<s>
A	O
barrel	B-Operating_System
processor	I-Operating_System
is	O
a	O
CPU	B-General_Concept
that	O
switches	O
between	O
threads	B-Operating_System
of	O
execution	O
on	O
every	O
cycle	B-General_Concept
.	O
</s>
<s>
This	O
CPU	B-General_Concept
design	I-General_Concept
technique	O
is	O
also	O
known	O
as	O
"	O
interleaved	O
"	O
or	O
"	O
fine-grained	O
"	O
temporal	B-Operating_System
multithreading	I-Operating_System
.	O
</s>
<s>
Unlike	O
simultaneous	B-Operating_System
multithreading	I-Operating_System
in	O
modern	O
superscalar	B-General_Concept
architectures	I-General_Concept
,	O
it	O
generally	O
does	O
not	O
allow	O
execution	O
of	O
multiple	O
instructions	O
in	O
one	O
cycle	B-General_Concept
.	O
</s>
<s>
Like	O
preemptive	O
multitasking	B-Operating_System
,	O
each	O
thread	B-Operating_System
of	I-Operating_System
execution	I-Operating_System
is	O
assigned	O
its	O
own	O
program	B-General_Concept
counter	I-General_Concept
and	O
other	O
hardware	B-General_Concept
registers	I-General_Concept
(	O
each	O
thread	B-Operating_System
's	O
architectural	B-General_Concept
state	I-General_Concept
)	O
.	O
</s>
<s>
A	O
barrel	B-Operating_System
processor	I-Operating_System
can	O
guarantee	O
that	O
each	O
thread	B-Operating_System
will	O
execute	O
one	O
instruction	O
every	O
n	O
cycles	O
,	O
unlike	O
a	O
preemptive	O
multitasking	B-Operating_System
machine	O
,	O
that	O
typically	O
runs	O
one	O
thread	B-Operating_System
of	I-Operating_System
execution	I-Operating_System
for	O
tens	O
of	O
millions	O
of	O
cycles	O
,	O
while	O
all	O
other	O
threads	B-Operating_System
wait	O
their	O
turn	O
.	O
</s>
<s>
A	O
technique	O
called	O
C-slowing	B-General_Concept
can	O
automatically	O
generate	O
a	O
corresponding	O
barrel	B-Operating_System
processor	B-General_Concept
design	I-General_Concept
from	O
a	O
single-tasking	O
processor	B-General_Concept
design	I-General_Concept
.	O
</s>
<s>
An	O
n-way	O
barrel	B-Operating_System
processor	I-Operating_System
generated	O
this	O
way	O
acts	O
much	O
like	O
n	O
separate	O
multiprocessing	B-Operating_System
copies	O
of	O
the	O
original	O
single-tasking	O
processor	O
,	O
each	O
one	O
running	O
at	O
roughly	O
1/n	O
the	O
original	O
speed	O
.	O
</s>
<s>
One	O
of	O
the	O
earliest	O
examples	O
of	O
a	O
barrel	B-Operating_System
processor	I-Operating_System
was	O
the	O
I/O	O
processing	O
system	O
in	O
the	O
CDC	B-Device
6000	I-Device
series	I-Device
supercomputers	O
.	O
</s>
<s>
From	O
CDC	B-Device
6000	I-Device
series	I-Device
we	O
read	O
that	O
"	O
The	O
peripheral	O
processors	O
are	O
collectively	O
implemented	O
as	O
a	O
barrel	B-Operating_System
processor	I-Operating_System
.	O
</s>
<s>
One	O
motivation	O
for	O
barrel	B-Operating_System
processors	I-Operating_System
was	O
to	O
reduce	O
hardware	O
costs	O
.	O
</s>
<s>
Another	O
example	O
is	O
the	O
Honeywell	B-Device
800	I-Device
,	O
which	O
had	O
8	O
groups	O
of	O
registers	O
,	O
allowing	O
up	O
to	O
8	O
concurrent	O
programs	O
.	O
</s>
<s>
Barrel	B-Operating_System
processors	I-Operating_System
have	O
also	O
been	O
used	O
as	O
large-scale	O
central	O
processors	O
.	O
</s>
<s>
The	O
Tera	B-Device
MTA	I-Device
(	O
1988	O
)	O
was	O
a	O
large-scale	O
barrel	B-Operating_System
processor	I-Operating_System
design	O
with	O
128	O
threads	B-Operating_System
per	O
core	O
.	O
</s>
<s>
The	O
MTA	B-Device
architecture	O
has	O
seen	O
continued	O
development	O
in	O
successive	O
products	O
,	O
such	O
as	O
the	O
Cray	B-Device
Urika-GD	I-Device
,	O
originally	O
introduced	O
in	O
2012	O
(	O
as	O
the	O
YarcData	O
uRiKA	O
)	O
and	O
targeted	O
at	O
data-mining	O
applications	O
.	O
</s>
<s>
Barrel	B-Operating_System
processors	I-Operating_System
are	O
also	O
found	O
in	O
embedded	B-Architecture
systems	I-Architecture
,	O
where	O
they	O
are	O
particularly	O
useful	O
for	O
their	O
deterministic	O
real-time	B-General_Concept
thread	B-Operating_System
performance	O
.	O
</s>
<s>
An	O
example	O
is	O
the	O
XMOS	O
XCore	O
XS1	O
(	O
2007	O
)	O
,	O
a	O
four-stage	O
barrel	B-Operating_System
processor	I-Operating_System
with	O
eight	O
threads	B-Operating_System
per	O
core	O
.	O
</s>
<s>
When	O
the	O
XS1	O
is	O
programmed	O
in	O
the	O
'	O
XC	O
 '	O
language	O
,	O
software	O
controlled	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
may	O
be	O
implemented	O
.	O
</s>
<s>
Barrel	B-Operating_System
processors	I-Operating_System
have	O
also	O
been	O
used	O
in	O
specialized	O
devices	O
such	O
as	O
the	O
eight-thread	O
Ubicom	O
IP3023	O
network	O
I/O	O
processor	O
(	O
2004	O
)	O
.	O
</s>
<s>
Some	O
8-bit	O
microcontrollers	B-Architecture
by	O
Padauk	O
Technology	O
feature	O
barrel	B-Operating_System
processors	I-Operating_System
with	O
up	O
to	O
8	O
threads	B-Operating_System
per	O
core	O
.	O
</s>
<s>
A	O
single-tasking	O
processor	O
spends	O
a	O
lot	O
of	O
time	O
idle	O
,	O
not	O
doing	O
anything	O
useful	O
whenever	O
a	O
cache	B-General_Concept
miss	O
or	O
pipeline	B-General_Concept
stall	I-General_Concept
occurs	O
.	O
</s>
<s>
Advantages	O
to	O
employing	O
barrel	B-Operating_System
processors	I-Operating_System
over	O
single-tasking	O
processors	O
include	O
:	O
</s>
<s>
The	O
ability	O
to	O
do	O
useful	O
work	O
on	O
the	O
other	O
threads	B-Operating_System
while	O
the	O
stalled	O
thread	B-Operating_System
is	O
waiting	O
.	O
</s>
<s>
Designing	O
an	O
n-way	O
barrel	B-Operating_System
processor	I-Operating_System
with	O
an	O
n-deep	O
pipeline	B-General_Concept
is	O
much	O
simpler	O
than	O
designing	O
a	O
single-tasking	O
processor	O
because	O
a	O
barrel	B-Operating_System
processor	I-Operating_System
never	O
has	O
a	O
pipeline	B-General_Concept
stall	I-General_Concept
and	O
does	O
n't	O
need	O
feed-forward	O
circuits	O
.	O
</s>
<s>
For	O
real-time	B-General_Concept
applications	I-General_Concept
,	O
a	O
barrel	B-Operating_System
processor	I-Operating_System
can	O
guarantee	O
that	O
a	O
"	O
real-time	B-General_Concept
"	O
thread	B-Operating_System
can	O
execute	O
with	O
precise	O
timing	O
,	O
no	O
matter	O
what	O
happens	O
to	O
the	O
other	O
threads	B-Operating_System
,	O
even	O
if	O
some	O
other	O
thread	B-Operating_System
locks	B-Operating_System
up	I-Operating_System
in	O
an	O
infinite	B-Algorithm
loop	I-Algorithm
or	O
is	O
continuously	O
interrupted	O
by	O
hardware	O
interrupts	O
.	O
</s>
<s>
There	O
are	O
a	O
few	O
disadvantages	O
to	O
barrel	B-Operating_System
processors	I-Operating_System
.	O
</s>
<s>
The	O
state	O
of	O
each	O
thread	B-Operating_System
must	O
be	O
kept	O
on-chip	O
,	O
typically	O
in	O
registers	O
,	O
to	O
avoid	O
costly	O
off-chip	O
context	O
switches	O
.	O
</s>
<s>
Either	O
all	O
threads	B-Operating_System
must	O
share	O
the	O
same	O
cache	B-General_Concept
,	O
which	O
slows	O
overall	O
system	O
performance	O
,	O
or	O
there	O
must	O
be	O
one	O
unit	O
of	O
cache	B-General_Concept
for	O
each	O
execution	O
thread	B-Operating_System
,	O
which	O
can	O
significantly	O
increase	O
the	O
transistor	O
count	O
and	O
thus	O
the	O
cost	O
of	O
such	O
a	O
CPU	B-General_Concept
.	O
</s>
<s>
However	O
,	O
in	O
hard	O
real-time	B-General_Concept
embedded	B-Architecture
systems	I-Architecture
where	O
barrel	B-Operating_System
processors	I-Operating_System
are	O
often	O
found	O
,	O
memory	O
access	O
costs	O
are	O
typically	O
calculated	O
assuming	O
worst-case	O
cache	B-General_Concept
behavior	O
,	O
so	O
this	O
is	O
a	O
minor	O
concern	O
.	O
</s>
<s>
Some	O
barrel	B-Operating_System
processors	I-Operating_System
such	O
as	O
the	O
XMOS	O
XS1	O
do	O
not	O
have	O
a	O
cache	B-General_Concept
at	O
all	O
.	O
</s>
