Interrupt latency
Another factor of concern for realtime systems is the amount of time taken between the generation of the hardware interrupt and the first line of code executed by the ISR.
Atomic operations
Some convenience functions are defined that allow you to perform atomic operations (i.e., operations that are guaranteed to be indivisible or uninterruptible). Using these functions alleviates the need to disable and enable interrupts around certain small, well-defined operations with variables.