An x
on an input means undefined, that is it is either 0
or 1
.
If both inputs to a mux are at the same value it does not matter what the select value is, the output is still defined as it has to be one of the inputs. In our example the select is x
but both inputs and output are 0, 4 then 5.
If the inputs are not the same, the output is x
, it is undefined as it could be either of the input values.