<s>
A	O
re-order	B-General_Concept
buffer	I-General_Concept
(	O
ROB	O
)	O
is	O
a	O
hardware	O
unit	O
used	O
in	O
an	O
extension	O
to	O
the	O
Tomasulo	B-General_Concept
algorithm	I-General_Concept
to	O
support	O
out-of-order	B-General_Concept
and	O
speculative	B-General_Concept
instruction	O
execution	O
.	O
</s>
<s>
The	O
buffer	O
is	O
a	O
circular	B-Data_Structure
buffer	I-Data_Structure
(	O
to	O
provide	O
a	O
FIFO	B-Operating_System
instruction	O
ordering	O
queue	O
)	O
implemented	O
as	O
an	O
array/vector	B-Data_Structure
(	O
which	O
allows	O
recording	O
of	O
results	O
against	O
instructions	O
as	O
they	O
complete	O
out	O
of	O
order	O
)	O
.	O
</s>
<s>
There	O
are	O
three	O
stages	O
to	O
the	O
Tomasulo	B-General_Concept
algorithm	I-General_Concept
:	O
"	O
Issue	O
"	O
,	O
"	O
Execute	O
"	O
,	O
"	O
Write	O
Result	O
"	O
.	O
</s>
<s>
The	O
"	O
Write	O
Result	O
"	O
stage	O
is	O
modified	O
to	O
place	O
results	O
in	O
the	O
re-order	B-General_Concept
buffer	I-General_Concept
.	O
</s>
<s>
Each	O
instruction	O
is	O
tagged	O
in	O
the	O
reservation	B-General_Concept
station	I-General_Concept
with	O
its	O
index	O
in	O
the	O
ROB	O
for	O
this	O
purpose	O
.	O
</s>
<s>
The	O
consequences	O
of	O
the	O
re-order	B-General_Concept
buffer	I-General_Concept
include	O
precise	O
exceptions	B-General_Concept
and	O
easy	O
rollback	B-Application
control	O
of	O
target	B-General_Concept
address	I-General_Concept
mis-predictions	I-General_Concept
(	O
branch	O
or	O
jump	O
)	O
.	O
</s>
<s>
When	O
jump	O
prediction	O
is	O
not	O
correct	O
or	O
a	O
nonrecoverable	O
exception	O
is	O
encountered	O
in	O
the	O
instruction	O
stream	O
,	O
the	O
ROB	O
is	O
cleared	O
of	O
all	O
instructions	O
(	O
by	O
setting	O
the	O
circular	B-Data_Structure
queue	I-Data_Structure
tail	O
to	O
the	O
head	O
)	O
and	O
reservation	B-General_Concept
stations	I-General_Concept
are	O
re-initialized	O
.	O
</s>
