Hardware/software systems provide flexible implementation of microprocessors, microcontrollers and various peripheral devices and supply the ability to perform almost any functional algorithm that can be simply modified or replaced. For reliability estimation of hardware/software systems we must take into account not only the loss of efficiency, which was caused by the hardware failures, but the abnormal functioning, that was caused by software failures. All these features of hardware/software systems must be considered in their models for reliability indexes estimation.

Development of mathematical models of hardware/software system begins with establishing the cause-and-effect relationships of its efficiency violations. Basing on these cause-and-effect relationships the structural-automatic model, which is formalized representation of structure and behavior of hardware/software system, was developed. This structural-automatic model and software module ASNA allows obtaining the analytical mathematical model of hardware/software system as graph of states and transitions. Development of structural-automatic model involves solving the following tasks: forming the vector of states, forming the set of formal parameters, defining the basic events and forming the tree of modification rules of vector’s of states components.

Features of forming the vector of states are that the hardware state affects the software state and on the contrary, so the hardware/software system’s state must be represented by two vector’s of states components: one to display the hardware state and the other to display the software state. The set of formal parameters contains all constants and their values, which are used for constructing the mathematical model. Basing on the information of Crittercism, IBM, Microsoft and Ballista OS Robustness Test Suite the intensities of hardware and software faults and failures were calculated. Other required initial data were obtained too. Also, 12 basic events, which can be occurred, and development of the tree of modification rules of vector’s of states components were submitted in the model of hardware/software system. The specification of states and transitions was given and the graph of states and transitions was presented. From the developed model of the system’s behavior we can clearly distinguish three types of states, which are related with faults and failures of hardware and of software separately as well as of hardware and software collectively.

Using the developed model the reliability indexes of non-reserved hardware/software system, which provides software restart when it hangs up, were researched. This mathematical model allows us to get standardized parameters of reliability and availability and other reliability indexes, which can be required by designers or engineers for comparison the competitive variants of hardware/software systems with different hardware and software configurations.