In VHDL, what does an unconstrained array's index range default to when passed as an argument to a function/procedure?

Question

I'm using a function which returns a user-defined unconstrained array:

type t_array is array(integer range <>) of std_logic_vector(7 downto 0);

This array is then passed to a procedure. Nowhere in my procedure or function have I explicitly defined the range of the array.

What would the array's index range default to when it's passed as an argument?

Answer 1

It will default to whatever you associate with the argument. So, in this example:

library IEEE;
use IEEE.std_logic_1164.all;

entity E is
end entity ;

architecture A of E is
  type t_array is array(integer range <>) of std_logic_vector(7 downto 0);
  procedure P (constant I : in t_array) is
  begin
    report integer'image(I'left);
    report integer'image(I'right);
  end procedure;
begin
  process
    variable V : t_array(0 to 9);
  begin
    P(V);
    wait;
  end process;
end architecture A;

This will be displayed:

# EXECUTION:: NOTE   : 0
# EXECUTION:: Time: 0 ps,  Iteration: 0,  Instance: /E,  Process: line__19.
# EXECUTION:: NOTE   : 9
# EXECUTION:: Time: 0 ps,  Iteration: 0,  Instance: /E,  Process: line__19.

https://www.edaplayground.com/x/2zNy

---

Given that you don't know what the range of the input will be, rather than peppering your code with 'left and 'right attributes and getting in a complete pickle with to and downto, it sometimes helps to normalise the input, eg:

  procedure P (constant I : in t_array) is
    variable normalised_I : t_array(1 to I'length) := I;
  begin
    ...
  end procedure;