CDC / RDC · All levels

Reconvergence & Gray Coding: Tricky Q&A

Senior interview and review questions for Reconvergence & Gray Coding.

Section Q&A bank

Use these drills after completing all topics in Reconvergence & Gray Coding. Answer with crossing class, mechanism, risk, evidence, owner, and signoff decision.

Why can synchronized branches still fail after merge?

diagram

[INT][CDC-RDC][RECONVERGENCE-GRAY]

Q: Why can synchronized branches still fail after merge?

A:
Different synchronization latency creates illegal transient combinations at reconvergence logic.

FOLLOW-UP TRAP: Assuming branch-level sync guarantees merge safety.

What does gray code guarantee?

diagram

[INT][CDC-RDC][RECONVERGENCE-GRAY]

Q: What does gray code guarantee?

A:
One-bit transition between adjacent states, reducing sampled ambiguity in pointer crossings.

FOLLOW-UP TRAP: Believing gray code alone proves FIFO correctness.

How do you debug intermittent reconvergence bugs?

diagram

[INT][CDC-RDC][RECONVERGENCE-GRAY]

Q: How do you debug intermittent reconvergence bugs?

A:
Align traces by event windows, map fanin cones, and prove protocol state legality around failure.

FOLLOW-UP TRAP: Debugging only one branch in isolation.

Q&A drill guide

diagram

INTENT -> FAILURE MODE -> EVIDENCE -> FIX/WAIVER -> REGRESSION

Sketch while answering

diagram

RECONVERGENCE

branch A sync --                 -> merge logic -> illegal combination window
branch B sync --/

Key takeaways

State crossing class, assumptions, and owner with every issue.
Run structural and dynamic regressions after each fix.

Common pitfalls

Treating all warnings as equivalent risk.
Waiving issues without containment evidence.
Skipping reset and reconvergence stress after CDC fixes.

Practice this lesson