The software that creates a virtual machine (VM) environment in a computer (for the fundamental concept, see virtual machine). In a regular, non-virtual computer, the operating system is the master control program, which manages the execution of all applications and acts as an interface between the apps and the hardware. The OS has the highest privilege level in the machine, known as "ring 0" (see ring).In a VM environment, the VM monitor (VMM) becomes the master control program with the highest privilege level, and the VMM manages one or more "guest operating systems." Each guest OS manages its own applications in a separate "virtual machine" (VM) in the computer, sometimes called a "guest OS stack."What Gets VirtualizedThe VM monitor (VMM) is an interface between the guest OS and the hardware. It intercepts calls to the peripheral devices and memory tables from each guest OS and intercedes on its behalf (see virtual memory). In reverse, when a disk or SSD write creates an interrupt, the VM monitor injects that interrupt into the appropriate guest OS. Following are the major monitor types.