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////////////////////////////////////////////////////////////////////////////////
//
//  XGATE Coprocessor - XGATE RISC Processor Core
//
//  Author: Bob Hayes
//          rehayes@opencores.org
//
//  Downloaded from: http://www.opencores.org/projects/xgate.....
//
////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2009, Robert Hayes
//
// This source file is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published
// by the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Supplemental terms.
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Neither the name of the <organization> nor the
//       names of its contributors may be used to endorse or promote products
//       derived from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY Robert Hayes ''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 Robert Hayes 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.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.
////////////////////////////////////////////////////////////////////////////////
// 45678901234567890123456789012345678901234567890123456789012345678901234567890
 
module xgate_risc #(parameter MAX_CHANNEL = 127)    // Max XGATE Interrupt Channel Number
 (
  output reg [15:0] xgr1,
  output reg [15:0] xgr2,
  output reg [15:0] xgr3,
  output reg [15:0] xgr4,
  output reg [15:0] xgr5,
  output reg [15:0] xgr6,
  output reg [15:0] xgr7,
  output     [15:0] xgate_address,
  output     [15:0] write_mem_data,   // Data for Memory write
  output            write_mem_strb_l, // Strobe for writing low data byte
  output            write_mem_strb_h, // Strobe for writing high data bye
  output reg                 zero_flag,
  output reg                 negative_flag,
  output reg                 carry_flag,
  output reg                 overflow_flag,
  output reg          [ 6:0] xgchid,
  output reg [MAX_CHANNEL:0] xgif,          // XGATE Interrupt Flag
  output              [ 7:0] host_semap,    // Semaphore status for host
 
 
  input      [15:0] read_mem_data,
  input      [15:0] perif_data,
  input             risc_clk,
  input             async_rst_b,
  input             xge,            // XGATE Module Enable
  input             xgfrz,          // Stop XGATE in Freeze Mode
  input             xgdbg,          // XGATE Debug Mode
  input             xgss,           // XGATE Single Step
  input      [15:1] xgvbr,          // XGATE vector Base Address Register
  input      [ 6:0] int_req,        // Encoded interrupt request
  input             write_xgsem,    // Write Strobe for XGSEM register
  input             write_xgccr,    // Write Strobe for XGATE Condition Code Register
  input             write_xgpc,     // Write Strobe for XGATE Program Counter
  input             write_xgr7,     // Write Strobe for XGATE Data Register R7
  input             write_xgr6,     // Write Strobe for XGATE Data Register R6
  input             write_xgr5,     // Write Strobe for XGATE Data Register R5
  input             write_xgr4,     // Write Strobe for XGATE Data Register R4
  input             write_xgr3,     // Write Strobe for XGATE Data Register R3
  input             write_xgr2,     // Write Strobe for XGATE Data Register R2
  input             write_xgr1,     // Write Strobe for XGATE Data Register R1
  input             clear_xgif_7,   // Strobe for decode to clear interrupt flag bank 7
  input             clear_xgif_6,   // Strobe for decode to clear interrupt flag bank 6
  input             clear_xgif_5,   // Strobe for decode to clear interrupt flag bank 5
  input             clear_xgif_4,   // Strobe for decode to clear interrupt flag bank 4
  input             clear_xgif_3,   // Strobe for decode to clear interrupt flag bank 3
  input             clear_xgif_2,   // Strobe for decode to clear interrupt flag bank 2
  input             clear_xgif_1,   // Strobe for decode to clear interrupt flag bank 1
  input             clear_xgif_0,   // Strobe for decode to clear interrupt flag bank 0
  input      [15:0] clear_xgif_data // Data for decode to clear interrupt flag
);
 
  integer i, j;  // Loop counters for decode of XGATE Interrupt Register
  integer k;     // Loop counter for Bit Field Insert decode
  integer bfi, bfii, bfix;     // Loop counter for Bit Field Insert function
 
  // State machine sequence
  parameter [2:0]      //synopsys enum state_info
       IDLE    = 3'b000,      // waiting for interrupt
       CONT    = 3'b001,      // Instruction processing state, first state
       STALL   = 3'b010,      // Second step in 2 state instruction (Memory Read/Write)
       DEBUG   = 3'b011,      //
       BOOT_1  = 3'b100,      //
       BOOT_2  = 3'b101,      //
       BOOT_3  = 3'b110,      //
       UNDEF5  = 3'b111;      //
 
 
  // Semaphore states
  parameter [1:0] NO_LOCK = 2'b00,
                  RISC_LOCK = 2'b10,
                  HOST_LOCK = 2'b11;
 
 
  reg  [ 2:0] cpu_state;
  reg  [ 2:0] next_cpu_state;    // Pseudo Register,
  reg         load_next_inst;    // Pseudo Register,
  reg  [15:0] program_counter;
  reg  [15:0] next_pc;           // Pseudo Register,
  reg  [15:0] alu_result;        // Pseudo Register,
  reg  [15:0] op_code;
  reg         ena_rd_low_byte;   // Pseudo Register,
  reg         ena_rd_high_byte;  // Pseudo Register,
 
  reg         data_access;   // Pseudo Register, RAM access in proccess
  reg         data_write;    // Pseudo Register, RAM access is write operation
  reg         data_word_op;  // Pseudo Register, RAM access operation is 16 bits(word)
  reg  [15:0] data_address;  // Pseudo Register, Address for RAM data read or write
  reg  [15:0] load_data;
 
  reg  [ 6:0] set_irq_flag;
 
  reg         next_zero;      // Pseudo Register,
  reg         next_negative;  // Pseudo Register,
  reg         next_carry;     // Pseudo Register,
  reg         next_overflow;  // Pseudo Register,
 
  reg         set_semaph;     // Pseudo Register,
  reg         clear_semaph;   // Pseudo Register,
  reg  [ 2:0] semaph_risc;    // Pseudo Register,
  reg  [ 7:0] semap_risc_bit; // Pseudo Register,
  wire [ 7:0] risc_semap;     // Semaphore status bit for RISC
  wire        semaph_stat;    // Return Status of Semaphore bit
 
  reg  [15:0] rd_data;        // Pseudo Register,
  reg  [15:0] rs1_data;       // Pseudo Register,
  reg  [15:0] rs2_data;       // Pseudo Register,
 
  wire [ 2:0] wrt_reg_sel;
  reg         sel_rd_field;  // Pseudo Register,
  reg         wrt_sel_xgr1;  // Pseudo Register,
  reg         wrt_sel_xgr2;  // Pseudo Register,
  reg         wrt_sel_xgr3;  // Pseudo Register,
  reg         wrt_sel_xgr4;  // Pseudo Register,
  reg         wrt_sel_xgr5;  // Pseudo Register,
  reg         wrt_sel_xgr6;  // Pseudo Register,
  reg         wrt_sel_xgr7;  // Pseudo Register,
 
  reg [MAX_CHANNEL:0] xgif_d;
 
  reg  [15:0] shift_in;
  wire [15:0] shift_out;
  wire        shift_rollover;
  reg         shift_left;
  reg  [ 4:0] shift_ammount;
  reg  [15:0] shift_filler;
 
  wire        start_thread;
 
 
 
  assign xgate_address = data_access ? data_address : program_counter;
 
  assign write_mem_strb_l = data_access && data_write && (data_word_op || !data_address[0]);
  assign write_mem_strb_h = data_access && data_write && (data_word_op ||  data_address[0]);
  assign write_mem_data   = (write_mem_strb_l || write_mem_strb_h) ? rd_data : 16'b0;
 
  assign start_thread = xge && (|int_req);
 
  // Decode register select for RD and RS
  always @*
    begin
      case (op_code[10:8])
        3'b001 : rd_data = xgr1;
        3'b010 : rd_data = xgr2;
        3'b011 : rd_data = xgr3;
        3'b100 : rd_data = xgr4;
        3'b101 : rd_data = xgr5;
        3'b110 : rd_data = xgr6;
        3'b111 : rd_data = xgr7;
        default : rd_data = 16'h0;  // XGR0 is always Zero
      endcase
    end
 
  assign wrt_reg_sel = sel_rd_field ? op_code[10:8] : op_code[4:2];
 
  // Decode register write select for eather RD or RI/RS2
  always @*
    begin
      wrt_sel_xgr1 = (cpu_state == BOOT_2);
      wrt_sel_xgr2 = 1'b0;
      wrt_sel_xgr3 = 1'b0;
      wrt_sel_xgr4 = 1'b0;
      wrt_sel_xgr5 = 1'b0;
      wrt_sel_xgr6 = 1'b0;
      wrt_sel_xgr7 = 1'b0;
      case (wrt_reg_sel)
        3'b001 : wrt_sel_xgr1 = 1'b1;
        3'b010 : wrt_sel_xgr2 = 1'b1;
        3'b011 : wrt_sel_xgr3 = 1'b1;
        3'b100 : wrt_sel_xgr4 = 1'b1;
        3'b101 : wrt_sel_xgr5 = 1'b1;
        3'b110 : wrt_sel_xgr6 = 1'b1;
        3'b111 : wrt_sel_xgr7 = 1'b1;
      endcase
    end
 
  // Decode register select for RS1 and RB
  always @*
    case (op_code[7:5])
      3'b001 : rs1_data = xgr1;
      3'b010 : rs1_data = xgr2;
      3'b011 : rs1_data = xgr3;
      3'b100 : rs1_data = xgr4;
      3'b101 : rs1_data = xgr5;
      3'b110 : rs1_data = xgr6;
      3'b111 : rs1_data = xgr7;
      default : rs1_data = 16'h0;  // XGR0 is always Zero
    endcase
 
  // Decode register select for RS2 and RI
  always @*
    case (op_code[4:2])
      3'b001 : rs2_data = xgr1;
      3'b010 : rs2_data = xgr2;
      3'b011 : rs2_data = xgr3;
      3'b100 : rs2_data = xgr4;
      3'b101 : rs2_data = xgr5;
      3'b110 : rs2_data = xgr6;
      3'b111 : rs2_data = xgr7;
      default : rs2_data = 16'h0;  // XGR0 is always Zero
    endcase
 
  reg [15:0] bf_mux_mask;  // Mask for controlling mux's in Bit Field Insert Instructions
  // Decode mux select mask for Bit Field Insert Instructions
  always @*
    begin
      k = 0;
      while (k < 16)
        begin
          bf_mux_mask[k] = 1'b0;
          if ((k >= rs2_data[3:0]) && (k <= rs2_data[3:0] + rs2_data[7:4]))
            bf_mux_mask[k] = 1'b1;
          k = k + 1;
        end
    end
 
 
  //  CPU State Register
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      cpu_state  <= IDLE;
    else
      cpu_state  <= next_cpu_state;
 
  //  CPU Instruction Register
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      op_code  <= 16'h0000;
    else
      op_code  <= load_next_inst ? read_mem_data : op_code;
 
  //  Active Channel Latch
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      xgchid  <= 7'b0;
    else
      xgchid  <= (cpu_state == IDLE) ? int_req : xgchid;
 
  //  CPU Read Data Buffer Register
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      load_data  <= 16'h0000;
    else
      load_data  <= (data_access && !data_write) ? read_mem_data : load_data;
 
  //  Program Counter Register
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      program_counter  <= 16'h0000;
    else
      program_counter  <= next_pc;
 
  //  ALU Flag Bits
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      begin
        carry_flag    <= 1'b0;
        overflow_flag <= 1'b0;
        zero_flag     <= 1'b0;
        negative_flag <= 1'b0;
      end
    else
      begin
        carry_flag    <= write_xgccr ? perif_data[0] : next_carry;
        overflow_flag <= write_xgccr ? perif_data[1] : next_overflow;
        zero_flag     <= write_xgccr ? perif_data[2] : next_zero;
        negative_flag <= write_xgccr ? perif_data[3] : next_negative;
      end
 
  //  Interrupt Flag next value
  always @*
      begin
        j = 0;
        i = 0;
        while (j <= MAX_CHANNEL)
          begin
            if (j < 16)
              xgif_d[j]  = (~(clear_xgif_0 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (15 < j < 32)
              xgif_d[j]  = (~(clear_xgif_1 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (31 < j < 48)
              xgif_d[j]  = (~(clear_xgif_2 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (47 < j < 64)
              xgif_d[j]  = (~(clear_xgif_3 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (63 < j < 80)
              xgif_d[j]  = (~(clear_xgif_4 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (79 < j < 96)
              xgif_d[j]  = (~(clear_xgif_5 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (95 < j < 112)
              xgif_d[j]  = (~(clear_xgif_6 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (111 < j < 128)
              xgif_d[j]  = (~(clear_xgif_7 && clear_xgif_data[i]) && xgif[j]) || (set_irq_flag == j);
            if (i < 15)
              i = i + 1;
            else
              i = 0;
            j = j + 1;
          end
      end
 
  //  Interrupt Flag Registers
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      xgif  <= 0;
    else
      xgif  <= xgif_d;
 
 
  //  RISC Data Registers
  always @(posedge risc_clk or negedge async_rst_b)
    if ( !async_rst_b )
      begin
        xgr1 <= 16'b0;
        xgr2 <= 16'b0;
        xgr3 <= 16'b0;
        xgr4 <= 16'b0;
        xgr5 <= 16'b0;
        xgr6 <= 16'b0;
        xgr7 <= 16'b0;
      end
    else
      begin
        xgr1 <= write_xgr1 ? perif_data :
                {((wrt_sel_xgr1 && ena_rd_high_byte) ? alu_result[15:8] : xgr1[15:8]),
                 ((wrt_sel_xgr1 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr1[ 7:0])};
        xgr2 <= write_xgr2 ? perif_data :
                {((wrt_sel_xgr2 && ena_rd_high_byte) ? alu_result[15:8] : xgr2[15:8]),
                 ((wrt_sel_xgr2 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr2[ 7:0])};
        xgr3 <= write_xgr3 ? perif_data :
                {((wrt_sel_xgr3 && ena_rd_high_byte) ? alu_result[15:8] : xgr3[15:8]),
                 ((wrt_sel_xgr3 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr3[ 7:0])};
        xgr4 <= write_xgr4 ? perif_data :
                {((wrt_sel_xgr4 && ena_rd_high_byte) ? alu_result[15:8] : xgr4[15:8]),
                 ((wrt_sel_xgr4 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr4[ 7:0])};
        xgr5 <= write_xgr5 ? perif_data :
                {((wrt_sel_xgr5 && ena_rd_high_byte) ? alu_result[15:8] : xgr5[15:8]),
                 ((wrt_sel_xgr5 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr5[ 7:0])};
        xgr6 <= write_xgr6 ? perif_data :
                {((wrt_sel_xgr6 && ena_rd_high_byte) ? alu_result[15:8] : xgr6[15:8]),
                 ((wrt_sel_xgr6 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr6[ 7:0])};
        xgr7 <= write_xgr7 ? perif_data :
                {((wrt_sel_xgr7 && ena_rd_high_byte) ? alu_result[15:8] : xgr7[15:8]),
                 ((wrt_sel_xgr7 && ena_rd_low_byte)  ? alu_result[ 7:0] : xgr7[ 7:0])};
      end
 
  // V � Vector fetch: always an aligned word read, lasts for at least one RISC core cycle
  // P � Program word fetch: always an aligned word read, lasts for at least one RISC core cycle
  // r � 8-bit data read: lasts for at least one RISC core cycle
  // R � 16-bit data read: lasts for at least one RISC core cycle
  // w � 8-bit data write: lasts for at least one RISC core cycle
  // W � 16-bit data write: lasts for at least one RISC core cycle
  // A � Alignment cycle: no read or write, lasts for zero or one RISC core cycles
  // f � Free cycle: no read or write, lasts for one RISC core cycles
  // Special Cases
  // PP/P � Branch: PP if branch taken, P if not taken
 
  always @*
    begin
      ena_rd_low_byte  = 1'b0;
      ena_rd_high_byte = 1'b0;
 
      next_cpu_state = CONT;
      load_next_inst = 1'b1;
      next_pc        = program_counter + 16'h0002;
 
      next_zero      = zero_flag;
      next_negative  = negative_flag;
      next_carry     = carry_flag;
      next_overflow  = overflow_flag;
 
      alu_result     = 16'h0000;
      sel_rd_field   = 1'b1;
 
      data_access    = 1'b0;
      data_word_op   = 1'b0;
      data_write     = 1'b0;
      data_address   = 16'h0000;
 
      shift_left     = 1'b0;
      shift_ammount  = 5'b0_0000;
      shift_filler   = 16'h0000;
      shift_in       = rd_data;
 
      set_irq_flag   = 7'b0;
 
      set_semaph    = 1'b0;
      clear_semaph  = 1'b0;
      semaph_risc   = 3'b0;
 
  casez ({cpu_state, op_code})
 
      {IDLE, 16'b????????????????} :
         begin
           next_cpu_state   = start_thread ? BOOT_1 : IDLE;
           next_pc          = program_counter;
           load_next_inst   = 1'b0;
         end
 
      // Output RAM address for Program Counter
      {BOOT_1, 16'b????????????????} :
         begin
           next_cpu_state   = BOOT_2;
           next_pc          = program_counter;
           load_next_inst   = 1'b0;
           data_access      = 1'b1;
           data_word_op     = 1'b1;
           data_address     = {xgvbr, 1'b0} + {7'b0, xgchid, 2'b0};
         end
 
      // Load PC value from data buffer register
      // Output RAM address for initial variable pointer
      {BOOT_2, 16'b????????????????} :
         begin
           next_cpu_state   = BOOT_3;
           next_pc          = load_data;
           load_next_inst   = 1'b0;
           data_access      = 1'b1;
           data_word_op     = 1'b1;
           data_address     = {xgvbr, 1'b0} + {7'b0, xgchid, 2'b0} + 2;
         end
 
      // Load R1 with initial variable pointer from data buffer register
      // Load first instruction to op_code register
      // Increment Program Counter
      {BOOT_3, 16'b????????????????} :
         begin
           next_cpu_state   = CONT;
           load_next_inst   = 1'b1;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = load_data;
         end
 
      {DEBUG, 16'b????????????????} :
         begin
           next_cpu_state = xge ? DEBUG : IDLE;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Return to Scheduler and Others
      // -----------------------------------------------------------------------
 
      // Instruction = BRK, Op Code =  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
      // Cycles - PAff
      {CONT, 16'b0000000000000000} :
         begin
           next_cpu_state   = DEBUG;
           next_pc          = program_counter;
           load_next_inst   = 1'b0;
         end
 
      // Instruction = NOP, Op Code =  0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0
      {CONT, 16'b0000000100000000} :
         begin
         end
 
      // Instruction = RTS, Op Code =  0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0
      // Cycles - PA
      {CONT, 16'b0000001000000000} :
         begin
           next_cpu_state   = IDLE;
           next_pc          = program_counter;
           load_next_inst   = 1'b0;
         end
 
      // Instruction = SIF, Op Code =  0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0
      // Sets the Interrupt Flag of the current channel (XGCHID) will be set.
      // Cycles - PA
      {CONT, 16'b0000001100000000} :
         begin
           set_irq_flag = xgchid;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Semaphore Instructions
      // -----------------------------------------------------------------------
 
      // Instruction = CSEM IMM3, Op Code =  0 0 0 0 0 IMM3 1 1 1 1 0 0 0 0
      // Unlocks a semaphore that was locked by the RISC core.
      // Cycles - PA
      {CONT, 16'b00000???11110000} :
         begin
           clear_semaph = 1'b1;
           semaph_risc  = op_code[10:8];
         end
 
      // Instruction = CSEM RS, Op Code =  0 0 0 0 0 RS 1 1 1 1 0 0 0 1
      // Unlocks a semaphore that was locked by the RISC core.
      // In monadic address mode, bits RS[2:0] select the semaphore to be cleared.
      // Cycles - PA
      {CONT, 16'b00000???11110001} :
         begin
           clear_semaph = 1'b1;
           semaph_risc  = rd_data[2:0];
         end
 
      // Instruction = SSEM IMM3, Op Code =  0 0 0 0 0 IMM3 1 1 1 1 0 0 1 0
      // Attempts to set a semaphore. The state of the semaphore will be stored in the Carry-Flag:
      // 1 = Semaphore is locked by the RISC core
      // 0 = Semaphore is locked by the S12X_CPU
      // Cycles - PA
      {CONT, 16'b00000???11110010} :
         begin
           set_semaph  = 1'b1;
           semaph_risc = op_code[10:8];
 
           next_carry    = semaph_stat;
         end
 
      // Instruction = SSEM RS, Op Code =  0 0 0 0 0 RS 1 1 1 1 0 0 1 1
      // Attempts to set a semaphore. The state of the semaphore will be stored in the Carry-Flag:
      // 1 = Semaphore is locked by the RISC core
      // 0 = Semaphore is locked by the S12X_CPU
      // In monadic address mode, bits RS[2:0] select the semaphore to be set.
      // Cycles - PA
      {CONT, 16'b00000???11110011} :
         begin
           set_semaph  = 1'b1;
           semaph_risc = rd_data[2:0];
 
           next_carry    = semaph_stat;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Single Register Instructions
      // -----------------------------------------------------------------------
 
      // Instruction = SEX RD, Op Code =  0 0 0 0 0 RD 1 1 1 1 0 1 0 0
      // SEX - Sign Extend Byte to Word
      // Cycles - P
      {CONT, 16'b00000???11110100} :
         begin
           ena_rd_high_byte = 1'b1;
 
           alu_result    = {{8{rd_data[7]}}, rd_data[7:0]};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = PAR RD, Op Code =  0 0 0 0 0 RD 1 1 1 1 0 1 0 1
      // PAR - Calculate Parity
      // Set Carry Flag on odd number of bits
      // Cycles - P
      {CONT, 16'b00000???11110101} :
         begin
           next_zero     = !(|rd_data);
           next_negative = 1'b0;
           next_carry    = ^rd_data;
           next_overflow = 1'b0;
         end
 
      // Instruction = JAL RD, Op Code =  0 0 0 0 0 RD 1 1 1 1 0 1 1 0
      // Jump And Link
      // PC + $0002 => RD; RD => PC
      // Jumps to the address stored in RD and saves the return address in RD.
      // Cycles - PP
      {CONT, 16'b00000???11110110} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = program_counter;
           next_pc          = rd_data;
         end
 
      {STALL, 16'b00000???11110110} :
         begin
           next_cpu_state   = CONT;
         end
 
      // Instruction = SIF RS, Op Code =  0 0 0 0 0 RS 1 1 1 1 0 1 1 1
      // Sets the Interrupt Flag associated with the channel id number
      // contained in RS[6:0] is set. The content of RS[15:7] is ignored
      // Cycles - P
      {CONT, 16'b00000???11110111} :
         begin
           set_irq_flag = rd_data[6:0];
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Special Move instructions
      // -----------------------------------------------------------------------
 
      // Instruction = TFR RD,CCR, Op Code =  0 0 0 0 0 RD 1 1 1 1 1 0 0 0
      // Transfer from and to Special Registers
      // TFR RD,CCR: CCR => RD[3:0], 0 => RD[15:4]
      // Cycles - P
      {CONT, 16'b00000???11111000} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = {12'b0, negative_flag, zero_flag, overflow_flag, carry_flag};
         end
 
      // Instruction = TFR CCR,RS, Op Code =  0 0 0 0 0 RS 1 1 1 1 1 0 0 1
      // Transfer from and to Special Registers
      // TFR CCR,RD: RD[3:0] => CCR
      // Cycles - P
      {CONT, 16'b00000???11111001} :
         begin
           next_negative = rd_data[3];
           next_zero     = rd_data[2];
           next_overflow = rd_data[1];
           next_carry    = rd_data[0];
         end
 
      // Instruction = TFR RD,PC, Op Code =  0 0 0 0 0 RD 1 1 1 1 1 0 1 0
      // Transfer from and to Special Registers
      // TFR RD,PC: PC+4 => RD
      // Cycles - P
      {CONT, 16'b00000???11111010} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = next_pc;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Shift instructions Dyadic
      // -----------------------------------------------------------------------
 
      // Instruction = BFFO RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 0 0 0
      // BFFO - Bit Field Find First One
      // FirstOne (RS) => RD
      // Cycles - P
      {CONT, 16'b00001??????10000} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           casez (rs1_data)
             16'b1???_????_????_???? : alu_result = 16'h000f;
             16'b01??_????_????_???? : alu_result = 16'h000e;
             16'b001?_????_????_???? : alu_result = 16'h000d;
             16'b0001_????_????_???? : alu_result = 16'h000c;
             16'b0000_1???_????_???? : alu_result = 16'h000b;
             16'b0000_01??_????_???? : alu_result = 16'h000a;
             16'b0000_001?_????_???? : alu_result = 16'h0009;
             16'b0000_0001_????_???? : alu_result = 16'h0008;
             16'b0000_0000_1???_???? : alu_result = 16'h0007;
             16'b0000_0000_01??_???? : alu_result = 16'h0006;
             16'b0000_0000_001?_???? : alu_result = 16'h0005;
             16'b0000_0000_0001_???? : alu_result = 16'h0004;
             16'b0000_0000_0000_1??? : alu_result = 16'h0003;
             16'b0000_0000_0000_01?? : alu_result = 16'h0002;
             16'b0000_0000_0000_001? : alu_result = 16'h0001;
             16'b0000_0000_0000_0001 : alu_result = 16'h0000;
           endcase
           next_zero     = !(|alu_result);
           next_negative = 1'b0;
           next_carry    = !(|rs1_data);
           next_overflow = 1'b0;
         end
 
      // Instruction = ASR RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 0 0 1
      // ASR - Arithmetic Shift Right
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10001} :
         begin
           shift_ammount  = {|rs1_data[15:4], rs1_data[3:0]};
           shift_filler   = {16{rd_data[15]}};
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = |rs1_data ? shift_rollover : carry_flag;
           next_overflow    = rd_data[15] ^ alu_result[15];  // Table and text disagree
         end
 
      // Instruction = CSL RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 0 1 0
      // CSL - Logical Shift Left with Carry
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10010} :
         begin
           shift_left     = 1'b1;
           shift_ammount  = {|rs1_data[15:4], rs1_data[3:0]};
           shift_filler   = {16{carry_flag}};
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = |rs1_data ? shift_rollover : carry_flag;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = CSR RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 0 1 1
      // Logical Shift Right with Carry
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10011} :
         begin
           shift_ammount  = {|rs1_data[15:4], rs1_data[3:0]};
           shift_filler   = {16{carry_flag}};
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = |rs1_data ? shift_rollover : carry_flag;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = LSL RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 1 0 0
      // Logical Shift Left
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10100} :
         begin
           shift_left     = 1'b1;
           shift_ammount  = {|rs1_data[15:4], rs1_data[3:0]};
           shift_filler   = 16'h0000;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = |rs1_data ? shift_rollover : carry_flag;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = LSR RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 1 0 1
      // Logical Shift Right
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10101} :
         begin
           shift_ammount  = {|rs1_data[15:4], rs1_data[3:0]};
           shift_filler   = 16'h0000;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = |rs1_data ? shift_rollover : carry_flag;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = ROL RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 1 1 0
      // Rotate Left
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10110} :
         begin
           shift_left     = 1'b1;
           shift_ammount  = {1'b0, rs1_data[3:0]};
           shift_filler   = rd_data;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_overflow    = 1'b0;
         end
 
      // Instruction = ROR RD, RS, Op Code =  0 0 0 0 1 RD RS 1 0 1 1 1
      // Rotate Right
      // n = RS
      // Cycles - P
      {CONT, 16'b00001??????10111} :
         begin
           shift_ammount  = {1'b0, rs1_data[3:0]};
           shift_filler   = rd_data;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_overflow    = 1'b0;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Shift instructions immediate
      // -----------------------------------------------------------------------
 
      // Instruction = ASR RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 0 0 1
      // ASR - Arithmetic Shift Right
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1001} :
         begin
           shift_ammount  = {!(|op_code[7:4]), op_code[7:4]};
           shift_filler   = {16{rd_data[15]}};
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = shift_rollover;
           next_overflow    = rd_data[15] ^ alu_result[15];  // Table and text disagree
         end
 
      // Instruction = CSL RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 0 1 0
      // CSL - Logical Shift Left with Carry
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1010} :
         begin
           shift_left     = 1'b1;
           shift_ammount  = {!(|op_code[7:4]), op_code[7:4]};
           shift_filler   = {16{carry_flag}};
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = shift_rollover;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = CSR RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 0 1 1
      // CSR - Logical Shift Right with Carry
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1011} :
         begin
           shift_ammount  = {!(|op_code[7:4]), op_code[7:4]};
           shift_filler   = {16{carry_flag}};
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = shift_rollover;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = LSL RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 1 0 0
      // LSL - Logical Shift Left
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1100} :
         begin
           shift_left     = 1'b1;
           shift_ammount  = {!(|op_code[7:4]), op_code[7:4]};
           shift_filler   = 16'h0000;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = shift_rollover;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = LSR RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 1 0 1
      // LSR - Logical Shift Right
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1101} :
         begin
           shift_ammount  = {!(|op_code[7:4]), op_code[7:4]};
           shift_filler   = 16'h0000;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_carry       = shift_rollover;
           next_overflow    = rd_data[15] ^ alu_result[15];
         end
 
      // Instruction = ROL RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 1 1 0
      // ROL - Rotate Left
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1110} :
         begin
           shift_left     = 1'b1;
           shift_ammount  = {1'b0, op_code[7:4]};
           shift_filler   = rd_data;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_overflow    = 1'b0;
         end
 
      // Instruction = ROR RD, #IMM4, Op Code =  0 0 0 0 1 RD IMM4 1 1 1 1
      // ROR - Rotate Right
      // n = IMM4
      // Cycles - P
      {CONT, 16'b00001???????1111} :
         begin
           shift_ammount  = {1'b0, op_code[7:4]};
           shift_filler   = rd_data;
 
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           alu_result       = shift_out;
           next_zero        = !(|alu_result);
           next_negative    = alu_result[15];
           next_overflow    = 1'b0;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Logical Triadic
      // -----------------------------------------------------------------------
 
      // Instruction = AND RD, RS1, RS2, Op Code =  0 0 0 1 0 RD RS1 RS2 0 0
      // AND - Logical AND
      // RS1 & RS2 => RD
      // Cycles - P
      {CONT, 16'b00010?????????00} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           alu_result    = rs1_data & rs2_data;
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = OR RD, RS1, RS2, Op Code =  0 0 0 1 0 RD RS1 RS2 1 0
      // OR - Logical OR
      // RS1 | RS2 => RD
      // Cycles - P
      {CONT, 16'b00010?????????10} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           alu_result    = rs1_data | rs2_data;
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
          end
 
      // Instruction = XNOR RD, RS1, RS2, Op Code =  0 0 0 1 0 RD RS1 RS2 1 1
      // XNOR - Logical Exclusive NOR
      // ~(RS1 ^ RS2) => RD
      // Cycles - P
      {CONT, 16'b00010?????????11} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           alu_result    = ~(rs1_data ^ rs2_data);
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Arithmetic Triadic
      // -----------------------------------------------------------------------
 
      // Instruction = SUB RD, RS1, RS2, Op Code =  0 0 0 1 1 RD RS1 RS2 0 0
      // SUB - Subtract without Carry
      // RS1 - RS2 => RD
      // Cycles - P
      {CONT, 16'b00011?????????00} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           {next_carry, alu_result}    = rs1_data - rs2_data;
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = (rs1_data[15] && !rs2_data[15] && !alu_result[15]) || (!rs1_data[15] && rs2_data[15] && alu_result[15]);
         end
 
      // Instruction = SBC RD, RS1, RS2, Op Code =  0 0 0 1 1 RD RS1 RS2 0 1
      // SBC - Subtract with Carry
      // RS1 - RS2 - C => RD
      // Cycles - P
      {CONT, 16'b00011?????????01} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           {next_carry, alu_result}    = rs1_data - rs2_data - {15'b0, carry_flag};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = (rs1_data[15] && !rs2_data[15] && !alu_result[15]) || (!rs1_data[15] && rs2_data[15] && alu_result[15]);
         end
 
      // Instruction = ADD RD, RS1, RS2, Op Code =  0 0 0 1 1 RD RS1 RS2 1 0
      // ADD - ADD without carry
      // RS1 + RS2 => RD
      // Cycles - P
      {CONT, 16'b00011?????????10} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           {next_carry, alu_result}    = rs1_data + rs2_data;
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = (rs1_data[15] && rs2_data[15] && !alu_result[15]) || (!rs1_data[15] && !rs2_data[15] && alu_result[15]);
         end
 
      // Instruction = ADC RD, RS1, RS2, Op Code =  0 0 0 1 1 RD RS1 RS2 1 1
      // ADC - add with carry
      // RS1 + RS2 + C => RD
      // Cycles - P
      {CONT, 16'b00011?????????11} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           {next_carry, alu_result}    = rs1_data + rs2_data + {15'b0, carry_flag};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = (rs1_data[15] && rs2_data[15] && !alu_result[15]) || (!rs1_data[15] && !rs2_data[15] && alu_result[15]);
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Branches
      // -----------------------------------------------------------------------
 
      // Instruction = BCC REL9, Op Code =  0 0 1 0 0 0 0 REL9
      // Branch if Carry Cleared
      // If C = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010000?????????} :
         begin
           if (!carry_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010000?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BCS REL9, Op Code =  0 0 1 0 0 0 1 REL9
      // Branch if Carry Set
      // If C = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010001?????????} :
         begin
           if (carry_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010001?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BNE REL9, Op Code =  0 0 1 0 0 1 0 REL9
      // Branch if Not Equal
      // If Z = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010010?????????} :
         begin
           if (!zero_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010010?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BEQ REL9, Op Code =  0 0 1 0 0 1 1 REL9
      // Branch if Equal
      // If Z = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010011?????????} :
         begin
           if (zero_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010011?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BPL REL9, Op Code =  0 0 1 0 1 0 0 REL9
      // Branch if Plus
      // If N = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010100?????????} :
         begin
           if (!negative_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010100?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BMI REL9, Op Code =  0 0 1 0 1 0 1 REL9
      // Branch if Minus
      // If N = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010101?????????} :
         begin
           if (negative_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010101?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BVC REL9, Op Code =  0 0 1 0 1 1 0 REL9
      // Branch if Overflow Cleared
      // If V = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010110?????????} :
         begin
           if (!overflow_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010110?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BVS REL9, Op Code =  0 0 1 0 1 1 1 REL9
      // Branch if Overflow Set
      // If V = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0010111?????????} :
         begin
           if (overflow_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0010111?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BHI REL9, Op Code =  0 0 1 1 0 0 0 REL9
      // Branch if Higher
      // If C | Z = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0011000?????????} :
         begin
           if (!(carry_flag || zero_flag))
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0011000?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BLS REL9, Op Code =  0 0 1 1 0 0 1 REL9
      // Branch if Lower or Same
      // If C | Z = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0011001?????????} :
         begin
           if (carry_flag || zero_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0011001?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BGE REL9, Op Code =  0 0 1 1 0 1 0 REL9
      // Branch if Greater than or Equal to Zero
      // If N ^ V = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0011010?????????} :
         begin
           if (!(negative_flag ^ overflow_flag))
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0011010?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BLT REL9, Op Code =  0 0 1 1 0 1 1 REL9
      // Branch if Lower than Zero
      // If N ^ V = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0011011?????????} :
         begin
           if (negative_flag ^ overflow_flag)
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0011011?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BGT REL9, Op Code =  0 0 1 1 1 0 0 REL9
      // Branch if Greater than Zero
      // If Z | (N ^ V) = 0, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0011100?????????} :
         begin
           if (!(zero_flag || (negative_flag ^ overflow_flag)))
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0011100?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BLE REL9, Op Code =  0 0 1 1 1 0 1 REL9
      // Branch if Less or Equal to Zero
      // If Z | (N ^ V) = 1, then PC + $0002 + (REL9 << 1) => PC
      // Cycles - PP/P
      {CONT, 16'b0011101?????????} :
         begin
           if (zero_flag || (negative_flag ^ overflow_flag))
             begin
               next_cpu_state = STALL;
               load_next_inst = 1'b0;
               next_pc        = program_counter + {{6{op_code[8]}}, op_code[8:0], 1'b0};
             end
         end
 
      {STALL, 16'b0011101?????????} :
         begin
               next_cpu_state = CONT;
         end
 
      // Instruction = BRA REL10, Op Code =  0 0 1 1 1 1 REL10
      // Branch Always, signed offset
      // PC + $0002 + (REL10 << 1) => PC
      // Cycles - PP
      {CONT, 16'b001111??????????} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter + {{5{op_code[9]}}, op_code[9:0], 1'b0};
         end
 
      {STALL, 16'b001111??????????} :
         begin
           next_cpu_state = CONT;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Load and Store Instructions
      // -----------------------------------------------------------------------
 
      // Instruction = LDB RD, (RB, #OFFS5), Op Code =  0 1 0 0 0 RD RB OFFS5
      // Load Byte from Memory, unsigned offset
      // M[RB, #OFFS5] => RD.L; $00 => RD.H
      // RB field == RS1 field
      // Cycles - Pr
      {CONT, 16'b01000???????????} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_address   = rs1_data + {11'b0, op_code[4:0]};
         end
 
      {STALL, 16'b01000???????????} :
         begin
           next_cpu_state   = CONT;
           alu_result       = {8'h00, load_data[15:8]};
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = LDW RD, (RB, #OFFS5), Op Code =  0 1 0 0 1 RD RB OFFS5
      // Load Word from Memory, unsigned offset
      // M[RB, #OFFS5] => RD
      // RB field == RS1 field
      // Cycles - PR
      {CONT, 16'b01001???????????} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_address   = rs1_data + {11'b0, op_code[4:0]};
           data_word_op   = 1'b1;
         end
 
      {STALL, 16'b01001???????????} :
         begin
           next_cpu_state   = CONT;
           alu_result       = load_data;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = STB RS, (RB, #OFFS5), Op Code =  0 1 0 1 0 RS RB OFFS5
      // Store Byte to Memory, unsigned offset
      // RS.L => M[RB, #OFFS5]
      // RS field == RD fieild, RB field == RS1 field
      // Cycles - Pw
      {CONT, 16'b01010???????????} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_write     = 1'b1;
           data_address   = rs1_data + {11'b0, op_code[4:0]};
         end
 
      {STALL, 16'b01010???????????} :
         begin
           next_cpu_state = CONT;
         end
 
 
      // Instruction = STW RS, (RB, #OFFS5), Op Code =  0 1 0 1 1 RS RB OFFS5
      // Store Word to Memory, unsigned offset
      // RS => M[RB, #OFFS5]
      // RS field == RD fieild, RB field == RS1 field
      // Cycles - PW
      {CONT, 16'b01011???????????} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_write     = 1'b1;
           data_address   = rs1_data + {11'b0, op_code[4:0]};
           data_word_op   = 1'b1;
         end
 
      {STALL, 16'b01011???????????} :
         begin
           next_cpu_state = CONT;
         end
 
      // Instruction = LDB RD, (RB, RI), Op Code =  0 1 1 0 0 RD RB RI 0 0
      // Load Byte from Memory
      // M[RB, RI] => RD.L; $00 => RD.H
      // RB field == RS1 field, RI field == RS2 field
      // Cycles - Pr
      {CONT, 16'b01100?????????00} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_address   = rs1_data + rs2_data;
         end
 
      {STALL, 16'b01100?????????00} :
         begin
           next_cpu_state   = CONT;
           alu_result       = {8'h00, load_data[15:8]};
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = LDW RD, (RB, RI), Op Code =  0 1 1 0 1 RD RB RI 0 0
      // Load Word from Memory
      // M[RB, RI] => RD
      // RB field == RS1 field, RI field == RS2 field
      // Cycles - PR
      {CONT, 16'b01101?????????00} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_address   = rs1_data + rs2_data;
           data_word_op   = 1'b1;
         end
 
      {STALL, 16'b01101?????????00} :
         begin
           next_cpu_state   = CONT;
           alu_result       = load_data;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = STB RS, (RB, RI), Op Code =  0 1 1 1 0 RS RB RI 0 0
      // RS.L => M[RB, RI]
      // Store Byte to Memory
      // RS field == RD fieild, RB field == RS1 field, RI field == RS2 field
      // Cycles - Pw
      {CONT, 16'b01110?????????00} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_write     = 1'b1;
           data_address   = rs1_data + rs2_data;
         end
 
      {STALL, 16'b01110?????????00} :
         begin
           next_cpu_state   = CONT;
         end
 
      // Instruction = STW RS, (RB, RI), Op Code =  0 1 1 1 1 RS RB RI 0 0
      // Store Word to Memory
      // RS => M[RB, RI]
      // RS field == RD fieild, RB field == RS1 field, RI field == RS2 field
      // Cycles - PW
      {CONT, 16'b01111?????????00} :
         begin
           next_cpu_state = STALL;
           load_next_inst = 1'b0;
           next_pc        = program_counter;
           data_access    = 1'b1;
           data_write     = 1'b1;
           data_address   = rs1_data + rs2_data;
           data_word_op   = 1'b1;
         end
 
      {STALL, 16'b01111?????????00} :
         begin
           next_cpu_state   = CONT;
         end
 
      // Instruction = LDB RD, (RB, RI+), Op Code =  0 1 1 0 0 RD RB RI 0 1
      // Load Byte from Memory
      // M[RB, RI] => RD.L; $00 => RD.H; RI+1 => RI
      //  If the same general purpose register is used as index (RI) and destination register (RD),
      //  the content of the register will not be incremented after the data move: M[RB, RI] => RD.L; $00 => RD.H
      // RB field == RS1 field, RI field == RS2 field
      // Cycles - Pr
      {CONT, 16'b01100?????????01} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           data_address     = rs1_data + rs2_data;
           alu_result       = rs2_data + 16'h0001;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01100?????????01} :
         begin
           next_cpu_state   = CONT;
           alu_result       = {8'h00, load_data[15:8]};
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = LDW RD, (RB, RI+), Op Code =  0 1 1 0 1 RD RB RI 0 1
      // Load Word from Memory
      // M[RB, RI] => RD; RI+2 => RI
      //  If the same general purpose register is used as index (RI) and destination register (RD),
      //  the content of the register will not be incremented after the data move: M[RB, RI] => RD
      // RB field == RS1 field, RI field == RS2 field
      // Cycles - PR
      {CONT, 16'b01101?????????01} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           data_address     = rs1_data + rs2_data;
           data_word_op     = 1'b1;
           alu_result       = rs2_data + 16'h0002;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01101?????????01} :
         begin
           next_cpu_state   = CONT;
           alu_result       = load_data;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = STB RS, (RB, RI+), Op Code =  0 1 1 1 0 RS RB RI 0 1
      // Store Byte to Memory
      // RS.L => M[RB, RI]; RI+1 => RI
      // RS field == RD fieild, RB field == RS1 field, RI field == RS2 field
      // Cycles - Pw
      {CONT, 16'b01110?????????01} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           data_write       = 1'b1;
           data_address     = rs1_data + rs2_data;
           alu_result       = rs2_data + 16'h0001;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01110?????????01} :
         begin
           next_cpu_state   = CONT;
         end
 
      // Instruction = STW RS, (RB, RI+), Op Code =  0 1 1 1 1 RS RB RI 0 1
      // Store Word to Memory
      // RS => M[RB, RI]; RI+2 => RI
      // RS field == RD fieild, RB field == RS1 field, RI field == RS2 field
      // Cycles - PW
      {CONT, 16'b01111?????????01} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           data_write       = 1'b1;
           data_word_op     = 1'b1;
           data_address     = rs1_data + rs2_data;
           alu_result       = rs2_data + 16'h0002;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01111?????????01} :
         begin
           next_cpu_state   = CONT;
         end
 
      // Instruction = LDB RD, (RB, -RI), Op Code =  0 1 1 0 0 RD RB RI 1 0
      // Load Byte from Memory
      // RI-1 => RI; M[RS, RI]  => RD.L; $00 => RD.H
      // RB field == RS1 field, RI field == RS2 field
      // Cycles - Pr
      {CONT, 16'b01100?????????10} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           alu_result       = rs2_data + 16'hffff;
           data_address     = rs1_data + alu_result;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01100?????????10} :
         begin
           next_cpu_state   = CONT;
           alu_result       = {8'h00, load_data[15:8]};
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = LDW RD, (RB, -RI), Op Code =  0 1 1 0 1 RD RB RI 1 0
      // Load Word from Memory
      // RI-2 => RI; M[RS, RI] => RD
      // RB field == RS1 field, RI field == RS2 field
      // Cycles - PR
      {CONT, 16'b01101?????????10} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           alu_result       = rs2_data + 16'hfffe;
           data_address     = rs1_data + alu_result;
           data_word_op     = 1'b1;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01101?????????10} :
         begin
           next_cpu_state   = CONT;
           alu_result       = load_data;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      // Instruction = STB RS, (RB, -RI), Op Code =  0 1 1 1 0 RS RB RI 1 0
      // Store Byte to Memory
      // RI-1 => RI; RS.L => M[RB, RI]
      //  If the same general purpose register is used as index (RI) and source register (RS),
      //  the unmodified content of the source register is written to the memory: RS.L => M[RB, RS-1]; RS-1 => RS
      // RS field == RD fieild, RB field == RS1 field, RI field == RS2 field
      // Cycles - Pw
      {CONT, 16'b01110?????????10} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           data_write       = 1'b1;
           alu_result       = rs2_data + 16'hffff;
           data_address     = rs1_data + alu_result;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01110?????????10} :
         begin
           next_cpu_state   = CONT;
         end
 
      // Instruction = STW RS, (RB, -RI), Op Code =  0 1 1 1 1 RS RB RI 1 0
      // Store Word to Memory
      // RI-2 => RI; RS => M[RB, RI]
      //  If the same general purpose register is used as index (RI) and source register (RS),
      //  the unmodified content of the source register is written to the memory: RS => M[RB, RS-2]; RS-2 => RS
      // RS field == RD fieild, RB field == RS1 field, RI field == RS2 field
      // Cycles - PW
      {CONT, 16'b01111?????????10} :
         begin
           next_cpu_state   = STALL;
           load_next_inst   = 1'b0;
           next_pc          = program_counter;
           data_access      = 1'b1;
           data_write       = 1'b1;
           data_word_op     = 1'b1;
           alu_result       = rs2_data + 16'hfffe;
           data_address     = rs1_data + alu_result;
           sel_rd_field     = 1'b0;
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
         end
 
      {STALL, 16'b01111?????????10} :
         begin
           next_cpu_state   = CONT;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Bit Field Instructions
      // -----------------------------------------------------------------------
 
      // Instruction = BFEXT RD, RS1, RS2, Op Code =  0 1 1 0 0 RD RS1 RS2 1 1
      // BFEXT - Bit Field Extract
      // RS1[(o+w):o] => RD[w:0]; 0 => RD[15:(w+1)]
      // Cycles - P
      {CONT, 16'b01100?????????11} :
         begin
           ena_rd_low_byte = 1'b1;
           ena_rd_high_byte = 1'b1;
           shift_ammount  = {1'b0, rs2_data[3:0]};
           for (bfi = 0; bfi <= 15; bfi = bfi + 1)
             shift_in[bfi] = bf_mux_mask[bfi] ? rs1_data[bfi] : 1'b0;
 
           alu_result    = shift_out;
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = BFINS RD, RS1, RS2, Op Code =  0 1 1 0 1 RD RS1 RS2 1 1
      // BFINS - Bit Field Insert
      // RS1[w:0] => RD[(w+o):o
      // Cycles - P
      {CONT, 16'b01101?????????11} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           shift_left     = 1'b1;
           shift_ammount  = {1'b0, rs2_data[3:0]};
           shift_in       = rs1_data;
 
           for (bfi = 0; bfi <= 15; bfi = bfi + 1)
             alu_result[bfi] = bf_mux_mask[bfi] ? shift_out[bfi] : rd_data[bfi];
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = BFINSI RD, RS1, RS2, Op Code =  0 1 1 1 0 RD RS1 RS2 1 1
      // BFINSI - Bit Field Insert and Invert
      // !RS1[w:0] => RD[w+o:o]
      // Cycles - P
      {CONT, 16'b01110?????????11} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           shift_left     = 1'b1;
           shift_ammount  = {1'b0, rs2_data[3:0]};
           shift_in       = rs1_data;
 
           for (bfi = 0; bfi <= 15; bfi = bfi + 1)
             alu_result[bfi] = bf_mux_mask[bfi] ? !shift_out[bfi] : rd_data[bfi];
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = BFINSX RD, RS1, RS2, Op Code =  0 1 1 1 1 RD RS1 RS2 1 1
      // BFINSX - Bit Field Insert and XNOR
      // !(RS1[w:0] ^ RD[w+o:o]) => RD[w+o:o]
      // Cycles - P
      {CONT, 16'b01111?????????11} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
           shift_left     = 1'b1;
           shift_ammount  = {1'b0, rs2_data[3:0]};
           shift_in       = rs1_data;
 
           for (bfi = 0; bfi <= 15; bfi = bfi + 1)
             alu_result[bfi] = bf_mux_mask[bfi] ? !(shift_out[bfi] ^ rd_data[bfi]) : rd_data[bfi];
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
        end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Logic Immediate Instructions
      // -----------------------------------------------------------------------
 
      // Instruction = ANDL RD, #IMM8, Op Code =  1 0 0 0 0 RD IMM8
      // AND - Logical AND
      // RD.L & IMM8 => RD.L
      // Cycles - P
      {CONT, 16'b10000???????????} :
         begin
           ena_rd_low_byte = 1'b1;
 
           alu_result    = rd_data & {8'b0, op_code[7:0]};
           next_zero     = !(|alu_result[7:0]);
           next_negative = alu_result[7];
           next_overflow = 1'b0;
         end
 
      // Instruction = ANDH RD, #IMM8, Op Code =  1 0 0 0 1 RD IMM8
      // AND - Logical AND
      // RD.H & IMM8 => RD.H
      // Cycles - P
      {CONT, 16'b10001???????????} :
         begin
           ena_rd_high_byte = 1'b1;
 
           alu_result    = rd_data & {op_code[7:0], 8'b0};
           next_zero     = !(|alu_result[15:8]);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = BITL RD, #IMM8, Op Code =  1 0 0 1 0 RD IMM8
      // RD.L & IMM8 => NONE
      {CONT, 16'b10010???????????} :
         begin
 
           next_zero     = !(|(rd_data[7:0] & op_code[7:0]));
           next_negative = rd_data[7] && op_code[7];
           next_overflow = 1'b0;
         end
 
      // Instruction = BITH RD, #IMM8, Op Code =  1 0 0 1 1 RD IMM8
      // RD.H & IMM8 => NONE
      {CONT, 16'b10011???????????} :
         begin
 
           next_zero     = !(|(rd_data[15:8] & op_code[7:0]));
           next_negative = rd_data[15] && op_code[7];
           next_overflow = 1'b0;
         end
 
      // Instruction = ORL RD, #IMM8, Op Code =  1 0 1 0 0 RD IMM8
      // OR - Logical OR
      // RD.L | IMM8 => RD.L
      {CONT, 16'b10100???????????} :
         begin
           ena_rd_low_byte = 1'b1;
 
           alu_result    = rd_data | {8'b0, op_code[7:0]};
           next_zero     = !(|alu_result[7:0]);
           next_negative = alu_result[7];
           next_overflow = 1'b0;
         end
 
      // Instruction = ORH RD, #IMM8, Op Code =  1 0 1 0 1 RD IMM8
      // OR - Logical OR
      // RD.H | IMM8 => RD.H
      {CONT, 16'b10101???????????} :
         begin
           ena_rd_high_byte = 1'b1;
 
           alu_result    = rd_data | {op_code[7:0], 8'b0};
           next_zero     = !(|alu_result[15:8]);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // Instruction = XNORL RD, #IMM8, Op Code =  1 0 1 1 0 RD IMM8
      // XNOR - Logical Exclusive NOR
      // ~(RD.L ^ IMM8) => RD.L
      {CONT, 16'b10110???????????} :
         begin
           ena_rd_low_byte = 1'b1;
 
           alu_result    = ~(rd_data ^ {8'b0, op_code[7:0]});
           next_zero     = !(|alu_result[7:0]);
           next_negative = alu_result[7];
           next_overflow = 1'b0;
         end
 
      // Instruction = XNORH RD, #IMM8, Op Code =  1 0 1 1 1 RD IMM8
      // XNOR - Logical Exclusive NOR
      // ~(RD.H ^ IMM8) => RD.H
      {CONT, 16'b10111???????????} :
         begin
           ena_rd_high_byte = 1'b1;
 
           alu_result    = ~(rd_data ^ {op_code[7:0], 8'b0});
           next_zero     = !(|alu_result[15:8]);
           next_negative = alu_result[15];
           next_overflow = 1'b0;
         end
 
      // -----------------------------------------------------------------------
      // Instruction Group -- Arithmetic Immediate Instructions
      // -----------------------------------------------------------------------
 
      // Instruction = SUBL RD, #IMM8, Op Code =  1 1 0 0 0 RD IMM8
      // SUB - Subtract without Carry
      // RD - $00:IMM8 => RD
      // Cycles - P
      {CONT, 16'b11000???????????} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           alu_result    = rd_data - {8'b0, op_code[7:0]};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_carry    = (!rd_data[7] && op_code[7]) || (!rd_data[7] && alu_result[7]) || (op_code[7] && alu_result[7]);
           next_overflow = (rd_data[7] && !op_code[7] && !alu_result[7]) || (!rd_data[7] && op_code[7] && alu_result[7]);
         end
 
      // Instruction = SUBH RD, #IMM8, Op Code =  1 1 0 0 1 RD IMM8
      // SUB - Subtract without Carry
      // RD - IMM8:$00 => RD
      // Cycles - P
      {CONT, 16'b11001???????????} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           {next_carry, alu_result} = rd_data - {op_code[7:0], 8'b0};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = (rd_data[15] && !op_code[7] && !alu_result[15]) || (!rd_data[15] && op_code[7] && alu_result[15]);
         end
 
      // Instruction = CMPL RS, #IMM8, Op Code =  1 1 0 1 0 RS IMM8
      // RS.L - IMM8 => NONE, only condition code flags get updated
      // RS field == RD field
      // Cycles - P
      {CONT, 16'b11010???????????} :
         begin
 
           alu_result    = {8'b0, rd_data[7:0] - op_code[7:0]};
           next_zero     = !(|alu_result[7:0]);
           next_negative = alu_result[7];
           next_carry    = (!rd_data[7] && op_code[7]) || (!rd_data[7] && alu_result[7]) || (op_code[7] && alu_result[7]);
           next_overflow = (rd_data[7] && !op_code[7] && !alu_result[7]) || (!rd_data[7] && op_code[7] && alu_result[7]);
         end
 
      // Instruction = CPCH RS, #IMM8, Op Code =  1 1 0 1 1 RS IMM8
      // RS.H - IMM8 - C => NONE, only condition code flags get updated
      // RS field == RD field
      // Cycles - P
      {CONT, 16'b11011???????????} :
         begin
 
           alu_result    = {rd_data[15:8], 8'b0} - {op_code[7:0], 8'b0} - {7'b0, carry_flag, 8'b0};
           next_zero     = !(|alu_result[15:8]) && zero_flag;
           next_negative = alu_result[15];
           next_carry    = (!rd_data[15] && op_code[7]) || (!rd_data[15] && alu_result[15]) || (op_code[7] && alu_result[15]);
           next_overflow = (rd_data[15] && !op_code[7] && !alu_result[15]) || (!rd_data[15] && op_code[7] && alu_result[15]);
         end
 
      // Instruction = ADDL RD, #IMM8, Op Code =  1 1 1 0 0 RD IMM8
      // ADD - ADD without carry
      // RD + $00:IMM8 => RD
      // Cycles - P
      {CONT, 16'b11100???????????} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           alu_result    = rd_data + {8'b0, op_code[7:0]};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_carry    = (rd_data[7] && op_code[7]) || (rd_data[7] && !alu_result[7]) || (op_code[7] && !alu_result[7]);
           next_overflow = (!rd_data[7] && !op_code[7] && alu_result[7]) || (rd_data[7] && op_code[7] && !alu_result[7]);
         end
 
      // Instruction = ADDH RD, #IMM8, Op Code =  1 1 1 0 1 RD IMM8
      // ADD - ADD without carry
      // RD + IMM8:$00 => RD
      // Cycles - P
      {CONT, 16'b11101???????????} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           {next_carry, alu_result} = rd_data + {op_code[7:0], 8'b0};
           next_zero     = !(|alu_result);
           next_negative = alu_result[15];
           next_overflow = (rd_data[15] && op_code[7] && !alu_result[15]) || (!rd_data[15] && !op_code[7] && alu_result[15]);
         end
 
      // Instruction = LDL RD, #IMM8, Op Code =  1 1 1 1 0 RD IMM8
      // IMM8 => RD.L; $00 => RD.H, High Byte is cleared
      // Cycles - P
      {CONT, 16'b11110???????????} :
         begin
           ena_rd_low_byte  = 1'b1;
           ena_rd_high_byte = 1'b1;
 
           alu_result    = {8'b0, op_code[7:0]};
         end
 
      // Instruction = LDH RD, #IMM8, Op Code =  1 1 1 1 1 RD IMM8
      // IMM8 => RD.H, Low Byte is unchanged
      // Cycles - P
      {CONT, 16'b11111???????????} :
         begin
           ena_rd_high_byte = 1'b1;
 
           alu_result    = {op_code[7:0], 8'b0};
         end
      default :
        begin
          $display("\nOP Code Error\n");
        end
    endcase
    end  // always
 
  xgate_barrel_shift barrel_shift(
    // outputs
    .shift_out( shift_out ),
    .shift_rollover( shift_rollover ),
    // inputs
    .shift_left( shift_left ),
    .shift_ammount( shift_ammount ),
    .shift_in( shift_in ),
    .shift_filler( shift_filler )
  );
 
  // Three to Eight line decoder
  always @*
    case (semaph_risc)
      4'h0 : semap_risc_bit = 8'b0000_0001;
      4'h1 : semap_risc_bit = 8'b0000_0010;
      4'h2 : semap_risc_bit = 8'b0000_0100;
      4'h3 : semap_risc_bit = 8'b0000_1000;
      4'h4 : semap_risc_bit = 8'b0001_0000;
      4'h5 : semap_risc_bit = 8'b0010_0000;
      4'h6 : semap_risc_bit = 8'b0100_0000;
      4'h7 : semap_risc_bit = 8'b1000_0000;
    endcase
 
  assign semaph_stat = |risc_semap;
 
  // Semaphore Bit
  semaphore_bit semaphore_0(
    // outputs
    .host_status( host_semap[0] ),
    .risc_status( risc_semap[0] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[0] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[8] ),
    .host_bit( perif_data[0] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_1(
    // outputs
    .host_status( host_semap[1] ),
    .risc_status( risc_semap[1] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[1] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[9] ),
    .host_bit( perif_data[1] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_2(
    // outputs
    .host_status( host_semap[2] ),
    .risc_status( risc_semap[2] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[2] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[10] ),
    .host_bit( perif_data[2] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_3(
    // outputs
    .host_status( host_semap[3] ),
    .risc_status( risc_semap[3] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[3] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[11] ),
    .host_bit( perif_data[2] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_4(
    // outputs
    .host_status( host_semap[4] ),
    .risc_status( risc_semap[4] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[4] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[12] ),
    .host_bit( perif_data[4] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_5(
    // outputs
    .host_status( host_semap[5] ),
    .risc_status( risc_semap[5] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[5] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[13] ),
    .host_bit( perif_data[5] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_6(
    // outputs
    .host_status( host_semap[6] ),
    .risc_status( risc_semap[6] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[6] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[14] ),
    .host_bit( perif_data[6] )
  );
 
  // Semaphore Bit
  semaphore_bit semaphore_7(
    // outputs
    .host_status( host_semap[7] ),
    .risc_status( risc_semap[7] ),
    // inputs
    .risc_clk( risc_clk ),
    .async_rst_b( async_rst_b ),
    .risc_bit_sel( semap_risc_bit[7] ),
    .csem( clear_semaph ),
    .ssem( set_semaph ),
    .host_wrt( write_xgsem ),
    .host_bit_mask( perif_data[15] ),
    .host_bit( perif_data[7] )
  );
 
 
  endmodule  // xgate_inst_decode
 
// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
 
module xgate_barrel_shift (
  output reg [15:0] shift_out,
  output reg        shift_rollover,
 
  input             shift_left,
  input      [ 4:0] shift_ammount,
  input      [15:0] shift_in,
  input      [15:0] shift_filler
  );
 
  always @*
    casez ({shift_left, shift_ammount})
     // Start Right Shifts
      6'b0_0_0000 :
        begin
          shift_out      = shift_in;
          shift_rollover = 1'b0;
        end
      6'b0_0_0001 :
        begin
          shift_out      = {shift_filler[ 0], shift_in[15: 1]};
          shift_rollover =  shift_in[ 0];
        end
      6'b0_0_0010 :
        begin
          shift_out      = {shift_filler[ 1:0], shift_in[15: 2]};
          shift_rollover =  shift_in[ 1];
        end
      6'b0_0_0011 :
        begin
          shift_out      = {shift_filler[ 2:0], shift_in[15: 3]};
          shift_rollover =  shift_in[ 2];
        end
      6'b0_0_0100 :
        begin
          shift_out      = {shift_filler[ 3:0], shift_in[15: 4]};
          shift_rollover =  shift_in[ 3];
        end
      6'b0_0_0101 :
        begin
          shift_out      = {shift_filler[ 4:0], shift_in[15: 5]};
          shift_rollover =  shift_in[ 4];
        end
      6'b0_0_0110 :
        begin
          shift_out      = {shift_filler[ 5:0], shift_in[15: 6]};
          shift_rollover =  shift_in[ 5];
        end
      6'b0_0_0111 :
        begin
          shift_out      = {shift_filler[ 6:0], shift_in[15: 7]};
          shift_rollover =  shift_in[ 6];
        end
      6'b0_0_1000 :
        begin
          shift_out      = {shift_filler[ 7:0], shift_in[15: 8]};
          shift_rollover =  shift_in[ 7];
        end
      6'b0_0_1001 :
        begin
          shift_out      = {shift_filler[ 8:0], shift_in[15: 9]};
          shift_rollover =  shift_in[ 8];
        end
      6'b0_0_1010 :
        begin
          shift_out      = {shift_filler[ 9:0], shift_in[15:10]};
          shift_rollover =  shift_in[ 9];
        end
      6'b0_0_1011 :
        begin
          shift_out      = {shift_filler[10:0], shift_in[15:11]};
          shift_rollover =  shift_in[10];
        end
      6'b0_0_1100 :
        begin
          shift_out      = {shift_filler[11:0], shift_in[15:12]};
          shift_rollover =  shift_in[11];
        end
      6'b0_0_1101 :
        begin
          shift_out      = {shift_filler[12:0], shift_in[15:13]};
          shift_rollover =  shift_in[12];
        end
      6'b0_0_1110 :
        begin
          shift_out      = {shift_filler[13:0], shift_in[15:14]};
          shift_rollover =  shift_in[13];
        end
      6'b0_0_1111 :
        begin
          shift_out      = {shift_filler[14:0], shift_in[15]};
          shift_rollover =  shift_in[14];
        end
      6'b0_1_???? :
        begin
          shift_out      = shift_filler[15:0];
          shift_rollover = shift_in[15];
        end
 
     // Start Left Shifts
 
      6'b1_0_0000 :
        begin
          shift_out      =  shift_in;
          shift_rollover  = 1'b0;
        end
      6'b1_0_0001 :
        begin
          shift_out      = {shift_in[14:0], shift_filler[15]};
          shift_rollover =  shift_in[15];
        end
      6'b1_0_0010 :
        begin
          shift_out      = {shift_in[13:0], shift_filler[15:14]};
          shift_rollover =  shift_in[14];
        end
      6'b1_0_0011 :
        begin
          shift_out      = {shift_in[12:0], shift_filler[15:13]};
          shift_rollover =  shift_in[13];
        end
      6'b1_0_0100 :
        begin
          shift_out      = {shift_in[11:0], shift_filler[15:12]};
          shift_rollover =  shift_in[12];
        end
      6'b1_0_0101 :
        begin
          shift_out      = {shift_in[10:0], shift_filler[15:11]};
          shift_rollover =  shift_in[11];
        end
      6'b1_0_0110 :
        begin
          shift_out      = {shift_in[ 9:0], shift_filler[15:10]};
          shift_rollover =  shift_in[10];
        end
      6'b1_0_0111 :
        begin
          shift_out      = {shift_in[ 8:0], shift_filler[15: 9]};
          shift_rollover =  shift_in[ 9];
        end
      6'b1_0_1000 :
        begin
          shift_out      = {shift_in[ 7:0], shift_filler[15: 8]};
          shift_rollover =  shift_in[ 8];
        end
      6'b1_0_1001 :
        begin
          shift_out      = {shift_in[ 6:0], shift_filler[15: 7]};
          shift_rollover =  shift_in[ 7];
        end
      6'b1_0_1010 :
        begin
          shift_out      = {shift_in[ 5:0], shift_filler[15: 6]};
          shift_rollover =  shift_in[ 6];
        end
      6'b1_0_1011 :
        begin
          shift_out      = {shift_in[ 4:0], shift_filler[15: 5]};
          shift_rollover =  shift_in[ 5];
        end
      6'b1_0_1100 :
        begin
          shift_out      = {shift_in[ 3:0], shift_filler[15 :4]};
          shift_rollover =  shift_in[ 4];
        end
      6'b1_0_1101 :
        begin
          shift_out      = {shift_in[ 2:0], shift_filler[15: 3]};
          shift_rollover =  shift_in[ 3];
        end
      6'b1_0_1110 :
        begin
          shift_out      = {shift_in[ 1:0], shift_filler[15: 2]};
          shift_rollover =  shift_in[ 2];
        end
      6'b1_0_1111 :
        begin
          shift_out      = {shift_in[ 0], shift_filler[15: 1]};
          shift_rollover =  shift_in[ 1];
        end
      6'b1_1_???? :
        begin
          shift_out      = shift_filler[15: 0];
          shift_rollover = shift_in[ 0];
        end
    endcase
 
 
endmodule  // xgate_barrel_shift
 
// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
 
module semaphore_bit #(parameter NO_LOCK   = 2'b00,
                       parameter RISC_LOCK = 2'b10,
                       parameter HOST_LOCK = 2'b11)
  (
  output      host_status,   // Return status for Host processor
  output      risc_status,   // Return Status for RISC processor
 
  input       risc_clk,      // Semaphore register clock
  input       async_rst_b,   // Async reset signal
  input       risc_bit_sel,  // Bit selected by RISC
  input       csem,          // RISC Clear Semaphore Bit
  input       ssem,          // RISC Set Semaphore Bit
  input       host_wrt,      // Host write to the Semaphore Register
  input       host_bit_mask, // Host mask bit written to the Semaphore bit
  input       host_bit       // Host bit written to the Semaphore bit
  );
 
  reg [1:0] next_semap_state;
  reg [1:0] semap_state;
 
  assign host_status = semap_state == HOST_LOCK;
  assign risc_status = ssem && risc_bit_sel && ((semap_state == RISC_LOCK) || (next_semap_state == RISC_LOCK));
 
  always @(posedge risc_clk or negedge async_rst_b)
    if (!async_rst_b)
      semap_state <= NO_LOCK;
    else
      semap_state <= next_semap_state;
 
  always @*
    begin
      case(semap_state)
        NO_LOCK:
          begin
            if (host_wrt && host_bit_mask && host_bit)
              next_semap_state = HOST_LOCK;
            else if (ssem && risc_bit_sel)
              next_semap_state = RISC_LOCK;
            else
              next_semap_state = NO_LOCK;
          end
        RISC_LOCK:
          begin
            if (csem && risc_bit_sel)
              next_semap_state = NO_LOCK;
          end
        HOST_LOCK:
          begin
            if (host_wrt && host_bit_mask && !host_bit)
              next_semap_state = NO_LOCK;
          end
        default:
          next_semap_state = NO_LOCK;
      endcase
    end
 
endmodule  // semaphore_bit
 
 
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[/] [xgate/] [trunk/] [rtl/] [verilog/] [xgate_risc.v] - Blame information for rev 2

Line No.	Rev	Author	Line
1	2	rehayes	`////////////////////////////////////////////////////////////////////////////////`
2			`//`
3			`// XGATE Coprocessor - XGATE RISC Processor Core`
4			`//`
5			`// Author: Bob Hayes`
6			`// rehayes@opencores.org`
7			`//`
8			`// Downloaded from: http://www.opencores.org/projects/xgate.....`
9			`//`
10			`////////////////////////////////////////////////////////////////////////////////`
11			`// Copyright (c) 2009, Robert Hayes`
12			`//`
13			`// This source file is free software: you can redistribute it and/or modify`
14			`// it under the terms of the GNU Lesser General Public License as published`
15			`// by the Free Software Foundation, either version 3 of the License, or`
16			`// (at your option) any later version.`
17			`//`
18			`// Supplemental terms.`
19			`// * Redistributions of source code must retain the above copyright`
20			`// notice, this list of conditions and the following disclaimer.`
21			`// * Neither the name of the <organization> nor the`
22			`// names of its contributors may be used to endorse or promote products`
23			`// derived from this software without specific prior written permission.`
24			`//`
25			`// THIS SOFTWARE IS PROVIDED BY Robert Hayes ''AS IS'' AND ANY`
26			`// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED`
27			`// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE`
28			`// DISCLAIMED. IN NO EVENT SHALL Robert Hayes BE LIABLE FOR ANY`
29			`// DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES`
30			`// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;`
31			`// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND`
32			`// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT`
33			`// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS`
34			`// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.`
35			`//`
36			`// You should have received a copy of the GNU General Public License`
37			`// along with this program. If not, see <http://www.gnu.org/licenses/>.`
38			`////////////////////////////////////////////////////////////////////////////////`
39			`// 45678901234567890123456789012345678901234567890123456789012345678901234567890`
40