In reply to chr_sue:
So, in my scenario,it will be
serial_write in0(clk); // interface instance
cpu_write DUT(.clk(clk),.reset(in0.reset), .enable(in0.enable), .sin(in0.sin),.addr(in0.addr),.data(in0.data),.count(in0.count));
" bind cpu_write cpu_if b0(en)";
I checked it also,but getting same error:undefined variable b0