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-- Copyright (c)2006, 2011, 2012, 2013, 2015, 2019, 2020 Jeremy Seth Henry

-- All rights reserved.

--

-- Redistribution and use in source and binary forms, with or without

-- modification, are permitted provided that the following conditions are met:

--     * Redistributions of source code must retain the above copyright

--       notice, this list of conditions and the following disclaimer.

--     * Redistributions in binary form must reproduce the above copyright

--       notice, this list of conditions and the following disclaimer in the

--       documentation and/or other materials provided with the distribution,

--       where applicable (as part of a user interface, debugging port, etc.)

--

-- THIS SOFTWARE IS PROVIDED BY JEREMY SETH HENRY ``AS IS'' AND ANY

-- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

-- WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

-- DISCLAIMED. IN NO EVENT SHALL JEREMY SETH HENRY BE LIABLE FOR ANY

-- DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES

-- (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;

-- LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND

-- ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

-- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

-- THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

--

-- VHDL Units :  o8_cpu

-- Description:  VHDL model of a RISC 8-bit processor core loosely based on the

--            :   V8/ARC uRISC instruction set. Requires Open8_pkg.vhd

--            :

-- Notes      :  Generic definitions

--            :

--            :  Program_Start_Addr sets the initial value of the program

--            :   counter.

--            :

--            :  ISR_Start_Addr sets the location of the interrupt service

--            :   vector table. There are 8 service vectors, or 16 bytes, which

--            :   must be allocated to either ROM or RAM.

--            :

--            :  Stack_Start_Address sets the initial (reset) value of the

--            :   stack pointer. Also used for the RSP instruction if

--            :   Allow_Stack_Address_Move is false.

--            :

--            :  Allow_Stack_Address_Move, when set true, allows the RSP to be

--            :   programmed via thet RSP instruction. If enabled, the

--            :   instruction changes into TSX or TXS based on the flag

--            :   specified by Stack_Xfer_Flag. If the flag is '0', RSP will

--            :   copy the current stack pointer to R1:R0 (TSX). If the flag

--            :   is '1', RSP will copy R1:R0 to the stack pointer (TXS). This

--            :   allows the processor to backup and restore stack pointers

--            :   in a multi-process environment. Note that no flags are

--            :   modified by either form of this instruction.

--            :

--            :  Stack_Xfer_Flag instructs the core to use the specified ALU

--            :   flag to alter the behavior of the RSP instruction when

--            :   Allow_Stack_Address_Move is set TRUE, otherwise it's ignored.

--            :   While technically any of the status bits may be used, the

--            :   intent was to use FL_GP[1,2,3,4], as these are not modified

--            :   by ordinary ALU operations.

--            :

--            :  The Enable_Auto_Increment generic can be used to modify the

--            :   indexed instructions such that specifying an odd register

--            :   will use the next lower register pair, post-incrementing the

--            :   value in that pair. IOW, specifying STX R1 will instead

--            :   result in STX R0++, or R0 = {R1:R0}; {R1:R0} + 1

--            :

--            :  BRK_Implements_WAI modifies the BRK instruction such that it

--            :   triggers the wait for interrupt state, but without triggering

--            :   a soft interrupt in lieu of its normal behavior, which is to

--            :   insert several dead clock cycles - essentially a long NOP

--            :

--            :  Enable_NMI overrides the mask bit for interrupt 0, creating a

--            :   non-maskable interrupt at the highest priority. To remain

--            :   true to the original core, this should be set false.

--            :

--            :  Sequential_Interrupts, when set, prevents interrupt service

--            :   routines from  being interrupted by postponing an later

--            :   interrupts until the I bit is cleared (usually with an RTI,

--            :   but a CLP PSR_I will also work). This is potentially

--            :   dangerous, as it means a lower-priority ISR can "hog" the CPU

--            :   by failing to return. However, it can also prevent the

--            :   condition of an ISR interrupting itself until it causes a

--            :   memory fault. (For example, an interrupt source that whose

--            :   period is shorter than the ISR service time) Note that this

--            :   setting alters the way the pending logic works, so it affects

--            :   all interrupts, including the NMI. If this is set, special

--            :   care should be taken to make sure ISRs are short and always

--            :   execute an RTI at the end.

--            :

--            :  RTI_Ignores_GP_Flags alters the set of flag bits restored

--            :   after an interrupt. By default, all of the flag bits are put

--            :   back to their original state. If this flag is set true, only

--            :   the lower four bits are restored, allowing ISR code to alter

--            :   the GP flags persistently.

--            :

--            :  Supervisor_Mode, when set, disables the STP PSR_I instruction

--            :   preventing code from setting the I bit. When enabled, only

--            :   interrupts can set the I bit, allowing for more robust memory

--            :   protection by preventing errant code execution from

--            :   inadvertently entering an interrupt state.

--            :

--            :   This setting also sets I bit at startup so that any

--            :   initialization code may be run in an ISR context, initially

--            :   bypassing memory protection. Init code should clear the I bit

--            :   when done;

--            :

--            :  Unsigned_Index_Offsets alters the way offsets are added to

--            :   [Rn+1:Rn] during LDO/STO instructions. The original, default

--            :   behavior treats these offsets as signed values, allowing

--            :   instructions to offset by -128 to +127 from [Rn+1:Rn].

--            :   Setting this generic to TRUE will switch to unsigned offsets,

--            :   switching the range to 0 to 255 instead.

--            :

--            :  Default_Interrupt_Mask sets the intial/reset value of the

--            :   interrupt mask. To remain true to the original core, which

--            :   had no interrupt mask, this should be set to x"FF". Otherwise

--            :   it can be initialized to any value. Note that Enable_NMI

--            :   will logically force the LSB high.

--            :

--            :  Reset_Level determines whether the processor registers reset

--            :   on a high or low level from higher logic.

--            :

--            : Architecture notes

--            :  This model deviates from the original ISA in a few important

--            :   ways.

--            :

--            :  First, there is only one set of registers. Interrupt service

--            :   routines must explicitely preserve context since the the

--            :   hardware doesn't. This was done to decrease size and code

--            :   complexity. Older code that assumes this behavior will not

--            :   execute correctly on this processor model.

--            :

--            :  Second, this model adds an additional pipeline stage between

--            :   the instruction decoder and the ALU. Unfortunately, this

--            :   means that the instruction stream has to be restarted after

--            :   any math instruction is executed, implying that any ALU

--            :   instruction now has a latency of 2 instead of 0. The

--            :   advantage is that the maximum frequency has gone up

--            :   significantly, as the ALU code is vastly more efficient.

--            :   As an aside, this now means that all math instructions,

--            :   including MUL (see below) and UPP have the same instruction

--            :   latency.

--            :

--            :  Third, the original ISA, also a soft core, had two reserved

--            :   instructions, USR and USR2. These have been implemented as

--            :   DBNZ, and MUL respectively.

--            :

--            :  DBNZ decrements the specified register and branches if the

--            :   result is non-zero. The instruction effectively executes a

--            :   DEC Rn instruction prior to branching, so the same flags will

--            :   be set.

--            :

--            :  MUL places the result of R0 * Rn into R1:R0. Instruction

--            :   latency is identical to other ALU instructions. Only the Z

--            :   flag is set, since there is no defined overflow or "negative

--            :   16-bit values"

--            :

--            :  Fourth, indexed load/store instructions now have an (optional)

--            :   ability to post-increment their index registers. If enabled,

--            :   using an odd operand for LDO,LDX, STO, STX will cause the

--            :   register pair to be incremented after the storage access.

--            :

--            :  Fifth, the RSP instruction has been (optionally) altered to

--            :   allow the stack pointer to be sourced from R1:R0.

--            :

--            :  Sixth, the BRK instruction can optionally implement a WAI,

--            :   which is the same as the INT instruction without the soft

--            :   interrupt, as a way to put the processor to "sleep" until the

--            :   next external interrupt.

--            :

--            :  Seventh, the original CPU model had 8 non-maskable interrupts

--            :   with priority. This model has the same 8 interrupts, but

--            :   allows software to mask them (with an additional option to

--            :   override the highest priority interrupt, making it the NMI.)

--            :

--            :  Lastly, previous unmapped instructions in the OP_STK opcode

--            :   were repurposed to support a new interrupt mask.

--            :   SMSK and GMSK transfer the contents of R0 (accumulator)

--            :   to/from the interrupt mask register. SMSK is immediate, while

--            :   GMSK has the same overhead as a math instruction.

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      07/19/06 Design Start

-- Seth Henry      01/18/11 Fixed BTT instruction to match V8

-- Seth Henry      07/22/11 Fixed interrupt transition logic to avoid data

--                           corruption issues.

-- Seth Henry      07/26/11 Optimized logic in ALU, stack pointer, and data

--                           path sections.

-- Seth Henry      07/27/11 Optimized logic for timing, merged blocks into

--                           single entity.

-- Seth Henry      09/20/11 Added BRK_Implements_WAI option, allowing the

--                           processor to wait for an interrupt instead of the

--                           normal BRK behavior.

-- Seth Henry      12/20/11 Modified core to allow WAI_Cx state to idle

--                           the bus entirely (Rd_Enable is low)

-- Seth Henry      02/03/12 Replaced complex interrupt controller with simpler,

--                           faster logic that simply does priority encoding.

-- Seth Henry      08/06/13 Removed HALT functionality

-- Seth Henry      10/29/15 Fixed inverted carry logic in CMP and SBC instrs

-- Seth Henry      12/19/19 Renamed to o8_cpu to fit "theme"

-- Seth Henry      03/09/20 Modified RSP instruction to work with a CPU flag

--                           allowing true backup/restore of the stack pointer

-- Seth Henry      03/11/20 Split the address logic from the main state machine

--                           in order to simplify things and eliminate

--                           redundancies. Came across and fixed a problem with

--                           the STO instruction when Enable_Auto_Increment is

--                           NOT set.

-- Seth Henry      03/12/20 Rationalized the naming of the CPU flags to match

--                           the assembler names. Also fixed an issue where

--                           the I bit wasn't being cleared after interrupts.

--                          Simplified the program counter logic to only use

--                           the offset for increments, redefining the

--                           original modes as fixed offset values.

--                          Modified the ALU section with a new ALU operation

--                           for GMSK. This allowed the .data field to be

--                           removed and Operand1 used in its place, which

--                           simplified the logic a great deal.

-- Seth Henry      03/16/20 Added CPU_Halt input back, only now as an input to

--                           the instruction decode state, where it acts as a

--                           modified form of the BRK instruction that holds

--                           state until CPU_Halt is deasserted. This has a

--                           much smaller impact on Fmax/complexity than the

--                           original clock enable, but imposes a mild impact

--                           due to the need to reset the instruction pipeline

-- Seth Henry      03/17/20 Added generic to control whether RTI full restores

--                           the flags, including the general purpose ones, or

--                           only the core ALU flags (Z, N, and C). Also

--                           brought out copies of the GP flags for external

--                           connection.

-- Seth Henry      04/09/20 Added a compile time setting to block interrupts

--                           while the I bit is set to avoid reentering ISRs

--                           This may slightly affect timing, as this will

--                           potentially block higher priority interrupts

--                           until the lower priority ISR returns or clears

--                           the I bit.

--                          Also added the I bit to the exported flags for

--                           use in memory protection schemes.

-- Seth Henry      04/16/20 Modified to use new Open8 bus record. Also added

--                           reset and usec_tick logic to drive utility

--                           signals. Also added Halt_Ack output.

-- Seth Henry      05/20/20 Added two new generics to alter the way the I bit

--                           is handled. The Supervisor_Mode setting disables

--                           STP PSR_I from being executed, preventing it

--                           from being set outside of an ISR. The

--                           Default_Int_Flag setting allows the I bit to

--                           start set so that initialization code can run,

--                           but not be hijacked later to corrupt any memory

--                           write protection later.

-- Seth Henry      05/21/20 Supervisor_Mode now protects the interrupt mask

--                           and stack pointer as well.

-- Seth Henry      05/24/20 Removed the Default_Int_Flag, as it is covered by

--                           Supervisor_Mode. If Supervisor_Mode isn't set,

--                           code can simply use STP to set the bit

-- Seth Henry      06/09/20 Added ability to use unsigned index offsets for

--                           LDO/STO. Also pipelined the address calculation

--                           for indexed instructions, reducing the final

--                           address generator to a multiplexor fed only by

--                           registers.

-- Seth Henry      07/10/20 Fixed a bug in the LDO/LDX logic where the register

--                           pair wasn't being incremented properly due to a

--                           missing UPP2 signal to the ALU.

-- Seth Henry      10/21/20 Modified the write data path to use separate

--                           enumerated states rather than reuse the .reg field

--                           to improve performance.

library ieee;

  use ieee.std_logic_1164.all;

  use ieee.std_logic_unsigned.all;

  use ieee.std_logic_arith.all;

  use ieee.std_logic_misc.all;

library work;

  use work.Open8_pkg.all;

entity o8_cpu is

  generic(

    Program_Start_Addr       : ADDRESS_TYPE := x"0000"; -- Initial PC location

    ISR_Start_Addr           : ADDRESS_TYPE := x"FFF0"; -- Bottom of ISR vec's

    Stack_Start_Addr         : ADDRESS_TYPE := x"03FF"; -- Top of Stack

    Allow_Stack_Address_Move : boolean      := false;   -- Use Normal v8 RSP

    Stack_Xfer_Flag          : integer      := PSR_GP4; -- GP4 modifies RSP

    Enable_Auto_Increment    : boolean      := false;   -- Modify indexed instr

    BRK_Implements_WAI       : boolean      := false;   -- BRK -> Wait for Int

    Enable_NMI               : boolean      := true;    -- Force INTR0 enabled

    Sequential_Interrupts    : boolean      := false;   -- Interruptable ISRs

    RTI_Ignores_GP_Flags     : boolean      := false;   -- RTI sets all flags

    Supervisor_Mode          : boolean      := false;   -- I bit is restricted

    Unsigned_Index_Offsets   : boolean      := false;   -- Offsets are signed

    Default_Interrupt_Mask   : DATA_TYPE    := x"FF";   -- Enable all Ints

    Clock_Frequency          : real                     -- Clock Frequency

);

  port(

    Clock                    : in  std_logic;

    PLL_Locked               : in  std_logic;

    --

    Halt_Req                 : in  std_logic := '0';

    Halt_Ack                 : out std_logic;

    --

    Open8_Bus                : out OPEN8_BUS_TYPE;

    Rd_Data                  : in  DATA_TYPE;

    Interrupts               : in  INTERRUPT_BUNDLE := x"00"

);

end entity;

architecture behave of o8_cpu is

  signal Reset_q             : std_logic := Reset_Level;

  signal Reset               : std_logic := Reset_Level;

  constant USEC_VAL          : integer := integer(Clock_Frequency / 1000000.0);

  constant USEC_WDT          : integer := ceil_log2(USEC_VAL - 1);

  constant USEC_DLY          : std_logic_vector :=

                                conv_std_logic_vector(USEC_VAL - 1, USEC_WDT);

  signal uSec_Cntr           : std_logic_vector( USEC_WDT - 1 downto 0 );

  signal uSec_Tick           : std_logic;

  signal CPU_Next_State      : CPU_STATES := IPF_C0;

  signal CPU_State           : CPU_STATES := IPF_C0;

  signal CPU_Halt_Req        : std_logic := '0';

  signal CPU_Halt_Ack        : std_logic := '0';

  signal Cache_Ctrl          : CACHE_MODES := CACHE_IDLE;

  signal Opcode              : OPCODE_TYPE := (others => '0');

  signal SubOp, SubOp_p1     : SUBOP_TYPE  := (others => '0');

  signal Prefetch            : DATA_TYPE   := x"00";

  signal Operand1, Operand2  : DATA_TYPE   := x"00";

  signal Instr_Prefetch      : std_logic   := '0';

  signal PC_Ctrl             : PC_CTRL_TYPE;

  signal Program_Ctr         : ADDRESS_TYPE := x"0000";

  signal ALU_Ctrl            : ALU_CTRL_TYPE;

  signal Regfile             : REGFILE_TYPE;

  signal Flags               : FLAG_TYPE;

  signal Mult                : ADDRESS_TYPE := x"0000";

  signal SP_Ctrl             : SP_CTRL_TYPE;

  signal Stack_Ptr           : ADDRESS_TYPE := x"0000";

  signal DP_Ctrl             : DATA_CTRL_TYPE;

  signal INT_Ctrl            : INT_CTRL_TYPE;

  signal Ack_D, Ack_Q, Ack_Q1: std_logic   := '0';

  signal Int_Req, Int_Ack    : std_logic   := '0';

  signal Set_Mask            : std_logic   := '0';

  signal Int_Mask            : DATA_TYPE   := x"00";

  signal i_Ints              : INTERRUPT_BUNDLE := x"00";

  signal Pending             : INTERRUPT_BUNDLE := x"00";

  signal Wait_for_FSM        : std_logic := '0';

  signal Wait_for_ISR        : std_logic := '0';

  alias  ISR_Addr_Base       is ISR_Start_Addr(15 downto 4);

  signal ISR_Addr_Offset     : std_logic_vector(3 downto 0) := x"0";

  constant INT_VECTOR_0      : std_logic_vector(3 downto 0) := x"0";

  constant INT_VECTOR_1      : std_logic_vector(3 downto 0) := x"2";

  constant INT_VECTOR_2      : std_logic_vector(3 downto 0) := x"4";

  constant INT_VECTOR_3      : std_logic_vector(3 downto 0) := x"6";

  constant INT_VECTOR_4      : std_logic_vector(3 downto 0) := x"8";

  constant INT_VECTOR_5      : std_logic_vector(3 downto 0) := x"A";

  constant INT_VECTOR_6      : std_logic_vector(3 downto 0) := x"C";

  constant INT_VECTOR_7      : std_logic_vector(3 downto 0) := x"E";

  signal IDX_Offset_SX       : std_logic := '0';

  signal IDX_Offset          : ADDRESS_TYPE := x"0000";

  signal IDX_Sel_l           : std_logic_vector(2 downto 0) := "000";

  signal IDX_Sel_h           : std_logic_vector(2 downto 0) := "000";

  signal IDX_Reg_l           : integer := 0;

  signal IDX_Reg_h           : integer := 0;

  signal IDX_NoOffset_Calc   : ADDRESS_TYPE := x"0000";

  signal IDX_Offset_Calc     : ADDRESS_TYPE := x"0000";

begin

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

-- Reset & uSec Tick

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

  CPU_Reset_Sync: process( Clock, PLL_Locked )

  begin

    if( PLL_Locked = '0' )then

      Reset_q                <= Reset_Level;

      Reset                  <= Reset_Level;

    elsif( rising_edge(Clock) )then

      Reset_q                <= not Reset_Level;

      Reset                  <= Reset_q;

    end if;

  end process;

  uSec_Tick_proc: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      uSec_Cntr              <= USEC_DLY;

      uSec_Tick              <= '0';

    elsif( rising_edge( Clock ) )then

      uSec_Cntr              <= uSec_Cntr - 1;

      if( or_reduce(uSec_Cntr) = '0' )then

        uSec_Cntr            <= USEC_DLY;

      end if;

      uSec_Tick              <= nor_reduce(uSec_Cntr);

    end if;

  end process;

  Open8_Bus.Clock            <= Clock;

  Open8_Bus.Reset            <= Reset;

  Open8_Bus.uSec_Tick        <= uSec_Tick;

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

-- Address bus selection/generation logic

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

  -- Address selection logic based on current CPU state. This is combinatorial,

  --  as adding pipeline registration would add a clock cycle to every instr,

  --  without really adding the Fmax to compensate.

  Address_Logic: process(CPU_State, Operand1, Operand2, IDX_NoOffset_Calc,

                         IDX_Offset_Calc, ISR_Addr_Offset, Stack_Ptr,

                         Program_Ctr )

  begin

    case( CPU_State )is

      when LDA_C2 | STA_C2 =>

        Open8_Bus.Address    <= Operand2 & Operand1;

      when LDX_C1 | STX_C1 =>

        Open8_Bus.Address    <= IDX_NoOffset_Calc;

      when LDO_C2 | STO_C2 =>

        Open8_Bus.Address    <= IDX_Offset_Calc;

      when ISR_C1 | ISR_C2 =>

        Open8_Bus.Address    <= ISR_Addr_Base & ISR_Addr_Offset;

      when PSH_C1 | POP_C1 |

           ISR_C3 | JSR_C1 | JSR_C2 |

           RTS_C1 | RTS_C2 | RTS_C3 =>

        Open8_Bus.Address    <= Stack_Ptr;

      when others =>

        Open8_Bus.Address    <= Program_Ctr;

    end case;

  end process;

  -- The original model treated the offset to LDO/STO as a signed value

  --  allowing access to locations -128 to +127 from [Rn+1:Rn]. This isn't

  --  always helpful, so the generic allows the CPU to use unsigned math

  --  for the offsets. This makes the range 0 to +255 instead.

  IDX_Offset_SX <= '0' when Unsigned_Index_Offsets else Operand1(7);

  IDX_Offset(15 downto 8)    <= (others => IDX_Offset_SX);

  IDX_Offset(7 downto 0)     <= Operand1;

  -- Enable_Auto_Increment uses the LSB to determine whether or not to

  --  do the auto-increment, so we need to lock the LSB for each operand

  --  if it is enabled. This forces [ODD:EVEN] pairing.

  IDX_Sel_l <= (SubOp(2 downto 1) & '0') when Enable_Auto_Increment else

               SubOp;

  IDX_Sel_h <= (SubOp(2 downto 1) & '1') when Enable_Auto_Increment else

               SubOp_p1;

  IDX_Reg_l <= conv_integer(IDX_Sel_l);

  IDX_Reg_h <= conv_integer(IDX_Sel_h);

  -- Pipeline registers for the indexed and indexed with offset addresses.

  Idx_Addr_Calc_proc: process( Clock, Reset )

    variable Reg, Reg_1      : integer range 0 to 7 := 0;

  begin

    if( Reset = Reset_Level )then

      IDX_NoOffset_Calc      <= x"0000";

      IDX_Offset_Calc        <= x"0000";

    elsif( rising_edge(Clock))then

      IDX_NoOffset_Calc      <= (Regfile(IDX_Reg_h) & Regfile(IDX_Reg_l));

      IDX_Offset_Calc        <= (Regfile(IDX_Reg_h) & Regfile(IDX_Reg_l)) +

                                IDX_Offset;

    end if;

  end process;

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

-- Combinatorial portion of CPU finite state machine

-- State Logic / Instruction Decoding & Execution

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

  State_Logic: process(CPU_State, Flags, Int_Mask, CPU_Halt_Req, Opcode,

                       SubOp , SubOp_p1, Operand1, Operand2, Int_Req )

    variable Reg             : integer range 0 to 7 := 0;

  begin

    CPU_Next_State           <= CPU_State;

    Cache_Ctrl               <= CACHE_IDLE;

    --

    PC_Ctrl.Oper             <= PC_INCR;

    PC_Ctrl.Offset           <= PC_IDLE;

    --

    ALU_Ctrl.Oper            <= ALU_IDLE;

    ALU_Ctrl.Reg             <= ACCUM;

    --

    SP_Ctrl.Oper             <= SP_IDLE;

    --

    DP_Ctrl.Src              <= DATA_RD_MEM;

    DP_Ctrl.Reg              <= ACCUM;

    --

    INT_Ctrl.Mask_Set        <= '0';

    INT_Ctrl.Soft_Ints       <= x"00";

    INT_Ctrl.Incr_ISR        <= '0';

    Ack_D                    <= '0';

    --

    Reg                     := conv_integer(SubOp);

    --

    CPU_Halt_Ack             <= '0';

    case CPU_State is

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

-- Initial Instruction fetch & decode

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

      when IPF_C0 =>

        CPU_Next_State       <= IPF_C1;

        PC_Ctrl.Offset       <= PC_NEXT;

      when IPF_C1 =>

        CPU_Next_State       <= IPF_C2;

        PC_Ctrl.Offset       <= PC_NEXT;

      when IPF_C2 =>

        CPU_Next_State       <= IDC_C0;

        Cache_Ctrl           <= CACHE_INSTR;

        PC_Ctrl.Offset       <= PC_NEXT;

      when IDC_C0 =>

        CPU_Next_State       <= IDC_C0;

        Cache_Ctrl           <= CACHE_INSTR;

        case Opcode is

          when OP_PSH =>

            CPU_Next_State   <= PSH_C1;

            Cache_Ctrl       <= CACHE_PREFETCH;

            PC_Ctrl.Offset   <= PC_REV1;

            DP_Ctrl.Src      <= DATA_WR_REG;

            DP_Ctrl.Reg      <= SubOp;

          when OP_POP =>

            CPU_Next_State   <= POP_C1;

            Cache_Ctrl       <= CACHE_PREFETCH;

            PC_Ctrl.Offset   <= PC_REV2;

            SP_Ctrl.Oper     <= SP_POP;

          when OP_BR0 | OP_BR1 =>

            CPU_Next_State   <= BRN_C1;

            Cache_Ctrl       <= CACHE_OPER1;

            PC_Ctrl.Offset   <= PC_NEXT;

          when OP_DBNZ =>

            CPU_Next_State   <= DBNZ_C1;

            Cache_Ctrl       <= CACHE_OPER1;

            PC_Ctrl.Offset   <= PC_NEXT;

            ALU_Ctrl.Oper    <= ALU_DEC;

            ALU_Ctrl.Reg     <= SubOp;

          when OP_INT =>

            PC_Ctrl.Offset   <= PC_NEXT;

            -- Make sure the requested interrupt is actually enabled first.

            --  Also, unlike CPU_Halt, the INT instruction is actually being

            --  executed, so go ahead and increment the program counter before

            --  pausing so the CPU restarts on the next instruction.

            if( Int_Mask(Reg) = '1' )then

              CPU_Next_State <= WAI_Cx;

              INT_Ctrl.Soft_Ints(Reg) <= '1';

            end if;

          when OP_STK =>

            case SubOp is

              when SOP_RSP  =>

                PC_Ctrl.Offset <= PC_NEXT;

                if( not Allow_Stack_Address_Move )then

                  -- The default behavior for this instruction is to simply

                  --  repoint the SP to the HDL default

                  SP_Ctrl.Oper    <= SP_CLR;

                end if;

                if( Allow_Stack_Address_Move and

                    Flags(Stack_Xfer_Flag) = '1' )then

                  -- If RSP is set to allow SP moves, and the specified flag

                  --  is true, then signal the stack pointer logic to load

                  --  from R1:R0

                  SP_Ctrl.Oper    <= SP_SET;

                end if;

                if( Allow_Stack_Address_Move and

                    Flags(Stack_Xfer_Flag) = '0')then

                  -- If RSP is set to allow SP moves, and the specified flag

                  --  is false, then signal the ALU to copy the stack pointer

                  --  to R1:R0

                  ALU_Ctrl.Oper   <= ALU_RSP;

                end if;

              when SOP_RTS | SOP_RTI =>

                CPU_Next_State    <= RTS_C1;

                Cache_Ctrl        <= CACHE_IDLE;

                SP_Ctrl.Oper      <= SP_POP;

              when SOP_BRK  =>

                if( BRK_Implements_WAI )then

                  -- If BRK_Implements_WAI, then jump to the WAI_Cx and

                  --  increment the PC similar to an ISR flow.

                  CPU_Next_State  <= WAI_Cx;

                  PC_Ctrl.Offset  <= PC_NEXT;

                else

                -- If Break is implemented normally, back the PC up by

                --  2 and return through IPF_C0 in order to execute a 3

                --  clock cycle delay

                  CPU_Next_State  <= BRK_C1;

                  PC_Ctrl.Offset  <= PC_REV2;

                end if;

              when SOP_JMP  =>

                CPU_Next_State    <= JMP_C1;

                Cache_Ctrl        <= CACHE_OPER1;

              when SOP_SMSK =>

                PC_Ctrl.Offset    <= PC_NEXT;

                INT_Ctrl.Mask_Set <= '1';

              when SOP_GMSK =>

                PC_Ctrl.Offset    <= PC_NEXT;

                ALU_Ctrl.Oper     <= ALU_GMSK;

              when SOP_JSR =>

                CPU_Next_State    <= JSR_C1;

                Cache_Ctrl        <= CACHE_OPER1;

                DP_Ctrl.Src       <= DATA_WR_PC_H;

              when others => null;

            end case;

          when OP_MUL =>

            CPU_Next_State   <= MUL_C1;

            -- Multiplication requires a single clock cycle to calculate PRIOR

            --  to the ALU writing the result to registers. As a result, this

            --  state needs to idle the ALU initially, and back the PC up by 1

            -- We can get away with only 1 extra clock by pre-fetching the

            --  next instruction, though.

            Cache_Ctrl       <= CACHE_PREFETCH;

            PC_Ctrl.Offset   <= PC_REV1;

            -- Note that both the multiply process AND ALU process need the

            --  source register for Rn (R1:R0 = R0 * Rn). Assert ALU_Ctrl.reg

            --  now, but hold off on the ALU command until the next state.

            ALU_Ctrl.Oper    <= ALU_IDLE;

            ALU_Ctrl.Reg     <= SubOp;

          when OP_UPP =>

            CPU_Next_State   <= UPP_C1;

            Cache_Ctrl       <= CACHE_PREFETCH;

            PC_Ctrl.Offset   <= PC_REV1;

            ALU_Ctrl.Oper    <= Opcode;

            ALU_Ctrl.Reg     <= SubOp;

          when OP_LDA =>

            CPU_Next_State   <= LDA_C1;

            Cache_Ctrl       <= CACHE_OPER1;

          when OP_LDI =>

            CPU_Next_State   <= LDI_C1;

            Cache_Ctrl       <= CACHE_OPER1;

            PC_Ctrl.Offset   <= PC_NEXT;

          when OP_LDO =>

            CPU_Next_State   <= LDO_C1;

            Cache_Ctrl       <= CACHE_OPER1;

            PC_Ctrl.Offset   <= PC_REV2;

          when OP_LDX =>

            CPU_Next_State   <= LDX_C1;

            Cache_Ctrl       <= CACHE_PREFETCH;

            PC_Ctrl.Offset   <= PC_REV2;

          when OP_STA =>

            CPU_Next_State   <= STA_C1;

            Cache_Ctrl       <= CACHE_OPER1;

          when OP_STO =>

            CPU_Next_State   <= STO_C1;

            Cache_Ctrl       <= CACHE_OPER1;

            PC_Ctrl.Offset   <= PC_REV1;

          when OP_STX =>

            CPU_Next_State   <= STX_C1;

            Cache_Ctrl       <= CACHE_PREFETCH;

            PC_Ctrl.Offset   <= PC_REV2;

            DP_Ctrl.Src      <= DATA_WR_REG;

            DP_Ctrl.Reg      <= ACCUM;

          when OP_STP =>

            PC_Ctrl.Offset   <= PC_NEXT;

            if( Supervisor_Mode )then

              if( SubOp /= PSR_I )then

                ALU_Ctrl.Oper  <= Opcode;

                ALU_Ctrl.Reg   <= SubOp;

              end if;

            else

              ALU_Ctrl.Oper  <= Opcode;

              ALU_Ctrl.Reg   <= SubOp;

            end if;