VLSI DV Interview Puzzles · All levels

Enum Safety with $cast

Predict cast success bits and enum value after each attempt.

Puzzle

Difficulty: Medium · Puzzle 4 of 6 · Topic: String and Casting Puzzles

Predict cast success bits and enum value after each attempt.

Code

systemverilog

typedef enum int {IDLE=0, BUSY=1, DONE=2} state_e;

module p4;
  int raw1 = 2;
  int raw2 = 3;
  state_e s = IDLE;
  initial begin
    $display("c1=%0d s=%s", $cast(s, raw1), s.name());
    $display("c2=%0d s=%s", $cast(s, raw2), s.name());
  end
endmodule

Hint

`$cast` to enum succeeds only for legal enumerator values. On failure, destination keeps previous value.

Step-by-step solution

diagram

1) `raw1=2` is legal enum value DONE, so first cast returns 1 and `s` becomes DONE.
2) `raw2=3` is not a legal enumerator, so second cast returns 0.
3) Failed cast does not modify destination, so `s` stays DONE.

Answer

Answer: First line: c1=1 s=DONE; second line: c2=0 s=DONE

Why candidates get it wrong

People assume failed `$cast` zeroes destination or sets it to first enum literal.

Interviewer follow-up

How would a static cast `state_e'(raw2)` behave differently?

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