Information Technology Reference
In-Depth Information
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Figure1.4: An operating system virtual machine
of physical memory, network bandwidth, and disk. Further, since the operating
system must decide how to split the fixed set of resources among the various
applications running at each moment, a particular application will have differ-
ent amounts of resources from time to time, even when running on the same
hardware. While a few applications might be designed to take advantage of a
computer's specic hardware conguration and their specic resource assign-
ment, most programmers want to use a higher level of abstraction.
We have just discussed one example of this: a uniprocessor can run only one
program at a time, yet most operating systems allow multiple applications to
appear to the user to be running at the same time. The operating system does so
through a concept called virtualization. Virtualization provides an application
Definition: virtualization
with the illusion of resources that are not physically present. For example, the
operating system can present to each application the abstraction that it has an
entire processor dedicated to it, even though at a physical level there may be only
a single processor shared among all the applications running on the computer.
With the right hardware and operating system support, most physical resources
can be virtualized: examples include the processor, memory, screen space, disk,
and the network. Even the type of processor can be virtualized, to allow the
same, unmodified application to be run on a smartphone, tablet, and laptop
computer.
Pushing this a step further, some operating systems virtualize the entire
computer, to run the operating system as an application running on top of
another operating system (see Figure 1.4). This is called creating a virtual
machine. The operating system running in the virtual machine, called the guest
Definition: virtual
machine
operating system, thinks it is running on a real, physical machine, but this is
Definition: guest
operating system
