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'h'1, 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.
thiskeyword 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 ; initial begin for(int i = 0; i < $size (pkt0); i++) begin pkt0[i] = new (); pkt0[i].display (); end end endmodule
Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1 Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1 Header = 0x1, Encode = 0, Mode = 0x5, Stop = 1
Since each myPacket object had no arguments in the constructor
new(), default values are applied.
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
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 a class and use that in the testbench.
virtual class Base; bit [7:0] data; bit enable; endclass