<s>
In	O
computing	O
,	O
a	O
system	B-Operating_System
virtual	I-Operating_System
machine	I-Operating_System
is	O
a	O
virtual	B-Architecture
machine	I-Architecture
(	O
VM	B-Application
)	O
that	O
provides	O
a	O
complete	O
system	B-Device
platform	I-Device
and	O
supports	O
the	O
execution	B-General_Concept
of	O
a	O
complete	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
.	O
</s>
<s>
These	O
usually	O
emulate	B-Application
an	O
existing	O
architecture	B-General_Concept
,	O
and	O
are	O
built	O
with	O
the	O
purpose	O
of	O
either	O
providing	O
a	O
platform	B-Device
to	O
run	O
programs	O
where	O
the	O
real	O
hardware	B-Architecture
is	O
not	O
available	O
for	O
use	O
(	O
for	O
example	O
,	O
executing	O
on	O
otherwise	O
obsolete	O
platforms	B-Device
)	O
,	O
or	O
of	O
having	O
multiple	O
instances	O
of	O
virtual	B-Architecture
machines	I-Architecture
leading	O
to	O
more	O
efficient	O
use	O
of	O
computing	O
resources	O
,	O
both	O
in	O
terms	O
of	O
energy	O
consumption	O
and	O
cost	O
effectiveness	O
(	O
known	O
as	O
hardware	B-General_Concept
virtualization	I-General_Concept
,	O
the	O
key	O
to	O
a	O
cloud	B-Architecture
computing	I-Architecture
environment	O
)	O
,	O
or	O
both	O
.	O
</s>
<s>
A	O
VM	B-Application
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
machine	O
"	O
.	O
</s>
<s>
System	B-Operating_System
virtual	I-Operating_System
machine	I-Operating_System
advantages	O
:	O
</s>
<s>
Multiple	O
OS	O
environments	O
can	O
co-exist	O
on	O
the	O
same	O
primary	O
hard	O
drive	O
,	O
with	O
a	O
virtual	O
partition	O
that	O
allows	O
sharing	O
of	O
files	O
generated	O
in	O
either	O
the	O
"	O
host	O
"	O
operating	B-General_Concept
system	I-General_Concept
or	O
"	O
guest	O
"	O
virtual	O
environment	O
.	O
</s>
<s>
Adjunct	O
software	O
installations	O
,	O
wireless	O
connectivity	O
,	O
and	O
remote	O
replication	O
,	O
such	O
as	O
printing	O
and	O
faxing	O
,	O
can	O
be	O
generated	O
in	O
any	O
of	O
the	O
guest	O
or	O
host	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Application	O
provisioning	O
,	O
maintenance	O
,	O
high	O
availability	O
and	O
disaster	O
recovery	O
are	O
inherent	O
in	O
the	O
virtual	B-Architecture
machine	I-Architecture
software	O
selected	O
.	O
</s>
<s>
Can	O
provide	O
emulated	B-Application
hardware	B-Architecture
environments	O
different	O
from	O
the	O
host	O
's	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	O
)	O
,	O
through	O
emulation	B-Application
or	O
by	O
using	O
just-in-time	O
compilation	O
.	O
</s>
<s>
The	O
main	O
disadvantages	O
of	O
VMs	B-Application
are	O
:	O
</s>
<s>
A	O
virtual	B-Architecture
machine	I-Architecture
is	O
less	O
efficient	O
than	O
an	O
actual	O
machine	O
when	O
it	O
accesses	O
the	O
host	O
hard	O
drive	O
indirectly	O
.	O
</s>
<s>
When	O
multiple	O
VMs	B-Application
are	O
concurrently	O
running	O
on	O
the	O
hard	O
drive	O
of	O
the	O
actual	O
host	O
,	O
adjunct	O
virtual	B-Architecture
machines	I-Architecture
may	O
exhibit	O
a	O
varying	O
and/or	O
unstable	O
performance	O
(	O
speed	O
of	O
execution	B-General_Concept
and	O
malware	O
protection	O
)	O
.	O
</s>
<s>
This	O
depends	O
on	O
the	O
data	O
load	O
imposed	O
on	O
the	O
system	O
by	O
other	O
VMs	B-Application
,	O
unless	O
the	O
selected	O
VM	B-Application
software	O
provides	O
temporal	B-General_Concept
isolation	I-General_Concept
among	I-General_Concept
virtual	I-General_Concept
machines	I-General_Concept
.	O
</s>
<s>
Malware	O
protections	O
for	O
VMs	B-Application
are	O
not	O
necessarily	O
compatible	O
with	O
the	O
"	O
host	O
"	O
,	O
and	O
may	O
require	O
separate	O
software	O
.	O
</s>
<s>
Multiple	O
VMs	B-Application
running	O
their	O
own	O
guest	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
are	O
frequently	O
engaged	O
for	O
server	B-Application
consolidation	O
in	O
order	O
to	O
avoid	O
interference	O
from	O
separate	O
VMs	B-Application
on	O
the	O
same	O
actual	O
machine	O
platform	B-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
motivation	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
CMS	B-Operating_System
.	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
actual	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
"	O
memory	O
pages	O
"	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
known	O
as	O
Kernel	B-Application
SamePage	I-Application
Merging	I-Application
.	O
</s>
<s>
This	O
is	O
particularly	O
useful	O
for	O
read-only	O
pages	O
,	O
such	O
as	O
those	O
that	O
contain	O
code	O
segments	O
;	O
in	O
particular	O
,	O
that	O
would	O
be	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	B-Application
servers	I-Application
,	O
middleware	O
components	O
,	O
etc	O
.	O
</s>
<s>
The	O
guest	B-General_Concept
operating	I-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	B-Architecture
,	O
thereby	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
Microsoft	B-Application
Windows	I-Application
,	O
Linux	B-Application
,	O
or	O
previous	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	B-General_Concept
operating	I-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	O
.	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
,	O
and	O
may	O
include	O
improved	O
debugging	O
access	O
and	O
faster	O
reboots	O
.	O
</s>
<s>
Different	O
virtualization	B-General_Concept
techniques	O
are	O
used	O
,	O
based	O
on	O
the	O
desired	O
usage	O
.	O
</s>
<s>
Native	O
execution	B-General_Concept
is	O
based	O
on	O
direct	O
virtualization	B-General_Concept
of	O
the	O
underlying	O
raw	O
hardware	B-Architecture
,	O
thus	O
it	O
provides	O
multiple	O
"	O
instances	O
"	O
of	O
the	O
same	O
architecture	B-General_Concept
a	O
real	O
machine	O
is	O
based	O
on	O
,	O
capable	O
of	O
running	O
complete	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Some	O
virtual	B-Architecture
machines	I-Architecture
can	O
also	O
emulate	B-Application
different	O
architectures	O
and	O
allow	O
execution	B-General_Concept
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	B-General_Concept
.	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
kernel	B-Operating_System
's	O
support	O
for	O
multiple	O
isolated	O
user	B-Operating_System
space	I-Operating_System
instances	O
,	O
which	O
are	O
usually	O
called	O
containers	B-Application
and	O
may	O
look	O
and	O
feel	O
like	O
real	O
machines	O
to	O
the	O
end	O
users	O
.	O
</s>
<s>
Some	O
computer	B-General_Concept
architectures	I-General_Concept
are	O
capable	O
of	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
,	O
which	O
enables	O
efficient	O
full	B-Architecture
virtualization	I-Architecture
by	O
using	O
virtualization-specific	O
hardware	B-Architecture
capabilities	O
,	O
primarily	O
from	O
the	O
host	O
CPUs	B-Device
.	O
</s>
<s>
This	O
approach	O
is	O
described	O
as	O
full	B-Architecture
virtualization	I-Architecture
of	O
the	O
hardware	B-Architecture
,	O
and	O
can	O
be	O
implemented	O
using	O
a	O
type	O
1	O
or	O
type	O
2	O
hypervisor	B-Operating_System
:	O
a	O
type	O
1	O
hypervisor	B-Operating_System
runs	O
directly	O
on	O
the	O
hardware	B-Architecture
,	O
and	O
a	O
type	O
2	O
hypervisor	B-Operating_System
runs	O
on	O
another	O
operating	B-General_Concept
system	I-General_Concept
,	O
such	O
as	O
Linux	B-Application
or	O
Windows	O
.	O
</s>
<s>
Each	O
virtual	B-Architecture
machine	I-Architecture
can	O
run	O
any	O
operating	B-General_Concept
system	I-General_Concept
supported	O
by	O
the	O
underlying	O
hardware	B-Architecture
.	O
</s>
<s>
Users	O
can	O
thus	O
run	O
two	O
or	O
more	O
different	O
"	O
guest	O
"	O
operating	B-General_Concept
systems	I-General_Concept
simultaneously	O
,	O
in	O
separate	O
"	O
private	O
"	O
virtual	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
The	O
pioneer	O
system	O
using	O
this	O
concept	O
was	O
IBM	O
's	O
CP-40	B-Application
,	O
the	O
first	O
(	O
1967	O
)	O
version	O
of	O
IBM	O
's	O
CP/CMS	B-Application
(	O
1967	O
–	O
1972	O
)	O
and	O
the	O
precursor	O
to	O
IBM	O
's	O
VM	B-Application
family	I-Application
(	O
1972	O
–	O
present	O
)	O
.	O
</s>
<s>
With	O
the	O
VM	B-Application
architecture	B-General_Concept
,	O
most	O
users	O
run	O
a	O
relatively	O
simple	O
interactive	B-General_Concept
computing	I-General_Concept
single-user	O
operating	B-General_Concept
system	I-General_Concept
,	O
CMS	B-Operating_System
,	O
as	O
a	O
"	O
guest	O
"	O
on	O
top	O
of	O
the	O
VM	B-Application
control	O
program	O
(	O
VM-CP	B-Application
)	O
.	O
</s>
<s>
This	O
approach	O
kept	O
the	O
CMS	B-Operating_System
design	O
simple	O
,	O
as	O
if	O
it	O
were	O
running	O
alone	O
;	O
the	O
control	O
program	O
quietly	O
provides	O
multitasking	O
and	O
resource	O
management	O
services	O
"	O
behind	O
the	O
scenes	O
"	O
.	O
</s>
<s>
In	O
addition	O
to	O
CMS	B-Operating_System
communication	O
and	O
other	O
system	O
tasks	O
are	O
performed	O
by	O
multitasking	O
VMs	B-Application
(	O
RSCS	O
,	O
GCS	O
,	O
TCP/IP	O
,	O
UNIX	O
)	O
,	O
and	O
users	O
can	O
run	O
any	O
of	O
the	O
other	O
IBM	O
operating	B-General_Concept
systems	I-General_Concept
,	O
such	O
as	O
MVS	B-Application
,	O
even	O
a	O
new	O
CP	O
itself	O
or	O
now	O
z/OS	B-Application
.	O
</s>
<s>
Even	O
the	O
simple	O
CMS	B-Operating_System
could	O
be	O
run	O
in	O
a	O
threaded	O
environment	O
(	O
LISTSERV	O
,	O
TRICKLE	O
)	O
.	O
</s>
<s>
z/VM	B-Application
is	O
the	O
current	O
version	O
of	O
VM	B-Application
,	O
and	O
is	O
used	O
to	O
support	O
hundreds	O
or	O
thousands	O
of	O
virtual	B-Architecture
machines	I-Architecture
on	O
a	O
given	O
mainframe	O
.	O
</s>
<s>
Some	O
installations	O
use	O
Linux	B-Application
on	I-Application
IBM	I-Application
Z	I-Application
to	O
run	O
Web	B-Application
servers	I-Application
,	O
where	O
Linux	B-Application
runs	O
as	O
the	O
operating	B-General_Concept
system	I-General_Concept
within	O
many	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
Full	B-Architecture
virtualization	I-Architecture
is	O
particularly	O
helpful	O
in	O
operating	B-General_Concept
system	I-General_Concept
development	O
,	O
when	O
experimental	O
new	O
code	O
can	O
be	O
run	O
at	O
the	O
same	O
time	O
as	O
older	O
,	O
more	O
stable	O
,	O
versions	O
,	O
each	O
in	O
a	O
separate	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
The	O
process	O
can	O
even	O
be	O
recursive	O
:	O
IBM	O
debugged	O
new	O
versions	O
of	O
its	O
virtual	B-Architecture
machine	I-Architecture
operating	B-General_Concept
system	I-General_Concept
,	O
VM	B-Application
,	O
in	O
a	O
virtual	B-Architecture
machine	I-Architecture
running	O
under	O
an	O
older	O
version	O
of	O
VM	B-Application
,	O
and	O
even	O
used	O
this	O
technique	O
to	O
simulate	O
new	O
hardware	B-Architecture
.	O
</s>
<s>
The	O
standard	O
x86	B-Operating_System
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
as	O
used	O
in	O
the	O
modern	O
PCs	O
does	O
not	O
actually	O
meet	O
the	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
virtualization	I-Architecture
requirements	I-Architecture
.	O
</s>
<s>
Notably	O
,	O
there	O
is	O
no	O
execution	B-General_Concept
mode	O
where	O
all	O
sensitive	O
machine	O
instructions	O
always	O
trap	O
,	O
which	O
would	O
allow	O
per-instruction	O
virtualization	B-General_Concept
.	O
</s>
<s>
Despite	O
these	O
limitations	O
,	O
several	O
software	O
packages	O
have	O
managed	O
to	O
provide	O
virtualization	B-General_Concept
on	I-General_Concept
the	I-General_Concept
x86	I-General_Concept
architecture	I-General_Concept
,	O
even	O
though	O
dynamic	O
recompilation	O
of	O
privileged	O
code	O
,	O
as	O
first	O
implemented	O
by	O
VMware	O
,	O
incurs	O
some	O
performance	O
overhead	O
as	O
compared	O
to	O
a	O
VM	B-Application
running	O
on	O
a	O
natively	O
virtualizable	O
architecture	B-General_Concept
such	O
as	O
the	O
IBM	B-Device
System/370	I-Device
or	O
Motorola	B-Device
MC68020	I-Device
.	O
</s>
<s>
By	O
now	O
,	O
several	O
other	O
software	O
packages	O
such	O
as	O
Virtual	B-Application
PC	I-Application
,	O
VirtualBox	B-Operating_System
,	O
Parallels	B-Operating_System
Workstation	I-Operating_System
and	O
Virtual	O
Iron	O
manage	O
to	O
implement	O
virtualization	B-General_Concept
on	O
x86	B-Operating_System
hardware	B-Architecture
.	O
</s>
<s>
Intel	O
and	O
AMD	O
have	O
introduced	O
features	O
to	O
their	O
x86	B-Operating_System
processors	O
to	O
enable	O
virtualization	B-General_Concept
in	I-General_Concept
hardware	I-General_Concept
.	O
</s>
<s>
As	O
well	O
as	O
virtualization	B-General_Concept
of	O
the	O
resources	O
of	O
a	O
single	O
machine	O
,	O
multiple	O
independent	O
nodes	O
in	O
a	O
cluster	O
can	O
be	O
combined	O
and	O
accessed	O
as	O
a	O
single	O
virtual	O
NUMA	B-Operating_System
machine	O
.	O
</s>
<s>
Virtual	B-Architecture
machines	I-Architecture
can	O
also	O
perform	O
the	O
role	O
of	O
an	O
emulator	B-Application
,	O
allowing	O
software	O
applications	O
and	O
operating	B-General_Concept
systems	I-General_Concept
written	O
for	O
another	O
computer	O
processor	O
architecture	B-General_Concept
to	O
be	O
run	O
.	O
</s>
<s>
Operating-system-level	B-Application
virtualization	I-Application
is	O
a	O
server	B-General_Concept
virtualization	I-General_Concept
technology	O
which	O
virtualizes	O
servers	O
on	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
kernel	B-Operating_System
)	O
layer	O
.	O
</s>
<s>
It	O
can	O
be	O
thought	O
of	O
as	O
partitioning	O
:	O
a	O
single	O
physical	O
server	B-Application
is	O
sliced	O
into	O
multiple	O
small	O
partitions	O
(	O
otherwise	O
called	O
virtual	O
environments	O
(	O
VE	O
)	O
,	O
virtual	B-General_Concept
private	I-General_Concept
servers	I-General_Concept
(	O
VPS	O
)	O
,	O
guests	O
,	O
zones	O
,	O
etc	O
.	O
</s>
<s>
)	O
;	O
each	O
such	O
partition	O
looks	O
and	O
feels	O
like	O
a	O
real	O
server	B-Application
,	O
from	O
the	O
point	O
of	O
view	O
of	O
its	O
users	O
.	O
</s>
<s>
For	O
example	O
,	O
Solaris	O
Zones	O
supports	O
multiple	O
guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
running	O
under	O
the	O
same	O
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
Solaris10	O
.	O
</s>
<s>
Guest	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
can	O
use	O
the	O
same	O
kernel	B-Operating_System
level	O
with	O
the	O
same	O
operating	B-General_Concept
system	I-General_Concept
version	O
,	O
or	O
can	O
be	O
a	O
separate	O
copy	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
with	O
a	O
different	O
kernel	B-Operating_System
version	O
using	O
Solaris	O
Kernel	B-Operating_System
Zones	O
.	O
</s>
<s>
Solaris	O
native	O
Zones	O
also	O
requires	O
that	O
the	O
host	O
operating	B-General_Concept
system	I-General_Concept
is	O
a	O
version	O
of	O
Solaris	O
;	O
other	O
operating	B-General_Concept
systems	I-General_Concept
from	O
other	O
manufacturers	O
are	O
not	O
supported	O
.	O
</s>
<s>
However	O
,	O
Solaris	O
Branded	O
Zones	O
would	O
need	O
to	O
be	O
used	O
to	O
have	O
other	O
operating	B-General_Concept
systems	I-General_Concept
as	O
zones	O
.	O
</s>
<s>
Another	O
example	O
is	O
System	O
Workload	O
Partitions	O
(	O
WPARs	O
)	O
,	O
introduced	O
in	O
version	O
6.1	O
of	O
the	O
IBM	O
AIX	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
level	O
architecture	B-General_Concept
has	O
low	O
overhead	O
that	O
helps	O
to	O
maximize	O
efficient	O
use	O
of	O
server	B-Application
resources	O
.	O
</s>
<s>
The	O
virtualization	B-General_Concept
introduces	O
only	O
a	O
negligible	O
overhead	O
and	O
allows	O
running	O
hundreds	O
of	O
virtual	B-General_Concept
private	I-General_Concept
servers	I-General_Concept
on	O
a	O
single	O
physical	O
server	B-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
approaches	O
such	O
as	O
full	B-Architecture
virtualization	I-Architecture
(	O
like	O
VMware	O
)	O
and	O
paravirtualization	O
(	O
like	O
Xen	B-Operating_System
or	O
UML	B-Application
)	O
cannot	O
achieve	O
such	O
level	O
of	O
density	O
,	O
due	O
to	O
overhead	O
of	O
running	O
multiple	O
kernels	B-Operating_System
.	O
</s>
<s>
From	O
the	O
other	O
side	O
,	O
operating	B-Application
system-level	I-Application
virtualization	I-Application
does	O
not	O
allow	O
running	O
different	O
operating	B-General_Concept
systems	I-General_Concept
(	O
i.e.	O
,	O
different	O
kernels	B-Operating_System
)	O
,	O
although	O
different	O
libraries	O
,	O
distributions	O
,	O
etc	O
.	O
</s>
<s>
Different	O
virtualization	B-General_Concept
techniques	O
are	O
used	O
,	O
based	O
on	O
the	O
desired	O
usage	O
.	O
</s>
<s>
Native	O
execution	B-General_Concept
is	O
based	O
on	O
direct	O
virtualization	B-General_Concept
of	O
the	O
underlying	O
raw	O
hardware	B-Architecture
,	O
thus	O
it	O
provides	O
multiple	O
"	O
instances	O
"	O
of	O
the	O
same	O
architecture	B-General_Concept
a	O
real	O
machine	O
is	O
based	O
on	O
,	O
capable	O
of	O
running	O
complete	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Some	O
virtual	B-Architecture
machines	I-Architecture
can	O
also	O
emulate	B-Application
different	O
architectures	O
and	O
allow	O
execution	B-General_Concept
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	B-General_Concept
.	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
kernel	B-Operating_System
's	O
support	O
for	O
multiple	O
isolated	O
user	B-Operating_System
space	I-Operating_System
instances	O
,	O
which	O
are	O
usually	O
called	O
containers	B-Application
and	O
may	O
look	O
and	O
feel	O
like	O
real	O
machines	O
to	O
the	O
end	O
users	O
.	O
</s>
<s>
Some	O
computer	B-General_Concept
architectures	I-General_Concept
are	O
capable	O
of	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
,	O
which	O
enables	O
efficient	O
full	B-Architecture
virtualization	I-Architecture
by	O
using	O
virtualization-specific	O
hardware	B-Architecture
capabilities	O
,	O
primarily	O
from	O
the	O
host	O
CPUs	B-Device
.	O
</s>
<s>
Examples	O
of	O
virtualization-enabled	O
hardware	B-Architecture
include	O
the	O
following	O
:	O
</s>
<s>
Boston	O
Circuits	O
gCore	O
(	O
grid-on-chip	O
)	O
with	O
16	O
ARC	B-Application
750D	O
cores	O
and	O
Time-machine	O
hardware	B-General_Concept
virtualization	I-General_Concept
module	O
.	O
</s>
<s>
x86	B-Operating_System
:	O
</s>
