Atmel SAM4S Series, Preliminary Data Sheet

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11100B–ATARM–31-Jul-12
SAM4S Series [Preliminary]
Memory Types
• 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.
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.
Additional Memory Attributes
• 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, the 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.
11.4.2.2 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. Nor-
mally, if correct program execution depends on two memory accesses completing in program
order, the software must insert a memory barrier instruction between the memory access
instructions, see
“Software Ordering of Memory Accesses” .
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 is described below.
Table 11-3. Ordering of the Memory Accesses Caused by Two Instructions
A2
Normal
Access
Device Access Strongly-
ordered
Access A1 Non-shareable Shareable
Normal Access – – – –
Device access, non-shareable – < – <
Device access, shareable – – < <
Strongly-ordered access – < < <