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Virtual memory provides the illusion of physical memory. The abstraction allows a process to assume that its memory space both is contiguous and begins at location 0. This simplifies the process' view of memory and hides the underlying physical locations of the process' memory. The physical memory corresponding to the virtual memory need not be contiguous. Indeed, some of the locations in virtual memory may have no corresponding physical addresses until the process references them.
Like virtual memory, a virtual machine provides the illusion of a physical machine. The abstraction allows operating systems to assume that they are running directly on the hardware. This allows one to run the operating system, and allows the operating system to run processes, with no changes in either the operating system or the programs. A second, lower "virtual machine monitor" runs directly on the hardware and provides the illusion of hardware to the operating systems run above it. The physical machine may support many virtual machines, each running its own operating system.
This chapter reviews the structure of a virtual machine.
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