Now that you have the k1, k2 and k3 circuits defined, instantiate one of each in the "main" circuit". To do this, double click "main" so that you are in the edit area of the "main" circuit. Then single left click the k circuit that you want to instantiate, and move the mouse cursor to the main circuit editing area.

Yes! Indeed, this is no different from how you instantiate built-in components of Logisim.

Now wire up these k components. Connect the appropriate tunnels/wires to the input pins of the k circuits. This is where labeling can really come in handy so you can identify ports of the k circuits.

After wiring all the k circuits, you now have all the carry bits: k 0 , k 1 , k 2 , k 3.

From the previous, the tunnel labeled Q should have all the q (intermediate sum) bits.

This means you have all the bits to compute the sum. However, because the Q tunnel is a bundle of 3 bits, where as the k circuits only output a single bit, the Q and K bits cannot connect to a gate readily.

Use a splitter in reverse as a merger to merge some of the K bits so that the output of the merger is a bundle of 3 bits. You can now connect the Q tunnel and the output of the merger to a single multi-bit gate to compute the sum ("S") bits. Connect the output of this gate directly to a multi-bit output pin as sum.

There should be a "leftover" k bit that is not used in the merger. This k bit connects to its own single-bit output pin as the "carry out" of the "main" circuit.

Now enter the following as X: 011

Enter the following as Y: 101

and the following as k0

1

Using the poke tool to poke the wire coming out of the merger of the K bits to the gate that computes sum bits. What is the value to the right of the slash?