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

Application Level Memory Model
Non-shareable Normal memory
For a Normal memory region, the Non-shareable attribute identifies Normal memory that is likely to be
accessed only by a single processor.
A region of Normal memory with the Non-shareable attribute does not have any requirement to make data
accesses by different observers coherent, unless the memory is non-cacheable. If other observers share the
memory system, software must use cache maintenance operations if the presence of caches might lead to
coherency issues when communicating between the observers. This cache maintenance requirement is in
addition to the barrier operations that are required to ensure memory ordering.
For Non-shareable Normal memory, the Load-Exclusive and Store-Exclusive synchronization primitives do
not take account of the possibility of accesses by more than one observer.
Shareable, Inner Shareable, and Outer Shareable Normal memory
For Normal memory, the Shareable and Outer Shareable memory attributes describe Normal memory that
is expected to be accessed by multiple processors or other system masters:
In a VMSA implementation, Normal memory that has the Shareable attribute but not the Outer
Shareable attribute assigned is described as having the Inner Shareable attribute.
In a PMSA implementation, no distinction is made between Inner Shareable and Outer Shareable
Normal memory, and you cannot assign the Outer Shareable attribute to Normal memory regions.
A region of Normal memory with the Shareable attribute is one for which data accesses to memory by
different observers within the same shareability domain are coherent.
The Outer Shareable attribute is introduced in ARMv7, and can be applied only to a Normal memory region
in a VMSA implementation that has the Shareable attribute assigned. It creates three levels of shareability
for a Normal memory region:
Non-shareable
A Normal memory region that does not have the Shareable attribute assigned.
Inner Shareable
A Normal memory region that has the Shareable attribute assigned, but not the Outer
Shareable attribute.
Outer Shareable
A Normal memory region that has both the Shareable and the Outer Shareable attributes
assigned.
These attributes can be used to define sets of observers for which the shareability attributes make the data
or unified caches transparent for data accesses. The sets of observers that are affected by the shareability
attributes are described as shareability domains. The details of the use of these attributes are
system-specific. Example A3-1 on page A3-31 shows how they might be used:
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