<s>
In	O
computing	O
on	O
Microsoft	O
platforms	O
,	O
WoW64	B-Application
(	O
Windows	B-Application
32-bit	I-Application
on	I-Application
Windows	I-Application
64-bit	I-Application
)	O
is	O
a	O
subsystem	O
of	O
the	O
Windows	B-Application
operating	I-Application
system	I-Application
capable	O
of	O
running	O
32-bit	O
applications	O
on	O
64-bit	B-Device
Windows	B-Application
.	O
</s>
<s>
It	O
is	O
included	O
in	O
all	O
64-bit	B-Device
versions	O
of	O
Windows	B-Application
—	O
including	O
Windows	B-Operating_System
XP	I-Operating_System
Professional	I-Operating_System
x64	I-Operating_System
Edition	I-Operating_System
,	O
IA-64	B-General_Concept
and	O
x64	B-Device
versions	O
of	O
Windows	B-Application
Server	I-Application
2003	I-Application
,	O
as	O
well	O
as	O
x64	B-Device
versions	O
of	O
Windows	B-Application
Vista	I-Application
,	O
Windows	B-Device
Server	I-Device
2008	I-Device
,	O
Windows	B-Device
7	I-Device
,	O
Windows	B-Application
8	I-Application
,	O
Windows	B-Device
Server	I-Device
2012	I-Device
,	O
Windows	B-Device
8.1	I-Device
,	O
Windows	B-Operating_System
10	I-Operating_System
,	O
Windows	B-Device
Server	I-Device
2016	I-Device
,	O
Windows	B-Device
Server	I-Device
2019	I-Device
,	O
Windows	B-Application
11	I-Application
,	O
Windows	B-Device
Server	I-Device
2022	I-Device
,	O
and	O
Wine	B-Application
.	O
</s>
<s>
as	O
well	O
as	O
ARM64	B-Architecture
versions	O
of	O
Windows	B-Operating_System
10	I-Operating_System
,	O
Windows	B-Application
11	I-Application
and	O
Windows	B-Device
Server	I-Device
2022	I-Device
,	O
except	O
in	O
Windows	B-Device
Server	I-Device
Server	O
Core	O
where	O
it	O
is	O
an	O
optional	O
component	O
,	O
and	O
Windows	B-Application
Nano	O
Server	O
where	O
it	O
is	O
not	O
included	O
.	O
</s>
<s>
WoW64	B-Application
aims	O
to	O
take	O
care	O
of	O
many	O
of	O
the	O
differences	O
between	O
32-bit	B-Application
Windows	I-Application
and	O
64-bit	B-Device
Windows	B-Application
,	O
particularly	O
involving	O
structural	O
changes	O
to	O
Windows	B-Application
itself	O
.	O
</s>
<s>
The	O
WoW64	B-Application
subsystem	O
comprises	O
a	O
lightweight	O
compatibility	B-General_Concept
layer	I-General_Concept
that	O
has	O
similar	O
interfaces	O
on	O
all	O
64-bit	B-Device
versions	O
of	O
Windows	B-Application
.	O
</s>
<s>
It	O
aims	O
to	O
create	O
a	O
32-bit	O
environment	O
that	O
provides	O
the	O
interfaces	O
required	O
to	O
run	O
unmodified	O
32-bit	B-Application
Windows	I-Application
applications	O
on	O
a	O
64-bit	B-Device
system	O
.	O
</s>
<s>
WOW64	B-Application
is	O
implemented	O
using	O
several	O
DLLs	O
,	O
some	O
of	O
which	O
include	O
:	O
</s>
<s>
Wow64cpu.dll	O
,	O
which	O
takes	O
care	O
of	O
switching	O
the	O
processor	O
from	O
32-bit	O
to	O
64-bit	B-Device
mode	O
.	O
</s>
<s>
This	O
is	O
used	O
in	O
x86-64	B-Device
implementations	O
of	O
Windows	B-Application
only	O
.	O
</s>
<s>
Other	O
DLLs	O
and	O
binaries	O
are	O
included	O
for	O
Itanium	B-General_Concept
and	O
ARMv8	O
64-bit	B-Device
architectures	I-Device
to	O
provide	O
emulation	O
to	O
x86	B-Operating_System
or	O
for	O
32-bit	O
entry	O
points	O
if	O
the	O
architecture	O
has	O
a	O
native	O
32-bit	O
operating	O
mode	O
.	O
</s>
<s>
Despite	O
its	O
outwardly	O
similar	O
appearance	O
on	O
all	O
versions	O
of	O
64-bit	B-Device
Windows	B-Application
,	O
WoW64	B-Application
's	O
implementation	O
varies	O
depending	O
on	O
the	O
target	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
version	O
of	O
64-bit	B-Device
Windows	B-Application
developed	O
for	O
the	O
Intel	O
Itanium	B-General_Concept
2	O
processor	O
(	O
known	O
as	O
the	O
IA-64	B-General_Concept
architecture	O
)	O
uses	O
Wow64win.dll	O
to	O
set	O
up	O
the	O
emulation	O
of	O
x86	B-Operating_System
instructions	O
within	O
the	O
Itanium2	O
's	O
unique	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
This	O
emulation	O
is	O
a	O
much	O
more	O
computationally	O
expensive	O
task	O
than	O
the	O
Wow64win.dll	O
'	O
s	O
functions	O
on	O
the	O
x86-64	B-Device
architecture	O
,	O
which	O
switches	O
the	O
processor	O
hardware	O
from	O
its	O
64-bit	B-Device
mode	O
to	O
compatibility	O
mode	O
when	O
it	O
becomes	O
necessary	O
to	O
execute	O
a	O
32-bit	O
thread	B-Operating_System
,	O
and	O
then	O
handles	O
the	O
switch	O
back	O
to	O
64-bit	B-Device
mode	O
.	O
</s>
<s>
The	O
WoW64	B-Application
subsystem	O
also	O
handles	O
other	O
key	O
aspects	O
of	O
running	O
32-bit	O
applications	O
.	O
</s>
<s>
It	O
is	O
involved	O
in	O
managing	O
the	O
interaction	O
of	O
32-bit	O
applications	O
with	O
the	O
Windows	B-Application
components	O
such	O
as	O
the	O
Registry	B-Protocol
,	O
which	O
has	O
distinct	O
keys	O
for	O
64-bit	B-Device
and	O
32-bit	O
applications	O
.	O
</s>
<s>
Some	O
Registry	B-Protocol
keys	I-Protocol
are	O
mapped	O
from	O
64-bit	B-Device
to	O
their	O
32-bit	O
equivalents	O
,	O
while	O
others	O
have	O
their	O
contents	O
mirrored	O
,	O
depending	O
on	O
the	O
edition	O
of	O
Windows	B-Application
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
uses	O
the	O
%SystemRoot%	O
\system32	O
directory	O
for	O
its	O
64-bit	B-Device
library	O
and	O
executable	O
files	O
.	O
</s>
<s>
This	O
is	O
done	O
for	O
backward	O
compatibility	O
reasons	O
,	O
as	O
many	O
legacy	B-Device
applications	I-Device
are	O
hardcoded	B-Application
to	O
use	O
that	O
path	O
.	O
</s>
<s>
When	O
executing	O
32-bit	O
applications	O
,	O
WoW64	B-Application
transparently	O
redirects	O
32-bit	O
DLLs	O
to	O
%SystemRoot%	O
\SysWoW64,	O
which	O
contains	O
32-bit	O
libraries	O
and	O
executables	O
.	O
</s>
<s>
32-bit	O
applications	O
are	O
generally	O
not	O
aware	O
that	O
they	O
are	O
running	O
on	O
a	O
64-bit	B-Device
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
There	O
are	O
two	O
Program	B-Application
Files	I-Application
directories	O
each	O
visible	O
to	O
both	O
32-bit	O
and	O
64-bit	B-Device
applications	O
.	O
</s>
<s>
The	O
directory	O
that	O
stores	O
the	O
32	O
bit	O
files	O
is	O
called	O
Program	B-Application
Files	I-Application
(	O
x86	B-Operating_System
)	O
to	O
differentiate	O
between	O
the	O
two	O
,	O
while	O
the	O
64	B-Device
bit	I-Device
maintains	O
the	O
traditional	O
Program	B-Application
Files	I-Application
name	O
without	O
any	O
additional	O
qualifier	O
.	O
</s>
<s>
32-bit	O
applications	O
that	O
include	O
only	O
32-bit	O
kernel-mode	O
device	B-Application
drivers	I-Application
,	O
or	O
that	O
plug	O
into	O
the	O
process	O
space	O
of	O
components	O
that	O
are	O
implemented	O
purely	O
as	O
64-bit	B-Device
processes	O
(	O
e.g.	O
</s>
<s>
Windows	B-Application
Explorer	O
)	O
cannot	O
be	O
executed	O
on	O
a	O
64-bit	B-Device
platform	O
.	O
</s>
<s>
The	O
SysWOW64	B-Application
folder	O
located	O
in	O
the	O
Windows	B-Application
folder	O
on	O
the	O
OS	O
drive	O
contains	O
several	O
applications	O
to	O
support	O
32-bit	O
applications	O
(	O
e.g.	O
</s>
<s>
cmd.exe	O
,	O
odbcad32.exe	O
,	O
to	O
register	O
ODBC	B-Application
connections	O
for	O
32-bit	O
applications	O
)	O
.	O
</s>
<s>
16-bit	O
legacy	B-Device
applications	I-Device
for	O
MS-DOS	O
and	O
early	O
versions	O
of	O
Windows	B-Application
are	O
usually	O
incompatible	O
with	O
64-bit	B-Device
versions	O
of	O
Windows	B-Application
Vista	I-Application
,	O
7	O
,	O
8	O
,	O
and	O
10	O
,	O
but	O
can	O
be	O
run	O
on	O
a	O
64-bit	B-Device
Windows	B-Application
OS	I-Application
via	O
virtualization	O
software	O
.	O
</s>
<s>
32-bit	O
versions	O
of	O
Windows	B-Application
XP	O
,	O
Vista	B-Application
,	O
7	O
,	O
8	O
,	O
and	O
10	O
on	O
the	O
other	O
hand	O
,	O
can	O
usually	O
run	O
16-bit	O
applications	O
with	O
few	O
to	O
no	O
problems	O
.	O
</s>
<s>
16-bit	O
applications	O
cannot	O
be	O
directly	O
run	O
under	O
x64	B-Device
editions	I-Device
of	O
Windows	B-Application
,	O
because	O
the	O
CPU	O
does	O
not	O
support	O
VM86	B-Application
mode	I-Application
when	O
running	O
in	O
x64	B-Device
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
is	O
implemented	O
as	O
both	O
a	O
32-bit	O
and	O
a	O
64-bit	B-Device
application	O
because	O
of	O
the	O
large	O
number	O
of	O
32-bit	O
ActiveX	B-Application
components	I-Application
on	O
the	O
Internet	O
that	O
would	O
not	O
be	O
able	O
to	O
plug	O
into	O
the	O
64-bit	B-Device
version	O
.	O
</s>
<s>
Previously	O
,	O
the	O
32-bit	O
version	O
was	O
used	O
by-default	O
and	O
it	O
was	O
difficult	O
to	O
set	O
the	O
64-bit	B-Device
version	O
to	O
be	O
the	O
default	O
browser	O
.	O
</s>
<s>
This	O
changed	O
in	O
Internet	B-Application
Explorer	I-Application
10	O
,	O
which	O
ran	O
32-bit	O
add-ons	O
inside	O
a	O
64-bit	B-Device
session	O
,	O
eliminating	O
the	O
need	O
to	O
switch	O
between	O
the	O
two	O
versions	O
.	O
</s>
<s>
If	O
a	O
user	O
was	O
to	O
go	O
into	O
the	O
32-bit	O
folder	O
(	O
typically	O
C:\Program	O
Files	O
(	O
x86	B-Operating_System
)	O
\Internet	O
Explorer	O
)	O
and	O
double-click	O
the	O
iexplore.exe	B-Application
file	O
there	O
,	O
the	O
64-bit	B-Device
version	O
will	O
still	O
load	O
.	O
</s>
<s>
In	O
Internet	B-Application
Explorer	I-Application
9	O
and	O
previous	O
,	O
this	O
would	O
load	O
only	O
the	O
32-bit	O
version	O
.	O
</s>
<s>
,	O
a	O
bug	O
in	O
the	O
translation	O
layer	O
of	O
the	O
x64	B-Device
version	O
of	O
WoW64	B-Application
also	O
renders	O
all	O
32-bit	O
applications	O
that	O
rely	O
on	O
the	O
Windows	B-Library
API	I-Library
function	O
GetThreadContext	O
incompatible	O
.	O
</s>
<s>
Such	O
applications	O
include	O
application	O
debuggers	O
,	O
call	B-General_Concept
stack	I-General_Concept
tracers	O
(	O
e.g.	O
</s>
<s>
IDEs	O
displaying	O
call	B-General_Concept
stack	I-General_Concept
)	O
and	O
applications	O
that	O
use	O
garbage	B-General_Concept
collection	I-General_Concept
(	O
GC	O
)	O
engines	O
.	O
</s>
<s>
One	O
of	O
the	O
more	O
widely	O
used	O
but	O
affected	O
GC	O
engines	O
is	O
the	O
Boehm	B-Language
GC	I-Language
.	O
</s>
<s>
It	O
is	O
also	O
used	O
as	O
the	O
default	O
garbage	B-General_Concept
collector	I-General_Concept
of	O
the	O
equally	O
popular	O
Mono	B-Application
.	O
</s>
<s>
While	O
Mono	B-Application
has	O
introduced	O
a	O
new	O
(	O
but	O
optional	O
)	O
GC	O
as	O
of	O
October	O
2010	O
called	O
SGen-GC	O
,	O
it	O
performs	O
stack	O
scanning	O
in	O
the	O
same	O
manner	O
as	O
Boehm	B-Language
GC	I-Language
,	O
thus	O
also	O
making	O
it	O
incompatible	O
under	O
WoW64	B-Application
.	O
</s>
<s>
According	O
to	O
Microsoft	O
,	O
32-bit	O
software	O
running	O
under	O
WOW64	B-Application
has	O
similar	O
performance	O
to	O
executing	O
under	O
32-bit	B-Application
Windows	I-Application
,	O
but	O
with	O
fewer	O
threads	B-Operating_System
possible	O
and	O
other	O
overheads	O
.	O
</s>
<s>
A	O
32-bit	O
application	O
can	O
be	O
given	O
a	O
full	O
4gigabytes	O
of	O
virtual	B-Architecture
memory	I-Architecture
on	O
a	O
64-bit	B-Device
system	O
,	O
whereas	O
on	O
a	O
32-bit	O
system	O
,	O
some	O
of	O
this	O
addressable	O
memory	O
is	O
lost	O
because	O
it	O
is	O
used	O
by	O
the	O
kernel	B-Operating_System
and	O
memory-mapped	O
peripherals	O
such	O
as	O
the	O
display	B-Device
adaptor	I-Device
,	O
typically	O
resulting	O
in	O
apps	O
being	O
able	O
to	O
use	O
either	O
2GB	O
or	O
3GB	O
of	O
RAM	O
at	O
most	O
.	O
</s>
