Testbench.v

module top ();

reg             clk;    
reg             rst;    
reg             req3;   
reg             req2;   
reg             req1;   
reg             req0;   
wire            gnt3;   
wire            gnt2;   
wire            gnt1;   
wire            gnt0;  

// Clock generator
always #1 clk = ~clk;

initial begin
  $dumpfile ("arbiter.vcd");
  $dumpvars();
  clk = 0;
  rst = 1;
  req0 = 0;
  req1 = 0;
  req2 = 0;
  req3 = 0;
  #10 rst = 0;
  repeat (1) @ (posedge clk);
  req0 <= 1;
  repeat (1) @ (posedge clk);
  req0 <= 0;
  repeat (1) @ (posedge clk);
  req0 <= 1;
  req1 <= 1;
  repeat (1) @ (posedge clk);
  req2 <= 1;
  req1 <= 0;
  repeat (1) @ (posedge clk);
  req3 <= 1;
  req2 <= 0;
  repeat (1) @ (posedge clk);
  req3 <= 0;
  repeat (1) @ (posedge clk);
  req0 <= 0;
  repeat (1) @ (posedge clk)
  #10 $finish;
end 

// Connect the DUT
arbiter U (
 clk,    
 rst,    
 req3,   
 req2,   
 req1,   
 req0,   
 gnt3,   
 gnt2,   
 gnt1,   
 gnt0   
);

endmodule