The Abstraction: Files and Directories

Thus far we have seen the development of two key operating system abstractions: the process, which is a virtualization of the CPU, and the address space, which is a virtualization of memory. In tandem, these two abstractions allow a program to run as if it is in its own pri-vate, isolated world; as if it has its own processor (or processors); as if it has its own memory. This illusion makes programming the sys-tem much easier and thus is prevalent today not only on desktops and servers but increasingly on all programmable platforms includ-ing mobile phones and the like. In this section, we add one more critical piece to the virtualiza-tion puzzle: persistent storage. A persistent-storage device, such as a classic hard disk drive or a more modern solid-state storage de-vice, stores information permanently (or at least, for a long time). Unlike memory, whose contents are lost when there is a power loss, a persistent-storage device keeps such data intact. Thus, the OSmus