Instruction Execution Timing
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
9-5
9.1.7
Branch Prediction
In this example, the blt instruction is dependent on the cmpi instruction. Nevertheless, the BPU predicts
the correct path and allows an overlap of its bubbles with those of lwz. When cmpi writes back, the BPU
reevaluates the decision. If the prediction is correct, no more action is taken and execution continues.
Instructions on the predicted path cannot be dispatched before the condition is resolved.
while:
mulli
r3,r12,r4
addi
r4,3(r0)
...
lwz r12,64
(r2)
cmpi
0,r12,3
addic
r6,r5,1
blt
cr0,while
...
Figure 9-8. Branch Prediction Timing
9.2
Instruction Timing List
Table 9-1
summarizes instruction execution timings in terms of latency and blockage of the appropriate
execution unit. A serializing instruction blocks all execution units.
Table 9-1. Instruction Execution Timing
Instructions
Latency
Blockage
Unit
Serializing
Branch:
b
,
ba
,
bl
,
bla
,
bc
,
bca
,
bcl
,
bcla
,
bclr
,
bclrl
,
bcctr
,
bcctl
Taken 2
2
BPU
No
Not taken 1
1
System call:
sc
,
rfi
Ser 2
—
Yes
CR logical:
crand
,
crxor
,
cror
,
crnand
,
crnor
,
crandc
,
creqv
,
crorc
,
mcrf
1
1
BPU
No
addic
lwz
lwz
cmpi
blt
Bubble
addic
mulli
cmpi
cmpi
Bubble
mulli
lwz
Bubble
lwz
lwz
blt
lwz
blt
cmp
addic
addic
mulli
Fetch
Decode
Read + Execute
Writeback
L Address Drive
L Data
Load Writeback
Branch Decode
Branch Execute
addi
Branch Final
Decision
blt
GCLK1
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