-- instructionMemory.vhd
-- Created on: Di 26. Sep 07:43:20 CEST 2023
-- Author(s): Yannick Reiß
-- Content: Instruction memory; Read and write operations are controlled externally.
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

-- Entity instructionMemory: Currently ROM; TODO: Add write enable when implementing a bus.
entity instructionMemory is

    port(
        clk                 :   in  std_logic;      -- clock with speed of board clock; Read on clock cycle
        instructionAddr     :   in  std_logic_vector(7 downto 0); -- We start with 256 instructions

        instruction         :   out std_logic_vector(2 downto 0)  -- instruction in current cell
    );
end instructionMemory;

-- Architecture arch of instructionMemory: read on every clock cycle to instruction.
architecture arch of instructionMemory is
    
    type imem is array(0 to 255) of std_logic_vector(2 downto 0);
    -- [+.]
    signal memory : imem := (b"110", b"010", b"101", b"111", others => "000");
begin
    -- Process clk_read
    clk_read : process (clk) -- runs only, when clk changed
    begin

        if rising_edge(clk) then

            instruction <= memory(to_integer(unsigned(instructionAddr)));

        end if;
    end process;

end arch;