Re: Add16

Posted by cadet1620 on Jan 08, 2015; 1:21pm
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Add16-tp4028597p4028607.html

You can use a FullAdder as a Half adder by setting the carry-in to zero:

    FullAdder (a=a[0], b=b[0], c=false, sum=out[0], carry=c0);
    ...

Common real world adder parts have a pin for the bit 0 carry-in.  This makes them chainable to make even wider adders.  Given an Add16c chip:

    IN a[16], b[16], c;
    OUT out[16], carry;
PARTS:
    FullAdder (a=a[0], b=b[0], c=c, sum=out[0], carry=c0);
    ...
    FullAdder (a=a[15], b=b[15], c=c14, sum=out[0], carry=carry);

You could use it to make a 32-bit adder:

    Add16c (a=a[0..15],  b=b[0..15],  c=false, out=out[0..15], carry=c15); 
    Add16c (a=a[16..31], b=b[16..31], c=c15,   out=out[16..31]); 

(Note that 32-bit parts are not supported by the simulator.)


Here's something to think about.  What operation does this chip implement, and how does it work?

CHIP Something16 {
    IN  a[16], b[16];
    OUT out[16];
PARTS:
    Not16 (in=b, out=notB);
    Add16c (a=a, b=notB, c=true, out=out);
}

--Mark