OS I,Lec3: Virtual Machine

1.1 Virtual Machines
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 (Chapter 5) and virtual-memory techniques (Chapter 9), an operating system host can create the illusion that a process has its own processor with its own (virtual) memory. The virtual machine provides an interface that is identical to the underlying bare hardware. Each guest process is provided with a (virtual) copy of the underlying computer (Figure 2.17). Usually, the guest process is in fact an operating system, and that is how a single physical machine can run multiple operating systems concurrently, each in its own virtual machine.
1.1.1 Benefits
There are several reasons for creating a virtual machine. Most of them are fundamentally related to being able to share the same hardware yet run several different execution environments (that is, different operating systems) concurrently.
One important advantage is that the host system is protected from the virtual machines, just as the virtual machines are protected from each other. Two approaches to provide sharing have been implemented, however. First, it is possible to share a file-system volume and thus to share files. Second, it is possible to define a network of virtual machines and enable each machine to send information over the virtual communication network. The network is modeled after physical communication networks but is implemented in software.