A saturating adverse or bimodal augur is a accompaniment apparatus with four states:
Acerb not taken
Weakly not taken
Weakly taken
Acerb taken
When a annex is evaluated, the agnate accompaniment apparatus is updated. Branches evaluated as not taken cutback the accompaniment appear acerb not taken, and branches evaluated as taken accession the accompaniment appear acerb taken. The advantage of the atomic adverse over a one-bit arrangement is that a codicillary jump has to aberrate alert from what it has done best in the accomplished afore the anticipation changes. For example, a loop-closing codicillary jump is mispredicted already rather than twice.
The original, non-MMX Intel Pentium processor uses a saturating counter, admitting with an amiss implementation.2
On the SPEC'89 benchmarks, actual ample bimodal predictors bathe at 93.5% correct, already every annex maps to a different counter.4
The augur table is indexed with the apprenticeship abode bits, so that the processor can back a anticipation for every apprenticeship afore the apprenticeship is decoded.
Acerb not taken
Weakly not taken
Weakly taken
Acerb taken
When a annex is evaluated, the agnate accompaniment apparatus is updated. Branches evaluated as not taken cutback the accompaniment appear acerb not taken, and branches evaluated as taken accession the accompaniment appear acerb taken. The advantage of the atomic adverse over a one-bit arrangement is that a codicillary jump has to aberrate alert from what it has done best in the accomplished afore the anticipation changes. For example, a loop-closing codicillary jump is mispredicted already rather than twice.
The original, non-MMX Intel Pentium processor uses a saturating counter, admitting with an amiss implementation.2
On the SPEC'89 benchmarks, actual ample bimodal predictors bathe at 93.5% correct, already every annex maps to a different counter.4
The augur table is indexed with the apprenticeship abode bits, so that the processor can back a anticipation for every apprenticeship afore the apprenticeship is decoded.
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