In below example we can understand how we can enable or disable a specific constraint whenever we need to do.

class rmode; 

  rand int a; 

  rand int b; 

  constraint ct_a { a == 50;} 

  constraint ct_b { b == 100;} 

endclass 

program main; 

  rmode rnd = new(); 

  initial begin 

    void'(rnd.randomize()); 

    $display(" 1 : a : %0d,  b : %0d ",rnd.a, rnd.b);   

    rnd.ct_a.constraint_mode(0); //disable constraint ct_a

    void'(rnd.randomize()); 

    $display(" 2 : a : %0d,  b : %0d ",rnd.a, rnd.b);  

    rnd.ct_a.constraint_mode(1); //enable constraint ct_a

    void'(rnd.randomize());

    $display(" 3 : a : %0d,  b : %0d ",rnd.a, rnd.b); 

  end 

endprogram

 

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