Subversion Repositories tinycpu

--Core module. 

--This module is basically connects everything and decodes the opcodes.

--The only thing above this is toplevel.vhd which actually sets the pinout for the FPGA

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

use work.tinycpu.all;

entity core is

  port(

    --memory interface 

    MemAddr: out std_logic_vector(15 downto 0); --memory address (in bytes)

    MemWW: out std_logic; --memory writeword

    MemWE: out std_logic; --memory writeenable

    MemIn: in std_logic_vector(15 downto 0);

    MemOut: out std_logic_vector(15 downto 0);

    --general interface

    Clock: in std_logic;

    Reset: in std_logic; --When this is high, CPU will reset within 1 clock cycles. 

    --Enable: in std_logic; --When this is high, the CPU executes as normal, when low the CPU stops at the next clock cycle(maintaining all state)

    Hold: in std_logic; --when high, CPU pauses execution and places Memory interfaces into high impendance state so the memory can be used by other components

    HoldAck: out std_logic; --when high, CPU acknowledged hold and buses are in high Z

    --todo: port interface

    --debug ports:

    DebugIR: out std_logic_vector(15 downto 0); --current instruction

    DebugIP: out std_logic_vector(7 downto 0); --current IP

    DebugCS: out std_logic_vector(7 downto 0); --current code segment

    DebugTR: out std_logic; --current value of TR

    DebugR0: out std_logic_vector(7 downto 0)

   );

end core;

architecture Behavioral of core is

  component fetch is

    port(

      Enable: in std_logic;

      AddressIn: in std_logic_vector(15 downto 0);

      Clock: in std_logic;

      DataIn: in std_logic_vector(15 downto 0); --interface from memory

      IROut: out std_logic_vector(15 downto 0);

      AddressOut: out std_logic_vector(15 downto 0) --interface to memory

    );

  end component;

  component alu is

    port(

      Op: in std_logic_vector(4 downto 0);

      DataIn1: in std_logic_vector(7 downto 0);

      DataIn2: in std_logic_vector(7 downto 0);

      DataOut: out std_logic_vector(7 downto 0);

      TR: out std_logic

    );

  end component;

  component carryover is

    port(

      EnableCarry: in std_logic; --When disabled, SegmentIn goes to SegmentOut

      DataIn: in std_logic_vector(7 downto 0);

      SegmentIn: in std_logic_vector(7 downto 0);

      Addend: in std_logic_vector(7 downto 0); --How much to increase DataIn by (as a signed number). Believe it or not, that's the actual word for what we need.

      DataOut: out std_logic_vector(7 downto 0);

      SegmentOut: out std_logic_vector(7 downto 0);

      Clock: in std_logic

    );

  end component;

  component registerfile is

  port(

    WriteEnable: in regwritetype;

    DataIn: in regdatatype;

    Clock: in std_logic;

    DataOut: out regdatatype

  );

  end component;

  constant REGIP: integer := 7;

  constant REGSP: integer := 6;

  constant REGSS: integer := 15;

  constant REGES: integer := 14;

  constant REGDS: integer := 13;

  constant REGCS: integer := 12;

  type ProcessorState is (

    ResetProcessor,

    FirstFetch1, --the fetcher needs two clock cycles to catch up

    FirstFetch2,

    Execute,

    WaitForMemory,

    HoldMemory

  );

  signal state: ProcessorState;

  signal HeldState: ProcessorState; --state the processor was in when HOLD was activated

  --carryout signals

  signal CarryCS: std_logic;

  signal CarrySS: std_logic;

  signal IPAddend: std_logic_vector(7 downto 0);

  signal SPAddend: std_logic_vector(7 downto 0);

  signal IPCarryOut: std_logic_vector(7 downto 0);

  signal CSCarryOut: std_logic_vector(7 downto 0);

  --register signals

  signal regWE:regwritetype;

  signal regIn: regdatatype;

  signal regOut: regdatatype;

  --fetch signals

  signal fetchEN: std_logic;

  signal IR: std_logic_vector(15 downto 0);

  --control signals

  signal InReset: std_logic;

  --opcode shortcut signals

  signal opmain: std_logic_vector(3 downto 0);

  signal opimmd: std_logic_vector(7 downto 0);

  signal opcond1: std_logic; --first conditional bit

  signal opcond2: std_logic; --second conditional bit

  signal opreg1: std_logic_vector(2 downto 0);

  signal opreg2: std_logic_vector(2 downto 0);

  signal opreg3: std_logic_vector(2 downto 0);

  signal opseges: std_logic; --use ES segment

  signal fetcheraddress: std_logic_vector(15 downto 0);

begin

  reg: registerfile port map(

    WriteEnable => regWE,

    DataIn => regIn,

    Clock => Clock,

    DataOut => regOut

  );

  carryovercs: carryover port map(

    EnableCarry => CarryCS,

    DataIn => regIn(REGIP),

    SegmentIn => regIn(REGCS),

    Addend => IPAddend,

    DataOut => IPCarryOut,

    SegmentOut => CSCarryOut,

    Clock => Clock

  );

  fetcher: fetch port map(

    Enable => fetchEN,

    AddressIn => fetcheraddress,

    Clock => Clock,

    DataIn => MemIn,

    IROut => IR,

    AddressOut => MemAddr --this component supports tristate, so no worries about an intermediate signal

  );

  fetcheraddress <= regIn(REGCS) & regIn(REGIP);

  --opcode shortcuts

  opmain <= IR(15 downto 12);

  opimmd <= IR(7 downto 0);

  opcond1 <= IR(8);

  opcond2 <= IR(7);

  opreg1 <= IR(11 downto 9);

  opreg3 <= IR(2 downto 0);

  opreg2 <= IR(5 downto 3);

  opseges <= IR(6);

  --debug ports

  DebugCS <= regOut(REGCS);

  DebugIP <= regOut(REGIP);

  DebugR0 <= regOut(0);

  DebugIR <= IR;

  decode: process(Clock, Hold, state, IR, inreset, reset, regin, regout, IPCarryOut, CSCarryOut)

  begin

    if rising_edge(Clock) then

    --states

      if reset='1' and hold='0' then

        InReset <= '1';

        state <= ResetProcessor;

        HoldAck <= '0';

        CarryCS <= '1';

        CarrySS <= '0';

        regWE <= (others => '1');

        regIn <= (others => "00000000");

        regIn(REGCS) <= x"01";

        IPAddend <= x"00";

        fetchEN <= '1';

        --finish up

      elsif InReset='1' and reset='0' and Hold='0' then --reset is done, start executing

        InReset <= '0';

        fetchEN <= '1';

        state <= FirstFetch1;

      elsif Hold = '1' and (state=HoldMemory or state=Execute or state=ResetProcessor) then

        --do not hold immediately if waiting on memory or if waiting on the first fetch of an instruction after reset

        state <= HoldMemory;

        HoldAck <= '1';

        FetchEN <= '0';

        MemAddr <= "ZZZZZZZZZZZZZZZZ";

        MemOut <= "ZZZZZZZZZZZZZZZZ";

        MemWE <= 'Z';

        MemWW <= 'Z';

      elsif Hold='0' and state=HoldMemory then

        if reset='1' or InReset='1' then

          state <= ResetProcessor;

        else

          state <= Execute;

        end if;

        FetchEN <= '1';

      elsif state=FirstFetch1 then --we have to let IR get loaded before we can execute.

        --regWE <= (others => '0');

        fetchEN <= '1'; --already enabled, but anyway

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

        state <= FirstFetch2;

      elsif state=FirstFetch2 then

        state <= Execute;

        IPAddend <= x"02";

        SPAddend <= x"00"; --no addend unless pushing or popping

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

        regIn(REGIP) <= IPCarryOut;

        regWE(REGIP) <= '1';

        regWE(REGCS) <= '1';

        regIn(REGCS) <= CSCarryOut;

      end if;

      if state=Execute then

        fetchEN <= '1';

        --reset to "usual"

        IPAddend <= x"02";

        SPAddend <= x"00"; --no addend unless pushing or popping

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

        regIn(REGIP) <= IPCarryOut;

        regWE(REGIP) <= '1';

        regWE(REGCS) <= '1';

        regIn(REGCS) <= CSCarryOut;

        MemWE <= '0';

        MemWW <= '0';

        --actual decoding

        case opmain is

          when "0000" => --mov reg,imm

            RegIn(to_integer(unsigned(opreg1))) <= opimmd;

            RegWE(to_integer(unsigned(opreg1))) <= '1';

          when others =>

            --synthesis off

            report "Not implemented" severity error;

            --synthesis on

        end case;

      end if;

    end if;

  end process;

end Behavioral;