In the case of a volatile object, the last store may not
be explicit in the program.

and

A volatile declaration may be used to describe an object
corresponding to a memory-mapped input/output port or an
object  accessed  by  an   asynchronously   interrupting
function.   Actions  on objects so declared shall not be
``optimized out''  by  an  implementation  or  reordered
except   as   permitted  by  the  rules  for  evaluating
expressions.