ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Protected Memory System Architecture (PMSA)
The Execute Never (XN) attribute and instruction prefetching
Each memory region can be tagged as not containing executable code. If the Execute never (XN) bit is set
to 1, any attempt to execute an instruction in that region results in a Permission fault, and the implementation
must not access the region to prefetch instructions speculatively. If the XN bit is 0, code can execute from
that memory region.
Note
The XN bit acts as an additional permission check. The address must also have a valid read access
permission.
In ARMv7, all regions of memory that contain read-sensitive peripherals must be marked as XN to avoid
the possibility of a speculative prefetch accessing the locations.
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