-- stack.vhd
-- Date:   Mon Mar  4 09:40:36 2024
-- Author:    Yannick Reiß
-- E-Mail:    yannick.reiss@nickr.eu
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;

entity stack is
  port(
    clk            : in  std_logic;     -- clk
    enable         : in  std_logic;     -- on enable do op
    op             : in  std_logic;     -- 0 = pop, 1 = push
    data_store     : in  std_logic_vector(7 downto 0);  -- Byte to store
    alert_overflow : out std_logic;  -- Signal when stack pointer is at limit
    data_out       : out std_logic_vector(7 downto 0);  -- Byte from memory
    block_parity   : out std_logic      -- parity over all bits
    );
end stack;

architecture Mem of stack is
  type memblock is array(0 to 63) of std_logic_vector(7 downto 0);

  signal memory          : memblock                     := (others => (others => '0'));
  signal stack_pointer   : std_logic_vector(5 downto 0) := (others => '0');
  signal s_parity        : std_logic                    := '0';
begin

  mem_logic : process(all)
  begin
    if rising_edge(clk) then
      if enable = '1' then

        -- read or write memory
        if op = '1' then
          memory(to_integer(unsigned(stack_pointer))) <= data_store;
        end if;

        -- adjust stack pointer
        if op = '1' then
        -- stack_pointer <= std_logic_vector(unsigned(stack_pointer) + 1);
        else
        -- stack_pointer <= std_logic_vector(unsigned(stack_pointer) - 1);
        end if;
      end if;
    end if;
  end process mem_logic;

  set_parity : process(memory, s_parity)
  begin
    for i in memory'range loop
      for j in memory(i)'range loop
        s_parity <= s_parity xor memory(i)(j);
      end loop;
    end loop;
  end process set_parity;

  set_limit : process(clk)
  begin
    if rising_edge(clk) then
      if unsigned(stack_pointer) = 63 then
        alert_overflow <= '1';
      else
        alert_overflow <= '0';
      end if;
    end if;
  end process set_limit;

  block_parity <= s_parity;
  data_out     <= memory(to_integer(unsigned(stack_pointer) - 1));
end Mem;