Virtual Machine (VM) is a system that enables multiple operating systems concurrently run on a single physical server, providing for much more effective utilization of the underlying hardware. In a virtual machine, the central processor chips isolate an area of memory from the rest of the system and the operating systems and applications run in a “protected mode” environment. If a program freezes in one Virtual Machine it will not affect the operation of the programs and operating systems running outside of that Virtual Machine.

There are four major schools of thought in the virtual machine architecture. The first is a nearly one-to-one mapping, represented by the IBM virtual machine model. The second lineage consists of a mapping of every instruction in the machine with a virtual instruction, represented by the Java virtual machine. The Unix virtual machine model and the OSI virtual machine model map some of the instructions directly, while some others are calls to the operating system functions directly.

In the case of the real computer systems, the operating system components known as device drivers control hardware resources, translating operating system instructions into a specific device control language. Drivers are developed with an assumption of exclusive device ownership, which typically precludes the possibility of running more than one operating system concurrently on a computer. Virtual machine is one of technologies overcome this limitation. Virtualization involves redirecting interactions with device resources at lower levels in such a way that higher-level application layers are unaffected. With Virtual machine, customers can run multiple operating systems concurrently on a single physical system.

Microsoft Virtual Server 2005 is based on the OSI virtual machine architecture with the following key points:

• The host operating system, such as Windows Server 2003, manages the host system.
• Virtual machine operating system, such as Virtual Server 2005, provides a VMM virtualization layer that manages virtual machines, providing the software infrastructure for hardware emulation.
• Each virtual machine consists of a set of virtualized devices, the virtual hardware for each virtual machine.
• A guest operating system and applications run in the virtual machine without the knowledge that the network adapter they interact with through Virtual Server is only a software simulation of a physical Ethernet device.

Virtual Machine

Related Terms: Virtual Server, Microsoft Virtual Server 2005, Java virtual machine, IBM virtual machine, UNIX virtual machine, OSI virtual machine