The volatile data-type qualifier prevents an object from being
  stored in a machine register, forcing it to be allocated in memory.
  This data-type qualifier is useful for declaring data that is to be
  accessed asynchronously.  A device driver application often uses
  volatile data storage.  Like const, you can specify the volatile
  data-type qualifier with any of the storage-class specifiers or
  modifiers with the exception of the register storage class.