<s>
An	O
asymmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
AMP	O
or	O
ASMP	B-Operating_System
)	O
system	O
is	O
a	O
multiprocessor	B-Operating_System
computer	O
system	O
where	O
not	O
all	O
of	O
the	O
multiple	O
interconnected	O
central	O
processing	O
units	O
(	O
CPUs	O
)	O
are	O
treated	O
equally	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
system	O
might	O
allow	O
(	O
either	O
at	O
the	O
hardware	O
or	O
operating	B-General_Concept
system	I-General_Concept
level	O
)	O
only	O
one	O
CPU	O
to	O
execute	O
operating	B-General_Concept
system	I-General_Concept
code	O
or	O
might	O
allow	O
only	O
one	O
CPU	O
to	O
perform	O
I/O	O
operations	O
.	O
</s>
<s>
Other	O
AMP	O
systems	O
might	O
allow	O
any	O
CPU	O
to	O
execute	O
operating	B-General_Concept
system	I-General_Concept
code	O
and	O
perform	O
I/O	O
operations	O
,	O
so	O
that	O
they	O
were	O
symmetric	O
with	O
regard	O
to	O
processor	O
roles	O
,	O
but	O
attached	O
some	O
or	O
all	O
peripherals	O
to	O
particular	O
CPUs	O
,	O
so	O
that	O
they	O
were	O
asymmetric	O
with	O
respect	O
to	O
the	O
peripheral	O
attachment	O
.	O
</s>
<s>
Asymmetric	B-Operating_System
multiprocessing	I-Operating_System
was	O
the	O
only	O
method	O
for	O
handling	O
multiple	O
CPUs	O
before	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
SMP	O
)	O
was	O
available	O
.	O
</s>
<s>
Notable	O
early	O
AMP	O
offerings	O
by	O
computer	O
manufacturers	O
were	O
the	O
Burroughs	B-Device
B5000	I-Device
,	O
the	O
DECsystem-1055	B-Device
,	O
and	O
the	O
IBM	B-Application
System/360	I-Application
model	O
65MP	O
.	O
</s>
<s>
The	O
problem	O
with	O
adding	O
a	O
second	O
CPU	O
to	O
a	O
computer	O
system	O
was	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
had	O
been	O
developed	O
for	O
single-CPU	O
systems	O
,	O
and	O
extending	O
it	O
to	O
handle	O
multiple	O
CPUs	O
efficiently	O
and	O
reliably	O
took	O
a	O
long	O
time	O
.	O
</s>
<s>
To	O
fill	O
this	O
gap	O
,	O
operating	B-General_Concept
systems	I-General_Concept
intended	O
for	O
single	O
CPUs	O
were	O
initially	O
extended	O
to	O
provide	O
minimal	O
support	O
for	O
a	O
second	O
CPU	O
.	O
</s>
<s>
In	O
this	O
minimal	O
support	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
ran	O
on	O
the	O
“	O
boot	O
”	O
processor	O
,	O
with	O
the	O
other	O
only	O
allowed	O
to	O
run	O
user	O
programs	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
the	O
Burroughs	B-Device
B5000	I-Device
,	O
the	O
second	O
processor	O
's	O
hardware	O
was	O
not	O
capable	O
of	O
running	O
"	O
control	O
state	O
"	O
code	O
.	O
</s>
<s>
Other	O
systems	O
allowed	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
run	O
on	O
all	O
processors	O
,	O
but	O
either	O
attached	O
all	O
the	O
peripherals	O
to	O
one	O
processor	O
or	O
attached	O
particular	O
peripherals	O
to	O
particular	O
processors	O
.	O
</s>
<s>
An	O
option	O
on	O
the	O
Burroughs	B-Device
B5000	I-Device
was	O
“	O
Processor	O
B	O
”	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
ran	O
only	O
on	O
Processor	O
A	O
.	O
</s>
<s>
When	O
there	O
was	O
a	O
user	O
job	O
to	O
be	O
executed	O
,	O
it	O
might	O
be	O
run	O
on	O
Processor	O
B	O
,	O
but	O
when	O
that	O
job	O
tried	O
to	O
access	O
the	O
operating	B-General_Concept
system	I-General_Concept
the	O
processor	O
halted	O
and	O
signaled	O
Processor	O
A	O
.	O
</s>
<s>
The	O
requested	O
operating	B-General_Concept
system	I-General_Concept
service	O
was	O
then	O
run	O
on	O
Processor	O
A	O
.	O
</s>
<s>
When	O
a	O
job	O
on	O
Processor	O
2	O
required	O
an	O
operating	B-General_Concept
system	I-General_Concept
service	O
it	O
would	O
be	O
rescheduled	O
on	O
Processor	O
1	O
,	O
which	O
was	O
responsible	O
for	O
both	O
initiating	O
I/O	O
processor	O
activity	O
and	O
responding	O
to	O
interrupts	O
indicating	O
completion	O
.	O
</s>
<s>
In	O
practice	O
,	O
this	O
meant	O
that	O
while	O
user	O
jobs	O
could	O
run	O
on	O
either	O
Processor	O
1	O
or	O
Processor	O
2	O
and	O
could	O
access	O
intrinsic	O
library	O
routines	O
that	O
did	O
n't	O
require	O
kernel	O
support	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
would	O
schedule	O
them	O
on	O
the	O
latter	O
whenever	O
possible	O
.	O
</s>
<s>
Control	O
Data	O
Corporation	O
offered	O
two	O
configurations	O
of	O
its	O
CDC	B-Device
6000	I-Device
series	I-Device
that	O
featured	O
two	O
central	O
processors	O
.	O
</s>
<s>
These	O
systems	O
were	O
organized	O
quite	O
differently	O
from	O
the	O
other	O
multiprocessors	B-Operating_System
in	O
this	O
article	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
ran	O
on	O
the	O
peripheral	O
processors	O
,	O
while	O
the	O
user	O
's	O
application	O
ran	O
on	O
the	O
CPUs	O
.	O
</s>
<s>
Thus	O
,	O
the	O
terms	O
ASMP	B-Operating_System
and	O
SMP	O
do	O
not	O
properly	O
apply	O
to	O
these	O
multiprocessors	B-Operating_System
.	O
</s>
<s>
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
offered	O
a	O
dual-processor	O
version	O
of	O
its	O
DECsystem-1050	B-Device
which	O
used	O
two	O
KA10	O
processors	O
;	O
all	O
peripherals	O
were	O
attached	O
to	O
one	O
processor	O
,	O
the	O
primary	O
processor	O
,	O
and	O
the	O
primary	O
processor	O
ran	O
the	O
operating	B-General_Concept
system	I-General_Concept
code	O
.	O
</s>
<s>
This	O
offering	O
was	O
extended	O
to	O
the	O
KL-10	B-Device
and	O
KS-10	B-Device
processors	O
in	O
the	O
PDP-10	B-Device
line	O
;	O
in	O
those	O
systems	O
,	O
the	O
boot	O
CPU	O
is	O
designated	O
the	O
"	O
policy	O
CPU	O
"	O
,	O
which	O
runs	O
the	O
command	O
interpreter	O
,	O
swaps	O
jobs	O
in	O
and	O
out	O
of	O
memory	O
,	O
and	O
performs	O
a	O
few	O
other	O
functions	O
;	O
other	O
operating	B-General_Concept
system	I-General_Concept
functions	O
,	O
and	O
I/O	O
,	O
can	O
be	O
performed	O
by	O
any	O
of	O
the	O
processors	O
,	O
and	O
if	O
the	O
policy	O
processor	O
fails	O
,	O
another	O
processor	O
takes	O
over	O
as	O
the	O
policy	O
processor	O
.	O
</s>
<s>
Digital	O
Equipment	O
Corporation	O
developed	O
,	O
but	O
never	O
released	O
,	O
a	O
multiprocessor	B-Operating_System
PDP-11	B-Device
,	O
the	O
PDP-11/74	O
,	O
running	O
a	O
multiprocessor	B-Operating_System
version	O
of	O
RSX-11M	B-Application
.	O
</s>
<s>
In	O
that	O
system	O
,	O
either	O
processor	O
could	O
run	O
operating	B-General_Concept
system	I-General_Concept
code	O
,	O
and	O
could	O
perform	O
I/O	O
,	O
but	O
not	O
all	O
peripherals	O
were	O
accessible	O
to	O
all	O
processors	O
;	O
most	O
peripherals	O
were	O
attached	O
to	O
one	O
or	O
the	O
other	O
of	O
the	O
CPUs	O
,	O
so	O
that	O
a	O
processor	O
to	O
which	O
a	O
peripheral	O
was	O
n't	O
attached	O
would	O
,	O
when	O
it	O
needed	O
to	O
perform	O
an	O
I/O	O
operation	O
on	O
that	O
peripheral	O
,	O
request	O
the	O
processor	O
to	O
which	O
the	O
peripheral	O
was	O
attached	O
to	O
perform	O
the	O
operation	O
.	O
</s>
<s>
DEC	O
's	O
first	O
multi-processor	B-Operating_System
VAX	B-Device
system	O
,	O
the	O
VAX-11/782	O
,	O
was	O
an	O
asymmetric	O
dual-processor	O
system	O
;	O
only	O
the	O
first	O
processor	O
had	O
access	O
to	O
the	O
I/O	O
devices	O
.	O
</s>
<s>
Two	O
options	O
were	O
available	O
for	O
the	O
IBM	B-Device
System/370	I-Device
Model	I-Device
168	I-Device
for	O
attaching	O
a	O
second	O
processor	O
.	O
</s>
<s>
One	O
was	O
the	O
IBM	O
3062	O
Attached	O
Processing	O
Unit	O
,	O
in	O
which	O
the	O
second	O
processor	O
had	O
no	O
access	O
to	O
the	O
channels	O
,	O
and	O
was	O
therefore	O
similar	O
to	O
the	O
B5000	O
's	O
Processor	O
B	O
or	O
the	O
second	O
processor	O
on	O
a	O
VAX-11/782	O
.	O
</s>
<s>
The	O
other	O
option	O
offered	O
a	O
complete	O
second	O
CPU	O
,	O
and	O
was	O
thus	O
more	O
like	O
the	O
System/360	B-Application
model	O
65MP	O
.	O
</s>
