Windows Nt 4.0 as a Rtos

To minimize the time spent in the interrupt service routine (ISR), NT introduces the concept of deferred procedure calls (DPC). The priority of these DPCs is higher than the priority of user and system threads, but all DPCs run at the same priority. This makes that all DPCs are queued in FIFO order, and that the DPC of a high priority interrupt will only be executed when all the DPCs that were queued before it are finished executing. This leads to variable and unpredictable response times, which is incompatible with the fifth requirement.