Of the four levels the two bits of interrupt priority implemented in your M0 core '0' is the highest, and '3' is the lowest. Note that the two 'implemented bits' are the HIGHEST TWO of the potentially-8-bit priority registers, so watch what 'tools' you use to see if you need to supply '3' or '192' to get the 'lowest' installed --- you are looking for something like a '<< (8 - ARM_INTERRUPT_LEVEL_BITS)' factor to tell you to use the 'zero to three' values.
