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
- Separate – and different sizes – BPT and BTB
- BPT. If it predicts taken then go to BTB (see next slide)
- Power PC 620: 2K entries BPT + 256 entries BTB
- HP PA-8000: 256*3 BPT + 32 (fully-associative) BTB