UM10462 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2016. All rights reserved.
User manual Rev. 5.5 — 21 December 2016 442 of 523
NXP Semiconductors
UM10462
Chapter 24: LPC11U3x/2x/1x Appendix ARM Cortex-M0
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.
Execute Never (XN) — Means the processor prevents instruction accesses. A HardFault
exception is generated on executing an instruction fetched from an XN region of memory.
24.3.2.2 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 any re-ordering 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 24–24.3.2.4
.
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.
< — Means that accesses are observed in program order, that is, A1 is always observed
before A2.
24.3.2.3 Behavior of memory accesses
The behavior of accesses to each region in the memory map is:
Fig 78. Memory ordering restrictions
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