63
11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
Notes:
1. See
“Memory Regions, Types and Attributes”
for more information.
2. WT = Write through, no write allocate. WBWA = Write back, write allocate. See the
for more information.
Instruction Prefetch And Branch Prediction
The Cortex-M4 processor:
• prefetches instructions ahead of execution
• speculatively prefetches from branch target addresses.
11.4.2.4
Software Ordering of Memory Accesses
The order of instructions in the program flow does not always guarantee the order of the corre-
sponding memory transactions. This is because:
• the processor can reorder some memory accesses to improve efficiency, providing this does
not affect the behavior of the instruction sequence.
• the processor has multiple bus interfaces
• memory or devices in the memory map have different wait states
• some memory accesses are buffered or speculative.
“Memory System Ordering of Memory Accesses”
describes the cases where the memory sys-
tem guarantees the order of memory accesses. Otherwise, if the order of memory accesses is
critical, the software must include memory barrier instructions to force that ordering. The proces-
sor provides the following memory barrier instructions:
DMB
The Data Memory Barrier (DMB) instruction ensures that outstanding memory transactions com-
plete before subsequent memory transactions. See
.
DSB
The Data Synchronization Barrier (DSB) instruction ensures that outstanding memory transac-
tions complete before subsequent instructions execute. See
.
ISB
The Instruction Synchronization Barrier (ISB) ensures that the effect of all completed memory
transactions is recognizable by subsequent instructions. See
0x60000000-
0x7FFFFFFF
External RAM
Normal
-
WBWA
0x80000000-
0x9FFFFFFF
WT
0xA0000000-
0xBFFFFFFF
External device
Device
Shareable
-
0xC0000000-
0xDFFFFFFF
Non-shareable
0xE0000000-
0xE00FFFFF
Private Peripheral
Bus
Strongly- ordered
Shareable
-
0xE0100000-
0xFFFFFFFF
Vendor-specific
device
Device
-
-
Table 11-5.
Memory Region Shareability Policies (Continued)
Address Range
Memory Region
Memory Type
Shareability
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