A modern computer consists of one or more processors, some main memory,disks, printers, a keyboard, a mouse, a display, network interfaces, and various other input/output devices. All in all, a complex system. If every application programmer had to understand how all these things work in detail, no code would ever get written. Furthermore, managing all these components and using them optimally is an exceedingly challenging job. For this reason, computers are equipped with a layer of software called the operating system, whose job is to provide user programs with a better, simpler, cleaner, model of the computer and to handle managing all the resources just mentioned. These systems are the subject of this book. Most readers will have had some experience with an operating system such as Windows, Linux, FreeBSD, or Max OS X, but appearances can be deceiving. The program that users interact with, usually called the shell when it is text based and the GUI (Graphical User Interface)—which is pronounced "gooey"—when uses icons, is actually not part of the operating system although it uses the operating system to get its work done.