We use a address decoder to decode the address that are present in address bus

As for example, consider a memory module of 16 location and each location can store 4 bit of information
                     The size of address bus is   4 bit and the size of the data bus is  4 bit
                     The size of address decoder is   4 X 16.

There is a control signal named R/W.
                       If   R/W  =  0,    we perform a  READ    operation and
                       if   R/W  =  1,    we perform a  WRITE   operation        

If the contents of address bus is  0101  and contents of data bus is 1100 and R/W = 1, then 1100 will be
written in location 5.

If the contents of address bus is 1011 and R/W=0, then the contents of location 1011 will be placed in data bus.

In next section, we will explain how to perform memory access operation in our small hypothetical computer.