4.5.5. MPU Region Attribute and Size Register

The MPU_RASR defines the region size and memory attributes of the MPU region specified by the MPU_RNR, and enables that region and any subregions. See the register summary in Table 4.38 for its attributes.

MPU_RASR is accessible using word or halfword accesses:

The bit assignments are:

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Table 4.43. MPU_RASR bit assignments

BitsNameFunction
[31:29]-Reserved.
[28]XN

Instruction access disable bit:

0 = instruction fetches enabled

1 = instruction fetches disabled.

[27]-Reserved.
[26:24]APAccess permission field, see Table 4.47.
[23:22] - Reserved.
[21:19, 17, 16]TEX, C, BMemory access attributes, see Table 4.45.
[18]S

Shareable bit, see Table 4.45.

[15:8]SRD

Subregion disable bits. For each bit in this field:

0 = corresponding sub-region is enabled

1 = corresponding sub-region is disabled

See Subregions for more information.

Region sizes of 128 bytes and less do not support subregions. When writing the attributes for such a region, write the SRD field as 0x00.

[7:6]-Reserved.
[5:1]SIZESpecifies the size of the MPU protection region. The minimum permitted value is 3 (0b00010). See SIZE field values for more information.
[0]ENABLERegion enable bit.

For information about access permission, see MPU access permission attributes.

SIZE field values

The SIZE field defines the size of the MPU memory region specified by the RNR. as follows:

    (Region size in bytes) = 2(SIZE+1)

The smallest permitted region size is 32B, corresponding to a SIZE value of 4. Table 4.44 gives example SIZE values, with the corresponding region size and value of N in the MPU_RBAR.

Table 4.44. Example SIZE field values

SIZE valueRegion sizeValue of N [a]Note
0b00100 (4)32B5Minimum permitted size
0b01001 (9)1KB10-
0b10011 (19)1MB20-
0b11101 (29)1GB30-
0b11111 (31)4GB32Maximum possible size

[a] In the MPU_RBAR, see MPU Region Base Address Register.


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