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-- Copyright (c)2006, 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_alu16

-- Description:  Provides a mix of common 16-bit math functions to accelerate

--            :   math operations on the Open8 microprocessor core.

--

-- Notes      :  All output registers are updated by writing to offset 0x1F.

--

--            :  The math unit is busy when the MSB of the status

--            :   register is high, and done/ready when it reads low.

--            :  Almost Equal checks to see if Addend 1 is no more, or less,

--            :   addend 2 within the specified tolerance. For example,

--            :   addend_1 = 2 is almost equal to addend_2 = -1 with a

--            :   tolerance of 3, but not a tolerance of 1. Actual function is

--            :   AE = '1' when (A1 <= A2 + T) and (A1 >= A2 - T) else '0'

--            :   This is an inherently signed function.

--            : Signed Overflow/Underflow is logically equivalent to

--            :  S_Sum/Dif(16) xor S_Sum/Dif(15), since the apparent result

--            :  changes sign while the internal sign bit doesn't. For example

--            :  |8000| will result in (-)8000 due to the way the internal

--            :  logic handles sign extension. Thus, the O bit should be

--            :  checked when performing signed math

--            : Decimal Adjust converts the contents of a register into its BCD

--            :  equivalent. This can be used to get the base 10 representation

--            :  of a value for conversion to ASCII. There are two variants,

--            :  Byte and Word. Note that conversion times are fairly long,

--            :  since we are repeatedly issuing division commands, but it is

--            :  still faster than software emulation.

--            : The Byte conversion only operates on the lower 8 bits, and

--            :  sets the Z and N flags. The N flag is only set when SDAB is

--            :  used and the signed value of the register is negative. The O

--            :  bit is set if the upper byte of the register is non-zero, but

--            :  does not actually result in an calculation error.

--            :  Examples:

--            :  UDAB 0x00FE -> 0x0254, Flags -> 0x0

--            :  SDAB 0x00FE -> 0x0002, Flags -> 0x4

--            : The Word conversion uses the entire 16-bit word, and uses the

--            :  Flags register to hold the Tens of Thousands place. Note that

--            :  the N flag is still used for signed conversions, while it may

--            :  be used as a data bit for unsigned conversions.

--            :  Examples:

--            :  UDAW 0xD431 -> 0x4321, Flags -> 0x5 ('0', MSB, MSB-1, MSB-2)

--            :  SDAW 0xD431 -> 0x1216, Flags -> 0x5 ('0', N  , MSB  , MSB-1)

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x00   AAAAAAAA Register 0 ( 7:0)                  (RW)

--   0x01   AAAAAAAA Register 0 (15:8)                  (RW)

--   0x02   AAAAAAAA Register 1 ( 7:0)                  (RW)

--   0x03   AAAAAAAA Register 1 (15:8)                  (RW)

--   0x04   AAAAAAAA Register 2 ( 7:0)                  (RW)

--   0x05   AAAAAAAA Register 2 (15:8)                  (RW)

--   0x06   AAAAAAAA Register 3 ( 7:0)                  (RW)

--   0x07   AAAAAAAA Register 3 (15:8)                  (RW)

--   0x08   AAAAAAAA Register 4 ( 7:0)                  (RW)

--   0x09   AAAAAAAA Register 4 (15:8)                  (RW)

--   0x0A   AAAAAAAA Register 5 ( 7:0)                  (RW)

--   0x0B   AAAAAAAA Register 5 (15:8)                  (RW)

--   0x0C   AAAAAAAA Register 6 ( 7:0)                  (RW)

--   0x0D   AAAAAAAA Register 6 (15:8)                  (RW)

--   0x0E   AAAAAAAA Register 7 ( 7:0)                  (RW)

--   0x0F   AAAAAAAA Register 7 (15:8)                  (RW)

--   0x10   -------- Reserved                           (--)

--   0x11   -------- Reserved                           (--)

--   0x12   -------- Reserved                           (--)

--   0x13   -------- Reserved                           (--)

--   0x14   -------- Reserved                           (--)

--   0x15   -------- Reserved                           (--)

--   0x16   -------- Reserved                           (--)

--   0x17   -------- Reserved                           (--)

--   0x18   -------- Reserved                           (--)

--   0x19   -------- Reserved                           (--)

--   0x1A   -------- Reserved                           (--)

--   0x1B   -------- Reserved                           (--)

--   0x1C   AAAAAAAA Tolerance  ( 7:0)                  (RW)

--   0x1D   AAAAAAAA Tolerance  (15:8)                  (RW)

--   0x1E   BBBBBAAA Instruction Register               (RW)

--                   A = Operand (register select)

--                   B = Opcode  (instruction select)

--   0x1F   E---DCBA Status & Flags                     (RW)

--                   A = Zero Flag

--                   B = Carry Flag

--                   C = Negative Flag

--                   D = Overflow / Error Flag

--                   E = Busy Flag (1 = busy, 0 = idle)

--

-- Instruction Map:

-- OP_T0X  "0000 0xxx" : Transfer R0 to Rx    R0      -> Rx (Sets Z,N)

-- OP_TX0  "0000 1xxx" : Transfer Rx to R0    Rx      -> R0 (Sets Z,N)

-- OP_CLR  "0001 0xxx" : Set Rx to 0          0x00    -> Rx (Sets Z,N)

--

-- OP_IDIV "0001 1xxx" : Integer Division     R0/Rx   -> Q:R0, R:Rx

--

-- OP_UMUL "0010 0xxx" : Unsigned Multiply    R0*Rx   -> R1:R0 (Sets Z)

-- OP_UADD "0010 1xxx" : Unsigned Addition    R0+Rx   -> R0 (Sets N,Z,C)

-- OP_UADC "0011 0xxx" : Unsigned Add w/Carry R0+Rx+C -> R0 (Sets N,Z,C)

-- OP_USUB "0011 1xxx" : Unsigned Subtraction R0-Rx   -> R0 (Sets N,Z,C)

-- OP_USBC "0100 0xxx" : Unsigned Sub w/Carry R0-Rx-C -> R0 (Sets N,Z,C)

-- OP_UCMP "0100 1xxx" : Unsigned Compare     R0-Rx - Sets N,Z,C only

--

-- OP_SMUL "0101 0xxx" : Signed Multiply      R0*Rx   -> R1:R0 (Sets N,Z)

-- OP_SADD "0101 1xxx" : Signed Addition      R0+Rx   -> R0 (Sets N,Z,O)

-- OP_SSUB "0110 0xxx" : Signed Subtraction   R0-Rx   -> R0 (Sets N,Z,O)

-- OP_SCMP "0110 1xxx" : Signed Compare       R0-Rx - Sets N,Z,O only

-- OP_SMAG "0111 0xxx" : Signed Magnitude     |Rx|    -> R0 (Sets Z,O)

-- OP_SNEG "0111 1xxx" : Signed Negation      -Rx     -> R0 (Sets N,Z,O)

--

-- OP_ACMP "1000 0xxx" : Signed Almost Equal (see description)

-- OP_SCRY "1000 1---" : Set the carry bit   (ignores operand)

--

-- OP_UDAB "1001 0xxx" : Decimal Adjust Byte (see description)

-- OP_SDAB "1001 1xxx" : Decimal Adjust Byte (see description)

-- OP_UDAW "1010 0xxx" : Decimal Adjust Word (see description)

-- OP_SDAW "1010 1xxx" : Decimal Adjust Word (see description)

-- OP_RSVD "1011 0---" : Reserved

-- OP_BSWP "1011 1xxx" : Byte Swap (Swaps upper and lower bytes)

-- OP_BOR  "1100 0xxx" : Bitwise Logical OR   Rx or  R0 -> R0

-- OP_BAND "1100 1xxx" : Bitwise Logical AND  Rx and R0 -> R0

-- OP_BXOR "1101 0xxx" : Bitwise Logical XOR  Rx xor R0 -> R0

--

-- OP_BINV "1101 1xxx" : Bitwise logical NOT #Rx      -> Rx

-- OP_BSFL "1110 0xxx" : Logical Shift Left   Rx<<1,0 -> Rx

-- OP_BROL "1110 1xxx" : Logical Rotate Left  Rx<<1,C -> Rx,C

-- OP_BSFR "1111 0xxx" : Logical Shift Right  0,Rx>>1 -> Rx

-- OP_BROR "1111 1xxx" : Logical Rotate Right C,Rx>>1 -> Rx,C

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      07/19/06 Design Start

-- Seth Henry      03/13/15 Added "Almost Equal" instruction

-- Seth Henry      12/19/19 Renamed to o8_alu16 to fit "theme"

-- Seth Henry      04/10/20 Comment and code cleanup

-- Seth Henry      04/16/20 Modified to use Open8 bus record

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_misc.all;

library work;

  use work.open8_pkg.all;

entity o8_alu16 is

generic(

  Address                    : ADDRESS_TYPE

);

port(

  Open8_Bus                  : in  OPEN8_BUS_TYPE;

  Rd_Data                    : out DATA_TYPE;

  Interrupt                  : out std_logic

);

end entity;

architecture behave of o8_alu16 is

  alias Clock                is Open8_Bus.Clock;

  alias Reset                is Open8_Bus.Reset;

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

  -- Opcode Definitions (should match the table above)

  -- Register Manipulation

  constant OP_T0X            : std_logic_vector(4 downto 0) := "00000";

  constant OP_TX0            : std_logic_vector(4 downto 0) := "00001";

  constant OP_CLR            : std_logic_vector(4 downto 0) := "00010";

  -- Integer Division

  constant OP_IDIV           : std_logic_vector(4 downto 0) := "00011";

  -- Unsigned Math Operations

  constant OP_UMUL           : std_logic_vector(4 downto 0) := "00100";

  constant OP_UADD           : std_logic_vector(4 downto 0) := "00101";

  constant OP_UADC           : std_logic_vector(4 downto 0) := "00110";

  constant OP_USUB           : std_logic_vector(4 downto 0) := "00111";

  constant OP_USBC           : std_logic_vector(4 downto 0) := "01000";

  constant OP_UCMP           : std_logic_vector(4 downto 0) := "01001";

  -- Signed Math Operations

  constant OP_SMUL           : std_logic_vector(4 downto 0) := "01010";

  constant OP_SADD           : std_logic_vector(4 downto 0) := "01011";

  constant OP_SSUB           : std_logic_vector(4 downto 0) := "01100";

  constant OP_SCMP           : std_logic_vector(4 downto 0) := "01101";

  constant OP_SMAG           : std_logic_vector(4 downto 0) := "01110";

  constant OP_SNEG           : std_logic_vector(4 downto 0) := "01111";

  -- Signed Almost Equal

  constant OP_ACMP           : std_logic_vector(4 downto 0) := "10000";

  -- Carry Flag set/clear

  constant OP_SCRY           : std_logic_vector(4 downto 0) := "10001";

  -- (Un)Signed Decimal Adjust Byte

  constant OP_UDAB           : std_logic_vector(4 downto 0) := "10010";

  constant OP_SDAB           : std_logic_vector(4 downto 0) := "10011";

  -- (Un)Signed Decimal Adjust Word

  constant OP_UDAW           : std_logic_vector(4 downto 0) := "10100";

  constant OP_SDAW           : std_logic_vector(4 downto 0) := "10101";

  -- Reserved for future use

  constant OP_RSVD           : std_logic_vector(4 downto 0) := "10110";

  -- Byte Swap ( U <> L )

  constant OP_BSWP           : std_logic_vector(4 downto 0) := "10111";

  -- Bitwise Boolean Operations (two operand)

  constant OP_BOR            : std_logic_vector(4 downto 0) := "11000";

  constant OP_BAND           : std_logic_vector(4 downto 0) := "11001";

  constant OP_BXOR           : std_logic_vector(4 downto 0) := "11010";

  -- In-place Bitwise Boolean Operations (single operand)

  constant OP_BINV           : std_logic_vector(4 downto 0) := "11011";

  constant OP_BSFL           : std_logic_vector(4 downto 0) := "11100";

  constant OP_BROL           : std_logic_vector(4 downto 0) := "11101";

  constant OP_BSFR           : std_logic_vector(4 downto 0) := "11110";

  constant OP_BROR           : std_logic_vector(4 downto 0) := "11111";

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

  constant User_Addr         : std_logic_vector(15 downto 5):=

                                 Address(15 downto 5);

  alias Comp_Addr            is Open8_Bus.Address(15 downto 5);

  signal Reg_Addr            : std_logic_vector(4 downto 0) :=

                                (others => '0');

  signal Addr_Match          : std_logic := '0';

  signal Wr_En               : std_logic := '0';

  signal Wr_Data_q           : DATA_TYPE := (others => '0');

  signal Rd_En               : std_logic := '0';

  type REG_ARRAY is array( 0 to 7 ) of std_logic_vector(15 downto 0);

  signal regfile             : REG_ARRAY := (

                                 x"0000",x"0000",x"0000",x"0000",

                                 x"0000",x"0000",x"0000",x"0000");

  signal Start               : std_logic := '0';

  signal Opcode              : std_logic_vector(4 downto 0) :=

                                (others => '0');

  signal Operand_Sel         : std_logic_vector(2 downto 0) :=

                                (others => '0');

  signal Tolerance           : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal High_Tol            : signed(16 downto 0) := (others => '0');

  signal Low_Tol             : signed(16 downto 0) := (others => '0');

  signal Almost_Equal        : std_logic := '0';

  constant FLAG_Z            : integer := 0;

  constant FLAG_C            : integer := 1;

  constant FLAG_N            : integer := 2;

  constant FLAG_O            : integer := 3;

  signal Flags               : std_logic_vector(3 downto 0) :=

                                (others => '0');

  type ALU_STATES is ( IDLE, LOAD, EXECUTE,   IDIV_INIT, IDIV_WAIT,

                       DAW_INIT,   DAB_INIT,  DAA_WAIT1, DAA_STEP2,

                       DAA_WAIT2,  DAA_STEP3, DAA_WAIT3, DAA_STEP4,

                       STORE );

  signal alu_ctrl            : ALU_STATES := IDLE;

  signal Busy                : std_logic := '0';

  signal Busy_q              : std_logic := '0';

  signal Operand_1           : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal Operand_2           : std_logic_vector(15 downto 0) :=

                                (others => '0');

  alias  Dividend            is Operand_1;

  alias  Divisor             is Operand_2;

  alias  u_Operand_1         is Operand_1;

  alias  u_Operand_2         is Operand_2;

  alias  u_Addend_1          is Operand_1;

  alias  u_Addend_2          is Operand_2;

  signal s_Operand_1         : signed(16 downto 0) := (others => '0');

  signal s_Operand_2         : signed(16 downto 0) := (others => '0');

  alias  s_Addend_1          is S_Operand_1;

  alias  s_Addend_2          is S_Operand_2;

  signal u_accum             : std_logic_vector(16 downto 0) :=

                               (others => '0');

  alias  u_data              is u_accum(15 downto 0);

  alias  u_sign              is u_accum(15);

  alias  u_carry             is u_accum(16);

  signal u_prod              : std_logic_vector(31 downto 0) :=

                                (others => '0');

  signal s_accum             : signed(16 downto 0) := (others => '0');

  alias  s_data              is s_accum(15 downto 0);

  alias  s_sign              is s_accum(15);

  alias  s_ovf               is s_accum(16);

  signal s_prod              : signed(33 downto 0) := (others => '0');

  signal IDIV_Start          : std_logic := '0';

  signal IDIV_Busy           : std_logic := '0';

  constant DIV_WIDTH         : integer := 16; -- Width of Operands

  signal q                   : std_logic_vector(DIV_WIDTH*2-1 downto 0) :=

                                (others => '0');

  signal diff                : std_logic_vector(DIV_WIDTH downto 0) :=

                                (others => '0');

  signal count               : integer range 0 to DIV_WIDTH + 1 := 0;

  signal Quotient_i          : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal Quotient            : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal Remainder_i         : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal Remainder           : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal DAA_intreg          : std_logic_vector(15 downto 0) :=

                                (others => '0');

  signal DAA_mode            : std_logic := '0';

  signal DAA_sign            : std_logic := '0';

  signal DAA_p4              : std_logic_vector(3 downto 0) :=

                                (others => '0');

  signal DAA_p3              : std_logic_vector(3 downto 0) :=

                                (others => '0');

  signal DAA_p2              : std_logic_vector(3 downto 0) :=

                                (others => '0');

  alias  DAA_p1              is Quotient(3 downto 0);

  alias  DAA_p0              is Remainder(3 downto 0);

  signal DAA_result          : std_logic_vector(19 downto 0) :=

                                (others => '0');

begin

  Addr_Match                 <= '1' when Comp_Addr = User_Addr else '0';

  -- Sign-extend the base operands to created operands for signed math

  S_Operand_1                <= signed(Operand_1(15) & Operand_1);

  S_Operand_2                <= signed(Operand_2(15) & Operand_2);

  -- Compute the tolerance bounds for the Almost Equal function

  High_Tol                   <= S_Operand_2 + signed('0' & Tolerance);

  Low_Tol                    <= S_Operand_2 - signed('0' & Tolerance);

  -- Combinational logic for the Decimal Adjust logic

  DAA_result                 <= DAA_p4 & DAA_p3 & DAA_p2 & DAA_p1 & DAA_p0;

  -- Combinational logic for the division logic

  diff                       <= ('0' & Q(DIV_WIDTH*2-2 downto DIV_WIDTH-1)) -

                                ('0' & Divisor);

  Quotient_i                 <= q(DIV_WIDTH-1 downto 0);

  Remainder_i                <= q(DIV_WIDTH*2-1 downto DIV_WIDTH);

  ALU_proc: process( Clock, Reset )

    variable Reg_Sel         : integer;

    variable Oper_Sel        : integer;

  begin

    if( Reset = Reset_Level )then

      Wr_En                  <= '0';

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

      Rd_En                  <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

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

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

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

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

      Start                  <= '0';

      Busy_q                 <= '0';

      Interrupt              <= '0';

      for i in 0 to 7 loop

        regfile(i)           <= (others => '0');

      end loop;

      alu_ctrl               <= IDLE;

      Operand_1              <= (others => '0');

      Operand_2              <= (others => '0');

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

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

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

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

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

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

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

      Almost_Equal           <= '0';

      Busy                   <= '0';

      DAA_mode               <= '0';

      DAA_sign               <= '0';

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

      DAA_p4                 <= (others => '0');

      DAA_p3                 <= (others => '0');

      DAA_p2                 <= (others => '0');

      IDIV_Start             <= '0';

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

      count                  <= DIV_WIDTH;

      IDIV_Busy              <= '0';

    elsif( rising_edge(Clock) )then

      -- For convenience, convert these to integers and assign them to

      --  variables

      Reg_Sel                := conv_integer(Reg_Addr(3 downto 1));

      Oper_Sel               := conv_integer(Operand_Sel);

      Wr_En                  <= Addr_Match and Open8_Bus.Wr_En;

      Wr_Data_q              <= Open8_Bus.Wr_Data;

      Reg_Addr               <= Open8_Bus.Address(4 downto 0);

      Start                  <= '0';

      if( Wr_En = '1' )then

        case( Reg_Addr )is

          -- Even addresses go to the lower byte of the register

          when "00000" | "00010" | "00100" | "00110" |

               "01000" | "01010" | "01100" | "01110" =>

            regfile(Reg_Sel)(7 downto 0) <= Wr_Data_q;

          -- Odd addresses go to the upper byte of the register

          when "00001" | "00011" | "00101" | "00111" |

               "01001" | "01011" | "01101" | "01111" =>

            regfile(Reg_Sel)(15 downto 8)<= Wr_Data_q;

          when "11100" => -- 0x1C -> Tolerance.l

            Tolerance(7 downto 0) <= Wr_Data_q;

          when "11101" => -- 0x1D -> Tolerance.u

            Tolerance(15 downto 8) <= Wr_Data_q;

          when "11110" => -- 0x1E -> Opcode register

            Opcode           <= Wr_Data_q(7 downto 3);

            Operand_Sel      <= Wr_Data_q(2 downto 0);

          when "11111" => -- 0x1F -> Status/Start register

            Start            <= '1';

          when others => null;

        end case;

      end if;

      Rd_Data                <= OPEN8_NULLBUS;

      Rd_En                  <= Addr_Match and Open8_Bus.Rd_En;

      if( Rd_En = '1' )then

        case( Reg_Addr )is

          when "00000" | "00010" | "00100" | "00110" |

               "01000" | "01010" | "01100" | "01110" =>

            Rd_Data          <= regfile(Reg_Sel)(7 downto 0);

          when "00001" | "00011" | "00101" | "00111" |

               "01001" | "01011" | "01101" | "01111" =>

            Rd_Data          <= regfile(Reg_Sel)(15 downto 8);

          when "11100" => -- 0x1C -> Tolerance.l

            Rd_Data          <= Tolerance(7 downto 0);

          when "11101" => -- 0x1D -> Tolerance.u

            Rd_Data          <= Tolerance(15 downto 8);

          when "11110" => -- 0x1E -> Opcode register

            Rd_Data          <= Opcode & Operand_Sel;

          when "11111" => -- 0x1F -> Flags & Status register

            Rd_Data          <= Busy & "000" & Flags;

          when others => null;

        end case;

      end if;

      Busy                   <= '1';

      IDIV_Start             <= '0';

      case( alu_ctrl )is

        when IDLE =>

          Busy               <= '0';

          if( Start = '1' )then

            alu_ctrl         <= LOAD;

          end if;

        -- Load the operands from the reg file. We also check for specific

        --  opcodes to set the DAA mode (signed vs unsigned). This is the only

        --  place where we READ the register file outside of the bus interface

        when LOAD =>

          Operand_1          <= regfile(0);

          Operand_2          <= regfile(Oper_Sel);

          DAA_mode           <= '0';

          if( Opcode = OP_SDAW or Opcode = OP_SDAB )then

            DAA_mode         <= '1';

          end if;

          alu_ctrl           <= EXECUTE;

        -- Now that the operands are loaded, we can execute the actual math

        --  operations. We do it with separate operand registers to pipeline

        --  the logic.

        when EXECUTE =>

          alu_ctrl           <= STORE;

          case( Opcode)is

            when OP_T0X =>

              u_accum        <= '0' & Operand_1;

            when OP_TX0 =>

              u_accum        <= '0' & Operand_2;

            when OP_CLR | OP_SCRY =>

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

            when OP_BSWP =>

              u_accum        <= '0' &

                                Operand_2(7 downto 0) &

                                Operand_2(15 downto 8);

            when OP_SMAG =>

              s_accum        <= S_Operand_2;

              if( S_Operand_2 < 0)then

                s_accum      <= -S_Operand_2;

              end if;

            when OP_SNEG =>

              s_accum        <= -S_Operand_2;

            when OP_SMUL =>

              s_prod         <= S_Operand_1 * S_Operand_2;

            when OP_UMUL =>

              u_prod         <= U_Operand_1 * U_Operand_2;

            when OP_SADD =>

              s_accum        <= S_Addend_1  + S_Addend_2;

            when OP_UADD =>

              u_accum        <= ('0' & Operand_1) +

                                ('0' & Operand_2);

            when OP_UADC =>

              u_accum        <= ('0' & Operand_1) +

                                ('0' & Operand_2) +

                                Flags(FLAG_C);

            when OP_SSUB | OP_SCMP =>

              s_accum        <= S_Addend_1 - S_Addend_2;

            when OP_USUB | OP_UCMP =>

              u_accum        <= ('0' & U_Addend_1) -

                                ('0' & U_Addend_2);

            when OP_USBC =>

              u_accum        <= ('0' & U_Addend_1) -

                                ('0' & U_Addend_2) -

                                Flags(FLAG_C);

            when OP_ACMP =>

              -- Perform the function

              -- AE = '1' when (A1 <= A2 + T) and (A1 >= A2 - T) else '0'

              Almost_Equal   <= '0';

              if( (S_Addend_1 <= High_Tol) and

                  (S_Addend_1 >= Low_Tol) )then

                Almost_Equal <= '1';

              end if;

            when OP_BINV =>

              u_accum        <= '0' & (not U_Operand_1);

            when OP_BSFL =>

              u_accum        <= U_Operand_1 & '0';

            when OP_BROL =>

              u_accum        <= U_Operand_1 & Flags(FLAG_C);

            when OP_BSFR =>

              u_accum        <= "00" & U_Operand_1(15 downto 1);

            when OP_BROR =>

              u_accum        <= U_Operand_1(0) & Flags(FLAG_C) &

                                U_Operand_1(15 downto 1);

            when OP_BOR  =>

              u_accum        <= '0' & (U_Operand_1 or U_Operand_2);

            when OP_BAND =>

              u_accum        <= '0' & (U_Operand_1 and U_Operand_2);

            when OP_BXOR =>

              u_accum        <= '0' & (U_Operand_1 xor U_Operand_2);

        -- Division unit has a longer latency, so we need to wait for its busy

        --  signal to return low before storing results. Trigger the engine,

        --  and then jump to the wait state for it to finish

            when OP_IDIV =>

              IDIV_Start     <= '1';

              alu_ctrl       <= IDIV_INIT;

        -- Decimal Adjust Word initialization