
7-2
G2 PowerPC Core Reference Manual
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MOTOROLA
Terminology and Conventions
the dispatch mechanism, without either being passed to an execution unit and or
given a position in the CQ.
Fetch—The process of bringing instructions from memory (such as a cache or
system memory) into the instruction queue.
Finish—Finishing occurs in the last cycle of execution. In this cycle, the CQ entry
is updated to indicate that the instruction has finished executing.
Folding (branch folding)—The replacement of a branch instruction with target
instructions and any instructions along the not-taken path, when a branch is either
taken or predicted as taken.
Latency—The number of clock cycles necessary to execute an instruction and make
ready the results of that execution for a subsequent instruction.
Pipeline—In 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 simultaneously—analogous 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.
Although an individual instruction may take many cycles to complete (the number
of cycles is called instruction latency), pipelining makes it possible to overlap the
processing so that the throughput (number of instructions completed per cycle) is
greater than if pipelining were not implemented.
Program order—The order of instructions in an executing program. More
specifically, this term is used to refer to the original order in which program
instructions are fetched into the instruction queue from the cache.
Rename register—Temporary buffers used by instructions that have finished
execution but have not completed.
Reservation station—A buffer between the dispatch and execute stages that allows
instructions to be dispatched even though the results of instructions on which the
dispatched instruction may depend are not available.
Retirement—Removal of the completed instruction from the CQ.
Stage—The term stage is used in two different senses, depending on whether the
pipeline is being discussed as a physical entity or a sequence of events. In the latter
case, a stage is an element in the pipeline during which certain actions are
performed, such as decoding the instruction, performing an arithmetic operation, or
writing back the results. A stage is typically described as taking a processor clock
cycle to perform its operation; however, some events (such as dispatch and
write-back) happen instantaneously, and may be thought to occur at the end of the
stage.
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