11.5.1 Serving a High- or Low-Priority Interrupt While the Main Routine Code Is Executing
When a high- or low-priority interrupt is requested while
                                    the main routine code is executing, the main routine execution
                                    is halted and the ISR is addressed. Upon a return from the ISR
                                    (by executing the RETFIE instruction), the main
                                    routine resumes execution. 
