Here's the detailed logic of the bitwise operation of the Hexadecimal Substraction Calculator (http://everycircuit.com/circuit/4556788742225920).
The inputs are number A (top left), followed that number B (the result is equal to A-B) and finally the borrow of bit N-1.
The upmost LED represents the state of bit N after the operations and the other one represents the borrow of bit N.
It's the same circuit as the Bitwise Addition Circuit, with the difference that input A (the upmost logic source) is inverted before entering the AND gates.
Here's the truth table of the circuit :
A = Bit N of number A
B = Bit N of number B
C = Borrow of bit N-1
Y = Result of bit N
Z = Borrow of bit N
ABC-YZ
000-00
001-11
010-11
011-01
100-10
101-00
110-00
111-11
