Substituting libraries

Now suppose we have two Logisim circuits that are supposed to do the same thing. As an instructor, you might have had students complete an assignment: You have one file containing your solution, but you have several student files containing their work. Maybe the assignment was to build a two-bit adder.

I'll imagine that we have two files, named master.circ and query.circ. Each file contains a circuit named Adder2 (it's important that the circuit to test be named exactly the same), whose appearance is the following.

| Adder2 in master.circ | Adder2 in query.circ | | | |

As you can see, the master circuit uses Logisim's built-in adder, while the query circuit uses two subcircuits representing a half adder and a full adder (which themselves are built up of simple gates). For the purpose of our example, the query circuit has a stupid error: The carry from the half adder is not connected into the full adder.

We build our testing circuit into a different file test.circ. There, we load master.circ as a Logisim Library | Project | → | Load Library | → | Logisim Library | , and we insert its 2-bit adder as a subcircuit. We could execute this circuit directly to get the desired output for a perfect solution.

java -jar logisim-evolution.jar test.circ -tty table

But we want to execute the circuit using query.circ rather than master.circ as the loaded library. The naive approach would be to open Logisim and load that library instead; or you might simply remove the master.circ file and rename query.circ to be named master.circ instead. But Logisim includes a handy -sub option that temporarily replace one file by another during that session — without making any changes on disk.

java -jar logisim-evolution.jar test.circ -tty table -sub master.circ query.circ

The output you would see from this is shown below; it is of course different from what we saw in the previous section because the library common to adder2 has been replaced in the test circuit by that of query.circ erroneous.

Copy

00 00 0E0
01 00 0E1
10 00 EE0
11 00 EE1
00 01 0E1
01 01 0E0
10 01 EE1
11 01 EE0
00 10 EE0
01 10 EE1
10 10 1E0
11 10 1E1
00 11 EE1
01 11 EE0
10 11 1E1
11 11 1E0

Next: Other verification options.