Like in C++, in Systemverilog we can create virtual class for abstract class, this abstract class can contain abstract methods(virtual methods). Virtual methods provide prototypes for subroutines, all of the information generally found on the first line of a method declaration: the encapsulation criteria, the type and number of arguments, and the return type if it is needed. Later, when subclasses override virtual methods, they must follow the prototype exactly. Thus, all versions of the virtual method will look identical in all subclasses.

Methods of normal classes can also be declared virtual. In this case, the method must have a body. If the method does have a body, then the class can be instantiated, as can its subclasses. However, if the subclass overrides the virtual method, then the new method must exactly match the superclass¿s prototype.

We offen call this abstract virtual class as base objects.

   1 program class_virtual;
   2   // Virtual class for body of any driver
   3   virtual class verif;
   4     // This starts all the threads
   5     virtual task startSim();
   6     endtask
   7     // This stops all the threads
   8     virtual task stopSim();
   9     endtask
  10     // This prints all the stats
  11     virtual task printStats();
  12     endtask
  13     // This check if driver is done or not
  14     virtual function bit isDone ();
  15       begin
  16         isDone = 0;
  17       end
  18     endfunction
  19     // set the driver config
  20     virtual task setConfig(integer item, integer value);
  21     endtask
  22     virtual function integer getConfig(integer item);
  23       begin
  24         getConfig = 32'hDEAD_BEAF;
  25       end
  26     endfunction
  27   endclass
  28   // ethernet inherits verif
  29   class ethernet extends verif;
  30     integer min_frame_size;
  31     integer max_frame_size;
  32     function new();
  33       begin
  34         min_frame_size = 32'h40;
  35         max_frame_size = 32'h200;
  36       end
  37     endfunction
  38     task startSim();
  39       begin
  40         $write("Starting Simulation\n");
  41       end
  42     endtask
  43     task stopSim();
  44       begin
  45         $write("Stopping Simulation\n");
  46       end
  47     endtask
  48     task printStats();
  49       begin
  50         $write("Sent normal   frames %d\n",100);
  51         $write("Sent runt     frames %d\n",1);
  52         $write("Sent oversize frames %d\n",1);
  53       end
  54     endtask
  55     function bit isDone();
  56       begin
  57         isDone = 1;
  58       end
  59     endfunction
  60     task setConfig(integer item, integer value);
  61       begin
  62         case(item)
  63            0 : min_frame_size = value;
  64            1 : max_frame_size = value;
  65         endcase
  66       end
  67     endtask
  68     function integer getConfig(integer item);
  69       begin
  70         case(item) 
  71            0 : getConfig = min_frame_size;
  72            1 : getConfig = max_frame_size;
  73            default :  begin
  74                      $write("Calling super.setConfig\n");
  75                       getConfig  = super.getConfig(item);
  76            end
  77         endcase
  78        end
  79     endfunction
  80   endclass
  81   
  82   class ethernet2 extends ethernet;
  83     integer min_ipg;
  84     function new();
  85       begin
  86         min_ipg = 32'hc;
  87       end
  88     endfunction
  89     task setConfig(integer item, integer value);
  90       begin
  91         case(item) 
  92            2 : min_ipg = value;
  93            default : begin
  94                      $write("Calling super.setConfig\n");
  95                      super.setConfig(item,value);
  96            end
  97         endcase
  98       end
  99     endtask
 100     function integer getConfig(integer item);
 101       begin
 102         case(item) 
 103            2 : getConfig = min_ipg;
 104            default :  begin
 105                      $write("Calling super.setConfig\n");
 106                       getConfig  = super.getConfig(item);
 107                     end
 108         endcase
 109       end
 110     endfunction
 111   endclass
 112  
 113 
 114   initial begin 
 115     ethernet2 eth = new();
 116     eth.setConfig(0,32'h100);
 117     eth.setConfig(2,32'h24);
 118     $write ("Value of min_frame is %0x\n", eth.getConfig(0));
 119     $write ("Value of max_frame is %0x\n", eth.getConfig(1));
 120     $write ("Value of min_ipg   is %0x\n", eth.getConfig(2));
 121     $write ("Value of unknown   is %0x\n", eth.getConfig(3));
 122   
 123     eth.startSim();
 124     while (eth.isDone() == 0) begin
 125       #1 ;
 126     end
 127     eth.stopSim();
 128     eth.printStats();
 129   end
 130 
 131 endprogram

 Calling super.setConfig
 Calling super.setConfig
 Value of min_frame is 100
 Calling super.setConfig
 Value of max_frame is 200
 Value of min_ipg   is 24
 Calling super.setConfig
 Calling super.setConfig
 Value of unknown   is deadbeaf
 Starting Simulation
 Stopping Simulation
 Sent normal   frames         100
 Sent runt     frames           1
 Sent oversize frames           1

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Deepak Kumar Tala - All rights reserved

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