Platform Virtualization

There are several approaches to platform virtualization:

• Emulation of Simulation
• Native Virtualization and Full Virtualization
• Hardware Enabled Virtualization
• Partial Virtualization
• Paravirtualization
• Cross Platform Virtualization

Emulation or Simulation

The virtual machine simulates the complete hardware, allowing an unmodified "guest" OS for a completely different CPU to be run.

This approach enables the creation of software for new processors before they were physically available.

Examples include Bochs, PearPC, PowerPC version of Virtual PC, QEMU without acceleration, and the Hercules emulator.

Emulation is implemented using a variety of techniques, from state machines to the use of dynamic recompilation on a full virtualization platform.

Native Virtualization and Full Virtualization

The virtual machine simulates enough hardware to allow an unmodified "guest" OS (one designed for the same CPU) to be run in isolation. This approach was pioneered in 1966 with IBM CP-40 and CP-67, predecessors of VM family.

Examples outside mainframe include: Parallels Workstation, Parallels Desktop for Mac, VirtualBox, Virtual Iron, Virtual PC, Virtual Server, VMware Workstation, VMware Server (formerly GSX Server), QEMU, Adeos, Mac-on-Linux, Win4BSD, Win4Lin Pro, and Egenera vBlade technology.

Hardware Enabled Virtualization

the hardware provides architectural support that facilitates building a virtual machine monitor and allows guest OSes to be run in isolation [3]. In 2005 and 2006, Intel and AMD provided additional hardware to support virtualization. Examples include Linux KVM, VMware Workstation, VMware Fusion, Microsoft Virtual PC, Xen, Parallels Desktop for Mac, and Parallels Workstation.

Partial Virtualization

The virtual machine simulates multiple instances of much of an underlying hardware environment, particularly address spaces.

Such an environment supports resource sharing and process isolation, but does not allow separate "guest" operating system instances.

Although not generally viewed as a virtual machine it was used in such systems as CTSS, the experimental IBM M44/44X, and systems like MVS.

More recent systems, such as Microsoft Windows and Linux use this basic approach.

Paravirtualization

The virtual machine does not necessarily simulate hardware, but instead of, or in addition to, offers a special API that can only be used by modifying the "guest" OS.

This system call to the hypervisor is called a "hypercall" in TRANGO and Xen; it is implemented via a DIAG ("diagnose") hardware instruction in IBM's CMS under VM [the origin of the term hypervisor].

Examples include: VMware ESX Server, IBM's LPARs[4],Win4Lin 9x, Sun's Logical Domains, z/VM,[citation needed] and TRANGO.

Cross-platform Virtualization

Running an application compiled for a specific CPU and operating system on a platform with a different CPU and operating system, without modification to source code or binaries.

Examples include: Apple Rosetta and Transitive QuickTransit.

Also See

Operating System [OS] Virtualization

Application Virtualization

NEXT: Server Virtualization

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