SystemVerilog Class

What are classes ?

class is a user-defined datatype, an OOP construct, that can be used to encapsulate data (property) and tasks/functions (methods) which operate on the data. Here's an example:

class myPacket;
	bit [2:0]  header;
	bit        encode;
	bit [2:0]  mode;
	bit [7:0]  data;
	bit        stop;
	
	function new (bit [2:0] header = 3'h1, bit [2:0] mode = 5);
		this.header = header;
		this.encode = 0;
		this.mode   = mode;
		this.stop   = 1;
	endfunction
	
	function display ();
		$display ("Header = 0x%0h, Encode = %0b, Mode = 0x%0h, Stop = %0b", 
		           this.header, this.encode, this.mode, this.stop);
	endfunction
endclass

There are a few key things to note in the example above:

• function new () is called the constructor and is automatically called upon object creation.
• this keyword is used to refer to the current class. Normally used within a class to refer to its own properties/methods.
• display () is a function, and rightly so, because displaying values does not consume simulation time.
• function new () has default values to the arguments, and hence line 6 (below) will create a packet object with values [3'h1, 0, 2'h3, 1]

How can I access signals within a class ?

To do that, you have to create an object of the class, which can be used as a handle to its properties and methods.

module tb_top;
	myPacket pkt0, pkt1;
		
	initial begin
		pkt0 = new (3'h2, 2'h3);
		pkt0.display ();
		
		pkt1 = new ();
		pkt1.display ();
	end
endmodule 

Header = 0x2, Encode = 0, Mode = 0x3, Stop = 1
Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1

How do I create an array of classes ?

An array of classes can be created in a way similar to how you create an int type array.

module tb_top;
	myPacket pkt0 [3];
	
	initial begin
    	for(int i = 0; i < $size (pkt0); i++) begin
   	   		pkt0[i] = new ();
       		pkt0[i].display ();
   		end
   	end
endmodule

Since each myPacket object had no arguments in the constructor new(), default values are applied.

Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1
Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1
Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1

What is inheritance ?

Let's say you wanted to have a class with all the properties/methods of myPacket and be able to add more stuff in it without changing myPacket, the best way to do so is by inheritance. In the example below, networkPacket inherits the properties/methods of myPacket using the extend keyword. To call the functions of base class (myPacket), use super keyword.

class networkPkt extends myPacket;
	bit        parity;
	bit [1:0]  crc;
	
	function new ();
		super.new ();
		this.parity = 1;
		this.crc = 3;
	endfunction
	
	function display ();
		super.display();
		$display ("Parity = %0b, CRC = 0x%0h", this.parity, this.crc);
	endfunction
endclass

What is an abstract/virtual class ?

If you create an abstract class using the virtual keyword, then you cannot create an object of the class. This is useful if you don't want others to create an object of the class and instead force users to keep the abstract class as the base and extend it to create child classes for their purpose.

// Creation of base class object is invalid
virtual class Base;
   bit [7:0]   data;
   bit         enable;
   
endclass

// Creation of child class object is valid
class Child extends Base;
  // User definition
endclass