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RANDOMIZATION CONTROLABILITY

Controlability

Additional to the controllability feauters supported by SystemVerilog, following are more points with which controlabiity can be achieved.

In the following example, MACROS MIN_D and MAX_D are defined. Set the MIN and MAX values in the pre_randomize as shown. As MIN_D and MAX_D are macros, they can be assigned from command line. Biggest disadvantage for the method shown below is dynamic controllability.

EXAMPLE:
   `define MAX_D 100
   `define MIN_D 50
   class Base;
     rand integer Var;
     integer MIN,MAX;
     constraint randge { Var < MAX ; Var > MIN ;}
     function void pre_randomize ();
       this.MIN = `MIN_D;
       this.MAX = `MAX_D;
       $display( " PRE_RANDOMIZE : MIN = %0d , MAX = %0d ",MIN,MAX);
     endfunction
   endclass

   program inhe_42;
     Base obj;
     initial
     begin
       obj = new();
       for(int i=0 ; i < 100 ; i++)
         if(obj.randomize())
           $display(" Randomization sucsessfull : Var = %0d ",obj.Var);
         else
           $display("Randomization failed");
     end
   endprogram

RESULTS:

#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 91
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 93
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 77
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 68
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 67
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 52
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 71
#  PRE_RANDOMIZE : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 98
...etc.

As in this example,a single object is created and randomized 100 times. Due to this,pre_reandomize is called 100 times, which may not be preferred.
By assigning the values while declaration itself this can be avoided. Simpler way to achieve the above logic.

EXAMPLE:
`define MAX_D 100
`define MIN_D 50
class Base;
  rand integer Var;
  integer MIN = `MIN_D;
  integer MAX = `MAX_D;
  constraint range { Var < MAX ; Var > MIN ;}
endclass

program inhe_43;
  Base obj;
  initial
  begin
    obj = new();
    for(int i=0 ; i < 100 ; i++)
      if(obj.randomize())
        $display(" Randomization sucsessfull : Var = %0d ",obj.Var);
      else
        $display("Randomization failed");
  end
endprogram

RESULTS:

#  Randomization sucsessfull : Var = 91
#  Randomization sucsessfull : Var = 93
#  Randomization sucsessfull : Var = 77
#  Randomization sucsessfull : Var = 68
#  Randomization sucsessfull : Var = 67
#  Randomization sucsessfull : Var = 52
#  Randomization sucsessfull : Var = 71
#  Randomization sucsessfull : Var = 98
#  Randomization sucsessfull : Var = 69
#  Randomization sucsessfull : Var = 70
...etc.

With the above approach also, dynamic controlability is lost. For dynamic controllability, define a task, pass this values as arguments when ever the changed is needed.

EXAMPLE:
class Base;
  rand integer Var;
  integer MIN = 10,MAX = 20;  // Define default values,If function set is not called,with this it will work
  constraint randge { Var < MAX ; Var > MIN ;}
  task set (integer MIN,integer MAX);
    this.MIN = MIN;
    this.MAX = MAX;
    $display( " SET : MIN = %0d , MAX = %0d ",MIN,MAX);
  endtask
endclass

program inhe_44;
  Base obj;

  initial
  begin
    obj = new();
    obj.set(0,100) ;
    for(int i=0 ; i < 5 ; i++)
      if(obj.randomize())
        $display(" Randomization sucsessfull : Var = %0d ",obj.Var);
      else
        $display("Randomization failed");

    obj.set(50,100) ;
    for(int i=0 ; i < 5 ; i++)
      if(obj.randomize())
        $display(" Randomization sucsessfull : Var = %0d ",obj.Var);
      else
        $display("Randomization failed");
  end
endprogram
RESULTS:

#  SET : MIN = 0 , MAX = 100
#  Randomization sucsessfull : Var = 24
#  Randomization sucsessfull : Var = 68
#  Randomization sucsessfull : Var = 43
#  Randomization sucsessfull : Var = 11
#  Randomization sucsessfull : Var = 4
#  SET : MIN = 50 , MAX = 100
#  Randomization sucsessfull : Var = 52
#  Randomization sucsessfull : Var = 71
#  Randomization sucsessfull : Var = 98
#  Randomization sucsessfull : Var = 69
#  Randomization sucsessfull : Var = 70

More simpler way to dynamically modifying the constraints is by modifying the data members of class via object reference.

EXAMPLE:
class Base;
  rand integer Var;
  integer MIN = 20,MAX =30;
  constraint randge { Var < MAX ; Var > MIN ;}
endclass

program inhe_45;
  Base obj;

  initial
  begin
    obj = new();
    obj.MIN = 0;
    obj.MAX = 100;
    for(int i=0 ; i < 5 ; i++)
    if(obj.randomize())
      $display(" Randomization sucsessfull : Var = %0d ",obj.Var);
    else
      $display("Randomization failed");
      $display("MIN and MAX changed");

    obj.MIN = 50;
    obj.MAX = 100;
    for(int i=0 ; i < 5 ; i++)
      if(obj.randomize())
        $display(" Randomization sucsessfull : Var = %0d ",obj.Var);
      else
        $display("Randomization failed");
  end
endprogram

RESULTS:

#  Randomization sucsessfull : Var = 24
#  Randomization sucsessfull : Var = 68
#  Randomization sucsessfull : Var = 43
#  Randomization sucsessfull : Var = 11
#  Randomization sucsessfull : Var = 4
# MIN and MAX changed
#  Randomization sucsessfull : Var = 52
#  Randomization sucsessfull : Var = 71
#  Randomization sucsessfull : Var = 98
#  Randomization sucsessfull : Var = 69
#  Randomization sucsessfull : Var = 70

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