<s>
In	O
software	B-General_Concept
engineering	I-General_Concept
,	O
a	O
compatibility	B-General_Concept
layer	I-General_Concept
is	O
an	O
interface	O
that	O
allows	O
binaries	O
for	O
a	O
legacy	O
or	O
foreign	O
system	O
to	O
run	O
on	O
a	O
host	O
system	O
.	O
</s>
<s>
This	O
translates	O
system	B-Operating_System
calls	I-Operating_System
for	O
the	O
foreign	O
system	O
into	O
native	O
system	B-Operating_System
calls	I-Operating_System
for	O
the	O
host	O
system	O
.	O
</s>
<s>
With	O
some	O
libraries	B-Library
for	O
the	O
foreign	O
system	O
,	O
this	O
will	O
often	O
be	O
sufficient	O
to	O
run	O
foreign	O
binaries	O
on	O
the	O
host	O
system	O
.	O
</s>
<s>
A	O
hardware	O
compatibility	B-General_Concept
layer	I-General_Concept
consists	O
of	O
tools	O
that	O
allow	O
hardware	O
emulation	B-Application
.	O
</s>
<s>
Windows	B-Application
's	O
application	O
compatibility	B-General_Concept
layers	I-General_Concept
to	O
attempt	O
to	O
run	O
poorly	O
written	O
applications	O
or	O
those	O
written	O
for	O
earlier	O
versions	O
of	O
the	O
platform	O
.	O
</s>
<s>
Lina	B-Language
,	O
which	O
runs	O
some	O
Linux	B-Application
binaries	O
on	O
Windows	B-Application
,	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
and	O
Unix-like	B-Operating_System
systems	I-Operating_System
with	O
native	O
look	O
and	O
feel	O
.	O
</s>
<s>
KernelEX	O
,	O
which	O
runs	O
some	O
Windows	B-Application
2000/XP	O
programs	O
on	O
Windows	B-Application
98/Me	O
.	O
</s>
<s>
Executor	B-Operating_System
,	O
which	O
runs	O
68k-based	O
"	O
classic	O
"	O
Mac	O
OS	O
programs	O
in	O
Windows	B-Application
,	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
and	O
Linux	B-Application
.	O
</s>
<s>
Anbox	B-Application
,	O
an	O
Android	B-Application
compatibility	B-General_Concept
layer	I-General_Concept
for	O
Linux	B-Application
.	O
</s>
<s>
Hybris	B-Application
,	O
library	O
that	O
translates	O
Bionic	O
into	O
glibc	O
calls	O
.	O
</s>
<s>
Darling	B-Language
,	O
a	O
translation	O
layer	O
that	O
attempts	O
to	O
run	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
and	O
Darwin	O
binaries	O
on	O
Linux	B-Application
.	O
</s>
<s>
Windows	B-Application
Subsystem	O
for	O
Linux	B-Application
v1	O
,	O
which	O
runs	O
Linux	B-Application
binaries	O
on	O
Windows	B-Application
via	O
a	O
compatibility	B-General_Concept
layer	I-General_Concept
which	O
translates	O
Linux	B-Application
system	B-Operating_System
calls	I-Operating_System
into	O
native	O
windows	B-Application
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
Cygwin	B-Language
,	O
a	O
POSIX-compatible	O
environment	O
that	O
runs	O
natively	O
on	O
Windows	B-Application
.	O
</s>
<s>
Rosetta	O
2	O
,	O
Apple	O
's	O
translation	O
layer	O
bundled	O
with	O
macOS	B-Operating_System
Big	O
Sur	O
to	O
allow	O
x86-64	O
exclusive	O
applications	O
to	O
run	O
on	O
ARM	O
hardware	O
.	O
</s>
<s>
ACL	O
for	O
Tizen	O
allows	O
Android	B-Application
apps	O
to	O
natively	O
execute	O
on	O
Tizen	O
phones	O
.	O
</s>
<s>
Compatibility	B-General_Concept
layer	I-General_Concept
in	O
kernel	B-Operating_System
:	O
</s>
<s>
FreeBSD	B-Operating_System
's	O
Linux	B-Application
compatibility	B-General_Concept
layer	I-General_Concept
,	O
which	O
enables	O
binaries	O
built	O
specifically	O
for	O
Linux	B-Application
to	O
run	O
on	O
FreeBSD	B-Operating_System
the	O
same	O
way	O
as	O
the	O
native	O
FreeBSD	B-Operating_System
API	O
layer	O
.	O
</s>
<s>
FreeBSD	B-Operating_System
also	O
has	O
some	O
Unix-like	B-Operating_System
system	I-Operating_System
emulations	B-Application
,	O
including	O
NDIS	O
,	O
NetBSD	B-Device
,	O
PECoff	O
,	O
SVR4	O
,	O
and	O
different	O
CPU	B-General_Concept
versions	O
of	O
FreeBSD	B-Operating_System
.	O
</s>
<s>
NetBSD	B-Device
has	O
several	O
Unix-like	B-Operating_System
system	I-Operating_System
emulations	B-Application
.	O
</s>
<s>
Windows	B-Application
Subsystem	O
for	O
Linux	B-Application
provides	O
a	O
Linux-compatible	O
kernel	B-Operating_System
interface	O
developed	O
by	O
Microsoft	O
.	O
</s>
<s>
(	O
aka	O
COMPAT_PECOFF	O
)	O
has	O
Win32	B-Library
compatible	B-Device
layer	O
for	O
NetBSD	B-Device
.	O
</s>
<s>
On	O
RSTS/E	B-Operating_System
for	O
the	O
PDP-11	B-Device
series	O
of	O
minicomputers	B-Architecture
,	O
programs	O
written	O
to	O
run	O
on	O
the	O
RT-11	B-Operating_System
operating	O
system	O
could	O
run	O
(	O
without	O
recompiling	O
)	O
on	O
RSTS	B-Operating_System
through	O
the	O
RT-11	B-Operating_System
Run-Time	O
System	O
having	O
its	O
EMT	O
flag	O
set	O
,	O
meaning	O
that	O
an	O
RT-11	B-Operating_System
EMT	B-Operating_System
instruction	I-Operating_System
that	O
matches	O
a	O
RSTS	B-Operating_System
EMT	O
is	O
diverted	O
to	O
the	O
RT-11	B-Operating_System
Run-Time	O
System	O
which	O
translates	O
them	O
to	O
the	O
equivalent	O
RSTS	B-Operating_System
EMT	O
.	O
</s>
<s>
Programs	O
written	O
to	O
take	O
advantage	O
of	O
RSTS	B-Operating_System
directly	O
(	O
or	O
calls	O
to	O
RSTS	B-Operating_System
within	O
the	O
Run-Time	O
system	O
itself	O
)	O
signal	O
this	O
by	O
having	O
a	O
second	O
EMT	B-Operating_System
instruction	I-Operating_System
(	O
usually	O
EMT	O
255	O
)	O
immediately	O
before	O
the	O
actual	O
RSTS	B-Operating_System
EMT	O
code	O
.	O
</s>
<s>
A	O
compatibility	B-General_Concept
layer	I-General_Concept
avoids	O
both	O
the	O
complexity	O
and	O
the	O
speed	O
penalty	O
of	O
full	O
hardware	O
emulation	B-Application
.	O
</s>
<s>
some	O
Linux	B-Application
applications	O
running	O
on	O
FreeBSD	B-Operating_System
's	O
Linux	B-Application
compatibility	B-General_Concept
layer	I-General_Concept
may	O
perform	O
better	O
than	O
the	O
same	O
applications	O
on	O
Red	O
Hat	O
Linux	B-Application
.	O
</s>
<s>
Benchmarks	O
are	O
occasionally	O
run	O
on	O
Wine	B-Application
to	O
compare	O
it	O
to	O
Windows	B-Application
NT-based	O
operating	O
systems	O
.	O
</s>
<s>
Even	O
on	O
similar	O
systems	O
,	O
the	O
details	O
of	O
implementing	O
a	O
compatibility	B-General_Concept
layer	I-General_Concept
can	O
be	O
quite	O
intricate	O
and	O
troublesome	O
;	O
a	O
good	O
example	O
is	O
the	O
IRIX	B-Operating_System
binary	O
compatibility	B-General_Concept
layer	I-General_Concept
in	O
the	O
MIPS	B-Device
architecture	I-Device
version	O
of	O
NetBSD	B-Device
.	O
</s>
<s>
A	O
compatibility	B-General_Concept
layer	I-General_Concept
requires	O
the	O
host	O
system	O
's	O
CPU	B-General_Concept
to	O
be	O
(	O
upwardly	O
)	O
compatible	B-Device
to	O
that	O
of	O
the	O
foreign	O
system	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
Microsoft	B-Application
Windows	I-Application
compatibility	B-General_Concept
layer	I-General_Concept
is	O
not	O
possible	O
on	O
PowerPC	B-Architecture
hardware	O
because	O
Windows	B-Application
requires	O
an	O
x86	B-Operating_System
CPU	B-General_Concept
.	O
</s>
<s>
In	O
this	O
case	O
full	O
emulation	B-Application
is	O
needed	O
.	O
</s>
<s>
Hardware	O
compatibility	B-General_Concept
layers	I-General_Concept
involve	O
tools	O
that	O
allow	O
hardware	O
emulation	B-Application
.	O
</s>
<s>
Some	O
hardware	O
compatibility	B-General_Concept
layers	I-General_Concept
involve	O
breakout	B-Protocol
boxes	I-Protocol
because	O
breakout	B-Protocol
boxes	I-Protocol
can	O
provide	O
compatibility	B-Device
for	O
certain	O
computer	B-General_Concept
buses	I-General_Concept
that	O
are	O
otherwise	O
incompatible	O
with	O
the	O
machine	O
.	O
</s>
