Introduction to Operating Systems（操作系统概论）.pdfVIP

2 Introduction to Operating Systems If you are taking an undergraduate operating systems course, you should already have some idea of what a computer program does when it runs. If not, this book (and the corresponding course) is going to be difficult — so you should probably stop reading this book, or run to the nearest bookstore and quickly consume the necessary background material be- fore continuing (both Patt/Patel [PP03] and particularly Bryant/O’Hallaron [BOH10] are pretty great books). So what happens when a program runs? Well, a running program does one very simple thing: it executes in- structions. Many millions (and these days, even billions) of times ev- ery second, the processor fetches an instruction from memory, decodes it (i.e., figures out which instruction this is), and executes it (i.e., it does the thing that it is supposed to do, like add two numbers together, access memory, check a condition, jump to a function, and so forth). After it is done with this instruction, the processor moves on to the next instruction, 1 and so on, and so on, until the program finally completes . Thus, we have just described the basics of the Von Neumann model of 2 computing . Sounds simple, right? But in this class, we will be learning that while a program runs, a lot of other wild things are going on with the primary goal of making the system easy to use. There is a body of software, in fact, that is responsible for making it easy to run programs (even allowing you to seemingly run many at the same time), allowing programs to share memory, enabling programs to interact with devices, and other fun stuff like that. That body of software 1Of course, modern processors do many bizarre and frightening things underneath the hood to make programs run faster, e.g., executing multiple instructions at once,