System verilog distribution constraint

SystemVerilog
, ,
1

Consider the below code :

class packet;
  rand bit addr;

  constraint addr_range { addr dist {1 := 60, 0 :=40 }; }
endclass

module constr_dist;
  initial begin
    packet pkt;
    pkt = new();
    $display("------------------------------------");
    repeat(10) begin
      pkt.randomize();
      $display("\taddr = %0d",pkt.addr);
    end
    $display("------------------------------------");
  end
endmodule

When i do randomize with dist constraint i see that distribution of 1 and 0 is not exactly 60% - 40% everytime. It gives me a distribution with a variance. Is it possible to somehow get the exact distribution? What probability distribution does system verilog use while randomizing with dist constraints?

2
3

Hi Dave,

Even with 2000 repeats I am still getting a variance of about 5%. I guess there is no way to get an exact distribution through dist constraint.

4

A distribution is a statistical probability of value occuring in a range. Each time you call randomize, the previous results have no effect on the new result, the probabilities remain the same.

Take a simpler example where the dist probability is 1 := 50, 0 :=50. If you call randomize twice, there are 4 possible outcomes with equal probability: 0-0, 0-1, 1-0, and 1-1. Only 2 out of 4 outcomes gives you an exact 0-1or 1-0 distribution, or 50% chance of seeing an exact distribution.

If you increase the random repetition to 4 times, then there are 16 possible outcomes with only 6 of them giving you an exact distribution of 2 0’s and 2 1’s. or 37.5%. 2000 repetitions, the probability goes down to 2.5%. The probability keeps getting lower as you increase the repetition count, but at the same time, the average distribution keeps getting closer to 50/50.

5

Thank you Dave! That makes sense.