Memory has two control wires, MA (memory active) and WR (write). If MA=0, the memory is isolated from all of its inputs and outputs. If MA=1 and WR=0, the selected word of memory is copied into onto MEM-OUT, which can then be latched into the MBR register when MBR-LD goes high. If MA=1 and WR=1, the value in the MBR is copied into the selected word of memory. Of course, the value in the MAR register determines which word is read or written.