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Instruction Pipeline and Execution Timing
e200z3 Power Architecture Core Reference Manual, Rev. 2
6-8
Freescale Semiconductor
Figure 6-5. Basic Pipeline Flow, Single-Cycle Instructions
6.3.4
Basic Load and Store Instruction Pipeline Operation
The EA calculations for load and store instructions are performed in the decode stage. The memory access
occurs in the execution stage.
If a load instruction is followed by a dependent ALU instruction, the load data is driven from the memory
in the MEM stage and feed-forwarded into the dependent ALU instruction in the following cycle. As a
result, there is no load-to-use pipeline bubble.
shows the instruction flow for a load instruction
followed by a dependent add instruction.
Figure 6-6. A Load Followed by a Dependent Add Instruction
Back-to-back load/store instructions are executed in a pipelined fashion, provided that their EA
calculations are not dependent on their previous load instructions.
shows the basic pipeline flow
for two back-to-back load instructions. In this case, the second load does not depend on its previous load
data for its EA calculation. Notice that the memory access of the first load instruction overlaps in time with
the EA calculation of the second load instruction.
IFETCH
DECODE
EXECUTE
First Inst.
Time Slot
FFwd/WB
Second Instruction
Third Instruction
IFETCH
DECODE
EXECUTE
FFwd/WB
IFETCH
DECODE
EXECUTE
FFwd/WB
DEC/EA
MEM
First Load Instruction
Time Slot
MEM
Second Add Instruction
DECODE
EXECUTE
MEM
IFETCH
IFETCH
Feedforward
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