<s>
A	O
hybrid	B-Operating_System
kernel	I-Operating_System
is	O
an	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
architecture	O
that	O
attempts	O
to	O
combine	O
aspects	O
and	O
benefits	O
of	O
microkernel	B-Operating_System
and	O
monolithic	B-Operating_System
kernel	I-Operating_System
architectures	O
used	O
in	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
The	O
traditional	O
kernel	B-Operating_System
categories	O
are	O
monolithic	B-Operating_System
kernels	I-Operating_System
and	O
microkernels	B-Operating_System
(	O
with	O
nanokernels	O
and	O
exokernels	B-Operating_System
seen	O
as	O
more	O
extreme	O
versions	O
of	O
microkernels	B-Operating_System
)	O
.	O
</s>
<s>
The	O
"	O
hybrid	O
"	O
category	O
is	O
controversial	O
,	O
due	O
to	O
the	O
similarity	O
of	O
hybrid	B-Operating_System
kernels	I-Operating_System
and	O
ordinary	O
monolithic	B-Operating_System
kernels	I-Operating_System
;	O
the	O
term	O
has	O
been	O
dismissed	O
by	O
Linus	O
Torvalds	O
as	O
simple	O
marketing	O
.	O
</s>
<s>
The	O
idea	O
behind	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
is	O
to	O
have	O
a	O
kernel	B-Operating_System
structure	O
similar	O
to	O
that	O
of	O
a	O
microkernel	B-Operating_System
,	O
but	O
to	O
implement	O
that	O
structure	O
in	O
the	O
manner	O
of	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
In	O
contrast	O
to	O
a	O
microkernel	B-Operating_System
,	O
all	O
(	O
or	O
nearly	O
all	O
)	O
operating	B-General_Concept
system	I-General_Concept
services	O
in	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
are	O
still	O
in	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
There	O
are	O
none	O
of	O
the	O
reliability	O
benefits	O
of	O
having	O
services	O
in	O
user	B-Operating_System
space	I-Operating_System
,	O
as	O
with	O
a	O
microkernel	B-Operating_System
.	O
</s>
<s>
However	O
,	O
just	O
as	O
with	O
an	O
ordinary	O
monolithic	B-Operating_System
kernel	I-Operating_System
,	O
there	O
is	O
none	O
of	O
the	O
performance	O
overhead	O
for	O
message	B-Architecture
passing	I-Architecture
and	O
context	O
switching	O
between	O
kernel	B-Operating_System
and	O
user	B-Operating_System
mode	I-Operating_System
that	O
normally	O
comes	O
with	O
a	O
microkernel	B-Operating_System
.	O
</s>
<s>
One	O
prominent	O
example	O
of	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
is	O
the	O
Microsoft	B-Device
Windows	I-Device
NT	I-Device
kernel	B-Operating_System
that	O
powers	O
all	O
operating	B-General_Concept
systems	I-General_Concept
in	O
the	O
Windows	B-Device
NT	I-Device
family	I-Device
,	O
up	O
to	O
and	O
including	O
Windows	B-Application
11	I-Application
and	O
Windows	B-Device
Server	I-Device
2022	I-Device
,	O
and	O
powers	O
Windows	O
Phone	O
8	O
,	O
Windows	B-Protocol
Phone	I-Protocol
8.1	I-Protocol
,	O
and	O
Xbox	B-Device
One	I-Device
.	O
</s>
<s>
Windows	B-Device
NT	I-Device
was	O
the	O
first	O
Windows	O
operating	B-General_Concept
system	I-General_Concept
based	O
on	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
The	O
hybrid	B-Operating_System
kernel	I-Operating_System
was	O
designed	O
as	O
a	O
modified	O
microkernel	B-Operating_System
,	O
influenced	O
by	O
the	O
Mach	B-Operating_System
microkernel	I-Operating_System
developed	O
by	O
Richard	O
Rashid	O
at	O
Carnegie	O
Mellon	O
University	O
,	O
but	O
without	O
meeting	O
all	O
of	O
the	O
criteria	O
of	O
a	O
pure	O
microkernel	B-Operating_System
.	O
</s>
<s>
NT-based	B-Device
Windows	O
is	O
classified	O
as	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
(	O
or	O
a	O
macrokernel	O
)	O
rather	O
than	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
because	O
the	O
emulation	O
subsystems	O
run	O
in	O
user-mode	O
server	O
processes	O
,	O
rather	O
than	O
in	O
kernel	B-Operating_System
mode	O
as	O
on	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
,	O
and	O
further	O
because	O
of	O
the	O
large	O
number	O
of	O
design	O
goals	O
which	O
resemble	O
design	O
goals	O
of	O
Mach	B-Operating_System
(	O
in	O
particular	O
the	O
separation	O
of	O
OS	O
personalities	O
from	O
a	O
general	O
kernel	B-Operating_System
design	I-Operating_System
)	O
.	O
</s>
<s>
Conversely	O
,	O
the	O
reason	O
NT	O
is	O
not	O
a	O
microkernel	B-Operating_System
system	O
is	O
because	O
most	O
of	O
the	O
system	O
components	O
run	O
in	O
the	O
same	O
address	B-General_Concept
space	I-General_Concept
as	O
the	O
kernel	B-Operating_System
,	O
as	O
would	O
be	O
the	O
case	O
with	O
a	O
monolithic	B-Operating_System
design	O
(	O
in	O
a	O
traditional	O
monolithic	B-Operating_System
design	O
,	O
there	O
would	O
not	O
be	O
a	O
microkernel	B-Operating_System
per	O
se	O
,	O
but	O
the	O
kernel	B-Operating_System
would	O
implement	O
broadly	O
similar	O
functionality	O
to	O
NT	O
's	O
microkernel	B-Operating_System
and	O
kernel-mode	O
subsystems	O
)	O
.	O
</s>
<s>
The	O
primary	O
operating	B-General_Concept
system	I-General_Concept
personality	O
on	O
Windows	O
is	O
the	O
Windows	B-Library
API	I-Library
,	O
which	O
is	O
always	O
present	O
.	O
</s>
<s>
For	O
performance	O
reasons	O
,	O
however	O
,	O
in	O
version	O
4.0	O
and	O
later	O
,	O
these	O
modules	O
(	O
which	O
are	O
often	O
implemented	O
in	O
user	B-Operating_System
mode	I-Operating_System
even	O
on	O
monolithic	B-Operating_System
systems	O
,	O
especially	O
those	O
designed	O
without	O
internal	O
graphics	O
support	O
)	O
run	O
as	O
a	O
kernel-mode	O
subsystem	O
.	O
</s>
<s>
Applications	O
that	O
run	O
on	O
NT	O
are	O
written	O
to	O
one	O
of	O
the	O
OS	O
personalities	O
(	O
usually	O
the	O
Windows	B-Library
API	I-Library
)	O
,	O
and	O
not	O
to	O
the	O
native	O
NT	O
API	B-General_Concept
for	O
which	O
documentation	O
is	O
not	O
publicly	O
available	O
(	O
with	O
the	O
exception	O
of	O
routines	O
used	O
in	O
device	O
driver	O
development	O
)	O
.	O
</s>
<s>
An	O
OS	O
personality	O
is	O
implemented	O
via	O
a	O
set	O
of	O
user-mode	O
DLLs	O
(	O
see	O
Dynamic-link	O
library	O
)	O
,	O
which	O
are	O
mapped	O
into	O
application	O
processes	O
 '	O
address	B-General_Concept
spaces	I-General_Concept
as	O
required	O
,	O
together	O
with	O
an	O
emulation	O
subsystem	O
server	O
process	O
(	O
as	O
described	O
previously	O
)	O
.	O
</s>
<s>
Applications	O
access	O
system	O
services	O
by	O
calling	O
into	O
the	O
OS	O
personality	O
DLLs	O
mapped	O
into	O
their	O
address	B-General_Concept
spaces	I-General_Concept
,	O
which	O
in	O
turn	O
call	O
into	O
the	O
NT	O
run-time	O
library	O
(	O
ntdll.dll	O
)	O
,	O
also	O
mapped	O
into	O
the	O
process	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
The	O
NT	O
run-time	O
library	O
services	O
these	O
requests	O
by	O
trapping	O
into	O
kernel	B-Operating_System
mode	O
to	O
either	O
call	O
kernel-mode	O
Executive	O
routines	O
or	O
make	O
Local	B-Operating_System
Procedure	I-Operating_System
Calls	I-Operating_System
(	O
LPCs	O
)	O
to	O
the	O
appropriate	O
user-mode	O
subsystem	O
server	O
processes	O
,	O
which	O
in	O
turn	O
use	O
the	O
NT	O
API	B-General_Concept
to	O
communicate	O
with	O
application	O
processes	O
,	O
the	O
kernel-mode	O
subsystems	O
and	O
each	O
other	O
.	O
</s>
<s>
XNU	B-Operating_System
is	O
the	O
kernel	B-Operating_System
that	O
Apple	O
Inc	O
.	O
acquired	O
and	O
developed	O
for	O
use	O
in	O
the	O
macOS	B-Application
,	O
iOS	B-Application
,	O
watchOS	B-Device
,	O
and	O
tvOS	B-Operating_System
operating	B-General_Concept
systems	I-General_Concept
and	O
released	O
as	O
free	B-Application
and	I-Application
open	I-Application
source	I-Application
software	I-Application
as	O
part	O
of	O
the	O
Darwin	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
.	O
</s>
<s>
XNU	B-Operating_System
is	O
an	O
acronym	O
for	O
X	B-Operating_System
is	I-Operating_System
Not	I-Operating_System
Unix	I-Operating_System
.	O
</s>
<s>
Originally	O
developed	O
by	O
NeXT	O
for	O
the	O
NeXTSTEP	B-Application
operating	B-General_Concept
system	I-General_Concept
,	O
XNU	B-Operating_System
was	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
combining	O
version	O
2.5	O
of	O
the	O
Mach	B-Operating_System
kernel	I-Operating_System
with	O
components	O
from	O
4.3BSD	O
and	O
an	O
object-oriented	O
API	B-General_Concept
for	O
writing	O
drivers	O
called	O
Driver	O
Kit	O
.	O
</s>
<s>
After	O
Apple	O
acquired	O
NeXT	O
,	O
the	O
Mach	B-Operating_System
component	O
was	O
upgraded	O
to	O
OSFMK	O
7.3	O
,	O
which	O
is	O
a	O
microkernel	B-Operating_System
.	O
</s>
<s>
Apple	O
uses	O
a	O
heavily	O
modified	O
OSFMK	O
7.3	O
functioning	O
as	O
a	O
hybrid	B-Operating_System
kernel	I-Operating_System
with	O
parts	O
of	O
FreeBSD	B-Operating_System
included	O
.	O
</s>
<s>
(	O
OSFMK	O
7.3	O
includes	O
applicable	O
code	O
from	O
the	O
University	O
of	O
Utah	O
Mach	B-Operating_System
4	O
kernel	B-Operating_System
and	O
applicable	O
code	O
from	O
the	O
many	O
Mach	B-Operating_System
3.0	I-Operating_System
variants	O
that	O
forked	B-Application
off	O
from	O
the	O
original	O
Carnegie	O
Mellon	O
University	O
Mach	B-Operating_System
3.0	I-Operating_System
kernel	B-Operating_System
.	O
)	O
</s>
<s>
The	O
BSD	B-Operating_System
components	O
were	O
upgraded	O
with	O
code	O
from	O
the	O
FreeBSD	B-Operating_System
project	I-Operating_System
and	O
the	O
Driver	O
Kit	O
was	O
replaced	O
with	O
a	O
C++	B-Language
API	B-General_Concept
for	O
writing	O
drivers	O
called	O
I/O	O
Kit	O
.	O
</s>
<s>
Like	O
some	O
other	O
modern	O
kernels	B-Operating_System
,	O
XNU	B-Operating_System
is	O
a	O
hybrid	O
,	O
containing	O
features	O
of	O
both	O
monolithic	B-Operating_System
and	O
microkernels	B-Operating_System
,	O
attempting	O
to	O
make	O
the	O
best	O
use	O
of	O
both	O
technologies	O
,	O
such	O
as	O
the	O
message	B-Architecture
passing	I-Architecture
capability	O
of	O
microkernels	B-Operating_System
enabling	O
greater	O
modularity	O
and	O
larger	O
portions	O
of	O
the	O
OS	O
to	O
benefit	O
from	O
protected	B-General_Concept
memory	I-General_Concept
,	O
as	O
well	O
as	O
retaining	O
the	O
speed	O
of	O
monolithic	B-Operating_System
kernels	I-Operating_System
for	O
certain	O
critical	O
tasks	O
.	O
</s>
