Hello world program

To illustrate the general purpose of VHDL, the following block 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
begin
  process
    variable l : line;
  begin
    write (l, String'("Hello world!"));
    writeline (output, l);
    wait;
  end process;
end behaviour;

Tip

  • Both .vhdl and .vhd extensions are used for VHDL source files, while .v is used for Verilog.

    • Since, extension .vhd is also interpreted as a Virtual Hard Disk file format, some users prefer .vhdl, to avoid ambiguity. This is the case with GHDL’s codebase. However, in order to maintain backward-compatibility with legacy DOS systems, other users prefer .vhd.
  • 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. Command -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!

Hint

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. These kind of object files can be compiled into foreign programs (see Linking GHDL object files to Ada/C).
  • 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.

Hint

-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.