<s>
In	O
computers	O
,	O
hardware	B-General_Concept
performance	I-General_Concept
counters	I-General_Concept
(	O
HPC	O
)	O
,	O
or	O
hardware	B-General_Concept
counters	I-General_Concept
are	O
a	O
set	O
of	O
special-purpose	O
registers	O
built	O
into	O
modern	O
microprocessors	B-Architecture
to	O
store	O
the	O
counts	O
of	O
hardware-related	O
activities	O
within	O
computer	O
systems	O
.	O
</s>
<s>
Advanced	O
users	O
often	O
rely	O
on	O
those	O
counters	O
to	O
conduct	O
low-level	O
performance	O
analysis	O
or	O
tuning	B-Device
.	O
</s>
<s>
The	O
number	O
of	O
available	O
hardware	B-General_Concept
counters	I-General_Concept
in	O
a	O
processor	O
is	O
limited	O
while	O
each	O
CPU	B-General_Concept
model	O
might	O
have	O
a	O
lot	O
of	O
different	O
events	O
that	O
a	O
developer	O
might	O
like	O
to	O
measure	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
processors	O
to	O
implement	O
such	O
counter	O
and	O
an	O
associated	O
instruction	O
RDPMC	O
to	O
access	O
it	O
was	O
the	O
Intel	B-General_Concept
Pentium	I-General_Concept
,	O
but	O
they	O
were	O
not	O
documented	O
until	O
Terje	O
Mathisen	O
wrote	O
an	O
article	O
about	O
reverse	O
engineering	O
them	O
in	O
Byte	O
July	O
1994	O
.	O
</s>
<s>
The	O
following	O
table	O
shows	O
some	O
examples	O
of	O
CPUs	O
and	O
the	O
number	O
of	O
available	O
hardware	B-General_Concept
counters	I-General_Concept
:	O
</s>
<s>
Compared	O
to	O
software	O
profilers	O
,	O
hardware	B-General_Concept
counters	I-General_Concept
provide	O
low-overhead	O
access	O
to	O
a	O
wealth	O
of	O
detailed	O
performance	O
information	O
related	O
to	O
CPU	B-General_Concept
's	O
functional	O
units	O
,	O
caches	O
and	O
main	O
memory	O
etc	O
.	O
</s>
<s>
However	O
,	O
the	O
types	O
and	O
meanings	O
of	O
hardware	B-General_Concept
counters	I-General_Concept
vary	O
from	O
one	O
kind	O
of	O
architecture	O
to	O
another	O
due	O
to	O
the	O
variation	O
in	O
hardware	O
organizations	O
.	O
</s>
<s>
Modern	O
superscalar	B-General_Concept
processors	I-General_Concept
schedule	O
and	O
execute	O
multiple	O
instructions	O
out-of-order	B-General_Concept
at	O
one	O
time	O
.	O
</s>
<s>
AMD	O
's	O
implementation	O
of	O
IBS	O
provides	O
hardware	B-General_Concept
counters	I-General_Concept
for	O
both	O
fetch	O
sampling	O
(	O
the	O
front	O
of	O
the	O
superscalar	B-General_Concept
pipeline	O
)	O
and	O
op	O
sampling	O
(	O
the	O
back	O
of	O
the	O
pipeline	O
)	O
.	O
</s>
