Unable to link PLI with system verilog based code

In reply to kiru2006:

I have found some issues in do file itself.After fixing that I am able to load the “sup_drv.sl”.But after that simulator is showing “Error :Expected system function , not system task ‘$sup_drv’” .
Any clue why this error ?
code snippet of how I am using $sup_drv :

wire [3:0] io_sup_drv;
assign io_sup_drv = $sup_drv(io1,io2);

Thanks
Kiran