Editing Virtual machine (section)

=== System virtual machines ===
{{main|System virtual machine}}
{{See also|Hardware virtualization|comparison of platform virtualization software}}

A 'virtual machine' was originally defined by [[Popek and Goldberg virtualization requirements|Popek and Goldberg]] as "an efficient, isolated duplicate of a real computer machine."<ref name="Popek"/> Current use includes virtual machines that have no direct correspondence to any real hardware.<ref name="Smith_Nair_05"/> The physical, "real-world" hardware running the VM is generally referred to as the 'host', and the virtual machine emulated on that machine is generally referred to as the 'guest'. A host can emulate several guests, each of which can emulate different [[Operating system|operating systems]] and hardware platforms.

The desire to run multiple operating systems was the initial motive for virtual machines, so as to allow time-sharing among several single-tasking operating systems. In some respects, a system virtual machine can be considered a generalization of the concept of [[virtual memory]] that historically preceded it. IBM's [[CP/CMS]], the first systems to allow [[full virtualization]], implemented [[time sharing]] by providing each user with a single-user operating system, the [[Conversational Monitor System]] (CMS). Unlike virtual memory, a system virtual machine entitled the user to write privileged instructions in their code. This approach had certain advantages, such as adding input/output devices not allowed by the standard system.<ref name="Smith_Nair_05"/>

As technology evolves virtual memory for purposes of virtualization, new systems of [[memory overcommitment]] may be applied to manage memory sharing among multiple virtual machines on one computer operating system. It may be possible to share ''memory pages'' that have identical contents among multiple virtual machines that run on the same physical machine, what may result in mapping them to the same physical page by a technique termed [[kernel same-page merging]] (KSM). This is especially useful for read-only pages, such as those holding code segments, which is the case for multiple virtual machines running the same or similar software, software libraries, web servers, [[middleware]] components, etc. The guest operating systems do not need to be compliant with the host hardware, thus making it possible to run different operating systems on the same computer (e.g., [[Microsoft Windows|Windows]], [[Linux]], or prior versions of an operating system) to support future software.<ref name="Oliphant"/>

The use of virtual machines to support separate guest operating systems is popular in regard to [[embedded system]]s. A typical use would be to run a [[real-time operating system]] simultaneously with a preferred complex operating system, such as Linux or Windows. Another use would be for novel and unproven software still in the developmental stage, so it runs inside a [[Sandbox (software development)|sandbox]]. Virtual machines have other advantages for operating system development and may include improved debugging access and faster reboots.<ref name="vmwarez_2006"/>

Multiple VMs running their own guest operating system are frequently engaged for server consolidation.<ref name="vmware_2007"/>