<s>
VxD	B-Device
is	O
the	O
device	B-Application
driver	I-Application
model	O
used	O
in	O
Microsoft	O
Windows/386	O
2.x	O
,	O
the	O
386	O
enhanced	O
mode	O
of	O
Windows	B-Application
3.x	I-Application
,	O
Windows	B-Application
9x	O
,	O
and	O
to	O
some	O
extent	O
also	O
by	O
the	O
Novell	O
DOS	B-Device
7	O
,	O
OpenDOS	O
7.01	O
,	O
and	O
DR-DOS	O
7.02	O
(	O
and	O
higher	O
)	O
multitasker	O
(	O
TASKMGR	O
)	O
.	O
</s>
<s>
VxDs	B-Device
have	O
access	O
to	O
the	O
memory	O
of	O
the	O
kernel	B-Operating_System
and	O
all	O
running	O
processes	O
,	O
as	O
well	O
as	O
raw	O
access	O
to	O
the	O
hardware	O
.	O
</s>
<s>
Starting	O
with	O
Windows	B-Device
98	I-Device
,	O
Windows	B-Application
Driver	I-Application
Model	I-Application
was	O
the	O
recommended	O
driver	O
model	O
to	O
write	O
drivers	O
for	O
,	O
with	O
the	O
VxD	B-Device
driver	O
model	O
still	O
being	O
supported	O
for	O
backward	O
compatibility	O
,	O
until	O
Windows	B-Device
Me	I-Device
.	O
</s>
<s>
The	O
name	O
"	O
VxD	B-Device
"	O
is	O
an	O
abbreviation	O
for	O
"	O
virtual	O
xxx	O
driver	O
"	O
,	O
where	O
"	O
xxx	O
"	O
is	O
some	O
class	O
of	O
hardware	O
device	O
.	O
</s>
<s>
It	O
derives	O
from	O
the	O
fact	O
that	O
most	O
drivers	O
had	O
filenames	O
of	O
the	O
form	O
vxxxd.386	O
in	O
Windows	B-Application
3.x	I-Application
.	O
</s>
<s>
VxDs	B-Device
under	O
Windows	B-Application
3.x	I-Application
usually	O
have	O
the	O
filename	O
extension	O
.386	B-Application
,	O
while	O
those	O
under	O
Windows	B-Application
9x	O
have	O
.vxd	B-Application
.	O
</s>
<s>
VxDs	B-Device
written	O
for	O
Windows	B-Application
3.x	I-Application
can	O
be	O
used	O
under	O
Windows	B-Application
9x	O
but	O
not	O
vice	O
versa	O
.	O
</s>
<s>
Prior	O
to	O
the	O
advent	O
of	O
Windows	B-Application
,	O
DOS	B-Device
applications	O
would	O
either	O
communicate	O
directly	O
with	O
the	O
various	O
pieces	O
of	O
hardware	O
(	O
responding	O
to	O
interrupts	O
,	O
reading	O
and	O
writing	O
device	O
memory	O
etc	O
.	O
)	O
</s>
<s>
or	O
go	O
through	O
a	O
DOS	B-Device
device	B-Application
driver	I-Application
.	O
</s>
<s>
As	O
DOS	B-Device
was	O
not	O
multitasking	O
,	O
each	O
application	O
would	O
have	O
exclusive	O
and	O
complete	O
control	O
over	O
the	O
hardware	O
while	O
running	O
.	O
</s>
<s>
Though	O
Windows	B-Application
applications	I-Application
do	O
n't	O
often	O
communicate	O
directly	O
with	O
hardware	O
,	O
it	O
was	O
the	O
only	O
way	O
for	O
Windows	B-Application
drivers	O
;	O
and	O
still	O
is	O
in	O
the	O
real	O
and	O
standard	O
modes	O
of	O
Windows	B-Application
3.x	I-Application
.	O
</s>
<s>
Windows/386	O
and	O
onward	O
allowed	O
multiple	O
DOS	B-Device
applications	O
to	O
execute	O
concurrently	O
by	O
executing	O
each	O
within	O
its	O
own	O
virtual	O
machine	O
.	O
</s>
<s>
To	O
share	O
physical	O
resources	O
among	O
these	O
virtual	O
machines	O
,	O
Microsoft	O
introduced	O
virtual	O
device	B-Application
drivers	I-Application
.	O
</s>
<s>
Instead	O
of	O
a	O
machine	B-Protocol
port	I-Protocol
representing	O
an	O
actual	O
device	O
,	O
it	O
would	O
represent	O
a	O
"	O
virtual	O
"	O
device	O
,	O
which	O
could	O
be	O
managed	O
by	O
the	O
operating	O
system	O
.	O
</s>
<s>
Although	O
Windows	B-Device
98	I-Device
introduced	O
the	O
Windows	B-Application
Driver	I-Application
Model	I-Application
(	O
WDM	O
)	O
,	O
VxD	B-Device
device	B-Application
drivers	I-Application
can	O
be	O
used	O
under	O
Windows	B-Device
98	I-Device
and	O
Windows	B-Device
Me	I-Device
.	O
</s>
<s>
Using	O
VxD	B-Device
drivers	O
instead	O
of	O
WDM	O
drivers	O
in	O
Windows	B-Application
9x	O
resulted	O
in	O
advanced	O
ACPI	B-Device
states	O
like	O
hibernation	O
being	O
unavailable	O
.	O
</s>
<s>
VxDs	B-Device
are	O
not	O
usable	O
in	O
Windows	B-Device
NT	I-Device
or	O
its	O
descendants	O
.	O
</s>
<s>
Windows	O
NT-based	O
operating	O
systems	O
from	O
3.1	B-Device
to	O
4.0	B-Device
must	O
use	O
drivers	O
written	O
specifically	O
for	O
them	O
,	O
otherwise	O
known	O
as	O
the	O
Windows	B-Device
NT	I-Device
Driver	O
Model	O
.	O
</s>
<s>
Starting	O
with	O
Windows	B-Application
2000	I-Application
,	O
Windows	O
NT-based	O
operating	O
systems	O
have	O
adopted	O
the	O
Windows	B-Application
Driver	I-Application
Model	I-Application
from	O
Windows	B-Device
98	I-Device
.	O
</s>
<s>
VxDs	B-Device
should	O
not	O
be	O
confused	O
with	O
the	O
similarly	O
named	O
NTVDM-specific	O
'	O
VDDs	O
 '	O
(	O
Virtual	O
Device	B-Application
Drivers	I-Application
)	O
,	O
which	O
provide	O
a	O
method	O
of	O
emulating	O
direct	O
I/O	O
under	O
a	O
Windows	B-Device
NT	I-Device
"	O
DOS	B-Device
Box	O
"	O
.	O
</s>
