microwatt/plru.vhdl

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

entity plru is
    generic (
	BITS : positive := 2
	)
	;
    port (
	clk    : in std_ulogic;
	rst    : in std_ulogic;

	acc    : in std_ulogic_vector(BITS-1 downto 0);
	acc_en : in std_ulogic;
	lru    : out std_ulogic_vector(BITS-1 downto 0)
	);
end entity plru;

architecture rtl of plru is
    constant count : positive := 2 ** BITS - 1;
    subtype node_t is integer range 0 to count;
    type tree_t is array(node_t) of std_ulogic;

    signal tree: tree_t;
begin

    -- XXX Check if we can turn that into a little ROM instead that
    -- takes the tree bit vector and returns the LRU. See if it's better
    -- in term of FPGA resouces usage...
    get_lru: process(all)
	variable node : node_t;
    begin
	node := 0;
	for i in 0 to BITS-1 loop
--	    report "GET: i:" & integer'image(i) & " node:" & integer'image(node) & " val:" & std_ulogic'image(tree(node));
	    lru(BITS-1-i) <= tree(node);
	    if i /= BITS-1 then
		node := node * 2;
		if tree(node) = '1' then
		    node := node + 2;
		else
		    node := node + 1;
		end if;
	    end if;
	end loop;
    end process;

    update_lru: process(clk)
	variable node : node_t;
	variable abit : std_ulogic;
    begin
	if rising_edge(clk) then
	    if rst = '1' then
		tree <= (others => '0');
	    elsif acc_en = '1' then
		node := 0;
		for i in 0 to BITS-1 loop
		    abit := acc(BITS-1-i);
		    tree(node) <= not abit;
--		    report "UPD: i:" & integer'image(i) & " node:" & integer'image(node) & " val" & std_ulogic'image(not abit);
		    if i /= BITS-1 then
			node := node * 2;
			if abit = '1' then
			    node := node + 2;
			else
			    node := node + 1;
			end if;
		    end if;
		end loop;
	    end if;	    
	end if;
    end process;
end;