Branch Prediction And Resolution; Figure 6-9. Branch Completion - IBM PowerPC 750GX User Manual

User's Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor

Figure 6-9. Branch Completion

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
Instruction Timing
Page 228 of 377
Branch Completion
(LR/CTR Write-Back)
Clock 0 Clock 1
IQ5 add5
IQ4 add4
IQ3 add3 add5
IQ2 bc add4
IQ1 add2 add3
IQ0 add1 bc
CQ5
CQ4
CQ3
CQ2
CQ1 add2
CQ0 add1
Clock 2 Clock 3
add7 add9
add6 add8
add5 add7
add4 add6
add3 add5
bc add4
gx_06.fm.(1.2)
March 27, 2006