<s>
In	O
computing	O
,	O
a	O
"	O
virtual	B-Architecture
machine	I-Architecture
"	O
(	O
VM	B-Application
)	O
is	O
the	O
virtualization	B-General_Concept
or	O
emulation	B-Application
of	O
a	O
computer	O
system	O
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
are	O
based	O
on	O
computer	B-General_Concept
architectures	I-General_Concept
and	O
provide	O
the	O
functionality	O
of	O
a	O
physical	O
computer	O
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
differ	O
and	O
are	O
organized	O
by	O
their	O
function	O
,	O
shown	O
here	O
:	O
</s>
<s>
System	B-Operating_System
virtual	I-Operating_System
machines	I-Operating_System
(	O
also	O
called	O
full	B-Architecture
virtualization	I-Architecture
VMs	B-Application
)	O
provide	O
a	O
substitute	O
for	O
a	O
real	O
machine	O
.	O
</s>
<s>
They	O
provide	O
the	O
functionality	O
needed	O
to	O
execute	O
entire	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
A	O
hypervisor	B-Operating_System
uses	O
native	B-Language
execution	I-Language
to	O
share	O
and	O
manage	O
hardware	O
,	O
allowing	O
for	O
multiple	O
environments	O
that	O
are	O
isolated	O
from	O
one	O
another	O
yet	O
exist	O
on	O
the	O
same	O
physical	O
machine	O
.	O
</s>
<s>
Modern	O
hypervisors	B-Operating_System
use	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
,	O
with	O
virtualization-specific	O
hardware	O
features	O
on	O
the	O
host	O
CPUs	B-Device
providing	O
assistance	O
to	O
hypervisors	B-Operating_System
.	O
</s>
<s>
Process	O
virtual	B-Architecture
machines	I-Architecture
are	O
designed	O
to	O
execute	O
computer	O
programs	O
in	O
a	O
platform-independent	O
environment	O
.	O
</s>
<s>
Some	O
virtual	B-Architecture
machine	I-Architecture
emulators	B-Application
,	O
such	O
as	O
QEMU	B-Application
and	O
video	O
game	O
console	O
emulators	B-Application
,	O
are	O
designed	O
to	O
also	O
emulate	O
(	O
or	O
"	O
virtually	O
imitate	O
"	O
)	O
different	O
system	O
architectures	O
,	O
thus	O
allowing	O
execution	O
of	O
software	O
applications	O
and	O
operating	B-General_Concept
systems	I-General_Concept
written	O
for	O
another	O
CPU	B-Device
or	O
architecture	O
.	O
</s>
<s>
Operating-system-level	B-Application
virtualization	I-Application
allows	O
the	O
resources	O
of	O
a	O
computer	O
to	O
be	O
partitioned	O
via	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
A	O
"	O
virtual	B-Architecture
machine	I-Architecture
"	O
was	O
originally	O
defined	O
by	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
as	O
"	O
an	O
efficient	O
,	O
isolated	O
duplicate	O
of	O
a	O
real	O
computer	O
machine.	O
"	O
</s>
<s>
Current	O
use	O
includes	O
virtual	B-Architecture
machines	I-Architecture
that	O
have	O
no	O
direct	O
correspondence	O
to	O
any	O
real	O
hardware	O
.	O
</s>
<s>
The	O
physical	O
,	O
"	O
real-world	O
"	O
hardware	O
running	O
the	O
VM	B-Application
is	O
generally	O
referred	O
to	O
as	O
the	O
'	O
host	O
 '	O
,	O
and	O
the	O
virtual	B-Architecture
machine	I-Architecture
emulated	B-Application
on	O
that	O
machine	O
is	O
generally	O
referred	O
to	O
as	O
the	O
'	O
guest	O
 '	O
.	O
</s>
<s>
A	O
host	O
can	O
emulate	O
several	O
guests	O
,	O
each	O
of	O
which	O
can	O
emulate	O
different	O
operating	B-General_Concept
systems	I-General_Concept
and	O
hardware	B-Device
platforms	I-Device
.	O
</s>
<s>
The	O
desire	O
to	O
run	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
was	O
the	O
initial	O
motive	O
for	O
virtual	B-Architecture
machines	I-Architecture
,	O
so	O
as	O
to	O
allow	O
time-sharing	B-General_Concept
among	O
several	O
single-tasking	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
In	O
some	O
respects	O
,	O
a	O
system	B-Operating_System
virtual	I-Operating_System
machine	I-Operating_System
can	O
be	O
considered	O
a	O
generalization	O
of	O
the	O
concept	O
of	O
virtual	B-Architecture
memory	I-Architecture
that	O
historically	O
preceded	O
it	O
.	O
</s>
<s>
IBM	O
's	O
CP/CMS	B-Application
,	O
the	O
first	O
systems	O
to	O
allow	O
full	B-Architecture
virtualization	I-Architecture
,	O
implemented	O
time	B-General_Concept
sharing	I-General_Concept
by	O
providing	O
each	O
user	O
with	O
a	O
single-user	O
operating	B-General_Concept
system	I-General_Concept
,	O
the	O
Conversational	B-Operating_System
Monitor	I-Operating_System
System	I-Operating_System
(	O
CMS	O
)	O
.	O
</s>
<s>
Unlike	O
virtual	B-Architecture
memory	I-Architecture
,	O
a	O
system	B-Operating_System
virtual	I-Operating_System
machine	I-Operating_System
entitled	O
the	O
user	O
to	O
write	O
privileged	O
instructions	O
in	O
their	O
code	O
.	O
</s>
<s>
As	O
technology	O
evolves	O
virtual	B-Architecture
memory	I-Architecture
for	O
purposes	O
of	O
virtualization	B-General_Concept
,	O
new	O
systems	O
of	O
memory	B-General_Concept
overcommitment	I-General_Concept
may	O
be	O
applied	O
to	O
manage	O
memory	O
sharing	O
among	O
multiple	O
virtual	B-Architecture
machines	I-Architecture
on	O
one	O
computer	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
.	O
</s>
<s>
It	O
may	O
be	O
possible	O
to	O
share	O
memory	O
pages	O
that	O
have	O
identical	O
contents	O
among	O
multiple	O
virtual	B-Architecture
machines	I-Architecture
that	O
run	O
on	O
the	O
same	O
physical	O
machine	O
,	O
what	O
may	O
result	O
in	O
mapping	O
them	O
to	O
the	O
same	O
physical	O
page	O
by	O
a	O
technique	O
termed	O
kernel	B-Application
same-page	I-Application
merging	I-Application
(	O
KSM	O
)	O
.	O
</s>
<s>
This	O
is	O
especially	O
useful	O
for	O
read-only	O
pages	O
,	O
such	O
as	O
those	O
holding	O
code	O
segments	O
,	O
which	O
is	O
the	O
case	O
for	O
multiple	O
virtual	B-Architecture
machines	I-Architecture
running	O
the	O
same	O
or	O
similar	O
software	O
,	O
software	O
libraries	O
,	O
web	O
servers	O
,	O
middleware	O
components	O
,	O
etc	O
.	O
</s>
<s>
The	O
guest	O
operating	B-General_Concept
systems	I-General_Concept
do	O
not	O
need	O
to	O
be	O
compliant	O
with	O
the	O
host	O
hardware	O
,	O
thus	O
making	O
it	O
possible	O
to	O
run	O
different	O
operating	B-General_Concept
systems	I-General_Concept
on	O
the	O
same	O
computer	O
(	O
e.g.	O
,	O
Windows	B-Application
,	O
Linux	B-Application
,	O
or	O
prior	O
versions	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
)	O
to	O
support	O
future	O
software	O
.	O
</s>
<s>
The	O
use	O
of	O
virtual	B-Architecture
machines	I-Architecture
to	O
support	O
separate	O
guest	O
operating	B-General_Concept
systems	I-General_Concept
is	O
popular	O
in	O
regard	O
to	O
embedded	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
A	O
typical	O
use	O
would	O
be	O
to	O
run	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
simultaneously	O
with	O
a	O
preferred	O
complex	O
operating	B-General_Concept
system	I-General_Concept
,	O
such	O
as	O
Linux	B-Application
or	O
Windows	B-Application
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
have	O
other	O
advantages	O
for	O
operating	B-General_Concept
system	I-General_Concept
development	O
and	O
may	O
include	O
improved	O
debugging	O
access	O
and	O
faster	O
reboots	O
.	O
</s>
<s>
Multiple	O
VMs	B-Application
running	O
their	O
own	O
guest	O
operating	B-General_Concept
system	I-General_Concept
are	O
frequently	O
engaged	O
for	O
server	O
consolidation	O
.	O
</s>
<s>
A	O
process	O
VM	B-Application
,	O
sometimes	O
called	O
an	O
application	O
virtual	B-Architecture
machine	I-Architecture
,	O
or	O
Managed	O
Runtime	O
Environment	O
(	O
MRE	O
)	O
,	O
runs	O
as	O
a	O
normal	O
application	O
inside	O
a	O
host	O
OS	O
and	O
supports	O
a	O
single	O
process	O
.	O
</s>
<s>
Its	O
purpose	O
is	O
to	O
provide	O
a	O
platform-independent	O
programming	O
environment	O
that	O
abstracts	O
away	O
details	O
of	O
the	O
underlying	O
hardware	O
or	O
operating	B-General_Concept
system	I-General_Concept
and	O
allows	O
a	O
program	O
to	O
execute	O
in	O
the	O
same	O
way	O
on	O
any	O
platform	B-Device
.	O
</s>
<s>
A	O
process	O
VM	B-Application
provides	O
a	O
high-level	O
abstraction	O
that	O
of	O
a	O
high-level	B-Language
programming	I-Language
language	I-Language
(	O
compared	O
to	O
the	O
low-level	O
ISA	O
abstraction	O
of	O
the	O
system	O
VM	B-Application
)	O
.	O
</s>
<s>
Process	O
VMs	B-Application
are	O
implemented	O
using	O
an	O
interpreter	B-Application
;	O
performance	O
comparable	O
to	O
compiled	B-Language
programming	O
languages	O
can	O
be	O
achieved	O
by	O
the	O
use	O
of	O
just-in-time	O
compilation	B-Language
.	O
</s>
<s>
This	O
type	O
of	O
VM	B-Application
has	O
become	O
popular	O
with	O
the	O
Java	B-Language
programming	I-Language
language	I-Language
,	O
which	O
is	O
implemented	O
using	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
.	O
</s>
<s>
Other	O
examples	O
include	O
the	O
Parrot	B-Language
virtual	I-Language
machine	I-Language
and	O
the	O
.NET	B-Application
Framework	I-Application
,	O
which	O
runs	O
on	O
a	O
VM	B-Application
called	O
the	O
Common	O
Language	O
Runtime	O
.	O
</s>
<s>
All	O
of	O
them	O
can	O
serve	O
as	O
an	O
abstraction	B-Architecture
layer	I-Architecture
for	O
any	O
computer	O
language	O
.	O
</s>
<s>
A	O
special	O
case	O
of	O
process	O
VMs	B-Application
are	O
systems	O
that	O
abstract	O
over	O
the	O
communication	O
mechanisms	O
of	O
a	O
(	O
potentially	O
heterogeneous	O
)	O
computer	B-Architecture
cluster	I-Architecture
.	O
</s>
<s>
Such	O
a	O
VM	B-Application
does	O
not	O
consist	O
of	O
a	O
single	O
process	O
,	O
but	O
one	O
process	O
per	O
physical	O
machine	O
in	O
the	O
cluster	O
.	O
</s>
<s>
Unlike	O
other	O
process	O
VMs	B-Application
,	O
these	O
systems	O
do	O
not	O
provide	O
a	O
specific	O
programming	O
language	O
,	O
but	O
are	O
embedded	O
in	O
an	O
existing	O
language	O
;	O
typically	O
such	O
a	O
system	O
provides	O
bindings	O
for	O
several	O
languages	O
(	O
e.g.	O
,	O
C	B-Language
and	O
Fortran	B-Application
)	O
.	O
</s>
<s>
Examples	O
are	O
Parallel	B-Operating_System
Virtual	I-Operating_System
Machine	I-Operating_System
(	O
PVM	B-Operating_System
)	O
and	O
Message	B-Application
Passing	I-Application
Interface	I-Application
(	O
MPI	O
)	O
.	O
</s>
<s>
They	O
are	O
not	O
strictly	O
virtual	B-Architecture
machines	I-Architecture
because	O
the	O
applications	O
running	O
on	O
top	O
still	O
have	O
access	O
to	O
all	O
OS	O
services	O
and	O
are	O
therefore	O
not	O
confined	O
to	O
the	O
system	O
model	O
.	O
</s>
<s>
Both	O
system	B-Operating_System
virtual	I-Operating_System
machines	I-Operating_System
and	O
process	O
virtual	B-Architecture
machines	I-Architecture
date	O
to	O
the	O
1960s	O
and	O
continue	O
to	O
be	O
areas	O
of	O
active	O
development	O
.	O
</s>
<s>
System	B-Operating_System
virtual	I-Operating_System
machines	I-Operating_System
grew	O
out	O
of	O
time-sharing	B-General_Concept
,	O
as	O
notably	O
implemented	O
in	O
the	O
Compatible	B-Operating_System
Time-Sharing	I-Operating_System
System	I-Operating_System
(	O
CTSS	O
)	O
.	O
</s>
<s>
Time-sharing	B-General_Concept
allowed	O
multiple	O
users	O
to	O
use	O
a	O
computer	O
concurrently	B-Architecture
:	O
each	O
program	O
appeared	O
to	O
have	O
full	O
access	O
to	O
the	O
machine	O
,	O
but	O
only	O
one	O
program	O
was	O
executed	O
at	O
the	O
time	O
,	O
with	O
the	O
system	O
switching	O
between	O
programs	O
in	O
time	O
slices	O
,	O
saving	O
and	O
restoring	O
state	O
each	O
time	O
.	O
</s>
<s>
This	O
evolved	O
into	O
virtual	B-Architecture
machines	I-Architecture
,	O
notably	O
via	O
IBM	O
's	O
research	O
systems	O
:	O
the	O
M44/44X	B-Device
,	O
which	O
used	O
partial	B-General_Concept
virtualization	I-General_Concept
,	O
and	O
the	O
CP-40	B-Application
and	O
SIMMON	B-Operating_System
,	O
which	O
used	O
full	B-Architecture
virtualization	I-Architecture
,	O
and	O
were	O
early	O
examples	O
of	O
hypervisors	B-Operating_System
.	O
</s>
<s>
The	O
first	O
widely	O
available	O
virtual	B-Architecture
machine	I-Architecture
architecture	O
was	O
the	O
CP-67/CMS	O
(	O
see	O
History	B-Application
of	I-Application
CP/CMS	I-Application
for	O
details	O
)	O
.	O
</s>
<s>
An	O
important	O
distinction	O
was	O
between	O
using	O
multiple	O
virtual	B-Architecture
machines	I-Architecture
on	O
one	O
host	O
system	O
for	O
time-sharing	B-General_Concept
,	O
as	O
in	O
M44/44X	B-Device
and	O
CP-40	B-Application
,	O
and	O
using	O
one	O
virtual	B-Architecture
machine	I-Architecture
on	O
a	O
host	O
system	O
for	O
prototyping	O
,	O
as	O
in	O
SIMMON	B-Operating_System
.	O
</s>
<s>
Emulators	B-Application
,	O
with	O
hardware	O
emulation	B-Application
of	O
earlier	O
systems	O
for	O
compatibility	O
,	O
date	O
back	O
to	O
the	O
IBM	B-Application
System/360	I-Application
in	O
1963	O
,	O
while	O
the	O
software	B-Application
emulation	I-Application
(	O
then-called	O
"	O
simulation	O
"	O
)	O
predates	O
it	O
.	O
</s>
<s>
Process	O
virtual	B-Architecture
machines	I-Architecture
arose	O
originally	O
as	O
abstract	O
platforms	B-Device
for	O
an	O
intermediate	O
language	O
used	O
as	O
the	O
intermediate	B-Application
representation	I-Application
of	O
a	O
program	O
by	O
a	O
compiler	B-Language
;	O
early	O
examples	O
date	O
to	O
around	O
1966	O
.	O
</s>
<s>
An	O
early	O
1966	O
example	O
was	O
the	O
O-code	B-Language
machine	I-Language
,	O
a	O
virtual	B-Architecture
machine	I-Architecture
that	O
executes	O
O-code	B-Language
(	O
object	O
code	O
)	O
emitted	O
by	O
the	O
front	O
end	O
of	O
the	O
BCPL	B-Language
compiler	B-Language
.	O
</s>
<s>
This	O
abstraction	O
allowed	O
the	O
compiler	B-Language
to	O
be	O
easily	O
ported	O
to	O
a	O
new	O
architecture	O
by	O
implementing	O
a	O
new	O
back	O
end	O
that	O
took	O
the	O
existing	O
O-code	B-Language
and	O
compiled	B-Language
it	O
to	O
machine	B-Language
code	I-Language
for	O
the	O
underlying	O
physical	O
machine	O
.	O
</s>
<s>
The	O
Euler	B-Language
language	O
used	O
a	O
similar	O
design	O
,	O
with	O
the	O
intermediate	O
language	O
named	O
P	O
(	O
portable	O
)	O
.	O
</s>
<s>
This	O
was	O
popularized	O
around	O
1970	O
by	O
Pascal	B-Application
,	O
notably	O
in	O
the	O
Pascal-P	O
system	O
(	O
1973	O
)	O
and	O
Pascal-S	O
compiler	B-Language
(	O
1975	O
)	O
,	O
in	O
which	O
it	O
was	O
termed	O
p-code	B-Language
and	O
the	O
resulting	O
machine	O
as	O
a	O
p-code	B-Language
machine	I-Language
.	O
</s>
<s>
This	O
has	O
been	O
influential	O
,	O
and	O
virtual	B-Architecture
machines	I-Architecture
in	O
this	O
sense	O
have	O
been	O
often	O
generally	O
called	O
p-code	B-Language
machines	I-Language
.	O
</s>
<s>
In	O
addition	O
to	O
being	O
an	O
intermediate	O
language	O
,	O
Pascal	B-Application
p-code	B-Language
was	O
also	O
executed	O
directly	O
by	O
an	O
interpreter	B-Application
implementing	O
the	O
virtual	B-Architecture
machine	I-Architecture
,	O
notably	O
in	O
UCSD	B-Language
Pascal	I-Language
(	O
1978	O
)	O
;	O
this	O
influenced	O
later	O
interpreters	B-Application
,	O
notably	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
(	O
JVM	B-Language
)	O
.	O
</s>
<s>
Another	O
early	O
example	O
was	O
SNOBOL4	B-Application
(	O
1967	O
)	O
,	O
which	O
was	O
written	O
in	O
the	O
SNOBOL	B-Application
Implementation	O
Language	O
(	O
SIL	O
)	O
,	O
an	O
assembly	O
language	O
for	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
which	O
was	O
then	O
targeted	O
to	O
physical	O
machines	O
by	O
transpiling	O
to	O
their	O
native	O
assembler	O
via	O
a	O
macro	B-Language
assembler	I-Language
.	O
</s>
<s>
Process	O
virtual	B-Architecture
machines	I-Architecture
were	O
a	O
popular	O
approach	O
to	O
implementing	O
early	O
microcomputer	O
software	O
,	O
including	O
Tiny	O
BASIC	O
and	O
adventure	O
games	O
,	O
from	O
one-off	O
implementations	O
such	O
as	O
Pyramid	B-Application
2000	I-Application
to	O
a	O
general-purpose	O
engine	O
like	O
Infocom	O
's	O
z-machine	O
,	O
which	O
Graham	O
Nelson	O
argues	O
is	O
"	O
possibly	O
the	O
most	O
portable	O
virtual	B-Architecture
machine	I-Architecture
ever	O
created	O
"	O
.	O
</s>
<s>
Significant	O
advances	O
occurred	O
in	O
the	O
implementation	O
of	O
Smalltalk-80	B-Application
,	O
</s>
<s>
which	O
pushed	O
just-in-time	O
(	O
JIT	O
)	O
compilation	B-Language
forward	O
as	O
an	O
implementation	O
approach	O
that	O
uses	O
process	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Later	O
notable	O
Smalltalk	B-Application
VMs	B-Application
were	O
VisualWorks	B-Application
,	O
the	O
Squeak	O
Virtual	B-Architecture
Machine	I-Architecture
,	O
</s>
<s>
and	O
Strongtalk	B-Language
.	O
</s>
<s>
A	O
related	O
language	O
that	O
produced	O
a	O
lot	O
of	O
virtual	B-Architecture
machine	I-Architecture
innovation	O
was	O
the	O
Self	B-Operating_System
programming	I-Operating_System
language	I-Operating_System
,	O
which	O
pioneered	O
adaptive	O
optimization	O
and	O
generational	O
garbage	O
collection	O
.	O
</s>
<s>
These	O
techniques	O
proved	O
commercially	O
successful	O
in	O
1999	O
in	O
the	O
HotSpot	B-Language
Java	B-Language
virtual	I-Language
machine	I-Language
.	O
</s>
<s>
Other	O
innovations	O
include	O
having	O
a	O
register-based	O
virtual	B-Architecture
machine	I-Architecture
,	O
to	O
better	O
match	O
the	O
underlying	O
hardware	O
,	O
rather	O
than	O
a	O
stack-based	O
virtual	B-Architecture
machine	I-Architecture
,	O
which	O
is	O
a	O
closer	O
match	O
for	O
the	O
programming	O
language	O
;	O
in	O
1995	O
,	O
this	O
was	O
pioneered	O
by	O
the	O
Dis	B-Application
virtual	B-Architecture
machine	I-Architecture
for	O
the	O
Limbo	B-Application
language	O
.	O
</s>
<s>
OpenJ9	O
is	O
an	O
alternative	O
for	O
HotSpot	B-Language
JVM	B-Language
in	O
OpenJDK	O
and	O
is	O
an	O
open	O
source	O
eclipse	O
project	O
claiming	O
better	O
startup	O
and	O
less	O
resource	O
consumption	O
compared	O
to	O
HotSpot	B-Language
.	O
</s>
<s>
In	O
full	B-Architecture
virtualization	I-Architecture
,	O
the	O
virtual	B-Architecture
machine	I-Architecture
simulates	O
enough	O
hardware	O
to	O
allow	O
an	O
unmodified	O
"	O
guest	O
"	O
OS	O
(	O
one	O
designed	O
for	O
the	O
same	O
instruction	B-General_Concept
set	I-General_Concept
)	O
to	O
be	O
run	O
in	O
isolation	O
.	O
</s>
<s>
This	O
approach	O
was	O
pioneered	O
in	O
1966	O
with	O
the	O
IBM	B-Application
CP-40	I-Application
and	O
CP-67	B-Application
,	O
predecessors	O
of	O
the	O
VM	B-Application
family	O
.	O
</s>
<s>
Examples	O
outside	O
the	O
mainframe	O
field	O
include	O
Parallels	B-Operating_System
Workstation	I-Operating_System
,	O
Parallels	B-Language
Desktop	I-Language
for	I-Language
Mac	I-Language
,	O
VirtualBox	B-Operating_System
,	O
Virtual	O
Iron	O
,	O
Oracle	B-Application
VM	I-Application
,	O
Virtual	B-Application
PC	I-Application
,	O
Virtual	B-General_Concept
Server	I-General_Concept
,	O
Hyper-V	O
,	O
VMware	B-Operating_System
Fusion	I-Operating_System
,	O
VMware	B-Operating_System
Workstation	I-Operating_System
,	O
VMware	B-Operating_System
Server	I-Operating_System
(	O
discontinued	O
,	O
formerly	O
called	O
GSX	B-Operating_System
Server	I-Operating_System
)	O
,	O
VMware	B-Operating_System
ESXi	I-Operating_System
,	O
QEMU	B-Application
,	O
Adeos	B-Operating_System
,	O
Mac-on-Linux	O
,	O
Win4BSD	O
,	O
Win4Lin	O
Pro	O
,	O
and	O
Egenera	O
vBlade	O
technology	O
.	O
</s>
<s>
In	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
,	O
the	O
hardware	O
provides	O
architectural	O
support	O
that	O
facilitates	O
building	O
a	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
and	O
allows	O
guest	O
OSes	O
to	O
be	O
run	O
in	O
isolation	O
.	O
</s>
<s>
Hardware-assisted	B-General_Concept
virtualization	I-General_Concept
was	O
first	O
introduced	O
on	O
the	O
IBM	O
System/370	O
in	O
1972	O
,	O
for	O
use	O
with	O
VM/370	B-Application
,	O
the	O
first	O
virtual	B-Architecture
machine	I-Architecture
operating	B-General_Concept
system	I-General_Concept
offered	O
by	O
IBM	O
as	O
an	O
official	O
product	O
.	O
</s>
<s>
In	O
2005	O
and	O
2006	O
,	O
Intel	O
and	O
AMD	O
provided	O
additional	O
hardware	O
to	O
support	O
virtualization	B-General_Concept
.	O
</s>
<s>
Sun	O
Microsystems	O
(	O
now	O
Oracle	B-Application
Corporation	I-Application
)	O
added	O
similar	O
features	O
in	O
their	O
UltraSPARC	B-Device
T-Series	I-Device
processors	O
in	O
2005	O
.	O
</s>
<s>
Examples	O
of	O
virtualization	B-General_Concept
platforms	B-Device
adapted	O
to	O
such	O
hardware	O
include	O
KVM	B-Application
,	O
VMware	B-Operating_System
Workstation	I-Operating_System
,	O
VMware	B-Operating_System
Fusion	I-Operating_System
,	O
Hyper-V	O
,	O
Windows	B-Application
Virtual	I-Application
PC	I-Application
,	O
Xen	B-Operating_System
,	O
Parallels	B-Language
Desktop	I-Language
for	I-Language
Mac	I-Language
,	O
Oracle	B-Application
VM	I-Application
Server	O
for	O
SPARC	O
,	O
VirtualBox	B-Operating_System
and	O
Parallels	B-Operating_System
Workstation	I-Operating_System
.	O
</s>
<s>
In	O
2006	O
,	O
first-generation	O
32	O
-	O
and	O
64-bit	O
x86	O
hardware	O
support	O
was	O
found	O
to	O
rarely	O
offer	O
performance	O
advantages	O
over	O
software	O
virtualization	B-General_Concept
.	O
</s>
<s>
In	O
operating-system-level	B-Application
virtualization	I-Application
,	O
a	O
physical	O
server	O
is	O
virtualized	B-General_Concept
at	O
the	O
operating	B-General_Concept
system	I-General_Concept
level	O
,	O
enabling	O
multiple	O
isolated	O
and	O
secure	O
virtualized	B-General_Concept
servers	O
to	O
run	O
on	O
a	O
single	O
physical	O
server	O
.	O
</s>
<s>
The	O
"	O
guest	O
"	O
operating	B-General_Concept
system	I-General_Concept
environments	O
share	O
the	O
same	O
running	O
instance	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
as	O
the	O
host	O
system	O
.	O
</s>
<s>
Thus	O
,	O
the	O
same	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
is	O
also	O
used	O
to	O
implement	O
the	O
"	O
guest	O
"	O
environments	O
,	O
and	O
applications	O
running	O
in	O
a	O
given	O
"	O
guest	O
"	O
environment	O
view	O
it	O
as	O
a	O
stand-alone	O
system	O
.	O
</s>
<s>
The	O
pioneer	O
implementation	O
was	O
FreeBSD	B-Language
jails	I-Language
;	O
other	O
examples	O
include	O
Docker	B-Language
,	O
Solaris	O
Containers	B-Application
,	O
OpenVZ	B-Language
,	O
Linux-VServer	B-Application
,	O
LXC	B-Application
,	O
AIX	O
Workload	B-Operating_System
Partitions	I-Operating_System
,	O
Parallels	B-Language
Virtuozzo	O
Containers	B-Application
,	O
and	O
iCore	B-Application
Virtual	I-Application
Accounts	I-Application
.	O
</s>
