Compaq EV68A specifications Choice Predictor, Instruction-Stream Translation Buffer

Models: EV68A

1 356
Download 356 pages 47.63 Kb
Page 33
Image 33

21264/EV68A Microarchitecture

Figure 2–4 Global Predictor

Global

Path

History

12

Index

Global

 

Predictor

 

 

 

 

4K x 2

 

 

2

 

 

2

 

1

 

+/-

Global Branch Prediction

FM-05812.AI4

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 2–5shows 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

Global

Path

History

12

Choice

Predictor

4K x 2

2

2 1

Choice Prediction

FM-05813.AI4

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.

21264/EV68A Hardware Reference Manual

Internal Architecture 2–5

Page 33
Image 33
Compaq EV68A specifications Choice Predictor, Instruction-Stream Translation Buffer