<s>
An	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
is	O
system	B-Application
software	I-Application
that	O
manages	O
computer	B-Architecture
hardware	I-Architecture
and	O
software	O
resources	O
,	O
and	O
provides	O
common	O
services	B-Operating_System
for	O
computer	B-Application
programs	I-Application
.	O
</s>
<s>
Time-sharing	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
schedule	O
tasks	O
for	O
efficient	O
use	O
of	O
the	O
system	O
and	O
may	O
also	O
include	O
accounting	O
software	O
for	O
cost	O
allocation	O
of	O
processor	O
time	O
,	O
mass	B-Device
storage	I-Device
,	O
printing	O
,	O
and	O
other	O
resources	O
.	O
</s>
<s>
For	O
hardware	B-Architecture
functions	O
such	O
as	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
and	O
memory	B-General_Concept
allocation	I-General_Concept
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
acts	O
as	O
an	O
intermediary	O
between	O
programs	O
and	O
the	O
computer	B-Architecture
hardware	I-Architecture
,	O
although	O
the	O
application	O
code	O
is	O
usually	O
executed	O
directly	O
by	O
the	O
hardware	B-Architecture
and	O
frequently	O
makes	O
system	B-Operating_System
calls	I-Operating_System
to	O
an	O
OS	O
function	O
or	O
is	O
interrupted	O
by	O
it	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
are	O
found	O
on	O
many	O
devices	O
that	O
contain	O
a	O
computer	O
from	O
cellular	O
phones	O
and	O
video	O
game	O
consoles	O
to	O
web	B-Application
servers	I-Application
and	O
supercomputers	B-Architecture
.	O
</s>
<s>
The	O
dominant	O
general-purpose	O
personal	B-Device
computer	I-Device
operating	B-General_Concept
system	I-General_Concept
is	O
Microsoft	B-Application
Windows	I-Application
with	O
a	O
market	O
share	O
of	O
around	O
74.99	O
%	O
.	O
</s>
<s>
macOS	B-Application
by	O
Apple	O
Inc	O
.	O
is	O
in	O
second	O
place	O
(	O
14.84	O
%	O
)	O
,	O
and	O
the	O
varieties	O
of	O
Linux	B-Application
are	O
collectively	O
in	O
third	O
place	O
(	O
2.81	O
%	O
)	O
.	O
</s>
<s>
In	O
the	O
mobile	B-Operating_System
sector	O
(	O
including	O
smartphones	B-Application
and	O
tablets	B-Device
)	O
,	O
Android	B-Application
's	I-Application
share	O
is	O
70.82	O
%	O
in	O
the	O
year	O
2020	O
.	O
</s>
<s>
According	O
to	O
third	O
quarter	O
2016	O
data	O
,	O
Android	B-Application
's	I-Application
share	O
on	O
smartphones	B-Application
is	O
dominant	O
with	O
87.5	O
percent	O
with	O
a	O
growth	O
rate	O
of	O
10.3	O
percent	O
per	O
year	O
,	O
followed	O
by	O
Apple	O
's	O
iOS	B-Application
with	O
12.1	O
percent	O
with	O
per	O
year	O
decrease	O
in	O
market	O
share	O
of	O
5.2	O
percent	O
,	O
while	O
other	O
operating	B-General_Concept
systems	I-General_Concept
amount	O
to	O
just	O
0.3	O
percent	O
.	O
</s>
<s>
Linux	B-Application
distributions	I-Application
are	O
dominant	O
in	O
the	O
server	B-Application
and	O
supercomputing	B-Architecture
sectors	O
.	O
</s>
<s>
Other	O
specialized	O
classes	O
of	O
operating	B-General_Concept
systems	I-General_Concept
(	O
special-purpose	O
operating	B-General_Concept
systems	I-General_Concept
)	O
,	O
such	O
as	O
embedded	B-Architecture
and	O
real-time	B-General_Concept
systems	I-General_Concept
,	O
exist	O
for	O
many	O
applications	O
.	O
</s>
<s>
Security-focused	O
operating	B-General_Concept
systems	I-General_Concept
also	O
exist	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
have	O
low	O
system	O
requirements	O
(	O
e.g.	O
</s>
<s>
light-weight	B-Operating_System
Linux	I-Operating_System
distribution	I-Operating_System
)	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
require	O
installation	O
or	O
may	O
come	O
pre-installed	O
with	O
purchased	O
computers	O
(	O
OEM-installation	O
)	O
,	O
whereas	O
others	O
may	O
run	O
directly	O
from	O
media	O
(	O
i.e.	O
</s>
<s>
A	O
single-tasking	O
system	O
can	O
only	O
run	O
one	O
program	O
at	O
a	O
time	O
,	O
while	O
a	O
multi-tasking	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
allows	O
more	O
than	O
one	O
program	O
to	O
be	O
running	O
concurrently	B-Architecture
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
time-sharing	B-General_Concept
,	O
where	O
the	O
available	O
processor	O
time	O
is	O
divided	O
between	O
multiple	O
processes	O
.	O
</s>
<s>
These	O
processes	O
are	O
each	O
interrupted	O
repeatedly	O
in	O
time	O
slices	O
by	O
a	O
task-scheduling	O
subsystem	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Multi-tasking	B-Operating_System
may	O
be	O
characterized	O
in	O
preemptive	B-Operating_System
and	O
cooperative	O
types	O
.	O
</s>
<s>
In	O
preemptive	B-Operating_System
multitasking	B-Operating_System
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
slices	O
the	O
CPU	B-General_Concept
time	O
and	O
dedicates	O
a	O
slot	O
to	O
each	O
of	O
the	O
programs	O
.	O
</s>
<s>
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
such	O
as	O
Linux	B-Application
—	O
as	O
well	O
as	O
non-Unix-like	O
,	O
such	O
as	O
AmigaOS	B-Application
—	O
support	O
preemptive	B-Operating_System
multitasking	B-Operating_System
.	O
</s>
<s>
Cooperative	B-Operating_System
multitasking	I-Operating_System
is	O
achieved	O
by	O
relying	O
on	O
each	O
process	B-Operating_System
to	O
provide	O
time	O
to	O
the	O
other	O
processes	O
in	O
a	O
defined	O
manner	O
.	O
</s>
<s>
16-bit	B-Device
versions	O
of	O
Microsoft	B-Application
Windows	I-Application
used	O
cooperative	O
multi-tasking	B-Operating_System
;	O
32-bit	O
versions	O
of	O
both	O
Windows	B-Device
NT	I-Device
and	O
Win9x	O
used	O
preemptive	B-Operating_System
multi-tasking	B-Operating_System
.	O
</s>
<s>
Single-user	B-Operating_System
operating	B-General_Concept
systems	I-General_Concept
have	O
no	O
facilities	O
to	O
distinguish	O
users	O
but	O
may	O
allow	O
multiple	O
programs	O
to	O
run	O
in	O
tandem	B-Application
.	O
</s>
<s>
A	O
multi-user	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
extends	O
the	O
basic	O
concept	O
of	O
multi-tasking	B-Operating_System
with	O
facilities	O
that	O
identify	O
processes	O
and	O
resources	O
,	O
such	O
as	O
disk	O
space	O
,	O
belonging	O
to	O
multiple	B-Operating_System
users	I-Operating_System
,	O
and	O
the	O
system	O
permits	O
multiple	B-Operating_System
users	I-Operating_System
to	O
interact	O
with	O
the	O
system	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Time-sharing	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
schedule	O
tasks	O
for	O
efficient	O
use	O
of	O
the	O
system	O
and	O
may	O
also	O
include	O
accounting	O
software	O
for	O
cost	O
allocation	O
of	O
processor	O
time	O
,	O
mass	B-Device
storage	I-Device
,	O
printing	O
,	O
and	O
other	O
resources	O
to	O
multiple	B-Operating_System
users	I-Operating_System
.	O
</s>
<s>
A	O
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
manages	O
a	O
group	O
of	O
distinct	O
,	O
networked	B-Architecture
computers	O
and	O
makes	O
them	O
appear	O
to	O
be	O
a	O
single	O
computer	O
,	O
as	O
all	O
computations	O
are	O
distributed	B-Architecture
(	O
divided	O
amongst	O
the	O
constituent	O
computers	O
)	O
.	O
</s>
<s>
Embedded	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
are	O
designed	O
to	O
be	O
used	O
in	O
embedded	B-Architecture
computer	I-Architecture
systems	I-Architecture
.	O
</s>
<s>
PDAs	B-Application
)	O
.	O
</s>
<s>
Windows	B-Operating_System
CE	I-Operating_System
and	O
Minix	B-Application
3	I-Application
are	O
some	O
examples	O
of	O
embedded	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
A	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
is	O
an	O
operating	B-General_Concept
system	I-General_Concept
that	O
guarantees	O
to	O
process	B-Operating_System
events	O
or	O
data	O
by	O
a	O
specific	O
moment	O
in	O
time	O
.	O
</s>
<s>
A	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
may	O
be	O
single	O
-	O
or	O
multi-tasking	B-Operating_System
,	O
but	O
when	O
multitasking	B-Operating_System
,	O
it	O
uses	O
specialized	O
scheduling	O
algorithms	O
so	O
that	O
a	O
deterministic	O
nature	O
of	O
behavior	O
is	O
achieved	O
.	O
</s>
<s>
Such	O
an	O
event-driven	O
system	O
switches	O
between	O
tasks	O
based	O
on	O
their	O
priorities	O
or	O
external	O
events	O
,	O
whereas	O
time-sharing	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
switch	O
tasks	O
based	O
on	O
clock	O
interrupts	B-Application
.	O
</s>
<s>
A	O
library	O
operating	B-General_Concept
system	I-General_Concept
is	O
one	O
in	O
which	O
the	O
services	B-Operating_System
that	O
a	O
typical	O
operating	B-General_Concept
system	I-General_Concept
provides	O
,	O
such	O
as	O
networking	O
,	O
are	O
provided	O
in	O
the	O
form	O
of	O
libraries	B-Library
and	O
composed	O
with	O
the	O
application	O
and	O
configuration	O
code	O
to	O
construct	O
a	O
unikernel	B-Operating_System
:	O
a	O
specialized	O
,	O
single	B-Operating_System
address	I-Operating_System
space	I-Operating_System
,	O
machine	O
image	O
that	O
can	O
be	O
deployed	O
to	O
cloud	O
or	O
embedded	B-Architecture
environments	O
.	O
</s>
<s>
Basic	B-Application
operating	I-Application
system	I-Application
features	O
were	O
developed	O
in	O
the	O
1950s	O
,	O
such	O
as	O
resident	B-Application
monitor	I-Application
functions	O
that	O
could	O
automatically	O
run	O
different	O
programs	O
in	O
succession	O
to	O
speed	O
up	O
processing	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
did	O
not	O
exist	O
in	O
their	O
modern	O
and	O
more	O
complex	O
forms	O
until	O
the	O
early	O
1960s	O
.	O
</s>
<s>
Hardware	B-Architecture
features	O
were	O
added	O
,	O
that	O
enabled	O
use	O
of	O
runtime	B-Library
libraries	I-Library
,	O
interrupts	B-Application
,	O
and	O
parallel	B-Operating_System
processing	I-Operating_System
.	O
</s>
<s>
When	O
personal	B-Device
computers	I-Device
became	O
popular	O
in	O
the	O
1980s	O
,	O
operating	B-General_Concept
systems	I-General_Concept
were	O
made	O
for	O
them	O
similar	O
in	O
concept	O
to	O
those	O
used	B-Operating_System
on	I-Operating_System
larger	O
computers	O
.	O
</s>
<s>
In	O
the	O
1940s	O
,	O
the	O
earliest	O
electronic	O
digital	O
systems	O
had	O
no	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
After	O
programmable	O
general-purpose	O
computers	O
were	O
invented	O
,	O
machine	O
languages(consisting of strings of the binary digits 0 and 1 on punched paper tape )	O
were	O
introduced	O
that	O
sped	O
up	O
the	O
programming	O
process	B-Operating_System
(	O
Stern	O
,	O
1981	O
)	O
.	O
</s>
<s>
The	O
program	O
would	O
be	O
loaded	O
into	O
the	O
machine	O
,	O
and	O
the	O
machine	O
would	O
be	O
set	O
to	O
work	O
until	O
the	O
program	O
completed	O
or	O
crashed	B-General_Concept
.	O
</s>
<s>
It	O
is	O
said	O
that	O
Alan	O
Turing	O
was	O
a	O
master	O
of	O
this	O
on	O
the	O
early	O
Manchester	B-Device
Mark	I-Device
1	I-Device
machine	O
,	O
and	O
he	O
was	O
already	O
deriving	O
the	O
primitive	O
conception	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
from	O
the	O
principles	O
of	O
the	O
universal	O
Turing	O
machine	O
.	O
</s>
<s>
Later	O
machines	O
came	O
with	O
libraries	B-Library
of	O
programs	O
,	O
which	O
would	O
be	O
linked	O
to	O
a	O
user	O
's	O
program	O
to	O
assist	O
in	O
operations	O
such	O
as	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
and	O
compiling	B-Language
(	O
generating	O
machine	B-Language
code	I-Language
from	O
human-readable	O
symbolic	B-Language
code	I-Language
)	O
.	O
</s>
<s>
This	O
was	O
the	O
genesis	O
of	O
the	O
modern-day	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
At	O
Cambridge	O
University	O
in	O
England	O
,	O
the	O
job	O
queue	B-Device
was	O
at	O
one	O
time	O
a	O
washing	O
line	O
(	O
clothesline	O
)	O
from	O
which	O
tapes	O
were	O
hung	O
with	O
different	O
colored	O
clothes-pegs	O
to	O
indicate	O
job	O
priority	O
.	O
</s>
<s>
By	O
the	O
late	O
1950s	O
,	O
programs	O
that	O
one	O
would	O
recognize	O
as	O
an	O
operating	B-General_Concept
system	I-General_Concept
were	O
beginning	O
to	O
appear	O
.	O
</s>
<s>
Often	O
pointed	O
to	O
as	O
the	O
earliest	O
recognizable	O
example	O
is	O
GM-NAA	B-Application
I/O	I-Application
,	O
released	O
in	O
1956	O
on	O
the	O
IBM	B-Device
704	I-Device
.	O
</s>
<s>
The	O
first	O
known	O
example	O
that	O
actually	O
referred	O
to	O
itself	O
was	O
the	O
SHARE	B-Application
Operating	I-Application
System	I-Application
,	O
a	O
development	O
of	O
GM-NAA	B-Application
I/O	I-Application
,	O
released	O
in	O
1959	O
.	O
</s>
<s>
One	O
of	O
the	O
more	O
famous	O
examples	O
that	O
is	O
often	O
found	O
in	O
discussions	O
of	O
early	O
systems	O
is	O
the	O
Atlas	B-Application
Supervisor	I-Application
,	O
running	O
on	O
the	O
Atlas	B-Device
in	O
1962	O
.	O
</s>
<s>
It	O
was	O
referred	O
to	O
as	O
such	O
in	O
a	O
December	O
1961	O
article	O
describing	O
the	O
system	O
,	O
but	O
the	O
context	O
of	O
"	O
the	O
Operating	B-General_Concept
System	I-General_Concept
"	O
is	O
more	O
along	O
the	O
lines	O
of	O
"	O
the	O
system	O
operates	O
in	O
the	O
fashion	O
"	O
.	O
</s>
<s>
The	O
Atlas	B-Device
team	O
itself	O
used	O
the	O
term	O
"	O
supervisor	O
"	O
,	O
which	O
was	O
widely	O
used	O
along	O
with	O
"	O
monitor	O
"	O
.	O
</s>
<s>
Through	O
the	O
1950s	O
,	O
many	O
major	O
features	O
were	O
pioneered	O
in	O
the	O
field	O
of	O
operating	B-General_Concept
systems	I-General_Concept
on	O
mainframe	B-Architecture
computers	I-Architecture
,	O
including	O
batch	B-General_Concept
processing	I-General_Concept
,	O
input/output	B-General_Concept
interrupting	B-Application
,	O
buffering	B-General_Concept
,	O
multitasking	B-Operating_System
,	O
spooling	B-Device
,	O
runtime	B-Library
libraries	I-Library
,	O
link-loading	B-Application
,	O
and	O
programs	O
for	O
sorting	B-Algorithm
records	O
in	O
files	B-Operating_System
.	O
</s>
<s>
These	O
features	O
were	O
included	O
or	O
not	O
included	O
in	O
application	O
software	O
at	O
the	O
option	O
of	O
application	O
programmers	O
,	O
rather	O
than	O
in	O
a	O
separate	O
operating	B-General_Concept
system	I-General_Concept
used	O
by	O
all	O
applications	O
.	O
</s>
<s>
In	O
1959	O
,	O
the	O
SHARE	B-Application
Operating	I-Application
System	I-Application
was	O
released	O
as	O
an	O
integrated	O
utility	O
for	O
the	O
IBM	B-Device
704	I-Device
,	O
and	O
later	O
in	O
the	O
709	B-Device
and	O
7090	B-Device
mainframes	B-Architecture
,	O
although	O
it	O
was	O
quickly	O
supplanted	O
by	O
IBSYS/IBJOB	B-Operating_System
on	O
the	O
709	B-Device
,	O
7090	B-Device
and	O
7094	O
,	O
which	O
in	O
turn	O
influenced	O
the	O
later	O
7040-PR-150	O
(	O
7040/7044	B-Device
)	O
and	O
1410-PR-155	O
(	O
1410/7010	O
)	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
During	O
the	O
1960s	O
,	O
IBM	O
's	O
OS/360	B-Application
introduced	O
the	O
concept	O
of	O
a	O
single	O
OS	O
spanning	O
an	O
entire	O
product	O
line	O
,	O
which	O
was	O
crucial	O
for	O
the	O
success	O
of	O
the	O
System/360	B-Application
machines	O
.	O
</s>
<s>
IBM	O
's	O
current	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
are	O
distant	B-Application
descendants	I-Application
of	O
this	O
original	O
system	O
and	O
modern	O
machines	O
are	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
applications	O
written	O
for	O
OS/360	B-Application
.	O
</s>
<s>
OS/360	B-Application
also	O
pioneered	O
the	O
concept	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
keeps	O
track	O
of	O
all	O
of	O
the	O
system	O
resources	O
that	O
are	O
used	O
,	O
including	O
program	O
and	O
data	O
space	O
allocation	O
in	O
main	B-Architecture
memory	I-Architecture
and	O
file	O
space	O
in	O
secondary	O
storage	O
,	O
and	O
file	B-Application
locking	I-Application
during	O
updates	O
.	O
</s>
<s>
When	O
a	O
process	B-Operating_System
is	O
terminated	O
for	O
any	O
reason	O
,	O
all	O
of	O
these	O
resources	O
are	O
re-claimed	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
alternative	O
CP-67	B-Application
system	O
for	O
the	O
S/360	B-Device
-67	I-Device
started	O
a	O
whole	O
line	O
of	O
IBM	O
operating	B-General_Concept
systems	I-General_Concept
focused	O
on	O
the	O
concept	O
of	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
Other	O
operating	B-General_Concept
systems	I-General_Concept
used	B-Operating_System
on	I-Operating_System
IBM	B-Application
S/360	I-Application
series	O
mainframes	B-Architecture
included	O
systems	O
developed	O
by	O
IBM	O
:	O
DOS/360	O
(	O
Disk	B-Operating_System
Operating	I-Operating_System
System	I-Operating_System
)	O
,	O
TSS/360	B-Application
(	O
Time	B-General_Concept
Sharing	I-General_Concept
System	O
)	O
,	O
TOS/360	B-Application
(	O
Tape	O
Operating	B-General_Concept
System	I-General_Concept
)	O
,	O
BOS/360	B-Application
(	O
Basic	B-Application
Operating	I-Application
System	I-Application
)	O
,	O
and	O
ACP	B-Application
(	O
Airline	B-Application
Control	I-Application
Program	I-Application
)	O
,	O
as	O
well	O
as	O
a	O
few	O
non-IBM	O
systems	O
:	O
MTS	B-Application
(	O
Michigan	B-Application
Terminal	I-Application
System	I-Application
)	O
,	O
MUSIC	B-Application
(	O
Multi-User	B-Operating_System
System	I-Operating_System
for	O
Interactive	O
Computing	B-Operating_System
)	O
,	O
and	O
ORVYL	B-Application
(	O
Stanford	O
Timesharing	B-General_Concept
System	O
)	O
.	O
</s>
<s>
Control	O
Data	O
Corporation	O
developed	O
the	O
SCOPE	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
in	O
the	O
1960s	O
,	O
for	O
batch	B-General_Concept
processing	I-General_Concept
.	O
</s>
<s>
In	O
cooperation	O
with	O
the	O
University	O
of	O
Minnesota	O
,	O
the	O
Kronos	B-Operating_System
and	O
later	O
the	O
NOS	B-Operating_System
operating	B-General_Concept
systems	I-General_Concept
were	O
developed	O
during	O
the	O
1970s	O
,	O
which	O
supported	O
simultaneous	O
batch	O
and	O
timesharing	B-General_Concept
use	O
.	O
</s>
<s>
Like	O
many	O
commercial	O
timesharing	B-General_Concept
systems	O
,	O
its	O
interface	O
was	O
an	O
extension	O
of	O
the	O
Dartmouth	O
BASIC	B-Application
operating	I-Application
systems	I-Application
,	O
one	O
of	O
the	O
pioneering	O
efforts	O
in	O
timesharing	B-General_Concept
and	O
programming	O
languages	O
.	O
</s>
<s>
In	O
the	O
late	O
1970s	O
,	O
Control	O
Data	O
and	O
the	O
University	O
of	O
Illinois	O
developed	O
the	O
PLATO	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
used	O
plasma	O
panel	O
displays	O
and	O
long-distance	O
time	B-General_Concept
sharing	I-General_Concept
networks	O
.	O
</s>
<s>
Plato	B-Operating_System
was	O
remarkably	O
innovative	O
for	O
its	O
time	O
,	O
featuring	O
real-time	O
chat	O
,	O
and	O
multi-user	B-Operating_System
graphical	O
games	O
.	O
</s>
<s>
In	O
1961	O
,	O
Burroughs	O
Corporation	O
introduced	O
the	O
B5000	B-Device
with	O
the	O
MCP	B-Application
(	O
Master	B-Application
Control	I-Application
Program	I-Application
)	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
B5000	B-Device
was	O
a	O
stack	B-Application
machine	I-Application
designed	O
to	O
exclusively	O
support	O
high-level	O
languages	O
with	O
no	O
assembler	B-Language
;	O
indeed	O
,	O
the	O
MCP	B-Application
was	O
the	O
first	O
OS	O
to	O
be	O
written	O
exclusively	O
in	O
a	O
high-level	O
language	O
(	O
ESPOL	B-Language
,	O
a	O
dialect	O
of	O
ALGOL	B-Language
)	O
.	O
</s>
<s>
MCP	B-Application
also	O
introduced	O
many	O
other	O
ground-breaking	O
innovations	O
,	O
such	O
as	O
being	O
the	O
first	O
commercial	O
implementation	O
of	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
During	O
development	O
of	O
the	O
AS/400	B-Device
,	O
IBM	O
made	O
an	O
approach	O
to	O
Burroughs	O
to	O
license	O
MCP	B-Application
to	O
run	O
on	O
the	O
AS/400	B-Device
hardware	B-Architecture
.	O
</s>
<s>
This	O
proposal	O
was	O
declined	O
by	O
Burroughs	O
management	O
to	O
protect	O
its	O
existing	O
hardware	B-Architecture
production	O
.	O
</s>
<s>
MCP	B-Application
is	O
still	O
in	O
use	O
today	O
in	O
the	O
Unisys	O
company	O
's	O
MCP/ClearPath	B-Application
line	O
of	O
computers	O
.	O
</s>
<s>
UNIVAC	O
,	O
the	O
first	O
commercial	O
computer	O
manufacturer	O
,	O
produced	O
a	O
series	O
of	O
EXEC	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Like	O
all	O
early	O
main-frame	B-Architecture
systems	O
,	O
this	O
batch-oriented	B-General_Concept
system	O
managed	O
magnetic	O
drums	O
,	O
disks	B-Device
,	O
card	O
readers	O
and	O
line	O
printers	O
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
UNIVAC	O
produced	O
the	O
Real-Time	O
Basic	O
(	O
RTB	O
)	O
system	O
to	O
support	O
large-scale	O
time	B-General_Concept
sharing	I-General_Concept
,	O
also	O
patterned	O
after	O
the	O
Dartmouth	O
BC	O
system	O
.	O
</s>
<s>
General	O
Electric	O
developed	O
General	B-Application
Electric	I-Application
Comprehensive	I-Application
Operating	I-Application
Supervisor	I-Application
(	O
GECOS	B-Application
)	O
,	O
which	O
primarily	O
supported	O
batch	B-General_Concept
processing	I-General_Concept
.	O
</s>
<s>
After	O
its	O
acquisition	O
by	O
Honeywell	O
,	O
it	O
was	O
renamed	O
General	B-Application
Comprehensive	I-Application
Operating	I-Application
System	I-Application
(	O
GCOS	B-Application
)	O
.	O
</s>
<s>
Bell	O
Labs	O
,	O
General	O
Electric	O
and	O
MIT	O
developed	O
Multiplexed	B-Application
Information	I-Application
and	I-Application
Computing	I-Application
Service	I-Application
(	O
Multics	B-Application
)	O
,	O
which	O
introduced	O
the	O
concept	O
of	O
ringed	O
security	O
privilege	O
levels	O
.	O
</s>
<s>
Digital	O
Equipment	O
Corporation	O
developed	O
many	O
operating	B-General_Concept
systems	I-General_Concept
for	O
its	O
various	O
computer	O
lines	O
,	O
including	O
TOPS-10	B-Operating_System
and	O
TOPS-20	B-Operating_System
time-sharing	B-General_Concept
systems	I-General_Concept
for	O
the	O
36-bit	O
PDP-10	O
class	O
systems	O
.	O
</s>
<s>
Before	O
the	O
widespread	O
use	O
of	O
UNIX	B-Application
,	O
TOPS-10	B-Operating_System
was	O
a	O
particularly	O
popular	O
system	O
in	O
universities	O
,	O
and	O
in	O
the	O
early	O
ARPANET	O
community	O
.	O
</s>
<s>
RT-11	B-Operating_System
was	O
a	O
single-user	B-Operating_System
real-time	B-Operating_System
OS	I-Operating_System
for	O
the	O
PDP-11	B-Device
class	O
minicomputer	B-Architecture
,	O
and	O
RSX-11	B-Application
was	O
the	O
corresponding	O
multi-user	B-Operating_System
OS	O
.	O
</s>
<s>
From	O
the	O
late	O
1960s	O
through	O
the	O
late	O
1970s	O
,	O
several	O
hardware	B-Architecture
capabilities	O
evolved	O
that	O
allowed	O
similar	O
or	O
ported	O
software	O
to	O
run	O
on	O
more	O
than	O
one	O
system	O
.	O
</s>
<s>
Early	O
systems	O
had	O
utilized	O
microprogramming	B-Device
to	O
implement	O
features	O
on	O
their	O
systems	O
in	O
order	O
to	O
permit	O
different	O
underlying	O
computer	B-General_Concept
architectures	I-General_Concept
to	O
appear	O
to	O
be	O
the	O
same	O
as	O
others	O
in	O
a	O
series	O
.	O
</s>
<s>
The	O
enormous	O
investment	O
in	O
software	O
for	O
these	O
systems	O
made	O
since	O
the	O
1960s	O
caused	O
most	O
of	O
the	O
original	O
computer	O
manufacturers	O
to	O
continue	O
to	O
develop	O
compatible	O
operating	B-General_Concept
systems	I-General_Concept
along	O
with	O
the	O
hardware	B-Architecture
.	O
</s>
<s>
Notable	O
supported	O
mainframe	B-Architecture
operating	B-General_Concept
systems	I-General_Concept
include	O
:	O
</s>
<s>
The	O
first	O
microcomputers	B-Architecture
did	O
not	O
have	O
the	O
capacity	O
or	O
need	O
for	O
the	O
elaborate	O
operating	B-General_Concept
systems	I-General_Concept
that	O
had	O
been	O
developed	O
for	O
mainframes	B-Architecture
and	O
minis	O
;	O
minimalistic	O
operating	B-General_Concept
systems	I-General_Concept
were	O
developed	O
,	O
often	O
loaded	O
from	O
ROM	B-Device
and	O
known	O
as	O
monitors	B-Application
.	O
</s>
<s>
One	O
notable	O
early	O
disk	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
was	O
CP/M	B-Application
,	O
which	O
was	O
supported	O
on	O
many	O
early	O
microcomputers	B-Architecture
and	O
was	O
closely	O
imitated	O
by	O
Microsoft	O
's	O
MS-DOS	B-Application
,	O
which	O
became	O
widely	O
popular	O
as	O
the	O
operating	B-General_Concept
system	I-General_Concept
chosen	O
for	O
the	O
IBM	B-Device
PC	I-Device
(	O
IBM	O
's	O
version	O
of	O
it	O
was	O
called	O
IBM	B-Operating_System
DOS	I-Operating_System
or	O
PC	B-Operating_System
DOS	I-Operating_System
)	O
.	O
</s>
<s>
In	O
the	O
1980s	O
,	O
Apple	O
Computer	O
Inc	O
.	O
(	O
now	O
Apple	O
Inc	O
.	O
)	O
abandoned	O
its	O
popular	O
Apple	B-Device
II	I-Device
series	O
of	O
microcomputers	B-Architecture
to	O
introduce	O
the	O
Apple	B-Device
Macintosh	I-Device
computer	O
with	O
an	O
innovative	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
to	O
the	O
Mac	B-Application
OS	I-Application
.	O
</s>
<s>
The	O
introduction	O
of	O
the	O
Intel	B-General_Concept
80386	I-General_Concept
CPU	B-Architecture
chip	I-Architecture
in	O
October	O
1985	O
,	O
with	O
32-bit	O
architecture	O
and	O
paging	B-Architecture
capabilities	O
,	O
provided	O
personal	B-Device
computers	I-Device
with	O
the	O
ability	O
to	O
run	O
multitasking	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
like	O
those	O
of	O
earlier	O
minicomputers	B-Architecture
and	O
mainframes	B-Architecture
.	O
</s>
<s>
Microsoft	O
responded	O
to	O
this	O
progress	O
by	O
hiring	O
Dave	O
Cutler	O
,	O
who	O
had	O
developed	O
the	O
VMS	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
for	O
Digital	O
Equipment	O
Corporation	O
.	O
</s>
<s>
He	O
would	O
lead	O
the	O
development	O
of	O
the	O
Windows	B-Device
NT	I-Device
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
continues	O
to	O
serve	O
as	O
the	O
basis	O
for	O
Microsoft	O
's	O
operating	B-General_Concept
systems	I-General_Concept
line	O
.	O
</s>
<s>
Steve	O
Jobs	O
,	O
a	O
co-founder	O
of	O
Apple	O
Inc.	O
,	O
started	O
NeXT	O
Computer	O
Inc.	O
,	O
which	O
developed	O
the	O
NEXTSTEP	B-Application
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
NEXTSTEP	B-Application
would	O
later	O
be	O
acquired	O
by	O
Apple	O
Inc	O
.	O
and	O
used	O
,	O
along	O
with	O
code	O
from	O
FreeBSD	B-Operating_System
as	O
the	O
core	O
of	O
Mac	B-Application
OS	I-Application
X	I-Application
(	O
macOS	B-Application
after	O
latest	O
name	O
change	O
)	O
.	O
</s>
<s>
The	B-Application
GNU	I-Application
Project	I-Application
was	O
started	O
by	O
activist	O
and	O
programmer	O
Richard	O
Stallman	O
with	O
the	O
goal	O
of	O
creating	O
a	O
complete	O
free	B-Application
software	I-Application
replacement	O
to	O
the	O
proprietary	B-Application
UNIX	B-Application
operating	I-Application
system	I-Application
.	O
</s>
<s>
While	O
the	O
project	O
was	O
highly	O
successful	O
in	O
duplicating	O
the	O
functionality	O
of	O
various	O
parts	O
of	O
UNIX	B-Application
,	O
development	O
of	O
the	O
GNU	B-Application
Hurd	I-Application
kernel	B-Operating_System
proved	O
to	O
be	O
unproductive	O
.	O
</s>
<s>
In	O
1991	O
,	O
Finnish	O
computer	O
science	O
student	O
Linus	O
Torvalds	O
,	O
with	O
cooperation	O
from	O
volunteers	O
collaborating	O
over	O
the	O
Internet	O
,	O
released	O
the	O
first	O
version	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
It	O
was	O
soon	O
merged	O
with	O
the	O
GNU	O
user	B-Operating_System
space	I-Operating_System
components	O
and	O
system	B-Application
software	I-Application
to	O
form	O
a	O
complete	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Since	O
then	O
,	O
the	O
combination	O
of	O
the	O
two	O
major	O
components	O
has	O
usually	O
been	O
referred	O
to	O
as	O
simply	O
"	O
Linux	B-Application
"	O
by	O
the	O
software	O
industry	O
,	O
a	O
naming	O
convention	O
that	O
Stallman	O
and	O
the	B-Operating_System
Free	I-Operating_System
Software	I-Operating_System
Foundation	I-Operating_System
remain	O
opposed	O
to	O
,	O
preferring	O
the	O
name	O
GNU/Linux	B-Application
.	O
</s>
<s>
The	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
,	O
known	O
as	O
BSD	B-Operating_System
,	O
is	O
the	O
UNIX	B-Application
derivative	O
distributed	B-Architecture
by	O
the	O
University	O
of	O
California	O
,	O
Berkeley	O
,	O
starting	O
in	O
the	O
1970s	O
.	O
</s>
<s>
Freely	O
distributed	B-Architecture
and	O
ported	O
to	O
many	O
minicomputers	B-Architecture
,	O
it	O
eventually	O
also	O
gained	O
a	O
following	O
for	O
use	O
on	O
PCs	B-Device
,	O
mainly	O
as	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
and	O
OpenBSD	B-Operating_System
.	O
</s>
<s>
Unix	B-Application
was	O
originally	O
written	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
Ken	O
Thompson	O
wrote	O
B	B-Language
,	O
mainly	O
based	O
on	O
BCPL	B-Language
,	O
based	O
on	O
his	O
experience	O
in	O
the	O
MULTICS	B-Application
project	O
.	O
</s>
<s>
B	B-Language
was	O
replaced	O
by	O
C	B-Language
,	O
and	O
Unix	B-Application
,	O
rewritten	O
in	O
C	B-Language
,	O
developed	O
into	O
a	O
large	O
,	O
complex	O
family	O
of	O
inter-related	O
operating	B-General_Concept
systems	I-General_Concept
which	O
have	O
been	O
influential	O
in	O
every	O
modern	O
operating	B-General_Concept
system	I-General_Concept
(	O
see	O
History	O
)	O
.	O
</s>
<s>
The	O
Unix-like	B-Operating_System
family	O
is	O
a	O
diverse	O
group	O
of	O
operating	B-General_Concept
systems	I-General_Concept
,	O
with	O
several	O
major	O
sub-categories	O
including	O
System	B-Operating_System
V	I-Operating_System
,	O
BSD	B-Operating_System
,	O
and	O
Linux	B-Application
.	O
</s>
<s>
The	O
name	O
"	O
UNIX	B-Application
"	O
is	O
a	O
trademark	O
of	O
The	B-Application
Open	I-Application
Group	I-Application
which	O
licenses	O
it	O
for	O
use	O
with	O
any	O
operating	B-General_Concept
system	I-General_Concept
that	O
has	O
been	O
shown	O
to	O
conform	O
to	O
their	O
definitions	O
.	O
</s>
<s>
"	O
UNIX-like	B-Operating_System
"	O
is	O
commonly	O
used	O
to	O
refer	O
to	O
the	O
large	O
set	O
of	O
operating	B-General_Concept
systems	I-General_Concept
which	O
resemble	O
the	O
original	O
UNIX	B-Application
.	O
</s>
<s>
Unix-like	B-Operating_System
systems	I-Operating_System
run	O
on	O
a	O
wide	O
variety	O
of	O
computer	B-General_Concept
architectures	I-General_Concept
.	O
</s>
<s>
They	O
are	O
used	O
heavily	O
for	O
servers	B-Application
in	O
business	O
,	O
as	O
well	O
as	O
workstations	B-Device
in	O
academic	O
and	O
engineering	O
environments	O
.	O
</s>
<s>
Free	B-Application
UNIX	B-Application
variants	O
,	O
such	O
as	O
Linux	B-Application
and	O
BSD	B-Operating_System
,	O
are	O
popular	O
in	O
these	O
areas	O
.	O
</s>
<s>
Five	O
operating	B-General_Concept
systems	I-General_Concept
are	O
certified	O
by	O
The	B-Application
Open	I-Application
Group	I-Application
(	O
holder	O
of	O
the	O
Unix	B-Application
trademark	O
)	O
as	O
Unix	B-Application
.	O
</s>
<s>
HP	O
's	O
HP-UX	B-Application
and	O
IBM	O
's	O
AIX	B-Application
are	O
both	O
descendants	O
of	O
the	O
original	O
System	B-Operating_System
V	I-Operating_System
Unix	I-Operating_System
and	O
are	O
designed	O
to	O
run	O
only	O
on	O
their	O
respective	O
vendor	O
's	O
hardware	B-Architecture
.	O
</s>
<s>
In	O
contrast	O
,	O
Sun	O
Microsystems	O
's	O
Solaris	B-Application
can	O
run	O
on	O
multiple	O
types	O
of	O
hardware	B-Architecture
,	O
including	O
x86	B-Operating_System
and	O
SPARC	B-Architecture
servers	B-Application
,	O
and	O
PCs	B-Device
.	O
</s>
<s>
Apple	O
's	O
macOS	B-Application
,	O
a	O
replacement	O
for	O
Apple	O
's	O
earlier	O
(	O
non-Unix	O
)	O
classic	B-Application
Mac	I-Application
OS	I-Application
,	O
is	O
a	O
hybrid	O
kernel-based	O
BSD	B-Operating_System
variant	O
derived	O
from	O
NeXTSTEP	B-Application
,	O
Mach	B-Operating_System
,	O
and	O
FreeBSD	B-Operating_System
.	O
</s>
<s>
IBM	O
's	O
z/OS	B-Application
UNIX	B-Operating_System
System	I-Operating_System
Services	I-Operating_System
includes	O
a	O
shell	B-Operating_System
and	O
utilities	O
based	O
on	O
Mortice	O
Kerns	O
 '	O
InterOpen	O
products	O
.	O
</s>
<s>
Unix	B-Application
interoperability	O
was	O
sought	O
by	O
establishing	O
the	O
POSIX	O
standard	O
.	O
</s>
<s>
The	O
POSIX	O
standard	O
can	O
be	O
applied	O
to	O
any	O
operating	B-General_Concept
system	I-General_Concept
,	O
although	O
it	O
was	O
originally	O
created	O
for	O
various	O
Unix	B-Application
variants	O
.	O
</s>
<s>
A	O
subgroup	O
of	O
the	O
Unix	B-Application
family	O
is	O
the	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
family	O
,	O
which	O
includes	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
and	O
OpenBSD	B-Operating_System
.	O
</s>
<s>
These	O
operating	B-General_Concept
systems	I-General_Concept
are	O
most	O
commonly	O
found	O
on	O
webservers	B-Application
,	O
although	O
they	O
can	O
also	O
function	O
as	O
a	O
personal	B-Device
computer	I-Device
OS	O
.	O
</s>
<s>
The	O
Internet	O
owes	O
much	O
of	O
its	O
existence	O
to	O
BSD	B-Operating_System
,	O
as	O
many	O
of	O
the	O
protocols	O
now	O
commonly	O
used	O
by	O
computers	O
to	O
connect	O
,	O
send	O
and	O
receive	O
data	O
over	O
a	O
network	B-Architecture
were	O
widely	O
implemented	O
and	O
refined	O
in	O
BSD	B-Operating_System
.	O
</s>
<s>
The	O
World	O
Wide	O
Web	O
was	O
also	O
first	O
demonstrated	O
on	O
a	O
number	O
of	O
computers	O
running	O
an	O
OS	O
based	O
on	O
BSD	B-Operating_System
called	O
NeXTSTEP	B-Application
.	O
</s>
<s>
In	O
1974	O
,	O
University	O
of	O
California	O
,	O
Berkeley	O
installed	O
its	O
first	O
Unix	B-Application
system	I-Application
.	O
</s>
<s>
When	O
Berkeley	O
received	O
new	O
VAX	B-Device
computers	O
in	O
1978	O
with	O
Unix	B-Application
installed	O
,	O
the	O
school	O
's	O
undergraduates	O
modified	O
Unix	B-Application
even	O
more	O
in	O
order	O
to	O
take	O
advantage	O
of	O
the	O
computer	O
's	O
hardware	B-Architecture
possibilities	O
.	O
</s>
<s>
Many	O
schools	O
,	O
corporations	O
,	O
and	O
government	O
organizations	O
took	O
notice	O
and	O
started	O
to	O
use	O
Berkeley	O
's	O
version	O
of	O
Unix	B-Application
instead	O
of	O
the	O
official	O
one	O
distributed	B-Architecture
by	O
AT&T	O
.	O
</s>
<s>
Steve	O
Jobs	O
,	O
upon	O
leaving	O
Apple	O
Inc	O
.	O
in	O
1985	O
,	O
formed	O
NeXT	O
Inc.	O
,	O
a	O
company	O
that	O
manufactured	O
high-end	O
computers	O
running	O
on	O
a	O
variation	O
of	O
BSD	B-Operating_System
called	O
NeXTSTEP	B-Application
.	O
</s>
<s>
One	O
of	O
these	O
computers	O
was	O
used	O
by	O
Tim	O
Berners-Lee	O
as	O
the	O
first	O
webserver	B-Application
to	O
create	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
Developers	O
like	O
Keith	O
Bostic	O
encouraged	O
the	O
project	O
to	O
replace	O
any	O
non-free	O
code	O
that	O
originated	O
with	O
Bell	O
Labs	O
.	O
</s>
<s>
After	O
two	O
years	O
of	O
legal	O
disputes	O
,	O
the	O
BSD	B-Operating_System
project	O
spawned	O
a	O
number	O
of	O
free	B-Application
derivatives	O
,	O
such	O
as	O
NetBSD	B-Device
and	O
FreeBSD	B-Operating_System
(	O
both	O
in	O
1993	O
)	O
,	O
and	O
OpenBSD	B-Operating_System
(	O
from	O
NetBSD	B-Device
in	O
1995	O
)	O
.	O
</s>
<s>
macOS	B-Application
(	O
formerly	O
"	O
Mac	B-Application
OS	I-Application
X	I-Application
"	O
and	O
later	O
"	O
OS	O
X	O
"	O
)	O
is	O
a	O
line	O
of	O
open	O
core	O
graphical	O
operating	B-General_Concept
systems	I-General_Concept
developed	O
,	O
marketed	O
,	O
and	O
sold	O
by	O
Apple	O
Inc.	O
,	O
the	O
latest	O
of	O
which	O
is	O
pre-loaded	O
on	O
all	O
currently	O
shipping	O
Macintosh	B-Device
computers	O
.	O
</s>
<s>
macOS	B-Application
is	O
the	O
successor	O
to	O
the	O
original	O
classic	B-Application
Mac	I-Application
OS	I-Application
,	O
which	O
had	O
been	O
Apple	O
's	O
primary	O
operating	B-General_Concept
system	I-General_Concept
since	O
1984	O
.	O
</s>
<s>
Unlike	O
its	O
predecessor	O
,	O
macOS	B-Application
is	O
a	O
UNIX	B-Application
operating	I-Application
system	I-Application
built	O
on	O
technology	O
that	O
had	O
been	O
developed	O
at	O
NeXT	O
through	O
the	O
second	O
half	O
of	O
the	O
1980s	O
and	O
up	O
until	O
Apple	O
purchased	O
the	O
company	O
in	O
early	O
1997	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
was	O
first	O
released	O
in	O
1999	O
as	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
1.0	I-Operating_System
,	O
followed	O
in	O
March	O
2001	O
by	O
a	O
client	O
version	O
(	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
v10.0	I-Operating_System
"	I-Operating_System
Cheetah	I-Operating_System
"	I-Operating_System
)	O
.	O
</s>
<s>
Since	O
then	O
,	O
six	O
more	O
distinct	O
"	O
client	O
"	O
and	O
"	O
server	B-Application
"	O
editions	O
of	O
macOS	B-Application
have	O
been	O
released	O
,	O
until	O
the	O
two	O
were	O
merged	O
in	O
OS	B-Application
X	I-Application
10.7	I-Application
"	I-Application
Lion	I-Application
"	I-Application
.	O
</s>
<s>
Prior	O
to	O
its	O
merging	O
with	O
macOS	B-Application
,	O
the	O
server	B-Application
edition	O
macOS	B-Operating_System
Server	I-Operating_System
was	O
architecturally	B-Architecture
identical	O
to	O
its	O
desktop	O
counterpart	O
and	O
usually	O
ran	O
on	O
Apple	O
's	O
line	O
of	O
Macintosh	B-Device
server	B-Application
hardware	B-Architecture
.	O
</s>
<s>
macOS	B-Operating_System
Server	I-Operating_System
included	O
work	O
group	O
management	O
and	O
administration	O
software	O
tools	O
that	O
provide	O
simplified	O
access	O
to	O
key	O
network	B-General_Concept
services	I-General_Concept
,	O
including	O
a	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
,	O
a	O
Samba	B-Language
server	I-Language
,	O
an	O
LDAP	B-Protocol
server	B-Application
,	O
a	O
domain	B-Protocol
name	I-Protocol
server	I-Protocol
,	O
and	O
others	O
.	O
</s>
<s>
With	O
Mac	B-Application
OS	I-Application
X	I-Application
v10.7	I-Application
Lion	I-Application
,	O
all	O
server	B-Application
aspects	O
of	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
Server	I-Operating_System
have	O
been	O
integrated	O
into	O
the	O
client	O
version	O
and	O
the	O
product	O
re-branded	O
as	O
"	O
OS	O
X	O
"	O
(	O
dropping	O
"	O
Mac	O
"	O
from	O
the	O
name	O
)	O
.	O
</s>
<s>
The	O
server	B-Application
tools	O
are	O
now	O
offered	O
as	O
an	O
application	O
.	O
</s>
<s>
First	O
introduced	O
as	O
the	O
OpenEdition	O
upgrade	O
to	O
MVS/ESA	O
System	O
Product	O
Version	O
4	O
Release	O
3	O
,	O
announced	O
February	O
1993	O
with	O
support	O
for	O
POSIX	O
and	O
other	O
standards	O
.	O
</s>
<s>
z/OS	B-Application
UNIX	B-Operating_System
System	I-Operating_System
Services	I-Operating_System
is	O
built	O
on	O
top	O
of	O
MVS	B-Application
services	B-Operating_System
and	O
cannot	O
run	O
independently	O
.	O
</s>
<s>
While	O
IBM	O
initially	O
introduced	O
OpenEdition	O
to	O
satisfy	O
FIPS	O
requirements	O
,	O
several	O
z/OS	B-Application
component	O
now	O
require	O
UNIX	B-Application
services	B-Operating_System
,	O
e.g.	O
,	O
TCP/IP	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
originated	O
in	O
1991	O
,	O
as	O
a	O
project	O
of	O
Linus	O
Torvalds	O
,	O
while	O
a	O
university	O
student	O
in	O
Finland	O
.	O
</s>
<s>
He	O
posted	O
information	O
about	O
his	O
project	O
on	O
a	O
newsgroup	O
for	O
computer	O
students	O
and	O
programmers	O
,	O
and	O
received	O
support	O
and	O
assistance	O
from	O
volunteers	O
who	O
succeeded	O
in	O
creating	O
a	O
complete	O
and	O
functional	O
kernel	B-Operating_System
.	O
</s>
<s>
Linux	B-Application
is	O
Unix-like	B-Operating_System
,	O
but	O
was	O
developed	O
without	O
any	O
Unix	B-Application
code	O
,	O
unlike	O
BSD	B-Operating_System
and	O
its	O
variants	O
.	O
</s>
<s>
Because	O
of	O
its	O
open	O
license	O
model	O
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
code	O
is	O
available	O
for	O
study	O
and	O
modification	O
,	O
which	O
resulted	O
in	O
its	O
use	O
on	O
a	O
wide	O
range	O
of	O
computing	B-Operating_System
machinery	O
from	O
supercomputers	B-Architecture
to	O
smartwatches	B-Application
.	O
</s>
<s>
Although	O
estimates	O
suggest	O
that	O
Linux	B-Application
is	O
used	B-Operating_System
on	I-Operating_System
only	O
2.81	O
%	O
of	O
all	O
"	O
desktop	O
"	O
(	O
or	O
laptop	O
)	O
PCs	B-Device
,	O
it	O
has	O
been	O
widely	O
adopted	O
for	O
use	O
in	O
servers	B-Application
and	O
embedded	B-Architecture
systems	I-Architecture
such	O
as	O
cell	O
phones	O
.	O
</s>
<s>
Linux	B-Application
has	O
superseded	O
Unix	B-Application
on	O
many	O
platforms	B-Device
and	O
is	O
used	B-Operating_System
on	I-Operating_System
most	O
supercomputers	B-Architecture
including	O
the	O
top	O
385	O
.	O
</s>
<s>
Many	O
of	O
the	O
same	O
computers	O
are	O
also	O
on	O
Green500	B-Application
(	O
but	O
in	O
different	O
order	O
)	O
,	O
and	O
Linux	B-Application
runs	O
on	O
the	O
top	O
10	O
.	O
</s>
<s>
Linux	B-Application
is	O
also	O
commonly	O
used	B-Operating_System
on	I-Operating_System
other	O
small	O
energy-efficient	O
computers	O
,	O
such	O
as	O
smartphones	B-Application
and	O
smartwatches	B-Application
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
is	O
used	O
in	O
some	O
popular	O
distributions	O
,	O
such	O
as	O
Red	O
Hat	O
,	O
Debian	O
,	O
Ubuntu	B-Operating_System
,	O
Linux	B-Application
Mint	I-Application
and	O
Google	B-Application
's	I-Application
Android	I-Application
,	O
ChromeOS	B-Operating_System
,	O
and	O
ChromiumOS	B-Application
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
is	O
a	O
family	O
of	O
proprietary	B-Application
operating	B-General_Concept
systems	I-General_Concept
designed	O
by	O
Microsoft	O
Corporation	O
and	O
primarily	O
targeted	O
to	O
Intel	O
architecture	O
based	O
computers	O
,	O
with	O
an	O
estimated	O
88.9	O
percent	O
total	O
usage	O
share	O
on	O
Web	O
connected	O
computers	O
.	O
</s>
<s>
The	O
latest	O
version	O
is	O
Windows	B-Application
11	I-Application
.	O
</s>
<s>
In	O
2011	O
,	O
Windows	O
7	O
overtook	O
Windows	B-Application
XP	I-Application
as	O
the	O
most	O
common	O
version	O
in	O
use	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
was	O
first	O
released	O
in	O
1985	O
,	O
as	O
an	O
operating	B-Application
environment	I-Application
running	O
on	O
top	O
of	O
MS-DOS	B-Application
,	O
which	O
was	O
the	O
standard	O
operating	B-General_Concept
system	I-General_Concept
shipped	O
on	O
most	O
Intel	O
architecture	O
personal	B-Device
computers	I-Device
at	O
the	O
time	O
.	O
</s>
<s>
In	O
1995	O
,	O
Windows	B-Application
95	I-Application
was	O
released	O
which	O
only	O
used	O
MS-DOS	B-Application
as	O
a	O
bootstrap	O
.	O
</s>
<s>
For	O
backwards	B-General_Concept
compatibility	I-General_Concept
,	O
Win9x	O
could	O
run	O
real-mode	O
MS-DOS	B-Application
and	O
16-bit	B-Device
Windows	O
3.x	O
drivers	O
.	O
</s>
<s>
Windows	B-Device
ME	I-Device
,	O
released	O
in	O
2000	O
,	O
was	O
the	O
last	O
version	O
in	O
the	O
Win9x	O
family	O
.	O
</s>
<s>
Later	O
versions	O
have	O
all	O
been	O
based	O
on	O
the	O
Windows	B-Device
NT	I-Device
kernel	B-Operating_System
.	O
</s>
<s>
Current	O
client	O
versions	O
of	O
Windows	O
run	O
on	O
IA-32	B-Device
,	O
x86-64	B-Device
and	O
ARM	B-Architecture
microprocessors	I-Architecture
.	O
</s>
<s>
In	O
addition	O
Itanium	B-General_Concept
is	O
still	O
supported	O
in	O
older	O
server	B-Application
version	O
Windows	B-Device
Server	I-Device
2008	I-Device
R2	I-Device
.	O
</s>
<s>
In	O
the	O
past	O
,	O
Windows	B-Device
NT	I-Device
supported	O
additional	O
architectures	O
.	O
</s>
<s>
Server	B-Application
editions	O
of	O
Windows	O
are	O
widely	O
used	O
,	O
however	O
,	O
Windows	O
 '	O
usage	O
on	O
servers	B-Application
is	O
not	O
as	O
widespread	O
as	O
on	O
personal	B-Device
computers	I-Device
as	O
Windows	O
competes	O
against	O
Linux	B-Application
and	O
BSD	B-Operating_System
for	O
server	B-Application
market	O
share	O
.	O
</s>
<s>
ReactOS	B-Application
is	O
a	O
Windows-alternative	O
operating	B-General_Concept
system	I-General_Concept
,	O
which	O
is	O
being	O
developed	O
on	O
the	O
principles	O
of	O
Windows	O
without	O
using	O
any	O
of	O
Microsoft	O
's	O
code	O
.	O
</s>
<s>
There	O
have	O
been	O
many	O
operating	B-General_Concept
systems	I-General_Concept
that	O
were	O
significant	O
in	O
their	O
day	O
but	O
are	O
no	O
longer	O
so	O
,	O
such	O
as	O
AmigaOS	B-Application
;	O
OS/2	B-Application
from	O
IBM	O
and	O
Microsoft	O
;	O
classic	B-Application
Mac	I-Application
OS	I-Application
,	O
the	O
non-Unix	O
precursor	O
to	O
Apple	O
's	O
macOS	B-Application
;	O
BeOS	B-Application
;	O
XTS-300	O
;	O
RISC	B-Operating_System
OS	I-Operating_System
;	O
MorphOS	B-Operating_System
;	O
Haiku	B-Language
;	O
BareMetal	B-Operating_System
and	O
FreeMint	B-Application
.	O
</s>
<s>
Some	O
are	O
still	O
used	O
in	O
niche	O
markets	O
and	O
continue	O
to	O
be	O
developed	O
as	O
minority	O
platforms	B-Device
for	O
enthusiast	O
communities	O
and	O
specialist	O
applications	O
.	O
</s>
<s>
OpenVMS	B-Operating_System
,	O
formerly	O
from	O
DEC	O
,	O
is	O
still	O
under	O
active	O
development	O
by	O
VMS	B-Operating_System
Software	I-Operating_System
Inc	I-Operating_System
.	O
</s>
<s>
The	O
IBM	B-Application
i	I-Application
operating	B-General_Concept
system	I-General_Concept
for	O
IBM	B-Device
AS/400	I-Device
and	O
IBM	B-Device
Power	I-Device
Systems	I-Device
midrange	B-Device
computers	I-Device
is	O
also	O
still	O
being	O
used	O
and	O
developed	O
.	O
</s>
<s>
Yet	O
other	O
operating	B-General_Concept
systems	I-General_Concept
are	O
used	O
almost	O
exclusively	O
in	O
academia	O
,	O
for	O
operating	B-General_Concept
systems	I-General_Concept
education	O
or	O
to	O
do	O
research	O
on	O
operating	B-General_Concept
system	I-General_Concept
concepts	O
.	O
</s>
<s>
A	O
typical	O
example	O
of	O
a	O
system	O
that	O
fulfills	O
both	O
roles	O
is	O
MINIX	B-Operating_System
,	O
while	O
for	O
example	O
Singularity	B-Operating_System
is	O
used	O
purely	O
for	O
research	O
.	O
</s>
<s>
Another	O
example	O
is	O
the	O
Oberon	B-Application
System	I-Application
designed	O
at	O
ETH	O
Zürich	O
by	O
Niklaus	O
Wirth	O
,	O
Jürg	O
Gutknecht	O
and	O
a	O
group	O
of	O
students	O
at	O
the	O
former	O
Computer	O
Systems	O
Institute	O
in	O
the	O
1980s	O
.	O
</s>
<s>
Other	O
operating	B-General_Concept
systems	I-General_Concept
have	O
failed	O
to	O
win	O
significant	O
market	O
share	O
,	O
but	O
have	O
introduced	O
innovations	O
that	O
have	O
influenced	O
mainstream	O
operating	B-General_Concept
systems	I-General_Concept
,	O
not	O
least	O
Bell	O
Labs	O
 '	O
Plan	B-Operating_System
9	I-Operating_System
.	O
</s>
<s>
The	O
components	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
all	O
exist	O
in	O
order	O
to	O
make	O
the	O
different	O
parts	O
of	O
a	O
computer	O
work	O
together	O
.	O
</s>
<s>
All	O
user	O
software	O
needs	O
to	O
go	O
through	O
the	O
operating	B-General_Concept
system	I-General_Concept
in	O
order	O
to	O
use	O
any	O
of	O
the	O
hardware	B-Architecture
,	O
whether	O
it	O
be	O
as	O
simple	O
as	O
a	O
mouse	B-Device
or	O
keyboard	B-Device
or	O
as	O
complex	O
as	O
an	O
Internet	O
component	O
.	O
</s>
<s>
With	O
the	O
aid	O
of	O
firmware	B-Application
and	O
device	B-Application
drivers	I-Application
,	O
the	O
kernel	B-Operating_System
provides	O
the	O
most	O
basic	O
level	O
of	O
control	O
over	O
all	O
of	O
the	O
computer	O
's	O
hardware	B-Architecture
devices	O
.	O
</s>
<s>
It	O
manages	O
memory	O
access	O
for	O
programs	O
in	O
the	O
RAM	B-Architecture
,	O
it	O
determines	O
which	O
programs	O
get	O
access	O
to	O
which	O
hardware	B-Architecture
resources	O
,	O
it	O
sets	O
up	O
or	O
resets	O
the	O
CPU	B-General_Concept
's	O
operating	O
states	O
for	O
optimal	O
operation	O
at	O
all	O
times	O
,	O
and	O
it	O
organizes	O
the	O
data	O
for	O
long-term	O
non-volatile	B-General_Concept
storage	I-General_Concept
with	O
file	B-Application
systems	I-Application
on	O
such	O
media	O
as	O
disks	B-Device
,	O
tapes	O
,	O
flash	O
memory	O
,	O
etc	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
provides	O
an	O
interface	O
between	O
an	O
application	O
program	O
and	O
the	O
computer	B-Architecture
hardware	I-Architecture
,	O
so	O
that	O
an	O
application	O
program	O
can	O
interact	O
with	O
the	O
hardware	B-Architecture
only	O
by	O
obeying	O
rules	O
and	O
procedures	O
programmed	O
into	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
is	O
also	O
a	O
set	O
of	O
services	B-Operating_System
which	O
simplify	O
development	O
and	O
execution	O
of	O
application	O
programs	O
.	O
</s>
<s>
Executing	O
an	O
application	O
program	O
typically	O
involves	O
the	O
creation	O
of	O
a	O
process	B-Operating_System
by	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
,	O
which	O
assigns	O
memory	O
space	O
and	O
other	O
resources	O
,	O
establishes	O
a	O
priority	O
for	O
the	O
process	B-Operating_System
in	O
multi-tasking	B-Operating_System
systems	O
,	O
loads	O
program	O
binary	O
code	O
into	O
memory	O
,	O
and	O
initiates	O
execution	O
of	O
the	O
application	O
program	O
,	O
which	O
then	O
interacts	O
with	O
the	O
user	O
and	O
with	O
hardware	B-Architecture
devices	O
.	O
</s>
<s>
However	O
,	O
in	O
some	O
systems	O
an	O
application	O
can	O
request	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
execute	O
another	O
application	O
within	O
the	O
same	O
process	B-Operating_System
,	O
either	O
as	O
a	O
subroutine	O
or	O
in	O
a	O
separate	O
thread	O
,	O
e.g.	O
,	O
the	O
LINK	O
and	O
ATTACH	O
facilities	O
of	O
OS/360	B-Application
and	I-Application
successors	I-Application
..	O
</s>
<s>
An	O
interrupt	B-Application
(	O
also	O
known	O
as	O
abort	B-General_Concept
,	O
exception	B-General_Concept
,	O
fault	O
,	O
signal	B-Operating_System
and	O
trap	B-Application
)	O
provides	O
an	O
efficient	O
way	O
for	O
most	O
operating	B-General_Concept
systems	I-General_Concept
to	O
react	O
to	O
the	O
environment	O
.	O
</s>
<s>
Interrupts	B-Application
cause	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	B-General_Concept
)	O
to	O
have	O
a	O
control	O
flow	O
change	O
away	O
from	O
the	O
currently	O
running	O
program	O
to	O
an	O
interrupt	B-General_Concept
handler	I-General_Concept
,	O
also	O
known	O
as	O
an	O
interrupt	B-General_Concept
service	I-General_Concept
routine	I-General_Concept
(	O
ISR	O
)	O
.	O
</s>
<s>
An	O
interrupt	B-General_Concept
service	I-General_Concept
routine	I-General_Concept
may	O
cause	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	B-General_Concept
)	O
to	O
have	O
a	O
context	B-Operating_System
switch	I-Operating_System
.	O
</s>
<s>
The	O
details	O
of	O
how	O
a	O
computer	O
processes	O
an	O
interrupt	B-Application
vary	O
from	O
architecture	O
to	O
architecture	O
,	O
and	O
the	O
details	O
of	O
how	O
interrupt	B-General_Concept
service	I-General_Concept
routines	I-General_Concept
behave	O
vary	O
from	O
operating	B-General_Concept
system	I-General_Concept
to	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
However	O
,	O
several	O
interrupt	B-Application
functions	O
are	O
common	O
.	O
</s>
<s>
The	O
architecture	O
and	O
operating	B-General_Concept
system	I-General_Concept
must	O
:	O
</s>
<s>
transfer	O
control	O
to	O
an	O
interrupt	B-General_Concept
service	I-General_Concept
routine	I-General_Concept
.	O
</s>
<s>
save	O
the	O
state	O
of	O
the	O
currently	O
running	O
process	B-Operating_System
.	O
</s>
<s>
restore	O
the	O
state	O
after	O
the	O
interrupt	B-Application
is	O
serviced	O
.	O
</s>
<s>
A	O
software	O
interrupt	B-Application
is	O
a	O
message	O
to	O
a	O
process	B-Operating_System
that	O
an	O
event	O
has	O
occurred	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
a	O
hardware	B-Architecture
interrupt	B-Application
—	O
which	O
is	O
a	O
message	O
to	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	B-General_Concept
)	O
that	O
an	O
event	O
has	O
occurred	O
.	O
</s>
<s>
Software	O
interrupts	B-Application
are	O
similar	O
to	O
hardware	B-Architecture
interrupts	B-Application
—	O
there	O
is	O
a	O
change	O
away	O
from	O
the	O
currently	O
running	O
process	B-Operating_System
.	O
</s>
<s>
Similarly	O
,	O
both	O
hardware	B-Architecture
and	O
software	O
interrupts	B-Application
execute	O
an	O
interrupt	B-General_Concept
service	I-General_Concept
routine	I-General_Concept
.	O
</s>
<s>
Software	O
interrupts	B-Application
may	O
be	O
normally	O
occurring	O
events	O
.	O
</s>
<s>
It	O
is	O
expected	O
that	O
a	O
time	O
slice	O
will	O
occur	O
,	O
so	O
the	O
kernel	B-Operating_System
will	O
have	O
to	O
perform	O
a	O
context	B-Operating_System
switch	I-Operating_System
.	O
</s>
<s>
A	O
computer	B-Application
program	I-Application
may	O
set	O
a	O
timer	O
to	O
go	O
off	O
after	O
a	O
few	O
seconds	O
in	O
case	O
too	O
much	O
data	O
causes	O
an	O
algorithm	O
to	O
take	O
too	O
long	O
.	O
</s>
<s>
Software	O
interrupts	B-Application
may	O
be	O
error	O
conditions	O
,	O
such	O
as	O
a	O
malformed	O
machine	B-Language
instruction	I-Language
.	O
</s>
<s>
However	O
,	O
the	O
most	O
common	O
error	O
conditions	O
are	O
division	B-Algorithm
by	I-Algorithm
zero	I-Algorithm
and	O
accessing	B-Error_Name
an	I-Error_Name
invalid	I-Error_Name
memory	I-Error_Name
address	I-Error_Name
.	O
</s>
<s>
Users	O
can	O
send	O
messages	O
to	O
the	O
kernel	B-Operating_System
to	O
modify	O
the	O
behavior	O
of	O
a	O
currently	O
running	O
process	B-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
command-line	B-Application
environment	I-Application
,	O
pressing	O
the	O
interrupt	B-Application
character	O
(	O
usually	O
Control-C	B-Device
)	O
might	O
terminate	O
the	O
currently	O
running	O
process	B-Operating_System
.	O
</s>
<s>
To	O
generate	O
software	O
interrupts	B-Application
for	O
x86	B-Operating_System
CPUs	O
,	O
the	O
INT	B-Device
assembly	B-Language
language	I-Language
instruction	O
is	O
available	O
.	O
</s>
<s>
The	O
syntax	O
is	O
INT	B-Device
X	O
,	O
where	O
X	O
is	O
the	O
offset	O
number	O
(	O
in	O
hexadecimal	O
format	O
)	O
to	O
the	O
interrupt	B-General_Concept
vector	I-General_Concept
table	I-General_Concept
.	O
</s>
<s>
To	O
generate	O
software	O
interrupts	B-Application
in	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
the	O
kill(pid,signum )	O
system	B-Operating_System
call	I-Operating_System
will	O
send	O
a	O
signal	B-Operating_System
to	O
another	O
process	B-Operating_System
.	O
</s>
<s>
pid	O
is	O
the	O
process	B-Operating_System
identifier	I-Operating_System
of	O
the	O
receiving	O
process	B-Operating_System
.	O
</s>
<s>
signum	O
is	O
the	O
signal	B-Operating_System
number	O
(	O
in	O
mnemonic	O
format	O
)	O
to	O
be	O
sent	O
.	O
</s>
<s>
(	O
The	O
abrasive	O
name	O
of	O
kill	O
was	O
chosen	O
because	O
early	O
implementations	O
only	O
terminated	O
the	O
process	B-Operating_System
.	O
)	O
</s>
<s>
In	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
signals	O
inform	O
processes	O
of	O
the	O
occurrence	O
of	O
asynchronous	O
events	O
.	O
</s>
<s>
To	O
communicate	O
asynchronously	O
,	O
interrupts	B-Application
are	O
required	O
.	O
</s>
<s>
One	O
reason	O
a	O
process	B-Operating_System
needs	O
to	O
asynchronously	O
communicate	O
to	O
another	O
process	B-Operating_System
solves	O
a	O
variation	O
of	O
the	O
classic	O
reader/writer	B-Operating_System
problem	I-Operating_System
.	O
</s>
<s>
The	O
writer	O
receives	O
a	O
pipe	O
from	O
the	O
shell	B-Operating_System
for	O
its	O
output	O
to	O
be	O
sent	O
to	O
the	O
reader	O
's	O
input	O
stream	O
.	O
</s>
<s>
The	O
command-line	B-Application
syntax	O
is	O
alpha	O
|	O
bravo	O
.	O
</s>
<s>
alpha	O
will	O
write	O
to	O
the	O
pipe	O
when	O
its	O
computation	O
is	O
ready	O
and	O
then	O
sleep	O
in	O
the	O
wait	B-Operating_System
queue	B-Device
.	O
</s>
<s>
bravo	O
will	O
then	O
be	O
moved	O
to	O
the	O
ready	B-Operating_System
queue	I-Operating_System
and	O
soon	O
will	O
read	O
from	O
its	O
input	O
stream	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
will	O
generate	O
software	O
interrupts	B-Application
to	O
coordinate	O
the	O
piping	O
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
finishes	O
normally	O
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
has	O
an	O
error	O
exception	B-General_Concept
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
runs	O
out	O
of	O
a	O
system	O
resource	O
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
executes	O
an	O
illegal	O
instruction	O
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
sets	O
an	O
alarm	O
event	O
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
is	O
aborted	O
from	O
the	O
keyboard	B-Device
.	O
</s>
<s>
when	O
a	O
process	B-Operating_System
has	O
a	O
tracing	O
alert	O
for	O
debugging	O
.	O
</s>
<s>
Input/Output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
devices	O
are	O
slower	O
than	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
Therefore	O
,	O
it	O
would	O
slow	O
down	O
the	O
computer	O
if	O
the	O
CPU	B-General_Concept
had	O
to	O
wait	B-Operating_System
for	O
each	O
I/O	B-General_Concept
to	O
finish	O
.	O
</s>
<s>
Instead	O
,	O
a	O
computer	O
may	O
implement	O
interrupts	B-Application
for	O
I/O	B-General_Concept
completion	O
,	O
avoiding	O
the	O
need	O
for	O
polling	B-General_Concept
or	O
busy	B-Operating_System
waiting	I-Operating_System
.	O
</s>
<s>
Some	O
computers	O
require	O
an	O
interrupt	B-Application
for	O
each	O
character	O
or	O
word	O
,	O
costing	O
a	O
significant	O
amount	O
of	O
CPU	B-General_Concept
time	O
.	O
</s>
<s>
Direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	O
)	O
is	O
an	O
architecture	O
feature	O
to	O
allow	O
devices	O
to	O
bypass	O
the	O
CPU	B-General_Concept
and	O
access	O
main	B-Architecture
memory	I-Architecture
directly	O
.	O
</s>
<s>
(	O
Separate	O
from	O
the	O
architecture	O
,	O
a	O
device	O
may	O
perform	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
to	O
and	O
from	O
main	B-Architecture
memory	I-Architecture
either	O
directly	O
or	O
via	O
a	O
bus	O
.	O
)	O
</s>
<s>
When	O
a	O
computer	O
user	O
types	O
a	O
key	O
on	O
the	O
keyboard	B-Device
,	O
typically	O
the	O
character	O
appears	O
immediately	O
on	O
the	O
screen	O
.	O
</s>
<s>
Likewise	O
,	O
when	O
a	O
user	O
moves	O
a	O
mouse	B-Device
,	O
the	O
cursor	B-Application
immediately	O
moves	O
across	O
the	O
screen	O
.	O
</s>
<s>
Each	O
keystroke	O
and	O
mouse	B-Device
movement	O
generates	O
an	O
interrupt	B-Application
called	O
Interrupt-driven	O
I/O	B-General_Concept
.	O
</s>
<s>
An	O
interrupt-driven	O
I/O	B-General_Concept
occurs	O
when	O
a	O
process	B-Operating_System
causes	O
an	O
interrupt	B-Application
for	O
every	O
character	O
or	O
word	O
transmitted	O
.	O
</s>
<s>
Devices	O
such	O
as	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
solid	B-Device
state	I-Device
drives	I-Device
,	O
and	O
magnetic	O
tape	O
drives	O
can	O
transfer	O
data	O
at	O
a	O
rate	O
high	O
enough	O
that	O
interrupting	B-Application
the	O
CPU	B-General_Concept
for	O
every	O
byte	O
or	O
word	O
transferred	O
,	O
and	O
having	O
the	O
CPU	B-General_Concept
transfer	O
the	O
byte	O
or	O
word	O
between	O
the	O
device	O
and	O
memory	O
,	O
would	O
require	O
too	O
much	O
CPU	B-General_Concept
time	O
.	O
</s>
<s>
Data	O
is	O
,	O
instead	O
,	O
transferred	O
between	O
the	O
device	O
and	O
memory	O
independently	O
of	O
the	O
CPU	B-General_Concept
by	O
hardware	B-Architecture
such	O
as	O
a	O
channel	B-Device
or	O
a	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
controller	O
;	O
an	O
interrupt	B-Application
is	O
delivered	O
only	O
when	O
all	O
the	O
data	O
is	O
transferred	O
.	O
</s>
<s>
If	O
a	O
computer	B-Application
program	I-Application
executes	O
a	O
system	B-Operating_System
call	I-Operating_System
to	O
perform	O
a	O
block	O
I/O	B-General_Concept
write	O
operation	O
,	O
then	O
the	O
system	B-Operating_System
call	I-Operating_System
might	O
execute	O
the	O
following	O
instructions	O
:	O
</s>
<s>
Set	O
the	O
contents	O
of	O
the	O
CPU	B-General_Concept
's	O
registers	B-General_Concept
(	O
including	O
the	O
program	B-General_Concept
counter	I-General_Concept
)	O
into	O
the	O
process	B-Operating_System
control	I-Operating_System
block	I-Operating_System
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
maintains	O
this	O
table	O
to	O
keep	O
track	O
of	O
which	O
processes	O
are	O
waiting	O
for	O
which	O
devices	O
.	O
</s>
<s>
One	O
field	O
in	O
the	O
table	O
is	O
the	O
memory	B-General_Concept
address	I-General_Concept
of	O
the	O
process	B-Operating_System
control	I-Operating_System
block	I-Operating_System
.	O
</s>
<s>
Place	O
all	O
the	O
characters	O
to	O
be	O
sent	O
to	O
the	O
device	O
into	O
a	O
memory	B-Architecture
buffer	I-Architecture
.	O
</s>
<s>
Set	O
the	O
memory	B-General_Concept
address	I-General_Concept
of	O
the	O
memory	B-Architecture
buffer	I-Architecture
to	O
a	O
predetermined	O
device	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
Set	O
the	O
buffer	B-General_Concept
size	O
(	O
an	O
integer	O
)	O
to	O
another	O
predetermined	O
register	O
.	O
</s>
<s>
Execute	O
the	O
machine	B-Language
instruction	I-Language
to	O
begin	O
the	O
writing	O
.	O
</s>
<s>
Perform	O
a	O
context	B-Operating_System
switch	I-Operating_System
to	O
the	O
next	O
process	B-Operating_System
in	O
the	O
ready	B-Operating_System
queue	I-Operating_System
.	O
</s>
<s>
While	O
the	O
writing	O
takes	O
place	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
will	O
context	B-Operating_System
switch	I-Operating_System
to	O
other	O
processes	O
as	O
normal	O
.	O
</s>
<s>
When	O
the	O
device	O
finishes	O
writing	O
,	O
the	O
device	O
will	O
interrupt	B-Application
the	O
currently	O
running	O
process	B-Operating_System
by	O
asserting	O
an	O
interrupt	B-General_Concept
request	I-General_Concept
.	O
</s>
<s>
Upon	O
accepting	O
the	O
interrupt	B-General_Concept
request	I-General_Concept
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
will	O
:	O
</s>
<s>
Push	O
the	O
contents	O
of	O
the	O
program	B-General_Concept
counter	I-General_Concept
(	O
a	O
register	O
)	O
followed	O
by	O
the	O
status	B-General_Concept
register	I-General_Concept
onto	O
the	O
call	B-General_Concept
stack	I-General_Concept
.	O
</s>
<s>
Push	O
the	O
contents	O
of	O
the	O
other	O
registers	B-General_Concept
onto	O
the	O
call	B-General_Concept
stack	I-General_Concept
.	O
</s>
<s>
(	O
Alternatively	O
,	O
the	O
contents	O
of	O
the	O
registers	B-General_Concept
may	O
be	O
placed	O
in	O
a	O
system	O
table	O
.	O
)	O
</s>
<s>
The	O
integer	O
is	O
an	O
offset	O
to	O
the	O
interrupt	B-General_Concept
vector	I-General_Concept
table	I-General_Concept
.	O
</s>
<s>
Extract	O
the	O
process	B-Operating_System
control	I-Operating_System
block	I-Operating_System
.	O
</s>
<s>
Perform	O
a	O
context	B-Operating_System
switch	I-Operating_System
back	O
to	O
the	O
writing	O
process	B-Operating_System
.	O
</s>
<s>
When	O
the	O
writing	O
process	B-Operating_System
has	O
its	O
time	O
slice	O
expired	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
will	O
:	O
</s>
<s>
Pop	O
from	O
the	O
call	B-General_Concept
stack	I-General_Concept
the	O
registers	B-General_Concept
other	O
than	O
the	O
status	B-General_Concept
register	I-General_Concept
and	O
program	B-General_Concept
counter	I-General_Concept
.	O
</s>
<s>
Pop	O
from	O
the	O
call	B-General_Concept
stack	I-General_Concept
the	O
status	B-General_Concept
register	I-General_Concept
.	O
</s>
<s>
Pop	O
from	O
the	O
call	B-General_Concept
stack	I-General_Concept
the	O
address	O
of	O
the	O
next	B-General_Concept
instruction	I-General_Concept
,	O
and	O
set	O
it	O
back	O
into	O
the	O
program	B-General_Concept
counter	I-General_Concept
.	O
</s>
<s>
With	O
the	O
program	B-General_Concept
counter	I-General_Concept
now	O
reset	O
,	O
the	O
interrupted	O
process	B-Operating_System
will	O
resume	O
its	O
time	O
slice	O
.	O
</s>
<s>
CPUs	O
with	O
this	O
capability	O
offer	O
at	O
least	O
two	O
modes	O
:	O
user	B-Operating_System
mode	I-Operating_System
and	O
supervisor	O
mode	O
.	O
</s>
<s>
User	B-Operating_System
mode	I-Operating_System
operation	O
sets	O
limits	O
on	O
instruction	O
use	O
and	O
typically	O
disallows	O
direct	O
access	O
to	O
machine	O
resources	O
.	O
</s>
<s>
CPUs	O
might	O
have	O
other	O
modes	O
similar	O
to	O
user	B-Operating_System
mode	I-Operating_System
as	O
well	O
,	O
such	O
as	O
the	O
virtual	O
modes	O
in	O
order	O
to	O
emulate	O
older	O
processor	O
types	O
,	O
such	O
as	O
16-bit	B-Device
processors	I-Device
on	O
a	O
32-bit	O
one	O
,	O
or	O
32-bit	O
processors	O
on	O
a	O
64-bit	B-Device
one	O
.	O
</s>
<s>
Once	O
an	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
has	O
been	O
loaded	O
and	O
started	O
,	O
the	O
boundary	O
between	O
user	B-Operating_System
mode	I-Operating_System
and	O
supervisor	O
mode	O
(	O
also	O
known	O
as	O
kernel	B-Operating_System
mode	O
)	O
can	O
be	O
established	O
.	O
</s>
<s>
Supervisor	O
mode	O
is	O
used	O
by	O
the	O
kernel	B-Operating_System
for	O
low	O
level	O
tasks	O
that	O
need	O
unrestricted	O
access	O
to	O
hardware	B-Architecture
,	O
such	O
as	O
controlling	O
how	O
memory	O
is	O
accessed	O
,	O
and	O
communicating	O
with	O
devices	O
such	O
as	O
disk	B-General_Concept
drives	I-General_Concept
and	O
video	O
display	O
devices	O
.	O
</s>
<s>
User	B-Operating_System
mode	I-Operating_System
,	O
in	O
contrast	O
,	O
is	O
used	O
for	O
almost	O
everything	O
else	O
.	O
</s>
<s>
Application	O
programs	O
,	O
such	O
as	O
word	O
processors	O
and	O
database	O
managers	O
,	O
operate	O
within	O
user	B-Operating_System
mode	I-Operating_System
,	O
and	O
can	O
only	O
access	O
machine	O
resources	O
by	O
turning	O
control	O
over	O
to	O
the	O
kernel	B-Operating_System
,	O
a	O
process	B-Operating_System
which	O
causes	O
a	O
switch	O
to	O
supervisor	O
mode	O
.	O
</s>
<s>
Typically	O
,	O
the	O
transfer	O
of	O
control	O
to	O
the	O
kernel	B-Operating_System
is	O
achieved	O
by	O
executing	O
a	O
software	O
interrupt	B-Application
instruction	O
,	O
such	O
as	O
the	O
Motorola	O
68000	O
TRAP	B-Application
instruction	O
.	O
</s>
<s>
The	O
software	O
interrupt	B-Application
causes	O
the	O
processor	O
to	O
switch	O
from	O
user	B-Operating_System
mode	I-Operating_System
to	O
supervisor	O
mode	O
and	O
begin	O
executing	O
code	O
that	O
allows	O
the	O
kernel	B-Operating_System
to	O
take	O
control	O
.	O
</s>
<s>
In	O
user	B-Operating_System
mode	I-Operating_System
,	O
programs	O
usually	O
have	O
access	O
to	O
a	O
restricted	O
set	O
of	O
processor	O
instructions	O
,	O
and	O
generally	O
cannot	O
execute	O
any	O
instructions	O
that	O
could	O
potentially	O
cause	O
disruption	O
to	O
the	O
system	O
's	O
operation	O
.	O
</s>
<s>
In	O
supervisor	O
mode	O
,	O
instruction	O
execution	O
restrictions	O
are	O
typically	O
removed	O
,	O
allowing	O
the	O
kernel	B-Operating_System
unrestricted	O
access	O
to	O
all	O
machine	O
resources	O
.	O
</s>
<s>
The	O
term	O
"	O
user	B-Operating_System
mode	I-Operating_System
resource	O
"	O
generally	O
refers	O
to	O
one	O
or	O
more	O
CPU	B-General_Concept
registers	I-General_Concept
,	O
which	O
contain	O
information	O
that	O
the	O
running	O
program	O
is	O
n't	O
allowed	O
to	O
alter	O
.	O
</s>
<s>
Attempts	O
to	O
alter	O
these	O
resources	O
generally	O
cause	O
a	O
switch	O
to	O
supervisor	O
mode	O
,	O
where	O
the	O
operating	B-General_Concept
system	I-General_Concept
can	O
deal	O
with	O
the	O
illegal	O
operation	O
the	O
program	O
was	O
attempting	O
;	O
for	O
example	O
,	O
by	O
forcibly	O
terminating	O
(	O
"	O
killing	O
"	O
)	O
the	O
program	O
.	O
</s>
<s>
Among	O
other	O
things	O
,	O
a	O
multiprogramming	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
must	O
be	O
responsible	O
for	O
managing	O
all	O
system	O
memory	O
which	O
is	O
currently	O
in	O
use	O
by	O
the	O
programs	O
.	O
</s>
<s>
Cooperative	O
memory	B-General_Concept
management	I-General_Concept
,	O
used	O
by	O
many	O
early	O
operating	B-General_Concept
systems	I-General_Concept
,	O
assumes	O
that	O
all	O
programs	O
make	O
voluntary	O
use	O
of	O
the	O
kernel	B-Operating_System
's	O
memory	O
manager	O
,	O
and	O
do	O
not	O
exceed	O
their	O
allocated	O
memory	O
.	O
</s>
<s>
This	O
system	O
of	O
memory	B-General_Concept
management	I-General_Concept
is	O
almost	O
never	O
seen	O
any	O
more	O
,	O
since	O
programs	O
often	O
contain	O
bugs	B-Application
which	O
can	O
cause	O
them	O
to	O
exceed	O
their	O
allocated	O
memory	O
.	O
</s>
<s>
Malicious	O
programs	O
or	O
viruses	O
may	O
purposefully	O
alter	O
another	O
program	O
's	O
memory	O
,	O
or	O
may	O
affect	O
the	O
operation	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
itself	O
.	O
</s>
<s>
With	O
cooperative	O
memory	B-General_Concept
management	I-General_Concept
,	O
it	O
takes	O
only	O
one	O
misbehaved	O
program	O
to	O
crash	B-General_Concept
the	O
system	O
.	O
</s>
<s>
Memory	B-General_Concept
protection	I-General_Concept
enables	O
the	O
kernel	B-Operating_System
to	O
limit	O
a	O
process	B-Operating_System
 '	O
access	O
to	O
the	O
computer	O
's	O
memory	O
.	O
</s>
<s>
Various	O
methods	O
of	O
memory	B-General_Concept
protection	I-General_Concept
exist	O
,	O
including	O
memory	B-General_Concept
segmentation	I-General_Concept
and	O
paging	B-Architecture
.	O
</s>
<s>
All	O
methods	O
require	O
some	O
level	O
of	O
hardware	B-Architecture
support	O
(	O
such	O
as	O
the	O
80286	B-General_Concept
MMU	O
)	O
,	O
which	O
does	O
n't	O
exist	O
in	O
all	O
computers	O
.	O
</s>
<s>
In	O
both	O
segmentation	B-General_Concept
and	O
paging	B-Architecture
,	O
certain	O
protected	B-Application
mode	I-Application
registers	B-General_Concept
specify	O
to	O
the	O
CPU	B-General_Concept
what	O
memory	B-General_Concept
address	I-General_Concept
it	O
should	O
allow	O
a	O
running	O
program	O
to	O
access	O
.	O
</s>
<s>
Attempts	O
to	O
access	O
other	O
addresses	O
trigger	O
an	O
interrupt	B-Application
,	O
which	O
causes	O
the	O
CPU	B-General_Concept
to	O
re-enter	O
supervisor	O
mode	O
,	O
placing	O
the	O
kernel	B-Operating_System
in	O
charge	O
.	O
</s>
<s>
This	O
is	O
called	O
a	O
segmentation	B-Error_Name
violation	I-Error_Name
or	O
Seg-V	O
for	O
short	O
,	O
and	O
since	O
it	O
is	O
both	O
difficult	O
to	O
assign	O
a	O
meaningful	O
result	O
to	O
such	O
an	O
operation	O
,	O
and	O
because	O
it	O
is	O
usually	O
a	O
sign	O
of	O
a	O
misbehaving	O
program	O
,	O
the	O
kernel	B-Operating_System
generally	O
resorts	O
to	O
terminating	O
the	O
offending	O
program	O
,	O
and	O
reports	O
the	O
error	O
.	O
</s>
<s>
Windows	O
versions	O
3.1	O
through	O
ME	O
had	O
some	O
level	O
of	O
memory	B-General_Concept
protection	I-General_Concept
,	O
but	O
programs	O
could	O
easily	O
circumvent	O
the	O
need	O
to	O
use	O
it	O
.	O
</s>
<s>
A	O
general	B-Operating_System
protection	I-Operating_System
fault	I-Operating_System
would	O
be	O
produced	O
,	O
indicating	O
a	O
segmentation	B-Error_Name
violation	I-Error_Name
had	O
occurred	O
;	O
however	O
,	O
the	O
system	O
would	O
often	O
crash	B-General_Concept
anyway	O
.	O
</s>
<s>
The	O
use	O
of	O
virtual	B-Architecture
memory	I-Architecture
addressing	O
(	O
such	O
as	O
paging	B-Architecture
or	O
segmentation	B-General_Concept
)	O
means	O
that	O
the	O
kernel	B-Operating_System
can	O
choose	O
what	O
memory	O
each	O
program	O
may	O
use	O
at	O
any	O
given	O
time	O
,	O
allowing	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
use	O
the	O
same	O
memory	B-General_Concept
locations	I-General_Concept
for	O
multiple	O
tasks	O
.	O
</s>
<s>
If	O
a	O
program	O
tries	O
to	O
access	O
memory	O
that	O
is	O
n't	O
in	O
its	O
current	O
range	O
of	O
accessible	O
memory	O
,	O
but	O
nonetheless	O
has	O
been	O
allocated	O
to	O
it	O
,	O
the	O
kernel	B-Operating_System
is	O
interrupted	O
in	O
the	O
same	O
way	O
as	O
it	O
would	O
if	O
the	O
program	O
were	O
to	O
exceed	O
its	O
allocated	O
memory	O
.	O
</s>
<s>
(	O
See	O
section	O
on	O
memory	B-General_Concept
management	I-General_Concept
.	O
)	O
</s>
<s>
Under	O
UNIX	B-Application
this	O
kind	O
of	O
interrupt	B-Application
is	O
referred	O
to	O
as	O
a	O
page	B-General_Concept
fault	I-General_Concept
.	O
</s>
<s>
When	O
the	O
kernel	B-Operating_System
detects	O
a	O
page	B-General_Concept
fault	I-General_Concept
it	O
generally	O
adjusts	O
the	O
virtual	B-Architecture
memory	I-Architecture
range	O
of	O
the	O
program	O
which	O
triggered	O
it	O
,	O
granting	O
it	O
access	O
to	O
the	O
memory	O
requested	O
.	O
</s>
<s>
This	O
gives	O
the	O
kernel	B-Operating_System
discretionary	O
power	O
over	O
where	O
a	O
particular	O
application	O
's	O
memory	O
is	O
stored	O
,	O
or	O
even	O
whether	O
or	O
not	O
it	O
has	O
actually	O
been	O
allocated	O
yet	O
.	O
</s>
<s>
In	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
,	O
memory	O
which	O
is	O
accessed	O
less	O
frequently	O
can	O
be	O
temporarily	O
stored	O
on	O
a	O
disk	O
or	O
other	O
media	O
to	O
make	O
that	O
space	O
available	O
for	O
use	O
by	O
other	O
programs	O
.	O
</s>
<s>
This	O
is	O
called	O
swapping	B-Architecture
,	O
as	O
an	O
area	O
of	O
memory	O
can	O
be	O
used	O
by	O
multiple	O
programs	O
,	O
and	O
what	O
that	O
memory	O
area	O
contains	O
can	O
be	O
swapped	O
or	O
exchanged	O
on	O
demand	O
.	O
</s>
<s>
"	O
Virtual	B-Architecture
memory	I-Architecture
"	O
provides	O
the	O
programmer	O
or	O
the	O
user	O
with	O
the	O
perception	O
that	O
there	O
is	O
a	O
much	O
larger	O
amount	O
of	O
RAM	B-Architecture
in	O
the	O
computer	O
than	O
is	O
really	O
there	O
.	O
</s>
<s>
Multitasking	B-Operating_System
refers	O
to	O
the	O
running	O
of	O
multiple	O
independent	O
computer	B-Application
programs	I-Application
on	O
the	O
same	O
computer	O
,	O
giving	O
the	O
appearance	O
that	O
it	O
is	O
performing	O
the	O
tasks	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Since	O
most	O
computers	O
can	O
do	O
at	O
most	O
one	O
or	O
two	O
things	O
at	O
one	O
time	O
,	O
this	O
is	O
generally	O
done	O
via	O
time-sharing	B-General_Concept
,	O
which	O
means	O
that	O
each	O
program	O
uses	O
a	O
share	O
of	O
the	O
computer	O
's	O
time	O
to	O
execute	O
.	O
</s>
<s>
An	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
contains	O
a	O
scheduling	O
program	O
which	O
determines	O
how	O
much	O
time	O
each	O
process	B-Operating_System
spends	O
executing	O
,	O
and	O
in	O
which	O
order	O
execution	O
control	O
should	O
be	O
passed	O
to	O
programs	O
.	O
</s>
<s>
Control	O
is	O
passed	O
to	O
a	O
process	B-Operating_System
by	O
the	O
kernel	B-Operating_System
,	O
which	O
allows	O
the	O
program	O
access	O
to	O
the	O
CPU	B-General_Concept
and	O
memory	O
.	O
</s>
<s>
Later	O
,	O
control	O
is	O
returned	O
to	O
the	O
kernel	B-Operating_System
through	O
some	O
mechanism	O
,	O
so	O
that	O
another	O
program	O
may	O
be	O
allowed	O
to	O
use	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
This	O
so-called	O
passing	O
of	O
control	O
between	O
the	O
kernel	B-Operating_System
and	O
applications	O
is	O
called	O
a	O
context	B-Operating_System
switch	I-Operating_System
.	O
</s>
<s>
An	O
early	O
model	O
which	O
governed	O
the	O
allocation	O
of	O
time	O
to	O
programs	O
was	O
called	O
cooperative	B-Operating_System
multitasking	I-Operating_System
.	O
</s>
<s>
In	O
this	O
model	O
,	O
when	O
control	O
is	O
passed	O
to	O
a	O
program	O
by	O
the	O
kernel	B-Operating_System
,	O
it	O
may	O
execute	O
for	O
as	O
long	O
as	O
it	O
wants	O
before	O
explicitly	O
returning	O
control	O
to	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
means	O
that	O
a	O
malicious	O
or	O
malfunctioning	O
program	O
may	O
not	O
only	O
prevent	O
any	O
other	O
programs	O
from	O
using	O
the	O
CPU	B-General_Concept
,	O
but	O
it	O
can	O
hang	O
the	O
entire	O
system	O
if	O
it	O
enters	O
an	O
infinite	B-Algorithm
loop	I-Algorithm
.	O
</s>
<s>
Modern	O
operating	B-General_Concept
systems	I-General_Concept
extend	O
the	O
concepts	O
of	O
application	O
preemption	B-Operating_System
to	O
device	B-Application
drivers	I-Application
and	O
kernel	B-Operating_System
code	O
,	O
so	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
has	O
preemptive	B-Operating_System
control	O
over	O
internal	O
run-times	O
as	O
well	O
.	O
</s>
<s>
The	O
philosophy	O
governing	O
preemptive	B-Operating_System
multitasking	B-Operating_System
is	O
that	O
of	O
ensuring	O
that	O
all	O
programs	O
are	O
given	O
regular	O
time	O
on	O
the	O
CPU	B-General_Concept
.	O
</s>
<s>
This	O
implies	O
that	O
all	O
programs	O
must	O
be	O
limited	O
in	O
how	O
much	O
time	O
they	O
are	O
allowed	O
to	O
spend	O
on	O
the	O
CPU	B-General_Concept
without	O
being	O
interrupted	O
.	O
</s>
<s>
To	O
accomplish	O
this	O
,	O
modern	O
operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
make	O
use	O
of	O
a	O
timed	O
interrupt	B-Application
.	O
</s>
<s>
A	O
protected	B-Application
mode	I-Application
timer	O
is	O
set	O
by	O
the	O
kernel	B-Operating_System
which	O
triggers	O
a	O
return	O
to	O
supervisor	O
mode	O
after	O
the	O
specified	O
time	O
has	O
elapsed	O
.	O
</s>
<s>
(	O
See	O
above	O
sections	O
on	O
Interrupts	B-Application
and	O
Dual	O
Mode	O
Operation	O
.	O
)	O
</s>
<s>
On	O
many	O
single	B-Operating_System
user	I-Operating_System
operating	B-General_Concept
systems	I-General_Concept
cooperative	B-Operating_System
multitasking	I-Operating_System
is	O
perfectly	O
adequate	O
,	O
as	O
home	O
computers	O
generally	O
run	O
a	O
small	O
number	O
of	O
well	O
tested	O
programs	O
.	O
</s>
<s>
AmigaOS	B-Application
is	O
an	O
exception	B-General_Concept
,	O
having	O
preemptive	B-Operating_System
multitasking	B-Operating_System
from	O
its	O
first	O
version	O
.	O
</s>
<s>
Windows	B-Device
NT	I-Device
was	O
the	O
first	O
version	O
of	O
Microsoft	B-Application
Windows	I-Application
which	O
enforced	O
preemptive	B-Operating_System
multitasking	B-Operating_System
,	O
but	O
it	O
did	O
n't	O
reach	O
the	O
home	O
user	O
market	O
until	O
Windows	B-Application
XP	I-Application
(	O
since	O
Windows	B-Device
NT	I-Device
was	O
targeted	O
at	O
professionals	O
)	O
.	O
</s>
<s>
Access	O
to	O
data	O
stored	O
on	O
disks	B-Device
is	O
a	O
central	O
feature	O
of	O
all	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Computers	O
store	O
data	O
on	O
disks	B-Device
using	O
files	B-Operating_System
,	O
which	O
are	O
structured	O
in	O
specific	O
ways	O
in	O
order	O
to	O
allow	O
for	O
faster	O
access	O
,	O
higher	O
reliability	O
,	O
and	O
to	O
make	O
better	O
use	O
of	O
the	O
drive	O
's	O
available	O
space	O
.	O
</s>
<s>
The	O
specific	O
way	O
in	O
which	O
files	B-Operating_System
are	O
stored	O
on	O
a	O
disk	O
is	O
called	O
a	O
file	B-Application
system	I-Application
,	O
and	O
enables	O
files	B-Operating_System
to	O
have	O
names	O
and	O
attributes	O
.	O
</s>
<s>
It	O
also	O
allows	O
them	O
to	O
be	O
stored	O
in	O
a	O
hierarchy	O
of	O
directories	B-Application
or	O
folders	O
arranged	O
in	O
a	O
directory	B-Application
tree	I-Application
.	O
</s>
<s>
Early	O
operating	B-General_Concept
systems	I-General_Concept
generally	O
supported	O
a	O
single	O
type	O
of	O
disk	O
drive	O
and	O
only	O
one	O
kind	O
of	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Early	O
file	B-Application
systems	I-Application
were	O
limited	O
in	O
their	O
capacity	O
,	O
speed	O
,	O
and	O
in	O
the	O
kinds	O
of	O
file	O
names	O
and	O
directory	B-Architecture
structures	I-Architecture
they	O
could	O
use	O
.	O
</s>
<s>
These	O
limitations	O
often	O
reflected	O
limitations	O
in	O
the	O
operating	B-General_Concept
systems	I-General_Concept
they	O
were	O
designed	O
for	O
,	O
making	O
it	O
very	O
difficult	O
for	O
an	O
operating	B-General_Concept
system	I-General_Concept
to	O
support	O
more	O
than	O
one	O
file	B-Application
system	I-Application
.	O
</s>
<s>
While	O
many	O
simpler	O
operating	B-General_Concept
systems	I-General_Concept
support	O
a	O
limited	O
range	O
of	O
options	O
for	O
accessing	O
storage	O
systems	O
,	O
operating	B-General_Concept
systems	I-General_Concept
like	O
UNIX	B-Application
and	O
Linux	B-Application
support	O
a	O
technology	O
known	O
as	O
a	O
virtual	B-Application
file	I-Application
system	I-Application
or	O
VFS	O
.	O
</s>
<s>
An	O
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
UNIX	B-Application
supports	O
a	O
wide	O
array	O
of	O
storage	O
devices	O
,	O
regardless	O
of	O
their	O
design	O
or	O
file	B-Application
systems	I-Application
,	O
allowing	O
them	O
to	O
be	O
accessed	O
through	O
a	O
common	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
.	O
</s>
<s>
A	O
VFS	O
allows	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
provide	O
programs	O
with	O
access	O
to	O
an	O
unlimited	O
number	O
of	O
devices	O
with	O
an	O
infinite	O
variety	O
of	O
file	B-Application
systems	I-Application
installed	O
on	O
them	O
,	O
through	O
the	O
use	O
of	O
specific	O
device	B-Application
drivers	I-Application
and	O
file	B-Application
system	I-Application
drivers	O
.	O
</s>
<s>
A	O
connected	O
storage	B-General_Concept
device	I-General_Concept
,	O
such	O
as	O
a	O
hard	B-Device
drive	I-Device
,	O
is	O
accessed	O
through	O
a	O
device	B-Application
driver	I-Application
.	O
</s>
<s>
The	O
device	B-Application
driver	I-Application
understands	O
the	O
specific	O
language	O
of	O
the	O
drive	O
and	O
is	O
able	O
to	O
translate	O
that	O
language	O
into	O
a	O
standard	O
language	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
access	O
all	O
disk	B-General_Concept
drives	I-General_Concept
.	O
</s>
<s>
On	O
UNIX	B-Application
,	O
this	O
is	O
the	O
language	O
of	O
block	O
devices	O
.	O
</s>
<s>
When	O
the	O
kernel	B-Operating_System
has	O
an	O
appropriate	O
device	B-Application
driver	I-Application
in	O
place	O
,	O
it	O
can	O
then	O
access	O
the	O
contents	O
of	O
the	O
disk	O
drive	O
in	O
raw	O
format	O
,	O
which	O
may	O
contain	O
one	O
or	O
more	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
A	O
file	B-Application
system	I-Application
driver	O
is	O
used	O
to	O
translate	O
the	O
commands	O
used	O
to	O
access	O
each	O
specific	O
file	B-Application
system	I-Application
into	O
a	O
standard	O
set	O
of	O
commands	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
can	O
use	O
to	O
talk	O
to	O
all	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
Programs	O
can	O
then	O
deal	O
with	O
these	O
file	B-Application
systems	I-Application
on	O
the	O
basis	O
of	O
filenames	O
,	O
and	O
directories/folders	O
,	O
contained	O
within	O
a	O
hierarchical	O
structure	O
.	O
</s>
<s>
They	O
can	O
create	O
,	O
delete	O
,	O
open	O
,	O
and	O
close	O
files	B-Operating_System
,	O
as	O
well	O
as	O
gather	O
various	O
information	O
about	O
them	O
,	O
including	O
access	O
permissions	O
,	O
size	O
,	O
free	B-Application
space	O
,	O
and	O
creation	O
and	O
modification	O
dates	O
.	O
</s>
<s>
Various	O
differences	O
between	O
file	B-Application
systems	I-Application
make	O
supporting	O
all	O
file	B-Application
systems	I-Application
difficult	O
.	O
</s>
<s>
Allowed	O
characters	O
in	O
file	O
names	O
,	O
case	O
sensitivity	O
,	O
and	O
the	O
presence	O
of	O
various	O
kinds	O
of	O
file	B-Application
attributes	I-Application
makes	O
the	O
implementation	O
of	O
a	O
single	O
interface	O
for	O
every	O
file	B-Application
system	I-Application
a	O
daunting	O
task	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
tend	O
to	O
recommend	O
using	O
(	O
and	O
so	O
support	O
natively	O
)	O
file	B-Application
systems	I-Application
specifically	O
designed	O
for	O
them	O
;	O
for	O
example	O
,	O
NTFS	B-Application
in	O
Windows	O
and	O
ReiserFS	B-Application
,	O
Reiser4	B-Application
,	O
ext3	B-Application
,	O
ext4	B-Application
and	O
Btrfs	B-Operating_System
in	O
Linux	B-Application
.	O
</s>
<s>
However	O
,	O
in	O
practice	O
,	O
third	O
party	O
drivers	O
are	O
usually	O
available	O
to	O
give	O
support	O
for	O
the	O
most	O
widely	O
used	O
file	B-Application
systems	I-Application
in	O
most	O
general-purpose	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
(	O
for	O
example	O
,	O
NTFS	B-Application
is	O
available	O
in	O
Linux	B-Application
through	O
NTFS-3g	B-Application
,	O
and	O
ext2/3	O
and	O
ReiserFS	B-Application
are	O
available	O
in	O
Windows	O
through	O
third-party	O
software	O
)	O
.	O
</s>
<s>
Support	O
for	O
file	B-Application
systems	I-Application
is	O
highly	O
varied	O
among	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
,	O
although	O
there	O
are	O
several	O
common	O
file	B-Application
systems	I-Application
which	O
almost	O
all	O
operating	B-General_Concept
systems	I-General_Concept
include	O
support	O
and	O
drivers	O
for	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
vary	O
on	O
file	B-Application
system	I-Application
support	O
and	O
on	O
the	O
disk	O
formats	O
they	O
may	O
be	O
installed	O
on	O
.	O
</s>
<s>
Under	O
Windows	O
,	O
each	O
file	B-Application
system	I-Application
is	O
usually	O
limited	O
in	O
application	O
to	O
certain	O
media	O
;	O
for	O
example	O
,	O
CDs	O
must	O
use	O
ISO	B-Application
9660	I-Application
or	O
UDF	O
,	O
and	O
as	O
of	O
Windows	B-Application
Vista	I-Application
,	O
NTFS	B-Application
is	O
the	O
only	O
file	B-Application
system	I-Application
which	O
the	O
operating	B-General_Concept
system	I-General_Concept
can	O
be	O
installed	O
on	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
install	O
Linux	B-Application
onto	O
many	O
types	O
of	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
Unlike	O
other	O
operating	B-General_Concept
systems	I-General_Concept
,	O
Linux	B-Application
and	O
UNIX	B-Application
allow	O
any	O
file	B-Application
system	I-Application
to	O
be	O
used	O
regardless	O
of	O
the	O
media	O
it	O
is	O
stored	O
in	O
,	O
whether	O
it	O
is	O
a	O
hard	B-Device
drive	I-Device
,	O
a	O
disc	O
(	O
CD	O
,	O
DVD	O
...	O
)	O
,	O
a	O
USB	O
flash	O
drive	O
,	O
or	O
even	O
contained	O
within	O
a	O
file	O
located	O
on	O
another	O
file	B-Application
system	I-Application
.	O
</s>
<s>
A	O
device	B-Application
driver	I-Application
is	O
a	O
specific	O
type	O
of	O
computer	O
software	O
developed	O
to	O
allow	O
interaction	O
with	O
hardware	B-Architecture
devices	O
.	O
</s>
<s>
Typically	O
this	O
constitutes	O
an	O
interface	O
for	O
communicating	O
with	O
the	O
device	O
,	O
through	O
the	O
specific	O
computer	O
bus	O
or	O
communications	O
subsystem	O
that	O
the	O
hardware	B-Architecture
is	O
connected	O
to	O
,	O
providing	O
commands	O
to	O
and/or	O
receiving	O
data	O
from	O
the	O
device	O
,	O
and	O
on	O
the	O
other	O
end	O
,	O
the	O
requisite	O
interfaces	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
software	O
applications	O
.	O
</s>
<s>
It	O
is	O
a	O
specialized	O
hardware-dependent	O
computer	B-Application
program	I-Application
which	O
is	O
also	O
operating	B-General_Concept
system	I-General_Concept
specific	O
that	O
enables	O
another	O
program	O
,	O
typically	O
an	O
operating	B-General_Concept
system	I-General_Concept
or	O
applications	O
software	B-Architecture
package	I-Architecture
or	O
computer	B-Application
program	I-Application
running	O
under	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
,	O
to	O
interact	O
transparently	O
with	O
a	O
hardware	B-Architecture
device	O
,	O
and	O
usually	O
provides	O
the	O
requisite	O
interrupt	B-Application
handling	I-Application
necessary	O
for	O
any	O
necessary	O
asynchronous	O
time-dependent	O
hardware	B-Architecture
interfacing	O
needs	O
.	O
</s>
<s>
The	O
key	O
design	O
goal	O
of	O
device	B-Application
drivers	I-Application
is	O
abstraction	B-Application
.	O
</s>
<s>
Every	O
model	O
of	O
hardware	B-Architecture
(	O
even	O
within	O
the	O
same	O
class	O
of	O
device	O
)	O
is	O
different	O
.	O
</s>
<s>
Computers	O
and	O
their	O
operating	B-General_Concept
systems	I-General_Concept
cannot	O
be	O
expected	O
to	O
know	O
how	O
to	O
control	O
every	O
device	O
,	O
both	O
now	O
and	O
in	O
the	O
future	O
.	O
</s>
<s>
To	O
solve	O
this	O
problem	O
,	O
operating	B-General_Concept
systems	I-General_Concept
essentially	O
dictate	O
how	O
every	O
type	O
of	O
device	O
should	O
be	O
controlled	O
.	O
</s>
<s>
The	O
function	O
of	O
the	O
device	B-Application
driver	I-Application
is	O
then	O
to	O
translate	O
these	O
operating	B-General_Concept
system	I-General_Concept
mandated	O
function	O
calls	O
into	O
device	O
specific	O
calls	O
.	O
</s>
<s>
This	O
new	O
driver	O
ensures	O
that	O
the	O
device	O
appears	O
to	O
operate	O
as	O
usual	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
point	O
of	O
view	O
.	O
</s>
<s>
Under	O
versions	O
of	O
Windows	O
before	O
Vista	B-Application
and	O
versions	O
of	O
Linux	B-Application
before	O
2.6	O
,	O
all	O
driver	O
execution	O
was	O
co-operative	O
,	O
meaning	O
that	O
if	O
a	O
driver	O
entered	O
an	O
infinite	B-Algorithm
loop	I-Algorithm
it	O
would	O
freeze	O
the	O
system	O
.	O
</s>
<s>
More	O
recent	O
revisions	O
of	O
these	O
operating	B-General_Concept
systems	I-General_Concept
incorporate	O
kernel	B-Operating_System
preemption	B-Operating_System
,	O
where	O
the	O
kernel	B-Operating_System
interrupts	B-Application
the	O
driver	O
to	O
give	O
it	O
tasks	O
,	O
and	O
then	O
separates	O
itself	O
from	O
the	O
process	B-Operating_System
until	O
it	O
receives	O
a	O
response	O
from	O
the	O
device	B-Application
driver	I-Application
,	O
or	O
gives	O
it	O
more	O
tasks	O
to	O
do	O
.	O
</s>
<s>
Currently	O
most	O
operating	B-General_Concept
systems	I-General_Concept
support	O
a	O
variety	O
of	O
networking	O
protocols	O
,	O
hardware	B-Architecture
,	O
and	O
applications	O
for	O
using	O
them	O
.	O
</s>
<s>
This	O
means	O
that	O
computers	O
running	O
dissimilar	O
operating	B-General_Concept
systems	I-General_Concept
can	O
participate	O
in	O
a	O
common	O
network	B-Architecture
for	O
sharing	O
resources	O
such	O
as	O
computing	B-Operating_System
,	O
files	B-Operating_System
,	O
printers	O
,	O
and	O
scanners	O
using	O
either	O
wired	O
or	O
wireless	O
connections	O
.	O
</s>
<s>
Networks	O
can	O
essentially	O
allow	O
a	O
computer	O
's	O
operating	B-General_Concept
system	I-General_Concept
to	O
access	O
the	O
resources	O
of	O
a	O
remote	O
computer	O
to	O
support	O
the	O
same	O
functions	O
as	O
it	O
could	O
if	O
those	O
resources	O
were	O
connected	O
directly	O
to	O
the	O
local	O
computer	O
.	O
</s>
<s>
This	O
includes	O
everything	O
from	O
simple	O
communication	O
,	O
to	O
using	O
networked	B-Architecture
file	B-Application
systems	I-Application
or	O
even	O
sharing	O
another	O
computer	O
's	O
graphics	O
or	O
sound	O
hardware	B-Architecture
.	O
</s>
<s>
Some	O
network	B-General_Concept
services	I-General_Concept
allow	O
the	O
resources	O
of	O
a	O
computer	O
to	O
be	O
accessed	O
transparently	O
,	O
such	O
as	O
SSH	B-Protocol
which	O
allows	O
networked	B-Architecture
users	O
direct	O
access	O
to	O
a	O
computer	O
's	O
command	B-Application
line	I-Application
interface	I-Application
.	O
</s>
<s>
Client/server	O
networking	O
allows	O
a	O
program	O
on	O
a	O
computer	O
,	O
called	O
a	O
client	O
,	O
to	O
connect	O
via	O
a	O
network	B-Architecture
to	O
another	O
computer	O
,	O
called	O
a	O
server	B-Application
.	O
</s>
<s>
Servers	B-Application
offer	O
(	O
or	O
host	O
)	O
various	O
services	B-Operating_System
to	O
other	O
network	B-Architecture
computers	O
and	O
users	O
.	O
</s>
<s>
These	O
services	B-Operating_System
are	O
usually	O
provided	O
through	O
ports	O
or	O
numbered	O
access	O
points	O
beyond	O
the	O
server	B-Application
's	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
A	O
daemon	B-Operating_System
,	O
being	O
a	O
user	O
program	O
,	O
can	O
in	O
turn	O
access	O
the	O
local	O
hardware	B-Architecture
resources	O
of	O
that	O
computer	O
by	O
passing	O
requests	O
to	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Many	O
operating	B-General_Concept
systems	I-General_Concept
support	O
one	O
or	O
more	O
vendor-specific	O
or	O
open	O
networking	O
protocols	O
as	O
well	O
,	O
for	O
example	O
,	O
SNA	B-Architecture
on	O
IBM	O
systems	O
,	O
DECnet	B-Protocol
on	O
systems	O
from	O
Digital	O
Equipment	O
Corporation	O
,	O
and	O
Microsoft-specific	O
protocols	O
(	O
SMB	B-Operating_System
)	O
on	O
Windows	O
.	O
</s>
<s>
Specific	O
protocols	O
for	O
specific	O
tasks	O
may	O
also	O
be	O
supported	O
such	O
as	O
NFS	B-Protocol
for	O
file	B-Application
access	I-Application
.	O
</s>
<s>
Protocols	O
like	O
ESound	B-Language
,	O
or	O
esd	O
can	O
be	O
easily	O
extended	O
over	O
the	O
network	B-Architecture
to	O
provide	O
sound	O
from	O
local	O
applications	O
,	O
on	O
a	O
remote	O
system	O
's	O
sound	O
hardware	B-Architecture
.	O
</s>
<s>
A	O
modern	O
operating	B-General_Concept
system	I-General_Concept
provides	O
access	O
to	O
a	O
number	O
of	O
resources	O
,	O
which	O
are	O
available	O
to	O
software	O
running	O
on	O
the	O
system	O
,	O
and	O
to	O
external	O
devices	O
like	O
networks	O
via	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
must	O
be	O
capable	O
of	O
distinguishing	O
between	O
requests	O
which	O
should	O
be	O
allowed	O
to	O
be	O
processed	O
,	O
and	O
others	O
which	O
should	O
not	O
be	O
processed	O
.	O
</s>
<s>
To	O
establish	O
identity	O
there	O
may	O
be	O
a	O
process	B-Operating_System
of	O
authentication	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
especially	O
connections	O
from	O
the	O
network	B-Architecture
,	O
resources	O
may	O
be	O
accessed	O
with	O
no	O
authentication	O
at	O
all	O
(	O
such	O
as	O
reading	O
files	B-Operating_System
over	O
a	O
network	B-Architecture
share	O
)	O
.	O
</s>
<s>
Also	O
covered	O
by	O
the	O
concept	O
of	O
requester	O
identity	O
is	O
authorization	O
;	O
the	O
particular	O
services	B-Operating_System
and	O
resources	O
accessible	O
by	O
the	O
requester	O
once	O
logged	O
into	O
a	O
system	O
are	O
tied	O
to	O
either	O
the	O
requester	O
's	O
user	O
account	O
or	O
to	O
the	O
variously	O
configured	O
groups	O
of	O
users	O
to	O
which	O
the	O
requester	O
belongs	O
.	O
</s>
<s>
Internal	O
security	O
,	O
or	O
security	O
from	O
an	O
already	O
running	O
program	O
is	O
only	O
possible	O
if	O
all	O
possibly	O
harmful	O
requests	O
must	O
be	O
carried	O
out	O
through	O
interrupts	B-Application
to	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
If	O
programs	O
can	O
directly	O
access	O
hardware	B-Architecture
and	O
resources	O
,	O
they	O
cannot	O
be	O
secured	O
.	O
</s>
<s>
External	O
security	O
involves	O
a	O
request	O
from	O
outside	O
the	O
computer	O
,	O
such	O
as	O
a	O
login	O
at	O
a	O
connected	O
console	O
or	O
some	O
kind	O
of	O
network	B-Architecture
connection	O
.	O
</s>
<s>
External	O
requests	O
are	O
often	O
passed	O
through	O
device	B-Application
drivers	I-Application
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
kernel	B-Operating_System
,	O
where	O
they	O
can	O
be	O
passed	O
onto	O
applications	O
,	O
or	O
carried	O
out	O
directly	O
.	O
</s>
<s>
Security	O
of	O
operating	B-General_Concept
systems	I-General_Concept
has	O
long	O
been	O
a	O
concern	O
because	O
of	O
highly	O
sensitive	O
data	O
held	O
on	O
computers	O
,	O
both	O
of	O
a	O
commercial	O
and	O
military	O
nature	O
.	O
</s>
<s>
This	O
became	O
of	O
vital	O
importance	O
to	O
operating	B-General_Concept
system	I-General_Concept
makers	O
,	O
because	O
the	O
TCSEC	O
was	O
used	O
to	O
evaluate	O
,	O
classify	O
and	O
select	O
trusted	O
operating	B-General_Concept
systems	I-General_Concept
being	O
considered	O
for	O
the	O
processing	O
,	O
storage	O
and	O
retrieval	O
of	O
sensitive	O
or	O
classified	O
information	O
.	O
</s>
<s>
Network	B-General_Concept
services	I-General_Concept
include	O
offerings	O
such	O
as	O
file	O
sharing	O
,	O
print	O
services	B-Operating_System
,	O
email	O
,	O
web	O
sites	O
,	O
and	O
file	B-Protocol
transfer	I-Protocol
protocols	I-Protocol
(	O
FTP	O
)	O
,	O
most	O
of	O
which	O
can	O
have	O
compromised	O
security	O
.	O
</s>
<s>
At	O
the	O
front	O
line	O
of	O
security	O
are	O
hardware	B-Architecture
devices	O
known	O
as	O
firewalls	B-Application
or	O
intrusion	O
detection/prevention	O
systems	O
.	O
</s>
<s>
At	O
the	O
operating	B-General_Concept
system	I-General_Concept
level	O
,	O
there	O
are	O
a	O
number	O
of	O
software	O
firewalls	B-Application
available	O
,	O
as	O
well	O
as	O
intrusion	O
detection/prevention	O
systems	O
.	O
</s>
<s>
Most	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
include	O
a	O
software	O
firewall	B-Application
,	O
which	O
is	O
enabled	O
by	O
default	O
.	O
</s>
<s>
A	O
software	O
firewall	B-Application
can	O
be	O
configured	O
to	O
allow	O
or	O
deny	O
network	B-Architecture
traffic	O
to	O
or	O
from	O
a	O
service	O
or	O
application	O
running	O
on	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Therefore	O
,	O
one	O
can	O
install	O
and	O
be	O
running	O
an	O
insecure	O
service	O
,	O
such	O
as	O
Telnet	O
or	O
FTP	O
,	O
and	O
not	O
have	O
to	O
be	O
threatened	O
by	O
a	O
security	O
breach	O
because	O
the	O
firewall	B-Application
would	O
deny	O
all	O
traffic	O
trying	O
to	O
connect	O
to	O
the	O
service	O
on	O
that	O
port	O
.	O
</s>
<s>
An	O
alternative	O
strategy	O
,	O
and	O
the	O
only	O
sandbox	O
strategy	O
available	O
in	O
systems	O
that	O
do	O
not	O
meet	O
the	O
Popek	B-Architecture
and	I-Architecture
Goldberg	I-Architecture
virtualization	I-Architecture
requirements	I-Architecture
,	O
is	O
where	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
not	O
running	O
user	O
programs	O
as	O
native	B-Language
code	I-Language
,	O
but	O
instead	O
either	O
emulates	B-Application
a	O
processor	O
or	O
provides	O
a	O
host	O
for	O
a	O
p-code	B-Language
based	O
system	O
such	O
as	O
Java	B-Device
.	O
</s>
<s>
Internal	O
security	O
is	O
especially	O
relevant	O
for	O
multi-user	B-Operating_System
systems	I-Operating_System
;	O
it	O
allows	O
each	O
user	O
of	O
the	O
system	O
to	O
have	O
private	O
files	B-Operating_System
that	O
the	O
other	O
users	O
cannot	O
tamper	O
with	O
or	O
read	O
.	O
</s>
<s>
Internal	O
security	O
is	O
also	O
vital	O
if	O
auditing	O
is	O
to	O
be	O
of	O
any	O
use	O
,	O
since	O
a	O
program	O
can	O
potentially	O
bypass	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
inclusive	O
of	O
bypassing	O
auditing	O
.	O
</s>
<s>
Every	O
computer	O
that	O
is	O
to	O
be	O
operated	O
by	O
an	O
individual	O
requires	O
a	O
user	B-Application
interface	I-Application
.	O
</s>
<s>
The	O
user	B-Application
interface	I-Application
is	O
usually	O
referred	O
to	O
as	O
a	O
shell	B-Operating_System
and	O
is	O
essential	O
if	O
human	O
interaction	O
is	O
to	O
be	O
supported	O
.	O
</s>
<s>
The	O
user	B-Application
interface	I-Application
views	O
the	O
directory	B-Architecture
structure	I-Architecture
and	O
requests	O
services	B-Operating_System
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
that	O
will	O
acquire	O
data	O
from	O
input	B-Device
hardware	I-Device
devices	I-Device
,	O
such	O
as	O
a	O
keyboard	B-Device
,	O
mouse	B-Device
or	O
credit	O
card	O
reader	O
,	O
and	O
requests	O
operating	B-Operating_System
system	I-Operating_System
services	I-Operating_System
to	O
display	O
prompts	O
,	O
status	O
messages	O
and	O
such	O
on	O
output	B-Device
hardware	I-Device
devices	I-Device
,	O
such	O
as	O
a	O
video	B-Device
monitor	I-Device
or	O
printer	O
.	O
</s>
<s>
The	O
two	O
most	O
common	O
forms	O
of	O
a	O
user	B-Application
interface	I-Application
have	O
historically	O
been	O
the	O
command-line	B-Application
interface	I-Application
,	O
where	O
computer	O
commands	O
are	O
typed	O
out	O
line-by-line	O
,	O
and	O
the	O
graphical	B-Application
user	I-Application
interface	I-Application
,	O
where	O
a	O
visual	O
environment	O
(	O
most	O
commonly	O
a	O
WIMP	B-Application
)	O
is	O
present	O
.	O
</s>
<s>
Most	O
of	O
the	O
modern	O
computer	O
systems	O
support	O
graphical	B-Application
user	I-Application
interfaces	I-Application
(	O
GUI	B-Application
)	O
,	O
and	O
often	O
include	O
them	O
.	O
</s>
<s>
In	O
some	O
computer	O
systems	O
,	O
such	O
as	O
the	O
original	O
implementation	O
of	O
the	O
classic	B-Application
Mac	I-Application
OS	I-Application
,	O
the	O
GUI	B-Application
is	O
integrated	O
into	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
While	O
technically	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
is	O
not	O
an	O
operating	B-General_Concept
system	I-General_Concept
service	O
,	O
incorporating	O
support	O
for	O
one	O
into	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
can	O
allow	O
the	O
GUI	B-Application
to	O
be	O
more	O
responsive	O
by	O
reducing	O
the	O
number	O
of	O
context	B-Operating_System
switches	I-Operating_System
required	O
for	O
the	O
GUI	B-Application
to	O
perform	O
its	O
output	O
functions	O
.	O
</s>
<s>
Other	O
operating	B-General_Concept
systems	I-General_Concept
are	O
modular	B-Architecture
,	O
separating	O
the	O
graphics	O
subsystem	O
from	O
the	O
kernel	B-Operating_System
and	O
the	O
Operating	B-General_Concept
System	I-General_Concept
.	O
</s>
<s>
In	O
the	O
1980s	O
UNIX	B-Application
,	O
VMS	B-Operating_System
and	O
many	O
others	O
had	O
operating	B-General_Concept
systems	I-General_Concept
that	O
were	O
built	O
this	O
way	O
.	O
</s>
<s>
Linux	B-Application
and	O
macOS	B-Application
are	O
also	O
built	O
this	O
way	O
.	O
</s>
<s>
Modern	O
releases	O
of	O
Microsoft	B-Application
Windows	I-Application
such	O
as	O
Windows	B-Application
Vista	I-Application
implement	O
a	O
graphics	O
subsystem	O
that	O
is	O
mostly	O
in	O
user-space	B-Operating_System
;	O
however	O
the	O
graphics	O
drawing	O
routines	O
of	O
versions	O
between	O
Windows	B-Device
NT	I-Device
4.0	I-Device
and	O
Windows	B-Application
Server	I-Application
2003	I-Application
exist	O
mostly	O
in	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Windows	O
9x	O
had	O
very	O
little	O
distinction	O
between	O
the	O
interface	O
and	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Many	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
allow	O
the	O
user	O
to	O
install	O
or	O
create	O
any	O
user	B-Application
interface	I-Application
they	O
desire	O
.	O
</s>
<s>
The	O
X	O
Window	O
System	O
in	O
conjunction	O
with	O
GNOME	B-Application
or	O
KDE	B-Language
Plasma	I-Language
5	I-Language
is	O
a	O
commonly	O
found	O
setup	O
on	O
most	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
(	O
BSD	B-Operating_System
,	O
Linux	B-Application
,	O
Solaris	B-Application
)	O
systems	O
.	O
</s>
<s>
A	O
number	O
of	O
Windows	B-Application
shell	I-Application
replacements	I-Application
have	O
been	O
released	O
for	O
Microsoft	B-Application
Windows	I-Application
,	O
which	O
offer	O
alternatives	O
to	O
the	O
included	O
Windows	B-Application
shell	I-Application
,	O
but	O
the	O
shell	B-Operating_System
itself	O
cannot	O
be	O
separated	O
from	O
Windows	O
.	O
</s>
<s>
Numerous	O
Unix-based	O
GUIs	B-Application
have	O
existed	O
over	O
time	O
,	O
most	O
derived	O
from	O
X11	O
.	O
</s>
<s>
Competition	O
among	O
the	O
various	O
vendors	O
of	O
Unix	B-Application
(	O
HP	O
,	O
IBM	O
,	O
Sun	O
)	O
led	O
to	O
much	O
fragmentation	O
,	O
though	O
an	O
effort	O
to	O
standardize	O
in	O
the	O
1990s	O
to	O
COSE	B-Operating_System
and	O
CDE	B-Application
failed	O
for	O
various	O
reasons	O
,	O
and	O
were	O
eventually	O
eclipsed	O
by	O
the	O
widespread	O
adoption	O
of	O
GNOME	B-Application
and	O
K	B-Application
Desktop	I-Application
Environment	I-Application
.	O
</s>
<s>
Prior	O
to	O
free	O
software-based	O
toolkits	O
and	O
desktop	O
environments	O
,	O
Motif	O
was	O
the	O
prevalent	O
toolkit/desktop	O
combination	O
(	O
and	O
was	O
the	O
basis	O
upon	O
which	O
CDE	B-Application
was	O
developed	O
)	O
.	O
</s>
<s>
Graphical	B-Application
user	I-Application
interfaces	I-Application
evolve	O
over	O
time	O
.	O
</s>
<s>
For	O
example	O
,	O
Windows	O
has	O
modified	O
its	O
user	B-Application
interface	I-Application
almost	O
every	O
time	O
a	O
new	O
major	O
version	O
of	O
Windows	O
is	O
released	O
,	O
and	O
the	O
MacOS	B-Application
GUI	B-Application
changed	O
dramatically	O
with	O
the	O
introduction	O
of	O
MacOSX	O
in	O
1999	O
.	O
</s>
<s>
A	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
(	O
RTOS	B-Operating_System
)	O
is	O
an	O
operating	B-General_Concept
system	I-General_Concept
intended	O
for	O
applications	O
with	O
fixed	O
deadlines	O
(	O
real-time	B-General_Concept
computing	I-General_Concept
)	O
.	O
</s>
<s>
Such	O
applications	O
include	O
some	O
small	O
embedded	B-Architecture
systems	I-Architecture
,	O
automobile	O
engine	O
controllers	O
,	O
industrial	O
robots	O
,	O
spacecraft	O
,	O
industrial	O
control	O
,	O
and	O
some	O
large-scale	O
computing	B-Operating_System
systems	O
.	O
</s>
<s>
An	O
early	O
example	O
of	O
a	O
large-scale	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
was	O
Transaction	B-Operating_System
Processing	I-Operating_System
Facility	I-Operating_System
developed	O
by	O
American	O
Airlines	O
and	O
IBM	O
for	O
the	O
Sabre	B-Application
Airline	I-Application
Reservations	I-Application
System	I-Application
.	O
</s>
<s>
Embedded	B-Architecture
systems	I-Architecture
that	O
have	O
fixed	O
deadlines	O
use	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
such	O
as	O
VxWorks	B-Operating_System
,	O
PikeOS	B-Operating_System
,	O
eCos	B-Operating_System
,	O
QNX	B-Operating_System
,	O
MontaVista	B-Operating_System
Linux	I-Operating_System
and	O
RTLinux	B-Application
.	O
</s>
<s>
Windows	B-Operating_System
CE	I-Operating_System
is	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
that	O
shares	O
similar	O
APIs	B-Application
to	O
desktop	O
Windows	O
but	O
shares	O
none	O
of	O
desktop	O
Windows	O
 '	O
codebase	O
.	O
</s>
<s>
Symbian	B-Operating_System
OS	I-Operating_System
also	O
has	O
an	O
RTOS	B-Operating_System
kernel	B-Operating_System
(	O
EKA2	O
)	O
starting	O
with	O
version	O
8.0b	O
.	O
</s>
<s>
Some	O
embedded	B-Architecture
systems	I-Architecture
use	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
Palm	B-Operating_System
OS	I-Operating_System
,	O
BSD	B-Operating_System
,	O
and	O
Linux	B-Application
,	O
although	O
such	O
operating	B-General_Concept
systems	I-General_Concept
do	O
not	O
support	O
real-time	B-General_Concept
computing	I-General_Concept
.	O
</s>
<s>
A	O
hobby	O
operating	B-General_Concept
system	I-General_Concept
may	O
be	O
classified	O
as	O
one	O
whose	O
code	O
has	O
not	O
been	O
directly	O
derived	O
from	O
an	O
existing	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
has	O
few	O
users	O
and	O
active	O
developers	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
hobby	O
development	O
is	O
in	O
support	O
of	O
a	O
"	O
homebrew	O
"	O
computing	B-Operating_System
device	O
,	O
for	O
example	O
,	O
a	O
simple	O
single-board	B-Device
computer	I-Device
powered	O
by	O
a	O
6502	B-General_Concept
microprocessor	I-General_Concept
.	O
</s>
<s>
Operating	B-General_Concept
system	I-General_Concept
development	O
may	O
come	O
from	O
entirely	O
new	O
concepts	O
,	O
or	O
may	O
commence	O
by	O
modeling	O
an	O
existing	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Examples	O
of	O
a	O
hobby	O
operating	B-General_Concept
system	I-General_Concept
include	O
Syllable	B-Application
and	O
TempleOS	B-Application
.	O
</s>
<s>
Application	O
software	O
is	O
generally	O
written	O
for	O
use	O
on	O
a	O
specific	O
operating	B-General_Concept
system	I-General_Concept
,	O
and	O
sometimes	O
even	O
for	O
specific	O
hardware	B-Architecture
.	O
</s>
<s>
Unix	B-Application
was	O
the	O
first	O
operating	B-General_Concept
system	I-General_Concept
not	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
making	O
it	O
very	O
portable	B-Architecture
to	O
systems	O
different	O
from	O
its	O
native	O
PDP-11	B-Device
.	O
</s>
<s>
This	O
cost	O
in	O
supporting	O
operating	B-General_Concept
systems	I-General_Concept
diversity	O
can	O
be	O
avoided	O
by	O
instead	O
writing	O
applications	O
against	O
software	B-Device
platforms	I-Device
such	O
as	O
Java	B-Device
or	O
Qt	B-Language
.	O
</s>
<s>
These	O
abstractions	B-Application
have	O
already	O
borne	O
the	O
cost	O
of	O
adaptation	O
to	O
specific	O
operating	B-General_Concept
systems	I-General_Concept
and	O
their	O
system	B-Library
libraries	I-Library
.	O
</s>
<s>
Another	O
approach	O
is	O
for	O
operating	B-General_Concept
system	I-General_Concept
vendors	O
to	O
adopt	O
standards	O
.	O
</s>
<s>
For	O
example	O
,	O
POSIX	O
and	O
OS	B-Operating_System
abstraction	I-Operating_System
layers	I-Operating_System
provide	O
commonalities	O
that	O
reduce	O
porting	O
costs	O
.	O
</s>
