antonblanchard.microwatt/writeback.vhdl

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

library work;
use work.common.all;
use work.crhelpers.all;

entity writeback is
    port (
        clk          : in std_ulogic;

        e_in         : in Execute1ToWritebackType;
        l_in         : in Loadstore1ToWritebackType;

        w_out        : out WritebackToRegisterFileType;
        c_out        : out WritebackToCrFileType;

        complete_out : out std_ulogic
        );
end entity writeback;

architecture behaviour of writeback is
begin
    writeback_1: process(all)
        variable x : std_ulogic_vector(0 downto 0);
        variable y : std_ulogic_vector(0 downto 0);
        variable w : std_ulogic_vector(0 downto 0);
	variable cf: std_ulogic_vector(3 downto 0);
        variable zero : std_ulogic;
        variable sign : std_ulogic;
	variable scf  : std_ulogic_vector(3 downto 0);
    begin
        x(0) := e_in.valid;
        y(0) := l_in.valid;
        assert (to_integer(unsigned(x)) + to_integer(unsigned(y))) <= 1 severity failure;

        x(0) := e_in.write_enable;
        y(0) := l_in.write_enable;
        assert (to_integer(unsigned(x)) + to_integer(unsigned(y))) <= 1 severity failure;

        w(0) := e_in.write_cr_enable;
        x(0) := (e_in.write_enable and e_in.rc);
        assert (to_integer(unsigned(w)) + to_integer(unsigned(x))) <= 1 severity failure;

        w_out <= WritebackToRegisterFileInit;
        c_out <= WritebackToCrFileInit;

        complete_out <= '0';
        if e_in.valid = '1' or l_in.valid = '1' then
            complete_out <= '1';
        end if;

        if e_in.write_enable = '1' then
            w_out.write_reg <= e_in.write_reg;
            w_out.write_data <= e_in.write_data;
            w_out.write_enable <= '1';
        end if;

        if e_in.write_cr_enable = '1' then
            c_out.write_cr_enable <= '1';
            c_out.write_cr_mask <= e_in.write_cr_mask;
            c_out.write_cr_data <= e_in.write_cr_data;
        end if;

	if e_in.write_xerc_enable = '1' then
            c_out.write_xerc_enable <= '1';
            c_out.write_xerc_data <= e_in.xerc;
	end if;

	if l_in.write_enable = '1' then
            w_out.write_reg <= gpr_to_gspr(l_in.write_reg);
            w_out.write_data <= l_in.write_data;
            w_out.write_enable <= '1';
        end if;

        if l_in.rc = '1' then
            -- st*cx. instructions
            scf(3) := '0';
            scf(2) := '0';
            scf(1) := l_in.store_done;
            scf(0) := l_in.xerc.so;
            c_out.write_cr_enable <= '1';
            c_out.write_cr_mask <= num_to_fxm(0);
            c_out.write_cr_data(31 downto 28) <= scf;
        end if;

        -- Perform CR0 update for RC forms
        -- Note that loads never have a form with an RC bit, therefore this can test e_in.write_data
        if e_in.rc = '1' and e_in.write_enable = '1' then
            sign := e_in.write_data(63);
            zero := not (or e_in.write_data);
            c_out.write_cr_enable <= '1';
            c_out.write_cr_mask <= num_to_fxm(0);
	    cf(3) := sign;
	    cf(2) := not sign and not zero;
	    cf(1) := zero;
	    cf(0) := e_in.xerc.so;
	    c_out.write_cr_data(31 downto 28) <= cf;
        end if;
    end process;
end;