​GHDL is an open-source VHDL simulator. It works well and runs fast. The main problem with using it for development of firmware for specific FPGAs is the fact that vendor-specific IP cores can't easily be used. However, with care to use "inferred" IP whenever possible, much can be done with GHDL.

Simulator output can be written as text using std_logic_textio or other VHDL facilities, but also waveform data may be written in a ​VCD file and viewed by (for example) ​gtkwave.

Some notes to get started. On Ubuntu (20.04, at least):

  $ sudo apt install ghdl
  $ sudo apt install gtkwave

To compile and run a simulation:

  $ ghdl -a --ieee=synopsys tdc.vhd
  $ ghdl -a --ieee=synopsys tdc_tb.vhd
  $ ghdl -e --ieee=synopsys tdc_tb
  $ ghdl -r --ieee=synopsys tdc_tb --stop-time=1000ns --vcd=test.vcd
  $ gtkwave test.vcd

See ​http://ohm.bu.edu/~hazen/ghdl_gtkwave/vhdl_example for a simple-minded but working example of a TDC with a VHDL testbench. There's a makefile, which runs the above commands.

GTKwave is fairly intuitive, however I haven't found a way to automate the process of displaying all top-level waveforms. You have to select the top-level entity, select all the signals, click "Append" then zoom full. See below.

Select the top entity	Select one signal, Ctrl-A, click Append	Click the "zoom all" button

See also some helpful notes from René Doß:

You can found the article on my homepage: ​GHDL_uart_sim.pdf

And the example files: ​UART.zip