VLSI DV Interview Puzzles · All levels

Does #0 Beat NBA?

Predict both prints at time 0 and explain why they differ even though there is no real time advance.

Puzzle

Difficulty: Easy · Puzzle 2 of 6 · Topic: Event Scheduling Puzzles

Predict both prints at time 0 and explain why they differ even though there is no real time advance.

Code

systemverilog

module tb;
  int x;
  initial begin
    x = 0;
    x <= 1;
    #0 x = 2;
    $display("[%0t INACTIVE] x=%0d", $time, x);
    $strobe("[%0t POSTPONED] x=%0d", $time, x);
  end
endmodule

Hint

#0 lands in inactive; NBA updates happen after inactive.

Step-by-step solution

diagram

1) Active region schedules x<=1 for NBA.
2) #0 resumes in inactive and assigns x=2.
3) $display in inactive prints x=2 at t=0.
4) NBA region then updates x to 1.
5) $strobe runs in postponed and prints x=1 at t=0.

Answer

Answer: Inactive print is [0] x=2, postponed print is [0] x=1. #0 executes before NBA in the same time slot.

Why candidates get it wrong

Many engineers think #0 moves after NBA because it 'feels later'. It does not.

Interviewer follow-up

If you replace #0 with #1, what changes in ordering and printed time?

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