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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Instruction Timing
Page 228 of 377
gx_06.fm.(1.2)
March 27, 2006
In this example, the Branch Conditional (bc) instruction is encoded to decrement the CTR. It is predicted as
not-taken in clock cycle 0. In clock cycle 2, bc and add3 are both dispatched. In clock cycle 3, the architected
CTR is updated, and the bc instruction is retired from the completion queue.
6.4.1.3 Branch Prediction and Resolution
The 750GX supports the following two types of branch prediction:
• Static branch prediction—This is defined by the PowerPC Architecture as part of the encoding of branch
instructions.
• Dynamic branch prediction—This is a processor-specific mechanism implemented in hardware (in partic-
ular the branch history table, or BHT) that monitors branch instruction behavior and maintains a record
from which the next occurrence of the branch instruction is predicted.
When a conditional branch cannot be resolved due to a CR data dependency, the BPU predicts whether it will
be taken, and instruction fetching proceeds down the predicted path. If the branch prediction resolves as
incorrect, the instruction queue and all subsequently executed instructions are purged, instructions executed
prior to the predicted branch are allowed to complete, and instruction fetching resumes down the correct path.
The
750GX executes through two levels of prediction. Instructions from the first unresolved branch can
execute, but they cannot be retired until the branch is resolved. If a second branch instruction is encountered
in the predicted instruction stream, it can be predicted and instructions can be fetched, but not executed, from
the second branch. No action can be taken for a third branch instruction until at least one of the two previous
branch instructions is resolved.
The number of instructions that can be executed after the issue of a predicted branch instruction is limited by
the fact that no instruction executed after a predicted branch can actually update (be retired) the register files
or memory until the branch is resolved. That is, instructions can be issued and executed, but cannot be
retired from the completion unit. When an instruction following a predicted branch completes execution, it
Figure 6-9. Branch Completion
IQ5 add5
IQ4 add4
IQ3 add3
IQ2 bc
IQ1 add2
IQ0 add1
add5
add4
add3
bc
Branch Completion
(LR/CTR Write-Back)
CQ5
CQ4
CQ3
CQ2
CQ1
CQ0
add2
add1
Clock 0 Clock 1
add7
add6
add5
add4
add3
bc
Clock 2
add9
add8
add7
add6
add5
add4
Clock 3