<s>
In	O
computer	O
engineering	O
,	O
register	B-General_Concept
windows	I-General_Concept
are	O
a	O
feature	O
which	O
dedicates	O
registers	O
to	O
a	O
subroutine	O
by	O
dynamically	O
aliasing	O
a	O
subset	O
of	O
internal	O
registers	O
to	O
fixed	O
,	O
programmer-visible	O
registers	O
.	O
</s>
<s>
Register	B-General_Concept
windows	I-General_Concept
are	O
implemented	O
to	O
improve	O
the	O
performance	O
of	O
a	O
processor	O
by	O
reducing	O
the	O
number	O
of	O
stack	B-Application
operations	O
required	O
for	O
function	O
calls	O
and	O
returns	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
influential	O
features	O
of	O
the	O
Berkeley	B-General_Concept
RISC	I-General_Concept
design	O
,	O
they	O
were	O
later	O
implemented	O
in	O
instruction	B-General_Concept
set	I-General_Concept
architectures	I-General_Concept
such	O
as	O
AMD	B-General_Concept
Am29000	I-General_Concept
,	O
Intel	B-General_Concept
i960	I-General_Concept
,	O
Sun	O
Microsystems	O
SPARC	B-Architecture
,	O
and	O
Intel	O
Itanium	B-General_Concept
.	O
</s>
<s>
In	O
the	O
Berkeley	B-General_Concept
RISC	I-General_Concept
design	O
,	O
only	O
eight	O
registers	O
out	O
of	O
a	O
total	O
of	O
64	O
are	O
visible	O
to	O
the	O
programs	O
.	O
</s>
<s>
The	O
complete	O
set	O
of	O
registers	O
are	O
known	O
as	O
the	O
register	B-General_Concept
file	I-General_Concept
,	O
and	O
any	O
particular	O
set	O
of	O
eight	O
as	O
a	O
window	O
.	O
</s>
<s>
By	O
comparison	O
,	O
the	O
Sun	O
Microsystems	O
SPARC	B-Architecture
architecture	O
provides	O
simultaneous	O
visibility	O
into	O
four	O
sets	O
of	O
eight	O
registers	O
each	O
.	O
</s>
<s>
When	O
a	O
procedure	O
is	O
called	O
,	O
the	O
register	B-General_Concept
window	I-General_Concept
shifts	O
by	O
sixteen	O
registers	O
,	O
hiding	O
the	O
old	O
input	O
registers	O
and	O
old	O
local	O
registers	O
and	O
making	O
the	O
old	O
output	O
registers	O
the	O
new	O
input	O
registers	O
.	O
</s>
<s>
The	O
AMD	B-General_Concept
29000	I-General_Concept
improved	O
the	O
design	O
by	O
allowing	O
the	O
windows	O
to	O
be	O
of	O
variable	O
size	O
,	O
which	O
helps	O
utilization	O
in	O
the	O
common	O
case	O
where	O
fewer	O
than	O
eight	O
registers	O
are	O
needed	O
for	O
a	O
call	O
.	O
</s>
<s>
Similarly	O
,	O
the	O
IA-64	B-General_Concept
(	O
Itanium	B-General_Concept
)	O
architecture	O
used	O
variable-sized	O
windows	O
,	O
with	O
32	O
global	O
registers	O
and	O
96	O
for	O
the	O
windows	O
.	O
</s>
<s>
In	O
the	O
Infineon	O
C166	B-Device
architecture	O
,	O
most	O
registers	O
are	O
simply	O
locations	O
in	O
internal	O
RAM	O
which	O
have	O
the	O
additional	O
property	O
of	O
being	O
accessible	O
as	O
registers	O
.	O
</s>
<s>
Register	B-General_Concept
windows	I-General_Concept
also	O
provide	O
an	O
easy	O
upgrade	O
path	O
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
use	O
of	O
object-oriented	B-Language
programming	I-Language
often	O
results	O
in	O
a	O
greater	O
number	O
of	O
"	O
smaller	O
"	O
calls	O
,	O
which	O
can	O
be	O
accommodated	O
by	O
increasing	O
the	O
windows	O
from	O
eight	O
to	O
sixteen	O
for	O
instance	O
.	O
</s>
<s>
This	O
was	O
the	O
approach	O
used	O
in	O
the	O
SPARC	B-Architecture
,	O
which	O
has	O
included	O
more	O
register	B-General_Concept
windows	I-General_Concept
with	O
newer	O
generations	O
of	O
the	O
architecture	O
.	O
</s>
<s>
The	O
end	O
result	O
is	O
fewer	O
slow	O
register	B-General_Concept
window	I-General_Concept
spill	O
and	O
fill	O
operations	O
because	O
the	O
register	B-General_Concept
windows	I-General_Concept
overflow	O
less	O
often	O
.	O
</s>
<s>
Register	B-General_Concept
windows	I-General_Concept
are	O
not	O
the	O
only	O
way	O
to	O
improve	O
register	O
performance	O
.	O
</s>
<s>
The	O
group	O
at	O
Stanford	O
University	O
designing	O
the	O
MIPS	B-General_Concept
saw	O
the	O
Berkeley	O
work	O
and	O
decided	O
that	O
the	O
problem	O
was	O
not	O
a	O
shortage	O
of	O
registers	O
,	O
but	O
poor	O
utilization	O
of	O
the	O
existing	O
ones	O
.	O
</s>
<s>
They	O
instead	O
invested	O
more	O
time	O
in	O
their	O
compiler	B-Language
's	O
register	O
allocation	O
,	O
making	O
sure	O
it	O
wisely	O
used	O
the	O
larger	O
set	O
available	O
in	O
MIPS	B-General_Concept
.	O
</s>
<s>
In	O
the	O
end	O
,	O
with	O
modern	O
compilers	B-Language
,	O
MIPS	B-General_Concept
makes	O
better	O
use	O
of	O
its	O
register	O
space	O
even	O
during	O
procedure	O
calls	O
.	O
</s>
