Strings and Runes in VHDL

-- A VHDL string is a sequence of characters. VHDL uses the
-- IEEE.std_logic_1164 package for basic types and functions.

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity StringsAndCharacters is
end entity StringsAndCharacters;

architecture Behavioral of StringsAndCharacters is
    -- Define a string constant
    constant s : string := "Hello";
begin
    process
        variable char : character;
    begin
        -- Print the length of the string
        report "Len: " & integer'image(s'length);
        
        -- Print each character's ASCII value
        for i in s'range loop
            report "ASCII value of '" & s(i) & "': " & integer'image(character'pos(s(i)));
        end loop;
        
        -- Count characters (always equal to length in VHDL)
        report "Character count: " & integer'image(s'length);
        
        -- Iterate through characters
        for i in s'range loop
            report "Character '" & s(i) & "' at index " & integer'image(i);
        end loop;
        
        -- Demonstrate character comparison
        char := 'H';
        if char = 'H' then
            report "Found H";
        elsif char = 'e' then
            report "Found e";
        end if;
        
        wait;
    end process;
end architecture Behavioral;

In VHDL, strings are simpler than in some other languages. They are essentially arrays of characters. Here’s an explanation of the VHDL code and its concepts:

1. We define a constant string s with the value “Hello”.

2. To get the length of a string, we use the 'length attribute.

3. VHDL doesn’t have the concept of “runes” or Unicode code points. It typically uses ASCII characters. We can get the ASCII value of a character using character'pos().

4. Iterating through a string is done with a for loop, using the 'range attribute to get the valid index range.

5. In VHDL, the character count is always equal to the string length, as each character is represented by one array element.

6. VHDL uses single quotes for character literals and double quotes for string literals.

7. Character comparison is straightforward using the equality operator.

This VHDL code demonstrates basic string and character operations. Note that VHDL doesn’t have built-in support for UTF-8 encoding or Unicode, so handling non-ASCII characters would require additional libraries or custom implementations.

To run this VHDL code, you would typically use a VHDL simulator such as ModelSim or GHDL. The output would be in the form of report statements in the simulation log.

$ ghdl -a strings_and_characters.vhd
$ ghdl -e StringsAndCharacters
$ ghdl -r StringsAndCharacters
Len: 5
ASCII value of 'H': 72
ASCII value of 'e': 101
ASCII value of 'l': 108
ASCII value of 'l': 108
ASCII value of 'o': 111
Character count: 5
Character 'H' at index 1
Character 'e' at index 2
Character 'l' at index 3
Character 'l' at index 4
Character 'o' at index 5
Found H

This example demonstrates basic string and character handling in VHDL, which differs significantly from higher-level programming languages due to VHDL’s nature as a hardware description language.