Demands of operating systems for control systems ranges
from real-time systems to intelligent home appliance. However, what is
needed depends on their applications. This paper discusses OS characteristics
desired to meet above demands and claims that such OS consists of 
multi servers running on state-of-the-art $\mu$-kernel. 
Our OS is expected to support extensibility, reliability, and
highly-distributed processing functionalities. Porting of Minix multi-server OS
on L4 $\mu$-kernel is underway. Issues such as interrupt handling and 
protection, switching of address space, efficiency aspects, framework
to achieve distributed processing are also described.