A semiconductor dynamic memory device contains differential sense amplifiers for detecting the charge on bit line halves which are of the folded type. The sense amplifiers are multiplexed so that one of two opposite pairs of bit line halves are selected. The two opposite pairs share precharge and active pull-up circuits on one side of the array, and share column output lines on the opposite side. Thus, the multiplex circuitry operates not only for selecting one side or the other for sensing, but also for coupling precharge and boost voltages or read/write data back and forth from one side of the sense amplifier to the other. The active pull-up circuits are activated in both read and write portions of a read-modify-write cycle.