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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.
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.
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 to support future software.
