Simple models of fault tolerant software

Fault tree models that can be used for qualitative and quantitative analysis of fault-tolerant software are presented. There are several advantages in addition to the intrinsic beauty of simplicity. The implications of the conclusions drawn from the model are easier for the reader to understand. The qualitative effects of the input parameters are easier to deduce. A reader can develop a model of a similar system and be confident of the results. Most importantly, a simple fault tree model of fault-tolerant software can be combined more easily with an analysis of the hardware structure on which it executes. This combination will facilitate the integrated analysis of fault-tolerant hardware and software systems.<<ETX>>