Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 148

The MPC8xx Core
Figure 3-4 represents the sequencer data path.
Read/Write
Busses
The instruction unit executes branches in parallel with those instructions that must be dispatched to an
execution unit. Ideally, an instruction is dispatched to an execution unit every clock cycle, even when
branches are in the code. The IQ also eliminates stalls due to instruction fetches that miss in the instruction
cache or that generate a page fault. All instructions are fetched into the IQ, and all instructions except
branch instructions are dispatched to the execution units when they reach IQ0. Branches enter the queue
to mark watchpoints. See
prevent the issue of nonbranch instructions unless they come in pairs, the performance impact of entering
branches in the IQ is negligible.
The core also implements a branch reservation station and static branch prediction so branches can be
resolved as early as possible. The reservation station allows a branch instruction to pass from the IQ before
its condition is ready. With the branch out of the way, fetching can continue as the branch is evaluated.
Static branch prediction (defined by the PowerPC UISA) determines which instruction stream is
prefetched while the branch is being resolved. When the branch operand becomes available, it is forwarded
to the BPU and the condition is evaluated. The static branch prediction mechanism is shown in
Branch Type
BC with negative offset
BC with positive offset
BCLR or BCCTR (LR or CTR) address ready
BCLR or BCCTR (LR or CTR) address not ready
B (unconditional branch)
3-8
Instruction Memory System
Instruction Address Generator
CC Unit
Execution Units and Registers Files
Figure 3-4. Sequencer Data Path
Chapter 53, "System Development and Debugging."
Table 3-1. Static Branch Prediction
MPC885 PowerQUICC Family Reference Manual, Rev. 2
32-Bit
Instruction Buffer
32-Bit
Branch Instruction
Condition
Queue (4)
Evaluation (IQ)
32-Bit
Default Prediction (y=0)
Taken
Fall through
Fall through
Wait
Taken
Because branches do not
Table 3-1.
Modified Prediction (y=1)
Fall through
Taken
Taken
Wait
Taken
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