<s>
Unified	B-Operating_System
Parallel	I-Operating_System
C	I-Operating_System
(	O
UPC	O
)	O
is	O
an	O
extension	O
of	O
the	O
C	B-Language
programming	I-Language
language	I-Language
designed	O
for	O
high-performance	B-Architecture
computing	I-Architecture
on	O
large-scale	O
parallel	B-Operating_System
machines	I-Operating_System
,	O
including	O
those	O
with	O
a	O
common	O
global	B-Application
address	I-Application
space	I-Application
(	O
SMP	B-Operating_System
and	O
NUMA	B-Operating_System
)	O
and	O
those	O
with	O
distributed	B-Operating_System
memory	I-Operating_System
(	O
e.g.	O
</s>
<s>
clusters	B-Architecture
)	O
.	O
</s>
<s>
The	O
programmer	B-Application
is	O
presented	O
with	O
a	O
single	O
partitioned	B-Application
global	I-Application
address	I-Application
space	I-Application
;	O
where	O
shared	O
variables	O
may	O
be	O
directly	O
read	O
and	O
written	O
by	O
any	O
processor	B-General_Concept
,	O
but	O
each	O
variable	O
is	O
physically	O
associated	O
with	O
a	O
single	O
processor	B-General_Concept
.	O
</s>
<s>
UPC	O
uses	O
a	O
single	B-Operating_System
program	I-Operating_System
,	I-Operating_System
multiple	I-Operating_System
data	I-Operating_System
(	O
SPMD	B-Operating_System
)	O
model	O
of	O
computation	O
in	O
which	O
the	O
amount	O
of	O
parallelism	B-Operating_System
is	O
fixed	O
at	O
program	O
startup	O
time	O
,	O
typically	O
with	O
a	O
single	B-Operating_System
thread	I-Operating_System
of	O
execution	O
per	O
processor	B-General_Concept
.	O
</s>
<s>
In	O
order	O
to	O
express	O
parallelism	B-Operating_System
,	O
UPC	O
extends	O
ISO	O
C	B-Language
99	O
with	O
the	O
following	O
constructs	O
:	O
</s>
<s>
The	O
UPC	O
language	O
evolved	O
from	O
experiences	O
with	O
three	O
other	O
earlier	O
languages	O
that	O
proposed	O
parallel	B-Operating_System
extensions	O
to	O
ISO	O
C	B-Language
99	O
:	O
AC	O
,	O
Split-C	B-Language
,	O
and	O
Parallel	B-Operating_System
C	I-Operating_System
preprocessor	O
(	O
PCP	O
)	O
.	O
</s>
<s>
UPC	O
combines	O
the	O
programmability	O
advantages	O
of	O
the	O
shared	O
memory	O
programming	O
paradigm	O
and	O
the	O
control	O
over	O
data	O
layout	O
and	O
performance	O
of	O
the	O
message	B-Architecture
passing	I-Architecture
programming	I-Architecture
paradigm	O
.	O
</s>
