<s>
In	O
computing	O
,	O
a	O
windowing	B-Application
system	I-Application
(	O
or	O
window	B-Application
system	I-Application
)	O
is	O
software	O
that	O
manages	O
separately	O
different	O
parts	O
of	O
display	O
screens	O
.	O
</s>
<s>
It	O
is	O
a	O
type	O
of	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
which	O
implements	O
the	O
WIMP	B-Application
(	O
windows	O
,	O
icons	O
,	O
menus	O
,	O
pointer	B-Device
)	O
paradigm	O
for	O
a	O
user	B-Library
interface	I-Library
.	O
</s>
<s>
Each	O
currently	O
running	O
application	B-Application
is	O
assigned	O
a	O
usually	O
resizable	O
and	O
usually	O
rectangular	O
surface	O
of	O
the	O
display	O
to	O
present	O
its	O
GUI	B-Application
to	O
the	O
user	O
;	O
these	O
windows	O
may	O
overlap	O
each	O
other	O
,	O
as	O
opposed	O
to	O
a	O
tiling	O
interface	O
where	O
they	O
are	O
not	O
allowed	O
to	O
overlap	O
.	O
</s>
<s>
The	O
programming	O
of	O
both	O
the	O
window	O
decoration	O
and	O
of	O
available	O
widgets	O
inside	O
of	O
the	O
window	O
,	O
which	O
are	O
graphical	O
elements	O
for	O
direct	O
user	O
interaction	O
,	O
such	O
as	O
sliders	O
,	O
buttons	O
,	O
etc.	O
,	O
is	O
eased	O
and	O
simplified	O
through	O
the	O
use	O
of	O
widget	B-Library
toolkits	I-Library
.	O
</s>
<s>
The	O
main	O
component	O
of	O
any	O
windowing	B-Application
system	I-Application
is	O
usually	O
called	O
the	O
display	O
server	O
,	O
although	O
alternative	O
denominations	O
such	O
as	O
window	O
server	O
or	O
compositor	O
are	O
also	O
in	O
use	O
.	O
</s>
<s>
Any	O
application	B-Application
that	O
runs	O
and	O
presents	O
its	O
GUI	B-Application
in	O
a	O
window	O
,	O
is	O
a	O
client	O
of	O
the	O
display	O
server	O
.	O
</s>
<s>
It	O
receives	O
all	O
the	O
input	O
from	O
the	O
kernel	B-Operating_System
,	O
that	O
the	O
kernel	B-Operating_System
receives	O
from	O
all	O
attached	O
input	B-Device
devices	I-Device
,	O
such	O
as	O
keyboard	B-Device
,	O
pointing	B-Device
devices	I-Device
,	O
or	O
touchscreen	B-Device
and	O
transmits	O
it	O
to	O
the	O
correct	O
client	O
.	O
</s>
<s>
The	O
display	O
server	O
is	O
also	O
responsible	O
for	O
the	O
output	O
of	O
the	O
clients	O
to	O
the	O
computer	B-Device
monitor	I-Device
.	O
</s>
<s>
The	O
output	O
of	O
sound	O
is	O
usually	O
not	O
managed	O
by	O
the	O
display	O
server	O
,	O
but	O
the	O
sound	O
volume	O
is	O
usually	O
handled	O
through	O
GUI	B-Application
applets	O
and	O
it	O
is	O
the	O
display	O
server	O
who	O
decides	O
which	O
applications	O
are	O
on	O
top	O
.	O
</s>
<s>
A	O
windowing	B-Application
system	I-Application
enables	O
the	O
computer	O
user	O
to	O
work	O
with	O
several	O
programs	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Each	O
program	O
presents	O
its	O
GUI	B-Application
in	O
its	O
own	O
window	O
,	O
which	O
is	O
generally	O
a	O
rectangular	O
area	O
of	O
the	O
screen	O
.	O
</s>
<s>
From	O
a	O
programmer	B-Application
's	O
point	O
of	O
view	O
,	O
a	O
windowing	B-Application
system	I-Application
implements	O
graphical	O
primitives	O
.	O
</s>
<s>
It	O
provides	O
an	O
abstraction	O
of	O
the	O
graphics	O
hardware	O
for	O
use	O
by	O
higher-level	O
elements	O
of	O
the	O
graphical	B-Application
interface	I-Application
such	O
as	O
a	O
window	B-Application
manager	I-Application
.	O
</s>
<s>
A	O
display	O
server	O
protocol	O
can	O
be	O
network	O
capable	O
or	O
even	O
network	B-General_Concept
transparent	I-General_Concept
,	O
facilitating	O
the	O
implementation	O
of	O
thin	B-Device
clients	I-Device
.	O
</s>
<s>
The	O
display	O
server	O
is	O
a	O
key	O
component	O
in	O
any	O
graphical	B-Application
user	I-Application
interface	I-Application
,	O
specifically	O
the	O
windowing	B-Application
system	I-Application
.	O
</s>
<s>
One	O
example	O
of	O
a	O
display	O
server	O
is	O
the	O
X.Org	B-Operating_System
Server	I-Operating_System
,	O
which	O
runs	O
on	O
top	O
of	O
the	O
kernel	B-Operating_System
(	O
usually	O
a	O
Unix-like	B-Operating_System
kernel	B-Operating_System
,	O
such	O
as	O
Linux	B-Application
or	O
BSD	B-Operating_System
)	O
.	O
</s>
<s>
It	O
receives	O
user	O
input	O
data	O
(	O
e.g.	O
</s>
<s>
from	O
evdev	B-Application
on	O
Linux	B-Application
)	O
and	O
passes	O
it	O
to	O
one	O
of	O
its	O
clients	O
.	O
</s>
<s>
The	O
display	O
server	O
also	O
receives	O
data	O
from	O
its	O
clients	O
;	O
it	O
processes	O
the	O
data	O
,	O
it	O
does	O
the	O
compositing	O
and	O
on	O
Linux	B-Application
it	O
passes	O
the	O
data	O
to	O
one	O
of	O
three	O
kernel	B-Operating_System
components	O
DRM	B-Application
,	O
gem	B-Application
or	O
KMS	B-Application
driver	I-Application
.	O
</s>
<s>
The	O
component	O
writes	O
the	O
data	O
into	O
the	O
framebuffer	B-Algorithm
and	O
content	O
of	O
the	O
framebuffer	B-Algorithm
is	O
transmitted	O
to	O
the	O
connected	O
screen	O
and	O
displayed	O
.	O
</s>
<s>
X	B-Operating_System
relies	O
on	O
GLX	B-Device
.	O
</s>
<s>
One	O
of	O
the	O
implementations	O
of	O
display	O
server	O
concept	O
is	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
,	O
in	O
particular	O
its	O
actually	O
used	O
version	O
X.Org	B-Operating_System
Server	I-Operating_System
and	O
Xlib	B-Language
and	O
XCB	B-Language
client	O
libraries	O
.	O
</s>
<s>
The	O
X.Org	B-Operating_System
Server	I-Operating_System
is	O
a	O
display	O
server	O
,	O
but	O
in	O
its	O
current	O
implementation	O
it	O
relies	O
on	O
a	O
second	O
program	O
,	O
the	O
compositing	B-Operating_System
window	I-Operating_System
manager	I-Operating_System
,	O
to	O
do	O
the	O
compositing	O
.	O
</s>
<s>
Examples	O
are	O
Mutter	B-Language
or	O
KWin	B-Language
.	O
</s>
<s>
Notable	O
examples	O
of	O
display	O
servers	O
implementing	O
the	O
X11	B-Operating_System
display	O
server	O
protocol	O
are	O
X.Org	B-Operating_System
Server	I-Operating_System
,	O
XFree86	B-Device
,	O
XQuartz	B-Operating_System
and	O
Cygwin/X	B-Application
,	O
while	O
client	O
libraries	O
implementing	O
the	O
X11	B-Operating_System
display	O
server	O
protocol	O
are	O
Xlib	B-Language
and	O
XCB	B-Language
.	O
</s>
<s>
Display	O
servers	O
that	O
implement	O
the	O
Wayland	B-Language
display	I-Language
server	I-Language
protocol	I-Language
are	O
called	O
Wayland	B-Language
compositors	O
.	O
</s>
<s>
Like	O
any	O
display	O
server	O
,	O
a	O
Wayland	B-Language
compositor	O
is	O
responsible	O
for	O
handling	O
input	O
and	O
output	O
for	O
its	O
clients	O
and	O
–	O
in	O
contrast	O
to	O
X11	B-Operating_System
–	O
additionally	O
for	O
the	O
compositing	O
.	O
</s>
<s>
Examples	O
are	O
Weston	B-Application
,	O
Mutter	B-Language
,	O
KWin	B-Language
or	O
Enlightenment	B-Application
.	O
</s>
<s>
Wayland	B-Language
compositors	O
communicate	O
with	O
Wayland	B-Language
clients	O
over	O
the	O
Wayland	B-Language
display	I-Language
server	I-Language
protocol	I-Language
.	O
</s>
<s>
This	O
protocol	O
defines	O
that	O
clients	O
can	O
directly	O
write	O
data	O
into	O
the	O
framebuffer	B-Algorithm
using	O
the	O
EGL	B-Application
rendering	O
API	O
.	O
</s>
<s>
The	O
display	O
server	O
still	O
gets	O
to	O
decide	O
which	O
window	O
is	O
on	O
top	O
and	O
thus	O
visible	O
to	O
the	O
user	O
and	O
also	O
still	O
is	O
responsible	O
for	O
passing	O
data	O
regarding	O
to	O
input	B-Device
devices	I-Device
from	O
evdev	B-Application
to	O
its	O
clients	O
.	O
</s>
<s>
Wayland	B-Language
is	O
used	O
to	O
a	O
certain	O
degree	O
in	O
some	O
Linux	B-Application
desktop	O
distributions	O
,	O
such	O
as	O
Fedora	O
.	O
</s>
<s>
It	O
is	O
also	O
well	O
suited	O
for	O
mobile	O
computing	O
and	O
has	O
been	O
adopted	O
,	O
for	O
example	O
,	O
by	O
the	O
smartphone	O
-	O
and	O
tablet-focused	O
projects	O
Tizen	B-Application
,	O
Sailfish	B-Operating_System
OS	I-Operating_System
and	O
AsteroidOS	B-Device
.	O
</s>
<s>
An	O
implementation	O
of	O
Wayland	B-Language
is	O
available	O
under	O
the	B-License
MIT	I-License
License	I-License
,	O
the	O
libwayland-client	O
and	O
libwayland-server	O
libraries	O
.	O
</s>
<s>
There	O
is	O
an	O
ongoing	O
effort	O
to	O
add	O
Wayland	B-Language
support	O
to	O
ChromeOS	B-Operating_System
.	O
</s>
<s>
The	O
Mir	B-Language
display	I-Language
server	I-Language
comes	O
with	O
its	O
own	O
Mir	B-Language
display	I-Language
server	I-Language
protocol	O
which	O
is	O
different	O
from	O
those	O
used	O
by	O
X11	B-Operating_System
and	O
Wayland	B-Language
.	O
</s>
<s>
Mir	B-Language
additionally	O
supports	O
the	O
X11	B-Operating_System
protocol	O
.	O
</s>
<s>
It	O
was	O
developed	O
by	O
Canonical	O
and	O
was	O
intended	O
to	O
be	O
the	O
display	O
server	O
of	O
choice	O
for	O
Ubuntu	B-Operating_System
.	O
</s>
<s>
As	O
of	O
2017	O
,	O
it	O
has	O
been	O
replaced	O
with	O
the	O
Wayland	B-Language
display	O
server	O
for	O
desktop	O
editions	O
of	O
Ubuntu	B-Operating_System
.	O
</s>
<s>
There	O
are	O
implementations	O
of	O
the	O
Mir	B-Language
display	I-Language
server	I-Language
,	O
the	O
libmir-server	O
and	O
the	O
libmir-client	O
libraries	O
available	O
under	O
the	O
GPLv3	O
.	O
</s>
<s>
Google	B-Application
developed	O
a	O
display	O
server	O
called	O
SurfaceFlinger	O
for	O
Android	B-Application
(	O
another	O
Linux	B-Application
kernel-based	O
operating	O
system	O
primarily	O
for	O
mobile	O
devices	O
)	O
:	O
</s>
<s>
Everything	O
in	O
Android	B-Application
is	O
rendered	O
to	O
a	O
"	O
surface	O
"	O
;	O
"	O
surfaces	O
"	O
are	O
produced	O
by	O
applications	O
and	O
placed	O
into	O
a	O
queue	O
that	O
is	O
managed	O
by	O
SurfaceFlinger	O
.	O
</s>
<s>
Yet	O
another	O
Android-specific	O
solution	O
is	O
"	O
Gralloc	O
"	O
.	O
</s>
<s>
Gralloc	O
handles	O
device	O
memory	O
i.e.	O
</s>
<s>
it	O
does	O
allocation	O
,	O
arbitration	O
,	O
it	O
handles	O
synchronization	O
via	O
Android/Linux	O
fence	O
file	O
descriptors	O
.	O
</s>
<s>
Gralloc	O
competes	O
with	O
other	O
solutions	O
like	O
e.g.	O
</s>
<s>
Mesa	O
's	O
Generic	B-Application
Buffer	I-Application
Management	I-Application
(	O
GBM	O
)	O
or	O
Nvidia	O
's	O
EGLStreams	O
.	O
</s>
<s>
For	O
compositing	O
in	O
Android	B-Application
,	O
Surfaces	O
are	O
sent	O
to	O
SurfaceFlinger	O
,	O
which	O
uses	O
OpenGL	O
ES	B-Application
to	O
do	O
the	O
compositing	O
.	O
</s>
<s>
Hardware	O
Composer	O
HAL	O
(	O
HWC	O
)	O
was	O
introduced	O
in	O
Android	B-Application
3.0	O
and	O
has	O
evolved	O
steadily	O
over	O
the	O
years	O
.	O
</s>
<s>
For	O
Apple	O
's	O
macOS	B-Application
family	O
of	O
operating	O
systems	O
,	O
Quartz	O
Compositor	O
fulfils	O
the	O
tasks	O
of	O
a	O
display	O
server	O
and	O
of	O
a	O
window	B-Application
manager	I-Application
in	O
the	O
windowing	B-Application
system	I-Application
.	O
</s>
<s>
For	O
Microsoft	B-Device
Windows	I-Device
,	O
from	O
Windows	B-Application
Vista	I-Application
onward	O
,	O
Desktop	O
Window	B-Application
Manager	I-Application
enables	O
the	O
use	O
of	O
hardware	O
acceleration	O
to	O
render	O
the	O
graphical	B-Application
user	I-Application
interface	I-Application
.	O
</s>
<s>
Some	O
systems	O
such	O
as	O
Microsoft	B-Device
Windows	I-Device
(	O
XP	B-Application
,	O
9x	O
and	O
earlier	O
)	O
,	O
the	O
classic	B-Application
Mac	I-Application
OS	I-Application
(	O
version	O
9	B-Operating_System
and	O
earlier	O
)	O
,	O
and	O
Palm	B-Operating_System
OS	I-Operating_System
,	O
contain	O
a	O
windowing	B-Application
system	I-Application
which	O
is	O
integrated	O
with	O
the	O
OS	O
.	O
</s>
