VLSI DV Interview Puzzles · All levels

UVM Analysis Port Handle Reuse

Scoreboard queue entries appear to change after insertion. Locate the monitor bug.

Puzzle

Difficulty: Hard · Puzzle 4 of 6 · Topic: Debugging Riddles

Scoreboard queue entries appear to change after insertion. Locate the monitor bug.

Code

systemverilog

task run_phase(uvm_phase phase);
  tr t = tr::type_id::create("t");
  forever begin
    collect_fields(t);
    ap.write(t);
  end
endtask

Hint

Analysis ports pass object handles, not deep copies.

Step-by-step solution

diagram

1) Same object t is reused every iteration.
2) Subscribers store handle to t; later collect_fields overwrites same object.
3) Scoreboard then sees previously queued entries mutate, appearing as random corruption.
4) Fix: create new object each transaction or clone before write (ap.write(t.clone())).
5) Also ensure do_copy/do_clone perform deep copy for dynamic fields.

Answer

Answer: The monitor reuses one transaction handle; clone/new per sample before ap.write.

Why candidates get it wrong

Assuming `write()` implies by-value transfer.

Interviewer follow-up

Which fields require explicit deep-copy logic in `do_copy`?

Navigate