
MOTOROLA
Chapter 7. Instruction Timing
7-1
Chapter 7
Instruction Timing
This chapter describes how the G2 core processor fetches, dispatches, and executes
instructions and how it reports the results of instruction execution. It gives detailed
descriptions of how the G2 core execution units work, and how those units interact with
other parts of the processor, such as the instruction fetching mechanism, register files, and
caches. It gives examples of instruction sequences, showing potential bottlenecks and how
to minimize their effects. Finally, it includes tables that identify the unit that executes each
instruction implemented on the core, the latency for each instruction, and other information
that is useful for the assembly language programmer.
7.1
Terminology and Conventions
This section provides an alphabetical glossary of terms used in this chapter. These
definitions are provided as a review of commonly used terms and as a way to point out
specific ways these terms are used in this chapter.
Branch prediction—The 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 defines
a means for static branch prediction as part of the instruction encoding.
Branch resolution—The 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.
Completion—Completion occurs when an instruction has finished 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 G2 core,
fall-through branch instructions are removed from the instruction stream at dispatch.
That is, these instructions are allowed to fall through the instruction queue through
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