All requests for a cache line are screened by {HEAD ~ the blocker} associated with {TAIL ~ each home directory} 
All requests for a cache line are screened by { HEAD ~ the blocker } associated with { TAIL ~ each home directory }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL