Re: Truth Tables and resulting outputs...

Posted by cadet1620 on
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Truth-Tables-and-resulting-outputs-tp3904544p3905827.html

echoes wrote

* I see that you're highlighting the A and OUT bits above and then B and the OUT bits below it. To determine the OUT bit, are we supposed to determine the value of (A AND SEL) = OUT and then (B AND SEL) = OUT? The only thing I see is that OUT = the value of A if S = 0 and OUT = the value of B if S = 1.

When sel is 0, a AND sel is always 0, You need to do something to sel to get a variable that will work correctly with AND and a.

Sometimes it helps to add columns with intermediate results to your truth table:

	sel	a	b	|	x	(x a)	(sel b)	|	out
	0	0	0	|	1	0	0	|	0
	0	0	1	|	1	0	0	|	0
	0	1	0	|	1	1	0	|	1
	0	1	1	|	1	1	0	|	1
	1	0	0	|	0	0	0	|	0
	1	0	1	|	0	0	1	|	1
	1	1	0	|	0	0	0	|	0
	1	1	1	|	0	0	1	|	1

What logical operation do you use to get x from sel?
What logical operation do you use to combine (x AND a) and (sel AND b)?

--Mark