Analysis of the influence of processor hidden registers on the accuracy of fault injection techniques

Modern processors tend to increase the number of registers, being part of them not accessible by the instruction set. Traditionally, the effect of faults in these hidden registers has not been considered during system validation using fault injection. In this paper, a study of the importance of faults in hidden registers is performed. Firstly, we have analysed the sensitivity of hidden registers to faults in combinational logic. In a second phase, we have analysed the impact of the faults occurred in hidden registers on system behaviour. A broad set of permanent and transient faults have been injected into the models of two typical commercial microcontrollers, using a VHDL-based fault injection tool developed by our research group. The results obtained indicate that the incidence of hidden registers is not negligible, and in some cases is even notable. This fact suggests that widely used fault injection techniques such as SWIFI could not be enough to perform a full and representative validation of modern processors, and it would be necessary to complement with other fault injection techniques that have a higher degree of accessibility.