Fetch & Decode Front-End

Section goal

Instruction delivery bandwidth, prediction quality, and decode throughput that sustain back-end utilization.

How to study this section

1. Start with each topic hub to establish mechanism-first framing.

2. Use reports and debug pages to separate symptoms from root cause.

3. Practice worked examples and interview drills under fixed metadata.

4. Close with checklist and silicon impact before signoff claims.

Topics

1. instruction-fetch-bandwidth/ - Instruction Fetch Bandwidth

2. branch-prediction-basics/ - Branch Prediction Basics

3. decode-width-and-uop-cache/ - Decode Width and uOP Cache

4. front-end-bubbles-and-stalls/ - Front-End Bubbles and Stalls

Related topics

• CPU Design course hub

CPU deep dive

Front-end quality is proven by sustained rename feed under branchy and translation-heavy instruction streams.

Concept diagram

FRONT-END FLOW

I-cache/ITLB -> branch predict -> fetch queue -> decode/uOP cache -> rename

Metric graph

FRONT-END BOTTLENECK MIX

predictor redirects   █████
ITLB + I-cache stalls ████
decode backpressure   ███

Reports and artifacts

• fetch bandwidth timeline

• branch redirection profile

• uOP cache hit/miss report

• front-end bubble taxonomy

Mini case study

A code-layout change increased branch target aliasing; fetch redirect penalties doubled and retire IPC dropped 18%.

Debug branches

• Correlate MPKI spikes with queue underflow windows

• Audit decode throughput versus uOP-cache residency

• Confirm front-end fixes improve full CPI stack, not only fetch counters

Senior review question

Ask: which CPI/latency evidence proves this topic is truly closed beyond synthetic benchmarks?