PM0214 datasheet(28/245 Pages) STMICROELECTRONICS

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The Cortex-M4 processor

PM0214

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2.2.1

Memory regions, types and attributes

The memory map and the programming of the 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:

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:

2.2.2

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 2.2.4: Software ordering of memory

accesses on page 30.

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:

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.

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.

Table 12. Ordering of memory accesses(1)

1. - 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.

Normal access

Device access

Strongly

ordered

access

Non-shareable

Shareable

Normal access

Device access, non-shareable

Device access, shareable

Strongly ordered access

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