Compaq 21264 - Manual (Page 32)

2–4 Internal Architecture Alpha 21264/EV67 Hardware Reference Manual
21264/EV67 Microarchitecture
Figure 2–2 Branch Predictor
Local Predictor
The local predictor uses a 2-level table that holds the history of individual branches.
The 2-level table design approaches the prediction accuracy of a larger single-level
table while requiring fewer total bits of storage. Figure 2–3 shows how the local pre-
dictor generates a prediction. Bits [11:2] of the VPC of the current branch are used as
the index to a 1K entry table in which each entry is a 10-bit value. This 10-bit value is
used as the index to a 1K entry table of 3-bit saturating counters. The value of the satu-
rating counter determines the predication, taken/not-taken, of the current branch.
Figure 2–3 Local Predictor
Global Predictor
The global predictor is indexed by a global history of all recent branches. The global
predictor correlates the local history of the current branch with all recent branches. Fig-
ure 2–4 shows how the global predictor generates a prediction. The global path history
is comprised of the taken/not-taken state of the 12 most-recent branches. These 12
states are used to form an index into a 4K entry table of 2-bit saturating counters. The
value of the saturating counter determines the predication, taken/not-taken, of the cur-
rent branch.
Local
Predictor
Global
Predictor
Choice
Predictor
Predicted
Branch
Address
FM-05810.AI4
Local
History
Table
1K x 10
FM-05811.AI4
Local
Predictor
1K x 3
+/-
10
Index
VPC[11:2]
Local Branch Prediction
1
3
10
3