Describing existing circuits

When used to describe an existing circuit, output values are (of course) either 0 or 1. Suppose for instance that we wish to make a truth table for the following circuit:

All we need to do to establish a truth table for this circuit is to compute the output value for the circuit for each possible combination of input values. We obtain the following truth table:

  w x y | a b

  -----------

  0 0 0 | 0 1

  0 0 1 | 0 1

  0 1 0 | 1 1

  0 1 1 | 1 0

  1 0 0 | 1 1

  1 0 1 | 1 1

  1 1 0 | 1 1

  1 1 1 | 1 0