Subversion Repositories lxp32

---------------------------------------------------------------------

-- Instruction cache

--

-- Part of the LXP32 CPU

--

-- Copyright (c) 2016 by Alex I. Kuznetsov

--

-- A simple single-page buffer providing both caching and

-- prefetching capabilities. Useful for high-latency memory,

-- such as external SDRAM.

---------------------------------------------------------------------

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

entity lxp32_icache is

        generic(

                BURST_SIZE: integer;

                PREFETCH_SIZE: integer

        );

        port(

                clk_i: in std_logic;

                rst_i: in std_logic;

                lli_re_i: in std_logic;

                lli_adr_i: in std_logic_vector(29 downto 0);

                lli_dat_o: out std_logic_vector(31 downto 0);

                lli_busy_o: out std_logic;

                wbm_cyc_o: out std_logic;

                wbm_stb_o: out std_logic;

                wbm_cti_o: out std_logic_vector(2 downto 0);

                wbm_bte_o: out std_logic_vector(1 downto 0);

                wbm_ack_i: in std_logic;

                wbm_adr_o: out std_logic_vector(29 downto 0);

                wbm_dat_i: in std_logic_vector(31 downto 0)

        );

end entity;

architecture rtl of lxp32_icache is

signal lli_adr_reg: std_logic_vector(lli_adr_i'range);

signal lli_adr_mux: std_logic_vector(lli_adr_i'range);

signal ram_waddr: std_logic_vector(7 downto 0);

signal ram_raddr: std_logic_vector(7 downto 0);

signal ram_re: std_logic;

signal ram_we: std_logic;

signal read_base: unsigned(21 downto 0);

signal read_offset: unsigned(7 downto 0);

signal init: std_logic:='0';

signal burst1: std_logic;

signal terminate_burst: std_logic;

signal near_miss: std_logic:='0';

signal prefetch_distance: unsigned(7 downto 0);

signal wrap_cnt: integer range 0 to 3:=0;

signal burst_cnt: integer range 0 to BURST_SIZE:=0;

signal wb_stb: std_logic:='0';

signal wb_cti: std_logic_vector(2 downto 0);

-- Note: the following five signals are zero-initialized for

-- simulation only, to suppress warnings from numeric_std.

-- This initialization is not required for synthesis.

signal current_base: unsigned(21 downto 0):=(others=>'0');

signal current_offset: unsigned(7 downto 0):=(others=>'0');

signal prev_base: unsigned(21 downto 0):=(others=>'0');

signal next_base: unsigned(21 downto 0):=(others=>'0');

signal start_offset: unsigned(7 downto 0):=(others=>'0');

signal hitc: std_logic;

signal hitp: std_logic;

signal miss: std_logic:='0';

begin

assert PREFETCH_SIZE>=4

        report "PREFETCH_SIZE cannot be less than 4"

        severity failure;

assert BURST_SIZE>=4

        report "BURST_SIZE cannot be less than 4"

        severity failure;

assert PREFETCH_SIZE+BURST_SIZE<=128

        report "PREFETCH_SIZE and BURST_SIZE combined cannot be greater than 128"

        severity failure;

process (clk_i) is

begin

        if rising_edge(clk_i) then

                if miss='0' then

                        lli_adr_reg<=lli_adr_i;

                end if;

        end if;

end process;

lli_adr_mux<=lli_adr_i when miss='0' else lli_adr_reg;

read_base<=unsigned(lli_adr_mux(29 downto 8));

read_offset<=unsigned(lli_adr_mux(7 downto 0));

-- Cache RAM

ram_waddr<=std_logic_vector(current_offset);

ram_raddr<=std_logic_vector(read_offset);

ram_we<=wb_stb and wbm_ack_i;

ram_re<=lli_re_i or miss;

ram_inst: entity work.lxp32_ram256x32(rtl)

        port map(

                wclk_i=>clk_i,

                we_i=>ram_we,

                waddr_i=>ram_waddr,

                wdata_i=>wbm_dat_i,

                rclk_i=>clk_i,

                re_i=>ram_re,

                raddr_i=>ram_raddr,

                rdata_o=>lli_dat_o

        );

-- Determine hit/miss

-- This cache uses a single ring buffer. Address in buffer corresponds

-- to the lower 8 bits of the full address. The part of the buffer that

-- is higher than current_offset represents a previous block ("p"), the

-- other part represents a current block ("c").

hitc<='1' when read_base=current_base and read_offset<current_offset and

        ((wrap_cnt=1 and read_offset>=start_offset) or

        wrap_cnt=2 or wrap_cnt=3) else '0';

hitp<='1' when read_base=prev_base and read_offset>current_offset and

        ((wrap_cnt=2 and read_offset>=start_offset) or

        wrap_cnt=3) else '0';

process (clk_i) is

begin

        if rising_edge(clk_i) then

                if rst_i='1' then

                        miss<='0';

                else

                        if hitc='0' and hitp='0' and ram_re='1' then

                                miss<='1';

                        else

                                miss<='0';

                        end if;

                end if;

        end if;

end process;

lli_busy_o<=miss;

-- Set INIT flag when the first lli_re_i signal is detected

process (clk_i) is

begin

        if rising_edge(clk_i) then

                if rst_i='1' then

                        init<='0';

                elsif lli_re_i='1' then

                        init<='1';

                end if;

        end if;

end process;

-- Fill cache

prefetch_distance<=current_offset-read_offset;

-- Note: "near_miss" signal prevents cache invalidation when difference

-- between the requested address and the currently fetched address 

-- is too small (and, therefore, the requested data will be fetched soon

-- without invalidation).

process (clk_i) is

begin

        if rising_edge(clk_i) then

                if rst_i='1' then

                        near_miss<='0';

                elsif wrap_cnt>0 and read_offset-current_offset<=to_unsigned(BURST_SIZE/2,8) and

                        ((read_base=current_base and read_offset>=current_offset) or

                        (read_base=next_base and read_offset<current_offset))

                then

                        near_miss<='1';

                else

                        near_miss<='0';

                end if;

        end if;

end process;

terminate_burst<='1' when burst_cnt<BURST_SIZE-1 and miss='1' and

        (burst_cnt>2 or burst1='0') and near_miss='0' else '0';

process (clk_i) is

begin

        if rising_edge(clk_i) then

                if rst_i='1' then

                        burst_cnt<=0;

                        wb_stb<='0';

                        wrap_cnt<=0;

                else

                        if burst_cnt=0 and init='1' then

                                if miss='1' and near_miss='0' then

                                        wb_stb<='1';

                                        wb_cti<="010";

                                        current_offset<=read_offset;

                                        start_offset<=read_offset;

                                        current_base<=read_base;

                                        next_base<=read_base+1;

                                        burst_cnt<=1;

                                        burst1<='1';

                                        wrap_cnt<=1;

                                elsif prefetch_distance<to_unsigned(PREFETCH_SIZE,8) or near_miss='1' then

                                        wb_stb<='1';

                                        wb_cti<="010";

                                        burst_cnt<=1;

                                        burst1<='0';

                                end if;

                        else

                                if wbm_ack_i='1' then

                                        current_offset<=current_offset+1;

                                        if current_offset=X"FF" then

                                                current_base<=next_base;

                                                next_base<=next_base+1;

                                                prev_base<=current_base;

                                                if wrap_cnt<3 then

                                                        wrap_cnt<=wrap_cnt+1;

                                                end if;

                                        end if;

                                        if burst_cnt=BURST_SIZE-1 or terminate_burst='1' then

                                                burst_cnt<=BURST_SIZE;

                                                wb_cti<="111";

                                        elsif burst_cnt<BURST_SIZE-1 then

                                                burst_cnt<=burst_cnt+1;

                                                wb_cti<="010";

                                        else

                                                if miss='1' and near_miss='0' then

                                                        wb_stb<='1';

                                                        wb_cti<="010";

                                                        current_offset<=read_offset;

                                                        start_offset<=read_offset;

                                                        current_base<=read_base;

                                                        next_base<=read_base+1;

                                                        burst_cnt<=1;

                                                        burst1<='1';

                                                        wrap_cnt<=1;

                                                elsif prefetch_distance<to_unsigned(PREFETCH_SIZE,8) or near_miss='1' then

                                                        wb_stb<='1';

                                                        wb_cti<="010";

                                                        burst_cnt<=1;

                                                        burst1<='0';

                                                else

                                                        burst_cnt<=0;

                                                        wb_stb<='0';

                                                end if;

                                        end if;

                                end if;

                        end if;

                end if;

        end if;

end process;

wbm_cyc_o<=wb_stb;

wbm_stb_o<=wb_stb;

wbm_cti_o<=wb_cti;

wbm_bte_o<="00";

wbm_adr_o<=std_logic_vector(current_base&current_offset);

end architecture;