Re: Full-Adder (3 Gates)

Posted by ouverson on May 20, 2019; 6:28pm
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Full-Adder-3-Gates-tp4029696p4033182.html

As Half-Adders have 2 inputs and 2 outputs I figured (looking at my diagram) that a carry bit will need to be "dropped", as Full-Adder has 3 inputs. I recall Noam and Shimon mentioning this carry bit "dropping off" in the video.

No matter how I wire the 3 Half-Adders, the test fails? I figured that the 1st Half-Adder was where to set carry to true (I also tried false.)

Frankly, I thought that there would be an implementation where there would be 1 or 2 Half-Adders and some way to negate sum and carry, depending on whether a was 0 or 1 - as that was the way the truth table seemed to lead - logically.