
Glossary-2
MPC7400 RISC Microprocessor Users Manual
undeTned results for a given operation may vary among
implementations and between execution attempts in the same
implementation.
Although the architecture does not prescribe the exact behavior for
when results are allowed to be boundedly undeTned, the results of
executing instructions in contexts where results are allowed to be
boundedly undeTned are constrained to ones that could have been
achieved by executing an arbitrary sequence of deTned instructions,
in valid form, starting in the state the machine was in before
attempting to execute the given instruction.
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.
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 deTnes 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.
Burst
. A multiple-beat data transfer whose total size is typically equal to a
cache block.
Cache
. High-speed memory containing recently accessed data and/or
instructions (subset of main memory).
Cache block
. A small region of contiguous memory that is copied from
memory into a
cache
. The size of a cache block may vary among
processors; the maximum block size is one
page
. In PowerPC
processors,
cache coherency
is maintained on a cache-block basis.
Note that the term cache block is often used interchangeably with
cache line.
C