Hi,

I have to design a circuit in Logisim that can multiply two 8-bit signed binary integers in two's complement format, producing a 16-bit signed result. I would like to do this using all combinational logic. I am a novice to circuit design and I am unsure of how to go about doing this. Attached is the document I have thus far. I would like to use a series of carry-save adders to produce each of the partial products and at the end invert the partial products so that it is in two's complement format.

 

Since you are a novice (as we all were at one time) I would recommend first focusing on getting a circuit to work, no matter how brute-force it might be, especially since you are doing this in simulation where gates are dirt cheap. So forget, for now, about carry-save adders and the like.

Ask yourself how YOU do it on paper with a pen an pencil. Use that as your starting point. So take out a sheet of paper and multiply two 4-bit unsigned values together, such as 1011 and 0110, and see if you can see a simple way to build a circuit that produces an 8-bit result. Now consider the impact of the 2's compliment representation and how you could accommodate that -- again, be as brute force as you need to be.

Once you understand how that first solution works, you are in a position to start seeing the patterns in the problem and its solution which puts you in a better position to understand more sophisticated approaches.