What is extern in system verilog and its purpose?

Hi Everyone !!!,

Today I am going to share the ideas about extern and its use case.

In System verilog, extern indicates that the body of the methods(task/function/constraint implementaion ) is to be found on outside of the class.

By using scope resolution operator also we can access the methods in outside of the class. Here the difference is if any method declared as extern it must be implemented in outside of the class other wise it will throw the compilation error. 

Coding Examples:

    

                    class methods;

                         rand int data1;

                         rand int data2;

                         constraint protocol1;  //**** implicit form--> it throws an warning message if we didn't implemnt the constraint in outside of the class****//

                         extern constraint protocol2; //*** explicit form--> we must implement the function otherwise it leads to compilation error

                  endclass

//*** Before the method name class name should  be specified with scope resolution operator to specify which class the method corresponds to ***//

                  constraint methods::protocol1{data1 inside{-4,5,7};} // **if we didn't implement we will get the warning message as protocol1 is not defined***//

                  constraint methods::protocol2{data2 >=0;} //  ** if we didn't implement we will get the compilation error**//

                

                   module TB;

                         methods mth;

                         initial begin

                           mth=new();

                            repeat(5) begin

                             mth.randomize();

                           end

                         end

                   endmodule

Example 2:

              class methods;

                  bit[2:0] data;

                  bit[2:0] addr;

//Here the function is declared as extern. From this we can understand the computation method implementation contains outside of the class//

                  extern function void computation();

              endclass

              // Implentation of the function in done outside of the class

                   function void methods::computation();

                      $display(" %0d %0d",data,addr);

                   endfunction

          

                   module extern_method;

                          methods m;

                          initial begin

                             m=new();

                              m.addr=5;

                              m.data=4;

                              m.computation();

                          end

                  endmodule

Note:

    local/protected/virtual qualifiers are allowed in extern methods. For example in above program we can declare extern vitrual function void computation

    The number of arguements,arguments name and its type should  be matched with definition and implementation  of methods.

         

Hope this code will be useful to everyone !!!

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