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In this paper, we propose to implement hybrid operating systems based on two-level hardware interrupts. We analyze and model the worst-case real-time interrupt latency for RTAI and identify the key component for its optimization. Then, we propose our methodology to implement hybrid operating systems with two-level hardware interrupts by combining the real-time kernel and the time sharing OS (Operating System) kernel. Based on the methodology, we discuss the important issues for the implementation. Finally, we implement a hybrid system called RTLinux-THIN (Real-Time LINUX with Two-level Hardware INterrupts) on the ARM architecture by combining ARM Linux kernel 2.6.9 and μC/OS-II. We conduct experiments on a set of real application programs including mplayer, Bonnie, and iperf, and compare the interrupt latency and interrupt task distributions for RTLinux-THIN (with and without cache locking), RTAI, Linux, and Linux with RT patch on a hardware platform based on Intel PXA270 processor. The results show that our scheme not only provides an easy method for implementing hybrid systems but also achieves the performance improvement for both the time sharing and real-time subsystems.