•The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines.  This isolation, however, permits no direct sharing of resources. •A virtual-machine system is a perfect vehicle for operating-systems research and development.  System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation. •The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine