The physical memory-sizer claims to preserve memory contents
(specifically the contents of msgbuf). The loop writes different
values into two adjacent locations and reads the contents of the
first, to ensure that whatever is read back from the first location is
from memory and isn't just the first write persisting on the bus.
The loop preserved the value of the first location, but not the second,
resulting in the second test value ('ZZZZ') over-writing a word in msgbuf.
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