Exit in VHDL

Here’s an idiomatic VHDL example that demonstrates the concept of exiting a process:

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

entity ExitExample is
    Port ( clk : in STD_LOGIC;
           reset : in STD_LOGIC;
           output : out STD_LOGIC_VECTOR(7 downto 0));
end ExitExample;

architecture Behavioral of ExitExample is
    signal counter : unsigned(7 downto 0) := (others => '0');
begin
    process(clk, reset)
    begin
        if reset = '1' then
            counter <= (others => '0');
        elsif rising_edge(clk) then
            -- Increment counter
            counter <= counter + 1;
            
            -- Exit the process when counter reaches 100
            if counter = 100 then
                -- This is equivalent to using 'exit' in a loop
                -- VHDL doesn't have a direct equivalent to Go's os.Exit()
                report "Counter reached 100. Exiting process." severity failure;
            end if;
        end if;
    end process;

    output <= std_logic_vector(counter);
end Behavioral;

This VHDL code demonstrates a concept similar to exiting a program, although it’s important to note that VHDL, being a hardware description language, doesn’t have a direct equivalent to Go’s os.Exit() function.

Here’s what the code does:

1. We define an entity ExitExample with clock and reset inputs, and an 8-bit output.
2. In the architecture, we declare a counter signal.
3. The main process is sensitive to the clock and reset signals.
4. On each rising edge of the clock, we increment the counter.
5. When the counter reaches 100, we use a report statement with severity failure to simulate an “exit” condition.

The report statement with severity failure will cause the simulation to stop, which is the closest equivalent to exiting a program in VHDL. This is typically used for testbench purposes or to catch critical conditions during simulation.

To use this code:

1. Save it in a file named ExitExample.vhd.

2. Compile it using your VHDL compiler. For example, if you’re using GHDL:

   $ ghdl -a ExitExample.vhdl
   $ ghdl -e ExitExample

3. Create a testbench to simulate this entity and run it:

   $ ghdl -r ExitExample --stop-time=1us

You should see output similar to this when the simulation reaches the exit condition:

ExitExample.vhd:25:17:@1000ns:(report failure): Counter reached 100. Exiting process.

This example demonstrates how to simulate an “exit” condition in VHDL, which is conceptually similar to exiting a program in other languages, while adhering to VHDL’s hardware-oriented paradigm.