Minecraft wiki entry for Logic_gates, associated text: A control bit is added to the circuit such that when it is on, the unit subtracts, and when it is off the unit adds. After this, add XOR gates between the control bit and each B input. Route the output of each XOR to the B input of each adder. Finally, to make the unit Two's compliment compatible, a final XOR gate must be added between the control bit and the carry out of the most significant bit.