2 I/O DEVICES Graphics

Before delving into the main content of this part of the book (on persis-tence), we first introduce the concept of an input/output (I/O) device and show how the operating systemmight interact with such an entity. I/O is quite critical to computer systems, of course; imagine a program without any input (it produces the same result each time); now imagine a pro-gram with no output (what was the purpose of it running?). Clearly, for computer systems to be interesting, both input and output are required. And thus, our general problem