Editing Virtual machine (section)

=== Process virtual machines ===
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{{Redirect-distinguish|Application virtual machine|application virtualization}}
{{See also|Comparison of application virtualization software}}

A '''process virtual machine''', sometimes called an ''application virtual machine'', or ''Managed Runtime Environment'' (MRE), runs as a normal application inside a host OS and supports a single process. It is created when that process is started and deleted when it is closed. Its purpose is to provide a [[system platform|platform]]-independent programming environment that abstracts away details of the underlying hardware or operating system and allows a program to execute in the same way on any platform.{{Citation needed|date=November 2015}}

A process VM provides a high-level abstraction{{snd}} that of a [[high-level programming language]] (compared to the low-level ISA abstraction of the system VM). Process VMs are implemented using an [[Interpreter (computing)|interpreter]]; performance comparable to compiled programming languages can be achieved by the use of [[just-in-time compilation]].{{Citation needed|date=November 2015}}

This type of VM has become popular with the [[Java (programming language)|Java programming language]], which is implemented using the [[Java virtual machine]]. Other examples include the [[Parrot virtual machine]] and the [[.NET Framework]], which runs on a VM called the [[Common Language Runtime]]. All of them can serve as an [[abstraction layer]] for any computer language.{{Citation needed|date=November 2015}}

A special case of process VMs are systems that abstract over the communication mechanisms of a (potentially heterogeneous) [[computer cluster]]. Such a VM does not consist of a single process, but one process per physical machine in the cluster. They are designed to ease the task of programming concurrent applications by letting the programmer focus on algorithms rather than the communication mechanisms provided by the interconnect and the OS. They do not hide the fact that communication takes place, and as such do not attempt to present the cluster as a single machine.{{Citation needed|date=March 2013}}

Unlike other process VMs, these systems do not provide a specific programming language, but are embedded in an existing language; typically such a system provides bindings for several languages (e.g., [[C (programming language)|C]] and [[Fortran]]).{{Citation needed|date=March 2013}} Examples are [[Parallel Virtual Machine]] (PVM) and [[Message Passing Interface]] (MPI).