<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
full	B-Architecture
virtualization	I-Architecture
(	O
fv	O
)	O
employs	O
techniques	O
used	O
to	O
create	O
instances	O
of	O
an	O
environment	O
,	O
as	O
opposed	O
to	O
simulation	O
,	O
which	O
models	O
the	O
environment	O
;	O
or	O
emulation	B-Application
,	O
which	O
replicates	O
the	O
target	O
environment	O
such	O
as	O
certain	O
kinds	O
of	O
virtual	B-Architecture
machine	I-Architecture
environments	O
.	O
</s>
<s>
Full	B-Architecture
virtualization	I-Architecture
requires	O
that	O
every	O
salient	O
feature	O
of	O
the	O
hardware	O
be	O
reflected	O
into	O
one	O
of	O
several	O
virtual	B-Architecture
machines	I-Architecture
–	O
including	O
the	O
full	O
instruction	O
set	O
,	O
input/output	B-General_Concept
operations	O
,	O
interrupts	O
,	O
memory	O
access	O
,	O
and	O
whatever	O
other	O
elements	O
are	O
used	O
by	O
the	O
software	O
that	O
runs	O
on	O
the	O
bare	O
machine	O
,	O
and	O
that	O
is	O
intended	O
to	O
run	O
in	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
In	O
such	O
an	O
environment	O
,	O
any	O
software	O
capable	O
of	O
execution	O
on	O
the	O
raw	O
hardware	O
can	O
be	O
run	O
in	O
the	O
virtual	B-Architecture
machine	I-Architecture
and	O
,	O
in	O
particular	O
,	O
any	O
operating	O
systems	O
.	O
</s>
<s>
The	O
obvious	O
test	O
of	O
full	B-Architecture
virtualization	I-Architecture
is	O
whether	O
an	O
operating	O
system	O
intended	O
for	O
stand-alone	O
use	O
can	O
successfully	O
run	O
inside	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
This	O
is	O
a	O
modern	O
technique	O
developed	O
in	O
late	O
1990s	O
and	O
is	O
different	O
from	O
simulation	O
and	O
emulation	B-Application
.	O
</s>
<s>
The	O
cornerstone	O
of	O
full	B-Architecture
virtualization	I-Architecture
or	O
type-1	O
virtualization	O
is	O
a	O
hypervisor	B-Operating_System
or	O
Super	O
Operating	O
system	O
that	O
operates	O
at	O
a	O
higher	O
privilege	O
level	O
than	O
the	O
OS	O
.	O
</s>
<s>
This	O
Hypervisor	B-Operating_System
or	O
Super	O
OS	O
requires	O
two	O
key	O
features	O
to	O
provision	O
and	O
protect	O
virtualized	O
environments	O
.	O
</s>
<s>
See	O
Intel	O
VT-x	O
or	O
AMD-V	O
for	O
a	O
detailed	O
description	O
of	O
privilege	O
levels	O
for	O
Hypervisor	B-Operating_System
,	O
OS	O
and	O
User	O
modes	O
,	O
VMCS	O
,	O
VM-Exit	O
and	O
VM-Entry	O
.	O
</s>
<s>
This	O
virtualization	O
is	O
not	O
to	O
be	O
confused	O
with	O
IBM	O
Virtual	B-Architecture
Machine	I-Architecture
implementations	O
of	O
late	O
60	O
's	O
and	O
early	O
70	O
's	O
as	O
IBM	O
systems	O
architecture	O
supported	O
only	O
two	O
modes	O
of	O
Supervisor	O
and	O
Program	O
which	O
provided	O
no	O
security	O
or	O
separation	O
of	O
Virtual	B-Architecture
Machines	I-Architecture
.	O
</s>
<s>
Other	O
forms	O
of	O
platform	B-General_Concept
virtualization	I-General_Concept
allow	O
only	O
certain	O
or	O
modified	O
software	O
to	O
run	O
within	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
The	O
concept	O
of	O
full	B-Architecture
virtualization	I-Architecture
is	O
well	O
established	O
in	O
the	O
literature	O
,	O
but	O
it	O
is	O
not	O
always	O
referred	O
to	O
by	O
this	O
specific	O
term	O
;	O
see	O
platform	B-General_Concept
virtualization	I-General_Concept
for	O
terminology	O
.	O
</s>
<s>
An	O
important	O
example	O
of	O
Virtual	B-Architecture
Machines	I-Architecture
,	O
not	O
to	O
be	O
confused	O
with	O
Virtualization	O
implemented	O
by	O
emulation	B-Application
was	O
that	O
provided	O
by	O
the	O
control	O
program	O
of	O
IBM	O
's	O
CP/CMS	B-Application
operating	O
system	O
.	O
</s>
<s>
It	O
was	O
first	O
demonstrated	O
with	O
IBM	O
's	O
CP-40	B-Application
research	O
system	O
in	O
1967	O
,	O
then	O
distributed	O
via	O
open	B-License
source	I-License
in	O
CP/CMS	B-Application
in	O
1967	O
–	O
1972	O
,	O
and	O
re-implemented	O
in	O
IBM	O
's	O
VM	B-Application
family	I-Application
from	O
1972	O
to	O
the	O
present	O
.	O
</s>
<s>
Each	O
CP/CMS	B-Application
user	O
was	O
provided	O
a	O
simulated	O
,	O
stand-alone	O
computer	O
.	O
</s>
<s>
Each	O
such	O
virtual	B-Architecture
machine	I-Architecture
had	O
the	O
complete	O
capabilities	O
of	O
the	O
underlying	O
machine	O
,	O
and	O
(	O
for	O
its	O
user	O
)	O
the	O
virtual	B-Architecture
machine	I-Architecture
was	O
indistinguishable	O
from	O
a	O
private	O
system	O
.	O
</s>
<s>
The	O
result	O
was	O
a	O
single	O
machine	O
that	O
could	O
be	O
multiplexed	B-Architecture
among	O
many	O
users	O
.	O
</s>
<s>
Full	B-Architecture
virtualization	I-Architecture
is	O
possible	O
only	O
with	O
the	B-Architecture
right	I-Architecture
combination	I-Architecture
of	I-Architecture
hardware	I-Architecture
and	I-Architecture
software	I-Architecture
elements	O
.	O
</s>
<s>
For	O
example	O
,	O
it	O
was	O
not	O
possible	O
with	O
most	O
of	O
IBM	O
's	O
System/360	B-Application
series	O
with	O
the	O
exception	O
being	O
the	O
IBM	B-Device
System/360	I-Device
-67	I-Device
;	O
nor	O
was	O
it	O
possible	O
with	O
IBM	O
's	O
early	O
System/370	B-Device
system	O
.	O
</s>
<s>
IBM	O
added	O
virtual	O
memory	O
hardware	O
to	O
the	O
System/370	B-Device
series	O
in	O
1972	O
which	O
is	O
not	O
the	O
same	O
as	O
Intel	O
VT-x	O
Rings	O
providing	O
a	O
higher	O
privilege	O
level	O
for	O
Hypervisor	B-Operating_System
to	O
properly	O
control	O
Virtual	B-Architecture
Machines	I-Architecture
requiring	O
full	O
access	O
to	O
Supervisor	O
and	O
Program	O
or	O
User	O
modes	O
.	O
</s>
<s>
Similarly	O
,	O
full	B-Architecture
virtualization	I-Architecture
was	O
not	O
quite	O
possible	O
with	O
the	O
x86	O
platform	O
until	O
the	O
2005	O
–	O
2006	O
addition	O
of	O
the	O
AMD-V	O
and	O
Intel	O
VT-x	O
extensions	O
(	O
see	O
x86	B-General_Concept
virtualization	I-General_Concept
)	O
.	O
</s>
<s>
Many	O
platform	O
hypervisors	B-Operating_System
for	O
the	O
x86	O
platform	O
came	O
very	O
close	O
and	O
claimed	O
full	B-Architecture
virtualization	I-Architecture
even	O
prior	O
to	O
the	O
AMD-V	O
and	O
Intel	O
VT-x	O
additions	O
.	O
</s>
<s>
Examples	O
include	O
Adeos	B-Operating_System
,	O
Mac-on-Linux	O
,	O
Parallels	B-Language
Desktop	I-Language
for	I-Language
Mac	I-Language
,	O
Parallels	B-Operating_System
Workstation	I-Operating_System
,	O
VMware	B-Operating_System
Workstation	I-Operating_System
,	O
VMware	B-Operating_System
Server	I-Operating_System
(	O
formerly	O
GSX	B-Operating_System
Server	I-Operating_System
)	O
,	O
VirtualBox	B-Operating_System
,	O
Win4BSD	O
,	O
and	O
Win4Lin	O
Pro	O
.	O
</s>
<s>
VMware	O
,	O
for	O
instance	O
,	O
employs	O
a	O
technique	O
called	O
binary	O
translation	O
to	O
automatically	O
modify	O
x86	O
software	O
on-the-fly	O
to	O
replace	O
instructions	O
that	O
"	O
pierce	O
the	O
virtual	B-Architecture
machine	I-Architecture
"	O
with	O
a	O
different	O
,	O
virtual	B-Architecture
machine	I-Architecture
safe	O
sequence	O
of	O
instructions	O
;	O
this	O
technique	O
provides	O
the	O
appearance	O
of	O
full	B-Architecture
virtualization	I-Architecture
.	O
</s>
<s>
A	O
key	O
challenge	O
for	O
full	B-Architecture
virtualization	I-Architecture
is	O
the	O
interception	O
and	O
simulation	O
of	O
privileged	O
operations	O
,	O
such	O
as	O
I/O	B-General_Concept
instructions	O
.	O
</s>
<s>
The	O
effects	O
of	O
every	O
operation	O
performed	O
within	O
a	O
given	O
virtual	B-Architecture
machine	I-Architecture
must	O
be	O
kept	O
within	O
that	O
virtual	B-Architecture
machine	I-Architecture
virtual	O
operations	O
cannot	O
be	O
allowed	O
to	O
alter	O
the	O
state	O
of	O
any	O
other	O
virtual	B-Architecture
machine	I-Architecture
,	O
the	O
control	O
program	O
,	O
or	O
the	O
hardware	O
.	O
</s>
<s>
But	O
other	O
instructions	O
that	O
would	O
"	O
pierce	O
the	O
virtual	B-Architecture
machine	I-Architecture
"	O
cannot	O
be	O
allowed	O
to	O
execute	O
directly	O
;	O
they	O
must	O
instead	O
be	O
trapped	O
and	O
simulated	O
.	O
</s>
<s>
Such	O
instructions	O
either	O
access	O
or	O
affect	O
state	O
information	O
that	O
is	O
outside	O
the	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Full	B-Architecture
virtualization	I-Architecture
has	O
proven	O
highly	O
successful	O
for	O
:	O
</s>
<s>
emulating	B-Application
new	O
hardware	O
to	O
achieve	O
improved	O
reliability	O
,	O
security	O
,	O
and	O
productivity	O
.	O
</s>
