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6-2
MPC7400 RISC Microprocessor Users Manual
Terminology and Conventions
6.1 Terminology and Conventions
This section provides an alphabetical glossary of terms used in this chapter. These
deTnitions are provided as a review of commonly used terms and as a way to point out
speciTc ways these terms are used in this chapter.
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Branch predictionThe process of guessing whether a branch will be taken. Such
predictions can be correct or incorrect; the term predicted as it is used here does
not imply that the prediction is correct (successful). The PowerPC architecture
deTnes a means for static branch prediction as part of the instruction encoding.
Branch resolutionThe determination of whether a branch is taken or not taken. A
branch is said to be resolved when the processor can determine which instruction
path to take. If the branch is resolved as predicted, the instructions following the
predicted branch that may have been speculatively executed can complete (see
completion). If the branch is not resolved as predicted, instructions on the
mispredicted path, and any results of speculative execution, are purged from the
pipeline and fetching continues from the nonpredicted path.
CompletionCompletion occurs when an instruction has Tnished executing,
written back any results, and is removed from the completion queue (CQ). When an
instruction completes, it is guaranteed that this instruction and all previous
instructions can cause no exceptions.
Fall-through (branch fall-through)A not-taken branch. On the MPC7400,
fall-through branch instructions are removed from the instruction stream at dispatch.
That is, these instructions are allowed to fall through the instruction queue via the
dispatch mechanism, without either being passed to an execution unit and or given
a position in the CQ.
FetchThe process of bringing instructions from memory (such as a cache or
system memory) into the instruction queue. In this chapter, the fetch stage is
considered to end when the instruction is dispatched.
Folding (branch folding)The replacement with target instructions of a branch
instruction and any instructions along the not-taken path when a branch is either
taken or predicted as taken.
FinishFinishing occurs in the last cycle of execution. In this cycle, the CQ entry
is updated to indicate that the instruction has Tnished executing.
Latency The number of clock cycles necessary to execute an instruction and make
ready the results of that execution for a subsequent instruction.
PipelineIn the context of instruction timing, the term pipeline refers to the
interconnection of the stages. The events necessary to process an instruction are
broken into several cycle-length tasks to allow work to be performed on several
instructions simultaneouslyanalogous to an assembly line. As an instruction is
processed, it passes from one stage to the next. When it does, the stage becomes
available for the next instruction.
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