RE: [sv-ec] Question about seeding $urandom

Cliff,

You got lucky with respect to the syntax.
The BNF does not allow empty parentheses to either $random or $urandom, and for $random, the seed has to be an integral variable and may not be a constant. 
Some tools will not compile those cases.

Regards,
Shalom 

> -----Original Message-----
> From: owner-sv-ec@server.eda.org 
> [mailto:owner-sv-ec@server.eda.org] On Behalf Of Clifford E. Cummings
> Sent: Thursday, June 11, 2009 2:32 AM
> To: sv-ec@server.eda.org
> Subject: [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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