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This paper proposes a novel intra-task dynamic voltage scheduling (IntraDVS) framework for low-energy hard real-time applications. Based on a static timing analysis technique, the proposed approach controls the supply voltage within an individual task boundary. By fully exploiting all the slack times, a scheduled program by the proposed technique always completes its execution near the deadline, thus achieving a high energy reduction ratio. The problem formulation of IntraDVS is first presented and two heuristics are proposed: one based on worst-case execution information and the other on average-case execution information. In order to validate the effectiveness of the proposed heuristics, a software tool that automatically converts a DVS-unaware program into an equivalent low-energy program was built. In an experiment on a DVS-enabled system, the low-energy version of a Moving Pictures Expert Group (MPEG)-4 encoder/decoder consumed only 35%-51% of the energy consumption of the original program running on a fixed-voltage system with a power-down mode. The energy efficiency of the IntraDVS algorithms was also compared with that of task-level voltage scheduling algorithms. The experimental results show that the IntraDVS algorithm can be useful in multitask environments as well.