This can save a lot of hardware costs: e.g. Since the full performance of today's computers is rarely needed full-time, instead of running many such physical computers, one can "pack" many virtual machines onto a few powerful hosts and balance the loads between them. With a virtualizer such as VirtualBox, one can run software written for one operating system on another (say, Windows software on Linux) without having to reboot. There are several scenarios that make virtualization attractive: VirtualBox is also different from so-called "paravirtualization" solutions such as Xen, which require that the guest operating system be modified. Virtualizers such as VirtualBox, on the other hand, can achieve near-native performance for the guest code, but can only run guest code that was written for the same target hardware (such as 32-bit Linux on a 32-bit Windows host). While emulators theoretically allow running code written for one type of hardware on completely different hardware (say, running 64-bit code on 32-bit hardware), they are typically quite slow. Instead, every single machine instruction is translated ("emulated"). With that approach, as performed by programs such as BOCHS, guest code is not allowed to run directly on the host. This approach, often called "native virtualization", is different from mere emulation. Most of the guest code runs unmodified, directly on the host computer, and the guest operating system "thinks" it's running on real machine. The physical computer is then usually called the "host", while the virtual machine is often called a "guest". This environment, called a "virtual machine", is created by the virtualization software by intercepting access to certain hardware components and certain features. When we describe VirtualBox as a "virtualization" product, we refer to "full virtualization", that is, the particular kind of virtualization that allows an unmodified operating system with all of its installed software to run in a special environment, on top of your existing operating system.
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