15 Extern

Dec 29, 2012 | Extern, Method, Constraint

extern can be used to move class methods and constraints out of the body of the class. makes code more neat

extern a class method (and constraint) qualifier is used. extern can be used to move the implementation (i.e. declaration/body) of a method outside of a class. This can help make code more readable. Imagine a class that has many lines devoted to class and constraint declarations. Moving them outside (external to) the class can allow a quick skim of the class to see its contents and not be bothered with the particulars.

In this lesson, I experiment with explicit extern constraints, but my simulator version (irun 12.10-s010) does not support them.

extern - to move method and constraint bodies out of classes, for neater code

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// "extern"  see IEEE-1800-2009-SystemVerilog-std.pdf  pgs 144-5,444
//  moving methods/constraints out of classes

module dice_game;

class dice;
   rand int value;       
   int stats[1:6]  = {0,0,0,0,0,0};


   constraint fixed_odds;  //implicit external constraint (can skip declaration)
   extern constraint even; //explicit external constraint (cannot skip declaration)

   extern function void post_randomize();
   extern function void show_stats();

endclass


   constraint dice::even { (value%2)==0; }   
   //constraint dice::fixed_odds{ value dist {6,5,4,3,2,1}; }

   function void dice::post_randomize();
      $display(" dice roll is now = %0d", value);      
      stats[value]  = stats[value]+1;
   endfunction

   function void dice::show_stats();
      foreach (stats[iii]) begin
     $display("%3d : %3d of %3d : %2.2f%",
          iii, stats[iii], stats.sum(), (real'(stats[iii])/real'(stats.sum()))*100);
      end
   endfunction


   initial begin
      dice my_dice;
      my_dice  = new();

      repeat (100) begin
     assert (my_dice.randomize()) else  begin 
        $display(" ERROR: Mister SVS, randomization failed, miserably.");    
        $fatal(1);      
     end
      end

      my_dice.show_stats();
   end   
endmodule // dice_game

.
//run command

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irun -sv dice_game.sv