Quick Start Guide

In this chapter, you will learn how to use GHDL by working on a few examples.

The ‘Hello world’ program

To illustrate the general purpose of VHDL, here is a commented ‘Hello world’ program which is saved in a file named hello.vhdl:

--  Hello world program
use std.textio.all; -- Imports the standard textio package.

--  Defines a design entity, without any ports.
entity hello_world is
end hello_world;

architecture behaviour of hello_world is
    variable l : line;
    write (l, String'("Hello world!"));
    writeline (output, l);
  end process;
end behaviour;


  • Both .vhdl and .vhd extensions are used for VHDL source files, while .v is used for Verilog.
  • Unless you use especial characters, either UTF-8 or ISO-8859-1 encodings can be used. However, if you do, the latter should be used. The standard defines ASCII (7-bit encoding) or ISO Latin-1 (ISO-8859-1) as default. However, GHDL has a relaxing option, --mb-comments (multi byte), to allow UTF-8 or other encodings in comments.
  • First, you have to compile the file; this is called analysis of a design file in VHDL terms. Run ghdl -a hello.vhdl in the shell. This command creates or updates a file work-obj93.cf, which describes the library work.
  • Then, run ghdl -e hello_world in the shell. Option -e means elaborate, which is used to build a design, with the hello_world entity at the top of the hierarchy.
  • Last, you can directly launch the simulation running ghdl -r hello_world in the shell. The result of the simulation will be shown on screen:
Hello world!


If a GCC/LLVM variant of GHDL is used:

  • Analysis generates a file, hello.o, which is the object file corresponding to your VHDL program. This is not created with mcode.
  • The elaboration step is mandatory after running the analysis and prior to launching the simulation. This will generate an executable binary named hello_world.
  • As a result, -r is just a passthrough to the binary generated in the elaboration. Therefore, the executable can be run directly, ./hello_world. See -r for more informartion.


-e can be bypassed with mcode, since -r actually elaborates the design and saves it on memory before running the simulation. But you can still use it to check for some elaboration problems.

The heartbeat program

entity hello_world is
  port ( clk: out std_logic; )
end hearbeat;

architecture behaviour of hello_world is
  -- Clock process definition
  clk_process: process
    clk <= '0';
    wait for clk_period/2;
    clk <= '1';
    wait for clk_period/2;
  end process;
end behaviour;

A full adder

VHDL is generally used for hardware design. This example starts with a full adder described in a file named adder.vhdl:

entity adder is
  -- `i0`, `i1`, and the carry-in `ci` are inputs of the adder.
  -- `s` is the sum output, `co` is the carry-out.
  port (i0, i1 : in bit; ci : in bit; s : out bit; co : out bit);
end adder;

architecture rtl of adder is
  --  This full-adder architecture contains two concurrent assignments.
  --  Compute the sum.
  s <= i0 xor i1 xor ci;
  --  Compute the carry.
  co <= (i0 and i1) or (i0 and ci) or (i1 and ci);
end rtl;

You can analyze this design file, ghdl -a adder.vhdl, and try to execute the adder design. But this is useless, since nothing externally visible will happen. In order to check this full adder, a testbench has to be run. This testbench is very simple, since the adder is also simple: it checks exhaustively all inputs. Note that only the behaviour is tested, timing constraints are not checked. A file named adder_tb.vhdl contains the testbench for the adder:

--  A testbench has no ports.
entity adder_tb is
end adder_tb;

architecture behav of adder_tb is
  --  Declaration of the component that will be instantiated.
  component adder
    port (i0, i1 : in bit; ci : in bit; s : out bit; co : out bit);
  end component;

  --  Specifies which entity is bound with the component.
  for adder_0: adder use entity work.adder;
  signal i0, i1, ci, s, co : bit;
  --  Component instantiation.
  adder_0: adder port map (i0 => i0, i1 => i1, ci => ci,
                           s => s, co => co);

  --  This process does the real job.
    type pattern_type is record
      --  The inputs of the adder.
      i0, i1, ci : bit;
      --  The expected outputs of the adder.
      s, co : bit;
    end record;
    --  The patterns to apply.
    type pattern_array is array (natural range <>) of pattern_type;
    constant patterns : pattern_array :=
      (('0', '0', '0', '0', '0'),
       ('0', '0', '1', '1', '0'),
       ('0', '1', '0', '1', '0'),
       ('0', '1', '1', '0', '1'),
       ('1', '0', '0', '1', '0'),
       ('1', '0', '1', '0', '1'),
       ('1', '1', '0', '0', '1'),
       ('1', '1', '1', '1', '1'));
    --  Check each pattern.
    for i in patterns'range loop
      --  Set the inputs.
      i0 <= patterns(i).i0;
      i1 <= patterns(i).i1;
      ci <= patterns(i).ci;
      --  Wait for the results.
      wait for 1 ns;
      --  Check the outputs.
      assert s = patterns(i).s
        report "bad sum value" severity error;
      assert co = patterns(i).co
        report "bad carry out value" severity error;
    end loop;
    assert false report "end of test" severity note;
    --  Wait forever; this will finish the simulation.
  end process;
end behav;

As usual, you should analyze the design, ghdl -a adder_tb.vhdl.


Then, if required, elaborate the testbench: ghdl -e adder_tb. You do not need to specify which object files are required, since GHDL knows them and automatically adds them.

Now, it is time to run the testbench, ghdl -r adder_tb, and check the result on screen:

adder_tb.vhdl:52:7:(assertion note): end of test

If your design is rather complex, you’d like to inspect signals. Signal values can be dumped using multiple formats (see section Export waveforms for more information). The resulting file can be read with a wave viewer such as GtkWave.

As explained in the manual, GtkWave ‘relies on a post-mortem approach through the use of dumpfiles’. Therefore, you should first simulate your design and dump a waveform file, say VCD: ghdl -r adder_tb --vcd=adder.vcd. Then, you can view the dump: gtkwave adder.vcd.

See section Simulation options, for more details on other runtime options.

Starting with a design

Unless you are only studying VHDL, you will work with larger designs than the ones of the previous examples. Let’s see how to analyze and run a bigger design, such as the DLX model suite written by Peter Ashenden which is distributed under the terms of the GNU General Public License. A copy is kept on ghdl.free.fr/dlx.tar.gz .

  • First, untar the sources: tar zxvf dlx.tar.gz.


In order not to pollute the sources with the library, it is a good idea to create a work/ subdirectory for the WORK library. To any GHDL commands, we will add the --workdir=work option, so that all files generated by the compiler (except the executable) will be placed in this directory.

$ cd dlx
$ mkdir work
  • Then, we will run the dlx_test_behaviour design. We need to analyze all the design units for the design hierarchy, in the correct order. GHDL provides an easy way to do this, by importing the sources, ghdl -i --workdir=work *.vhdl.
  • GHDL knows all the design units of the DLX, but none of them has been analyzed. Run the make option, ghdl -m --workdir=work dlx_test_behaviour, which analyzes and elaborates a design. This creates many files in the work/ directory, and (GCC/LLVM only) the dlx_test_behaviour executable in the current directory.


The simulation needs to have a DLX program contained in the file dlx.out. This memory image will be loaded in the DLX memory. Just take one sample: cp test_loop.out dlx.out.

  • Now, you can run the test suite: ghdl -r --workdir=work dlx_test_behaviour. The test bench monitors the bus and displays each instruction executed. It finishes with an assertion of severity level note:

    dlx-behaviour.vhdl:395:11:(assertion note): TRAP instruction
     encountered, execution halted
  • Lastly, since the clock is still running, you have to manually stop the program with the C-c key sequence. This behavior prevents you from running the test bench in batch mode. However, you may force the simulator to stop when an assertion above or equal a certain severity level occurs. To do so, call run with this option instead: ghdl -r --workdir=work dlx_test_behaviour --assert-level=note`. With this option, the program stops just after the previous message:

    dlx-behaviour.vhdl:395:11:(assertion note): TRAP instruction
     encountered, execution halted
    error: assertion failed


If you want to make room on your hard drive, you can either:

  • Clean the design library with the GHDL command ghdl --clean --workdir=work. This removes the executable and all the object files. If you want to rebuild the design at this point, just do the make command as shown above.
  • Remove the design library with the GHDL command ghdl --remove --workdir=work. This removes the executable, all the object files and the library file. If you want to rebuild the design, you have to import the sources again and make the design.
  • Remove the work/ directory: rm -rf work. Only the executable is kept. If you want to rebuild the design, create the work/ directory, import the sources, and make the design.


Sometimes, a design does not fully follow the VHDL standards. For example it might use the badly engineered std_logic_unsigned package. GHDL supports this VHDL dialect through some options: --ieee=synopsys -fexplicit. See section IEEE library pitfalls, for more details.