VHDL “For” Loop Null Range

Question

I've been stuck at this problem for some hours now, and it seems I can't find the solution by searching i.e. didn't find anything here or on Google.

Here's my piece of code:

LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE IEEE.numeric_std;
USE work.arrays.ALL;

ENTITY parallel IS  
  PORT (clk:IN std_logic; text:IN INT_ARRAY(119 DOWNTO 0); result:OUT INT_MATRIX_2D);
END parallel;

ARCHITECTURE arch OF parallel IS
  COMPONENT unit_comparator IS
    PORT (letter:IN integer; difference:OUT integer);
  END COMPONENT;
  SIGNAL temp: INT_MATRIX_2D := (others => (others => 0));
  SIGNAL temp_differences: INT_ARRAY(119 DOWNTO 0) := (others => 0);
  BEGIN
    PROCESS(clk)
    BEGIN
      IF(rising_edge(clk))THEN
        FOR index IN 119 TO 1 LOOP
          temp(temp_differences(index))(temp_differences(index - 1)) <= 
          temp(temp_differences(index))(temp_differences(index - 1)) + 1;
        END LOOP;
        result <= temp;
      END IF;
    END PROCESS;
  wiring_loop: FOR index IN 119 DOWNTO 0 GENERATE
    wiring_unit: unit_comparator PORT MAP (text(index), temp_differences(index));
  END GENERATE;
END arch;

You see that "FOR index IN 119 TO 1 LOOP" ?

The compiler gives a "Range 119 to 0 is Null" warning (no doubt the whole thing doesn't work as it should), which I seem to have a problem understanding. If there is an integer assigned to "index" at each step through the loop, how can it become null (and it says it happens at every step!). I need a firm understanding rather than a plain solution to this. (Note: All of the modules and packages used are tested and work properly!)

Thank you!

Answer 1

Ranges need a direction that corresponds to their limits. You want 119 downto 1 or 1 to 119. 119 to 1 is not a range that is suitable for iterating through.