Have being working on real-time systems for almost four years since the beginning of my PhD, I have learnt a lot from the ground up about this research field. It is also becoming more and more clear to me what is this research really about. The most essential objective of real-time systems is to have timing guarantee, in which the tasks in the system, especially with hard real-time constraints, would meet their deadlines under any circumstances at execution time.
Real-Time Scheduling Theory has been developed over the last 40 years, since the first published work of Liu and Layland in 1973. Before that, real-times systems were designed with cyclic executives, which is in a ad-hoc manner and very difficult to maintain. In general, the theory is consist of task modelling, scheduling policy and schedulability test. Some other issues, e.g., resource management, response time analysis, worst-case execution time analysis, are also involved in the design of real-time systems.