Coverage InterviewQuestions You Should Master

Code coverage reports exercised implementation structure. Functional coverage reports whether authored behaviors, values, transitions, and combinations were observed. Neither says by itself that the observed result was correct.

Every instrumented construct can execute while important data combinations, orderings, error cases, or protocol outcomes remain absent or unchecked.

Each item needs a requirement or risk, measurable observation, sampling definition, checker relationship, target, owner, and policy for unreachable or excluded behavior.

Statement coverage records executed operations, branch coverage records control outcomes, and condition coverage examines Boolean operands or combinations that may be masked inside a taken branch.

A bit may toggle because of reset or incidental traffic without exercising its intended function, while a multi-bit field can have full bit toggles and still miss important encoded values.

First establish why it is unreachable in the verified configuration. Remove dead logic when appropriate, or use formal evidence and a narrowly reviewed exclusion when the behavior is intentionally impossible.

Partition values by behavioral consequence: normal classes, boundaries, modes, errors, and meaningful transitions. Numeric convenience is secondary to verification intent.

An unsized bin array expands a finite value set into fine-grained bins, while a fixed-count array distributes that set across the requested bounded number of bins.

Ignored values are intentionally removed from the coverage goal. Illegal-bin hits identify forbidden observations and are reported, but assertions and protocol checkers should remain the primary correctness mechanism.

Individual completion proves each value occurred somewhere. A cross reveals whether requirement-critical values occurred together in the same valid sample.

Cross fewer dimensions, group behaviorally equivalent values before crossing, sample only valid contexts, and remove combinations only when their irrelevance or impossibility is justified.

Confirm the combination is required and reachable first, then inspect constraints, generated transactions, monitor reconstruction, sampling conditions, and bin definitions before adding tests.

Use an event when the covergroup has one unambiguous stable sampling point. Use manual sampling when a transaction or its classification is assembled later or the correct sample context varies.

Usually in a dedicated subscriber or collector fed by monitor transactions. Coverage tied to a checked outcome may consume scoreboard-classified observations instead.

The DUT, interface, monitor, and coverage code may act in different scheduling regions. Sampling before the observed transaction is stable can capture the prior or partially updated state.

It represents the union of compatible bin hits across selected runs. It is meaningful only when the design, model, configuration, and merge policy are controlled.

Use instance-level visibility when replicated agents, channels, or interfaces have separate obligations. Aggregate type coverage is sufficient only when instances are truly interchangeable for the goal.

Goal sets a reporting target, weight changes contribution to an aggregate score, and auto_bin_max limits automatic partitioning. None of them creates verification evidence.

Planned reachable goals are hit, remaining holes have evidence-backed dispositions, exclusions are reviewed, and complementary checking and bug evidence support the release decision.

Validate the requirement and bin first, then inspect reachability, constraints, generated traffic, monitor reconstruction, sample conditions, and database merge status in that order.

Treat the mismatch as diagnostic evidence. Structural holes suggest unexercised RTL; functional holes suggest missing scenarios, coarse stimulus, or model and sampling gaps. Investigate both before waiving either.