This paper discusses the role of microprogramming in operating system design and shows several things: (1) advantages of the efficiency which may be gained from microcoded operating system primitives, (2) selecting the most appropriste primitives for implementation, and (3) an analysis of the tradeoffs among software, firmware, and hardware. The authors propose a practical approach of enhancing computer architecture level, from a view point of functional hierarchy of operating systems. In order to prove the advantages of this approach, we have designed and implemented an experimental abstract machine for an operating system nucleus. This research is an experimental design, and evaluation on its operating system nucleus machine,called HEART. HEART is a set of primitive and universal functions, and works as a nucleus of a multiprogrammed operating system. The research results of our approach are the followings: First, to clarify the properties of operating system nucleus, taking functional hierarchy of operating system into consideration. Second, to show the design of operating system nucleus based on novel concepts. Third, to confirm the possibility of implimenting operating system nucleus machine. Finally, we give a performance evaluation on microcoded HEART and the effectiveness of enhancing computer architecture level based on the properties of operating systems.