Hardware Reference
In-Depth Information
There are RAW data hazards from the second
L.D
to
MUL.D
,
ADD.D
, and
SUB.D
, from
MUL.D
to
DIV.D
, and from
SUB.D
to
ADD.D
. There is a WAR data hazard between
DIV.D
and
ADD.D
and
SUB.D
. Finally, there is a structural hazard on the add functional
unit for
ADD.D
and
SUB.D
. What the tables look like when
MUL.D
and
DIV.D
are ready
to write their results is shown in
Figures C.56
and
C.57
,
respectively.
FIGURE C.56
Scoreboard tables just before the
MUL.D
goes to write result
.
The
DIV.D
has not yet read either of its operands, since it has a dependence on
the result of the multiply. The
ADD.D
has read its operands and is in execution,
although it was forced to wait until the
SUB.D
finished to get the functional unit.
ADD.D
cannot proceed to write result because of the WAR hazard on F6, which
is used by the
DIV.D
. The Q fields are only relevant when a functional unit is
waiting for another unit.
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