If you want to use the Xilinx libraries this should help reduce some confusion.



iverilog -y%XILINX%/verilog/src/simprims/

or for Linux:

iverilog -y$XILINX/verilog/src/simprims/

to your iverilog command.  Similarly, the unimacro and unisims directory also have modules.

Then, run vvp, and generate a VCD or ltx from your testbench.

The SIMPRIMS modules all have a `timescale directive, the glbl.v file has a different one.