Virtual Machine

DEFINITIONS

Specifically, the term virtual machine has several distinct meanings:

Original meaning

The original meaning of virtual machine is that 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. This is nowadays much better referred to by using the terms Virtualization and Virtual Server s. 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 Platform s, 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 . One of the best known examples of an application virtual machine is Sun Microsystem 's Java Virtual Machine .

Hardware 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.

This can be done using three major ways:

full virtualization — the virtual machine simulates the complete hardware, allowing an unmodified OS for a completely different CPU to be run.

Paravirtualization — the virtual machine does not simulate hardware but instead offers a special API that requires OS modifications.

native virtualization — the virtual machine only partially simulates enough hardware to allow an unmodified OS to be run in isolation, but the guest OS must be designed for the same type of CPU.

Machine aggregation

A less common use of the term is to refer to a Computer Cluster consisting of many computers that have been aggregated together as a larger and more powerful "virtual" machine. In this case, the 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.

PVM ( Parallel Virtual Machine ) and MPI ( Message Passing Interface ) are two common software packages that permits a heterogeneous collection of Unix and/or Windows computers hooked together by a network and 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.

TECHNIQUES

Emulation of the underlying raw hardware (native virtualization)

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 Mainframe s, and in some which are used as Web Server s, the operating system running in each of many virtual machines is Linux .

The VMware , Parallels Workstation , 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 ).

Emulation of a non-native system

Virtual machines can also perform the role of an Emulator , allowing software applications and Operating Systems written for another Computer Processor architecture to be run.

Some virtual machines emulate hardware that only exists as a detailed specification. For example:

One of the first was the P-Code Machine specification, which allowed programmers to write Pascal Programs that would run on any computer running virtual machine software that correctly implemented the specification.

The specification of the Java Virtual Machine .

The Common Language Infrastructure virtual machine at the heart of the Microsoft .NET initiative.

This technique allows diverse computers to run any software written to that specification; only the virtual machine software itself must be written separately for each type of computer on which it runs.

LIST OF HARDWARE WITH VIRTUAL MACHINE SUPPORT