Fine-grained multithreading based on a natural model, such as dataflow model, is promising in achieving high efficiency and high programming productivity. In this paper, we discuss operating system issues for fine-grained multithread programs. We are developing an operating system called CEFOS based on a dataflow based computation model. A program on CEFOS consists of zero-wait threads which run to completion without suspension once started. Firing control among such threads is performed in a dataflow manner along with continuation relations in the program. Target platforms include Fuce processor, which is dedicated to fine-grained multithreading, and commodity processors such as Intel. x86. In this paper, after introducing our basic model and our operating system model, we discuss implementation issues on Fuce and commodity platforms. The evaluation results indicate that our approach on commodity platforms is effective in reducing overheads while our approach on a special architecture naturally exploit parallelism even in I/O handling.