Application Programming Interface (APIs)

Operating systems don't quite date back to the beginning of computing, but they go back far enough. Mainframe customers wrote the first ones in the late 1950s, with operating systems that we'd more clearly recognize as such today—including OS/360 from IBM and Unix from Bell Labs—following over the next couple of decades. An operating system performs a wide variety of useful functions in a system, but it's helpful to think of those as falling into three general categories. First, the operating system sits on top of a physical system and talks to the hardware. This insulates application software from many hardware implementation details...