Implement data storage interrupts

[microwatt.git] / dcache.vhdl

--
-- Set associative dcache write-through
--
-- TODO (in no specific order):
--
-- * See list in icache.vhdl
-- * Complete load misses on the cycle when WB data comes instead of
--   at the end of line (this requires dealing with requests coming in
--   while not idle...)
--
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

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

entity dcache is
    generic (
        -- Line size in bytes
        LINE_SIZE : positive := 64;
        -- Number of lines in a set
        NUM_LINES : positive := 32;
        -- Number of ways
        NUM_WAYS  : positive := 4;
        -- L1 DTLB entries per set
        TLB_SET_SIZE : positive := 64;
        -- L1 DTLB number of sets
        TLB_NUM_WAYS : positive := 2;
        -- L1 DTLB log_2(page_size)
        TLB_LG_PGSZ : positive := 12
        );
    port (
        clk          : in std_ulogic;
        rst          : in std_ulogic;

        d_in         : in Loadstore1ToDcacheType;
        d_out        : out DcacheToLoadstore1Type;

        stall_out    : out std_ulogic;

        wishbone_out : out wishbone_master_out;
        wishbone_in  : in wishbone_slave_out
        );
end entity dcache;

architecture rtl of dcache is
    -- BRAM organisation: We never access more than wishbone_data_bits at
    -- a time so to save resources we make the array only that wide, and
    -- use consecutive indices for to make a cache "line"
    --
    -- ROW_SIZE is the width in bytes of the BRAM (based on WB, so 64-bits)
    constant ROW_SIZE      : natural := wishbone_data_bits / 8;
    -- ROW_PER_LINE is the number of row (wishbone transactions) in a line
    constant ROW_PER_LINE  : natural := LINE_SIZE / ROW_SIZE;
    -- BRAM_ROWS is the number of rows in BRAM needed to represent the full
    -- dcache
    constant BRAM_ROWS     : natural := NUM_LINES * ROW_PER_LINE;

    -- Bit fields counts in the address

    -- REAL_ADDR_BITS is the number of real address bits that we store
    constant REAL_ADDR_BITS : positive := 56;
    -- ROW_BITS is the number of bits to select a row 
    constant ROW_BITS      : natural := log2(BRAM_ROWS);
    -- ROW_LINEBITS is the number of bits to select a row within a line
    constant ROW_LINEBITS  : natural := log2(ROW_PER_LINE);
    -- LINE_OFF_BITS is the number of bits for the offset in a cache line
    constant LINE_OFF_BITS : natural := log2(LINE_SIZE);
    -- ROW_OFF_BITS is the number of bits for the offset in a row
    constant ROW_OFF_BITS  : natural := log2(ROW_SIZE);
    -- INDEX_BITS is the number if bits to select a cache line
    constant INDEX_BITS    : natural := log2(NUM_LINES);
    -- SET_SIZE_BITS is the log base 2 of the set size
    constant SET_SIZE_BITS : natural := LINE_OFF_BITS + INDEX_BITS;
    -- TAG_BITS is the number of bits of the tag part of the address
    constant TAG_BITS      : natural := REAL_ADDR_BITS - SET_SIZE_BITS;
    -- WAY_BITS is the number of bits to select a way
    constant WAY_BITS     : natural := log2(NUM_WAYS);

    -- Example of layout for 32 lines of 64 bytes:
    --
    -- ..  tag    |index|  line  |
    -- ..         |   row   |    |
    -- ..         |     |---|    | ROW_LINEBITS  (3)
    -- ..         |     |--- - --| LINE_OFF_BITS (6)
    -- ..         |         |- --| ROW_OFF_BITS  (3)
    -- ..         |----- ---|    | ROW_BITS      (8)
    -- ..         |-----|        | INDEX_BITS    (5)
    -- .. --------|              | TAG_BITS      (45)

    subtype row_t is integer range 0 to BRAM_ROWS-1;
    subtype index_t is integer range 0 to NUM_LINES-1;
    subtype way_t is integer range 0 to NUM_WAYS-1;

    -- The cache data BRAM organized as described above for each way
    subtype cache_row_t is std_ulogic_vector(wishbone_data_bits-1 downto 0);

    -- The cache tags LUTRAM has a row per set. Vivado is a pain and will
    -- not handle a clean (commented) definition of the cache tags as a 3d
    -- memory. For now, work around it by putting all the tags
    subtype cache_tag_t is std_logic_vector(TAG_BITS-1 downto 0);
--    type cache_tags_set_t is array(way_t) of cache_tag_t;
--    type cache_tags_array_t is array(index_t) of cache_tags_set_t;
    constant TAG_RAM_WIDTH : natural := TAG_BITS * NUM_WAYS;
    subtype cache_tags_set_t is std_logic_vector(TAG_RAM_WIDTH-1 downto 0);
    type cache_tags_array_t is array(index_t) of cache_tags_set_t;

    -- The cache valid bits
    subtype cache_way_valids_t is std_ulogic_vector(NUM_WAYS-1 downto 0);
    type cache_valids_t is array(index_t) of cache_way_valids_t;

    -- Storage. Hopefully "cache_rows" is a BRAM, the rest is LUTs
    signal cache_tags   : cache_tags_array_t;
    signal cache_valids : cache_valids_t;

    attribute ram_style : string;
    attribute ram_style of cache_tags : signal is "distributed";

    -- L1 TLB.
    constant TLB_SET_BITS : natural := log2(TLB_SET_SIZE);
    constant TLB_WAY_BITS : natural := log2(TLB_NUM_WAYS);
    constant TLB_EA_TAG_BITS : natural := 64 - (TLB_LG_PGSZ + TLB_SET_BITS);
    constant TLB_TAG_WAY_BITS : natural := TLB_NUM_WAYS * TLB_EA_TAG_BITS;
    constant TLB_PTE_BITS : natural := 64;
    constant TLB_PTE_WAY_BITS : natural := TLB_NUM_WAYS * TLB_PTE_BITS;

    subtype tlb_way_t is integer range 0 to TLB_NUM_WAYS - 1;
    subtype tlb_index_t is integer range 0 to TLB_SET_SIZE - 1;
    subtype tlb_way_valids_t is std_ulogic_vector(TLB_NUM_WAYS-1 downto 0);
    type tlb_valids_t is array(tlb_index_t) of tlb_way_valids_t;
    subtype tlb_tag_t is std_ulogic_vector(TLB_EA_TAG_BITS - 1 downto 0);
    subtype tlb_way_tags_t is std_ulogic_vector(TLB_TAG_WAY_BITS-1 downto 0);
    type tlb_tags_t is array(tlb_index_t) of tlb_way_tags_t;
    subtype tlb_pte_t is std_ulogic_vector(TLB_PTE_BITS - 1 downto 0);
    subtype tlb_way_ptes_t is std_ulogic_vector(TLB_PTE_WAY_BITS-1 downto 0);
    type tlb_ptes_t is array(tlb_index_t) of tlb_way_ptes_t;
    type hit_way_set_t is array(tlb_way_t) of way_t;

    signal dtlb_valids : tlb_valids_t;
    signal dtlb_tags : tlb_tags_t;
    signal dtlb_ptes : tlb_ptes_t;
    attribute ram_style of dtlb_tags : signal is "distributed";
    attribute ram_style of dtlb_ptes : signal is "distributed";

    signal r0 : Loadstore1ToDcacheType;
    signal r0_valid : std_ulogic;

    -- Type of operation on a "valid" input
    type op_t is (OP_NONE,
                  OP_LOAD_HIT,      -- Cache hit on load
                  OP_LOAD_MISS,     -- Load missing cache
                  OP_LOAD_NC,       -- Non-cachable load
                  OP_BAD,           -- BAD: Cache hit on NC load/store
                  OP_STORE_HIT,     -- Store hitting cache
                  OP_STORE_MISS);   -- Store missing cache
                      
    -- Cache state machine
    type state_t is (IDLE,             -- Normal load hit processing
                     RELOAD_WAIT_ACK,  -- Cache reload wait ack
                     FINISH_LD_MISS,   -- Extra cycle after load miss
                     STORE_WAIT_ACK,   -- Store wait ack
                     NC_LOAD_WAIT_ACK);-- Non-cachable load wait ack


    --
    -- Dcache operations:
    --
    -- In order to make timing, we use the BRAMs with an output buffer,
    -- which means that the BRAM output is delayed by an extra cycle.
    --
    -- Thus, the dcache has a 2-stage internal pipeline for cache hits
    -- with no stalls.
    --
    -- All other operations are handled via stalling in the first stage.
    --
    -- The second stage can thus complete a hit at the same time as the
    -- first stage emits a stall for a complex op.
    --

    -- First stage register, contains state for stage 1 of load hits
    -- and for the state machine used by all other operations
    --
    type reg_stage_1_t is record
        -- Latch the complete request from ls1
        req              : Loadstore1ToDcacheType;

        -- Cache hit state
        hit_way          : way_t;
        hit_load_valid   : std_ulogic;

        -- Data buffer for "slow" read ops (load miss and NC loads).
        slow_data        : std_ulogic_vector(63 downto 0);
        slow_valid       : std_ulogic;

        -- Signal to complete a failed stcx.
        stcx_fail        : std_ulogic;

        -- Cache miss state (reload state machine)
        state            : state_t;
        wb               : wishbone_master_out;
        store_way        : way_t;
        store_row        : row_t;
        store_index      : index_t;

        -- Signals to complete with error
        error_done       : std_ulogic;
        tlb_miss         : std_ulogic;

        -- completion signal for tlbie
        tlbie_done       : std_ulogic;
    end record;

    signal r1 : reg_stage_1_t;

    -- Reservation information
    --
    type reservation_t is record
        valid : std_ulogic;
        addr  : std_ulogic_vector(63 downto LINE_OFF_BITS);
    end record;

    signal reservation : reservation_t;

    -- Async signals on incoming request
    signal req_index   : index_t;
    signal req_row     : row_t;
    signal req_hit_way : way_t;
    signal req_tag     : cache_tag_t;
    signal req_op      : op_t;
    signal req_data    : std_ulogic_vector(63 downto 0);
    signal req_laddr   : std_ulogic_vector(63 downto 0);

    signal early_req_row  : row_t;

    signal cancel_store : std_ulogic;
    signal set_rsrv     : std_ulogic;
    signal clear_rsrv   : std_ulogic;

    -- Cache RAM interface
    type cache_ram_out_t is array(way_t) of cache_row_t;
    signal cache_out   : cache_ram_out_t;

    -- PLRU output interface
    type plru_out_t is array(index_t) of std_ulogic_vector(WAY_BITS-1 downto 0);
    signal plru_victim : plru_out_t;
    signal replace_way : way_t;

    -- Wishbone read/write/cache write formatting signals
    signal bus_sel     : std_ulogic_vector(7 downto 0);

    -- TLB signals
    signal tlb_tag_way : tlb_way_tags_t;
    signal tlb_pte_way : tlb_way_ptes_t;
    signal tlb_valid_way : tlb_way_valids_t;
    signal tlb_req_index : tlb_index_t;
    signal tlb_hit : std_ulogic;
    signal tlb_hit_way : tlb_way_t;
    signal pte : tlb_pte_t;
    signal ra : std_ulogic_vector(REAL_ADDR_BITS - 1 downto 0);
    signal valid_ra : std_ulogic;

    -- TLB PLRU output interface
    type tlb_plru_out_t is array(tlb_index_t) of std_ulogic_vector(TLB_WAY_BITS-1 downto 0);
    signal tlb_plru_victim : tlb_plru_out_t;

    --
    -- Helper functions to decode incoming requests
    --

    -- Return the cache line index (tag index) for an address
    function get_index(addr: std_ulogic_vector(63 downto 0)) return index_t is
    begin
        return to_integer(unsigned(addr(SET_SIZE_BITS - 1 downto LINE_OFF_BITS)));
    end;

    -- Return the cache row index (data memory) for an address
    function get_row(addr: std_ulogic_vector(63 downto 0)) return row_t is
    begin
        return to_integer(unsigned(addr(SET_SIZE_BITS - 1 downto ROW_OFF_BITS)));
    end;

    -- Returns whether this is the last row of a line
    function is_last_row_addr(addr: wishbone_addr_type) return boolean is
        constant ones : std_ulogic_vector(ROW_LINEBITS-1 downto 0) := (others => '1');
    begin
        return addr(LINE_OFF_BITS-1 downto ROW_OFF_BITS) = ones;
    end;

    -- Returns whether this is the last row of a line
    function is_last_row(row: row_t) return boolean is
        variable row_v : std_ulogic_vector(ROW_BITS-1 downto 0);
        constant ones  : std_ulogic_vector(ROW_LINEBITS-1 downto 0) := (others => '1');
    begin
        row_v := std_ulogic_vector(to_unsigned(row, ROW_BITS));
        return row_v(ROW_LINEBITS-1 downto 0) = ones;
    end;

    -- Return the address of the next row in the current cache line
    function next_row_addr(addr: wishbone_addr_type) return std_ulogic_vector is
        variable row_idx : std_ulogic_vector(ROW_LINEBITS-1 downto 0);
        variable result  : wishbone_addr_type;
    begin
        -- Is there no simpler way in VHDL to generate that 3 bits adder ?
        row_idx := addr(LINE_OFF_BITS-1 downto ROW_OFF_BITS);
        row_idx := std_ulogic_vector(unsigned(row_idx) + 1);
        result := addr;
        result(LINE_OFF_BITS-1 downto ROW_OFF_BITS) := row_idx;
        return result;
    end;

    -- Return the next row in the current cache line. We use a dedicated
    -- function in order to limit the size of the generated adder to be
    -- only the bits within a cache line (3 bits with default settings)
    --
    function next_row(row: row_t) return row_t is
       variable row_v  : std_ulogic_vector(ROW_BITS-1 downto 0);
       variable row_idx : std_ulogic_vector(ROW_LINEBITS-1 downto 0);
       variable result : std_ulogic_vector(ROW_BITS-1 downto 0);
    begin
       row_v := std_ulogic_vector(to_unsigned(row, ROW_BITS));
       row_idx := row_v(ROW_LINEBITS-1 downto 0);
       row_v(ROW_LINEBITS-1 downto 0) := std_ulogic_vector(unsigned(row_idx) + 1);
       return to_integer(unsigned(row_v));
    end;

    -- Get the tag value from the address
    function get_tag(addr: std_ulogic_vector(REAL_ADDR_BITS - 1 downto 0)) return cache_tag_t is
    begin
        return addr(REAL_ADDR_BITS - 1 downto SET_SIZE_BITS);
    end;

    -- Read a tag from a tag memory row
    function read_tag(way: way_t; tagset: cache_tags_set_t) return cache_tag_t is
    begin
        return tagset((way+1) * TAG_BITS - 1 downto way * TAG_BITS);
    end;

    -- Write a tag to tag memory row
    procedure write_tag(way: in way_t; tagset: inout cache_tags_set_t;
                        tag: cache_tag_t) is
    begin
        tagset((way+1) * TAG_BITS - 1 downto way * TAG_BITS) := tag;
    end;

    -- Read a TLB tag from a TLB tag memory row
    function read_tlb_tag(way: tlb_way_t; tags: tlb_way_tags_t) return tlb_tag_t is
        variable j : integer;
    begin
        j := way * TLB_EA_TAG_BITS;
        return tags(j + TLB_EA_TAG_BITS - 1 downto j);
    end;

    -- Write a TLB tag to a TLB tag memory row
    procedure write_tlb_tag(way: tlb_way_t; tags: inout tlb_way_tags_t;
                            tag: tlb_tag_t) is
        variable j : integer;
    begin
        j := way * TLB_EA_TAG_BITS;
        tags(j + TLB_EA_TAG_BITS - 1 downto j) := tag;
    end;

    -- Read a PTE from a TLB PTE memory row
    function read_tlb_pte(way: tlb_way_t; ptes: tlb_way_ptes_t) return tlb_pte_t is
        variable j : integer;
    begin
        j := way * TLB_PTE_BITS;
        return ptes(j + TLB_PTE_BITS - 1 downto j);
    end;

    procedure write_tlb_pte(way: tlb_way_t; ptes: inout tlb_way_ptes_t; newpte: tlb_pte_t) is
        variable j : integer;
    begin
        j := way * TLB_PTE_BITS;
        ptes(j + TLB_PTE_BITS - 1 downto j) := newpte;
    end;

begin

    assert LINE_SIZE mod ROW_SIZE = 0 report "LINE_SIZE not multiple of ROW_SIZE" severity FAILURE;
    assert ispow2(LINE_SIZE)    report "LINE_SIZE not power of 2" severity FAILURE;
    assert ispow2(NUM_LINES)    report "NUM_LINES not power of 2" severity FAILURE;
    assert ispow2(ROW_PER_LINE) report "ROW_PER_LINE not power of 2" severity FAILURE;
    assert (ROW_BITS = INDEX_BITS + ROW_LINEBITS)
        report "geometry bits don't add up" severity FAILURE;
    assert (LINE_OFF_BITS = ROW_OFF_BITS + ROW_LINEBITS)
        report "geometry bits don't add up" severity FAILURE;
    assert (REAL_ADDR_BITS = TAG_BITS + INDEX_BITS + LINE_OFF_BITS)
        report "geometry bits don't add up" severity FAILURE;
    assert (REAL_ADDR_BITS = TAG_BITS + ROW_BITS + ROW_OFF_BITS)
        report "geometry bits don't add up" severity FAILURE;
    assert (64 = wishbone_data_bits)
        report "Can't yet handle a wishbone width that isn't 64-bits" severity FAILURE;

    -- Latch the request in r0 as long as we're not stalling
    stage_0 : process(clk)
    begin
        if rising_edge(clk) then
            if rst = '1' then
                r0.valid <= '0';
            elsif stall_out = '0' then
                r0 <= d_in;
            end if;
        end if;
    end process;

    -- Hold off the request in r0 when stalling,
    -- and cancel it if we get an error in a previous request.
    r0_valid <= r0.valid and not stall_out and not r1.error_done;

    -- TLB
    -- Operates in the second cycle on the request latched in r0.
    -- TLB updates write the entry at the end of the second cycle.
    tlb_read : process(clk)
        variable index : tlb_index_t;
    begin
        if rising_edge(clk) then
            if stall_out = '1' then
                -- keep reading the same thing while stalled
                index := tlb_req_index;
            else
                index := to_integer(unsigned(d_in.addr(TLB_LG_PGSZ + TLB_SET_BITS - 1
                                                       downto TLB_LG_PGSZ)));
            end if;
            tlb_valid_way <= dtlb_valids(index);
            tlb_tag_way <= dtlb_tags(index);
            tlb_pte_way <= dtlb_ptes(index);
        end if;
    end process;

    -- Generate TLB PLRUs
    maybe_tlb_plrus: if TLB_NUM_WAYS > 1 generate
    begin
        tlb_plrus: for i in 0 to TLB_SET_SIZE - 1 generate
            -- TLB PLRU interface
            signal tlb_plru_acc    : std_ulogic_vector(TLB_WAY_BITS-1 downto 0);
            signal tlb_plru_acc_en : std_ulogic;
            signal tlb_plru_out    : std_ulogic_vector(TLB_WAY_BITS-1 downto 0);
        begin
            tlb_plru : entity work.plru
                generic map (
                    BITS => TLB_WAY_BITS
                    )
                port map (
                    clk => clk,
                    rst => rst,
                    acc => tlb_plru_acc,
                    acc_en => tlb_plru_acc_en,
                    lru => tlb_plru_out
                    );

            process(tlb_req_index, tlb_hit, tlb_hit_way, tlb_plru_out)
            begin
                -- PLRU interface
                if tlb_hit = '1' and tlb_req_index = i then
                    tlb_plru_acc_en <= '1';
                else
                    tlb_plru_acc_en <= '0';
                end if;
                tlb_plru_acc <= std_ulogic_vector(to_unsigned(tlb_hit_way, TLB_WAY_BITS));
                tlb_plru_victim(i) <= tlb_plru_out;
            end process;
        end generate;
    end generate;

    tlb_search : process(all)
        variable hitway : tlb_way_t;
        variable hit : std_ulogic;
        variable eatag : tlb_tag_t;
    begin
        tlb_req_index <= to_integer(unsigned(r0.addr(TLB_LG_PGSZ + TLB_SET_BITS - 1
                                                     downto TLB_LG_PGSZ)));
        hitway := 0;
        hit := '0';
        eatag := r0.addr(63 downto TLB_LG_PGSZ + TLB_SET_BITS);
        for i in tlb_way_t loop
            if tlb_valid_way(i) = '1' and
                read_tlb_tag(i, tlb_tag_way) = eatag then
                hitway := i;
                hit := '1';
            end if;
        end loop;
        tlb_hit <= hit and r0_valid;
        tlb_hit_way <= hitway;
        pte <= read_tlb_pte(hitway, tlb_pte_way);
        valid_ra <= tlb_hit or not r0.virt_mode;
        if r0.virt_mode = '1' then
            ra <= pte(REAL_ADDR_BITS - 1 downto TLB_LG_PGSZ) &
                  r0.addr(TLB_LG_PGSZ - 1 downto 0);
        else
            ra <= r0.addr(REAL_ADDR_BITS - 1 downto 0);
        end if;
    end process;

    tlb_update : process(clk)
        variable tlbie : std_ulogic;
        variable tlbia : std_ulogic;
        variable tlbwe : std_ulogic;
        variable repl_way : tlb_way_t;
        variable eatag : tlb_tag_t;
        variable tagset : tlb_way_tags_t;
        variable pteset : tlb_way_ptes_t;
    begin
        if rising_edge(clk) then
            tlbie := '0';
            tlbia := '0';
            tlbwe := '0';
            if r0_valid = '1' and r0.tlbie = '1' then
                if r0.addr(11 downto 10) /= "00" then
                    tlbia := '1';
                elsif r0.addr(9) = '1' then
                    tlbwe := '1';
                else
                    tlbie := '1';
                end if;
            end if;
            if rst = '1' or tlbia = '1' then
                -- clear all valid bits at once
                for i in tlb_index_t loop
                    dtlb_valids(i) <= (others => '0');
                end loop;
            elsif tlbie = '1' then
                if tlb_hit = '1' then
                    dtlb_valids(tlb_req_index)(tlb_hit_way) <= '0';
                end if;
            elsif tlbwe = '1' then
                if tlb_hit = '1' then
                    repl_way := tlb_hit_way;
                else
                    repl_way := to_integer(unsigned(tlb_plru_victim(tlb_req_index)));
                end if;
                eatag := r0.addr(63 downto TLB_LG_PGSZ + TLB_SET_BITS);
                tagset := tlb_tag_way;
                write_tlb_tag(repl_way, tagset, eatag);
                dtlb_tags(tlb_req_index) <= tagset;
                pteset := tlb_pte_way;
                write_tlb_pte(repl_way, pteset, r0.data);
                dtlb_ptes(tlb_req_index) <= pteset;
                dtlb_valids(tlb_req_index)(repl_way) <= '1';
            end if;
        end if;
    end process;

    -- Generate PLRUs
    maybe_plrus: if NUM_WAYS > 1 generate
    begin
        plrus: for i in 0 to NUM_LINES-1 generate
            -- PLRU interface
            signal plru_acc    : std_ulogic_vector(WAY_BITS-1 downto 0);
            signal plru_acc_en : std_ulogic;
            signal plru_out    : std_ulogic_vector(WAY_BITS-1 downto 0);
            
        begin
            plru : entity work.plru
                generic map (
                    BITS => WAY_BITS
                    )
                port map (
                    clk => clk,
                    rst => rst,
                    acc => plru_acc,
                    acc_en => plru_acc_en,
                    lru => plru_out
                    );

            process(req_index, req_op, req_hit_way, plru_out)
            begin
                -- PLRU interface
                if (req_op = OP_LOAD_HIT or
                    req_op = OP_STORE_HIT) and req_index = i then
                    plru_acc_en <= '1';
                else
                    plru_acc_en <= '0';
                end if;
                plru_acc <= std_ulogic_vector(to_unsigned(req_hit_way, WAY_BITS));
                plru_victim(i) <= plru_out;
            end process;
        end generate;
    end generate;

    -- Cache request parsing and hit detection
    dcache_request : process(all)
        variable is_hit  : std_ulogic;
        variable hit_way : way_t;
        variable op      : op_t;
        variable opsel   : std_ulogic_vector(2 downto 0);
        variable go      : std_ulogic;
        variable s_hit   : std_ulogic;
        variable s_tag   : cache_tag_t;
        variable s_pte   : tlb_pte_t;
        variable s_ra    : std_ulogic_vector(REAL_ADDR_BITS - 1 downto 0);
        variable hit_set : std_ulogic_vector(TLB_NUM_WAYS - 1 downto 0);
        variable hit_way_set : hit_way_set_t;
    begin
        -- Extract line, row and tag from request
        req_index <= get_index(r0.addr);
        req_row <= get_row(r0.addr);
        req_tag <= get_tag(ra);

        -- Only do anything if not being stalled by stage 1
        go := r0_valid and not r0.tlbie;

        -- Calculate address of beginning of cache line, will be
        -- used for cache miss processing if needed
        --
        req_laddr <= (63 downto REAL_ADDR_BITS => '0') &
                     ra(REAL_ADDR_BITS - 1 downto LINE_OFF_BITS) &
                     (LINE_OFF_BITS-1 downto 0 => '0');

        -- Test if pending request is a hit on any way
        -- In order to make timing in virtual mode, when we are using the TLB,
        -- we compare each way with each of the real addresses from each way of
        -- the TLB, and then decide later which match to use.
        hit_way := 0;
        is_hit := '0';
        if r0.virt_mode = '1' then
            for j in tlb_way_t loop
                hit_way_set(j) := 0;
                s_hit := '0';
                s_pte := read_tlb_pte(j, tlb_pte_way);
                s_ra := s_pte(REAL_ADDR_BITS - 1 downto TLB_LG_PGSZ) &
                        r0.addr(TLB_LG_PGSZ - 1 downto 0);
                s_tag := get_tag(s_ra);
                for i in way_t loop
                    if go = '1' and cache_valids(req_index)(i) = '1' and
                        read_tag(i, cache_tags(req_index)) = s_tag and
                        tlb_valid_way(j) = '1' then
                        hit_way_set(j) := i;
                        s_hit := '1';
                    end if;
                end loop;
                hit_set(j) := s_hit;
            end loop;
            if tlb_hit = '1' then
                is_hit := hit_set(tlb_hit_way);
                hit_way := hit_way_set(tlb_hit_way);
            end if;
        else
            s_tag := get_tag(r0.addr(REAL_ADDR_BITS - 1 downto 0));
            for i in way_t loop
                if go = '1' and cache_valids(req_index)(i) = '1' and
                    read_tag(i, cache_tags(req_index)) = s_tag then
                    hit_way := i;
                    is_hit := '1';
                end if;
            end loop;
        end if;

        -- The way that matched on a hit               
        req_hit_way <= hit_way;

        -- The way to replace on a miss
        replace_way <= to_integer(unsigned(plru_victim(req_index)));

        -- Combine the request and cache his status to decide what
        -- operation needs to be done
        --
        op := OP_NONE;
        if go = '1' then
            if valid_ra = '1' then
                opsel := r0.load & r0.nc & is_hit;
                case opsel is
                    when "101" => op := OP_LOAD_HIT;
                    when "100" => op := OP_LOAD_MISS;
                    when "110" => op := OP_LOAD_NC;
                    when "001" => op := OP_STORE_HIT;
                    when "000" => op := OP_STORE_MISS;
                    when "010" => op := OP_STORE_MISS;
                    when "011" => op := OP_BAD;
                    when "111" => op := OP_BAD;
                    when others => op := OP_NONE;
                end case;
            else
                op := OP_BAD;
            end if;
        end if;
        req_op <= op;

        -- Version of the row number that is valid one cycle earlier
        -- in the cases where we need to read the cache data BRAM.
        -- If we're stalling then we need to keep reading the last
        -- row requested.
        if stall_out = '0' then
            early_req_row <= get_row(d_in.addr);
        else
            early_req_row <= req_row;
        end if;
    end process;

    -- Wire up wishbone request latch out of stage 1
    wishbone_out <= r1.wb;

    -- Generate stalls from stage 1 state machine
    stall_out <= '1' when r1.state /= IDLE else '0';

    -- Handle load-with-reservation and store-conditional instructions
    reservation_comb: process(all)
    begin
        cancel_store <= '0';
        set_rsrv <= '0';
        clear_rsrv <= '0';
        if r0_valid = '1' and r0.reserve = '1' then
            -- XXX generate alignment interrupt if address is not aligned
            -- XXX or if r0.nc = '1'
            if r0.load = '1' then
                -- load with reservation
                set_rsrv <= '1';
            else
                -- store conditional
                clear_rsrv <= '1';
                if reservation.valid = '0' or
                    r0.addr(63 downto LINE_OFF_BITS) /= reservation.addr then
                    cancel_store <= '1';
                end if;
            end if;
        end if;
    end process;

    reservation_reg: process(clk)
    begin
        if rising_edge(clk) then
            if rst = '1' or clear_rsrv = '1' then
                reservation.valid <= '0';
            elsif set_rsrv = '1' then
                reservation.valid <= '1';
                reservation.addr <= r0.addr(63 downto LINE_OFF_BITS);
            end if;
        end if;
    end process;

    -- Return data for loads & completion control logic
    --
    writeback_control: process(all)
    begin

        -- The mux on d_out.data defaults to the normal load hit case.
        d_out.valid <= '0';
        d_out.data <= cache_out(r1.hit_way);
        d_out.store_done <= '0';
        d_out.error <= '0';
        d_out.tlb_miss <= '0';

        -- We have a valid load or store hit or we just completed a slow
        -- op such as a load miss, a NC load or a store
        --
        -- Note: the load hit is delayed by one cycle. However it can still
        -- not collide with r.slow_valid (well unless I miscalculated) because
        -- slow_valid can only be set on a subsequent request and not on its
        -- first cycle (the state machine must have advanced), which makes
        -- slow_valid at least 2 cycles from the previous hit_load_valid.
        --

        -- Sanity: Only one of these must be set in any given cycle
        assert (r1.slow_valid and r1.stcx_fail) /= '1' report
            "unexpected slow_valid collision with stcx_fail"
            severity FAILURE;
        assert ((r1.slow_valid or r1.stcx_fail) and r1.hit_load_valid) /= '1' report
            "unexpected hit_load_delayed collision with slow_valid"
            severity FAILURE;

        -- Load hit case is the standard path
        if r1.hit_load_valid = '1' then
            report "completing load hit";
            d_out.valid <= '1';
        end if;

        -- error cases complete without stalling
        if r1.error_done = '1' then
            report "completing ld/st with error";
            d_out.error <= '1';
            d_out.tlb_miss <= r1.tlb_miss;
            d_out.valid <= '1';
        end if;

        -- tlbie is handled above and doesn't go through the cache state machine
        if r1.tlbie_done = '1' then
            report "completing tlbie";
            d_out.valid <= '1';
        end if;

        -- Slow ops (load miss, NC, stores)
        if r1.slow_valid = '1' then
            -- If it's a load, enable register writeback and switch
            -- mux accordingly
            --
            if r1.req.load then
                -- Read data comes from the slow data latch
                d_out.data <= r1.slow_data;
            end if;
            d_out.store_done <= '1';

            report "completing store or load miss";
            d_out.valid <= '1';
        end if;

        if r1.stcx_fail = '1' then
            d_out.store_done <= '0';
            d_out.valid <= '1';
        end if;

    end process;

    --
    -- Generate a cache RAM for each way. This handles the normal
    -- reads, writes from reloads and the special store-hit update
    -- path as well.
    --
    -- Note: the BRAMs have an extra read buffer, meaning the output
    --       is pipelined an extra cycle. This differs from the
    --       icache. The writeback logic needs to take that into
    --       account by using 1-cycle delayed signals for load hits.
    --
    rams: for i in 0 to NUM_WAYS-1 generate
        signal do_read  : std_ulogic;
        signal rd_addr  : std_ulogic_vector(ROW_BITS-1 downto 0);
        signal do_write : std_ulogic;
        signal wr_addr  : std_ulogic_vector(ROW_BITS-1 downto 0);
        signal wr_data  : std_ulogic_vector(wishbone_data_bits-1 downto 0);
        signal wr_sel   : std_ulogic_vector(ROW_SIZE-1 downto 0);
        signal dout     : cache_row_t;
    begin
        way: entity work.cache_ram
            generic map (
                ROW_BITS => ROW_BITS,
                WIDTH => wishbone_data_bits,
                ADD_BUF => true
                )
            port map (
                clk     => clk,
                rd_en   => do_read,
                rd_addr => rd_addr,
                rd_data => dout,
                wr_en   => do_write,
                wr_sel  => wr_sel,
                wr_addr => wr_addr,
                wr_data => wr_data
                );
        process(all)
            variable tmp_adr   : std_ulogic_vector(63 downto 0);
            variable reloading : boolean;
        begin
            -- Cache hit reads
            do_read <= '1';
            rd_addr <= std_ulogic_vector(to_unsigned(early_req_row, ROW_BITS));
            cache_out(i) <= dout;

            -- Write mux:
            --
            -- Defaults to wishbone read responses (cache refill),
            --
            -- For timing, the mux on wr_data/sel/addr is not dependent on anything
            -- other than the current state. Only the do_write signal is.
            --
            if r1.state = IDLE then
                -- In IDLE state, the only write path is the store-hit update case
                wr_addr  <= std_ulogic_vector(to_unsigned(req_row, ROW_BITS));
                wr_data  <= r0.data;
                wr_sel   <= r0.byte_sel;
            else
                -- Otherwise, we might be doing a reload or a DCBZ
                if r1.req.dcbz = '1' then
                    wr_data <= (others => '0');
                else
                    wr_data <= wishbone_in.dat;
                end if;
                wr_sel  <= (others => '1');
                wr_addr <= std_ulogic_vector(to_unsigned(r1.store_row, ROW_BITS));
            end if;

            -- The two actual write cases here
            do_write <= '0';
            reloading := r1.state = RELOAD_WAIT_ACK;
            if reloading and wishbone_in.ack = '1' and r1.store_way = i then
                do_write <= '1';
            end if;
            if req_op = OP_STORE_HIT and req_hit_way = i and cancel_store = '0' and
                r1.req.dcbz = '0' then
                assert not reloading report "Store hit while in state:" &
                    state_t'image(r1.state)
                    severity FAILURE;
                do_write <= '1';
            end if;
        end process;
    end generate;

    --
    -- Cache hit synchronous machine for the easy case. This handles load hits.
    -- It also handles error cases (TLB miss, cache paradox)
    --
    dcache_fast_hit : process(clk)
    begin
        if rising_edge(clk) then
            -- If we have a request incoming, we have to latch it as r0.valid
            -- is only set for a single cycle. It's up to the control logic to
            -- ensure we don't override an uncompleted request (for now we are
            -- single issue on load/stores so we are fine, later, we can generate
            -- a stall output if necessary).

            if req_op /= OP_NONE and stall_out = '0' then
                r1.req <= r0;
                report "op:" & op_t'image(req_op) &
                    " addr:" & to_hstring(r0.addr) &
                    " nc:" & std_ulogic'image(r0.nc) &
                    " idx:" & integer'image(req_index) &
                    " tag:" & to_hstring(req_tag) &
                    " way: " & integer'image(req_hit_way);
            end if;

            -- Fast path for load/store hits. Set signals for the writeback controls.
            if req_op = OP_LOAD_HIT then
                r1.hit_way <= req_hit_way;
                r1.hit_load_valid <= '1';
            else
                r1.hit_load_valid <= '0';
            end if;

            if req_op = OP_BAD then
                r1.error_done <= '1';
                r1.tlb_miss <= not valid_ra;
            else
                r1.error_done <= '0';
            end if;

            -- complete tlbies in the third cycle
            r1.tlbie_done <= r0_valid and r0.tlbie;
        end if;
    end process;

    --
    -- Every other case is handled by this state machine:
    --
    --   * Cache load miss/reload (in conjunction with "rams")
    --   * Load hits for non-cachable forms
    --   * Stores (the collision case is handled in "rams")
    --
    -- All wishbone requests generation is done here. This machine
    -- operates at stage 1.
    --
    dcache_slow : process(clk)
        variable tagset    : cache_tags_set_t;
        variable stbs_done : boolean;
    begin
        if rising_edge(clk) then
            -- On reset, clear all valid bits to force misses
            if rst = '1' then
                for i in index_t loop
                    cache_valids(i) <= (others => '0');
                end loop;
                r1.state <= IDLE;
                r1.slow_valid <= '0';
                r1.wb.cyc <= '0';
                r1.wb.stb <= '0';

                -- Not useful normally but helps avoiding tons of sim warnings
                r1.wb.adr <= (others => '0');
            else
                -- One cycle pulses reset
                r1.slow_valid <= '0';
                r1.stcx_fail <= '0';

                -- Main state machine
                case r1.state is
                when IDLE =>
                    case req_op is
                    when OP_LOAD_HIT =>
                        -- stay in IDLE state

                    when OP_LOAD_MISS =>
                        -- Normal load cache miss, start the reload machine
                        --
                        report "cache miss addr:" & to_hstring(r0.addr) &
                            " idx:" & integer'image(req_index) &
                            " way:" & integer'image(replace_way) &
                            " tag:" & to_hstring(req_tag);

                        -- Force misses on that way while reloading that line
                        cache_valids(req_index)(replace_way) <= '0';

                        -- Store new tag in selected way
                        for i in 0 to NUM_WAYS-1 loop
                            if i = replace_way then
                                tagset := cache_tags(req_index);
                                write_tag(i, tagset, req_tag);
                                cache_tags(req_index) <= tagset;
                            end if;
                        end loop;

                        -- Keep track of our index and way for subsequent stores.
                        r1.store_index <= req_index;
                        r1.store_way <= replace_way;
                        r1.store_row <= get_row(req_laddr);

                        -- Prep for first wishbone read. We calculate the address of
                        -- the start of the cache line and start the WB cycle
                        --
                        r1.wb.adr <= req_laddr(r1.wb.adr'left downto 0);
                        r1.wb.sel <= (others => '1');
                        r1.wb.we  <= '0';
                        r1.wb.cyc <= '1';
                        r1.wb.stb <= '1';

                        -- Track that we had one request sent
                        r1.state <= RELOAD_WAIT_ACK;

                    when OP_LOAD_NC =>
                        r1.wb.sel <= r0.byte_sel;
                        r1.wb.adr <= ra(r1.wb.adr'left downto 3) & "000";
                        r1.wb.cyc <= '1';
                        r1.wb.stb <= '1';
                        r1.wb.we <= '0';
                        r1.state <= NC_LOAD_WAIT_ACK;

                    when OP_STORE_HIT | OP_STORE_MISS =>
                        if r0.dcbz = '0' then
                            r1.wb.sel <= r0.byte_sel;
                            r1.wb.adr <= ra(r1.wb.adr'left downto 3) & "000";
                            r1.wb.dat <= r0.data;
                            if cancel_store = '0' then
                                r1.wb.cyc <= '1';
                                r1.wb.stb <= '1';
                                r1.wb.we <= '1';
                                r1.state <= STORE_WAIT_ACK;
                            else
                                r1.stcx_fail <= '1';
                                r1.state <= IDLE;
                            end if;
                        else
                            -- dcbz is handled much like a load miss except
                            -- that we are writing to memory instead of reading
                            r1.store_index <= req_index;
                            r1.store_row <= get_row(req_laddr);

                            if req_op = OP_STORE_HIT then
                                r1.store_way <= req_hit_way;
                            else
                                r1.store_way <= replace_way;

                                -- Force misses on the victim way while zeroing
                                cache_valids(req_index)(replace_way) <= '0';

                                -- Store new tag in selected way
                                for i in 0 to NUM_WAYS-1 loop
                                    if i = replace_way then
                                        tagset := cache_tags(req_index);
                                        write_tag(i, tagset, req_tag);
                                        cache_tags(req_index) <= tagset;
                                    end if;
                                end loop;
                            end if;

                            -- Set up for wishbone writes
                            r1.wb.adr <= req_laddr(r1.wb.adr'left downto 0);
                            r1.wb.sel <= (others => '1');
                            r1.wb.we <= '1';
                            r1.wb.dat <= (others => '0');
                            r1.wb.cyc <= '1';
                            r1.wb.stb <= '1';

                            -- Handle the rest like a load miss
                            r1.state <= RELOAD_WAIT_ACK;
                        end if;

                    -- OP_NONE and OP_BAD do nothing
                    -- OP_BAD was handled above already
                    when OP_NONE =>
                    when OP_BAD =>
                    end case;

                when RELOAD_WAIT_ACK =>
                    -- Requests are all sent if stb is 0
                    stbs_done := r1.wb.stb = '0';

                    -- If we are still sending requests, was one accepted ?
                    if wishbone_in.stall = '0' and not stbs_done then
                        -- That was the last word ? We are done sending. Clear
                        -- stb and set stbs_done so we can handle an eventual last
                        -- ack on the same cycle.
                        --
                        if is_last_row_addr(r1.wb.adr) then
                            r1.wb.stb <= '0';
                            stbs_done := true;
                        end if;

                        -- Calculate the next row address
                        r1.wb.adr <= next_row_addr(r1.wb.adr);
                    end if;

                    -- Incoming acks processing
                    if wishbone_in.ack = '1' then
                        -- Is this the data we were looking for ? Latch it so
                        -- we can respond later. We don't currently complete the
                        -- pending miss request immediately, we wait for the
                        -- whole line to be loaded. The reason is that if we
                        -- did, we would potentially get new requests in while
                        -- not idle, which we don't currently know how to deal
                        -- with.
                        --
                        if r1.store_row = get_row(r1.req.addr) and r1.req.dcbz = '0' then
                            r1.slow_data <= wishbone_in.dat;
                        end if;

                        -- Check for completion
                        if stbs_done and is_last_row(r1.store_row) then
                            -- Complete wishbone cycle
                            r1.wb.cyc <= '0';

                            -- Cache line is now valid
                            cache_valids(r1.store_index)(r1.store_way) <= '1';

                            -- Don't complete and go idle until next cycle, in
                            -- case the next request is for the last dword of
                            -- the cache line we just loaded.
                            r1.state <= FINISH_LD_MISS;
                        end if;

                        -- Increment store row counter
                        r1.store_row <= next_row(r1.store_row);
                    end if;

                when FINISH_LD_MISS =>
                    -- Write back the load data that we got
                    r1.slow_valid <= '1';
                    r1.state <= IDLE;
                    report "completing miss !";

                when STORE_WAIT_ACK | NC_LOAD_WAIT_ACK =>
                    -- Clear stb when slave accepted request
                    if wishbone_in.stall = '0' then
                        r1.wb.stb <= '0';
                    end if;

                    -- Got ack ? complete.
                    if wishbone_in.ack = '1' then
                        if r1.state = NC_LOAD_WAIT_ACK then
                            r1.slow_data <= wishbone_in.dat;
                        end if;
                        r1.state <= IDLE;
                        r1.slow_valid <= '1';
                        r1.wb.cyc <= '0';
                        r1.wb.stb <= '0';
                    end if;
                end case;
            end if;
        end if;
    end process;
end;