{TAIL ~ processor} means for reading data from {HEAD ~ said main memory means} and writing data into said main memory means 
{ TAIL ~ processor } means for reading data from { HEAD ~ said main memory means } and writing data into said main memory means
TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH