Let us consider the value of P=+x
Then the value of Q will be -x respectively as 1 as MSB,
Now according to IEEE format...,
P+Q=+0 and Q+P=-0
i.e., P+Q= 0000 0000 0000 0000 0000 0000 0000 0000
and Q+P= 1000 0000 0000 0000 0000 0000 0000 0000
The sign follows the first term in ambiguous cases.