Pointers in VHDL

VHDL doesn’t have direct support for pointers as found in languages like C or Go. However, we can demonstrate similar concepts using signals and variable assignments. In VHDL, we use signals for communication between processes and entities, which can be thought of as a form of reference passing.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity Pointers is
end Pointers;

architecture Behavioral of Pointers is
    signal i : integer := 1;
    
    -- This procedure takes a signal (similar to a reference)
    procedure zeroptr(signal x : inout integer) is
    begin
        x <= 0;
    end procedure;
    
    -- This procedure takes an integer value
    procedure zeroval(variable x : in integer) is
        variable temp : integer;
    begin
        temp := x;
        temp := 0;
        -- Note: This won't affect the original value
    end procedure;

begin
    process
        variable temp : integer;
    begin
        report "initial: " & integer'image(i);
        
        temp := i;
        zeroval(temp);
        report "zeroval: " & integer'image(i);
        
        zeroptr(i);
        wait for 1 ns;  -- Wait for signal to update
        report "zeroptr: " & integer'image(i);
        
        wait;
    end process;

end Behavioral;

In this VHDL example, we demonstrate concepts similar to value passing and reference passing:

1. We define a signal i which is similar to a global variable that can be accessed and modified by different processes.

2. The zeroval procedure takes an integer value. Changes to this value inside the procedure don’t affect the original variable, similar to pass-by-value in other languages.

3. The zeroptr procedure takes a signal, which is similar to pass-by-reference. Changes to this signal inside the procedure will affect the original signal.

4. In the main process, we first print the initial value of i.

5. We then call zeroval with a temporary variable. This doesn’t affect the original i signal.

6. Next, we call zeroptr with the i signal. This changes the value of i to 0.

7. We use wait for 1 ns to allow time for the signal assignment to take effect, as signal assignments in VHDL are not immediate.

8. Finally, we print the updated value of i.

To run this VHDL code, you would typically use a VHDL simulator like ModelSim or GHDL. The simulation would show the values of i changing over time, demonstrating the difference between value passing and signal (reference) passing in VHDL.

Note that VHDL doesn’t have a direct equivalent to printing memory addresses as seen in the original example, as VHDL is typically used for hardware description where the concept of memory addresses is not directly applicable.