The two selection lines S1 and S2 are connected to the selection inputs of all four multiplexers.
The selection lines choose the four bits of one register and transfer them into the four-line
When both of the select lines are at low logic, i.e. S1S0 = 00, the 0 data inputs of all four
multiplexers are selected and applied to the outputs that forms the bus. This, in turn, causes the
bus lines to receive the content of register A since the outputs of this register are connected to the
0 data inputs of the multiplexers.
Similarly, when S1S0 = 01, register B is selected, and the bus lines will receive the content
provided by register B.
The following function table shows the register that is selected by the bus for each of the four
possible binary values of the Selection line
A common busy system is shown below with the corresponding truth table: