MPC555
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MPC556
CENTRAL PROCESSING UNIT
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
3-3
3.3 Instruction Sequencer
The instruction sequencer provides centralized control over data flow between execu-
tion units and register files. The sequencer implements the basic instruction pipeline,
fetches instructions from the memory system, issues them to available execution
units, and maintains a state history so it can back the machine up in the event of an
exception.
The instruction sequencer fetches instructions from the burst buffer controller into the
instruction pre-fetch queue. The BPU extracts branch instructions from the pre-fetch
queue and uses static branch prediction on unresolved conditional branches to allow
the instruction unit to fetch instructions from a predicted target instruction stream while
a conditional branch is evaluated. The BPU folds out branch instructions for uncondi-
tional branches or conditional branches unaffected by instructions in the execution
stage.
Instructions issued beyond a predicted branch do not complete execution until the
branch is resolved, preserving the programming model of sequential execution. If
branch prediction is incorrect, the instruction unit flushes all predicted path instruc-
tions, and instructions are issued from the correct path.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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