The modules themselves supply the data which may be visible to a waveform file; the simulation engine purely provides access to it.

Exec file enhancements

The simulation engine supplies the following exec file enhancements to provide access to the VCD waveform files:

vcd_file_open( string handle, string filename )

Open a VCD waveform file with a given handle (for future reference) and a given filename. The filename should refer to a (potential) file that is writable.

vcd_file_close( string handle )

Close a VCD waveform file handle; this cleanly finishes the VCD file attached to the handle.

vcd_file_add( string handle, string signal or state, ... )

Add a signal or other described state to the specified VCD handle; the signal or state specifiied may be a global signal name, a state or signal within a module that a module exports, or it may be a module name in which case all the signals and states of that module are added to the VCD file.

vcd_file_enable( string handle )

Outputs the header for the specified VCD file, and enables its further output. This should be issued once after all the VCD file signal additions have been performed, before a simulation is stepped.

integer vcd_file_size( string handle )

Returns the size of the specified VCD file

So a simple example of the use of these commmands would be:

vcd_file_open my_handle "signals.vcd"
vcd_file_add my_handle input_0 input_1
vcd_file_add my_handle output_2
vcd_file_add my_handle module_1
vcd_file_enable my_handle
reset
step 50
vcd_file_close my_handle