Abstract: To design highly dependable safety critical real time operating system CRTOS 2.0,the status quo of dependability safeguard mechanisms of existing operating systems is firstly analyzed. Since safety critical systems requires that both temporal and spatial isolation safeguard among different processes be maintained,a novel approach for designing CRTOS 2.0 is put forward,which is based on temporal and spatial isolation safeguard mechanism. The spatial isolation safeguard mechanism uses hardware memory protection to isolate and protect kernel itself and user tasks from incorrect operation caused by accidental errors or malicious tampering,and the memory protection provides support for multiple protected virtual address spaces designed for worst-case performance. However,the purpose of temporal isolation safeguard mechanism is to prevent an application from overrunning its allocated quota of resource usage and delay the progress of other integrated application tasks. In order to realize temporal isolation safeguard mechanism,the traditional approach based on processor capacity reservation is improved,and a novel implementation approach based on two-level hierachical scheduling approach is brought forward and discussed in detail. The dependability of CRTOS 2.0 is enhanced with temporal and spatial isolation safeguard mechanisms.