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The hello world program

To illustrate the large purpose of VHDL, here is a commented VHDL "Hello world" program.

--  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
begin
   process
      variable l : line;
   begin
      l := new String'("Hello world!");
      writeline (output, l);
      wait;
   end process;
end behaviour;

Suppose this program is contained in the file hello.vhdl. First, you have to compile the file; this is called analysis of a design file in VHDL terms.

$ ghdl -a hello.vhdl
This command generates a file hello.o, which is the object file corresponding to your VHDL program. This command also creates or updates a file work-obj93.cf, which describes the library work.

Then, you have to build an executable file.

$ ghdl -e hello_world
The -e option means elaborate. With this option, GHDL creates code in order to elaborate a design, with the hello entity at the top of the hierarchy.

The result is an executable program called hello which can be run:

$ ./hello_world

and which should display:

Hello world!