parameter可用作在顶层模块中例化底层模块时传递参数的接口，localparam的作用域仅仅限于当前module，不能作为参数传递的接口。

`timescale 1ns/100ps
 
module mem (
 
  clka,
  wea,
  addra,
  dina,
 
  clkb,
  addrb,
  doutb);
 
  parameter       DATA_WIDTH = 16;
  parameter       ADDR_WIDTH =  5;
  localparam      DW = DATA_WIDTH - 1;
  localparam      AW = ADDR_WIDTH - 1;
 
  input           clka;
  input           wea;
  input   [AW:0]  addra;
  input   [DW:0]  dina;
 
  input           clkb;
  input   [AW:0]  addrb;
  output  [DW:0]  doutb;
 
  reg     [DW:0]  m_ram[0:((2**ADDR_WIDTH)-1)];
  reg     [DW:0]  doutb;
 
  always @(posedge clka) begin
    if (wea == 1'b1) begin
      m_ram[addra] <= dina;
    end
  end
 
  always @(posedge clkb) begin
    doutb <= m_ram[addrb];
  end
 
endmodule