{TAIL ~ a second constant current source} that is connected to the first bit line, {HEAD ~ the second constant current source} generating a reference current for the second memory cell column; and 
{ TAIL ~ a second constant current source } that is connected to the first bit line , { HEAD ~ the second constant current source } generating a reference current for the second memory cell column ; and
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH