The software fault failure rate bound is discussed and generalized for different reliability growth models. The fault introduction during testing and the fault removal efficiency are modeled to relax the two common assumptions made in software reliability models. Three approaches are introduced for the fault content estimation, and thus they are applied to software coverage estimation. A three-state nonhomogenous Markov model is constructed for software safety assessment. The two most important metrics for safety assessment, Steady State Safety and MTTUF, are estimated using the three-state Markov model. A case study is conducted to verify the theory proposed in the paper.