CSCI E-92: Application Note 2

Atomic access to multifield variables
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Access to time-of-day clock fields (for example, the two 32-bit words
that together make up a ms-since-the-epoch tick clock) in time-of-day
SVC routines needs to be atomic.  If an interrupt that would update
the time-of-day clock occurs between the accesses to those fields, an
incorrect time may be fetched or set.