d = a-b;
get MSB of d
no is +ve and a>b and so compare a and c
//Number d is positive and so we get a as greater than b
//Now compare a and c in similar fashion
no is -ve and a<b so compare b and c in similar fashion.
//Number d is negative and so we get a as less than b
//Now compare b and c in similar fashion
I hope this comments as explanation helps you to understand.