10-2
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part II. PowerPC Microprocessor Module
Figure 10-1. Data Cache Load Timing
10.1.2 Writeback Arbitration
In Figure 10-2, 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 10-2. Writeback Arbitration TimingÑExample 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 10-3. Writeback Arbitration TimingÑExample 2
lwz
sub
mulli
addi
addic
Fetch
Decode
Read + Execute
Writeback
L Address Drive
L Data
Load Write Back
lwz
sub
Bubble
addic
lwz
sub
sub
addic
ld
ld
ld
GCLK1
addic
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
Содержание MPC860 PowerQUICC
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