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