-- ********************************************************
-- * project:     extender
-- * filename:    extender.vhd
-- * author:      << insert your name here >>
-- * date:        MSOE Spring Quarter 2020
-- * provides:    a component to convert 8, 12, or 24-bit
-- *              ARMV4 constants to 32-bit ALU values 
-- * approach:    use a multiplexer to choose 32-bit values
-- *              created from smaller values
-- ********************************************************

-- use library packages 
-- std_logic_1164: 9-valued logic signal voltages 
library ieee;
use ieee.std_logic_1164.all;

-- function block symbol 
-- IMM8 is the 8-bit arithmetic immediate field input
-- IMM12 is the 12-bit load-store immediate field input
-- IMM24 is the 24-bit branch immediate field input
-- EXTS selects the size of extension based on instruction
-- IMM32 is the extended constant output for use by the ALU
entity EXTENDER is 
port(IMM8:  in std_logic_vector(7 downto 0);
     IMM12: in std_logic_vector(11 downto 0);
     IMM24: in std_logic_vector(23 downto 0);
     EXTS:  in std_logic_vector(1 downto 0);
     IMM32: out std_logic_vector(31 downto 0));
end entity EXTENDER;

-- circuit description 
architecture MULTIPLEXER of EXTENDER is 
begin

   -- truth table 
   -- START WITH IMM32 OUTPUT AS ALL ZEROS
   -- USE CONCATENATION & TO FORM OUTPUT
   -- EXTS  SELECTS THIS BEHAVIOR                                     
   -- 0     put IMM8 into the lower 8-bits of IMM32
   -- 1     put IMM12 into the lower 12 bits of IMM32
   -- 2     put IMM24 into IMM32(25 downto 2)
   --       and set IMM32(31 downto 26) = IMM24(23) 
   -- 3     unused: output IMM32 = 0
   with EXTS select 
   IMM32 <= B"000000000000000000000000"&IMM8(7 downto 0) when B"00", -- ALU
            B"                        "                   when B"01", -- complete
            
            IMM24(23)&IMM24(23)                           when B"10", -- complete
            B"                         "                  when others; -- complete
   
 end architecture MULTIPLEXER;