<s>
In	O
computing	O
,	O
an	O
abstraction	B-Architecture
layer	I-Architecture
or	O
abstraction	B-Architecture
level	I-Architecture
is	O
a	O
way	O
of	O
hiding	O
the	O
working	O
details	O
of	O
a	O
subsystem	O
.	O
</s>
<s>
Examples	O
of	O
software	O
models	O
that	O
use	O
layers	B-Architecture
of	I-Architecture
abstraction	I-Architecture
include	O
the	O
OSI	O
model	O
for	O
network	O
protocols	O
,	O
OpenGL	B-Application
,	O
and	O
other	O
graphics	B-Library
libraries	I-Library
,	O
which	O
allow	O
the	O
separation	O
of	O
concerns	O
to	O
facilitate	O
interoperability	O
and	O
platform	B-Operating_System
independence	I-Operating_System
.	O
</s>
<s>
Another	O
example	O
is	O
Media	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
an	O
abstraction	B-Architecture
layer	I-Architecture
is	O
a	O
generalization	O
of	O
a	O
conceptual	B-Application
model	I-Application
or	O
algorithm	O
,	O
away	O
from	O
any	O
specific	O
implementation	O
.	O
</s>
<s>
The	O
simplification	O
provided	O
by	O
a	O
good	O
abstraction	B-Architecture
layer	I-Architecture
allows	O
for	O
easy	O
reuse	O
by	O
distilling	O
a	O
useful	O
concept	O
or	O
design	O
pattern	O
so	O
that	O
situations	O
,	O
where	O
it	O
may	O
be	O
accurately	O
applied	O
,	O
can	O
be	O
quickly	O
recognized	O
.	O
</s>
<s>
A	O
layer	O
is	O
considered	O
to	O
be	O
on	O
top	O
of	O
another	O
if	O
it	O
depends	B-Application
on	O
it	O
.	O
</s>
<s>
Frequently	O
abstraction	B-Architecture
layers	I-Architecture
can	O
be	O
composed	O
into	O
a	O
hierarchy	O
of	O
abstraction	B-Architecture
levels	I-Architecture
.	O
</s>
<s>
The	O
OSI	O
model	O
comprises	O
seven	O
abstraction	B-Architecture
layers	I-Architecture
.	O
</s>
<s>
A	O
famous	O
aphorism	O
of	O
David	O
Wheeler	O
is	O
"	O
All	O
problems	O
in	O
computer	B-General_Concept
science	I-General_Concept
can	O
be	O
solved	O
by	O
another	O
level	O
of	O
indirection	O
"	O
.	O
</s>
<s>
In	O
a	O
computer	B-General_Concept
architecture	I-General_Concept
,	O
a	O
computer	O
system	O
is	O
usually	O
represented	O
as	O
consisting	O
of	O
several	O
abstraction	B-Architecture
levels	I-Architecture
such	O
as	O
:	O
</s>
<s>
Programmable	O
logic	O
is	O
often	O
considered	O
part	O
of	O
the	O
hardware	B-Architecture
,	O
while	O
the	O
logical	O
definitions	O
are	O
also	O
sometimes	O
seen	O
as	O
part	O
of	O
a	O
device	O
's	O
software	O
or	O
firmware	O
.	O
</s>
<s>
Firmware	O
may	O
include	O
only	O
low-level	O
software	O
,	O
but	O
can	O
also	O
include	O
all	O
software	O
,	O
including	O
an	O
operating	B-General_Concept
system	I-General_Concept
and	O
applications	O
.	O
</s>
<s>
The	O
software	O
layers	O
can	O
be	O
further	O
divided	O
into	O
hardware	B-Operating_System
abstraction	I-Operating_System
layers	I-Operating_System
,	O
physical	O
and	O
logical	O
device	O
drivers	O
,	O
repositories	O
such	O
as	O
filesystems	O
,	O
operating	B-General_Concept
system	I-General_Concept
kernels	O
,	O
middleware	O
,	O
applications	O
,	O
and	O
others	O
.	O
</s>
<s>
A	O
distinction	O
can	O
also	O
be	O
made	O
from	O
low-level	O
programming	O
languages	O
like	O
VHDL	B-Language
,	O
machine	B-Language
language	I-Language
,	O
assembly	B-Language
language	I-Language
to	O
a	O
compiled	B-Language
language	I-Language
,	O
interpreter	B-Application
,	O
and	O
script	B-Language
language	I-Language
.	O
</s>
<s>
In	O
the	O
Unix	O
operating	B-General_Concept
system	I-General_Concept
,	O
most	O
types	O
of	O
input	O
and	O
output	O
operations	O
are	O
considered	O
to	O
be	O
streams	O
of	O
bytes	O
read	O
from	O
a	O
device	O
or	O
written	O
to	O
a	O
device	O
.	O
</s>
<s>
This	O
stream	O
of	O
bytes	O
model	O
is	O
used	O
for	O
file	O
I/O	O
,	O
socket	O
I/O	O
,	O
and	O
terminal	O
I/O	O
in	O
order	O
to	O
provide	O
device	B-Operating_System
independence	I-Operating_System
.	O
</s>
<s>
The	O
device	O
's	O
physical	O
characteristics	O
are	O
mediated	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
which	O
in	O
turn	O
presents	O
an	O
abstract	B-Architecture
interface	I-Architecture
that	O
allows	O
the	O
programmer	O
to	O
read	O
and	O
write	O
bytes	O
from/to	O
the	O
device	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
then	O
performs	O
the	O
actual	O
transformation	O
needed	O
to	O
read	O
and	O
write	O
the	O
stream	O
of	O
bytes	O
to	O
the	O
device	O
.	O
</s>
<s>
Most	O
graphics	B-Library
libraries	I-Library
such	O
as	O
OpenGL	B-Application
provide	O
an	O
abstract	O
graphical	O
device	O
model	O
as	O
an	O
interface	O
.	O
</s>
<s>
The	O
specific	O
device	O
commands	O
for	O
a	O
plotter	B-Operating_System
are	O
different	O
from	O
the	O
device	O
commands	O
for	O
a	O
CRT	B-Device
monitor	I-Device
,	O
but	O
the	O
graphics	B-Library
library	I-Library
hides	O
the	O
implementation	O
and	O
device-dependent	O
details	O
by	O
providing	O
an	O
abstract	B-Architecture
interface	I-Architecture
which	O
provides	O
a	O
set	O
of	O
primitives	O
that	O
are	O
generally	O
useful	O
for	O
drawing	O
graphical	O
objects	O
.	O
</s>
