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4.5.6. MPU access permission attributes

This section describes the MPU access permission attributes. The access permission bits, C, B, S, AP, and XN, of the MPU_RASR, control access to the corresponding memory region. If an access is made to an area of memory without the required permissions, then the MPU generates a permission fault.

Table 4.32 shows the encodings for the C, B, and S access permission bits.

Table 4.32. C, B, and S encoding
CBSMemory typeShareabilityOther attributes
0 0 x[a]Strongly-orderedShareable-
1 x[a]DeviceShareable-
1 0 0NormalNot shareableOuter and inner write-through. No write allocate.
1Shareable
1 0NormalNot shareableOuter and inner write-back. No write allocate.
1Shareable

[a] The MPU ignores the value of this bit.


Table 4.33 shows the AP encodings that define the access permissions for privileged and unprivileged software.

Table 4.33. AP encoding
AP[2:0]

Privileged

permissions

Unprivileged

permissions

Description
000No accessNo accessAll accesses generate a permission fault.
001RWNo accessAccess from privileged software only.
010RWROWrites by unprivileged software generate a permission fault.
011RWRWFull access.
100UnpredictableUnpredictableReserved.
101RONo accessReads by privileged software only.
110RORORead only, by privileged or unprivileged software.
111RORORead only, by privileged or unprivileged software.

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