2.2.1. Memory regions, types and attributes

The memory map and programming the optional MPU splits the memory map into regions. Each region has a defined memory type, and some regions have additional memory attributes. The memory type and attributes determine the behavior of accesses to the region.

The memory types are:

Normal

The processor can re-order transactions for efficiency, or perform speculative reads.

Device and Strongly-Ordered

The processor preserves transaction order relative to other transactions to Device or Strongly-ordered memory.

The different ordering requirements for Device and Strongly-ordered memory mean that the external memory system can buffer a write to Device memory, but must not buffer a write to Strongly-ordered memory.

The additional memory attributes include:

Shareable

For a shareable memory region that is implemented, the memory system provides data synchronization between bus masters in a system with multiple bus masters, for example, a processor with a DMA controller.

Strongly-ordered memory is always shareable.

If multiple bus masters can access a non-shareable memory region, software must ensure data coherency between the bus masters.

Execute Never (XN)

Means the processor prevents instruction accesses. A fault exception is generated only on execution of an instruction executed from an XN region.

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