antonblanchard.microwatt/loadstore2.vhdl

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

library work;
use work.common.all;
use work.helpers.all;
use work.wishbone_types.all;

-- 2 cycle LSU
-- In this cycle we read or write any data and do sign extension and update if required.

entity loadstore2 is
    port (
        clk   : in std_ulogic;

        l_in  : in Loadstore1ToLoadstore2Type;
        w_out : out Loadstore2ToWritebackType;

        m_in  : in wishbone_slave_out;
        m_out : out wishbone_master_out
        );
end loadstore2;

architecture behave of loadstore2 is
    signal l_saved : Loadstore1ToLoadstore2Type;
    signal w_tmp   : Loadstore2ToWritebackType;
    signal m_tmp   : wishbone_master_out;
    signal dlength : std_ulogic_vector(3 downto 0);

    type state_t is (IDLE, WAITING_FOR_READ_ACK, WAITING_FOR_WRITE_ACK);
    signal state   : state_t := IDLE;

    function length_to_sel(length : in std_logic_vector(3 downto 0)) return std_ulogic_vector is
    begin
        case length is
            when "0001" =>
                return "00000001";
            when "0010" =>
                return "00000011";
            when "0100" =>
                return "00001111";
            when "1000" =>
                return "11111111";
            when others =>
                return "00000000";
        end case;
    end function length_to_sel;

    function wishbone_data_shift(address : in std_ulogic_vector(63 downto 0)) return natural is
    begin
        return to_integer(unsigned(address(2 downto 0))) * 8;
    end function wishbone_data_shift;

    function wishbone_data_sel(size : in std_logic_vector(3 downto 0); address : in std_logic_vector(63 downto 0)) return std_ulogic_vector is
    begin
        return std_ulogic_vector(shift_left(unsigned(length_to_sel(size)), to_integer(unsigned(address(2 downto 0)))));
    end function wishbone_data_sel;
begin

    w_out <= w_tmp;
    m_out <= m_tmp;

    loadstore2_0: process(clk)
    begin
        if rising_edge(clk) then

            w_tmp.valid <= '0';
            w_tmp.write_enable <= '0';
            w_tmp.write_reg <= (others => '0');
            w_tmp.write_len <= "1000";
            w_tmp.write_shift <= "000";
            w_tmp.sign_extend <= '0';
            w_tmp.byte_reverse <= '0';
            w_tmp.second_word <= '0';

            l_saved <= l_saved;

            case_0: case state is
                when IDLE =>
                    if l_in.valid = '1' then
                        m_tmp <= wishbone_master_out_init;

                        m_tmp.sel <= wishbone_data_sel(l_in.length, l_in.addr);
                        m_tmp.adr <= l_in.addr(63 downto 3) & "000";
                        m_tmp.cyc <= '1';
                        m_tmp.stb <= '1';

                        l_saved <= l_in;

                        if l_in.load = '1' then
                            m_tmp.we <= '0';

                            -- Load with update instructions write two GPR destinations.
                            -- We don't want the expense of two write ports, so make it
                            -- single in the pipeline and write back the update GPR now
                            -- and the load once we get the data back. We'll have to
                            -- revisit this when loads can take exceptions.
                            if l_in.update = '1' then
                                w_tmp.write_enable <= '1';
                                w_tmp.write_reg <= l_in.update_reg;
                                w_tmp.write_data <= l_in.addr;
                            end if;

                            state <= WAITING_FOR_READ_ACK;
                        else
                            m_tmp.we <= '1';

                            m_tmp.dat <= std_logic_vector(shift_left(unsigned(l_in.data), wishbone_data_shift(l_in.addr)));

                            assert l_in.sign_extend = '0' report "sign extension doesn't make sense for stores" severity failure;

                            state <= WAITING_FOR_WRITE_ACK;
                        end if;
                    end if;

                when WAITING_FOR_READ_ACK =>
                    if m_in.ack = '1' then
                        -- write data to register file
                        w_tmp.valid <= '1';
                        w_tmp.write_enable <= '1';
                        w_tmp.write_data <= m_in.dat;
                        w_tmp.write_reg <= l_saved.write_reg;
                        w_tmp.write_len <= l_saved.length;
                        w_tmp.write_shift <= l_saved.addr(2 downto 0);
                        w_tmp.sign_extend <= l_saved.sign_extend;
                        w_tmp.byte_reverse <= l_saved.byte_reverse;

                        m_tmp <= wishbone_master_out_init;
                        state <= IDLE;
                    end if;

                when WAITING_FOR_WRITE_ACK =>
                    if m_in.ack = '1' then
                        w_tmp.valid <= '1';
                        if l_saved.update = '1' then
                            w_tmp.write_enable <= '1';
                            w_tmp.write_reg <= l_saved.update_reg;
                            w_tmp.write_data <= l_saved.addr;
                        end if;

                        m_tmp <= wishbone_master_out_init;
                        state <= IDLE;
                    end if;
            end case;
        end if;
    end process;
end;