In general terms, a virtual machine in computer science software that creates an environment between the computer platform and the end user in which the end user can operate software.
Specifically, the term virtual machine has several distinct meanings:
The original meaning of virtual machine is the creation of a number of different identical execution environments on a single computer, each of which exactly emulates the host computer. This provides each user with the illusion of having an entire computer, but one that is their "private" machine, isolated from other users, all on a single physical machine. The host software which provides this capability is often referred to as a virtual machine monitor or hypervisor.
Application virtual machine
The second, and now more common, meaning of virtual machine is a piece of computer software that isolates the application being used by the user from the computer. Because versions of the virtual machine are written for various computer platforms, any application written for the virtual machine can be operated on any of the platforms, instead of having to produce separate versions of the application for each computer and operating system. The application is run on the computer using an interpreter or Just In Time compilation.
Operating system virtual machine
The term virtual machine is now also used to refer to the environment created by an emulator, where software is used to emulate an operating system for the end user, while the computer runs its own native operating system.
Parallel virtual machine
More recently, the term virtual machine is also used to refer to a Parallel Virtual Machine (PVM). In this case, the virtual machine software allows a single environment to be created spanning multiple computers, so that the end user appears to be using only one computer rather than several.
Emulation of the underlying raw hardware
Since each user can run whatever operating system they want, this type of virtual machine allows users to do things like run two different operating systems (sometimes referred to as "guests") on their "private" virtual computers. Also, experimental new versions of operating systems can be run at the same time as older, more stable, versions, each in a separate virtual machine. The process can even be recursive; IBM debugged new versions of its virtual machine operating system, VM, in a virtual machine running under an older version of VM.
One early user of this concept was the IBM VM/CMS time-sharing product, which used a relatively simple interactive computing single-user operating system, CMS, which ran on top of VM. In that way, CMS could be written simply, as if it were running alone, and the VM operating system quietly provided multitasking and resource management services behind the scenes.
Not all VM users had to run CMS, though; some preferred to run some form of OS/360 (or eventually MVS) in one or more virtual machines, to provide traditional batch processing services to those users who wanted that. VM is still used today on IBM mainframes, and in some which are used as Web servers, the operating system run in each of many virtual machines is Linux.
The plex86 and VMware packages do the same thing on modern PCs, trapping all hardware accesses and simulating all of a motherboard except for the processor.
The x86 processor architecture as used in modern PCs does not actually meet the Popek and Goldberg virtualization requirements. Notably, there is no execution mode where all privileged machine instructions always trap, which would allow per-instruction virtualization. As a result, VMware and similar virtualization software for the x86 must dynamically recompile privileged mode code. This technique incurs some performance overhead as compared to a VM running on a natively virtualizable architecture such as the IBM System/370 or Motorola MC68020. Intel and AMD have each announced plans to add x86 virtualization capabilities to future x86 processors (see Vanderpool).
Emulation of a non-native system
Some of this class of virtual machines are emulators; these allow software written for one machine to run on another. Emulation for computer systems can include emulation for both different machine architectures, and operating systems.
Others produce behaviors and capabilities of a machine that doesn't necessarily exist as an actual piece of hardware but may only be a detailed specification. For example, the p-Code machine specification (one of the first, used for support of Pascal) was a description of a specific set of capabilities and behaviors that programmers could use to write programs that would run on any computer running virtual machine software that correctly implemented the specification.
More modern examples include the specification of the Java virtual machine and the Common Language Infrastructure virtual machine is at the heart of the Microsoft .NET initiative.
These allow diverse computers all to run software written to that specification; the virtual machine software itself must be written separately for each type of computer on which it runs.
- Citations from CiteSeer
- TRANGO real-time embedded hypervisor
- The Reincarnation of Virtual Machines, Article on ACM Queue by Mendel Rosenblum, Co-Founder, VMware