Darwin

Apple released the Darwin operating system in 2000. It was derived from BSD and a number of other sources, including NextStep (which is why the Cocoa class names are generally prefixed with the letters NS), and contains the I/O Kit device driver API that saves you ever having to worry about getting keyboard or mouse input into the computer, and a million other things.

Info about Darwin is available at puredarwin.org, and if device drivers are your thing, Apple's I/O Kit documentation starts here: https://developer.apple.com/library/mac/documentation/DeviceDrivers/Conceptual/IOKitFundamentals/Introduction/Introduction.html.