RE: [sv-ec] $random(), $urandom()

> $random() is arguably prohibited by Syntax 20-14 which could
> be interpreted as a restriction on what's allowed by Annex A.

I think this is actually a merge issue.  The Syntax box was copied from Syntax 17-17 in Verilog-2005, in which language system function calls could not use empty parentheses.  But in SystemVerilog they can, and there's no reason for a special restriction to prohibit that for seedless calls to $random.

-- Brad


________________________________________
From: Brad Pierce
Sent: Thursday, June 11, 2009 10:31 AM
To: sv-ec@eda.org
Subject: RE: [sv-ec] $random(), $urandom()

Cliff,

I don't see any BNF restriction against $urandom(), unless you believe 18.13.1

       function int unsigned $urandom [ (int seed ) ] ;

in which case the 'int' keyword would also be required.

$random() is arguably prohibited by Syntax 20-14 which could be interpreted as a restriction on what's allowed by Annex A.

-- Brad

________________________________________
From: owner-sv-ec@eda.org [owner-sv-ec@eda.org] On Behalf Of Clifford E. Cummings [cliffc@sunburst-design.com]
Sent: Thursday, June 11, 2009 10:09 AM
To: sv-ec@eda.org
Subject: [sv-ec] $random(), $urandom()

Thanks, Shalom -

I have run into issues with empty parentheses in the past with one
implementation, but not with the two implementations that I tested yesterday.

I am not as certain about the integral number -vs- integral variable issue.

I did find text supporting your position in the third paragraph of
clause 20.15.1 (ballot draft):
The seed argument shall be an integral variable. The seed value
should be assigned to this variable prior to calling $random.
But I wonder if that is a mistake. Do you have access to any tools
that do not support integral numbers inside of the parentheses? Seems
like if one uses $random(integral_number), one would always get the
same value returned, which is what I observed. Adding the integral
variable allows $random to change the seed between calls.

Regarding $urandom(integral_number), from the second paragraph of
18.13.1 (ballot draft):
The seed is an optional argument that determines the sequence of
random numbers generated. The seed can be any integral expression.
The random number generator (RNG) shall generate the same sequence of
random numbers every time the same seed is used.
And the subsequent example shows: addr[32:1] = $urandom(254);

Comments?

Regards - Cliff


At 07:01 AM 6/11/2009, Bresticker, Shalom wrote:
>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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