Abstract
An efficient real-time operating system, PARTOS-11, has been developed for a low-speed and small-memory microcontroller, 68HC11. The size of the kernel is 2 Kbytes. Both hard real-time tasks and soft real-time tasks can be run in the system concurrently. The rate-monotonic policy is adopted for scheduling hard real time tasks, which guarantees that all tasks meet their deadline if the condition of Liu and Layland's theorem is met. A novel model, the Slack Sharing Server (SSS), was proposed and implemented. The SSS is a simple and efficient server for soft real time tasks running together with hard real time tasks in a small real-time embedded system.