UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 20 December 2013 738 of 841
NXP Semiconductors
UM10360
Chapter 34: Appendix: Cortex-M3 user guide
• Normal: The processor can re-order transactions for efficiency, or perform speculative
reads.
• Device: The processor preserves transaction order relative to other transactions to
Device or Strongly-ordered memory.
• Strongly-ordered: The processor preserves transaction order relative to all other
transactions.
The different ordering requirements for Device and Strongly-ordered memory mean that
the 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, 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. Any attempt to fetch an
instruction from an XN region causes a memory management fault exception.
34.3.2.2 Memory system ordering of memory accesses
For most memory accesses caused by explicit memory access instructions, the memory
system does not guarantee that the order in which the accesses complete matches the
program order of the instructions, providing this does not affect the behavior of the
instruction sequence. Normally, if correct program execution depends on two memory
accesses completing in program order, software must insert a memory barrier instruction
between the memory access instructions, see Section 34.3.2.4
.
However, the memory system does guarantee some ordering of accesses to Device and
Strongly-ordered memory. For two memory access instructions A1 and A2, if A1 occurs
before A2 in program order, the ordering of the memory accesses caused by two
instructions is:
Where:
‘—’ means that the memory system does not guarantee the ordering of the accesses.
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