<s>
In	O
computing	O
,	O
Windows	B-Application
on	I-Application
Windows	I-Application
(	O
commonly	O
referred	O
to	O
as	O
WOW	O
)	O
,	O
was	O
a	O
compatibility	B-General_Concept
layer	I-General_Concept
of	O
32-bit	O
versions	O
of	O
the	O
Windows	B-Device
NT	I-Device
family	I-Device
of	O
operating	B-General_Concept
systems	I-General_Concept
since	O
1993	O
with	O
the	O
release	O
of	O
Windows	B-Device
NT	I-Device
3.1	I-Device
,	O
which	O
extends	O
NTVDM	O
to	O
provide	O
limited	O
support	O
for	O
running	O
legacy	B-Device
16-bit	B-Device
programs	O
written	O
for	O
Windows	O
3.x	O
or	O
earlier	O
.	O
</s>
<s>
There	O
is	O
a	O
similar	O
subsystem	O
,	O
known	O
as	O
WoW64	B-Application
,	O
on	O
64-bit	O
Windows	O
versions	O
that	O
runs	O
32-bit	O
programs	O
.	O
</s>
<s>
This	O
subsystem	O
is	O
not	O
available	O
in	O
64-bit	O
editions	O
prior	O
to	O
Windows	B-Application
11	I-Application
(	O
including	O
Windows	B-Device
Server	I-Device
2008	I-Device
R2	I-Device
and	O
later	O
,	O
which	O
only	O
have	O
64-bit	O
editions	O
)	O
and	O
therefore	O
cannot	O
run	O
16-bit	B-Device
software	O
without	O
third-party	O
emulation	O
software	O
(	O
e.g.	O
</s>
<s>
DOSBox	B-Application
)	O
.	O
</s>
<s>
Windows	B-Operating_System
10	I-Operating_System
is	O
the	O
final	O
version	O
of	O
Windows	O
to	O
include	O
this	O
subsystem	O
.	O
</s>
<s>
This	O
subsystem	O
has	O
since	O
been	O
discontinued	O
,	O
as	O
Windows	B-Application
11	I-Application
dropped	O
support	O
for	O
32-bit	O
processors	O
.	O
</s>
<s>
Many	O
16-bit	B-Device
Windows	O
legacy	B-Device
programs	O
can	O
run	O
without	O
changes	O
on	O
newer	O
32-bit	O
editions	O
of	O
Windows	O
.	O
</s>
<s>
The	O
reason	O
designers	O
made	O
this	O
possible	O
was	O
to	O
allow	O
software	O
developers	O
time	O
to	O
remedy	O
their	O
software	O
during	O
the	O
industry	O
transition	O
from	O
Windows	B-Application
3.1x	I-Application
to	O
Windows	B-Application
95	I-Application
and	O
later	O
,	O
without	O
restricting	O
the	O
ability	O
for	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
upgraded	O
to	O
a	O
current	O
version	O
before	O
all	O
programs	O
used	O
by	O
a	O
customer	O
had	O
been	O
taken	O
care	O
of	O
.	O
</s>
<s>
The	O
Windows	O
9x	O
series	O
of	O
operating	B-General_Concept
systems	I-General_Concept
,	O
reflecting	O
their	O
roots	O
in	O
DOS	B-Device
,	O
functioned	O
as	O
hybrid	O
16	O
-	O
and	O
32-bit	O
systems	O
in	O
the	O
sense	O
that	O
the	O
underlying	O
operating	B-General_Concept
system	I-General_Concept
was	O
not	O
truly	O
32-bit	O
,	O
and	O
therefore	O
could	O
run	O
16-bit	B-Device
software	O
natively	O
without	O
requiring	O
any	O
special	O
emulation	O
;	O
Windows	B-Device
NT	I-Device
operating	B-General_Concept
systems	I-General_Concept
differ	O
significantly	O
from	O
Windows	O
9x	O
in	O
their	O
architecture	O
,	O
and	O
therefore	O
require	O
a	O
more	O
complex	O
solution	O
.	O
</s>
<s>
Two	O
separate	O
strategies	O
are	O
used	O
in	O
order	O
to	O
let	O
16-bit	B-Device
programs	O
run	O
on	O
32-bit	O
versions	O
of	O
Windows	O
(	O
with	O
some	O
runtime	O
limitations	O
)	O
.	O
</s>
<s>
They	O
are	O
called	O
thunking	B-Application
and	O
shimming	B-General_Concept
.	O
</s>
<s>
The	O
WOW	O
subsystem	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
in	O
order	O
to	O
provide	O
support	O
for	O
16-bit	B-Device
pointers	O
,	O
memory	O
models	O
and	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
All	O
16-bit	B-Device
programs	O
run	O
by	O
default	O
in	O
a	O
single	O
virtual	O
DOS	B-Device
machine	O
with	O
shared	O
memory	O
space	O
.	O
</s>
<s>
However	O
,	O
they	O
can	O
be	O
configured	O
to	O
run	O
in	O
their	O
own	O
separate	O
memory	O
space	O
,	O
in	O
which	O
case	O
each	O
16-bit	B-Device
process	O
has	O
its	O
own	O
dedicated	O
virtual	O
machine	O
.	O
</s>
<s>
The	O
separate	O
memory	O
space	O
increases	O
system	O
stability	O
by	O
preventing	O
buggy	O
16-bit	B-Device
programs	O
from	O
interfering	O
with	O
one	O
another	O
,	O
at	O
the	O
expense	O
of	O
reduced	O
16-bit	B-Device
inter-process	B-Operating_System
communication	I-Operating_System
and	O
increased	O
memory	O
utilization	O
.	O
</s>
<s>
The	O
WOWEXEC.EXE	O
process	O
on	O
a	O
Windows	B-Device
NT	I-Device
system	O
facilitates	O
Windows-on-Windows	O
.	O
</s>
<s>
In	O
addition	O
to	O
Windows-on-Windows	O
emulating	O
the	O
Windows	B-Application
95	I-Application
and	O
Windows	B-Device
98	I-Device
kernels	O
,	O
the	O
WIN.COM	O
file	O
emulates	O
a	O
Windows	O
3.x	O
kernel	O
for	O
NTVDM	O
,	O
which	O
runs	O
the	O
16-bit	B-Device
DOS-based	O
Windows	B-Application
applications	I-Application
on	O
Windows	B-Device
NT	I-Device
.	O
</s>
<s>
For	O
example	O
,	O
they	O
may	O
incorrectly	O
assume	O
full	O
write	O
access	O
to	O
the	O
whole	O
file	O
system	O
whereas	O
NTFS	B-Application
security	O
is	O
in	O
place	O
.	O
</s>
<s>
When	O
the	O
Windows	B-Application
95	I-Application
line	O
of	O
operating	B-General_Concept
systems	I-General_Concept
was	O
designed	O
,	O
a	O
key	O
requirement	O
was	O
for	O
the	O
file	O
system	O
to	O
keep	O
backward	O
compatibility	O
with	O
8.3	O
filenames	O
to	O
allow	O
legacy	B-Device
applications	I-Device
to	O
continue	O
to	O
work	O
on	O
the	O
platform	O
.	O
</s>
<s>
Windows	B-Application
95	I-Application
and	O
later	O
operating	B-General_Concept
systems	I-General_Concept
therefore	O
support	O
a	O
compatibility	O
mode	O
whereby	O
both	O
a	O
long	O
filename	O
and	O
a	O
short	O
filename	O
are	O
stored	O
in	O
the	O
File	B-Application
Allocation	I-Application
Table	I-Application
.	O
</s>
<s>
Furthermore	O
,	O
legacy	B-Device
applications	I-Device
that	O
attempt	O
to	O
access	O
hardware	O
directly	O
cannot	O
do	O
so	O
in	O
user	B-Operating_System
mode	I-Operating_System
.	O
</s>
<s>
Legacy	B-Device
applications	I-Device
may	O
also	O
fail	O
if	O
system	O
configuration	O
files	O
from	O
the	O
DOS	B-Device
and	O
Windows	O
9x	O
era	O
are	O
not	O
present	O
in	O
Windows	B-Device
NT	I-Device
based	O
kernels	O
,	O
hence	O
the	O
reason	O
for	O
zero-length	O
versions	O
of	O
files	O
like	O
AUTOEXEC.BAT	O
and	O
CONFIG.SYS	B-Device
having	O
to	O
be	O
carried	O
forward	O
on	O
operating	B-General_Concept
systems	I-General_Concept
that	O
do	O
not	O
use	O
them	O
.	O
</s>
<s>
A	O
considerable	O
number	O
of	O
shims	B-General_Concept
are	O
present	O
in	O
the	O
application	B-General_Concept
compatibility	I-General_Concept
layer	I-General_Concept
of	O
later	O
versions	O
of	O
Windows	O
to	O
intercept	O
and	O
modify	O
API	B-Application
calls	O
made	O
by	O
legacy	B-Device
applications	I-Device
that	O
were	O
written	O
with	O
a	O
different	O
set	O
of	O
assumptions	O
and	O
operating	B-General_Concept
system	I-General_Concept
best	O
practices	O
in	O
mind	O
.	O
</s>
<s>
These	O
fixes	O
are	O
updated	O
from	O
time-to-time	O
as	O
issues	O
are	O
discovered	O
in	O
popular	O
legacy	B-Device
applications	I-Device
that	O
are	O
still	O
in	O
use	O
.	O
</s>
