Virtual Machines

• The layered approach described in Section 1.6.2 is taken to its logical conclusion in the concept of a virtual machine. The fundamental idea behind a virtual machine is to abstract the hardware of a single computer (the CPU, memory, disk drives, network interface cards, and so forth) into several different execution environments, thereby creating the illusion that each separate execution environment is running its own private computer.
• By using CPU scheduling and virtual-memory techniques, an OS can create the illusion that a process has its own processor with its own (virtual) memory. Normally, a process has additional features, such as system calls and a file system, that are not provided by the bare hardware.
• The virtual-machine approach does not provide any such additional functionality but rather provides an interface that is identical to the underlying bare hardware. Each process is provided with a (virtual) copy of the underlying computer (see Fig. 16).

  Figure 16: System models. (a) Nonvirtual machine. (b) Virtual machine.

  
• There are several reasons for creating a virtual machine, all of which are fundamentally related to being able to share the same hardware yet run several different execution environments (that is, different OSs) concurrently.
• Despite the advantages of virtual machines, they received little attention for a number of years after they were first developed. Today, however, virtual machines are coming back into fashion as a means of solving system compatibility problems.