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Long Term Evolution (LTE) and LTE-Advanced Pro 305

4.18.2 Running an Operating System in a Virtual Machine
There are several things that make such a virtual environment very useful on a PC.
First, a complete operating system can be run in the virtual environment without any
modification of the normal operating system that runs directly on the real hardware.
An example would be a user with a Windows PC who occasionally would like to test
new software but does not want to do this on their ‘real’ operating system because
they are not sure whether it is safe or whether they would like to keep it installed after
trying it out. In such a scenario a virtual machine is used in which another Windows
operating system, referred to as the ‘guest operating system’, is executed. Here, soft-
ware installation and configuration changes can be tested without any changes to the
‘real’ operating system, referred to as the ‘host operating system’. Another example
would be if a user ran a Linux distribution such as Ubuntu as their main operating
system, i.e. as the host operating system, but every now and then needed to use a
Windows program that is not available on that platform. There are ways to
run Windows programs on Linux but running them in a virtual machine in which
the Windows operating system is installed is the better approach in many cases. In
both scenarios the guest operating system runs in a window on the host operating
system. Figure 4.31 shows how Windows runs as a guest in a window on a Linux host.
The guest operating system is not aware that its screen is being shown in a window, as
from its point of view it puts its graphical user interface via the (simulated) graphics
card to a real display. The guest operating system does not see a difference between
real and virtual hardware. Also, when the guest operating system writes something to
its (simulated) hard disk, the hypervisor translates that request and writes the content
into a large file on the physical hard drive. Again, the guest operating system is not
aware of these actions.

Figure 4.31 An Ubuntu Linux host running Windows as a guest operating system in a virtual machine.