<s>
Multiprocessing	B-Operating_System
is	O
the	O
use	O
of	O
two	O
or	O
more	O
central	B-General_Concept
processing	I-General_Concept
units	I-General_Concept
(	O
CPUs	B-Device
)	O
within	O
a	O
single	O
computer	O
system	O
.	O
</s>
<s>
There	O
are	O
many	O
variations	O
on	O
this	O
basic	O
theme	O
,	O
and	O
the	O
definition	O
of	O
multiprocessing	B-Operating_System
can	O
vary	O
with	O
context	O
,	O
mostly	O
as	O
a	O
function	O
of	O
how	O
CPUs	B-Device
are	O
defined	O
(	O
multiple	B-Architecture
cores	I-Architecture
on	O
one	O
die	O
,	O
multiple	O
dies	O
in	O
one	O
package	B-Algorithm
,	O
multiple	O
packages	O
in	O
one	O
system	B-General_Concept
unit	I-General_Concept
,	O
etc	O
.	O
</s>
<s>
According	O
to	O
some	O
on-line	O
dictionaries	O
,	O
a	O
multiprocessor	B-Operating_System
is	O
a	O
computer	O
system	O
having	O
two	O
or	O
more	O
processing	B-General_Concept
units	I-General_Concept
(	O
multiple	O
processors	O
)	O
each	O
sharing	O
main	O
memory	O
and	O
peripherals	O
,	O
in	O
order	O
to	O
simultaneously	O
process	O
programs	O
.	O
</s>
<s>
A	O
2009	O
textbook	O
defined	O
multiprocessor	B-Operating_System
system	O
similarly	O
,	O
but	O
noting	O
that	O
the	O
processors	O
may	O
share	O
"	O
some	O
or	O
all	O
of	O
the	O
system	O
’s	O
memory	O
and	O
I/O	O
facilities	O
"	O
;	O
it	O
also	O
gave	O
tightly	B-Operating_System
coupled	I-Operating_System
system	I-Operating_System
as	O
a	O
synonymous	O
term	O
.	O
</s>
<s>
At	O
the	O
operating	B-General_Concept
system	I-General_Concept
level	O
,	O
multiprocessing	B-Operating_System
is	O
sometimes	O
used	O
to	O
refer	O
to	O
the	O
execution	O
of	O
multiple	O
concurrent	B-Operating_System
processes	B-Operating_System
in	O
a	O
system	O
,	O
with	O
each	O
process	O
running	O
on	O
a	O
separate	O
CPU	B-Device
or	O
core	O
,	O
as	O
opposed	O
to	O
a	O
single	O
process	O
at	O
any	O
one	O
instant	O
.	O
</s>
<s>
When	O
used	O
with	O
this	O
definition	O
,	O
multiprocessing	B-Operating_System
is	O
sometimes	O
contrasted	O
with	O
multitasking	B-Operating_System
,	O
which	O
may	O
use	O
just	O
a	O
single	O
processor	O
but	O
switch	O
it	O
in	O
time	O
slices	O
between	O
tasks	O
(	O
i.e.	O
</s>
<s>
a	O
time-sharing	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
Multiprocessing	B-Operating_System
however	O
means	O
true	O
parallel	O
execution	O
of	O
multiple	O
processes	B-Operating_System
using	O
more	O
than	O
one	O
processor	O
.	O
</s>
<s>
Multiprocessing	B-Operating_System
does	O
n't	O
necessarily	O
mean	O
that	O
a	O
single	O
process	O
or	O
task	O
uses	O
more	O
than	O
one	O
processor	O
simultaneously	O
;	O
the	O
term	O
parallel	B-Operating_System
processing	I-Operating_System
is	O
generally	O
used	O
to	O
denote	O
that	O
scenario	O
.	O
</s>
<s>
Other	O
authors	O
prefer	O
to	O
refer	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
techniques	O
as	O
multiprogramming	O
and	O
reserve	O
the	O
term	O
multiprocessing	B-Operating_System
for	O
the	O
hardware	O
aspect	O
of	O
having	O
more	O
than	O
one	O
processor	O
.	O
</s>
<s>
The	O
remainder	O
of	O
this	O
article	O
discusses	O
multiprocessing	B-Operating_System
only	O
in	O
this	O
hardware	O
sense	O
.	O
</s>
<s>
In	O
Flynn	B-Operating_System
's	I-Operating_System
taxonomy	I-Operating_System
,	O
multiprocessors	B-Operating_System
as	O
defined	O
above	O
are	O
MIMD	B-Operating_System
machines	O
.	O
</s>
<s>
As	O
the	O
term	O
"	O
multiprocessor	B-Operating_System
"	O
normally	O
refers	O
to	O
tightly	B-Operating_System
coupled	I-Operating_System
systems	I-Operating_System
in	O
which	O
all	O
processors	O
share	O
memory	O
,	O
multiprocessors	B-Operating_System
are	O
not	O
the	O
entire	O
class	O
of	O
MIMD	B-Operating_System
machines	O
,	O
which	O
also	O
contains	O
message	B-Architecture
passing	I-Architecture
multicomputer	B-Operating_System
systems	O
.	O
</s>
<s>
Possibly	O
the	O
first	O
expression	O
of	O
the	O
idea	O
of	O
multiprocessing	B-Operating_System
was	O
written	O
by	O
Luigi	O
Federico	O
Menabrea	O
in	O
1842	O
,	O
about	O
Charles	O
Babbage	O
's	O
analytical	B-Device
engine	I-Device
(	O
as	O
translated	O
by	O
Ada	B-General_Concept
Lovelace	I-General_Concept
)	O
:	O
"	O
the	O
machine	O
can	O
be	O
brought	O
into	O
play	O
so	O
as	O
to	O
give	O
several	O
results	O
at	O
the	O
same	O
time	O
,	O
which	O
will	O
greatly	O
abridge	O
the	O
whole	O
amount	O
of	O
the	O
processes.	O
"	O
</s>
<s>
In	O
a	O
multiprocessing	B-Operating_System
system	O
,	O
all	O
CPUs	B-Device
may	O
be	O
equal	O
,	O
or	O
some	O
may	O
be	O
reserved	O
for	O
special	O
purposes	O
.	O
</s>
<s>
A	O
combination	O
of	O
hardware	O
and	O
operating	B-General_Concept
system	I-General_Concept
software	O
design	O
considerations	O
determine	O
the	O
symmetry	O
(	O
or	O
lack	O
thereof	O
)	O
in	O
a	O
given	O
system	O
.	O
</s>
<s>
For	O
example	O
,	O
hardware	O
or	O
software	O
considerations	O
may	O
require	O
that	O
only	O
one	O
particular	O
CPU	B-Device
respond	O
to	O
all	O
hardware	O
interrupts	O
,	O
whereas	O
all	O
other	O
work	O
in	O
the	O
system	O
may	O
be	O
distributed	O
equally	O
among	O
CPUs	B-Device
;	O
or	O
execution	O
of	O
kernel-mode	O
code	O
may	O
be	O
restricted	O
to	O
only	O
one	O
particular	O
CPU	B-Device
,	O
whereas	O
user-mode	O
code	O
may	O
be	O
executed	O
in	O
any	O
combination	O
of	O
processors	O
.	O
</s>
<s>
Multiprocessing	B-Operating_System
systems	O
are	O
often	O
easier	O
to	O
design	O
if	O
such	O
restrictions	O
are	O
imposed	O
,	O
but	O
they	O
tend	O
to	O
be	O
less	O
efficient	O
than	O
systems	O
in	O
which	O
all	O
CPUs	B-Device
are	O
utilized	O
.	O
</s>
<s>
Systems	O
that	O
treat	O
all	O
CPUs	B-Device
equally	O
are	O
called	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
SMP	O
)	O
systems	O
.	O
</s>
<s>
In	O
systems	O
where	O
all	O
CPUs	B-Device
are	O
not	O
equal	O
,	O
system	O
resources	O
may	O
be	O
divided	O
in	O
a	O
number	O
of	O
ways	O
,	O
including	O
asymmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
ASMP	B-Operating_System
)	O
,	O
non-uniform	B-Operating_System
memory	I-Operating_System
access	I-Operating_System
(	O
NUMA	B-Operating_System
)	O
multiprocessing	B-Operating_System
,	O
and	O
clustered	B-Architecture
multiprocessing	B-Operating_System
.	O
</s>
<s>
In	O
a	O
master/slave	O
multiprocessor	B-Operating_System
system	O
,	O
the	O
master	O
CPU	B-Device
is	O
in	O
control	O
of	O
the	O
computer	O
and	O
the	O
slave	O
CPU(s )	O
performs	O
assigned	O
tasks	O
.	O
</s>
<s>
The	O
CPUs	B-Device
can	O
be	O
completely	O
different	O
in	O
terms	O
of	O
speed	O
and	O
architecture	O
.	O
</s>
<s>
Some	O
(	O
or	O
all	O
)	O
of	O
the	O
CPUs	B-Device
can	O
have	O
share	O
common	O
bus	O
,	O
each	O
can	O
also	O
have	O
a	O
private	O
bus	O
(	O
for	O
private	O
resources	O
)	O
,	O
or	O
they	O
may	O
be	O
isolated	O
except	O
for	O
a	O
common	O
communications	O
pathway	O
.	O
</s>
<s>
Likewise	O
,	O
the	O
CPUs	B-Device
can	O
share	O
common	O
RAM	B-Architecture
and/or	O
have	O
private	O
RAM	B-Architecture
that	O
the	O
other	O
processor(s )	O
cannot	O
access	O
.	O
</s>
<s>
The	O
roles	O
of	O
master	O
and	O
slave	O
can	O
change	O
from	O
one	O
CPU	B-Device
to	O
another	O
.	O
</s>
<s>
An	O
early	O
example	O
of	O
a	O
master/slave	O
multiprocessor	B-Operating_System
system	O
is	O
the	O
Tandy/Radio	O
Shack	O
TRS-80	O
Model	O
16	O
desktop	O
computer	O
which	O
came	O
out	O
in	O
February	O
1982	O
and	O
ran	O
the	O
multi-user/multi	O
-tasking	O
Xenix	B-Application
operating	I-Application
system	I-Application
,	O
Microsoft	O
's	O
version	O
of	O
UNIX	O
(	O
called	O
TRS-XENIX	B-Application
)	O
.	O
</s>
<s>
The	O
Model	O
16	O
has	O
3	O
microprocessors	O
,	O
an	O
8-bit	O
Zilog	B-General_Concept
Z80	I-General_Concept
CPU	B-Device
running	O
at	O
4MHz	O
,	O
a	O
16-bit	O
Motorola	B-Device
68000	I-Device
CPU	B-Device
running	O
at	O
6MHz	O
and	O
an	O
Intel	B-Device
8021	I-Device
in	O
the	O
keyboard	O
.	O
</s>
<s>
When	O
the	O
system	O
was	O
booted	O
,	O
the	O
Z-80	B-General_Concept
was	O
the	O
master	O
and	O
the	O
Xenix	B-Application
boot	O
process	O
initialized	O
the	O
slave	O
68000	B-Device
,	O
and	O
then	O
transferred	O
control	O
to	O
the	O
68000	B-Device
,	O
whereupon	O
the	O
CPUs	B-Device
changed	O
roles	O
and	O
the	O
Z-80	B-General_Concept
became	O
a	O
slave	O
processor	O
that	O
was	O
responsible	O
for	O
all	O
I/O	O
operations	O
including	O
disk	O
,	O
communications	O
,	O
printer	O
and	O
network	O
,	O
as	O
well	O
as	O
the	O
keyboard	O
and	O
integrated	O
monitor	O
,	O
while	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
applications	O
ran	O
on	O
the	O
68000	B-Device
CPU	B-Device
.	O
</s>
<s>
The	O
Z-80	B-General_Concept
could	O
be	O
used	O
to	O
do	O
other	O
tasks	O
.	O
</s>
<s>
The	O
earlier	O
TRS-80	B-Device
Model	I-Device
II	I-Device
,	O
which	O
was	O
released	O
in	O
1979	O
,	O
could	O
also	O
be	O
considered	O
a	O
multiprocessor	B-Operating_System
system	O
as	O
it	O
had	O
both	O
a	O
Z-80	B-General_Concept
CPU	B-Device
and	O
an	O
Intel	B-Device
8021	I-Device
microprocessor	O
in	O
the	O
keyboard	O
.	O
</s>
<s>
The	O
8021	B-Device
made	O
the	O
Model	O
II	O
the	O
first	O
desktop	O
computer	O
system	O
with	O
a	O
separate	O
detachable	O
lightweight	O
keyboard	O
connected	O
with	O
by	O
a	O
single	O
thin	O
flexible	O
wire	O
,	O
and	O
likely	O
the	O
first	O
keyboard	O
to	O
use	O
a	O
dedicated	O
microprocessor	O
,	O
both	O
attributes	O
that	O
would	O
later	O
be	O
copied	O
years	O
later	O
by	O
Apple	O
and	O
IBM	O
.	O
</s>
<s>
In	O
multiprocessing	B-Operating_System
,	O
the	O
processors	O
can	O
be	O
used	O
to	O
execute	O
a	O
single	O
sequence	O
of	O
instructions	O
in	O
multiple	O
contexts	O
(	O
single	B-Device
instruction	I-Device
,	I-Device
multiple	I-Device
data	I-Device
or	O
SIMD	B-Device
,	O
often	O
used	O
in	O
vector	B-Operating_System
processing	I-Operating_System
)	O
,	O
multiple	O
sequences	O
of	O
instructions	O
in	O
a	O
single	O
context	O
(	O
multiple	B-Operating_System
instruction	I-Operating_System
,	I-Operating_System
single	I-Operating_System
data	I-Operating_System
or	O
MISD	O
,	O
used	O
for	O
redundancy	O
in	O
fail-safe	O
systems	O
and	O
sometimes	O
applied	O
to	O
describe	O
pipelined	B-General_Concept
processors	I-General_Concept
or	O
hyper-threading	B-Operating_System
)	O
,	O
or	O
multiple	O
sequences	O
of	O
instructions	O
in	O
multiple	O
contexts	O
(	O
multiple	B-Operating_System
instruction	I-Operating_System
,	I-Operating_System
multiple	I-Operating_System
data	I-Operating_System
or	O
MIMD	B-Operating_System
)	O
.	O
</s>
<s>
Tightly	O
coupled	O
multiprocessor	B-Operating_System
systems	O
contain	O
multiple	O
CPUs	B-Device
that	O
are	O
connected	O
at	O
the	O
bus	O
level	O
.	O
</s>
<s>
These	O
CPUs	B-Device
may	O
have	O
access	O
to	O
a	O
central	O
shared	O
memory	O
(	O
SMP	O
or	O
UMA	B-Operating_System
)	O
,	O
or	O
may	O
participate	O
in	O
a	O
memory	O
hierarchy	O
with	O
both	O
local	O
and	O
shared	O
memory	O
(	O
SM	O
)	O
(	O
NUMA	B-Operating_System
)	O
.	O
</s>
<s>
Intel	B-Device
Xeon	I-Device
processors	O
dominated	O
the	O
multiprocessor	B-Operating_System
market	O
for	O
business	O
PCs	O
and	O
were	O
the	O
only	O
major	O
x86	O
option	O
until	O
the	O
release	O
of	O
AMD	O
's	O
Opteron	B-General_Concept
range	O
of	O
processors	O
in	O
2004	O
.	O
</s>
<s>
Both	O
ranges	O
of	O
processors	O
had	O
their	O
own	O
onboard	O
cache	O
but	O
provided	O
access	O
to	O
shared	O
memory	O
;	O
the	O
Xeon	B-Device
processors	O
via	O
a	O
common	O
pipe	O
and	O
the	O
Opteron	B-General_Concept
processors	O
via	O
independent	O
pathways	O
to	O
the	O
system	O
RAM	B-Architecture
.	O
</s>
<s>
Chip	B-Architecture
multiprocessors	I-Architecture
,	O
also	O
known	O
as	O
multi-core	B-Architecture
computing	O
,	O
involves	O
more	O
than	O
one	O
processor	O
placed	O
on	O
a	O
single	O
chip	O
and	O
can	O
be	O
thought	O
of	O
the	O
most	O
extreme	O
form	O
of	O
tightly	O
coupled	O
multiprocessing	B-Operating_System
.	O
</s>
<s>
Loosely	B-Application
coupled	I-Application
multiprocessor	B-Operating_System
systems	O
(	O
often	O
referred	O
to	O
as	O
clusters	B-Architecture
)	O
are	O
based	O
on	O
multiple	O
standalone	O
relatively	O
low	O
processor	O
count	O
commodity	B-Device
computers	I-Device
interconnected	O
via	O
a	O
high	O
speed	O
communication	O
system	O
(	O
Gigabit	O
Ethernet	O
is	O
common	O
)	O
.	O
</s>
<s>
A	O
Linux	O
Beowulf	B-Operating_System
cluster	I-Operating_System
is	O
an	O
example	O
of	O
a	O
loosely	B-Application
coupled	I-Application
system	O
.	O
</s>
<s>
Tightly	B-Operating_System
coupled	I-Operating_System
systems	I-Operating_System
perform	O
better	O
and	O
are	O
physically	O
smaller	O
than	O
loosely	B-Application
coupled	I-Application
systems	O
,	O
but	O
have	O
historically	O
required	O
greater	O
initial	O
investments	O
and	O
may	O
depreciate	O
rapidly	O
;	O
nodes	O
in	O
a	O
loosely	B-Application
coupled	I-Application
system	O
are	O
usually	O
inexpensive	O
commodity	B-Device
computers	I-Device
and	O
can	O
be	O
recycled	O
as	O
independent	O
machines	O
upon	O
retirement	O
from	O
the	O
cluster	O
.	O
</s>
<s>
Tightly	B-Operating_System
coupled	I-Operating_System
systems	I-Operating_System
tend	O
to	O
be	O
much	O
more	O
energy-efficient	O
than	O
clusters	B-Architecture
.	O
</s>
<s>
This	O
is	O
because	O
a	O
considerable	O
reduction	O
in	O
power	O
consumption	O
can	O
be	O
realized	O
by	O
designing	O
components	O
to	O
work	O
together	O
from	O
the	O
beginning	O
in	O
tightly	B-Operating_System
coupled	I-Operating_System
systems	I-Operating_System
,	O
whereas	O
loosely	B-Application
coupled	I-Application
systems	O
use	O
components	O
that	O
were	O
not	O
necessarily	O
intended	O
specifically	O
for	O
use	O
in	O
such	O
systems	O
.	O
</s>
<s>
Loosely	B-Application
coupled	I-Application
systems	O
have	O
the	O
ability	O
to	O
run	O
different	O
operating	B-General_Concept
systems	I-General_Concept
or	O
OS	O
versions	O
on	O
different	O
systems	O
.	O
</s>
