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Forums > Functional Verification > SystemVerilog

Subject: A question about randomize()
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jt27560
Posts: 2
Online: User is Offline
10/01/2007 4:10 AM  
In the following code,  when BCDA.randomize() is excuted, it seems to me methd ADD() is also excuted at same time even though it is not explicitly called. How does randomize() work exactly? Thanks in advance!

//BCD addition testbench
//www.asichowto.com

module BCDAddition_tb();

class BCDAdd;
    rand logicΑ:0] BCD1,BCD2;
    logicΕ:0] result;
    
    function new();
        BCD1=0;
        BCD2=0;
    endfunction
    
    constraint c
    {
        BCD1<10;
        BCD2<10;
    }

    function void ADD();
        result=BCD1+BCD2;
        if(result>9)
           result=result+6;
    endfunction
endclass



BCDAdd BCDA;
logicΑ:0] b1,b2;
logicΕ:0] r;

BCDAddition U(b1,b2,r);

initial
begin
    repeat(10)
    begin
        BCDA=new;
        if(BCDA.randomize())
        begin
            b1=BCDA.BCD1;
            b2=BCDA.BCD2;
//            BCDA.ADD();
            if(r!=BCDA.result)
            begin
                $display("Class:  BCD1=%d BCD2=%d result=%d", BCDA.BCD1,BCDA.BCD2,BCDA.result);
                $display("Module: BCD1=%d BCD2=%d result=%d", b1,b2,r);
            end
        end
        #10ns;
    end
end
endmodule
stephenh
Posts: 77
Online: User is Offline
10/01/2007 4:29 AM  
Why do you think ADD() is called?
I don't see anything that calls it explicitly (ignoring the commented out line).
I pasted this code into a file and it ran without calling ADD()...
Steve H.
jt27560
Posts: 2
Online: User is Offline
10/02/2007 3:06 PM  
Thanks, Steve.

It does not display anything whether I comment out BCDA.ADD() or not, which gives me an impression that ADD() is executed too when randomize is executed.
I should use !== instead of !=. And should put a delay right after ADD();
kameade
Posts: 13
Online: User is Offline
10/02/2007 3:22 PM  
Hello,

In your example above, ADD is not called unless it is called specifically via the BCDA.ADD() function call. What's happening in your code is that the begin/end block after calling randomize() executes to completion before the BCDAddition() module instance is evaluated so you are getting strange results. If you put a delay after the b1/b2 assignments, you will get the expected behavior.

I hope this helps.

Kathleen
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Forums > Functional Verification > SystemVerilog > A question about randomize()


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