Abstract: In time-sensitive embedded systems, tasks need to meet their deadlines, and missing deadlines can significantly affect the quality of service or have catastrophic consequences. Compared with the general system, the research progress of real-time system is slow, even many basic concepts have not reached consensus. Precision timed (PRET) machine and real-time processing unit (RPU) are two existing real-time system solutions. Taking them as examples, we introduce the related concepts of real-time system, expound the problems in the development of real-time system, and compare the similarities and differences. The problems encountered at each level of the real-time system and the existing solutions are analyzed. At the application layer, the user needs an interface for periodic operations; in instruction set architecture (ISA) layer, resources provided by hardware should be fully utilized to provide sufficient semantic abstraction and timing precision to the upper layer. The hardware layer needs to support ISA's time attributes and time semantics, and to improve performance as much as possible while ensuring real-time performance. Research on real-time systems faces many challenges. In the process of designing and researching the existing real-time system, the key problem is that the time semantics of the upper application is difficult to keep consistent with the lower implementation.