So I was implementing a 16-bit square root function using this algorithm.

The components contains several 8-bit multipliers and 16-bit comparators, with gates addressing the cycle mechanism mentioned in the algorithm. Here's the highest level schematic:

All ARGs are exactly the same (The first one looks a bit large, but still the same):

On the top right is a 16-bit comparator, nothing wrong with it. The error is in the lower left 8-bit multiplier.

As you can see, one of the output wire is red, indicating some errors.

It turns out that it was from one of the full adder in the 8-bit multiplier, as such:




How come the output of that particular Full Adder becomes blue(unkown)?

After I investigate that full adder, all its wires are blue, which makes little sense to me, since its inputs aren't unknown at all!

Here is the circuit file.

  • \$\begingroup\$ Can you share your .circ file somewhere? And, as an aside: Is there some reason you aren't using bus wires anywhere? \$\endgroup\$
    – user39382
    Commented Apr 6, 2017 at 21:11
  • \$\begingroup\$ The Cin seem to be unknown if I understand correctly the last drawing. Is it connected? \$\endgroup\$
    – Eugene Sh.
    Commented Apr 6, 2017 at 22:06
  • \$\begingroup\$ @duskwuff Just added my .circ file. Oh, I was just not aware that bus is allowed in logisim.. \$\endgroup\$
    – xtt
    Commented Apr 7, 2017 at 1:25
  • \$\begingroup\$ @EugeneSh., yes it is connected. From the second last image you can see that its cin is green. I assume it is a bug as rfoster said.. \$\endgroup\$
    – xtt
    Commented Apr 7, 2017 at 1:26

1 Answer 1


I've noticed a similar bug in the past when working with logisim. Try saving the file, closing it, and reopening it. That should solve the problem with the misbehaving full adder.


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