<s>
An	O
instruction	B-General_Concept
window	I-General_Concept
in	O
computer	B-General_Concept
architecture	I-General_Concept
refers	O
to	O
the	O
set	O
of	O
instructions	O
which	O
can	O
execute	B-General_Concept
out-of-order	I-General_Concept
in	O
a	O
speculative	B-General_Concept
processor	O
.	O
</s>
<s>
In	O
particular	O
,	O
in	O
a	O
conventional	O
design	O
,	O
the	O
instruction	B-General_Concept
window	I-General_Concept
consists	O
of	O
all	O
instructions	O
which	O
are	O
in	O
the	O
re-order	B-General_Concept
buffer	I-General_Concept
(	O
ROB	O
)	O
.	O
</s>
<s>
In	O
such	O
a	O
processor	O
,	O
any	O
instruction	O
within	O
the	O
instruction	B-General_Concept
window	I-General_Concept
can	O
be	O
executed	O
when	O
its	O
operands	O
are	O
ready	O
.	O
</s>
<s>
The	O
instruction	B-General_Concept
window	I-General_Concept
has	O
a	O
finite	O
size	O
,	O
and	O
new	O
instructions	O
can	O
enter	O
the	O
window	O
(	O
usually	O
called	O
dispatch	O
or	O
allocate	O
)	O
only	O
when	O
other	O
instructions	O
leave	O
the	O
window	O
(	O
usually	O
called	O
retire	O
or	O
commit	O
)	O
.	O
</s>
<s>
Instructions	O
enter	O
and	O
leave	O
the	O
instruction	B-General_Concept
window	I-General_Concept
in	O
program	O
order	O
,	O
and	O
an	O
instruction	O
can	O
only	O
leave	O
the	O
window	O
when	O
it	O
is	O
the	O
oldest	O
instruction	O
in	O
the	O
window	O
and	O
it	O
has	O
been	O
completed	O
.	O
</s>
<s>
Hence	O
,	O
the	O
instruction	B-General_Concept
window	I-General_Concept
can	O
be	O
seen	O
as	O
a	O
sliding	O
window	O
in	O
which	O
the	O
instructions	O
can	O
become	O
out-of-order	O
.	O
</s>
<s>
All	O
execution	O
within	O
the	O
window	O
is	O
speculative	B-General_Concept
(	O
i.e.	O
,	O
side-effects	O
are	O
not	O
applied	O
outside	O
the	O
CPU	O
)	O
until	O
it	O
is	O
committed	O
in	O
order	O
to	O
support	O
asynchronous	O
exception	B-General_Concept
handling	I-General_Concept
like	O
interrupts	B-Application
.	O
</s>
<s>
The	O
instruction	B-General_Concept
window	I-General_Concept
is	O
distinct	O
from	O
pipelining	B-General_Concept
:	O
instructions	O
in	O
an	O
in-order	O
pipeline	O
are	O
not	O
in	O
an	O
instruction	B-General_Concept
window	I-General_Concept
in	O
the	O
conventionally	O
understood	O
sense	O
,	O
because	O
they	O
cannot	O
execute	O
out	O
of	O
order	O
with	O
respect	O
to	O
one	O
another	O
.	O
</s>
<s>
Out-of-order	O
processors	O
are	O
usually	O
built	O
around	O
pipelines	O
,	O
but	O
many	O
of	O
the	O
pipeline	O
stages	O
(	O
e.g.	O
,	O
front-end	O
instruction	O
fetch	O
and	O
decode	O
stages	O
)	O
are	O
not	O
considered	O
to	O
be	O
part	O
of	O
the	O
instruction	B-General_Concept
window	I-General_Concept
.	O
</s>
