7.1.3. Background regions

Overlapping regions increase the flexibility of how the regions can be mapped onto physical memory devices in the system. You can also use the overlapping properties to specify a background region. For example, you might have a number of physical memory areas sparsely distributed across the 4GB address space. If a programming error occurs, the processor might issue an address that does not fall into any defined region.

If the address that the processor issues falls outside any of the defined regions and the MPU is enabled, the MPU is hard-wired to abort the access. That is, all accesses for an address that is not mapped to a region in the MPU generate a background fault. You can override this behavior by programming region 0 as a 4GB background region. In this way, if the address does not fall into any of the other 11 regions, the attributes and access permissions you specified for region 0 control the access.

In Privileged modes, you can also override this behavior by setting the BR bit, bit [17], of the SCTLR. This causes Privileged accesses that fall outside any of the defined regions to use the default memory map.

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