Instruction Execution Timing
MPC885 PowerQUICC Family Reference Manual, Rev. 2
9-2
Freescale Semiconductor
9.1.2
Writeback Arbitration
In
Figure 9-2
, the addic instruction is dependent on the mulli result. Because the single-cycle instruction
sub has priority on the writeback bus over the mulli, the mulli writeback is delayed one clock and causes
a bubble in the execute stream.
mulli r12,r4,3
sub r3,r15,3
addic 4,r12,1
Figure 9-2. Writeback Arbitration Timing—Example 1
In this example, the addic instruction is dependent on sub rather than on mulli. Although the writeback
of the mulli is delayed two clocks, there is no bubble in the execution stream.
mulli r12,r4,3
sub r3,r15,3
addic r4,r3,1
Figure 9-3. Writeback Arbitration Timing—Example 2
9.1.3
Private Writeback Bus Load
In
Figure 9-4
, lwz and xor write back in the same clock since they use the writeback bus in two different
ticks (a tick = 1/4 of a processor clock).
lwz r12,64
(SP)
sub
r5,r5,3
cror
4,14,1
and
r3,r4.r5
xor
r4,r3,r5
ori
r6,r12.r3
mulli
sub
addic
Fetch
Decode
Read + Execute
Writeback
mulli
sub
Bubble
addic
mulli
sub, mulli
mulli
addic
sub
GCLK1
addic
mulli
sub
addic
Fetch
Decode
Read + Execute
Writeback
mulli
sub
addic
mulli
sub, mulli
mulli
addic
sub
GCLK1
addic
Summary of Contents for PowerQUICC MPC870
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