ARM_Computer/instructiondecoder.vhd at master · CodeOx/ARM_Computer · GitHub

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library IEEE;
use IEEE.STD_LOGIC_1164.ALL;


entity instructiondecoder is
    Port ( ins : in STD_LOGIC_VECTOR (31 downto 0);
           ins_type : out STD_LOGIC_VECTOR (1 downto 0);
           ins_subtype : out STD_LOGIC_VECTOR (2 downto 0);
           ins_variant : out STD_LOGIC_VECTOR (1 downto 0);
           undefined_encoding: out STD_LOGIC);
end instructiondecoder;
-- 00 DP
--      000 test/cmp
--      001 arith/logical
-- variants
--      00 imm
--      01 imm specified shift
--      10 reg specified shift

-- 01 DT
--      000 -> ldr
--      001 -> ldrh
--      010 -> ldrb
--      011 -> ldrsh
--      100 -> ldrsb
--      101 -> str
--      110 -> strh
--      111 -> strb
-- variants
--      00 reg specified offset
--      01 imm specified offset

-- 10 MUL/MLA
--      000 mul
--      001 mla

-- 11 B/BL/SWI_go/SWI_return
--      000 b
--      001 bl
--      010 SWI_go
--      011 SWI_ret
-- \    100 SWI_writeIns
--      111 SWI_exit

architecture Behavioral of instructiondecoder is

    signal undefined : STD_LOGIC := '0';

begin

    undefined_encoding <= undefined;

process (ins)
begin
    if ins(27 downto 26)="00" then
        if ins(25) = '1'then
            ins_type <= "00";
            ins_variant <="00";
            undefined <= '0';
            if ins(24 downto 23)="10" then
                ins_subtype<="000";
            else
                ins_subtype <="001";
            end if;


        else
            if ins(7 downto 4)="1001" then
                ins_type<= "10";
                undefined <= ins(23) or ins(24);
                ins_subtype <= "00" & ins(21);
            elsif ins(7)='1' and ins(4)='1' and ins(6 downto 5) /= "00" then
                ins_type<= "01";
                if ins(20)='1' then
                    if ins(6 downto 5) = "01" then
                        ins_subtype <="001";
                    elsif ins(6 downto 5) = "10" then
                        ins_subtype <= "100";
                    else
                        ins_subtype <= "011";
                    end if;
               else
                   ins_subtype <= "110";
               end if;
               ins_variant <= '0' & ins(22);
               undefined <= ins(5)and ins(6);

            else
                ins_type <= "00";
                if ins(25)='1' then
                    ins_variant <="00";
                elsif ins(4) <= '0' then
                    ins_variant <="01";
                else
                    ins_variant <="10";
                end if;

                if ins(11 downto 8)="1111" then
                    undefined <= '1';
                else
                    undefined <= '0';
                end if;

                if ins(24 downto 23)="10" then
                    ins_subtype<="000";
                else
                    ins_subtype <="001";
                end if;

            end if;
        end if;

    elsif ins(27 downto 26)="01" then
        ins_type<="01";
        if ins(20)='1' then
            if ins(22)='0' then
                ins_subtype <= "000";
            else
                ins_subtype <= "010";
            end if;
         else
            if ins(22)='0' then
                 ins_subtype <= "101";
             else
                 ins_subtype <= "111";
             end if;
         end if;
         ins_variant <= '0' & ins(25);
         undefined <= ins(25) and ins(4);

    elsif ins(27 downto 26)="10" then
        if ins(25)='0' then
            undefined <= '1';
        else
            ins_type <= "11";
            ins_subtype <="00" & ins (24);
            undefined <= '0';
        end if;
    else
        if ins(25 downto 24) = "11" then
            undefined <= '0';
            ins_type <= "11";
            if ins(23) = '0' then
                ins_subtype <= "010";
            elsif ins(23 downto 0) = "111111111111111111111111" then
                ins_subtype <= "111";
            elsif ins(23 downto 22) = "11" then
                ins_subtype <= "100";
            else
                ins_subtype <= "011";
            end if;
        else
            undefined <= '1';
        end if;
    end if;
end process;
end Behavioral;