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