Abstract: In mixed critical systems, applications with different security and time criticality share computing resources. Suffered from kinds of uncertainties in the system, designers need a design method with tight timing that can satisfy multiple design constraints such as function behavior certainty, timing behavior predictability and high computing performance at the same time, which challenges the theories and methods of existing computer architectures and programming languages. A real-time machine mo- del, 
RTM, and a time triggered instruction set, TTI, which support time semantics, are proposed as the important foundation in constructing a Multi-tier Tight Timing design method, MTTT. A helicopter flight control program is used as an example to illustrate the effectiveness of RTM and TTI instruction set.

Key words: mixed critical system, computer architecture, instruction set, programming model, time predictablity