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FPGA based soft processors are an attractive choice for implementing many embedded systems. As real-time operating systems are adopted in the development of many applications using soft processors, we propose COMA, a cooperative management scheme in this paper for energy efficient implementation of real-time operating systems on soft processors. By utilizing the configurability of soft processors, we tightly couple several customized energy management hardware peripherals to them. These hardware peripherals cooperatively manage tasks and interrupts together with the processor while retaining the real-time responsiveness of the operating system. More specifically, they perform the following functionalities: (1) control the clock distribution network for driving the processor, the hardware peripherals and the communication interfaces between them; (2) take over the task and interrupt management responsibility of the operating system when the processor is shut off; (3) selectively wake up the processor and the corresponding hardware components for task execution based on the configurations of the processor and the hardware resource requirements of the tasks. We implement a real-time operating system on a popular soft processor to illustrate our approach. We show the development of an embedded application on the operating system enhanced with our energy management techniques. Actual measurements on an FPGA board demonstrates that our energy management scheme leads to energy reductions ranging from 73.3% to 89.9% and 86.8% on the average for the various execution scenarios considered in this paper.