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library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
 
use work.mips_pkg.all;
 
entity mips_alu is
    port(
        clk             : in std_logic;
        reset           : in std_logic;
 
        -- function selection
        ac              : in t_alu_control;
 
        flags           : out t_alu_flags;
 
        -- '1' when inp1 is zero
        --flags.inp1_eq_zero    : out std_logic;
        -- '1' when inp1 is < 0 (bit 31 set)
        --flags.inp1_lt_zero    : out std_logic;
        -- '1' when (inp1 + ac.neg_sel(inp2) + ac.cy_in) < 0
        --flags.inp1_lt_inp2    : out std_logic;
        -- '1' when (inp1 + ac.neg_sel(inp2) + ac.cy_in) == 0
        --flags.inp1_eq_inp2    : out std_logic;
 
        -- data inputs
        inp1            : in std_logic_vector(31 downto 0);
        inp2            : in std_logic_vector(31 downto 0);
        -- data result output
        outp            : out std_logic_vector(31 downto 0)
    );
end;
 
architecture rtl of mips_alu is
 
subtype t_eword is std_logic_vector(32 downto 0);
 
signal inp2_neg :           t_word;
signal alu_eop1, alu_eop2 : t_eword;
signal sex1, sex2 :         std_logic;
signal alu_arith :          t_eword;
signal alu_shift :          t_word;
signal alu_logic_shift :    t_word;
signal alu_logic :          t_word;
 
signal less_than_zero :     std_logic;
signal final_mux_sel :      std_logic_vector(1 downto 0);
signal alu_temp :           t_word;
 
 
 
begin
 
 
with ac.neg_sel select inp2_neg <= 
    not inp2                        when "01",      -- nor, sub, etc.
    inp2(15 downto 0) & X"0000"     when "10",      -- lhi
    X"00000000"                     when "11",      -- zero
    inp2                            when others;    -- straight
 
sex1 <= inp1(31) when ac.arith_unsigned='0' else '0';
alu_eop1 <= sex1 & inp1;
sex2 <= inp2_neg(31) when ac.arith_unsigned='0' else '0';
alu_eop2 <= sex2 & inp2_neg;
alu_arith <= alu_eop1 + alu_eop2 + ac.cy_in;
 
with ac.logic_sel select alu_logic <= 
    inp1 and inp2_neg       when "00",
    inp1 or  inp2_neg       when "01",
    inp1 xor inp2_neg       when "10",
             inp2_neg       when others;
 
shifter : entity work.mips_shifter
    port map (
        d   => inp2,
        a   => ac.shift_amount,
        fn  => ac.shift_sel,
        r   => alu_shift
    );
 
 
with ac.use_logic select alu_logic_shift <= 
    alu_logic           when "01",
    not alu_logic       when "11",  -- used only by NOR instruction
    alu_shift           when others;
 
 
final_mux_sel(0) <= ac.use_arith when ac.use_slt='0' else less_than_zero;
final_mux_sel(1) <= ac.use_slt;
 
with final_mux_sel select alu_temp <= 
    alu_arith(31 downto 0)  when "01",
    alu_logic_shift         when "00",
    X"00000001"             when "11",
    X"00000000"             when others;
 
less_than_zero <= alu_arith(32);
 
flags.inp1_lt_zero <= inp1(31);
flags.inp1_lt_inp2 <= less_than_zero;
flags.inp1_eq_inp2 <= '1' when alu_arith(31 downto 0)=X"00000000" else '0';
flags.inp1_eq_zero <= '1' when inp1(31 downto 0)=X"00000000" else '0'; -- FIXME simplify
 
outp <= alu_temp;
 
end; --architecture rtl