stt@ada-uts (02/18/88)
Preemption is not required to implement the delay, even when multiple priorities are supported, because... "The execution of a delay statement ... suspends further execution of the task ... for *at least* the duration specified..." (LRM 9.6:1). The "at least" part allows the delay statement to result in a delay until the next "natural" scheduling point following its expiration. I certainly agree that this defeats the usual purpose for delay, and the LRM should have made statements indicating that the implementation must preempt at the first possible time after expiration, but alas, it didn't, and so implementations need not support preemption. Note however that multi-processor implementations must support preemption for another reason -- namely that a high priority task on one processor may start a middle priority task, and if some other processor is at that time running a lower priority task, it must be preempted (presuming that the new task can "sensibly be executed using the same physical processor" (LRM 9.8:4). Of course "sensibly" is not well defined, and so this preemption requirement might also be avoided by appropriate definition gymnastics. Tucker Taft c/o Intermetrics, Inc. Cambridge, MA 02138