Software Ordering Of Memory Accesses - ST STM32H7 Series Programming Manual

PM0214
2.2.4

Software ordering of memory accesses

The order of instructions in the program flow does not always guarantee the order of the
corresponding memory transactions. The reason for this is that:
•
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.
Section 2.2.2: Memory system ordering of memory accesses on page 29
cases where the memory system guarantees the order of memory accesses. Otherwise, if
the order of memory accesses is critical, software must include memory barrier instructions
to force that ordering. The processor provides the following memory barrier instructions:
DMB The Data Memory Barrier (DMB) instruction ensures that outstanding memory
transactions complete before subsequent memory transactions. See
page
DSB The Data Synchronization Barrier (DSB) instruction ensures that outstanding
memory transactions complete before subsequent instructions execute. See
on page
ISB The Instruction Synchronization Barrier (ISB) ensures that the effect of all
completed memory transactions is recognizable by subsequent instructions. See
ISB on page
Use memory barrier instructions in, for example:
•
Vector table. If the program changes an entry in the vector table, and then enables the
corresponding exception, use a DMB instruction between the operations. This ensures
that if the exception is taken immediately after being enabled the processor uses the
new exception vector.
•
Self-modifying code. If a program contains self-modifying code, use an ISB
instruction immediately after the code modification in the program. This ensures that
the subsequent instruction execution uses the updated program.
•
Memory map switching. If the system contains a memory map switching mechanism,
use a DSB instruction after switching the memory map in the program. This ensures
that the subsequent instruction execution uses the updated memory map.
• Dynamic exception priority change. When an exception priority has to change when
the exception is pending or active, use DSB instructions after the change. This ensures
that the change takes effect on completion of the DSB instruction.
•
Using a semaphore in multi-master system. If the system contains more than one
bus master, for example, if another processor is present in the system, each processor
must use a DMB instruction after any semaphore instructions, to ensure other bus
masters see the memory transactions in the order in which they were executed.
Memory accesses to Strongly-ordered memory, such as the system control block, do not
require the use of DMB instructions.
For MPU programming, use a DSB followed by an ISB instruction or exception return to
ensure that the new MPU configuration is used by subsequent instructions.
183.
184.
185.
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The Cortex-M4 processor
describes the
DMB on
DSB
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