A virtual machine is a piece of software that uses underlying hardware to emulate the behavior and functionality of another hardware. The same hardware can be used to emulate multiple hardware at the same time.
Use Cases and Examples
For example, the Oracle VirtualBox virtualization software makes it possible to run the Linux operating system on Windows at the same time using the same hardware.
Virtualization software is also used by web hosting companies to provision and allocate a single server's resources to multiple customers. This makes it a lot more profitable as the hosting company does not need to host an actual server per customer.
Virtual machine as a concept is also adopted by programming languages. For example, the Java virtual machine makes it possible to run Java code on a large array of operating systems and computer hardware.
This is made possible because the virtual machine itself behaves like a complete computer system and can run Java byte code. As long as the virtual machine has been implemented for a computer system, any java code will automatically run on that machine.
Many other operating systems require the code itself to be compiled/interpreted to machine code the computer system can execute and this is much harder to do and you can't always ensure the code will run the same way across different systems.
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