreshold leakage current even when the transistor is for switching. Estimates suggest a five- fold increase in leakage energy in every future generation. In modern microarchirectures, much of the leakage energy is dissipated in large on-chip cache memory structures with high transistor densities. While cache utilization varies both within and across applications, modern cache designs are fixed in size resulting in transistor leakage inefficiencies. This paper explores an integrated architectural and circuit level approach to reducing leakage energy in instruction caches (i-caches). At the architectural level, we propose the Dynamically Resizable i- cache (DRI i-cache), a novel i-cache design that dynamically resizes and adapts to an application's required size. At the circuit-level, we use gated-Vdd, a mechanism that effectively turns of the supply voltage to, and eliminates leakage in, the SRAM cells in a DRI i-cache's unused sections. Architectural and circuit-level simulation results indicate that a DRI i-cache successfully and robust exploits the cache size variability both within and across applications. Compared to a conventional i-cache using an aggressively-scaled threshold voltage a 64K DRI i-cache reduces on average both the leakage energy-delay product and cache size 62%, with less than 4% impact on execution time