<s>
A	O
Unix	B-Operating_System
architecture	I-Operating_System
is	O
a	O
computer	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
system	B-Architecture
architecture	I-Architecture
that	O
embodies	O
the	B-Operating_System
Unix	I-Operating_System
philosophy	I-Operating_System
.	O
</s>
<s>
It	O
may	O
adhere	O
to	O
standards	O
such	O
as	O
the	O
Single	O
UNIX	B-Application
Specification	O
(	O
SUS	O
)	O
or	O
similar	O
POSIX	O
IEEE	O
standard	O
.	O
</s>
<s>
No	O
single	O
published	O
standard	O
describes	O
all	O
Unix	B-Operating_System
architecture	I-Operating_System
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
—	O
this	O
is	O
in	O
part	O
a	O
legacy	O
of	O
the	O
Unix	B-Application
wars	O
.	O
</s>
<s>
There	O
are	O
many	O
systems	O
which	O
are	O
Unix-like	B-Operating_System
in	O
their	O
architecture	O
.	O
</s>
<s>
Notable	O
among	O
these	O
are	O
the	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
The	O
distinctions	O
between	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
systems	I-Operating_System
have	O
been	O
the	O
subject	O
of	O
heated	O
legal	O
battles	O
,	O
and	O
the	O
holders	O
of	O
the	O
UNIX	B-Application
brand	O
,	O
The	B-Application
Open	I-Application
Group	I-Application
,	O
object	O
to	O
"	O
Unix-like	B-Operating_System
"	O
and	O
similar	O
terms	O
.	O
</s>
<s>
For	O
distinctions	O
between	O
SUS	O
branded	O
UNIX	B-Operating_System
architectures	I-Operating_System
and	O
other	O
similar	O
architectures	O
,	O
see	O
Unix-like	B-Operating_System
.	O
</s>
<s>
A	O
Unix	B-Application
kernel	B-Operating_System
the	O
core	O
or	O
key	O
components	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
consists	O
of	O
many	O
kernel	B-Operating_System
subsystems	O
like	O
process	B-Operating_System
management	I-Operating_System
,	O
scheduling	O
,	O
file	O
management	O
,	O
device	O
management	O
,	O
network	O
management	O
,	O
memory	B-General_Concept
management	I-General_Concept
,	O
and	O
dealing	O
with	O
interrupts	O
from	O
hardware	O
devices	O
.	O
</s>
<s>
Concurrency	B-Operating_System
:	O
As	O
Unix	B-Application
is	O
a	O
multiprocessing	O
OS	O
,	O
many	O
processes	O
run	O
concurrently	O
to	O
improve	O
the	O
performance	O
of	O
the	O
system	O
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
(	O
VM	O
)	O
:	O
Memory	B-General_Concept
management	I-General_Concept
subsystem	O
implements	O
the	O
virtual	B-Architecture
memory	I-Architecture
concept	O
and	O
users	O
need	O
not	O
worry	O
about	O
the	O
executable	O
program	O
size	O
and	O
the	O
RAM	O
size	O
.	O
</s>
<s>
Paging	B-Architecture
:	O
It	O
is	O
a	O
technique	O
to	O
minimize	O
the	O
internal	O
as	O
well	O
as	O
the	O
external	O
fragmentation	O
in	O
the	O
physical	O
memory	O
.	O
</s>
<s>
Virtual	B-Application
file	I-Application
system	I-Application
(	O
VFS	O
)	O
:	O
A	O
VFS	O
is	O
a	O
file	O
system	O
used	O
to	O
help	O
the	O
user	O
to	O
hide	O
the	O
different	O
file	O
systems	O
complexities	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
provides	O
these	O
and	O
other	O
basic	O
services	O
:	O
interrupt	B-General_Concept
and	O
trap	O
handling	O
,	O
separation	O
between	O
user	O
and	O
system	O
space	O
,	O
system	B-Operating_System
calls	I-Operating_System
,	O
scheduling	O
,	O
timer	O
and	O
clock	O
handling	O
,	O
file	B-Application
descriptor	I-Application
management	O
.	O
</s>
<s>
Some	O
key	O
features	O
of	O
the	O
Unix	B-Operating_System
architecture	I-Operating_System
concept	O
are	O
:	O
</s>
<s>
Unix	B-Application
systems	I-Application
use	O
a	O
centralized	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
which	O
manages	O
system	O
and	O
process	O
activities	O
.	O
</s>
<s>
All	O
non-kernel	O
software	O
is	O
organized	O
into	O
separate	O
,	O
kernel-managed	O
processes	O
.	O
</s>
<s>
Unix	B-Application
systems	I-Application
are	O
preemptively	O
multitasking	O
:	O
multiple	O
processes	O
can	O
run	O
at	O
the	O
same	O
time	O
,	O
or	O
within	O
small	O
time	O
slices	O
and	O
nearly	O
at	O
the	O
same	O
time	O
,	O
and	O
any	O
process	O
can	O
be	O
interrupted	O
and	O
moved	O
out	O
of	O
execution	O
by	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
is	O
known	O
as	O
thread	B-Operating_System
management	I-Operating_System
.	O
</s>
<s>
Files	O
are	O
stored	O
on	O
disk	O
in	O
a	O
hierarchical	B-Application
file	I-Application
system	I-Application
,	O
with	O
a	O
single	O
top	O
location	O
throughout	O
the	O
system	O
(	O
root	O
,	O
or	O
"	O
/	O
"	O
)	O
,	O
with	O
both	O
files	O
and	O
directories	O
,	O
subdirectories	O
,	O
sub-subdirectories	O
,	O
and	O
so	O
on	O
below	O
it	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
everything	B-Application
is	I-Application
a	I-Application
file	I-Application
.	O
</s>
<s>
The	O
UNIX	B-Application
operating	I-Application
system	I-Application
supports	O
the	O
following	O
features	O
and	O
capabilities	O
:	O
</s>
<s>
The	O
UNIX-HATERS	O
Handbook	O
covers	O
some	O
of	O
these	O
design	O
features	O
as	O
failures	O
from	O
the	O
user	O
point	O
of	O
view	O
.	O
</s>
<s>
However	O
,	O
although	O
some	O
information	O
is	O
quite	O
dated	O
and	O
cannot	O
be	O
applied	O
to	O
modern	O
Unixes	B-Application
or	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
such	O
as	O
Linux	B-Application
,	O
Eric	O
S	O
.	O
Raymond	O
discovered	O
that	O
several	O
issues	O
are	O
still	O
prevailing	O
,	O
while	O
others	O
were	O
resolved	O
.	O
</s>
<s>
Raymond	O
concludes	O
that	O
not	O
all	O
concepts	O
behind	O
Unix	B-Application
can	O
be	O
deemed	O
as	O
non-functional	O
even	O
though	O
the	O
book	O
's	O
intention	O
may	O
have	O
been	O
to	O
portray	O
Unix	B-Application
as	O
inferior	O
without	O
encouraging	O
discussions	O
with	O
developers	O
to	O
actually	O
fix	O
the	O
issues	O
.	O
</s>
