Atmel SAM4S-XPLD Atmel ATSAM4S-XPLD ATSAM4S-XPLD Hoja De Datos

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SAM4S [DATASHEET]

11100E–ATARM–24-Jul-13



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.



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.

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, the software must insert a memory barrier instruction between the memory access
instructions, see 

.

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.

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.

Normal Access

–

–

–

–

Device access, non-
shareable

–

<

–

<

Device access, shareable

–

–

<

<

Strongly-ordered access

–

<

<

<