Rare conditions-an important cause of failures

Data gathered by the author as well as those published by others show that rarely executed code has a much higher failure rate expressed in terms of execution time than frequently executed code during the early operational period. More detailed analysis of the data shows that the inability to handle multiple rare conditions, such as response to hardware failures or exception conditions caused by the computer state, is a prominent cause of program failure in well-tested systems. Approaches to coping with this difficulty are discussed. We investigated differences in failure rate between rarely executed code (RC) and frequently executed code (FC) in the NASA/JPL Deep Space Network (DSN)