Compaq 21264 - Manual (Page 33)

Alpha 21264/EV67 Hardware Reference Manual Internal Architecture 2–5
21264/EV67 Microarchitecture
Figure 2–4 Global Predictor
Choice Predictor
The choice predictor monitors the history of the local and global predictors and chooses
the best of the two predictors for a particular branch. Figure 2–5 shows how the choice
predictor generates its choice of the result of the local or global prediction. The 12-bit
global path history (see Figure 2–4) is used to index a 4K entry table of 2-bit saturating
counters. The value of the saturating counter determines the choice between the outputs
of the local and global predictors.
Figure 2–5 Choice Predictor
2.1.1.3 Instruction-Stream Translation Buffer
The Ibox includes a 128-entry, fully-associative instruction-stream translation buffer
(ITB) that is used to store recently used instruction-stream (Istream) address transla-
tions and page protection information. Each of the entries in the ITB can map 1, 8, 64,
or 512 contiguous 8KB pages. The allocation scheme is round-robin.
The ITB supports an 8-bit ASN and contains an ASM bit. The Icache is virtually
addressed and contains the access-check information, so the ITB is accessed only for
Istream references that miss in the Icache.
Istream transactions to I/O address space are UNDEFINED.
Global
Path
History
FM-05812.AI4
Global
Predictor
4K x 2
+/-
12
Index
Global Branch Prediction
1
2
2
Global
Path
History
FM-05813.AI4
Choice
Predictor
4K x 2
12
Choice Prediction
1 2
2