Re: Full-Adder (3 Gates)

Posted by WBahn on May 20, 2019; 7:31pm
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Full-Adder-3-Gates-tp4029696p4033183.html


Step back to the basics. We tend to overly complicate things when often the answer is staring us in the face by just thinking how we would have added three numbers together in second grade.

You have three one-bit inputs (call them X, Y, and Z) that you want to add together. First add X and Y and then add Z to that result, one digit position at a time. It really is that simple.

You use one half-adder to add X and Y to get the subtotal.

You use one half-adder to add Z to the units digit of the subtotal and the carry to the next digit position.

You use one half-adder to add the carry from the prior operation to the 2's digit of the original subtotal.

Not the most efficient implementation, but the most direct in terms of mapping the logic to the task.