caching - How does a prefetch MSHR works? -

i have been reading miss status handling registers (mshr) , using sim-alpha experiments cache , noticed in parameters number of mshrs can configured, number of prefetch mshrs .

i have looked in article describes alpha 21264 microprocessor not mention it. neither have found online resource can definition.

does 1 knows how prefetch mshr works?

thanks , best regards.