Hardware interview practice
How to Build an Out-of-Order UVM Scoreboard
A DUT can return responses out of order and may reuse transaction IDs after reset. Which scoreboard design is most robust?
Answer choices
- A. Compare expected and actual queues strictly by arrival position
- B. Disable the monitor during reset but retain all pre-reset expectations indefinitely
- C. Sort all responses by payload value before comparing
- D. Match associative entries by a key that includes transaction ID and reset epoch, and define an explicit reset flush/cancel policy
Concise answer
The answer and the key reason
Deeper explanation
Work through the implementation and tradeoffs
Arrival-order queues assume responses return in request order, which the DUT explicitly does not guarantee. Instead, the scoreboard should index expected results by protocol identity and retrieve the corresponding entry when an actual response arrives. If a key can have multiple simultaneous requests, its value can hold a queue or another unique sequence field.
Reset makes the ID alone insufficient because the same numeric value may describe traffic from two different lifetimes. Incrementing an epoch on reset separates those lifetimes. The scoreboard must also document whether reset flushes, cancels, or later permits outstanding operations, then report unmatched, duplicate, missing, and late responses according to that policy.
What to remember
- Match by transaction identity
- Include a reset epoch
- Define reset disposition
Practice the complete prompt