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SystemVerilog Tutorial PART 14:  by Abhiram Rao

Classes in SystemVerilog- Part 4


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Virtual Class

Ok, the third cool thing about classes is the use of virtual  you can have virtual classes and/or virtual methods  a class that begins with the keyword virtual (a virtual class)  cannot have objects from it. But it can be used as a base  class and all the methods and variables are going to be the commonality  from all derived classes/objects. 


An example could be a fabric router, that has many kinds  of data formats. The base class contains all the common  data but that datatype would never be send into the router  by itself. All derived classes must have that common data,   a class that contains virtual methods (virtual task/ virtual function)  allows existing code to use the base class handles, but in fact  the derived objects methods are being used. This permits  a great deal of reuse, since the original code doesn't need  be modified when some functionally gets changed   an example is the VMM methodology, where the Bus Functional Models (xactors) act on data, but one can substitute a changed data and the BFM does not need to be modified to take this new data.  All of this is called polymorphism!


 ok, so now you are expecting to see some code!




program tb_top;

// here is a shape class, defining that there will be

// some shapes to draw, and they all must have

// a shape method to be used by the "BFM"


 virtual class Shapes;

    // every shape must have a total size, and a start coordinate

    int total_size;

    int start_x, start_y;

    bit[7:0] color;

    // every shape must implement draw and color actions

    virtual task draw();



    // by defining code here, it becomes optional for

    // derived methods to define it there.

    // because it is virtual, the derived object method

    // will override this call, unless super is used by

    // the derived object to call this one

    virtual task setcolor (bit [7:0] color);

       this.color = color;





class Circle extends Shapes;

  bit [30:0] circle_d_data;

  function new(int size = 4);

      total_size = size * 3;


  // here is how I would draw a circle

  // I keep it virtual so that one someone

  // can create a 3D circle and build upon

  // this example

  virtual task draw();

     // do the actual draw here

     $display("drawing a circle...\n");





class Square extends Shapes;

  int drawn = 0;

  function new(int sizex = 1, int sizey = 1);

      total_size = sizex * sizey;


  virtual task draw();

     // do the actual draw here

     $display("drawing a square...\n");

     drawn = 1;


  virtual task setcolor(bit [7:0] color);

     // here I need to do the base class stuff, but then

     // the work of updating the color of the square


     if (drawn == 1) begin

     $display("updating drawing...\n");



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Shapes s[2];

Circle c1;

Square s1;




   // initialize the objects, let base class handles

   // point to them also

   c1 = new(); // create a circle object

   s[0] = c1; // s[0] also points to that circle

   s1 = new(); // create a square object

   s[1] = s1; // watch out for sharp edges


   //s0.draw(); is the same as calling c1.draw(), but

   // the neat thing here is that a BFM only needs the

   // base class handle, and doesn't need to be modified

   // if the functionality or data features change!!




   // is the same as



   // but the base methods didn't need to be changed




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