Branch Prediction and Fetch · arch

Computer Architecture · All levels

4topics
4lessons

Fetch stays full only when prediction guesses both direction and target correctly. This topic covers predictor structures, MPKI and recovery math, fetch timing, and debug.

Question	Answered by	Wrong guess costs
Taken or not taken	Direction tables (bimodal, gshare)	Flush of wrong-path work
Where to if taken	BTB target, return stack, indirect predictor	Redirect delay plus refill
Was it worth it	MPKI and recovery math	CPI added per thousand instructions

Lesson	What it covers
Predictor structures	Direction tables, BTB targets and the return stack, with a runnable model.
MPKI and recovery math	MPKI, accuracy and penalty tied to CPI with hand-checkable numbers.
Fetch and recovery pipeline	Lookup latency, flush depth and refill, and flat IPC despite better accuracy.
Tuning, cost and interviews	Branch-type debug, storage cost math, ROI, and interview drills.

Check yourself A program executes 1M instructions with 40K mispredicts at 10 cycles each. What CPI is added?

MPKI is 40 and CPI added is 40 x 10 / 1000 = 0.40. The lesson on MPKI math shows each step.

Up nextPredictor Structures

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