Tasks in a real-time computing systems are com- monly periodic. Each job generated by the invocation of a periodic task has normally a deadline constraint by which it must complete its execution in all circumstances. However, the hardware may be subject to failures, faults may be present in software, energy to supply the system may be depleted and the processor may be overloaded transiently. Any of these exceptions involves a situation where it is no longer possible to meet the deadlines of all the jobs in the application. In this paper, we discuss a general model for the application task set that permits the operating system to correctly manage the above emergency situations in accordance with specific parameters statically attached to every task in addition to its classical timing parameters. The model is inspired by the well- known Deadline Mechanism and Skip-Over model. We introduce the BGW model where each job of any periodic task can get one of the three colours Black, Grey and White. A colour specifies that the job has to imperatively execute the primary version, the job has to execute at least one version among primary and alternate or the job may be discarded. We briefly discuss implementation issues for this new model.
