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# How Do Computers Add? Part 2

Have you ever wondered how computers and calculators—both of which are nothing more than mindless boxes of plastic, wires, and other strange parts—manage to add numbers? And so quickly! Math Dude has the second part of the story.

By
Jason Marshall, PhD
Episode #146

## The Rest of the Story…

Now that we know how XOR works (and even how to build an XOR gate), we’re well on our way towards understanding how computers and calculators add. In particular, since we know that computers work in binary, we know that they must first convert the decimal numbers we give them into their binary number equivalents. And we now know that half of the problem of adding these binary numbers up—the part that involves finding the so-called sum bit—is solved using an XOR gate.

But how do we solve the other half of the problem—the problem of finding the carry bit? And how do we then extend our adding machine so it can add bigger numbers? As we’ll see next time, we just need to make a few (relatively simple) additions to the machine we’re building. After that, you’ll finally know everything you ever wanted to know about how computers and calculators add!

## Wrap Up

Okay, that’s all the math we have time for today. Remember to become a fan of the Math Dude on Facebook where you’ll find lots of great math posted throughout the week. If you’re on Twitter, please follow me there, too. Finally, please send your math questions my way via Facebook, Twitter, or email at mathdude@quickanddirtytips.com.

Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!