Decoupled BPT and BTB
For a fixed real estate (i.e., fixed area on the chip):
- Increasing the number of entries implies less bits for history or no field for target instruction or fewer bits for tags (more aliasing)
- Increasing the number of entries implies better accuracy of prediction.
Decoupled design
- BPT. If it predicts taken then go to BTB (BTAC in Power PC 620)
- Power PC 620 2K BPT + 256 BTAC
- HP PA-8000 256*3 BPT + 32 (fully-associative) BTAC