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=== Virtual Machines machines (VMs) ==='''Virtual Machines machines''' emulate some real or fictional hardware, which in turn requires real resources from the Host ''host'' (the machine running the VMs). This approach, used by most System Emulators''system emulators'', allows the emulator to run an arbitrary Guest Operating System ''guest operating system'' without modifications because guest OS isn't is not aware that it’s it is not running on real hardware. The main issue with this approach is that some CPU instructions require additional privileges and may not be executed in user space thus requiring a Virtual Machines Monitor ''virtual machines monitor'' (VMM) to analyze executed code and make it safe on-the-fly. Hardware Emulation emulation approach is used by [http://www.vmware.com/ VMware] products, [http://parallels.com/ Parallels] and [http://www.microsoft.com/windowsserversystem/virtualserver/default.mspx Microsoft Virtual Server].