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-------------------------------------------------------------------------------- -- mips_cache.vhdl -- cache module -- -- This module contains both MIPS caches (I-Cache and D-Cache) combined with -- all the glue logic used to decode and interface external memories and -- devices, both synchronous and asynchronous. -- Everything that goes into or comes from the CPU passes through this module. -- -- The D-Cache is unimplemented in this version, and uses the logic from the -- stub cache module. -- -- Main cache parameters: -- -- I-Cache: 256 4-word lines, direct mapped. -- D-Cache: 256 4-word lines, direct mapped, write-through (UNIMPLEMENTED YET) -- -- The cache works mostly like the R3000 caches, except for the following -- traits: -- -- 1.- When bit CP0[12].17='0' (reset value) the cache is 'disabled'. In this -- state, ALL memory reads miss the cache and force a line refill -- even -- succesive reads from the same line will refill the entire line. This -- simplifies the cache logic a lot but slows uncached code a lot. Which means -- you should initialize the cache and enable it ASAP after reset. -- -- 2.- When bits CP0[12].17:16 = "01", the CPU can invalidate a cache line N -- by writing word N to ANY address. The address will be executed as normal AND -- the cache controller will invalidate I-Cache line N. -- -- Note that the standard behavior for bits 17 and 16 of the SR is not -- implemented at all -- no cache swapping, etc. -- -- 3.- In this version, all areas of memory are cacheable, except those mapped -- as MT_IO_SYNC or MT_UNMAPPEd in mips_pkg. -- Since you can enable or disable the cache at will this difference doesn't -- seem too important. -- There is a 'cacheable' flag in the t_range_attr record which is currently -- unused. -- -- 4.- The tag is only 14 bits long, which means the memory map is severely -- restricted in this version. See @note2. -- -- This is not the standard MIPS way but is compatible enough and above all it -- is simple. -- -------------------------------------------------------------------------------- -- NOTES: -- -- @note1: I-Cache initialization and tag format -- -- In the tag table (code_tag_table), tags are stored together with a 'valid' -- bit (MSB), which is '0' for VALID tags. -- When the CPU invalidates a line, it writes a '1' in the proper tag table -- entry together with the tag value. -- When tags are matched, the valid bit is matched against -- -- -- @note2: I-Cache tags and cache mirroring -- -- To save space in the I-Cache tag table, the tags are shorter than they -- should -- 14 bits instead of the 20 bits we would need to cover the -- entire 32-bit address: -- -- ___________ <-- These address bits are NOT in the tag -- / \ -- 31 .. 27| 26 .. 21 |20 .. 12|11 .. 4|3:2| -- +---------+-----------+-----------------+---------------+---+---+ -- | 5 | | 9 | 8 | 2 | | -- +---------+-----------+-----------------+---------------+---+---+ -- ^ ^ ^ ^- LINE_INDEX_SIZE -- 5 bits 9 bits LINE_NUMBER_SIZE -- -- Since bits 26 downto 21 are not included in the tag, there will be a -- 'mirror' effect in the cache. We have split the memory space -- into 32 separate blocks of 1MB which is obviously not enough but will do -- for the initial tests. -- In subsequen versions of the cache, the tag size needs to be enlarged AND -- some of the top bits might be omitted when they're not needed to implement -- the default memory map (namely bit 30 which is always '0'). -- -- -- @note3: Possible bug in Quartus-II and workaround -- -- I had to put a 'dummy' mux between the cache line store and the CPU in order -- to get rid of a quirk in Quartus-II synthseizer (V9.0 build 235). -- If we omit this extra dummy layer of logic the synth will fail to infer the -- tag table as a BRAM and will use logic fabric instead, crippling performance. -- The mux is otherwise useless and hits performance badly, but so far I haven't -- found any other way to overcome this bug, not even with the helop of the -- Altera support forum. -- -------------------------------------------------------------------------------- -- This module 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 or 8-bit wide static memory (SRAM or FLASH) -- 4.- External 16-bit wide SDRAM (NOT IMPLEMENTED YET) -- -- 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. -- -- This is a work in progress, based on the dummy cache module. -- -------------------------------------------------------------------------------- -- 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 (synthesis 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 PROBLEMS: -- -- 1.- All parameters hardcoded -- generics are almost ignored. -------------------------------------------------------------------------------- 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 is generic ( BRAM_ADDR_SIZE : integer := 10; -- BRAM address size SRAM_ADDR_SIZE : integer := 17; -- Static RAM/Flash address size -- these cache parameters are unused in this implementation, they're -- here for compatibility to the final cache module. LINE_SIZE : integer := 4; -- Line size in words CACHE_SIZE : integer := 256 -- I- and D- cache size in lines ); port( clk : in std_logic; reset : in std_logic; -- Interface to CPU core data_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; 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; ic_invalidate : in std_logic; -- Asserted for 1 cycle after code/data access to unmapped area unmapped : out 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; architecture direct of mips_cache is -- Address of line within line store constant LINE_NUMBER_SIZE : integer := log2(CACHE_SIZE); -- Address of word within line constant LINE_INDEX_SIZE : integer := log2(LINE_SIZE); -- Address of word within line store constant LINE_ADDR_SIZE : integer := LINE_NUMBER_SIZE+LINE_INDEX_SIZE; -- Code tag size, excluding valid bit -- FIXME should be a generic constant CODE_TAG_SIZE : integer := 14; -- Data tag size, excluding valid bit -- FIXME should be a generic constant DATA_TAG_SIZE : integer := 14; -- Wait state counter -- we're supporting static memory from 10 to >100 ns -- (0 to 7 wait states with realistic clock rates). subtype t_wait_state_counter is std_logic_vector(2 downto 0); -- State machine ---------------------------------------------------- type t_cache_state is ( idle, -- Cache is 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 ------------------------------------------------- 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; -- Refill word counters signal code_refill_ctr : integer range 0 to LINE_SIZE-1; signal data_refill_ctr : integer range 0 to LINE_SIZE-1; -- 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 ---------------------------------------------------------- subtype t_line_addr is std_logic_vector(LINE_NUMBER_SIZE-1 downto 0); subtype t_word_addr is std_logic_vector(LINE_ADDR_SIZE-1 downto 0); subtype t_code_tag is std_logic_vector(CODE_TAG_SIZE+1-1 downto 0); type t_code_tag_table is array(CACHE_SIZE-1 downto 0) of t_code_tag; type t_code_line_table is array((CACHE_SIZE*LINE_SIZE)-1 downto 0) of t_word; -- Code tag table (stores line tags) signal code_tag_table : t_code_tag_table := (others => "000000000000000"); -- Code line table (stores lines) signal code_line_table : t_code_line_table := (others => X"00000000"); -- Tag from code fetch address ('target' address, straight from CPU lines) signal code_tag : t_code_tag; -- Registered code_tag, used matching after reading from code_tag_table signal code_tag_reg : t_code_tag; -- Tag read from cache (will be matched against code_tag_reg) signal code_cache_tag : t_code_tag; -- Code cache line address for read and write ports signal code_line_addr : t_line_addr; -- Code cache word address (read from cache) signal code_word_addr : t_word_addr; -- Code cache word address (write to cache in refills) signal code_word_addr_wr : t_word_addr; -- This stuff is part of the workaround for @note3 attribute noprune: boolean; attribute noprune of code_word_addr : signal is true; attribute noprune of code_word_addr_wr : signal is true; attribute noprune of ps : signal is true; -- Word written into code cache signal code_refill_data : t_word; -- Address the code refill data is fetched from signal code_refill_addr : t_pc; -- code word read from cache signal code_cache_rd : t_word; -- raised when code_cache_rd is not valid due to a cache miss signal code_miss : std_logic; -- code_miss for accesses to CACHED areas with cache enabled signal code_miss_cached : std_logic; -- code_miss for accesses to UNCACHED areas OR with cache disabled signal code_miss_uncached : 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, code_refill_ctr, 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 <= 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 <= 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 code_refill_ctr/=0 then -- Still not finished refilling line, go for next word ns <= code_refill_bram_0; else -- 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; 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 code_refill_ctr/=0 and ws_wait_done='1' then -- Still not finished refilling line, go for next word ns <= code_refill_sram_0; else 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; 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 code_refill_ctr/=0 and ws_wait_done='1' then -- Still not finished refilling line, go for next word ns <= code_refill_sram8_0; else 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; 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 => if 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 data_ignore_write => -- Access to unmapped area. We have 1 cycle to do something. ns <= idle; when data_ignore_read => -- Access to unmapped area. We have 1 cycle to do something. 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. -- FIXME raise trap or flag or something 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; -------------------------------------------------------------------------------- -- Wait state logic -- 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'; -------------------------------------------------------------------------------- -- Refill word counters code_refill_word_counter: process(clk) begin if clk'event and clk='1' then if reset='1' or (code_miss='1' and ps=idle) then code_refill_ctr <= LINE_SIZE-1; else if (ps=code_refill_bram_2 or ps=code_refill_sram_1 or ps=code_refill_sram8_3) and ws_wait_done='1' and code_refill_ctr/=0 then code_refill_ctr <= code_refill_ctr-1; end if; end if; end if; end process code_refill_word_counter; -------------------------------------------------------------------------------- -- CPU interface registers and address decoding -------------------------------- -- Everything coming and going to the CPU is registered, so that the CPU has -- some timing marging. These are those registers. -- Besides, we have here a couple of read/write pending flags used to properly -- sequence the cache accesses (first fetch, then any pending r/w). 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_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_addr(31 downto 2); 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; -- The code refill address is that of the current code line, with the running -- refill counter appended: we will read all the words from the line in sequence -- (in REVERSE sequence, actually, see below). code_refill_addr <= code_rd_addr_reg(code_rd_addr_reg'high downto 4) & conv_std_logic_vector(code_refill_ctr,LINE_INDEX_SIZE); -- 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); data_rd_addr_mask <= data_rd_addr_reg(31 downto t_addr_decode'low); data_wr_addr_mask <= data_wr_addr_reg(31 downto t_addr_decode'low); code_rd_attr <= decode_addr(code_rd_addr_mask); data_rd_attr <= decode_addr(data_rd_addr_mask); data_wr_attr <= decode_addr(data_wr_addr_mask); -- Unmapped area access flag, raised for 1 cycle only after each wrong access with ps select unmapped <= '1' when code_crash, '1' when data_ignore_read, '1' when data_ignore_write, '0' when others; -------------------------------------------------------------------------------- -- BRAM interface (BRAM is FPGA Block RAM) -- BRAM address can come from code or data buses, we support code execution -- and data r/w from BRAM. -- (note both inputs to this mux are register outputs) bram_rd_addr <= data_rd_addr_reg(bram_rd_addr'high downto 2) when ps=data_refill_bram_0 else code_refill_addr(bram_rd_addr'high downto 2) ; bram_data_rd_vma <= '1' when ps=data_refill_bram_1 else '0'; -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- -- Code cache -- Most of the code cache is provisional, though it has already been tried on -- hardware. -- CPU is wired directly to cache output, no muxes -- or at least is SHOULD. -- Due to an apparent bug in Quartus-2 (V9.0 build 235), if we omit this extra -- dummy layer of logic the synth will fail to infer the tag table as a BRAM. -- (@note3) code_rd <= code_cache_rd when reset='0' else X"00000000"; -- Register here the requested code tag so we can compare it to the tag in the -- cache store. Note we register and match the 'line valid' bit together with -- the rest of the tag. code_tag_register: process(clk) begin if clk'event and clk='1' then -- Together with the tag value, we register the valid bit against which -- we will match after reading the tag table. -- The valid bit will be '0' for normal accesses or '1' when the cache -- is disabled OR we're invalidating lines. This ensures that the cache -- will miss in those cases. code_tag_reg <= (ic_invalidate or (not cache_enable)) & code_tag(code_tag'high-1 downto 0); end if; end process code_tag_register; -- The I-Cache misses when the tag in the cache is not the tag we want or -- it is not valid. code_miss_cached <= '1' when (code_tag_reg /= code_cache_tag) else '0'; -- When cache is disabled, ALL code fetches will miss uncached_code_miss_logic: process(clk) begin if clk'event and clk='1' then if reset='1' then code_miss_uncached <= '0'; else code_miss_uncached <= code_rd_vma; -- always miss end if; end if; end process uncached_code_miss_logic; -- Select the proper code_miss signal code_miss <= code_miss_uncached when cache_enable='0' else code_miss_cached; -- Code line address used for both read and write into the table code_line_addr <= -- when the CPU wants to invalidate I-Cache lines, the addr comes from the -- data bus (see @note1) data_wr(7 downto 0) when byte_we(3)='1' and ic_invalidate='1' -- otherwise the addr comes from the code address as usual else code_rd_addr(11 downto 4); code_word_addr <= code_rd_addr(11 downto 2); code_word_addr_wr <= code_line_addr & conv_std_logic_vector(code_refill_ctr,LINE_INDEX_SIZE); -- NOTE: the tag will be marked as INVALID ('1') when the CPU is invalidating -- code lines (@note1) code_tag <= (ic_invalidate) & code_rd_addr(31 downto 27) & code_rd_addr(11+CODE_TAG_SIZE-5 downto 11+1); code_tag_memory: process(clk) begin if clk'event and clk='1' then if ps=code_refill_bram_1 or ps=code_refill_sram8_3 or ps=code_refill_sram_1 then code_tag_table(conv_integer(code_line_addr)) <= code_tag; end if; code_cache_tag <= code_tag_table(conv_integer(code_line_addr)); end if; end process code_tag_memory; code_line_memory: process(clk) begin if clk'event and clk='1' then if ps=code_refill_bram_1 or ps=code_refill_sram8_3 or ps=code_refill_sram_1 then code_line_table(conv_integer(code_word_addr_wr)) <= code_refill_data; end if; code_cache_rd <= code_line_table(conv_integer(code_word_addr)); end if; end process code_line_memory; -- Code can only come from BRAM or SRAM (including 16- and 8- bit interfaces) with ps select code_refill_data <= bram_rd_data when code_refill_bram_1, sram_rd_data when others; -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- -- Data cache (unimplemented -- uses stub cache logic) -- CPU data input mux: direct cache output OR uncached io input with ps select data_rd <= io_rd_data when data_read_io_1, data_cache_rd when others; -- All the tag match logic is unfinished and will be simplified away in synth. -- The 'cache' is really a single register. data_cache_rd <= data_cache_store; data_cache_tag <= data_cache_tag_store; data_cache_memory: process(clk) begin if clk'event and clk='1' then if reset='1' then -- in the real hardware the tag store can't be reset and it's up -- to the SW to initialize the cache. data_cache_tag_store <= (others => '0'); data_cache_store <= (others => '0'); else -- Refill data cache if necessary if ps=data_refill_sram_1 or ps=data_refill_sram8_3 then data_cache_tag_store <= "01" & data_rd_addr_reg(t_data_tag'high-2 downto t_data_tag'low); data_cache_store <= sram_rd_data; elsif ps=data_refill_bram_1 then data_cache_tag_store <= "01" & data_rd_addr_reg(t_data_tag'high-2 downto t_data_tag'low); data_cache_store <= bram_rd_data; end if; end if; end if; end process data_cache_memory; -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- -- SRAM interface -- Note this signals are meant to be connected directly to FPGA pins (and then -- to a SRAM, of course). They are the only signals whose tco we care about. -- FIXME should add a SRAM CE\ signal -- SRAM address bus (except for LSB) comes from cpu code or data addr registers sram_address(sram_address'high downto 2) <= data_rd_addr_reg(sram_address'high downto 2) when (ps=data_refill_sram_0 or ps=data_refill_sram_1 or ps=data_refill_sram8_0 or ps=data_refill_sram8_1 or ps=data_refill_sram8_2 or ps=data_refill_sram8_3) else code_refill_addr(sram_address'high downto 2) when (ps=code_refill_sram_0 or ps=code_refill_sram_1 or ps=code_refill_sram8_0 or ps=code_refill_sram8_1 or ps=code_refill_sram8_2 or ps=code_refill_sram8_3) else data_wr_addr_reg(sram_address'high downto 2); -- SRAM addr bus LSB depends on the D-cache state because we read/write the -- halfwords sequentially in successive cycles. sram_address(1) <= '0' when (ps=data_writethrough_sram_0a or ps=data_writethrough_sram_0b or ps=data_writethrough_sram_0c or ps=data_refill_sram8_0 or ps=data_refill_sram8_1 or ps=data_refill_sram_0 or ps=code_refill_sram8_0 or ps=code_refill_sram8_1 or ps=code_refill_sram_0) else '1' when (ps=data_writethrough_sram_1a or ps=data_writethrough_sram_1b or ps=data_writethrough_sram_1c or ps=data_refill_sram8_2 or ps=data_refill_sram8_3 or ps=data_refill_sram_1 or ps=code_refill_sram8_2 or ps=code_refill_sram8_3 or ps=code_refill_sram_1) else '0'; -- The lowest addr bit will only be used when accessing byte-wide memory, and -- even when we're reading word-aligned code (because we need to read the four -- bytes one by one). sram_address(0) <= '0' when (ps=data_refill_sram8_0 or ps=data_refill_sram8_2 or ps=code_refill_sram8_0 or ps=code_refill_sram8_2) else '1'; -- SRAM databus (when used for output) comes from either hword of the data -- write register. with ps select sram_data_wr <= data_wr_reg(31 downto 16) when data_writethrough_sram_0a, data_wr_reg(31 downto 16) when data_writethrough_sram_0b, data_wr_reg(31 downto 16) when data_writethrough_sram_0c, data_wr_reg(15 downto 0) when data_writethrough_sram_1a, data_wr_reg(15 downto 0) when data_writethrough_sram_1b, data_wr_reg(15 downto 0) when data_writethrough_sram_1c, (others => 'Z') when others; -- The byte_we is split in two similarly. with ps select sram_byte_we_n <= not byte_we_reg(3 downto 2) when data_writethrough_sram_0b, not byte_we_reg(1 downto 0) when data_writethrough_sram_1b, "11" when others; -- SRAM OE\ is only asserted low for read cycles sram_oe_n <= '0' when (ps=data_refill_sram_0 or ps=data_refill_sram_1 or ps=data_refill_sram8_0 or ps=data_refill_sram8_1 or ps=data_refill_sram8_2 or ps=data_refill_sram8_3 or ps=code_refill_sram_0 or ps=code_refill_sram_1 or ps=code_refill_sram8_0 or ps=code_refill_sram8_1 or ps=code_refill_sram8_2 or ps=code_refill_sram8_3) else '1'; -- When reading from the SRAM, read word comes from read hword register and -- SRAM bus (read register is loaded in previous cycle). sram_rd_data <= sram_rd_data_reg & sram_data_rd(7 downto 0) when ps=data_refill_sram8_3 or ps=code_refill_sram8_3 else sram_rd_data_reg(31 downto 16) & sram_data_rd; sram_input_halfword_register: process(clk) begin if clk'event and clk='1' then if ps=data_refill_sram_0 or ps=code_refill_sram_0 then sram_rd_data_reg(31 downto 16) <= sram_data_rd; elsif ps=data_refill_sram8_0 or ps=code_refill_sram8_0 then sram_rd_data_reg(31 downto 24) <= sram_data_rd(7 downto 0); elsif ps=data_refill_sram8_1 or ps=code_refill_sram8_1 then sram_rd_data_reg(23 downto 16) <= sram_data_rd(7 downto 0); elsif ps=data_refill_sram8_2 or ps=code_refill_sram8_2 then sram_rd_data_reg(15 downto 8) <= sram_data_rd(7 downto 0); end if; end if; end process sram_input_halfword_register; -------------------------------------------------------------------------------- -- I/O interface -- IO is assumed to behave like synchronous memory io_byte_we <= byte_we_reg when ps=data_write_io_0 else "0000"; io_rd_addr <= data_rd_addr_reg; io_wr_addr <= data_wr_addr_reg; io_wr_data <= data_wr_reg; io_rd_vma <= '1' when ps=data_read_io_0 else '0'; -------------------------------------------------------------------------------- -- CPU stall control -- FIXME data_miss should be raised only on the cycle a data miss is detected, -- otherwise it overlaps data_wait data_miss <= read_pending; -- FIXME stub; will change with real D-Cache -- Stall the CPU when either state machine needs it mem_wait <= (code_wait or data_wait or -- code or data refill in course code_miss or data_miss -- code or data miss ) and not reset; -- FIXME stub -- Assert code_wait until the cycle where the CPU has valid code word on its -- code bus with ps select code_wait <= '1' when code_refill_bram_0, '1' when code_refill_bram_1, '1' when code_refill_bram_2, '1' when code_refill_sram_0, '1' when code_refill_sram_1, '1' when code_refill_sram8_0, '1' when code_refill_sram8_1, '1' when code_refill_sram8_2, '1' when code_refill_sram8_3, '0' when others; -- Assert data_wait until the cycle where the CPU has valid data word on its -- code bus AND no other operations are ongoing that may use the external buses. with ps select data_wait <= '1' when data_writethrough_sram_0a, '1' when data_writethrough_sram_0b, '1' when data_writethrough_sram_0c, '1' when data_writethrough_sram_1a, '1' when data_writethrough_sram_1b, '1' when data_writethrough_sram_1c, '1' when data_refill_sram_0, '1' when data_refill_sram_1, '1' when data_refill_sram8_0, '1' when data_refill_sram8_1, '1' when data_refill_sram8_2, '1' when data_refill_sram8_3, '1' when data_refill_bram_0, '1' when data_refill_bram_1, '1' when data_read_io_0, '0' when others; end architecture direct;
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