1 Introduction
There are two ways in which VirtualBox can achieve virtualization: either entirely
in software or,with newer processors, using certain hardware features.
•Forsome years, Intel and AMD processors have had support for so-called “hard-
ware virtualization”. This means that these processors aid virtualization soft-
ware such as VirtualBox in intercepting potentially dangerous operations that a
guest operating system may be attempting and in presenting virtual hardware to
a virtual machine.
These hardware features differ between Intel and AMD processors. Intel named
its technology VT-x;AMD calls theirs AMD-V.
Note: On most systems, the hardware virtualization features first need to be
enabled in the BIOS before VirtualBoxcan use them.
•As opposed to other virtualization software, for many usage scenarios,
VirtualBox does not require hardware virtualization features to be present.
Through sophisticated techniques, VirtualBox virtualizes many guest operating
systemsentirely in software. Thismeans that you can run virtual machines even
on older processors which do not support hardware virtualization.
Youcan select for each virtual machine individually whether VirtualBox should use
software or hardware virtualization. Prior to version 2.2, software virtualization was
the default; starting with version 2.2, VirtualBox will enable hardware virtualization
by default for new virtual machines that you create. (Existing virtual machines are
not automatically changed for compatibility reasons, and the default can of course be
changed for each virtual machine.)
Even though VirtualBox does not always require hardware virtualization, enabling
it is required in the following three scenarios:
•Certain rare guest operating systems like OS/2 make use of very esoteric pro-
cessor instructions that are not supported with our software virtualization. For
virtual machines that are configured to contain such an operating system, hard-
ware virtualization is enabled automatically.
•VirtualBox’s 64-bit guest support (added with version 2.0) and multiprocessing
(SMP, added with version 3.0) both require hardware virtualization to be en-
abled. (This is not much of a limitation since the vast majority of today’s 64-bit
andmulticore CPUs ship with hardware virtualization anyway; the exceptions to
this rule are e.g. older Intel Celeron and AMD Opteron CPUs.)
The reason for changing the default with version 2.2 is that the hardware has sig-
nificantly improved with the latest Intel and AMD processors, and VirtualBox has also
fine-tuned its hardware virtualization support to a degree that it is now faster than
software virtualization in many situations.
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