VLSI DV Interview Puzzles · All levels

join vs join_any vs join_none

Give the timeline of all prints and identify which children survive each block.

Puzzle

Difficulty: Easy · Puzzle 1 of 6 · Topic: Fork Join Puzzles

Give the timeline of all prints and identify which children survive each block.

Code

systemverilog

module p1;
  initial begin
    fork
      begin #5  $display("%0t A", $time); end
      begin #10 $display("%0t B", $time); end
    join
    $display("%0t join_done", $time);

    fork
      begin #5  $display("%0t C", $time); end
      begin #10 $display("%0t D", $time); end
    join_any
    $display("%0t join_any_done", $time);
    disable fork;

    fork
      begin #5  $display("%0t E", $time); end
      begin #10 $display("%0t F", $time); end
    join_none
    $display("%0t join_none_done", $time);
    #1 disable fork;
  end
endmodule

Hint

Analyze each fork block independently, but remember simulation time accumulates because they execute sequentially in one initial block.

Step-by-step solution

diagram

1) First block (`join`) waits for both children: A at t=5, B at t=10, then `join_done` at t=10.
2) Second block starts at t=10: C at t=15, parent resumes at `join_any` and prints `join_any_done` at t=15, then `disable fork` kills D.
3) Third block starts at t=15: `join_none_done` prints immediately at t=15, then parent waits #1 and kills both detached children at t=16 before E/F can print.

Answer

Answer: A@5, B@10, join_done@10, C@15, join_any_done@15, join_none_done@15; D/E/F never print

Why candidates get it wrong

People remember only 'parallel' and forget parent resumption semantics and explicit cleanup needs.

Interviewer follow-up

What changes if the third block removes `#1` and calls `disable fork;` immediately?

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