4.6.6. MPU access permission attributes

Note

Ignore this section if the optional MPU is not present in your implementation.

This section describes the MPU access permission attributes. The access permission bits, TEX, C, B, S, AP, and XN, of the 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 4.55 shows encodings for the TEX, C, B, and S access permission bits.

Table 4.55. TEX, C, B, and S encoding

TEXCBSMemory typeShareabilityOther attributes
0b00000x[a]Strongly OrderedShareable-
1x[a]DeviceShareable-
100NormalNot shareableOuter and inner write-through. No Write-Allocate.
1Shareable
10NormalNot shareableOuter and inner Write-Back. No Write-Allocate.
1Shareable
0b001000NormalNot shareableOuter and inner noncacheable.
1Shareable
1x[a]Reserved encoding-
10x[a]Contact the Arm partner for attributes used.-
10NormalNot shareableOuter and inner Write-Back. Write and read allocate.
1Shareable
0b01000x[a]DeviceNot shareableNonshared Device.
1x[a]Reserved encoding-
1x[a]x[a]Reserved encoding-
0b1BBAA0NormalNot shareable

Cached memory, BB = outer policy, AA = inner policy. See Table 4.56 for the encoding of the AA and BB bits.

1Shareable

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


Table 4.56 shows the cache policy for memory attribute encodings with a TEX value is in the range 4-7.

Table 4.56. Cache policy for memory attribute encoding

Encoding, AA or BBCorresponding cache policy
00Non-cacheable
01Write back, write and Read- Allocate
10Write through, no Write-Allocate
11Write back, no Write-Allocate

Table 4.57 shows the AP encodings that define the access permissions for privileged and unprivileged software.

Table 4.57. 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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