LM3S5C51 Datasheet, PDF (86/1266 Pages) Texas Instruments – Stellaris® LM3S5C51 Microcontroller

The Cortex-M3 Processor
2.4.2
2.4.3
2.4.4
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 the order 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
“Software Ordering of Memory Accesses” on page 86).
However, the memory system does guarantee ordering of accesses to Device and Strongly Ordered
memory. For two memory access instructions A1 and A2, if both A1 and A2 are accesses to either
Device or Strongly Ordered memory, and if A1 occurs before A2 in program order, A1 is always
observed before A2.
Behavior of Memory Accesses
Table 2-5 on page 86 shows the behavior of accesses to each region in the memory map. See
“Memory Regions, Types and Attributes” on page 85 for more information on memory types and
the XN attribute. Stellaris devices may have reserved memory areas within the address ranges
shown below (refer to Table 2-4 on page 83 for more information).
Table 2-5. Memory Access Behavior
Address Range
Memory Region
0x0000.0000 - 0x1FFF.FFFF Code
Memory Type Execute
Never
(XN)
Normal
-
0x2000.0000 - 0x3FFF.FFFF SRAM
Normal
-
0x4000.0000 - 0x5FFF.FFFF Peripheral
Device
XN
0x6000.0000 - 0x9FFF.FFFF External RAM
Normal
-
0xA000.0000 - 0xDFFF.FFFF External device Device
XN
0xE000.0000- 0xE00F.FFFF Private peripheral Strongly
XN
bus
Ordered
0xE010.0000- 0xFFFF.FFFF Reserved
-
-
Description
This executable region is for program code.
Data can also be stored here.
This executable region is for data. Code
can also be stored here. This region
includes bit band and bit band alias areas
(see Table 2-6 on page 88).
This region includes bit band and bit band
alias areas (see Table 2-7 on page 88).
This executable region is for data.
This region is for external device memory.
This region includes the NVIC, system
timer, and system control block.
-
The Code, SRAM, and external RAM regions can hold programs. However, it is recommended that
programs always use the Code region because the Cortex-M3 has separate buses that can perform
instruction fetches and data accesses simultaneously.
The MPU can override the default memory access behavior described in this section. For more
information, see “Memory Protection Unit (MPU)” on page 110.
The Cortex-M3 prefetches instructions ahead of execution and speculatively prefetches from branch
target addresses.
Software Ordering of Memory Accesses
The order of instructions in the program flow does not always guarantee the order of the
corresponding memory transactions for the following reasons:
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January 23, 2012
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