In this article, design issues for scalable operating systems suited to support efficient multiprogramming in large Transputer clusters are considered. Shortcomings of some current operating system approaches for Transputer based systems concerning efficiency, scalability, and multiprogramming support are discussed. After a brief overview of the new operating system COSY, the emphasis is laid on the design of the kernel entry mechanism which plays a key role for efficiency and which has not yet received the attention it deserves in the operating system literature. The kernel entry layer is an appropriate place to isolate most of the hardware dependent parts of a kernel. Its design is discussed in the context of an implementation on Transputers.