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Virtual Machine Technology

vVirtual Machine Features

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The key component in building this abstraction layer is commonly referred to as a VMM (virtual machine monitor) or, sometimes, a hypervisor. This software is responsible for sharing the computer's physical resources among the many VMs that could be running. The VMM is not an easy piece of software to get right because it must trick the guest OS into thinking it has control of the real hardware. To accomplish this, the VMM runs at processor privilege level Ring 0. The guest OS runs up a level, at Ring 1. Most modern OSs run user applications at Ring 3, where applications are prevented from trampling on or otherwise adversely affecting one another. Running the OS in Ring 1 lets the VMM trap some of the operations the guest OS is attempting (like accessing memory) and take corrective measures.


Another component to creating a VM is abstracting the hardware layer. The VM software must create virtual hardware devices, such as the IDE chip set and network and SCSI cards, to be consumed by the guest OS. Each vendor has specific devices its products will emulate. The software then translates these emulated devices to a device that is present on the physical hardware. By creating these virtual hardware devices, a guest OS can be copied to another computer running different hardware and still work. The VMM is responsible for redirecting virtual devices to physical devices. For example, the Microsoft products emulate an Intel 21141 network card; the VMware products emulate an AMD PCnet card.

Putting VMs to Work

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