What does “ ref ” mean in systemverilog?

Question

I found this in systemverilog:

task automatic xxx(ref xxxpackage bus,input interface ift);

I want to know the usage of ref. What is the advantage?

Answer 1

A ref argument is a variable passed by reference. This type of argument are not a copy but a reference to the original variable.

Arguments passed by reference are not copied into the subroutine area, rather, a reference to the original argument is passed to the subroutine. The subroutine can then access the argument data via the reference.

From section 13.5.2 in IEEE Std 1800-2012.

Answer 2

Hey bunch here's the explanation of the example that DAVE gave. Thanks a lot for the example Dave.

module top;
logic A,B;
task automatic mytask(inout logic arg1, ref logic arg2);
  #0 $display("%m %t arg1 %b arg2 %b",$time,arg1,arg2);
  // actual arguments have been set to 0
  #5 $display("%m %t arg1 %b arg2 %b",$time,arg1,arg2);
  #0 arg1 = 1; arg2 = 1;
  #5 $display("%m %t arg1 %b arg2 %b",$time,arg1,arg2);
endtask
initial #1 mytask(A,B);
initial begin
       A = 'z; B ='z;
       #2 A = 0; B = 0; // after call 
       // arguments have been set to 1
       #5 $display("%m %t A %b B %b",$time,A ,B);
       #5 $display("%m %t A %b B %b",$time,A ,B);
end
endmodule

/*Both the two 'initial' statements are running simultaneously*/
/* 1) At time t=0 A and B are set to z by second initial statement
   2) At time t=1 mytask(A,B) is called by first initial 
      statement, 
      the first display statements displays arg1 and arg2 =z as 
      set by A and B.
   3) t=3 the second initial statement sets A=0 and B=0, but only  
      A=0 is passed to arg 1 in the ongoing task since it is
      passed by reference, whereas B=0 can only be passed at the
      starting or the end of the task since it is passed by value 
      hence arg2 remains z.
   4) inside the task--At t=6 values of arg1 and arg2 are 
      displayed
   5) at t=6 the values of arg1 and arg2 are made 1.
   6) in the second initial statement at t=7 values of A and B
      is displayed, since arg2 is passed through reference
      therefore it becomes 1, whereas A remains zero until the end of
   the task.
   7) at t=11 the values of arg1 and arg2 are displayed. -- task 
      ends.
   8) Since the task is ended arg2 value is passed to B and is 
      displayed by the second initial statement at t=12. 
   */

I have explained as per the output displayed in the kernel and timing it shows hope this helps.

Answer 3

Normally, task and function arguments declared as input are copied by value upon entry to the routine, and arguments declared as output are copied by value upon returning from the routine. inout arguments are copied both upon entry and return from the routine. Arguments declared with ref are not copied but instead are references to the actual arguments used when making the call to the routine. There are much stricter data type compatibility rules when using ref arguments.

In a task that consumes time, a ref can be used instead of an inout to capture value changes that occur while the task is active. Remember that an inout argument is copied into the task when it is called, and copied out when the task returns. Here is an example that you should try.

module top;
logic A,B;
task automatic mytask(inout logic arg1, ref logic arg2);
  #0 $display("%m %t arg1 %b arg2 %b",$time,arg1,arg2);
  // actual arguments have been set to 0
  #5 $display("%m %t arg1 %b arg2 %b",$time,arg1,arg2);
  #0 arg1 = 1; arg2 = 1;
  #5 $display("%m %t arg1 %b arg2 %b",$time,arg1,arg2);
endtask
initial #1 mytask(A,B);
initial begin
       A = 'z; B ='z;
       #2 A = 0; B = 0; // after call 
       // arguments have been set to 1
       #5 $display("%m %t A %b B %b",$time,A ,B);
       #5 $display("%m %t A %b B %b",$time,A ,B);
end
endmodule

See the difference between the inout and pass by ref arguments.

Note that a class variable is a reference to a class handle already, so passing a class variable by reference rarely has any benefit. Also, in a function, the only benefit of a ref argument might be performance in passing large data structures like an array instead of using an input, output, or inout.