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-- mips_cache_stub.vhdl -- 1-word cache module

--

-- This module has the same interface and logic as a real cache but the cache

-- memory is just 1 word for each of code and data.

--

-- It interfaces the CPU to the following:

--

--  1.- Internal 32-bit-wide BRAM for read only

--  2.- Internal 32-bit I/O bus

--  3.- External 16-bit wide SRAM

--

-- The SRAM memory interface signals are meant to connect directly to FPGA pins

-- and all outputs are registered (tco should be minimal).

-- SRAM data inputs are NOT registered, though. They go through a couple muxes

-- before reaching the first register so watch out for tsetup.

--

-- Obviously this module provides no performance gain; on the contrary, by

-- coupling the CPU to slow external memory (16 bit bus) it actually slows it

-- down. The purpose of this module is just to test the SRAM interface and the

-- cache logic and timing.

--

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

-- External FPGA signals

--

-- This module has signals meant to connect directly to FPGA pins: the SRAM

-- interface. They are either direct register outputs or at most with an

-- intervening 2-mux, in order to minimize the Tco (clock-to-output).

--

-- The Tco of these signals has to be accounted for in the real SRAM interface.

-- For example, under Quartus-2 and with a Cyclone-2 grade -7 device, the

-- worst Tco for the SRAM data pins is below 5 ns, enough to use a 10ns SRAM

-- with a 20 ns clock cycle.

-- Anyway, you need to take care of this yourself (constraints).

--

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

-- Interface to CPU

--

-- 1.- All signals coming from the CPU are registered.

-- 2.- All CPU inputs come directly from a register, or at most have a 2-mux in

--     between.

--

-- This means this block will not degrade the timing performance of the system,

-- as long as its logic is shallower than the current bottleneck (the ALU).

--

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

-- KNOWN TROUBLE:

--

-- Apart from the very rough looks of the code, there's a few known problems:

--

-- 1.- Write cycles too long

--      In order to guarantee setup and hold times for WE controlled write

--      cycles, two extra clock cycles are inserted for each SRAM write access.

--      This is the most reliable way and the easiest but probably not the best.

--      Until I come up with something better, write cycles to SRAM are going

--      to be very slow.

--

-- 2.- Access to unmapped areas will crash the CPU

--      A couple states are missing in the state machine for handling accesses

--      to unmapped areas. I haven't yet decided how to handle that (return

--      zero, trigger trap, mirror another mapped area...).

--

-- 3.- Code refills from SRAM is unimplemented yet

--      To be done for sheer lack of time.

--

-- 4.- Does not work as a real 1-word cache yet

--      That functionality is still missing, all accesses 'miss'. It should be

--      implemented, as a way to test the real cache logic on a small scale.

--

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

use work.mips_pkg.all;

entity mips_cache_stub is

    generic (

        BRAM_ADDR_SIZE : integer := 10;

        SRAM_ADDR_SIZE : integer := 17

    );

    port(

        clk             : in std_logic;

        reset           : in std_logic;

        -- Interface to CPU core

        data_rd_addr    : in std_logic_vector(31 downto 0);

        data_rd         : out std_logic_vector(31 downto 0);

        data_rd_vma     : in std_logic;

        code_rd_addr    : in std_logic_vector(31 downto 2);

        code_rd         : out std_logic_vector(31 downto 0);

        code_rd_vma     : in std_logic;

        data_wr_addr    : in std_logic_vector(31 downto 2);

        byte_we         : in std_logic_vector(3 downto 0);

        data_wr         : in std_logic_vector(31 downto 0);

        mem_wait        : out std_logic;

        cache_enable    : in std_logic;

        -- interface to FPGA i/o devices

        io_rd_data      : in std_logic_vector(31 downto 0);

        io_rd_addr      : out std_logic_vector(31 downto 2);

        io_wr_addr      : out std_logic_vector(31 downto 2);

        io_wr_data      : out std_logic_vector(31 downto 0);

        io_rd_vma       : out std_logic;

        io_byte_we      : out std_logic_vector(3 downto 0);

        -- interface to synchronous 32-bit-wide FPGA BRAM (possibly used as ROM)

        bram_rd_data    : in std_logic_vector(31 downto 0);

        bram_wr_data    : out std_logic_vector(31 downto 0);

        bram_rd_addr    : out std_logic_vector(BRAM_ADDR_SIZE+1 downto 2);

        bram_wr_addr    : out std_logic_vector(BRAM_ADDR_SIZE+1 downto 2);

        bram_byte_we    : out std_logic_vector(3 downto 0);

        bram_data_rd_vma: out std_logic;

        -- interface to asynchronous 16-bit-wide or 8-bit-wide static memory

        sram_address    : out std_logic_vector(SRAM_ADDR_SIZE-1 downto 0);

        sram_data_rd    : in std_logic_vector(15 downto 0);

        sram_data_wr    : out std_logic_vector(15 downto 0);

        sram_byte_we_n  : out std_logic_vector(1 downto 0);

        sram_oe_n       : out std_logic

    );

end entity mips_cache_stub;

architecture stub of mips_cache_stub is

-- Wait state counter -- we're supporting static memory from 10 to >100 ns

subtype t_wait_state_counter is std_logic_vector(2 downto 0);

-- State machine ----------------------------------------------------

type t_cache_state is (

    idle,                       -- Cache hitting, control machine idle

    -- Code refill --------------------------------------------------

    code_refill_bram_0,         -- pc in bram_rd_addr

    code_refill_bram_1,         -- op in bram_rd

    code_refill_bram_2,         -- op in code_rd

    code_refill_sram_0,         -- rd addr in SRAM addr bus (low hword)

    code_refill_sram_1,         -- rd addr in SRAM addr bus (high hword)

    code_refill_sram8_0,        -- rd addr in SRAM addr bus (byte 0)

    code_refill_sram8_1,        -- rd addr in SRAM addr bus (byte 1)

    code_refill_sram8_2,        -- rd addr in SRAM addr bus (byte 2)

    code_refill_sram8_3,        -- rd addr in SRAM addr bus (byte 3)

    code_crash,                 -- tried to run from i/o or something like that

    -- Data refill & write-through ----------------------------------

    data_refill_sram_0,         -- rd addr in SRAM addr bus (low hword)

    data_refill_sram_1,         -- rd addr in SRAM addr bus (high hword)

    data_refill_sram8_0,        -- rd addr in SRAM addr bus (byte 0)

    data_refill_sram8_1,        -- rd addr in SRAM addr bus (byte 1)

    data_refill_sram8_2,        -- rd addr in SRAM addr bus (byte 2)

    data_refill_sram8_3,        -- rd addr in SRAM addr bus (byte 3)

    data_refill_bram_0,         -- rd addr in bram_rd_addr

    data_refill_bram_1,         -- rd data in bram_rd_data

    data_read_io_0,             -- rd addr on io_rd_addr, io_vma active

    data_read_io_1,             -- rd data on io_rd_data

    data_write_io_0,            -- wr addr & data in io_wr_*, io_byte_we active

    data_writethrough_sram_0a,  -- wr addr & data in SRAM buses (low hword)

    data_writethrough_sram_0b,  -- WE asserted

    data_writethrough_sram_0c,  -- WE deasserted

    data_writethrough_sram_1a,  -- wr addr & data in SRAM buses (high hword)

    data_writethrough_sram_1b,  -- WE asserted

    data_writethrough_sram_1c,  -- WE deasserted

    data_ignore_write,          -- hook for raising error flag FIXME untested

    data_ignore_read,           -- hook for raising error flag FIXME untested

    -- Other states -------------------------------------------------

    --code_wait_for_dcache,       -- wait for D-cache to stop using the buses

    bug                         -- caught an error in the state machine

   );

-- Cache state machine state register & next state

signal ps, ns :             t_cache_state;

-- Wait state down-counter, formally part of the state machine register

signal ws_ctr :             t_wait_state_counter;

-- Wait states for memory being accessed

signal ws_value :           t_wait_state_counter;

-- Asserted to initialize the wait state counter

signal load_ws_ctr :        std_logic;

-- Asserted when the wait state counter has reached zero

signal ws_wait_done :       std_logic;

-- CPU interface registers ------------------------------------------

signal data_rd_addr_reg :   t_pc;

signal data_wr_addr_reg :   t_pc;

signal code_rd_addr_reg :   t_pc;

signal data_wr_reg :        std_logic_vector(31 downto 0);

signal byte_we_reg :        std_logic_vector(3 downto 0);

-- SRAM interface ---------------------------------------------------

-- Stores first (high) HW read from SRAM

signal sram_rd_data_reg :   std_logic_vector(31 downto 8);

-- Data read from SRAM, valid in refill_1

signal sram_rd_data :       t_word;

-- I-cache -- most of this is unimplemented -------------------------

subtype t_code_tag is std_logic_vector(23 downto 2);

signal code_cache_tag :     t_code_tag;

signal code_cache_tag_store : t_code_tag;

signal code_cache_store :   t_word;

-- code word read from cache

signal code_cache_rd :      t_word;

-- raised whel code_cache_rd is not valid due to a cache miss

signal code_miss :          std_logic;

-- '1' when the I-cache state machine stalls the pipeline (mem_wait)

signal code_wait :          std_logic;

-- D-cache -- most of this is unimplemented -------------------------

subtype t_data_tag is std_logic_vector(23 downto 2);

signal data_cache_tag :     t_data_tag;

signal data_cache_tag_store : t_data_tag;

signal data_cache_store :   t_word;

-- active when there's a write waiting to be done

signal write_pending :      std_logic;

-- active when there's a read waiting to be done

signal read_pending :       std_logic;

-- data word read from cache

signal data_cache_rd :      t_word;

-- '1' when data_cache_rd is not valid due to a cache miss

signal data_miss :          std_logic;

-- '1' when the D-cache state machine stalls the pipeline (mem_wait)

signal data_wait :          std_logic;

-- Address decoding -------------------------------------------------

-- Address slices used to decode

signal code_rd_addr_mask :  t_addr_decode;

signal data_rd_addr_mask :  t_addr_decode;

signal data_wr_addr_mask :  t_addr_decode;

-- Memory map area being accessed for each of the 3 buses:

signal code_rd_attr :       t_range_attr;

signal data_rd_attr :       t_range_attr;

signal data_wr_attr :       t_range_attr;

begin

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

-- Cache control state machine

cache_state_machine_reg:

process(clk)

begin

   if clk'event and clk='1' then

        if reset='1' then

            ps <= idle;

        else

            ps <= ns;

        end if;

    end if;

end process cache_state_machine_reg;

-- Unified control state machine for I-Cache and D-cache -----------------------

control_state_machine_transitions:

process(ps, code_rd_vma, code_miss,

        data_wr_attr.mem_type, data_rd_attr.mem_type, code_rd_attr.mem_type,

        ws_wait_done,

        write_pending, read_pending)

begin

    case ps is

    when idle =>

        if code_miss='1' then

            case code_rd_attr.mem_type is

            when MT_BRAM        => ns <= code_refill_bram_0;

            when MT_SRAM_16B    => ns <= code_refill_sram_0;

            when MT_SRAM_8B     => ns <= code_refill_sram8_0;

            when others         => ns <= code_crash;

            end case;

        elsif write_pending='1' then

            case data_wr_attr.mem_type is

            when MT_BRAM        => ns <= data_ignore_write;

            when MT_SRAM_16B    => ns <= data_writethrough_sram_0a;

            when MT_IO_SYNC     => ns <= data_write_io_0;

            -- FIXME ignore write to undecoded area (clear pending flag)

            when others         => ns <= ps;

            end case;

        elsif read_pending='1' then

            case data_rd_attr.mem_type is

            when MT_BRAM        => ns <= data_refill_bram_0;

            when MT_SRAM_16B    => ns <= data_refill_sram_0;

            when MT_SRAM_8B     => ns <= data_refill_sram8_0;

            when MT_IO_SYNC     => ns <= data_read_io_0;

            -- FIXME ignore read from undecoded area (clear pending flag)

            when others         => ns <= data_ignore_read;

            end case;

        else

            ns <= ps;

        end if;

    -- Code refill states -------------------------------------------

    when code_refill_bram_0 =>

        ns <= code_refill_bram_1;

    when code_refill_bram_1 =>

        ns <= code_refill_bram_2;

    when code_refill_bram_2 =>

        -- If there's a data operation pending, do it now

        if write_pending='1' then

            case data_wr_attr.mem_type is

            when MT_BRAM        => ns <= data_ignore_write;

            when MT_SRAM_16B    => ns <= data_writethrough_sram_0a;

            when MT_IO_SYNC     => ns <= data_write_io_0;

            -- FIXME ignore write to undecoded area (clear pending flag)

            when others         => ns <= ps;

            end case;

        elsif read_pending='1' then

            case data_rd_attr.mem_type is

            when MT_BRAM        => ns <= data_refill_bram_0;

            when MT_SRAM_16B    => ns <= data_refill_sram_0;

            when MT_SRAM_8B     => ns <= data_refill_sram8_0;

            when MT_IO_SYNC     => ns <= data_read_io_0;

            -- FIXME ignore read from undecoded area (clear pending flag)

            when others         => ns <= data_ignore_read;

            end case;

        else

            ns <= idle;

        end if;

    when code_refill_sram_0 =>

        if ws_wait_done='1' then

            ns <= code_refill_sram_1;

        else

            ns <= ps;

        end if;

    when code_refill_sram_1 =>

        if ws_wait_done='1' then

            -- If there's a data operation pending, do it now

            if write_pending='1' then

                case data_wr_attr.mem_type is

                when MT_BRAM        => ns <= data_ignore_write;

                when MT_SRAM_16B    => ns <= data_writethrough_sram_0a;

                when MT_IO_SYNC     => ns <= data_write_io_0;

                -- FIXME ignore write to undecoded area (clear pending flag)

                when others         => ns <= ps;

                end case;

            elsif read_pending='1' then

                case data_rd_attr.mem_type is

                when MT_BRAM        => ns <= data_refill_bram_0;

                when MT_SRAM_16B    => ns <= data_refill_sram_0;

                when MT_SRAM_8B     => ns <= data_refill_sram8_0;

                when MT_IO_SYNC     => ns <= data_read_io_0;

                -- FIXME ignore read from undecoded area (clear pending flag)

                when others         => ns <= data_ignore_read;

                end case;

            else

                ns <= idle;

            end if;

        else

            ns <= ps;

        end if;

    when code_refill_sram8_0 =>

        if ws_wait_done='1' then

            ns <= code_refill_sram8_1;

        else

            ns <= ps;

        end if;

    when code_refill_sram8_1 =>

        if ws_wait_done='1' then

            ns <= code_refill_sram8_2;

        else

            ns <= ps;

        end if;

    when code_refill_sram8_2 =>

        if ws_wait_done='1' then

            ns <= code_refill_sram8_3;

        else

            ns <= ps;

        end if;

    when code_refill_sram8_3 =>

        if ws_wait_done='1' then

            -- If there's a data operation pending, do it now

            if write_pending='1' then

                case data_wr_attr.mem_type is

                when MT_BRAM        => ns <= data_ignore_write;

                when MT_SRAM_16B    => ns <= data_writethrough_sram_0a;

                when MT_IO_SYNC     => ns <= data_write_io_0;

                -- FIXME ignore write to undecoded area (clear pending flag)

                when others         => ns <= data_ignore_write;

                end case;

            elsif read_pending='1' then

                case data_rd_attr.mem_type is

                when MT_BRAM        => ns <= data_refill_bram_0;

                when MT_SRAM_16B    => ns <= data_refill_sram_0;

                when MT_SRAM_8B     => ns <= data_refill_sram8_0;

                when MT_IO_SYNC     => ns <= data_read_io_0;

                -- FIXME ignore read from undecoded area (clear pending flag)

                when others         => ns <= data_ignore_read;

                end case;

            else

                ns <= idle;

            end if;

        else

            ns <= ps;

        end if;

    -- Data refill & write-through states ---------------------------

    when data_write_io_0 =>

        ns <= idle;

    when data_read_io_0 =>

        ns <= data_read_io_1;

    when data_read_io_1 =>

        ns <= idle;

    when data_refill_sram8_0 =>

        if ws_wait_done='1' then

            ns <= data_refill_sram8_1;

        else

            ns <= ps;

        end if;

    when data_refill_sram8_1 =>

        if ws_wait_done='1' then

            ns <= data_refill_sram8_2;

        else

            ns <= ps;

        end if;

    when data_refill_sram8_2 =>

        if ws_wait_done='1' then

            ns <= data_refill_sram8_3;

        else

            ns <= ps;

        end if;

    when data_refill_sram8_3 =>

        if ws_wait_done='1' then

            ns <= idle;

        else

            ns <= ps;

        end if;

    when data_refill_sram_0 =>

        if ws_wait_done='1' then

            ns <= data_refill_sram_1;

        else

            ns <= ps;

        end if;

    when data_refill_sram_1 =>

        if ws_wait_done='1' then

            ns <= idle;

        else

            ns <= ps;

        end if;

    when data_refill_bram_0 =>

        ns <= data_refill_bram_1;

    when data_refill_bram_1 =>

        ns <= idle;

    when data_writethrough_sram_0a =>

        ns <= data_writethrough_sram_0b;

    when data_writethrough_sram_0b =>

        if ws_wait_done='1' then

            ns <= data_writethrough_sram_0c;

        else

            ns <= ps;

        end if;

    when data_writethrough_sram_0c =>

        ns <= data_writethrough_sram_1a;

    when data_writethrough_sram_1a =>

        ns <= data_writethrough_sram_1b;

    when data_writethrough_sram_1b =>

        if ws_wait_done='1' then

            ns <= data_writethrough_sram_1c;

        else

            ns <= ps;

        end if;

    when data_writethrough_sram_1c =>

        ns <= idle;

    when data_ignore_write =>

        ns <= idle;

    when data_ignore_read =>

        ns <= idle;

    -- Exception states (something went wrong) ----------------------

    when code_crash =>

        -- Attempted to fetch from i/o area. This is a software bug, probably,

        -- and should trigger a trap. We have 1 cycle to do something about it.

        -- After this cycle, back to normal.

        ns <= idle;

    when bug =>

        -- Something weird happened, we have 1 cycle to do something like raise

        -- an error flag, etc. After 1 cycle, back to normal.

        ns <= idle;

    when others =>

        -- We should never arrive here. If we do we handle it in state bug.

        ns <= bug;

    end case;

end process control_state_machine_transitions;

-- load wait state counter when we're entering the state we will wait on

load_ws_ctr <= '1' when

    (ns=code_refill_sram_0  and ps/=code_refill_sram_0) or

    (ns=code_refill_sram_1  and ps/=code_refill_sram_1) or

    (ns=code_refill_sram8_0 and ps/=code_refill_sram8_0) or

    (ns=code_refill_sram8_1 and ps/=code_refill_sram8_1) or

    (ns=code_refill_sram8_2 and ps/=code_refill_sram8_2) or

    (ns=code_refill_sram8_3 and ps/=code_refill_sram8_3) or

    (ns=data_refill_sram_0  and ps/=data_refill_sram_0) or

    (ns=data_refill_sram_1  and ps/=data_refill_sram_1) or

    (ns=data_refill_sram8_0 and ps/=data_refill_sram8_0) or

    (ns=data_refill_sram8_1 and ps/=data_refill_sram8_1) or

    (ns=data_refill_sram8_2 and ps/=data_refill_sram8_2) or

    (ns=data_refill_sram8_3 and ps/=data_refill_sram8_3) or

    (ns=data_writethrough_sram_0a) or

    (ns=data_writethrough_sram_1a)

    else '0';

-- select the wait state counter value as that of read address or write address

with ns select ws_value <=

    data_rd_attr.wait_states    when data_refill_sram_0,

    data_rd_attr.wait_states    when data_refill_sram_1,

    data_rd_attr.wait_states    when data_refill_sram8_0,

    data_rd_attr.wait_states    when data_refill_sram8_1,

    data_rd_attr.wait_states    when data_refill_sram8_2,

    data_rd_attr.wait_states    when data_refill_sram8_3,

    data_wr_attr.wait_states    when data_writethrough_sram_0a,

    data_wr_attr.wait_states    when data_writethrough_sram_1a,

    code_rd_attr.wait_states    when code_refill_sram_0,

    code_rd_attr.wait_states    when code_refill_sram_1,

    code_rd_attr.wait_states    when code_refill_sram8_0,

    code_rd_attr.wait_states    when code_refill_sram8_1,

    code_rd_attr.wait_states    when code_refill_sram8_2,

    code_rd_attr.wait_states    when code_refill_sram8_3,

    data_wr_attr.wait_states    when others;

wait_state_counter_reg:

process(clk)

begin

    if clk'event and clk='1' then

        if reset='1' then

            ws_ctr <= (others => '0');

        else

            if load_ws_ctr='1' then

                ws_ctr <= ws_value;

            elsif ws_wait_done='0' then

                ws_ctr <= ws_ctr - 1;

            end if;

        end if;

    end if;

end process wait_state_counter_reg;

ws_wait_done <= '1' when ws_ctr="000" else '0';

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

-- CPU interface registers and address decoding --------------------------------

-- Everything coming and going to the CPU is registered, so that the CPU has

-- some timing marging.

cpu_data_interface_registers:

process(clk)

begin

    if clk'event and clk='1' then

        if reset='1' then

            write_pending <= '0';

            read_pending <= '0';

            byte_we_reg <= "0000";

        else

            -- Raise 'read_pending' at 1st cycle of a data read, clear it when

            -- the read (and/or refill) operation has been done.

            -- data_rd_addr_reg always has the addr of any pending read

            if data_rd_vma='1' then

                read_pending <= '1';

                data_rd_addr_reg <= data_rd_addr(31 downto 2);

            elsif ps=data_refill_sram_1 or

                  ps=data_refill_sram8_3 or

                  ps=data_refill_bram_1 or

                  ps=data_read_io_0 or

                  ps=data_ignore_read then

                read_pending <= '0';

            end if;

            -- Raise 'write_pending' at the 1st cycle of a write, clear it when

            -- the write (writethrough actually) operation has been done.

            -- data_wr_addr_reg always has the addr of any pending write

            if byte_we/="0000" and ps=idle then

                byte_we_reg <= byte_we;

                data_wr_reg <= data_wr;

                data_wr_addr_reg <= data_wr_addr;

                write_pending <= '1';

            elsif ps=data_writethrough_sram_1b or

                  ps=data_write_io_0 or

                  ps=data_ignore_write then

                write_pending <= '0';

                byte_we_reg <= "0000";

            end if;

        end if;

    end if;

end process cpu_data_interface_registers;

cpu_code_interface_registers:

process(clk)

begin

    if clk'event and clk='1' then

        -- Register code fetch addresses only when they are valid; so that

        -- code_rd_addr_reg always holds the last fetch address.

        if code_rd_vma='1' then

            code_rd_addr_reg <= code_rd_addr;

        end if;

    end if;

end process cpu_code_interface_registers;

-- Address decoding ------------------------------------------------------------

-- Decoding is done on the high bits of the address only, there'll be mirroring.

-- Write to areas not explicitly decoded will be silently ignored. Reads will

-- get undefined data.

code_rd_addr_mask <= code_rd_addr_reg(31 downto t_addr_decode'low);