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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.
1 0NormalNot shareableOuter and inner write-back. No write allocate.

[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





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

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