containing the accessed cell will drop from vdd towards ground when the nmos device 421 or 422 turns on similarly the voltage of the other column will also drop from vdd towards ground as the reference cell 451 or 452 turns on now the voltage of the column with the accessed cell will drop at twice the rate that the voltage on the other column drops due to the reference cell having only one half of the drive current fig5 a shows the two column voltages dropping and the voltage differential u03b4v seen by sense amplifier 330 conversely if the accessed cell does not have an nmos device present storing a 1 in the upper rom or a 0 in the lower rom then the voltage on the column containing the accessed cell does not drop toward ground because nothing turns on to pull it down but the reference cell in the other column turns on and pulls the voltage towards ground see fig5 b again showing the differential voltage u03b4v seen by sense amplifier 330 but with opposite polarity from the case of fig5 a the two rows of reference cells have nmos devices in every column whereas the rows of cells storing data only have nmos devices in the columns with.