Operating system is large, but modularity is important thus a computer system is made up of layers. The hardware is the lowest level. The kernel running at the next level uses the hardware instructions to create a set of system calls. Virtual machine creates an efficient, dynamic, secure and portable object-oriented facility. Virtual machine provides an interface.
The resources of the physical computer are shared to create the virtual machines. For example if the physical machine has three disk drives to support seven virtual machines, it can not allocate a disk drive to each virtual machine, but it will provide virtual disks. Virtual machine concept is difficult to implement because underlying machine has two modes user-mode and monitor-mode. Virtual machine software can run in monitor mode and execute in user mode.
Benefits
Complete protection of the resources i.e., there is no direct sharing of resources, it is implemented by software
Virtual machine is completely isolated from all other virtual machines, so there are no security problems.
Virtual machine system is a perfect vehicle for operating systems research and development.
When to change operating system, current system must be stopped and taken out of use while changes are made and tested. This period is called “system development time”.
