A number of different approaches to virtualization have been developed over the years, each with inherent advantages and disadvantages. VMware Server 2.0 uses a concept known as ''software virtualization''. This, and the other virtualization methodologies, will be covered in detail in the remaining sections of this chapter.
== Guest Operating System Software Virtualization ==
Guest OS Software virtualization is perhaps the easiest concept to understand. In this scenario the physical host computer system runs a standard unmodified operating system such as Windows, Linux, Unix or MacOS X. Running on this operating system is a virtualization application which executes in much the same way as any other application such as a word processor or spreadsheet would run on the system. It is within this virtualization application that one or more virtual machines are created to run the guest operating systems on the host computer. The virtualization application is responsible for starting, stopping and managing each virtual machine and essentially controlling access to physical hardware resources on behalf of the individual virtual machines. The virtualization application also engages in a process known as ''binary rewriting'' which involves scanning the instruction stream of the executing guest system and replacing any privileged instructions with safe emulations. This has the effect of making the guest system think it is running directly on the system hardware, rather than in a virtual machine within an application.
Some examples of guest OS software virtualization technologies include VMware Server , VirtualPC and VirtualBox.
The following figure provides an illustration of guest OS software based virtualization:
