** GHDL
GHDL is a compiler and simulator for VHDL, a Hardware Description Language.

GHDL offers three machine code generation backends: one based on GCC, one using the LLVM compiler suite and a GHDL specific one called mcode. These are available in the ghdl-gcc, ghdl-llvm and ghdl-mcode packages respectively. Both the GCC and LLVM backends create highly optimized code for excellent simulation performance while simulations compiled with the GCC backend also allow coverage testing using gcov. The mcode backend creates less performant code but makes up for it with much faster compilation. It is therefore preferable for smaller projects without large or long running simulations.

Multiple backends can be installed at the same time and selected by either invoking the desired GHDL directly (as ghdl-gcc, ghdl-llvm or ghdl-mcode) or by providing a GHDL_BACKEND environment variable (containing gcc, llvm or mcode) while invoking ghdl.

ghdl --help
ghdl --clean
ghdl -a xxx.vhdl		analyse
ghdl -e unit_name		elaborate
ghdl -r unit_name		run/compile
Options:
	--work=NAME    	Name of the WORK library, i.e. ghdl -a --work=foo foo.vhdl
	--std=STD	 	Which VHDL standard (87|93|93c|00|02), i.e. ghdl -a --std=87 old.vhdl
--ieee=VER    		Which IEEE library (none|standard|synopsys|mentor), i.e. ghdl -a --ieee=synopsys broken.vhdl
--no-vital-checks 	Disable VITAL restriction checking, i.e. ghdl -a --no-vital-checks unsupported_vital.vhdl

Executables created by GHDL have addition simulation options. The most important ones are listed below:

--assert-level=LVL	stop simulation: none|note|warning|error|failure, i.e. ./touchy_design --assert-level=note
--stop-time=TIME    Stop simuation after TIME, i.e. ./design --stop-time=50ns
--vcd=FILENAME    	Dump VCD to FILENAME (a waveform dump, viewable with--for instance--gtkwave), i.e. ./design --vcd=design.vcd
--sdf=[TYPE=]PATH=FILENAME
    				Back annotate SDF onto design using TYPE (min|typ|max), instance PATH,
    				and SDF file FILENAME, i.e. ./sdf_design --sdf=typ=top/inst=inst.sdf

**GTKWave
zeigt .vcd Dateien an
gtkwave is a viewer for VCD (Value Change Dump) files which are usually created by digital circuit simulators.