10 Rand and Constraint Block

• rand - give a variable the ability to be randomized

• constraint block - constrain that randomization

• randomize() - to run randomization on an object

• pre_randomize() function - automatically run before randomization

• post_randomize() function - automatically run after randomization

As an intro to the constraint block, we create a class to describe a die (as in gaming dice that are rolled). The value of the die can be random, but we constrain the value with a constraint block, allowing values of [1,2,3,4,5,6].

pre_randomize() and post_randomize() are functions that are automatically run whenever randomization is performed on the object.

rand variable and constraint - representing value of a rolling die (dice).

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module dice_game;

class a_die;
   rand int value;

   constraint six_sides { value>=1 && value<=6; }

   function void pre_randomize();      
      $write(" die ready to be rolled ....");
   endfunction   

   function void post_randomize();      
      $display(" die roll is now = %0d", value);
   endfunction   
endclass


   initial begin
      a_die my_die;
      my_die  = new();

      repeat (4) begin
     my_die.randomize();     
      end

   end 

endmodule // dice_game

.
//run command

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irun -sv dice_roll.sv

Aside: We are intentionally ignoring the return value of randomize() in this example. Note the warning during compilation.