Cortex-M3 Processor (Reference Material)
UG0331 User Guide Revision 15.0
30
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. A fault exception is generated
only on execution of an instruction executed from an XN region.
3.5.2.2
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.
However, the memory system does guarantee some ordering of accesses to Device and Strongly-
ordered memory. The following figure shows the ordering of the memory accesses caused by two
instructions A1 and A2 if A1 occurs before A2 in program order.
Figure 12 •
Memory Ordering Restrictions
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.
3.5.2.3
Behavior of Memory Accesses
The following table provides information about the behavior of accesses to each region in the memory
map.
Table 17 •
Memory Access Behavior
Address range Memory region
Memory Type
1
XN
1
Description
0x00000000-
0x1FFFFFFF
Code
Normal
Executable region for program code. You can also put
data here.
0x20000000-
0x3FFFFFFF
SRAM
Normal
Executable region for data. You can also put code
here.This region includes bit band and bit band alias
areas, see
0x40000000-
0x5FFFFFFF
Peripheral
Device
XN
This region includes bit band and bit band alias areas,
see
0x60000000-
0x9FFFFFFF
External RAM
Normal
Executable region for data.
0xA0000000-
0xDFFFFFFF
External device
Device
XN
External Device memory.
Normal access
Device access, non-shareable
Device access, shareable
Strongly-ordered access
Normal
access
Non-shareable
Shareable
Strongly-
ordered
access
Device access
A1
A2
-
-
-
-
-
<
-
<
-
-
<
<
-
<
<
<
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