Environment Variables in VHDL

Environment variables are a universal mechanism for conveying configuration information to programs. Let’s look at how to set, get, and list environment variables in VHDL.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_TEXTIO.ALL;
use STD.TEXTIO.ALL;

entity EnvironmentVariables is
end EnvironmentVariables;

architecture Behavioral of EnvironmentVariables is
begin
    process
        variable l : line;
        variable foo_value : string(1 to 1);
        variable bar_value : string(1 to 1);
    begin
        -- To set a key/value pair, we can use the VHPI (VHDL Procedural Interface)
        -- However, for simplicity, we'll simulate setting an environment variable
        foo_value := "1";
        
        -- To get a value for a key, we can use the VHPI
        -- Here, we're simulating getting environment variables
        write(l, string'("FOO: "));
        write(l, foo_value);
        writeline(output, l);
        
        write(l, string'("BAR: "));
        write(l, bar_value);
        writeline(output, l);
        
        -- In VHDL, there's no direct equivalent to listing all environment variables
        -- However, we can simulate it by printing some predefined variables
        writeline(output, l); -- Print a blank line
        write(l, string'("VHDL_VERSION"));
        writeline(output, l);
        write(l, string'("TIME_RESOLUTION"));
        writeline(output, l);
        write(l, string'("FOREIGN"));
        writeline(output, l);
        
        wait;
    end process;
end Behavioral;

In this VHDL code, we’re simulating the behavior of environment variables, as VHDL doesn’t have a built-in concept of environment variables like many software programming languages do.

To set a key/value pair, we’re simply assigning a value to a variable. In a real-world scenario, you might use the VHDL Procedural Interface (VHPI) to interact with the host system’s environment variables.

To get a value for a key, we’re reading from our simulated variables. Again, in a real scenario, you’d use VHPI to read actual environment variables.

VHDL doesn’t have a direct equivalent to listing all environment variables. However, we’ve simulated this by printing some predefined VHDL environment settings.

Running this VHDL code in a simulator would produce output similar to:

FOO: 1
BAR: 

VHDL_VERSION
TIME_RESOLUTION
FOREIGN

Note that the actual behavior with environment variables in VHDL would depend on the specific simulation or synthesis environment you’re using, and might require the use of VHPI or vendor-specific extensions.

In VHDL, environment variables are not as commonly used as in software programming languages. Instead, VHDL typically uses generics, constants, or configuration declarations to pass configuration information to designs.