Virtual machine: Encyclopedia - Virtual machine

Virtual machine

In general terms, a virtual machine in computer science is 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:

Virtual machine - Definitions

Virtual machine - Original meaning

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.

Virtual machine - 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

Virtual machine - 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. This is done to allow applications written for one OS to be executed on a machine which runs a different OS; or to provide execution "sandboxes" which provide a greater level of isolation between processes than is achieved when running multiple processes on the same instance of an OS.

Virtual machine - Parallel virtual machine

PVM (Parallel Virtual Machine) is a software package that permits a heterogeneous collection of Unix and/or Windows computers hooked together by a network to be used as a single large parallel computer. Thus large computational problems can be solved more cost effectively by using the aggregate power and memory of many computers. The software is very portable.

PVM enables users to exploit their existing computer hardware to solve much larger problems at minimal additional cost. Hundreds of sites around the world are using PVM to solve important scientific, industrial, and medical problems in addition to PVM's use as an educational tool to teach parallel programming. With tens of thousands of users, PVM has become the de facto standard for distributed computing world-wide.

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.

Virtual machine - Techniques

Virtual machine - 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 VMware, twoOStwo and SVISTA packages do the same thing on modern PCs, trapping all hardware accesses and simulating all of a motherboard except for the processor.

On the other hand, plex86 can run only Linux under Linux using a specific patched kernel. It does not emulate a processor, but uses bochs for emulation of motherboard devices.

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 sensitive 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).

Virtual machine - 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 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.

Virtual machine - List of virtual machine software

• Application virtualization
  • Common Language Runtime - C#, Visual Basic .NET, J#, Managed C++
  • Forth virtual machine - Forth
  • Glulx - Glulx, Z-code
  • Inferno - Limbo
  • Java virtual machine - Java
  • Low Level Virtual Machine (LLVM) - currently C, C++, Stacker
  • Lua
  • Macromedia Flash Player - SWF
  • MMIX - MMIXAL
  • Neko virtual machine - currently Neko and haXe
  • O-code machine - BCPL
  • P-Code_machine - Pascal
  • Parrot - Perl 6
  • Perl virtual machine - Perl
  • Portable.NET - C#, Visual Basic .NET, J#, Managed C++
  • ScummVM - Scumm
  • SECD machine - ISWIM, Lispkit_Lisp
  • Smalltalk virtual machine - Smalltalk
  • Sqlite virtual machine - Sqlite opcodes
  • Squeak virtual machine - Squeak
  • TrueType virtual machine - TrueType
  • vx32 virtual machine - application-level virtualization for native x86 code
  • Warren Abstract Machine - Prolog
  • Z-machine - Z-Code
• Operating system virtualization
  • Bochs, portable open source x86 and AMD64 PCs emulator
  • FAUmachine
  • Microsoft Virtual PC
  • Microsoft Virtual Server
  • OPEN COLINUX - Linux inside Windows
  • QEMU
  • SVISTA
  • TRANGO Real-time Embedded Hypervisor
  • twoOStwo
  • User-mode Linux
  • Virtuozzo
  • VMWare
  • Xen

See also

• Gerrit Blaauw (A significant designer of the IBM/360, an early virtual computing architecture)
• Threaded code (A common implementation technique for application virtual machines)
• Computing
• LLVM (The University of Illinois's Low Level Virtual Machine, a compiler toolset)

Adapted from the Wikipedia article "Virtual machine", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki