<s>
VHD	O
(	O
Virtual	O
Hard	B-Device
Disk	I-Device
)	O
and	O
its	O
successor	O
VHDX	O
are	O
file	O
formats	O
representing	O
a	O
virtual	B-Application
hard	I-Application
disk	I-Application
drive	I-Application
(	O
HDD	O
)	O
.	O
</s>
<s>
They	O
may	O
contain	O
what	O
is	O
found	O
on	O
a	O
physical	O
HDD	O
,	O
such	O
as	O
disk	B-Application
partitions	I-Application
and	O
a	O
file	B-Application
system	I-Application
,	O
which	O
in	O
turn	O
can	O
contain	O
files	B-Operating_System
and	O
folders	O
.	O
</s>
<s>
They	O
are	O
typically	O
used	O
as	O
the	O
hard	B-Device
disk	I-Device
of	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
are	O
built	O
into	O
modern	O
versions	O
of	O
Windows	O
,	O
and	O
are	O
the	O
native	O
file	O
format	O
for	O
Microsoft	O
's	O
hypervisor	B-Operating_System
(	O
virtual	B-Architecture
machine	I-Architecture
system	O
)	O
,	O
Hyper-V	O
.	O
</s>
<s>
The	O
format	O
was	O
created	O
by	O
Connectix	O
for	O
their	O
Virtual	B-Application
PC	I-Application
product	O
,	O
known	O
as	O
Microsoft	B-Application
Virtual	I-Application
PC	I-Application
since	O
Microsoft	O
acquired	O
Connectix	O
in	O
2003	O
.	O
</s>
<s>
A	O
Virtual	O
Hard	B-Device
Disk	I-Device
allows	O
multiple	O
operating	O
systems	O
to	O
reside	O
on	O
a	O
single	O
host	B-Operating_System
machine	I-Operating_System
.	O
</s>
<s>
This	O
method	O
enables	O
developers	O
to	O
test	O
software	O
on	O
different	O
operating	O
systems	O
without	O
the	O
cost	O
or	O
hassle	O
of	O
installing	O
a	O
second	O
hard	B-Device
disk	I-Device
or	O
partitioning	B-Application
a	O
single	O
hard	B-Device
disk	I-Device
into	O
multiple	O
volumes	O
.	O
</s>
<s>
The	O
ability	O
to	O
directly	O
modify	O
a	O
virtual	B-Architecture
machine	I-Architecture
's	O
hard	B-Device
disk	I-Device
from	O
a	O
host	O
server	O
supports	O
many	O
applications	O
,	O
including	O
:	O
</s>
<s>
VHDs	O
are	O
implemented	O
as	O
files	B-Operating_System
that	O
reside	O
on	O
the	O
native	O
host	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
following	O
types	O
of	O
VHD	O
formats	O
are	O
supported	O
by	O
Microsoft	B-Application
Virtual	I-Application
PC	I-Application
and	O
Virtual	B-General_Concept
Server	I-General_Concept
:	O
</s>
<s>
Fixed	O
hard	B-Application
disk	I-Application
image	I-Application
:	O
a	O
file	O
that	O
is	O
allocated	O
to	O
the	O
size	O
of	O
the	O
virtual	O
disk	O
.	O
</s>
<s>
Fixed	O
VHDs	O
consist	O
of	O
a	O
raw	B-Device
disk	I-Device
image	O
followed	O
by	O
a	O
VHD	O
footer	O
(	O
512	O
or	O
formerly	O
511	O
bytes	O
)	O
.	O
</s>
<s>
Dynamic	O
hard	B-Application
disk	I-Application
image	I-Application
:	O
a	O
file	O
that	O
at	O
any	O
given	O
time	O
is	O
as	O
large	O
as	O
the	O
actual	O
data	O
written	O
to	O
it	O
,	O
plus	O
the	O
size	O
of	O
the	O
header	O
and	O
footer	O
.	O
</s>
<s>
Differencing	O
hard	B-Application
disk	I-Application
image	I-Application
:	O
a	O
set	O
of	O
modified	O
blocks	O
(	O
maintained	O
in	O
a	O
separate	O
file	O
referred	O
to	O
as	O
the	O
"	O
child	O
image	O
"	O
)	O
in	O
comparison	O
to	O
a	O
parent	O
image	O
.	O
</s>
<s>
The	O
Differencing	O
hard	B-Application
disk	I-Application
image	I-Application
format	O
allows	O
the	O
concept	O
of	O
Undo	O
Changes	O
:	O
when	O
enabled	O
,	O
all	O
changes	O
to	O
a	O
hard	B-Device
drive	I-Device
contained	O
within	O
a	O
VHD	O
(	O
the	O
parent	O
image	O
)	O
are	O
stored	O
in	O
a	O
separate	O
file	O
(	O
the	O
child	O
image	O
)	O
.	O
</s>
<s>
Linked	O
to	O
a	O
hard	B-Device
disk	I-Device
(	O
aka	O
pass-through	O
)	O
:	O
a	O
file	O
that	O
contains	O
a	O
link	O
to	O
a	O
physical	O
hard	B-Device
drive	I-Device
or	O
partition	B-Application
of	O
a	O
physical	O
hard	B-Device
drive	I-Device
.	O
</s>
<s>
Significant	O
benefits	O
result	O
from	O
the	O
ability	O
to	O
boot	O
a	O
physical	O
computer	O
from	O
a	O
virtual	O
hard	B-Device
drive	I-Device
:	O
</s>
<s>
Backup-and-Restore	O
:	O
Changes	O
to	O
the	O
contents	O
of	O
a	O
VHD	O
(	O
such	O
as	O
infection	O
by	O
a	O
virus	O
,	O
or	O
accidental	O
deletion	O
of	O
critical	O
files	B-Operating_System
)	O
are	O
easily	O
undone	O
.	O
</s>
<s>
Native	O
VHD	O
Boot	O
refers	O
to	O
the	O
ability	O
of	O
a	O
physical	O
computer	O
to	O
mount	B-Application
and	O
boot	O
from	O
an	O
operating	O
system	O
contained	O
within	O
a	O
VHD	O
.	O
</s>
<s>
Windows	B-Device
7	I-Device
Enterprise	O
and	O
Ultimate	O
editions	O
support	O
this	O
ability	O
,	O
both	O
with	O
and	O
without	O
a	O
host	O
operating	O
system	O
present	O
.	O
</s>
<s>
Windows	B-Device
Server	I-Device
2008	I-Device
R2	I-Device
is	O
also	O
compatible	O
with	O
this	O
feature	O
.	O
</s>
<s>
Even	O
,	O
there	O
is	O
a	O
technique	O
by	O
which	O
VHD	O
can	O
be	O
indexed	O
in	O
the	O
BCD	O
entry	O
of	O
windows	O
and	O
it	O
is	O
a	O
constructive	O
way	O
to	O
install	O
and	O
run	O
Windows	O
on	O
VHD	O
without	O
any	O
third-party	O
Virtual	B-Operating_System
box	I-Operating_System
.	O
</s>
<s>
About	O
127	O
GiB	O
is	O
also	O
the	O
limit	O
for	O
VHDs	O
in	O
Windows	B-Application
Virtual	I-Application
PC	I-Application
.	O
</s>
<s>
The	O
specification	O
does	O
not	O
discuss	O
cases	O
where	O
the	O
CHS	O
value	O
in	O
the	O
VHD	O
footer	O
does	O
not	O
match	O
the	O
(	O
virtual	O
)	O
CHS	O
geometry	O
in	O
the	O
Master	B-Device
Boot	I-Device
Record	I-Device
of	O
the	O
disk	B-Application
image	I-Application
in	O
the	O
VHD	O
.	O
</s>
<s>
Microsoft	B-General_Concept
Virtual	I-General_Concept
Server	I-General_Concept
(	O
also	O
Connectix	O
derived	O
)	O
has	O
this	O
limitation	O
using	O
virtual	O
IDE	O
drivers	O
but	O
2	O
TiB	O
if	O
virtual	O
RAID	O
or	O
virtual	O
SCSI	O
drivers	O
are	O
used	O
.	O
</s>
<s>
Virtual	O
Hard	B-Device
Disk	I-Device
format	O
was	O
initially	O
used	O
only	O
by	O
Microsoft	B-Application
Virtual	I-Application
PC	I-Application
(	O
and	O
Microsoft	B-General_Concept
Virtual	I-General_Concept
Server	I-General_Concept
)	O
.	O
</s>
<s>
Later	O
however	O
,	O
Microsoft	O
used	O
the	O
VHD	O
format	O
in	O
Hyper-V	O
,	O
the	O
hypervisor-based	O
virtualization	O
technology	O
of	O
Windows	B-Device
Server	I-Device
2008	I-Device
.	O
</s>
<s>
Microsoft	O
also	O
used	O
the	O
format	O
in	O
Complete	O
PC	O
Backup	B-Application
,	O
a	O
backup	B-Application
software	I-Application
component	I-Application
included	O
with	O
Windows	B-Application
Vista	I-Application
and	O
Windows	B-Device
7	I-Device
.	O
</s>
<s>
In	O
addition	O
,	O
Windows	B-Device
7	I-Device
and	O
Windows	B-Device
Server	I-Device
2008	I-Device
R2	I-Device
include	O
support	O
for	O
creating	O
,	O
mounting	B-Application
,	O
and	O
booting	O
from	O
VHD	O
files	B-Operating_System
.	O
</s>
<s>
The	O
Vista	B-Application
(	O
or	O
later	O
)	O
drive	O
manager	O
GUI	O
supports	O
a	O
subset	O
of	O
the	O
functions	O
in	O
the	O
diskpart	O
command	O
line	O
tool	O
.	O
</s>
<s>
VHDs	O
known	O
as	O
vdisk	O
in	O
diskpart	O
can	O
be	O
created	O
,	O
formatted	O
,	O
attached	O
(	O
mounted	O
)	O
,	O
detached	O
(	O
unmounted	O
)	O
,	O
merged	O
(	O
for	O
differencing	O
VHDs	O
)	O
,	O
and	O
compacted	O
(	O
for	O
VHDs	O
on	O
an	O
NTFS	B-Application
host	O
file	B-Application
system	I-Application
)	O
.	O
</s>
<s>
The	O
virtual	B-Architecture
machine	I-Architecture
additions	O
in	O
older	O
VPC	B-Application
versions	O
and	O
the	O
virtual	B-Architecture
machine	I-Architecture
integration	O
features	O
in	O
Windows	B-Application
Virtual	I-Application
PC	I-Application
contain	O
precompact	O
ISO	O
images	O
for	O
the	O
first	O
step	O
in	O
supported	O
guest	O
systems	O
.	O
</s>
<s>
Oracle	B-Operating_System
VirtualBox	I-Operating_System
,	O
part	O
of	O
Sun	O
xVM	O
line	O
of	O
Sun	O
Microsystems	O
supports	O
VHD	O
in	O
versions	O
2	O
and	O
later	O
.	O
</s>
<s>
Hyper-V	O
features	O
offline	O
VHD	O
manipulation	O
,	O
providing	O
administrators	O
with	O
the	O
ability	O
to	O
securely	O
access	O
files	B-Operating_System
within	O
a	O
VHD	O
without	O
having	O
to	O
instantiate	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
The	O
Windows	O
Disk	O
Management	O
MMC	O
plugin	O
can	O
directly	O
mount	B-Application
a	O
VHD	O
file	O
as	O
a	O
drive	O
letter	O
in	O
Windows	O
7/Server	O
2008	O
and	O
newer	O
.	O
</s>
<s>
For	O
situations	O
where	O
mounting	B-Application
a	O
VHD	O
within	O
the	O
operating	O
system	O
is	O
undesirable	O
,	O
several	O
programs	O
enable	O
software	O
developers	O
to	O
inspect	O
and	O
modify	O
VHD	O
files	B-Operating_System
,	O
including	O
.NET	O
DiscUtils	O
,	O
WinImage	O
,	O
and	O
R1soft	O
Hyper-V	O
VHD	O
Explorer	O
.	O
</s>
<s>
7-Zip	B-Language
supports	O
extraction	O
and	O
inspection	O
of	O
VHD	O
files	B-Operating_System
.	O
</s>
<s>
Virtual	B-General_Concept
Floppy	I-General_Concept
Disk	I-General_Concept
(	O
VFD	O
)	O
is	O
a	O
related	O
file	O
format	O
used	O
by	O
Microsoft	B-Application
Virtual	I-Application
PC	I-Application
,	O
Microsoft	O
Automated	O
Deployment	O
Services	O
and	O
Microsoft	B-General_Concept
Virtual	I-General_Concept
Server	I-General_Concept
2005	O
.	O
</s>
<s>
A	O
VFD	O
that	O
contains	O
an	O
image	O
of	O
a	O
720KB	O
low-density	O
,	O
1.44MB	O
high-density	O
or	O
1.68MB	O
DMF	B-Device
3.5-inch	O
floppy	B-Device
disk	I-Device
can	O
be	O
mounted	O
by	O
Virtual	B-Application
PC	I-Application
.	O
</s>
<s>
Other	O
virtual	B-Architecture
machine	I-Architecture
software	O
such	O
as	O
VMWare	B-Operating_System
Workstation	I-Operating_System
and	O
VMware	B-Operating_System
Player	I-Operating_System
can	O
mount	B-Application
raw	B-Device
floppy	B-Application
images	I-Application
in	O
the	O
same	O
way	O
.	O
</s>
<s>
Windows	B-Application
Virtual	I-Application
PC	I-Application
for	O
Windows	B-Device
7	I-Device
(	O
version	O
6.1	O
)	O
does	O
not	O
offer	O
a	O
user	O
interface	O
for	O
manipulating	O
virtual	B-General_Concept
floppy	I-General_Concept
disks	I-General_Concept
;	O
however	O
,	O
it	O
still	O
supports	O
physical	O
and	O
virtual	B-General_Concept
floppy	I-General_Concept
disks	I-General_Concept
via	O
scripting	B-Language
.	O
</s>
<s>
Under	O
Hyper-V	O
,	O
VFD	O
files	B-Operating_System
are	O
usable	O
through	O
the	O
VM	O
settings	O
for	O
Generation	O
1	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
Generation	O
2	O
virtual	B-Architecture
machines	I-Architecture
do	O
not	O
emulate	O
a	O
floppy	B-Device
controller	O
and	O
do	O
not	O
support	O
floppy	B-Device
disk	I-Device
images	O
.	O
</s>
<s>
VHDX	O
(	O
Virtual	O
Hard	B-Device
Disk	I-Device
v2	O
)	O
is	O
the	O
successor	O
format	O
to	O
VHD	O
.	O
</s>
<s>
For	O
disk	B-Application
images	I-Application
with	O
this	O
newer	O
format	O
the	O
filename	O
extension	O
vhdx	O
is	O
used	O
instead	O
of	O
vhd	O
.	O
</s>
