systemverilog中ref的所有用法是什么？举例说明

在 SystemVerilog 中，ref 是一个关键字，用于传递引用。下面是 ref 的几种用法及示例：

1. 传递引用型参数

module top;
  logic [3:0] a = 4'b0010;
  logic [3:0] b = 4'b0110;
  
  function void swap(ref logic [3:0] x, ref logic [3:0] y);
    logic [3:0] temp;
    temp = x;
    x = y;
    y = temp;
  endfunction
  
  initial begin
    $display("Before swap a=%b, b=%b", a, b);
    swap(a, b);
    $display("After swap a=%b, b=%b", a, b);
  end
endmodule

2. 传递引用型返回值

module top;
  logic [3:0] a = 4'b0010;
  logic [3:0] b = 4'b0110;
  
  function automatic ref logic [3:0] larger(ref logic [3:0] x, ref logic [3:0] y);
    if (x > y)
      return x;
    else
      return y;
  endfunction
  
  initial begin
    logic [3:0] &larger_value;
    larger_value = larger(a, b);
    $display("Larger value is %b", larger_value);
  end
endmodule

3. 按引用修改数组元素

module top;
  logic [31:0] mem [0:3];
  
  task modify(ref logic [31:0] mem_array [0:3], int index, ref logic [31:0] data);
    mem_array[index] = data;
  endtask
  
  initial begin
    logic [31:0] data = 32'h12345678;
    $display("Before modification mem[1] = %h", mem[1]);
    modify(mem, 1, data);
    $display("After modification mem[1] = %h", mem[1]);
  end
endmodule