Advanced Computer Architecture, Imperial
College 2001
7
N-bit BHT -
why does it work so well?
•n-bit
BHT predictor essentially based on a saturating counter: taken increments,
not-taken decrements
•predict taken if most significant bit is set
Predict
taken
Predict
not-taken
Predict
taken
Predict
not-taken
taken
not
taken
not taken
taken
taken
taken
not taken
not taken
11
10
01
00
•Most
branches are highly biased: either almost-always
taken, or
almost-always not-taken
•Works badly for branches which
aren’t
Often called the
“bimodal” predictor