full virtualization, the virtual machine simulates enough hardware to allow an unmodified "guest" OS designed for the same instruction set to be run in isolation. This approach was pioneered in 1966 with the IBM CP-40 and CP-67, predecessors of the VM family. Hardware-assisted virtualization In hardware-assisted virtualization, the hardware provides architectural support that facilitates building a virtual machine monitor and allows guest OSs to be run in isolation. Hardware-assisted virtualization was first introduced on the IBM System/370 in 1972, for use with VM/370, the first virtual machine operating system. In 2005 and 2006, Intel and AMD provided additional hardware to support virtualization. Sun Microsystems (now Oracle Corporation) added similar features in their UltraSPARC T-Series processors in 2005. In 2006, first-generation 32- and 64-bit x86 hardware support was found to rarely offer performance advantages over software virtualization. Paravirtualization In paravirtualization, the virtual machine does not necessarily simulate hardware, but instead (or in addition) offers a special API that can only be used by modifying the "guest" OS. For this to be possible, the "guest" OS's source code must be available. If the source code is available, it is sufficient to replace sensitive instructions with calls to VMM APIs (e.g.: "cli" with "vm_handle_cli()"), then re-compile the OS and use the new binaries. This system call to the hypervisor is called a "hypercall" in TRANGO and Xen; it is implemented via a DIAG ("diagnose") hardware instruction in IBM's CMS under VM (which was the origin of the term hypervisor).. Operating-system-level virtualization In operating-system-level virtualization, a physical server is virtualized at the operating system level, enabling multiple isolated and secure virtualized servers to run on a single physical server. The "guest" operating system environments share the same running instance of the operating system as the host system. Thus, the same operating system kernel is also used to implement the "guest" environments, and applications running in a given "guest" environment view it as a stand-alone system. Hardware virtualization disaster recovery A disaster recovery (DR) plan is often considered good practice for a hardware virtualization platform. DR of a virtualization environment can ensure high rate of availability during a wide range of situations that disrupt normal business operations. In situations where continued operations of hardware virtualization platforms is important, a disaster recovery plan can ensure hardware performance and maintenance requirements are met. A hardware virtualization disaster recovery plan involves both hardware and software protection by various