The MPU_RNR selects which memory region is referenced by the MPU_RBAR and MPU_RASR registers. See the register summary in Table 4.25 for its attributes. The bit assignments are:
Indicates the MPU region referenced by the MPU_RBAR and MPU_RASR registers.
The MPU supports 8 memory regions, so the permitted values of this field are 0-7.
Normally, you write the required region number to this register before accessing the MPU_RBAR or MPU_RASR. However you can change the region number by writing to the MPU_RBAR with the VALID bit set to 1, see MPU Region Base Address Register. This write updates the value of the REGION field.