Virtual signals

Virtual signals are aliases for combinations or subelements of signals written to the WLF file by the simulation kernel. They can be displayed in the Signals, List, and Wave windows, accessed by the examine command, and set using the force command. Virtual signals can be created via a menu in the Wave and List windows (Edit > Combine), or with the virtual signal command. Virtual signals can also be dragged and dropped from the Signals window to the Wave and List windows.

Virtual signals are automatically attached to the design region in the hierarchy that corresponds to the nearest common ancestor of all the elements of the virtual signal. The virtual signal command has an -install <region> option to specify where the virtual signal should be installed. This can be used to install the virtual signal in a user-defined region in order to reconstruct the original RTL hierarchy when simulating and driving a postsynthesis, gate-level implementation.

A virtual signal can be used to reconstruct RTL-level design buses that were broken down during synthesis. The virtual hide command can be used to hide the display of the broken-down bits if you don't want them cluttering up the Signals window.

If the virtual signal has elements from more than one WLF file, it will be automatically installed in the virtual region "virtuals:/Signals."

Virtual signals are not hierarchical - if two virtual signals are concatenated to become a third virtual signal, the resulting virtual signal will be a concatenation of all the subelements of the first two virtual signals.

The definitions of virtuals can be saved to a macro file using the virtual save command. By default, when quitting, ModelSim will append any newly-created virtuals (that have not been saved) to the virtuals.do file in the local directory.

If you have virtual signals displayed in the Wave or List window when you save the Wave or List format, you will need to execute the virtuals.do file (or some other equivalent) to restore the virtual signal definitions before you re-load the Wave or List format during a later run. There is one exception: "implicit virtuals" are automatically saved with the Wave or List format.

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