| Study of Real-time Operating System (RTOS) is now a hot topic. Compared with common OS, RTOS has the characteristic that its validity does not only depend on the logistic correctness, but also depends on outputting result in time. RTOS are wildly used in fields, such as defense, industrial automation control and communication. Today, applications need more strict time request. Operating systems that support real time are needed. With the booming of open-source projects, Linux has popularized and developed rapidly. Because of efforts made by users and developers around the world, Linux has become a stable and preeminent operating system. However, Linux is originally designed as a divided-time OS in general use and mainly aims at fairness and system throughput; it is quite different from real-time OS. According to the internal characters of Linux, it is quite possible for researchers to develop an open, standard and high efficient real-time OS which bases on Linux. Hence, how to enhance the real-time performance of Linux is the key point in the current research of real-time OS. After being released in early 2004, Linux 2.6 kernel has been proved to be stable and high efficient. Compared with 2.4 version kernel, 2.6 has been improved in real-time performance by adopting some new technologies, such as preemptible kernel, new O(1) schedule algorithm etc. Although the developer of RTOS can benefit from these changes, Linux 2.6 is still not a real-time kernel. All these changes make it possible to convert Linux to a RTOS. So the reforming of Linux kernel is needed which improve real-time performance. Since the Linux 2.6 kernel and its release version Fedora was available, after studying on it for several months carefully, some research and improvement about Linux time system are finally decided to be done in order to shorten the response time. Based on research results mentioned above, a new Linux kernel with high real-time performance is implemented by reforming its time system. The key achievement is adding a high-precision time system that coexists with the old one. The reformed kernel runs on PC for long time and meets the real-time demands in the microsecond level. The whole way of reforming the Linux kernel time system is given in this paper, including the common ideas about the Linux real-time realization and a new method in this field proposed in this paper, which adopts bi-time-system to enhance real-time performance. This method can provide some value of research of Linux real-time improvement. In the process of reforming the Linux time system, research is focused on the following three aspects. (1) Modifying the time interrupt services; (2) Multi-mode time interrupt; (3) Precise time system. At the end of this paper, experiment results are given. Some continual research aims on some limitation are also narrated.
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