[sv-ec] Question about seeding $urandom

Hi, All -

I think random falls under EC, so at the bottom is the question.

I was playing with $urandom and trying to seed it, and have run into 
problems related to my understanding of seeding this function.

The following code is also in an attached file: urandomtest.sv
(I run the nine different iterations using +define+RUN1 thru +define+RUN9)

program urandomtest;
   logic [7:0] a;
   int         seed;

   initial $monitor("a=%h", a);

   initial begin
     `ifdef RUN1
     repeat(20) #1 a = $random();
     `elsif RUN2
     repeat(20) #1 a = $random(145);
     `elsif RUN3
     seed = 145;
     repeat(20) #1 a = $random(seed);
     `elsif RUN4
     repeat(20) #1 a = $urandom();
     `elsif RUN5
     repeat(20) #1 a = $urandom(145);
     `elsif RUN6
     seed = 145;
     repeat(20) #1 a = $urandom(seed);
     `elsif RUN7
     repeat(20) #1 a = $urandom_range(145);
     `elsif RUN8
     repeat(20) #1 a = $urandom_range(100,145);
     `elsif RUN9
     seed = 145;
     repeat(20) #1 a = $urandom_range(seed);
     `endif
   end
endprogram

$random - normal Verilog behavior
RUN1 - $random() generates 20 random numbers

RUN2 - generates the same random number 20 times, each time using the 
seed value of 145.

RUN3 - generates 20 random numbers with the first number based on the 
"seed" value of 145.

$urandom - a few surprises

RUN4 - generates 20 urandom numbers.

RUN5 - generates the same urandom number 20 times, each time using 
the seed value of 145. In P1800-2009 Ballot Draft, clause 18.13.1, 
last sentence in the paragraph after the prototype function is a 
little misleading when it says: "The random number generator (RNG) 
shall generate the same sequence of random numbers every time the 
same seed is used."

RUN6 - generates the same urandom number 20 times, each time using 
the seed value of 145 (seeding behavior is different than Verilog's 
$random from RUN3).

$urandom_range - no big surprises

RUN7 - generates 20 urandom_range numbers between 0-145

RUN8 - generates 20 urandom_range numbers between 100-145

RUN9 - generates 20 urandom_range numbers between 0-145

Question - what is the correct way to seed the $urandom function when 
used in procedural code outside of a class??

Thanks for any feedback.

Regards - Cliff

----------------------------------------------------
Cliff Cummings - Sunburst Design, Inc.
14314 SW Allen Blvd., PMB 501, Beaverton, OR 97005
Phone: 503-641-8446 / FAX: 503-641-8486
cliffc@sunburst-design.com / www.sunburst-design.com
Expert Verilog, SystemVerilog, Synthesis and Verification Training

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