<s>
The	O
program	B-General_Concept
counter	I-General_Concept
(	O
PC	O
)	O
,	O
commonly	O
called	O
the	O
instruction	B-General_Concept
pointer	I-General_Concept
(	O
IP	O
)	O
in	O
Intel	B-Operating_System
x86	I-Operating_System
and	O
Itanium	B-General_Concept
microprocessors	I-General_Concept
,	O
and	O
sometimes	O
called	O
the	O
instruction	B-General_Concept
address	I-General_Concept
register	I-General_Concept
(	O
IAR	O
)	O
,	O
the	O
instruction	B-General_Concept
counter	I-General_Concept
,	O
or	O
just	O
part	O
of	O
the	O
instruction	O
sequencer	O
,	O
is	O
a	O
processor	B-General_Concept
register	I-General_Concept
that	O
indicates	O
where	O
a	O
computer	O
is	O
in	O
its	O
program	B-Application
sequence	O
.	O
</s>
<s>
Usually	O
,	O
the	O
PC	O
is	O
incremented	O
after	O
fetching	O
an	O
instruction	O
,	O
and	O
holds	O
the	O
memory	B-General_Concept
address	I-General_Concept
of	O
(	O
"	O
points	O
to	O
"	O
)	O
the	O
next	B-General_Concept
instruction	I-General_Concept
that	O
would	O
be	O
executed	O
.	O
</s>
<s>
These	O
include	O
branches	B-General_Concept
(	O
sometimes	O
called	O
jumps	O
)	O
,	O
subroutine	O
calls	O
,	O
and	O
returns	B-Language
.	O
</s>
<s>
A	O
branch	O
provides	O
that	O
the	O
next	B-General_Concept
instruction	I-General_Concept
is	O
fetched	O
from	O
elsewhere	O
in	O
memory	O
.	O
</s>
<s>
A	O
subroutine	O
call	O
not	O
only	O
branches	B-General_Concept
but	O
saves	O
the	O
preceding	O
contents	O
of	O
the	O
PC	O
somewhere	O
.	O
</s>
<s>
In	O
a	O
simple	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
,	O
the	O
PC	O
is	O
a	O
digital	O
counter	O
(	O
which	O
is	O
the	O
origin	O
of	O
the	O
term	O
"	O
program	B-General_Concept
counter	I-General_Concept
"	O
)	O
that	O
may	O
be	O
one	O
of	O
several	O
hardware	O
registers	B-General_Concept
.	O
</s>
<s>
The	O
instruction	B-General_Concept
cycle	I-General_Concept
begins	O
with	O
a	O
fetch	O
,	O
in	O
which	O
the	O
CPU	O
places	O
the	O
value	O
of	O
the	O
PC	O
on	O
the	O
address	B-Architecture
bus	I-Architecture
to	O
send	O
it	O
to	O
the	O
memory	O
.	O
</s>
<s>
The	O
memory	O
responds	O
by	O
sending	O
the	O
contents	O
of	O
that	O
memory	B-General_Concept
location	I-General_Concept
on	O
the	O
data	B-General_Concept
bus	I-General_Concept
.	O
</s>
<s>
(	O
This	O
is	O
the	O
stored-program	O
computer	O
model	O
,	O
in	O
which	O
a	O
single	O
memory	O
space	O
contains	O
both	O
executable	O
instructions	O
and	O
ordinary	O
data	O
.	O
)	O
</s>
<s>
At	O
some	O
point	O
in	O
this	O
cycle	O
,	O
the	O
PC	O
will	O
be	O
modified	O
so	O
that	O
the	O
next	B-General_Concept
instruction	I-General_Concept
executed	O
is	O
a	O
different	O
one	O
(	O
typically	O
,	O
incremented	O
so	O
that	O
the	O
next	B-General_Concept
instruction	I-General_Concept
is	O
the	O
one	O
starting	O
at	O
the	O
memory	B-General_Concept
address	I-General_Concept
immediately	O
following	O
the	O
last	O
memory	B-General_Concept
location	I-General_Concept
of	O
the	O
current	B-General_Concept
instruction	I-General_Concept
)	O
.	O
</s>
<s>
Like	O
other	O
processor	B-General_Concept
registers	I-General_Concept
,	O
the	O
PC	O
may	O
be	O
a	O
bank	O
of	O
binary	O
latches	O
,	O
each	O
one	O
representing	O
one	O
bit	O
of	O
the	O
value	O
of	O
the	O
PC	O
.	O
</s>
<s>
On	O
some	O
processors	O
,	O
the	O
width	O
of	O
program	B-General_Concept
counter	I-General_Concept
instead	O
depends	O
on	O
the	O
addressable	O
memory	O
;	O
for	O
example	O
,	O
some	O
AVR	B-Architecture
controllers	I-Architecture
have	O
a	O
PC	O
which	O
wraps	O
around	O
after	O
12	O
bits	O
.	O
</s>
<s>
To	O
identify	O
the	O
current	B-General_Concept
instruction	I-General_Concept
,	O
the	O
PC	O
may	O
be	O
combined	O
with	O
other	O
registers	B-General_Concept
that	O
identify	O
a	O
segment	B-General_Concept
or	O
page	B-General_Concept
.	O
</s>
<s>
Such	O
a	O
PC	O
is	O
central	O
to	O
the	O
von	B-Architecture
Neumann	I-Architecture
architecture	I-Architecture
.	O
</s>
<s>
The	O
resulting	O
“	O
von	O
Neumann	O
bottleneck	O
”	O
led	O
to	O
research	O
into	O
parallel	O
computing	O
,	O
including	O
non-von	O
Neumann	O
or	O
dataflow	B-Application
models	O
that	O
did	O
not	O
use	O
a	O
PC	O
;	O
for	O
example	O
,	O
rather	O
than	O
specifying	O
sequential	O
steps	O
,	O
the	O
high-level	O
programmer	O
might	O
specify	O
desired	O
function	B-Language
and	O
the	O
low-level	O
programmer	O
might	O
specify	O
this	O
using	O
combinatory	B-Application
logic	I-Application
.	O
</s>
<s>
Pipelining	B-General_Concept
,	O
in	O
which	O
different	O
hardware	O
in	O
the	O
CPU	O
executes	O
different	O
phases	O
of	O
multiple	O
instructions	O
simultaneously	O
.	O
</s>
<s>
The	O
very	B-General_Concept
long	I-General_Concept
instruction	I-General_Concept
word	I-General_Concept
(	O
VLIW	B-General_Concept
)	O
architecture	O
,	O
where	O
a	O
single	O
instruction	O
can	O
achieve	O
multiple	O
effects	O
.	O
</s>
<s>
Techniques	O
to	O
predict	O
out-of-order	B-General_Concept
execution	I-General_Concept
and	O
prepare	O
subsequent	O
instructions	O
for	O
execution	O
outside	O
the	O
regular	O
sequence	O
.	O
</s>
<s>
The	O
high-level	O
language	O
is	O
essentially	O
the	O
machine	O
language	O
of	O
a	O
virtual	O
machine	O
,	O
too	O
complex	O
to	O
be	O
built	O
as	O
hardware	O
but	O
instead	O
emulated	O
or	O
interpreted	B-Application
by	O
software	O
.	O
</s>
<s>
When	O
writing	O
a	O
multi-threaded	B-Operating_System
program	B-Application
,	O
the	O
programmer	O
may	O
write	O
each	O
thread	B-Operating_System
as	O
a	O
sequence	O
of	O
instructions	O
without	O
specifying	O
the	O
timing	O
of	O
any	O
instruction	O
relative	O
to	O
instructions	O
in	O
other	O
threads	B-Operating_System
.	O
</s>
<s>
In	O
event-driven	B-Application
programming	I-Application
,	O
the	O
programmer	O
may	O
write	O
sequences	O
of	O
instructions	O
to	O
respond	O
to	O
events	O
without	O
specifying	O
an	O
overall	O
sequence	O
for	O
the	O
program	B-Application
.	O
</s>
<s>
In	O
dataflow	B-Application
programming	I-Application
,	O
the	O
programmer	O
may	O
write	O
each	O
section	O
of	O
a	O
computing	O
pipeline	B-Operating_System
without	O
specifying	O
the	O
timing	O
relative	O
to	O
other	O
sections	O
.	O
</s>
<s>
Vendors	O
use	O
different	O
characters	O
to	O
symbolize	O
the	O
program	B-General_Concept
counter	I-General_Concept
in	O
assembly	B-Language
language	I-Language
programs	O
.	O
</s>
