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Describe arm memory types 

* From the arm infocenter
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Berkus Decker 2019-02-24 00:01:59 +02:00
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The Device memory type has several attributes:
G or nG - Gathering or non-Gathering. Multiple accesses to a device can be merged into a single transaction except for operations with memory ordering semantics, for example, memory barrier instructions, load acquire/store release.
R or nR - Reordering.
E or nE - Early Write Acknowledge (similar to bufferable).
Only four combinations of these attributes are valid:
Device-nGnRnE <-- "Strongly Ordered"
Device-nGnRE <-- "Device Memory"
Device-nGRE
Device-GRE
Typically peripheral control registers must be either `Device-nGnRE`, or `Device-nGnRnE`. This prevents reordering of the transactions in the programming sequences.
`Device-nGRE` and `Device-GRE` memory types can be useful for peripherals where memory access sequence and ordering does not affect results, for example, in bitmap or display buffers in a display interface. If the bus interface of such peripheral can only accept certain transfer sizes, the peripheral must be set to `Device-nGRE`.
Device memory is shareable, and must be cached.
[source](https://developer.arm.com/products/architecture/cpu-architecture/m-profile/docs/100699/latest/memory-type-definitions-in-armv8-m-architecture)