Schedule-Carrying Code

The interactions of real-time tasks with each other and with the environment can be specified in a platform-independent machine language called E code. E code is time safe if it can be scheduled on a given platform so that all its timing constraints are met. For specifying static, dynamic, and conditional schedules, we propose again an executable machine language, called S code. A compiler for real-time programs, then, consists of a platform-independent and a platform-dependent part. The former produces E code; the latter generates S code that ensures the time-safe execution of the E code. The run-time system consists of an implementation of the E machine, which interprets E code that manages interrupts from the environment, and of the S machine, which interprets S code that manages task execution on the processors. Generating nonpreemptive schedules for periodic tasks is NP-hard. However, for E code that specifies periodic tasks, and S code that specifies a corresponding nonpreemptive schedule, we show that time safety can be checked in linear time. This suggests the notion of schedulecarrying code (SCC), where E code is annotated with S code before being sent to an execution host. The host, if it does not trust the sender, can then check the time safety of the code at a cost that is far below the cost of generating a feasible schedule.