Reducing unpredictability in Ada executions

One of the most drastic examples of uncertainty of a program execution is surely that of an unrecoverab1e run-time error causing the failure of the whole program. This kind of failure is possible a1so in the case of Ada, and is allowed by the occurrence of a particular class of run-time errors for which there is no requirement on Ada compilers or interpreters to provide either a static or run-time detection. When one of these undetected errors occurs the program execution is said to become "erroneous" (i.e. no more governed by Ada rules) and and its effect is considered "unpredictable". An unrecoverable failure of the whole program (e.g. caused by a memory protection violation, or by a run-time system internal error) is just one of the possible symptoms of the program erroneousness. A study on the possible ways to cope with this kind of errors has been carried out inside the activity of the EEC MAP project n. 755 "SFD-APSE". In particular, the attention has been concentrated on the possibility of detecting at run time the occurrence of erroneous executions and on the possibility of recovering from them in a safe way. In this paper the project results are shortly summarized and the possible directions for further activity are pointed out.