.------U-------
Vmem---o----------| Vcc
       |          |  
       |          |  
       <          |  
       < R1?      |  
       <          |  
       |   SPDT   |  
         /--------| Axx (highest addr line)
       |          |  
       |          |  
       <          |  
       < R2?      |  
       <
       |
       |
      ===  GND
       =