<s>
In	O
computer	B-General_Concept
architecture	I-General_Concept
,	O
register	B-Architecture
renaming	I-Architecture
is	O
a	O
technique	O
that	O
abstracts	O
logical	O
registers	O
from	O
physical	O
registers	O
.	O
</s>
<s>
Every	O
logical	O
register	B-General_Concept
has	O
a	O
set	O
of	O
physical	O
registers	O
associated	O
with	O
it	O
.	O
</s>
<s>
When	O
a	O
machine	B-Language
language	I-Language
instruction	O
refers	O
to	O
a	O
particular	O
logical	O
register	B-General_Concept
,	O
the	O
processor	O
transposes	O
this	O
name	O
to	O
one	O
specific	O
physical	O
register	B-General_Concept
on	O
the	O
fly	O
.	O
</s>
<s>
This	O
technique	O
is	O
used	O
to	O
eliminate	O
false	O
data	B-Operating_System
dependencies	I-Operating_System
arising	O
from	O
the	O
reuse	O
of	O
registers	O
by	O
successive	O
instructions	O
that	O
do	O
not	O
have	O
any	O
real	O
data	B-Operating_System
dependencies	I-Operating_System
between	O
them	O
.	O
</s>
<s>
The	O
elimination	O
of	O
these	O
false	O
data	B-Operating_System
dependencies	I-Operating_System
reveals	O
more	O
instruction-level	B-Operating_System
parallelism	I-Operating_System
in	O
an	O
instruction	O
stream	O
,	O
which	O
can	O
be	O
exploited	O
by	O
various	O
and	O
complementary	O
techniques	O
such	O
as	O
superscalar	B-General_Concept
and	O
out-of-order	B-General_Concept
execution	I-General_Concept
for	O
better	O
performance	O
.	O
</s>
<s>
In	O
a	O
register	B-Application
machine	I-Application
,	O
programs	O
are	O
composed	O
of	O
instructions	O
which	O
operate	O
on	O
values	O
.	O
</s>
<s>
In	O
order	O
to	O
have	O
a	O
compact	O
instruction	O
encoding	O
,	O
most	O
processor	O
instruction	B-General_Concept
sets	I-General_Concept
have	O
a	O
small	O
set	O
of	O
special	O
locations	O
which	O
can	O
be	O
referred	O
to	O
by	O
special	O
names	O
:	O
registers	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
x86	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
has	O
8integer	O
registers	O
,	O
x86-64	B-Device
has	O
16	O
,	O
many	O
RISCs	B-Architecture
have	O
32	O
,	O
and	O
IA-64	B-General_Concept
has	O
128	O
.	O
</s>
<s>
In	O
smaller	O
processors	O
,	O
the	O
names	O
of	O
these	O
locations	O
correspond	O
directly	O
to	O
elements	O
of	O
a	O
register	B-General_Concept
file	I-General_Concept
.	O
</s>
<s>
Out-of-order	B-General_Concept
execution	I-General_Concept
has	O
been	O
used	O
in	O
most	O
recent	O
high-performance	O
CPUs	O
to	O
achieve	O
some	O
of	O
their	O
speed	O
gains	O
.	O
</s>
<s>
Consider	O
this	O
piece	O
of	O
code	O
running	O
on	O
an	O
out-of-order	B-General_Concept
CPU	I-General_Concept
:	O
</s>
<s>
The	O
program	O
will	O
run	O
faster	O
than	O
before	O
via	O
obliterating	O
any	O
stalling	O
due	O
to	O
a	O
false	O
data	B-Operating_System
dependency	I-Operating_System
.	O
</s>
<s>
On	O
targets	O
without	O
appropriate	O
data-flow	O
detection	O
good	O
compilers	O
would	O
detect	O
independent	O
instruction	O
sequences	O
and	O
choose	O
different	O
registers	O
during	O
code	B-Application
generation	I-Application
.	O
</s>
<s>
a	O
read	O
from	O
a	O
register	B-General_Concept
or	O
memory	O
location	O
must	O
return	O
the	O
value	O
placed	O
there	O
by	O
the	O
last	O
write	O
in	O
program	O
order	O
,	O
not	O
some	O
other	O
write	O
.	O
</s>
<s>
successive	O
writes	O
to	O
a	O
particular	O
register	B-General_Concept
or	O
memory	O
location	O
must	O
leave	O
that	O
location	O
containing	O
the	O
result	O
of	O
the	O
second	O
write	O
.	O
</s>
<s>
a	O
read	O
from	O
a	O
register	B-General_Concept
or	O
memory	O
location	O
must	O
return	O
the	O
last	O
prior	O
value	O
written	O
to	O
that	O
location	O
,	O
and	O
not	O
one	O
written	O
programmatically	O
after	O
the	O
read	O
.	O
</s>
<s>
This	O
is	O
a	O
sort	O
of	O
false	B-Operating_System
dependency	I-Operating_System
that	O
can	O
be	O
resolved	O
by	O
renaming	O
.	O
</s>
<s>
The	O
false	B-Operating_System
dependency	I-Operating_System
is	O
broken	O
and	O
additional	O
opportunities	O
for	O
out-of-order	B-General_Concept
execution	I-General_Concept
are	O
created	O
.	O
</s>
<s>
This	O
is	O
the	O
essential	O
concept	O
behind	O
register	B-Architecture
renaming	I-Architecture
.	O
</s>
<s>
Memory	O
locations	O
can	O
also	O
be	O
renamed	O
,	O
although	O
it	O
is	O
not	O
commonly	O
done	O
to	O
the	O
extent	O
practiced	O
in	O
register	B-Architecture
renaming	I-Architecture
.	O
</s>
<s>
The	O
Transmeta	B-General_Concept
Crusoe	I-General_Concept
processor	O
's	O
gated	O
store	O
buffer	O
is	O
a	O
form	O
of	O
memory	O
renaming	O
.	O
</s>
<s>
If	O
programs	O
refrained	O
from	O
reusing	O
registers	O
immediately	O
,	O
there	O
would	O
be	O
no	O
need	O
for	O
register	B-Architecture
renaming	I-Architecture
.	O
</s>
<s>
Some	O
instruction	B-General_Concept
sets	I-General_Concept
(	O
e.g.	O
,	O
IA-64	B-General_Concept
)	O
specify	O
very	O
large	O
numbers	O
of	O
registers	O
for	O
specifically	O
this	O
reason	O
.	O
</s>
<s>
In	O
loops	O
,	O
for	O
instance	O
,	O
successive	O
iterations	O
would	O
have	O
to	O
use	O
different	O
registers	O
,	O
which	O
requires	O
replicating	O
the	O
code	O
in	O
a	O
process	O
called	O
loop	B-Operating_System
unrolling	I-Operating_System
,	O
or	O
utilising	O
self-modifying	B-Application
code	I-Application
to	O
change	O
the	O
operand	O
targets	O
in	O
each	O
iteration	O
.	O
</s>
<s>
Large	O
numbers	O
of	O
registers	O
require	O
more	O
bits	O
for	O
specifying	O
a	O
register	B-General_Concept
as	O
an	O
operand	O
in	O
an	O
instruction	O
,	O
resulting	O
in	O
increased	O
code	O
size	O
.	O
</s>
<s>
Many	O
instruction	B-General_Concept
sets	I-General_Concept
historically	O
specified	O
smaller	O
numbers	O
of	O
registers	O
and	O
cannot	O
be	O
changed	O
while	O
still	O
retaining	O
backwards	O
compatibility	O
.	O
</s>
<s>
Machine	B-Language
language	I-Language
programs	O
specify	O
reads	O
and	O
writes	O
to	O
a	O
limited	O
set	O
of	O
registers	O
specified	O
by	O
the	O
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	O
)	O
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
Alpha	B-Device
ISA	O
specifies	O
32	O
integer	O
registers	O
,	O
each	O
64	O
bits	O
wide	O
,	O
and	O
32	O
floating-point	O
registers	O
,	O
each	O
64	O
bits	O
wide	O
.	O
</s>
<s>
Programs	O
written	O
for	O
processors	O
running	O
the	O
Alpha	B-Device
instruction	B-General_Concept
set	I-General_Concept
will	O
specify	O
operations	O
reading	O
and	O
writing	O
those	O
64	O
registers	O
.	O
</s>
<s>
One	O
particular	O
processor	O
which	O
implements	O
this	O
ISA	O
,	O
the	O
Alpha	B-General_Concept
21264	I-General_Concept
,	O
has	O
80	O
integer	O
and	O
72	O
floating-point	O
physical	O
registers	O
.	O
</s>
<s>
(	O
In	O
fact	O
,	O
there	O
are	O
even	O
more	O
locations	O
than	O
that	O
,	O
but	O
those	O
extra	O
locations	O
are	O
not	O
germane	O
to	O
the	O
register	B-Architecture
renaming	I-Architecture
operation	O
.	O
)	O
</s>
<s>
The	O
following	O
text	O
describes	O
two	O
styles	O
of	O
register	B-Architecture
renaming	I-Architecture
,	O
which	O
are	O
distinguished	O
by	O
the	O
circuit	O
which	O
holds	O
the	O
data	O
ready	O
for	O
an	O
execution	O
unit	O
.	O
</s>
<s>
Because	O
the	O
size	O
of	O
a	O
register	B-General_Concept
file	I-General_Concept
generally	O
grows	O
as	O
the	O
square	O
of	O
the	O
number	O
of	O
ports	O
,	O
the	O
rename	O
file	O
is	O
usually	O
physically	O
large	O
and	O
consumes	O
significant	O
power	O
.	O
</s>
<s>
In	O
the	O
tag-indexed	O
register	B-General_Concept
file	I-General_Concept
style	O
,	O
there	O
is	O
one	O
large	O
register	B-General_Concept
file	I-General_Concept
for	O
data	O
values	O
,	O
containing	O
one	O
register	B-General_Concept
for	O
every	O
tag	O
.	O
</s>
<s>
In	O
this	O
style	O
,	O
when	O
an	O
instruction	O
is	O
issued	O
to	O
an	O
execution	O
unit	O
,	O
the	O
tags	O
of	O
the	O
source	O
registers	O
are	O
sent	O
to	O
the	O
physical	B-General_Concept
register	I-General_Concept
file	I-General_Concept
,	O
where	O
the	O
values	O
corresponding	O
to	O
those	O
tags	O
are	O
read	O
and	O
sent	O
to	O
the	O
execution	O
unit	O
.	O
</s>
<s>
In	O
the	O
reservation	O
station	O
style	O
,	O
there	O
are	O
many	O
small	O
associative	O
register	B-General_Concept
files	I-General_Concept
,	O
usually	O
one	O
at	O
the	O
inputs	O
to	O
each	O
execution	O
unit	O
.	O
</s>
<s>
Each	O
operand	O
of	O
each	O
instruction	O
in	O
an	O
issue	O
queue	O
has	O
a	O
place	O
for	O
a	O
value	O
in	O
one	O
of	O
these	O
register	B-General_Concept
files	I-General_Concept
.	O
</s>
<s>
In	O
this	O
style	O
,	O
when	O
an	O
instruction	O
is	O
issued	O
to	O
an	O
execution	O
unit	O
,	O
the	O
register	B-General_Concept
file	I-General_Concept
entries	O
corresponding	O
to	O
the	O
issue	O
queue	O
entry	O
are	O
read	O
and	O
forwarded	O
to	O
the	O
execution	O
unit	O
.	O
</s>
<s>
The	O
committed	O
register	B-General_Concept
state	O
of	O
the	O
machine	O
.	O
</s>
<s>
RAM	O
indexed	O
by	O
logical	O
register	B-General_Concept
number	O
.	O
</s>
<s>
Typically	O
written	O
into	O
as	O
results	O
are	O
retired	O
or	O
committed	O
out	O
of	O
a	O
reorder	B-General_Concept
buffer	I-General_Concept
.	O
</s>
<s>
The	O
most	O
speculative	O
register	B-General_Concept
state	O
of	O
the	O
machine	O
.	O
</s>
<s>
RAM	O
indexed	O
by	O
logical	O
register	B-General_Concept
number	O
.	O
</s>
<s>
The	O
Intel	B-Device
P6	I-Device
group	O
's	O
term	O
for	O
Future	O
File	O
.	O
</s>
<s>
After	O
a	O
branch	O
misprediction	O
must	O
use	O
results	O
from	O
the	O
history	O
buffer	O
—	O
either	O
they	O
are	O
copied	O
,	O
or	O
the	O
future	O
file	O
lookup	O
is	O
disabled	O
and	O
the	O
history	O
buffer	O
is	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
(	O
CAM	B-Data_Structure
)	O
indexed	O
by	O
logical	O
register	B-General_Concept
number	O
.	O
</s>
<s>
It	O
differs	O
from	O
a	O
history	O
buffer	O
because	O
the	O
reorder	B-General_Concept
buffer	I-General_Concept
typically	O
comes	O
after	O
the	O
future	O
file	O
(	O
if	O
it	O
exists	O
)	O
and	O
before	O
the	O
architectural	O
register	B-General_Concept
file	I-General_Concept
.	O
</s>
<s>
Reorder	B-General_Concept
buffers	I-General_Concept
can	O
be	O
data-less	O
or	O
data-ful	O
.	O
</s>
<s>
In	O
Willamette	O
's	O
ROB	O
,	O
the	O
ROB	O
entries	O
point	O
to	O
registers	O
in	O
the	O
physical	B-General_Concept
register	I-General_Concept
file	I-General_Concept
(	O
PRF	O
)	O
,	O
and	O
also	O
contain	O
other	O
book	O
keeping	O
.	O
</s>
<s>
P6	B-Device
's	O
ROB	O
,	O
the	O
ROB	O
entries	O
contain	O
data	O
;	O
there	O
is	O
no	O
separate	O
PRF	O
.	O
</s>
<s>
ROBs	O
usually	O
do	O
n't	O
have	O
associative	O
logic	O
,	O
and	O
certainly	O
none	O
of	O
the	O
ROBs	O
designed	O
by	O
Andy	O
Glew	O
have	O
CAMs	B-Data_Structure
.	O
</s>
<s>
The	O
first	O
ROB	O
proposal	O
may	O
have	O
had	O
CAMs	B-Data_Structure
.	O
</s>
<s>
This	O
is	O
the	O
renaming	O
style	O
used	O
in	O
the	O
MIPS	O
R10000	B-General_Concept
,	O
the	O
Alpha	B-General_Concept
21264	I-General_Concept
,	O
and	O
in	O
the	O
FP	O
section	O
of	O
the	O
AMD	B-Architecture
Athlon	I-Architecture
.	O
</s>
<s>
In	O
the	O
renaming	O
stage	O
,	O
every	O
architectural	O
register	B-General_Concept
referenced	O
(	O
for	O
read	O
or	O
write	O
)	O
is	O
looked	O
up	O
in	O
an	O
architecturally-indexed	O
remap	O
file	O
.	O
</s>
<s>
For	O
read	O
operands	O
,	O
this	O
tag	O
takes	O
the	O
place	O
of	O
the	O
architectural	O
register	B-General_Concept
in	O
the	O
instruction	O
.	O
</s>
<s>
For	O
every	O
register	B-General_Concept
write	O
,	O
a	O
new	O
tag	O
is	O
pulled	O
from	O
a	O
free	O
tag	O
FIFO	O
,	O
and	O
a	O
new	O
mapping	O
is	O
written	O
into	O
the	O
remap	O
file	O
,	O
so	O
that	O
future	O
instructions	O
reading	O
the	O
architectural	O
register	B-General_Concept
will	O
refer	O
to	O
this	O
new	O
tag	O
.	O
</s>
<s>
The	O
previous	O
physical	O
register	B-General_Concept
allocated	O
for	O
that	O
architectural	O
register	B-General_Concept
is	O
saved	O
with	O
the	O
instruction	O
in	O
the	O
reorder	B-General_Concept
buffer	I-General_Concept
,	O
which	O
is	O
a	O
FIFO	O
that	O
holds	O
the	O
instructions	O
in	O
program	O
order	O
between	O
the	O
decode	O
and	O
graduation	O
stages	O
.	O
</s>
<s>
Issued	O
instructions	O
read	O
from	O
a	O
tag-indexed	O
physical	B-General_Concept
register	I-General_Concept
file	I-General_Concept
(	O
bypassing	O
just-broadcast	O
operands	O
)	O
and	O
then	O
execute	O
.	O
</s>
<s>
Execution	O
results	O
are	O
written	O
to	O
tag-indexed	O
physical	B-General_Concept
register	I-General_Concept
file	I-General_Concept
,	O
as	O
well	O
as	O
broadcast	O
to	O
the	O
bypass	O
network	O
preceding	O
each	O
functional	O
unit	O
.	O
</s>
<s>
Graduation	O
puts	O
the	O
previous	O
tag	O
for	O
the	O
written	O
architectural	O
register	B-General_Concept
into	O
the	O
free	O
queue	O
so	O
that	O
it	O
can	O
be	O
reused	O
for	O
a	O
newly	O
decoded	O
instruction	O
.	O
</s>
<s>
This	O
is	O
the	O
style	O
used	O
in	O
the	O
integer	O
section	O
of	O
the	O
AMD	B-Architecture
K7	I-Architecture
and	O
K8	O
designs	O
.	O
</s>
<s>
In	O
the	O
renaming	O
stage	O
,	O
every	O
architectural	O
register	B-General_Concept
referenced	O
for	O
reads	O
is	O
looked	O
up	O
in	O
both	O
the	O
architecturally-indexed	O
future	O
file	O
and	O
the	O
rename	O
file	O
.	O
</s>
<s>
The	O
future	O
file	O
read	O
gives	O
the	O
value	O
of	O
that	O
register	B-General_Concept
,	O
if	O
there	O
is	O
no	O
outstanding	O
instruction	O
yet	O
to	O
write	O
to	O
it	O
(	O
i.e.	O
,	O
it	O
's	O
ready	O
)	O
.	O
</s>
<s>
Register	B-General_Concept
writes	O
in	O
the	O
instruction	O
cause	O
a	O
new	O
,	O
non-ready	O
tag	O
to	O
be	O
written	O
into	O
the	O
rename	O
file	O
.	O
</s>
<s>
As	O
mentioned	O
earlier	O
,	O
the	O
reservation	O
station	O
register	B-General_Concept
files	I-General_Concept
are	O
usually	O
small	O
,	O
with	O
perhaps	O
eight	O
entries	O
.	O
</s>
<s>
Execution	O
results	O
are	O
written	O
to	O
the	O
reorder	B-General_Concept
buffer	I-General_Concept
,	O
to	O
the	O
reservation	O
stations	O
(	O
if	O
the	O
issue	O
queue	O
entry	O
has	O
a	O
matching	O
tag	O
)	O
,	O
and	O
to	O
the	O
future	O
file	O
if	O
this	O
is	O
the	O
last	O
instruction	O
to	O
target	O
that	O
architectural	O
register	B-General_Concept
(	O
in	O
which	O
case	O
register	B-General_Concept
is	O
marked	O
ready	O
)	O
.	O
</s>
<s>
Graduation	O
copies	O
the	O
value	O
from	O
the	O
reorder	B-General_Concept
buffer	I-General_Concept
into	O
the	O
architectural	O
register	B-General_Concept
file	I-General_Concept
.	O
</s>
<s>
The	O
sole	O
use	O
of	O
the	O
architectural	O
register	B-General_Concept
file	I-General_Concept
is	O
to	O
recover	O
from	O
exceptions	O
and	O
branch	O
mispredictions	O
.	O
</s>
<s>
Reservation	O
stations	O
have	O
better	O
latency	O
from	O
rename	O
to	O
execute	O
,	O
because	O
the	O
rename	O
stage	O
finds	O
the	O
register	B-General_Concept
values	O
directly	O
,	O
rather	O
than	O
finding	O
the	O
physical	O
register	B-General_Concept
number	O
,	O
and	O
then	O
using	O
that	O
to	O
find	O
the	O
value	O
.	O
</s>
<s>
Reservation	O
stations	O
also	O
have	O
better	O
latency	O
from	O
instruction	O
issue	O
to	O
execution	O
,	O
because	O
each	O
local	O
register	B-General_Concept
file	I-General_Concept
is	O
smaller	O
than	O
the	O
large	O
central	O
file	O
of	O
the	O
tag-indexed	O
scheme	O
.	O
</s>
<s>
The	O
physical	B-General_Concept
register	I-General_Concept
files	I-General_Concept
used	O
by	O
reservation	O
stations	O
usually	O
collapse	O
unused	O
entries	O
in	O
parallel	O
with	O
the	O
issue	O
queue	O
they	O
serve	O
,	O
which	O
makes	O
these	O
register	B-General_Concept
files	I-General_Concept
larger	O
in	O
aggregate	O
,	O
and	O
consume	O
more	O
power	O
,	O
and	O
more	O
complicated	O
than	O
the	O
simpler	O
register	B-General_Concept
files	I-General_Concept
used	O
in	O
a	O
tag-indexed	O
scheme	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
reservation	O
station	O
scheme	O
has	O
four	O
places	O
(	O
Future	O
File	O
,	O
Reservation	O
Station	O
,	O
Reorder	B-General_Concept
Buffer	I-General_Concept
and	O
Architectural	O
File	O
)	O
where	O
a	O
result	O
value	O
can	O
be	O
stored	O
,	O
whereas	O
the	O
tag-indexed	O
scheme	O
has	O
just	O
one	O
(	O
the	O
physical	B-General_Concept
register	I-General_Concept
file	I-General_Concept
)	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/360	I-Device
Model	I-Device
91	I-Device
was	O
an	O
early	O
machine	O
that	O
supported	O
out-of-order	B-General_Concept
execution	I-General_Concept
of	O
instructions	O
;	O
it	O
used	O
the	O
Tomasulo	B-General_Concept
algorithm	I-General_Concept
,	O
which	O
uses	O
register	B-Architecture
renaming	I-Architecture
.	O
</s>
<s>
The	O
POWER1	B-General_Concept
is	O
the	O
first	O
microprocessor	B-Architecture
that	O
used	O
register	B-Architecture
renaming	I-Architecture
and	O
out-of-order	B-General_Concept
execution	I-General_Concept
in	O
1990	O
.	O
</s>
<s>
The	O
original	O
R10000	B-General_Concept
design	O
had	O
neither	O
collapsing	O
issue	O
queues	O
nor	O
variable	O
priority	O
encoding	O
,	O
and	O
suffered	O
starvation	O
problems	O
as	O
a	O
result	O
—	O
the	O
oldest	O
instruction	O
in	O
the	O
queue	O
would	O
sometimes	O
not	O
be	O
issued	O
until	O
both	O
instruction	O
decode	O
stopped	O
completely	O
for	O
lack	O
of	O
rename	O
registers	O
,	O
and	O
every	O
other	O
instruction	O
had	O
been	O
issued	O
.	O
</s>
<s>
Later	O
revisions	O
of	O
the	O
design	O
starting	O
with	O
the	O
R12000	B-General_Concept
used	O
a	O
partially	O
variable	O
priority	O
encoder	O
to	O
mitigate	O
this	O
problem	O
.	O
</s>
<s>
Most	O
modern	O
machines	O
do	O
renaming	O
by	O
RAM	O
indexing	O
a	O
map	O
table	O
with	O
the	O
logical	O
register	B-General_Concept
number	O
.	O
</s>
<s>
E.g.	O
,	O
P6	B-Device
did	O
this	O
;	O
future	O
files	O
do	O
this	O
,	O
and	O
have	O
data	O
storage	O
in	O
the	O
same	O
structure	O
.	O
</s>
<s>
However	O
,	O
earlier	O
machines	O
used	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
(	O
CAM	B-Data_Structure
)	O
in	O
the	O
renamer	O
.	O
</s>
<s>
E.g.	O
,	O
the	O
HPSM	O
RAT	O
,	O
or	O
Register	B-General_Concept
Alias	I-General_Concept
Table	I-General_Concept
,	O
essentially	O
used	O
a	O
CAM	B-Data_Structure
on	O
the	O
logical	O
register	B-General_Concept
number	O
in	O
combination	O
with	O
different	O
versions	O
of	O
the	O
register	B-General_Concept
.	O
</s>
<s>
In	O
many	O
ways	O
,	O
the	O
story	O
of	O
out-of-order	O
microarchitecture	O
has	O
been	O
how	O
these	O
CAMs	B-Data_Structure
have	O
been	O
progressively	O
eliminated	O
.	O
</s>
<s>
Small	O
CAMs	B-Data_Structure
are	O
useful	O
;	O
large	O
CAMs	B-Data_Structure
are	O
impractical	O
.	O
</s>
<s>
The	O
P6	B-Device
microarchitecture	I-Device
was	O
the	O
first	O
microarchitecture	O
by	O
Intel	O
to	O
implement	O
both	O
out-of-order	B-General_Concept
execution	I-General_Concept
and	O
register	B-Architecture
renaming	I-Architecture
.	O
</s>
<s>
The	O
P6	B-Device
microarchitecture	I-Device
was	O
used	O
in	O
Pentium	O
Pro	O
,	O
Pentium	O
II	O
,	O
Pentium	O
III	O
,	O
Pentium	O
M	O
,	O
Core	O
,	O
and	O
Core	O
2	O
microprocessors	B-Architecture
.	O
</s>
<s>
The	O
Cyrix	B-General_Concept
M1	I-General_Concept
,	O
released	O
on	O
October	O
2	O
,	O
1995	O
,	O
was	O
the	O
first	O
x86	O
processor	O
to	O
use	O
register	B-Architecture
renaming	I-Architecture
and	O
out-of-order	B-General_Concept
execution	I-General_Concept
.	O
</s>
<s>
Other	O
x86	O
processors	O
(	O
such	O
as	O
NexGen	O
Nx686	O
and	O
AMD	O
K5	O
)	O
released	O
in	O
1996	O
also	O
featured	O
register	B-Architecture
renaming	I-Architecture
and	O
out-of-order	B-General_Concept
execution	I-General_Concept
of	O
RISC	B-Architecture
μ-operations	B-General_Concept
(	O
rather	O
than	O
native	O
x86	O
instructions	O
)	O
.	O
</s>
