The original direction of virtual machine is the creation of several environments of execution on only one computer, of which each one emulates the host-computer. That provides to each user the illusion to have a complete Ordinateur whereas each virtual machine is isolated from the different one. The software host which provides this functionality is often called supervisory or hypervisor . This concept goes than that further from simple the Temps divided S where each user has only one space of personal development, and not of a whole simulated machine.
VM/370 made it possible as of the the Seventies to have several simultaneous operating systems on same the Mainframe. To avoid useless duplications of code, one could define segments of memory divided in an invisible way (in mode of reading alone, of course) between two of these systems or more.
Windows NT and its successors incorporates a virtual machine to simulate an environment MS-DOS
Advantage of a virtual machineDuring the preparation of a migration, one can use simultaneously and without danger to the exploitation even in the event of crash landing system:
- the old machine and the news which is simulated above, or the reverse.
- the old system and the new one which is simulated above, or the reverse.
Safety : The virtual machines are completely isolated the ones from the others; moreover, in 2006, the majority of the viruses tested immediately if they turn in virtualized environment and renonçaient to act when it was the case. Nothing proves nevertheless this protection cannot be circumvented in one way or another, and an abundant literature is published permanently on this subject.
Facilitated extension : The number of virtual machines is managed quasi independently of the number of real machines, and a transparent way for the users. The statistics of load of the virtual machines make it possible to reorganize them on the real machines, like envisaging the dates on which to envisage extensions.
Virtual machine emulating of the operating systemsThe term of virtual machine is also for some time used in a very different direction to indicate an environment created by a emulator. This one is a software which emulates a Operating system for the end user. This software is a surcouche which is grafted on the native operating system.
- Virtual PC Gratuit
- VMware allows the virtualisation not only one PC Intel/AMD architecture (on which one can then use simultaneously of Windows and Linux, possibly of several different generations), but of all the peripherals of a network: a distant peripheral can appear if one wishes it like room!!
- VirtualBox Alternate with VMware and other competitors which passed under License LPG.
Parallel virtual machineMore recently, the term of virtual machine was used to indicate a parallel virtual Machine (PVM). In this case, a virtual machine creates an environment which seems to be only one computer whereas the resources of several computers are used.
- PVM is a virtual machine making it possible to carry out a program on several computers.
Applicative virtual machineIn its second direction, maintaining most common, a virtual machine indicates a software or interpreter which isolates the application used by the user from specificities of the computer, i.e. of those of his architecture or its operating system. This indirection makes it possible to the originator of an application to make it available on a great number of computers without the usual constraints to the drafting of a portable software turning directly on the computer. Technology JIT makes it possible in many cases the application to have performances comparable with a native application.
Windows XP and similar also turns in a virtualized environment, which is created by the layer HALL ( Hardware abstraction to bush-hammer ). In the event of physical change of machine, one can not change in theory that HALL without touching with the remainder of his Windows installation.
- the virtual Machine Java makes it possible to carry out code Java
- CLR, in the platform .NET
- Low Level Virtual Machine
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