The hard real-time serial receiving program which dealt with totally four different data frames was designed with four techniques: data frame picking-up in interrupt serve program, finite state machine theory, buffer sharing and code optimizing.

This paper presents a new hard real-time DVS algorithm.

There are many practical applications with the requirements of hard real-time and high availability, in military, aeronautics and astronautics, bank, weather processing and electronic commerce.

For hard real-time system(or tasks),it is imperative that no deadline be missed.

There are many short-transaction applications, with requirements of high-availability and hard real time, in military, industry and electronic commerce.

As the development of control systems ,more and more control systems require a kind of complex operating system with the hard real time features as well as the general operating system functions.

The paper discusses problems of scheduling policies, priority, time slice, running in kernel mode and spending by system when designing realtime multi-task system, and gives an example of real realization for some large hard real time application in the Digital UNIX environment.

In order to meet the requirement of hard realtime and high dependability of measurement and control software,the best operation system platform must be selected.