4-8
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part II. PowerPC Microprocessor Module
Figure 4-4. Sequencer Data Path
The instruction unit executes branches in parallel with those instructions that must be
dispatched to an execution unit. Ideally, a 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 Chapter 37, ÒSystem Development and Debugging.Ó Because branches do
not 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 (deÞned 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
Table 4-1.
Instruction Memory System
Instruction Address Generator
Instruction Buffer
Instruction
Queue (4)
(IQ)
Branch
Condition
Evaluation
CC Unit
Execution Units and Registers Files
32-Bit
32-Bit
32-Bit
Read/Write
Busses
Содержание MPC860 PowerQUICC
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