6.1. The Basics

Virtual machines, and the notion of virtualization in general, are all the rage these days. Whole companies have been built around particular virtual machines, and there is an array of choices from vendors and from the open source world for those who want to explore the world of virtual machines.

Most of the virtual machines^[17] that people are paying attention to virtualize at a very low level. These virtual machines provide a software layer between the raw hardware and the operating system, giving the OS a set of calls that allow that system to boot and interact with the hardware using a single software interface. Using such virtual machines, a single computer can run multiple operating systems at the same time, since the operating system now runs on a virtual rather than physical machine. These virtual machines are really virtual hardware environments. We programmers interact with these virtual machines in the same way that we interact with all pure hardware environments. Which is to say, not at all, except through the mediation of an operating system, which really provides us with a set of abstractions that we can use directly.

^[17] Examples of the sorts of virtual machines I’m talking about here are VMWare’s products (both server and desktop), Xen, VirtualBox, and the like. This is hardly an exhaustive list; more seem to appear weekly.