SAM3U_14 Datasheet, PDF (174/1187 Pages) ATMEL Corporation – From 64 to 256 Kbytes embedded Flash

Find the IPR number and byte offset for interrupt N as follows:
• the corresponding IPR number, M, is given by M = N DIV 4
• the byte offset of the required Priority field in this register is N MOD 4, where:
– byte offset 0 refers to register bits[7:0]
– byte offset 1 refers to register bits[15:8]
– byte offset 2 refers to register bits[23:16]
– byte offset 3 refers to register bits[31:24].
174 SAM3U Series
6430F–ATARM–21-Feb-12