`timescale 1ns/10ps   
module mul(//input
             clk,nrst,
             a,b,mul_en,
                          
             //output
             oen,product);                         
input  clk,nrst;
input  [7:0] a,b;
input  mul_en;  

output oen; //output enable
output [15:0] product;

reg [15:0] product;
reg [2:0] count;
wire load=(count==7);
wire oen =(count==0);
always@(posedge clk or negedge nrst)
begin
 if (~nrst)       count<= 3'b111;
 else if (mul_en) count<= count-1;
end 

wire [15:0] temp=(product<<1);
always@(posedge clk or negedge nrst) 
begin
 if (~nrst)              product<= 0;
 else if (mul_en)begin
      if (~load) begin
          if (b[count])  product<= temp+a;
          else           product<= temp;     
      end else if (load) begin
          if (~b[7])     product<= 0; 
          else if (b[7]) product<= a;
      end     
  end         
end      
                              
endmodule