I'm quite new to logism and circuits and I have an assignment about implementing a carry look ahead unit. I Just want some help in regards to clearing some of my confusions to carry look ahead units. So from what I understand, A CLA has 3 main parts to it, a Carry input, a Propagator and a Generator. Carry input can be calculated via the following formula:

\$C_{i+1} = G_i + P_iC_i\$

And \$G_i =\$ A XOR B, \$P_i=AB\$

And the what I really need to find out the rest of the carry inputs is just \$C_0\$

Now, as I am implementing my CLA unit in Logism, am I supposed to make different circuits for each carry input? Like for example, for \$C_1\$ I would implement the circuit of \$G_0 + P_0C_0\$ and then another for \$C_2\$, I would implement the circuit \$G_1 + P_1C_1\$, and then concatenate them via a splitter. I ask this because I tried just plugging every variable up to \$C_4\$ and implementing the long expression, only to get things wrong. Any helpful hints is appreciated. Please don't outright give a picture of the CLA implementation as I want to actually do this myself :)

edit: I managed to figure it out. I did indeed have to calculate each carry in and make a circuit for each expression.

  • \$\begingroup\$ please post an answer, then you can accept it \$\endgroup\$
    – jsotola
    Commented Feb 3, 2020 at 5:04
  • \$\begingroup\$ Do you know the concept of subcircuits in Logisim? Read the built-in tutorial for details. It might help you minimize the work. \$\endgroup\$ Commented Feb 3, 2020 at 7:42

1 Answer 1


What I did was calculate each Carry-in expression, and implement their circuits, before concatenating them all together.


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