One of the most successful hardware design approaches for energy efficiency is to provide run-time control of power vs. performance trade-off. Such an approach requires both ad hoc hardware and software interfaces. We aim to extend the SW perspective by taking into account all the features that are directly or indirectly exploited by Real Time Operative Systems (RT-OS). Operating System performances are highly context-dependent and, regardless of the underlying hardware, the range of services provided by the OS may vary from data base access to highly sophisticated real time features. Sometimes performances are affected by the OS implementation. A synergic approach that takes into account both the hardware and software perspec-tives of the system, will definitelly lead to optimal results.