= Interrupt-safe ring-buffer snapshot
When an <interrupt handler> is the sole writer of a fixed <ring buffer>, foreground code can obtain a coherent view by saving and masking the relevant interrupt, copying the count and valid entries, and restoring the earlier mask. Perform expensive calculations on the copy after leaving the <critical section>. Exclusion prevents the writer from changing the logical version during the copy; individually atomic loads alone do not give this guarantee. Correctness also requires compiler ordering at the boundaries, no unmasked competing writer, and a short enough mask interval that the peripheral does not lose events. Restoring the previous mask avoids accidentally enabling an interrupt disabled by an outer caller.
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