Compatability

I always wonder how difficult it would be to integrate older code into newer devices. People call this ability as "backward compatability" so they can integrate what's called legacy devices. For instance, a legacy device would be a car phone. We know that the car phone no longer exists, and has been replaced with cell phones. However, cell phones have had some code that have made it's way into some of the newer code. For instance, when you turn on a cell phone, there is code to make the back screen light up. This code was originally in the carphone!