So this is the truth table given for the Priority Encoder:
and this is its logic diagram:
I am extremely confused with the part where the outputs
y are labeled as
X (where D0 D1 D2 D3 are all zero). First of all, how could an output be a "don't care" term? What that would mean is that when D0 D1 D2 D3 are all zero,
y are either 1 or 0. But that does not make sense at all. How could an output have an indefinite value when we have definite inputs?
As you can see from the diagram,
x = D2 + D3 and
y = D3 + D1D2'. So if you put zeros to all inputs it is clear that
y should be 0. Then why is it shown as
X in the truth table?