tv80_mcode.v

//
// TV80 8-Bit Microprocessor Core
// Based on the VHDL T80 core by Daniel Wallner (jesus@opencores.org)
//
// Copyright (c) 2004,2007 Guy Hutchison (ghutchis@opencores.org)
//
// Permission is hereby granted, free of charge, to any person obtaining a 
// copy of this software and associated documentation files (the "Software"), 
// to deal in the Software without restriction, including without limitation 
// the rights to use, copy, modify, merge, publish, distribute, sublicense, 
// and/or sell copies of the Software, and to permit persons to whom the 
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included 
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 
// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 
// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 
// SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

module tv80_mcode 
  (/*AUTOARG*/
  // Outputs
  MCycles, TStates, Prefix, Inc_PC, Inc_WZ, IncDec_16, Read_To_Reg, 
  Read_To_Acc, Set_BusA_To, Set_BusB_To, ALU_Op, Save_ALU, PreserveC, 
  Arith16, Set_Addr_To, IORQ, Jump, JumpE, JumpXY, Call, RstP, LDZ, 
  LDW, LDSPHL, Special_LD, ExchangeDH, ExchangeRp, ExchangeAF, 
  ExchangeRS, I_DJNZ, I_CPL, I_CCF, I_SCF, I_RETN, I_BT, I_BC, I_BTR, 
  I_RLD, I_RRD, I_INRC, SetDI, SetEI, IMode, Halt, NoRead, Write, 
  // Inputs
  IR, ISet, MCycle, F, NMICycle, IntCycle
  );
  
  parameter             Mode   = 0;
  parameter             Flag_C = 0;
  parameter             Flag_N = 1;
  parameter             Flag_P = 2;
  parameter             Flag_X = 3;
  parameter             Flag_H = 4;
  parameter             Flag_Y = 5;
  parameter             Flag_Z = 6;
  parameter             Flag_S = 7;

  input [7:0]           IR;
  input [1:0]           ISet                    ;
  input [6:0]           MCycle                  ;
  input [7:0]           F                       ;
  input                 NMICycle                ;
  input                 IntCycle                ;
  output [2:0]          MCycles                 ;
  output [2:0]          TStates                 ;
  output [1:0]          Prefix                  ; // None,BC,ED,DD/FD
  output                Inc_PC                  ;
  output                Inc_WZ                  ;
  output [3:0]          IncDec_16               ; // BC,DE,HL,SP   0 is inc
  output                Read_To_Reg             ;
  output                Read_To_Acc             ;
  output [3:0]          Set_BusA_To     ; // B,C,D,E,H,L,DI/DB,A,SP(L),SP(M),0,F
  output [3:0]          Set_BusB_To     ; // B,C,D,E,H,L,DI,A,SP(L),SP(M),1,F,PC(L),PC(M),0
  output [3:0]          ALU_Op                  ;
  output                Save_ALU                ;
  output                PreserveC               ;
  output                Arith16                 ;
  output [2:0]          Set_Addr_To             ; // aNone,aXY,aIOA,aSP,aBC,aDE,aZI
  output                IORQ                    ;
  output                Jump                    ;
  output                JumpE                   ;
  output                JumpXY                  ;
  output                Call                    ;
  output                RstP                    ;
  output                LDZ                     ;
  output                LDW                     ;
  output                LDSPHL                  ;
  output [2:0]          Special_LD              ; // A,I;A,R;I,A;R,A;None
  output                ExchangeDH              ;
  output                ExchangeRp              ;
  output                ExchangeAF              ;
  output                ExchangeRS              ;
  output                I_DJNZ                  ;
  output                I_CPL                   ;
  output                I_CCF                   ;
  output                I_SCF                   ;
  output                I_RETN                  ;
  output                I_BT                    ;
  output                I_BC                    ;
  output                I_BTR                   ;
  output                I_RLD                   ;
  output                I_RRD                   ;
  output                I_INRC                  ;
  output                SetDI                   ;
  output                SetEI                   ;
  output [1:0]          IMode                   ;
  output                Halt                    ;
  output                NoRead                  ;
  output                Write   ;                

  // regs
  reg [2:0]             MCycles                 ;
  reg [2:0]             TStates                 ;
  reg [1:0]             Prefix                  ; // None,BC,ED,DD/FD
  reg                   Inc_PC                  ;
  reg                   Inc_WZ                  ;
  reg [3:0]             IncDec_16               ; // BC,DE,HL,SP   0 is inc
  reg                   Read_To_Reg             ;
  reg                   Read_To_Acc             ;
  reg [3:0]             Set_BusA_To     ; // B,C,D,E,H,L,DI/DB,A,SP(L),SP(M),0,F
  reg [3:0]             Set_BusB_To     ; // B,C,D,E,H,L,DI,A,SP(L),SP(M),1,F,PC(L),PC(M),0
  reg [3:0]             ALU_Op                  ;
  reg                   Save_ALU                ;
  reg                   PreserveC               ;
  reg                   Arith16                 ;
  reg [2:0]             Set_Addr_To             ; // aNone,aXY,aIOA,aSP,aBC,aDE,aZI
  reg                   IORQ                    ;
  reg                   Jump                    ;
  reg                   JumpE                   ;
  reg                   JumpXY                  ;
  reg                   Call                    ;
  reg                   RstP                    ;
  reg                   LDZ                     ;
  reg                   LDW                     ;
  reg                   LDSPHL                  ;
  reg [2:0]             Special_LD              ; // A,I;A,R;I,A;R,A;None
  reg                   ExchangeDH              ;
  reg                   ExchangeRp              ;
  reg                   ExchangeAF              ;
  reg                   ExchangeRS              ;
  reg                   I_DJNZ                  ;
  reg                   I_CPL                   ;
  reg                   I_CCF                   ;
  reg                   I_SCF                   ;
  reg                   I_RETN                  ;
  reg                   I_BT                    ;
  reg                   I_BC                    ;
  reg                   I_BTR                   ;
  reg                   I_RLD                   ;
  reg                   I_RRD                   ;
  reg                   I_INRC                  ;
  reg                   SetDI                   ;
  reg                   SetEI                   ;
  reg [1:0]             IMode                   ;
  reg                   Halt                    ;
  reg                   NoRead                  ;
  reg                   Write   ;                

  parameter             aNone   = 3'b111;
  parameter             aBC     = 3'b000;
  parameter             aDE     = 3'b001;
  parameter             aXY     = 3'b010;
  parameter             aIOA    = 3'b100;
  parameter             aSP     = 3'b101;
  parameter             aZI     = 3'b110;
  //    constant aNone  : std_logic_vector[2:0] = 3'b000;
  //    constant aXY    : std_logic_vector[2:0] = 3'b001;
  //    constant aIOA   : std_logic_vector[2:0] = 3'b010;
  //    constant aSP    : std_logic_vector[2:0] = 3'b011;
  //    constant aBC    : std_logic_vector[2:0] = 3'b100;
  //    constant aDE    : std_logic_vector[2:0] = 3'b101;
  //    constant aZI    : std_logic_vector[2:0] = 3'b110;

  function is_cc_true;
    input [7:0] FF;
    input [2:0] cc;
    begin
      if (Mode == 3 ) 
        begin
          case (cc)
            3'b000  : is_cc_true = FF[7] == 1'b0; // NZ
            3'b001  : is_cc_true = FF[7] == 1'b1; // Z
            3'b010  : is_cc_true = FF[4] == 1'b0; // NC
            3'b011  : is_cc_true = FF[4] == 1'b1; // C
            3'b100  : is_cc_true = 0;
            3'b101  : is_cc_true = 0;
            3'b110  : is_cc_true = 0;
            3'b111  : is_cc_true = 0;
          endcase
        end 
      else 
        begin
          case (cc)
            3'b000  : is_cc_true = FF[6] == 1'b0; // NZ
            3'b001  : is_cc_true = FF[6] == 1'b1; // Z
            3'b010  : is_cc_true = FF[0] == 1'b0; // NC
            3'b011  : is_cc_true = FF[0] == 1'b1; // C
            3'b100  : is_cc_true = FF[2] == 1'b0; // PO
            3'b101  : is_cc_true = FF[2] == 1'b1; // PE
            3'b110  : is_cc_true = FF[7] == 1'b0; // P
            3'b111  : is_cc_true = FF[7] == 1'b1; // M
          endcase
        end
    end
  endfunction // is_cc_true
  

  reg [2:0] DDD;
  reg [2:0] SSS;
  reg [1:0] DPAIR;
  
  always @ (/*AUTOSENSE*/F or IR or ISet or IntCycle or MCycle
            or NMICycle)
    begin
      DDD = IR[5:3];
      SSS = IR[2:0];
      DPAIR = IR[5:4];

      MCycles = 3'b001;
      if (MCycle[0] ) 
        begin
          TStates = 3'b100;
        end 
      else 
        begin
          TStates = 3'b011;
        end
      Prefix = 2'b00;
      Inc_PC = 1'b0;
      Inc_WZ = 1'b0;
      IncDec_16 = 4'b0000;
      Read_To_Acc = 1'b0;
      Read_To_Reg = 1'b0;
      Set_BusB_To = 4'b0000;
      Set_BusA_To = 4'b0000;
      ALU_Op = { 1'b0, IR[5:3] };
      Save_ALU = 1'b0;
      PreserveC = 1'b0;
      Arith16 = 1'b0;
      IORQ = 1'b0;
      Set_Addr_To = aNone;
      Jump = 1'b0;
      JumpE = 1'b0;
      JumpXY = 1'b0;
      Call = 1'b0;
      RstP = 1'b0;
      LDZ = 1'b0;
      LDW = 1'b0;
      LDSPHL = 1'b0;
      Special_LD = 3'b000;
      ExchangeDH = 1'b0;
      ExchangeRp = 1'b0;
      ExchangeAF = 1'b0;
      ExchangeRS = 1'b0;
      I_DJNZ = 1'b0;
      I_CPL = 1'b0;
      I_CCF = 1'b0;
      I_SCF = 1'b0;
      I_RETN = 1'b0;
      I_BT = 1'b0;
      I_BC = 1'b0;
      I_BTR = 1'b0;
      I_RLD = 1'b0;
      I_RRD = 1'b0;
      I_INRC = 1'b0;
      SetDI = 1'b0;
      SetEI = 1'b0;
      IMode = 2'b11;
      Halt = 1'b0;
      NoRead = 1'b0;
      Write = 1'b0;
      
      case (ISet)
        2'b00  :
          begin

            //----------------------------------------------------------------------------
            //
            //  Unprefixed instructions
            //
            //----------------------------------------------------------------------------

            casez (IR)
              // 8 BIT LOAD GROUP
              8'b01zzzzzz :
                begin
                  if (IR[5:0] == 6'b110110)
                    Halt = 1'b1;
                  else if (IR[2:0] == 3'b110)
                    begin
                      // LD r,(HL)
                      MCycles = 3'b010;
                      if (MCycle[0])
                        Set_Addr_To = aXY;
                      if (MCycle[1])
                        begin
                          Set_BusA_To[2:0] = DDD;
                          Read_To_Reg = 1'b1;
                        end
                    end // if (IR[2:0] == 3'b110)
                  else if (IR[5:3] == 3'b110)
                    begin
                      // LD (HL),r
                      MCycles = 3'b010;
                      if (MCycle[0])
                        begin
                          Set_Addr_To = aXY;
                          Set_BusB_To[2:0] = SSS;
                          Set_BusB_To[3] = 1'b0;
                        end
                      if (MCycle[1])
                        Write = 1'b1;
                    end // if (IR[5:3] == 3'b110)
                  else
                    begin
                      Set_BusB_To[2:0] = SSS;
                      ExchangeRp = 1'b1;
                      Set_BusA_To[2:0] = DDD;
                      Read_To_Reg = 1'b1;
                    end // else: !if(IR[5:3] == 3'b110)
                end // case: 8'b01zzzzzz                                    

              8'b00zzz110 :
                begin
                  if (IR[5:3] == 3'b110)
                    begin
                      // LD (HL),n
                      MCycles = 3'b011;
                      if (MCycle[1])
                        begin
                          Inc_PC = 1'b1;
                          Set_Addr_To = aXY;
                          Set_BusB_To[2:0] = SSS;
                          Set_BusB_To[3] = 1'b0;
                        end
                      if (MCycle[2])
                        Write = 1'b1;
                    end // if (IR[5:3] == 3'b110)
                  else
                    begin
                      // LD r,n
                      MCycles = 3'b010;
                      if (MCycle[1])
                        begin
                          Inc_PC = 1'b1;
                          Set_BusA_To[2:0] = DDD;
                          Read_To_Reg = 1'b1;
                        end
                    end
                end
              
              8'b00001010  :
                begin
                  // LD A,(BC)
                  MCycles = 3'b010;
                  if (MCycle[0])
                    Set_Addr_To = aBC;
                  if (MCycle[1])
                    Read_To_Acc = 1'b1;
                end // case: 8'b00001010
              
              8'b00011010  :
                begin
                  // LD A,(DE)
                  MCycles = 3'b010;
                  if (MCycle[0])
                    Set_Addr_To = aDE;
                  if (MCycle[1])
                    Read_To_Acc = 1'b1;
                end // case: 8'b00011010
              
              8'b00111010  :
                begin
                  if (Mode == 3 ) 
                    begin
                      // LDD A,(HL)
                      MCycles = 3'b010;
                      if (MCycle[0])
                        Set_Addr_To = aXY;
                      if (MCycle[1])
                        begin
                          Read_To_Acc = 1'b1;
                          IncDec_16 = 4'b1110;
                        end
                    end 
                  else 
                    begin
                      // LD A,(nn)
                      MCycles = 3'b100;
                      if (MCycle[1])
                        begin
                          Inc_PC = 1'b1;
                          LDZ = 1'b1;
                        end
                      if (MCycle[2])
                        begin
                          Set_Addr_To = aZI;
                          Inc_PC = 1'b1;
                        end
                      if (MCycle[3])
                        begin
                          Read_To_Acc = 1'b1;
                        end
                    end // else: !if(Mode == 3 )
                end // case: 8'b00111010
              
              8'b00000010  :
                begin
                  // LD (BC),A
                  MCycles = 3'b010;
                  if (MCycle[0])
                    begin
                      Set_Addr_To = aBC;
                      Set_BusB_To = 4'b0111;
                    end
                  if (MCycle[1])
                    begin
                      Write = 1'b1;
                    end
                end // case: 8'b00000010
              
              8'b00010010  :
                begin
                  // LD (DE),A
                  MCycles = 3'b010;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        Set_Addr_To = aDE;
                        Set_BusB_To = 4'b0111;
                      end
                    MCycle[1] :
                      Write = 1'b1;
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b00010010
              
              8'b00110010  :
                begin
                  if (Mode == 3 ) 
                    begin
                      // LDD (HL),A
                      MCycles = 3'b010;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          begin
                            Set_Addr_To = aXY;
                            Set_BusB_To = 4'b0111;
                          end
                        MCycle[1] :
                          begin
                            Write = 1'b1;
                            IncDec_16 = 4'b1110;
                          end
                        default :;
                      endcase // case(MCycle)
                      
                    end 
                  else 
                    begin
                      // LD (nn),A
                      MCycles = 3'b100;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        MCycle[2] :
                          begin
                            Set_Addr_To = aZI;
                            Inc_PC = 1'b1;
                            Set_BusB_To = 4'b0111;
                          end
                        MCycle[3] :
                          begin
                            Write = 1'b1;
                          end
                        default :;
                      endcase
                    end // else: !if(Mode == 3 )
                end // case: 8'b00110010
              

              // 16 BIT LOAD GROUP
              8'b00000001,8'b00010001,8'b00100001,8'b00110001  :
                begin
                  // LD dd,nn
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        Inc_PC = 1'b1;
                        Read_To_Reg = 1'b1;
                        if (DPAIR == 2'b11 ) 
                          begin
                            Set_BusA_To[3:0] = 4'b1000;
                          end 
                        else 
                          begin
                            Set_BusA_To[2:1] = DPAIR;
                            Set_BusA_To[0] = 1'b1;
                          end
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        Inc_PC = 1'b1;
                        Read_To_Reg = 1'b1;
                        if (DPAIR == 2'b11 ) 
                          begin
                            Set_BusA_To[3:0] = 4'b1001;
                          end 
                        else 
                          begin
                            Set_BusA_To[2:1] = DPAIR;
                            Set_BusA_To[0] = 1'b0;
                          end
                      end // case: 3
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b00000001,8'b00010001,8'b00100001,8'b00110001
              
              8'b00101010  :
                begin
                  if (Mode == 3 ) 
                    begin
                      // LDI A,(HL)
                      MCycles = 3'b010;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          Set_Addr_To = aXY;
                        MCycle[1] :
                          begin
                            Read_To_Acc = 1'b1;
                            IncDec_16 = 4'b0110;
                          end
                        
                        default :;
                      endcase
                    end 
                  else 
                    begin
                      // LD HL,(nn)
                      MCycles = 3'b101;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        MCycle[2] :
                          begin
                            Set_Addr_To = aZI;
                            Inc_PC = 1'b1;
                            LDW = 1'b1;
                          end
                        MCycle[3] :
                          begin
                            Set_BusA_To[2:0] = 3'b101; // L
                            Read_To_Reg = 1'b1;
                            Inc_WZ = 1'b1;
                            Set_Addr_To = aZI;
                          end
                        MCycle[4] :
                          begin
                            Set_BusA_To[2:0] = 3'b100; // H
                            Read_To_Reg = 1'b1;
                          end
                        default :;
                      endcase
                    end // else: !if(Mode == 3 )
                end // case: 8'b00101010
              
              8'b00100010  :
                begin
                  if (Mode == 3 ) 
                    begin
                      // LDI (HL),A
                      MCycles = 3'b010;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          begin
                            Set_Addr_To = aXY;
                            Set_BusB_To = 4'b0111;
                          end
                        MCycle[1] :
                          begin
                            Write = 1'b1;
                            IncDec_16 = 4'b0110;
                          end
                        default :;
                      endcase
                    end 
                  else 
                    begin
                      // LD (nn),HL
                      MCycles = 3'b101;
                      case (1'b1) // MCycle                        
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        
                        MCycle[2] :
                          begin
                            Set_Addr_To = aZI;
                            Inc_PC = 1'b1;
                            LDW = 1'b1;
                            Set_BusB_To = 4'b0101; // L
                          end
                        
                        MCycle[3] :
                          begin
                            Inc_WZ = 1'b1;
                            Set_Addr_To = aZI;
                            Write = 1'b1;
                            Set_BusB_To = 4'b0100; // H
                          end
                        MCycle[4] :                          
                          Write = 1'b1;
                        default :;
                      endcase
                    end // else: !if(Mode == 3 )
                end // case: 8'b00100010
              
              8'b11111001  :
                begin
                  // LD SP,HL
                  TStates = 3'b110;
                  LDSPHL = 1'b1;
                end
              
              8'b11zz0101 :
                begin
                  // PUSH qq
                  MCycles = 3'b011;
                  case (1'b1) // MCycle                    
                    MCycle[0] :
                      begin
                        TStates = 3'b101;
                        IncDec_16 = 4'b1111;
                        Set_Addr_To = aSP;
                        if (DPAIR == 2'b11 ) 
                          begin
                            Set_BusB_To = 4'b0111;
                          end 
                        else
                          begin
                            Set_BusB_To[2:1] = DPAIR;
                            Set_BusB_To[0] = 1'b0;
                            Set_BusB_To[3] = 1'b0;
                          end
                      end // case: 1
                    
                    MCycle[1] :
                      begin
                        IncDec_16 = 4'b1111;
                        Set_Addr_To = aSP;
                        if (DPAIR == 2'b11 ) 
                          begin
                            Set_BusB_To = 4'b1011;
                          end 
                        else 
                          begin
                            Set_BusB_To[2:1] = DPAIR;
                            Set_BusB_To[0] = 1'b1;
                            Set_BusB_To[3] = 1'b0;
                          end
                        Write = 1'b1;
                      end // case: 2
                    
                    MCycle[2] :
                      Write = 1'b1;
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b11000101,8'b11010101,8'b11100101,8'b11110101
              
              8'b11zz0001 :
                begin
                  // POP qq
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      Set_Addr_To = aSP;
                    MCycle[1] :
                      begin
                        IncDec_16 = 4'b0111;
                        Set_Addr_To = aSP;
                        Read_To_Reg = 1'b1;
                        if (DPAIR == 2'b11 ) 
                          begin
                            Set_BusA_To[3:0] = 4'b1011;
                          end 
                        else 
                          begin
                            Set_BusA_To[2:1] = DPAIR;
                            Set_BusA_To[0] = 1'b1;
                          end
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        IncDec_16 = 4'b0111;
                        Read_To_Reg = 1'b1;
                        if (DPAIR == 2'b11 ) 
                          begin
                            Set_BusA_To[3:0] = 4'b0111;
                          end 
                        else 
                          begin
                            Set_BusA_To[2:1] = DPAIR;
                            Set_BusA_To[0] = 1'b0;
                          end
                      end // case: 3
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b11000001,8'b11010001,8'b11100001,8'b11110001
              

              // EXCHANGE, BLOCK TRANSFER AND SEARCH GROUP
              8'b11101011  :
                begin
                  if (Mode != 3 ) 
                    begin
                      // EX DE,HL
                      ExchangeDH = 1'b1;
                    end
                end
              
              8'b00001000  :
                begin
                  if (Mode == 3 ) 
                    begin
                      // LD (nn),SP
                      MCycles = 3'b101;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        
                        MCycle[2] :
                          begin
                            Set_Addr_To = aZI;
                            Inc_PC = 1'b1;
                            LDW = 1'b1;
                            Set_BusB_To = 4'b1000;
                          end
                        
                        MCycle[3] :
                          begin
                            Inc_WZ = 1'b1;
                            Set_Addr_To = aZI;
                            Write = 1'b1;
                            Set_BusB_To = 4'b1001;
                          end
                        
                        MCycle[4] :
                          Write = 1'b1;
                        default :;
                      endcase
                    end 
                  else if (Mode < 2 ) 
                    begin
                      // EX AF,AF'
                      ExchangeAF = 1'b1;
                    end
                end // case: 8'b00001000
              
              8'b11011001  :
                begin
                  if (Mode == 3 ) 
                    begin
                      // RETI
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          Set_Addr_To = aSP;
                        MCycle[1] :
                          begin
                            IncDec_16 = 4'b0111;
                            Set_Addr_To = aSP;
                            LDZ = 1'b1;
                          end
                        
                        MCycle[2] :
                          begin
                            Jump = 1'b1;
                            IncDec_16 = 4'b0111;
                            I_RETN = 1'b1;
                            SetEI = 1'b1;
                          end
                        default :;
                      endcase
                    end 
                  else if (Mode < 2 ) 
                    begin
                      // EXX
                      ExchangeRS = 1'b1;
                    end
                end // case: 8'b11011001
              
              8'b11100011  :
                begin
                  if (Mode != 3 ) 
                    begin
                      // EX (SP),HL
                      MCycles = 3'b101;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          Set_Addr_To = aSP;
                        MCycle[1] :
                          begin
                            Read_To_Reg = 1'b1;
                            Set_BusA_To = 4'b0101;
                            Set_BusB_To = 4'b0101;
                            Set_Addr_To = aSP;
                          end
                        MCycle[2] :
                          begin
                            IncDec_16 = 4'b0111;
                            Set_Addr_To = aSP;
                            TStates = 3'b100;
                            Write = 1'b1;
                          end
                        MCycle[3] :
                          begin
                            Read_To_Reg = 1'b1;
                            Set_BusA_To = 4'b0100;
                            Set_BusB_To = 4'b0100;
                            Set_Addr_To = aSP;
                          end
                        MCycle[4] :
                          begin
                            IncDec_16 = 4'b1111;
                            TStates = 3'b101;
                            Write = 1'b1;
                          end
                        
                        default :;
                      endcase
                    end // if (Mode != 3 )
                end // case: 8'b11100011
              

              // 8 BIT ARITHMETIC AND LOGICAL GROUP
              8'b10zzzzzz :
                begin
                  if (IR[2:0] == 3'b110)
                    begin
                      // ADD A,(HL)
                      // ADC A,(HL)
                      // SUB A,(HL)
                      // SBC A,(HL)
                      // AND A,(HL)
                      // OR A,(HL)
                      // XOR A,(HL)
                      // CP A,(HL)
                      MCycles = 3'b010;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          Set_Addr_To = aXY;
                        MCycle[1] :
                          begin
                            Read_To_Reg = 1'b1;
                            Save_ALU = 1'b1;
                            Set_BusB_To[2:0] = SSS;
                            Set_BusA_To[2:0] = 3'b111;
                          end
                        
                        default :;
                      endcase // case(MCycle)
                    end // if (IR[2:0] == 3'b110)
                  else
                    begin
                      // ADD A,r
                      // ADC A,r
                      // SUB A,r
                      // SBC A,r
                      // AND A,r
                      // OR A,r
                      // XOR A,r
                      // CP A,r
                      Set_BusB_To[2:0] = SSS;
                      Set_BusA_To[2:0] = 3'b111;
                      Read_To_Reg = 1'b1;
                      Save_ALU = 1'b1;
                    end // else: !if(IR[2:0] == 3'b110)                  
                end // case: 8'b10000000,8'b10000001,8'b10000010,8'b10000011,8'b10000100,8'b10000101,8'b10000111,...
              
              8'b11zzz110 :
                begin
                  // ADD A,n
                  // ADC A,n
                  // SUB A,n
                  // SBC A,n
                  // AND A,n
                  // OR A,n
                  // XOR A,n
                  // CP A,n
                  MCycles = 3'b010;
                  if (MCycle[1] ) 
                    begin
                      Inc_PC = 1'b1;
                      Read_To_Reg = 1'b1;
                      Save_ALU = 1'b1;
                      Set_BusB_To[2:0] = SSS;
                      Set_BusA_To[2:0] = 3'b111;
                    end
                end
              
              8'b00zzz100 :
                begin
                  if (IR[5:3] == 3'b110)
                    begin
                      // INC (HL)
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          Set_Addr_To = aXY;
                        MCycle[1] :
                          begin
                            TStates = 3'b100;
                            Set_Addr_To = aXY;
                            Read_To_Reg = 1'b1;
                            Save_ALU = 1'b1;
                            PreserveC = 1'b1;
                            ALU_Op = 4'b0000;
                            Set_BusB_To = 4'b1010;
                            Set_BusA_To[2:0] = DDD;
                          end // case: 2
                        
                        MCycle[2] :
                          Write = 1'b1;
                        default :;
                      endcase // case(MCycle)
                    end // case: 8'b00110100
                  else
                    begin
                      // INC r
                      Set_BusB_To = 4'b1010;
                      Set_BusA_To[2:0] = DDD;
                      Read_To_Reg = 1'b1;
                      Save_ALU = 1'b1;
                      PreserveC = 1'b1;
                      ALU_Op = 4'b0000;
                    end
                end
              
              8'b00zzz101 :
                begin               
                  if (IR[5:3] == 3'b110)
                    begin
                      // DEC (HL)
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          Set_Addr_To = aXY;
                        MCycle[1] :
                          begin
                            TStates = 3'b100;
                            Set_Addr_To = aXY;
                            ALU_Op = 4'b0010;
                            Read_To_Reg = 1'b1;
                            Save_ALU = 1'b1;
                            PreserveC = 1'b1;
                            Set_BusB_To = 4'b1010;
                            Set_BusA_To[2:0] = DDD;
                          end // case: 2
                        
                        MCycle[2] :
                          Write = 1'b1;
                        default :;
                      endcase // case(MCycle)
                    end
                  else
                    begin
                      // DEC r
                      Set_BusB_To = 4'b1010;
                      Set_BusA_To[2:0] = DDD;
                      Read_To_Reg = 1'b1;
                      Save_ALU = 1'b1;
                      PreserveC = 1'b1;
                      ALU_Op = 4'b0010;
                    end
                end
              
              // GENERAL PURPOSE ARITHMETIC AND CPU CONTROL GROUPS
              8'b00100111  :
                begin
                  // DAA
                  Set_BusA_To[2:0] = 3'b111;
                  Read_To_Reg = 1'b1;
                  ALU_Op = 4'b1100;
                  Save_ALU = 1'b1;
                end
              
              8'b00101111  :
                // CPL
                I_CPL = 1'b1;
              
              8'b00111111  :
                // CCF
                I_CCF = 1'b1;
              
              8'b00110111  :
                // SCF
                I_SCF = 1'b1;
              
              8'b00000000  :
                begin
                  if (NMICycle == 1'b1 ) 
                    begin
                      // NMI
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          begin
                            TStates = 3'b101;
                            IncDec_16 = 4'b1111;
                            Set_Addr_To = aSP;
                            Set_BusB_To = 4'b1101;
                          end
                        
                        MCycle[1] :
                          begin
                            TStates = 3'b100;
                            Write = 1'b1;
                            IncDec_16 = 4'b1111;
                            Set_Addr_To = aSP;
                            Set_BusB_To = 4'b1100;
                          end
                        
                        MCycle[2] :
                          begin
                            TStates = 3'b100;
                            Write = 1'b1;
                          end
                        
                        default :;
                      endcase // case(MCycle)
                      
                    end 
                  else if (IntCycle == 1'b1 ) 
                    begin
                      // INT (IM 2)
                      MCycles = 3'b101;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          begin
                            LDZ = 1'b1;
                            TStates = 3'b101;
                            IncDec_16 = 4'b1111;
                            Set_Addr_To = aSP;
                            Set_BusB_To = 4'b1101;
                          end
                        
                        MCycle[1] :
                          begin
                            TStates = 3'b100;
                            Write = 1'b1;
                            IncDec_16 = 4'b1111;
                            Set_Addr_To = aSP;
                            Set_BusB_To = 4'b1100;
                          end
                        
                        MCycle[2] :
                          begin
                            TStates = 3'b100;
                            Write = 1'b1;
                          end
                        
                        MCycle[3] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        
                        MCycle[4] :
                          Jump = 1'b1;
                        default :;
                      endcase
                    end
                end // case: 8'b00000000
              
              8'b11110011  :
                // DI
                SetDI = 1'b1;
              
              8'b11111011  :
                // EI
                SetEI = 1'b1;

              // 16 BIT ARITHMETIC GROUP
              8'b00zz1001  :
                begin
                  // ADD HL,ss
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        NoRead = 1'b1;
                        ALU_Op = 4'b0000;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_BusA_To[2:0] = 3'b101;
                        case (IR[5:4])
                          0,1,2  :
                            begin
                              Set_BusB_To[2:1] = IR[5:4];
                              Set_BusB_To[0] = 1'b1;
                            end
                          
                          default :
                            Set_BusB_To = 4'b1000;
                        endcase // case(IR[5:4])
                        
                        TStates = 3'b100;
                        Arith16 = 1'b1;
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        NoRead = 1'b1;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        ALU_Op = 4'b0001;
                        Set_BusA_To[2:0] = 3'b100;
                        case (IR[5:4])
                          0,1,2  :
                            Set_BusB_To[2:1] = IR[5:4];
                          default :
                            Set_BusB_To = 4'b1001;
                        endcase
                        Arith16 = 1'b1;
                      end // case: 3
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b00001001,8'b00011001,8'b00101001,8'b00111001              
              
              8'b00zz0011 :
                begin
                  // INC ss
                  TStates = 3'b110;
                  IncDec_16[3:2] = 2'b01;
                  IncDec_16[1:0] = DPAIR;
                end
              
              8'b00zz1011 :
                begin
                  // DEC ss
                  TStates = 3'b110;
                  IncDec_16[3:2] = 2'b11;
                  IncDec_16[1:0] = DPAIR;
                end

              // ROTATE AND SHIFT GROUP
              8'b00000111,
                  // RLCA
                  8'b00010111,
                  // RLA
                  8'b00001111,
                  // RRCA
                  8'b00011111 :
                    // RRA
                    begin
                      Set_BusA_To[2:0] = 3'b111;
                      ALU_Op = 4'b1000;
                      Read_To_Reg = 1'b1;
                      Save_ALU = 1'b1;
                    end // case: 8'b00000111,...
              

              // JUMP GROUP
              8'b11000011  :
                begin
                  // JP nn
                  MCycles = 3'b011;
                  if (MCycle[1])
                    begin
                      Inc_PC = 1'b1;
                      LDZ = 1'b1;
                    end
                  
                  if (MCycle[2])
                    begin
                      Inc_PC = 1'b1;
                      Jump = 1'b1;
                    end
                  
                end // case: 8'b11000011
              
              8'b11zzz010  :
                begin
                  if (IR[5] == 1'b1 && Mode == 3 ) 
                    begin
                      case (IR[4:3])
                        2'b00  :
                          begin
                            // LD ($FF00+C),A
                            MCycles = 3'b010;
                            case (1'b1) // MCycle
                              MCycle[0] :
                                begin
                                  Set_Addr_To = aBC;
                                  Set_BusB_To   = 4'b0111;
                                end
                              MCycle[1] :
                                begin
                                  Write = 1'b1;
                                  IORQ = 1'b1;
                                end
                              
                              default :;
                            endcase // case(MCycle)
                          end // case: 2'b00
                        
                        2'b01  :
                          begin
                            // LD (nn),A
                            MCycles = 3'b100;
                            case (1'b1) // MCycle
                              MCycle[1] :
                                begin
                                  Inc_PC = 1'b1;
                                  LDZ = 1'b1;
                                end
                              
                              MCycle[2] :
                                begin
                                  Set_Addr_To = aZI;
                                  Inc_PC = 1'b1;
                                  Set_BusB_To = 4'b0111;
                                end
                              
                              MCycle[3] :
                                Write = 1'b1;
                              default :;
                            endcase // case(MCycle)
                          end // case: default :...
                        
                        2'b10  :
                          begin
                            // LD A,($FF00+C)
                            MCycles = 3'b010;
                            case (1'b1) // MCycle
                              MCycle[0] :
                                Set_Addr_To = aBC;
                              MCycle[1] :
                                begin
                                  Read_To_Acc = 1'b1;
                                  IORQ = 1'b1;
                                end
                              default :;
                            endcase // case(MCycle)
                          end // case: 2'b10
                        
                        2'b11  :
                          begin
                            // LD A,(nn)
                            MCycles = 3'b100;
                            case (1'b1) // MCycle
                              MCycle[1] :
                                begin
                                  Inc_PC = 1'b1;
                                  LDZ = 1'b1;
                                end
                              MCycle[2] :
                                begin
                                  Set_Addr_To = aZI;
                                  Inc_PC = 1'b1;
                                end
                              MCycle[3] :
                                Read_To_Acc = 1'b1;
                              default :;
                            endcase // case(MCycle)
                          end
                      endcase
                    end 
                  else 
                    begin
                      // JP cc,nn
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        MCycle[2] :
                          begin
                            Inc_PC = 1'b1;
                            if (is_cc_true(F, IR[5:3]) ) 
                              begin
                                Jump = 1'b1;
                              end
                          end
                        
                        default :;
                      endcase
                    end // else: !if(DPAIR == 2'b11 )
                end // case: 8'b11000010,8'b11001010,8'b11010010,8'b11011010,8'b11100010,8'b11101010,8'b11110010,8'b11111010
              
              8'b00011000  :
                begin
                  if (Mode != 2 ) 
                    begin
                      // JR e
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          Inc_PC = 1'b1;
                        MCycle[2] :
                          begin
                            NoRead = 1'b1;
                            JumpE = 1'b1;
                            TStates = 3'b101;
                          end
                        default :;
                      endcase
                    end // if (Mode != 2 )
                end // case: 8'b00011000

              // Conditional relative jumps (JR [C/NC/Z/NZ], e)
              8'b001zz000  :
                begin
                  if (Mode != 2 ) 
                    begin
                      MCycles = 3'd3;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;

                            case (IR[4:3])
                              0 : MCycles = (F[Flag_Z]) ? 3'd2 : 3'd3;
                              1 : MCycles = (!F[Flag_Z]) ? 3'd2 : 3'd3;
                              2 : MCycles = (F[Flag_C]) ? 3'd2 : 3'd3;
                              3 : MCycles = (!F[Flag_C]) ? 3'd2 : 3'd3;
                            endcase
                          end
                        
                        MCycle[2] :
                          begin
                            NoRead = 1'b1;
                            JumpE = 1'b1;
                            TStates = 3'd5;
                          end
                        default :;
                      endcase
                    end // if (Mode != 2 )
                end // case: 8'b00111000              
              
              8'b11101001  :
                // JP (HL)
                JumpXY = 1'b1;
              
              8'b00010000  :
                begin
                  if (Mode == 3 ) 
                    begin
                      I_DJNZ = 1'b1;
                    end 
                  else if (Mode < 2 ) 
                    begin
                      // DJNZ,e
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          begin
                            TStates = 3'b101;
                            I_DJNZ = 1'b1;
                            Set_BusB_To = 4'b1010;
                            Set_BusA_To[2:0] = 3'b000;
                            Read_To_Reg = 1'b1;
                            Save_ALU = 1'b1;
                            ALU_Op = 4'b0010;
                          end
                        MCycle[1] :
                          begin
                            I_DJNZ = 1'b1;
                            Inc_PC = 1'b1;
                          end
                        MCycle[2] :
                          begin
                            NoRead = 1'b1;
                            JumpE = 1'b1;
                            TStates = 3'b101;
                          end
                        default :;
                      endcase
                    end // if (Mode < 2 )
                end // case: 8'b00010000
              

              // CALL AND RETURN GROUP
              8'b11001101  :
                begin
                  // CALL nn
                  MCycles = 3'b101;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        Inc_PC = 1'b1;
                        LDZ = 1'b1;
                      end
                    MCycle[2] :
                      begin
                        IncDec_16 = 4'b1111;
                        Inc_PC = 1'b1;
                        TStates = 3'b100;
                        Set_Addr_To = aSP;
                        LDW = 1'b1;
                        Set_BusB_To = 4'b1101;
                      end
                    MCycle[3] :
                      begin
                        Write = 1'b1;
                        IncDec_16 = 4'b1111;
                        Set_Addr_To = aSP;
                        Set_BusB_To = 4'b1100;
                      end
                    MCycle[4] :
                      begin
                        Write = 1'b1;
                        Call = 1'b1;
                      end
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b11001101
              
              8'b11zzz100  :
                begin
                  if (IR[5] == 1'b0 || Mode != 3 ) 
                    begin
                      // CALL cc,nn
                      MCycles = 3'b101;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            LDZ = 1'b1;
                          end
                        MCycle[2] :
                          begin
                            Inc_PC = 1'b1;
                            LDW = 1'b1;
                            if (is_cc_true(F, IR[5:3]) ) 
                              begin
                                IncDec_16 = 4'b1111;
                                Set_Addr_To = aSP;
                                TStates = 3'b100;
                                Set_BusB_To = 4'b1101;
                              end 
                            else 
                              begin
                                MCycles = 3'b011;
                              end // else: !if(is_cc_true(F, IR[5:3]) )
                          end // case: 3
                        
                        MCycle[3] :
                          begin
                            Write = 1'b1;
                            IncDec_16 = 4'b1111;
                            Set_Addr_To = aSP;
                            Set_BusB_To = 4'b1100;
                          end
                        
                        MCycle[4] :
                          begin
                            Write = 1'b1;
                            Call = 1'b1;
                          end
                        
                        default :;
                      endcase
                    end // if (IR[5] == 1'b0 || Mode != 3 )
                end // case: 8'b11000100,8'b11001100,8'b11010100,8'b11011100,8'b11100100,8'b11101100,8'b11110100,8'b11111100
              
              8'b11001001  :
                begin
                  // RET
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        TStates = 3'b101;
                        Set_Addr_To = aSP;
                      end
                    
                    MCycle[1] :
                      begin
                        IncDec_16 = 4'b0111;
                        Set_Addr_To = aSP;
                        LDZ = 1'b1;
                      end
                    
                    MCycle[2] :
                      begin
                        Jump = 1'b1;
                        IncDec_16 = 4'b0111;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b11001001
              
              8'b11000000,8'b11001000,8'b11010000,8'b11011000,8'b11100000,8'b11101000,8'b11110000,8'b11111000  :
                begin                  
                  if (IR[5] == 1'b1 && Mode == 3 ) 
                    begin
                      case (IR[4:3])
                        2'b00  :
                          begin
                            // LD ($FF00+nn),A
                            MCycles = 3'b011;
                            case (1'b1) // MCycle
                              MCycle[1] :
                                begin
                                  Inc_PC = 1'b1;
                                  Set_Addr_To = aIOA;
                                  Set_BusB_To   = 4'b0111;
                                end
                              
                              MCycle[2] :
                                Write = 1'b1;
                              default :;
                            endcase // case(MCycle)
                          end // case: 2'b00
                        
                        2'b01  :
                          begin
                            // ADD SP,n
                            MCycles = 3'b011;
                            case (1'b1) // MCycle
                              MCycle[1] :
                                begin
                                  ALU_Op = 4'b0000;
                                  Inc_PC = 1'b1;
                                  Read_To_Reg = 1'b1;
                                  Save_ALU = 1'b1;
                                  Set_BusA_To = 4'b1000;
                                  Set_BusB_To = 4'b0110;
                                end
                              
                              MCycle[2] :
                                begin
                                  NoRead = 1'b1;
                                  Read_To_Reg = 1'b1;
                                  Save_ALU = 1'b1;
                                  ALU_Op = 4'b0001;
                                  Set_BusA_To = 4'b1001;
                                  Set_BusB_To = 4'b1110;        // Incorrect unsigned !!!!!!!!!!!!!!!!!!!!!
                                end
                              
                              default :;
                            endcase // case(MCycle)
                          end // case: 2'b01
                        
                        2'b10  :
                          begin
                            // LD A,($FF00+nn)
                            MCycles = 3'b011;
                            case (1'b1) // MCycle
                              MCycle[1] :
                                begin
                                  Inc_PC = 1'b1;
                                  Set_Addr_To = aIOA;
                                end
                              
                              MCycle[2] :
                                Read_To_Acc = 1'b1;
                              default :;
                            endcase // case(MCycle)
                          end // case: 2'b10
                        
                        2'b11  :
                          begin
                            // LD HL,SP+n       -- Not correct !!!!!!!!!!!!!!!!!!!
                            MCycles = 3'b101;
                            case (1'b1) // MCycle
                              MCycle[1] :
                                begin
                                  Inc_PC = 1'b1;
                                  LDZ = 1'b1;
                                end
                              
                              MCycle[2] :
                                begin
                                  Set_Addr_To = aZI;
                                  Inc_PC = 1'b1;
                                  LDW = 1'b1;
                                end
                              
                              MCycle[3] :
                                begin
                                  Set_BusA_To[2:0] = 3'b101; // L
                                  Read_To_Reg = 1'b1;
                                  Inc_WZ = 1'b1;
                                  Set_Addr_To = aZI;
                                end
                              
                              MCycle[4] :
                                begin
                                  Set_BusA_To[2:0] = 3'b100; // H
                                  Read_To_Reg = 1'b1;
                                end
                              
                              default :;
                            endcase // case(MCycle)
                          end // case: 2'b11
                        
                      endcase // case(IR[4:3])
                      
                    end 
                  else 
                    begin
                      // RET cc
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[0] :
                          begin
                            if (is_cc_true(F, IR[5:3]) )                              
                              begin
                                Set_Addr_To = aSP;
                              end 
                            else 
                              begin
                                MCycles = 3'b001;
                              end
                            TStates = 3'b101;
                          end // case: 1
                        
                        MCycle[1] :
                          begin
                            IncDec_16 = 4'b0111;
                            Set_Addr_To = aSP;
                            LDZ = 1'b1;
                          end
                        MCycle[2] :
                          begin
                            Jump = 1'b1;
                            IncDec_16 = 4'b0111;
                          end
                        default :;
                      endcase
                    end // else: !if(IR[5] == 1'b1 && Mode == 3 )
                end // case: 8'b11000000,8'b11001000,8'b11010000,8'b11011000,8'b11100000,8'b11101000,8'b11110000,8'b11111000
              
              8'b11000111,8'b11001111,8'b11010111,8'b11011111,8'b11100111,8'b11101111,8'b11110111,8'b11111111  :
                begin
                  // RST p
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        TStates = 3'b101;
                        IncDec_16 = 4'b1111;
                        Set_Addr_To = aSP;
                        Set_BusB_To = 4'b1101;
                      end
                    
                    MCycle[1] :
                      begin
                        Write = 1'b1;
                        IncDec_16 = 4'b1111;
                        Set_Addr_To = aSP;
                        Set_BusB_To = 4'b1100;
                      end
                    
                    MCycle[2] :
                      begin
                        Write = 1'b1;
                        RstP = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b11000111,8'b11001111,8'b11010111,8'b11011111,8'b11100111,8'b11101111,8'b11110111,8'b11111111
              
              // INPUT AND OUTPUT GROUP
              8'b11011011  :
                begin
                  if (Mode != 3 ) 
                    begin
                      // IN A,(n)
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            Set_Addr_To = aIOA;
                          end
                        
                        MCycle[2] :
                          begin
                            Read_To_Acc = 1'b1;
                            IORQ = 1'b1;
                          end
                        
                        default :;
                      endcase
                    end // if (Mode != 3 )
                end // case: 8'b11011011
              
              8'b11010011  :
                begin
                  if (Mode != 3 ) 
                    begin
                      // OUT (n),A
                      MCycles = 3'b011;
                      case (1'b1) // MCycle
                        MCycle[1] :
                          begin
                            Inc_PC = 1'b1;
                            Set_Addr_To = aIOA;
                            Set_BusB_To = 4'b0111;
                          end
                        
                        MCycle[2] :
                          begin
                            Write = 1'b1;
                            IORQ = 1'b1;
                          end
                        
                        default :;
                      endcase
                    end // if (Mode != 3 )
                end // case: 8'b11010011
              

              //----------------------------------------------------------------------------
              //----------------------------------------------------------------------------
              // MULTIBYTE INSTRUCTIONS
              //----------------------------------------------------------------------------
              //----------------------------------------------------------------------------

              8'b11001011  :
                begin
                  if (Mode != 2 ) 
                    begin
                      Prefix = 2'b01;
                    end
                end              

              8'b11101101  :
                begin
                  if (Mode < 2 ) 
                    begin
                      Prefix = 2'b10;
                    end
                end

              8'b11011101,8'b11111101  :
                begin
                  if (Mode < 2 ) 
                    begin
                      Prefix = 2'b11;
                    end
                end
              
            endcase // case(IR)
          end // case: 2'b00
        

        2'b01  :
          begin
            

            //----------------------------------------------------------------------------
            //
            //  CB prefixed instructions
            //
            //----------------------------------------------------------------------------
            
            Set_BusA_To[2:0] = IR[2:0];
            Set_BusB_To[2:0] = IR[2:0];
            
            casez (IR)
              8'b00000000,8'b00000001,8'b00000010,8'b00000011,8'b00000100,8'b00000101,8'b00000111,
              8'b00010000,8'b00010001,8'b00010010,8'b00010011,8'b00010100,8'b00010101,8'b00010111,
              8'b00001000,8'b00001001,8'b00001010,8'b00001011,8'b00001100,8'b00001101,8'b00001111,
              8'b00011000,8'b00011001,8'b00011010,8'b00011011,8'b00011100,8'b00011101,8'b00011111,
              8'b00100000,8'b00100001,8'b00100010,8'b00100011,8'b00100100,8'b00100101,8'b00100111,
              8'b00101000,8'b00101001,8'b00101010,8'b00101011,8'b00101100,8'b00101101,8'b00101111,
              8'b00110000,8'b00110001,8'b00110010,8'b00110011,8'b00110100,8'b00110101,8'b00110111,
              8'b00111000,8'b00111001,8'b00111010,8'b00111011,8'b00111100,8'b00111101,8'b00111111 :
                begin
                  // RLC r
                  // RL r
                  // RRC r
                  // RR r
                  // SLA r
                  // SRA r
                  // SRL r
                  // SLL r (Undocumented) / SWAP r
                  if (MCycle[0] ) begin
                    ALU_Op = 4'b1000;
                    Read_To_Reg = 1'b1;
                    Save_ALU = 1'b1;
                  end
                end // case: 8'b00000000,8'b00000001,8'b00000010,8'b00000011,8'b00000100,8'b00000101,8'b00000111,...
              
              8'b00zzz110  :
                begin
                  // RLC (HL)
                  // RL (HL)
                  // RRC (HL)
                  // RR (HL)
                  // SRA (HL)
                  // SRL (HL)
                  // SLA (HL)
                  // SLL (HL) (Undocumented) / SWAP (HL)
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0], MCycle[6] :
                      Set_Addr_To = aXY;
                    MCycle[1] :
                      begin
                        ALU_Op = 4'b1000;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_Addr_To = aXY;
                        TStates = 3'b100;
                      end
                    
                    MCycle[2] :
                      Write = 1'b1;
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b00000110,8'b00010110,8'b00001110,8'b00011110,8'b00101110,8'b00111110,8'b00100110,8'b00110110
              
              8'b01000000,8'b01000001,8'b01000010,8'b01000011,8'b01000100,8'b01000101,8'b01000111,
                  8'b01001000,8'b01001001,8'b01001010,8'b01001011,8'b01001100,8'b01001101,8'b01001111,
                  8'b01010000,8'b01010001,8'b01010010,8'b01010011,8'b01010100,8'b01010101,8'b01010111,
                  8'b01011000,8'b01011001,8'b01011010,8'b01011011,8'b01011100,8'b01011101,8'b01011111,
                  8'b01100000,8'b01100001,8'b01100010,8'b01100011,8'b01100100,8'b01100101,8'b01100111,
                  8'b01101000,8'b01101001,8'b01101010,8'b01101011,8'b01101100,8'b01101101,8'b01101111,
                  8'b01110000,8'b01110001,8'b01110010,8'b01110011,8'b01110100,8'b01110101,8'b01110111,
                  8'b01111000,8'b01111001,8'b01111010,8'b01111011,8'b01111100,8'b01111101,8'b01111111 :
                    begin
                      // BIT b,r
                      if (MCycle[0] )
                        begin
                          Set_BusB_To[2:0] = IR[2:0];
                          ALU_Op = 4'b1001;
                        end
                    end // case: 8'b01000000,8'b01000001,8'b01000010,8'b01000011,8'b01000100,8'b01000101,8'b01000111,...
              
              8'b01000110,8'b01001110,8'b01010110,8'b01011110,8'b01100110,8'b01101110,8'b01110110,8'b01111110  :
                begin
                  // BIT b,(HL)
                  MCycles = 3'b010;
                  case (1'b1) // MCycle
                    MCycle[0], MCycle[6] :
                      Set_Addr_To = aXY;
                    MCycle[1] :
                      begin
                        ALU_Op = 4'b1001;
                        TStates = 3'b100;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01000110,8'b01001110,8'b01010110,8'b01011110,8'b01100110,8'b01101110,8'b01110110,8'b01111110
              
              8'b11000000,8'b11000001,8'b11000010,8'b11000011,8'b11000100,8'b11000101,8'b11000111,
                  8'b11001000,8'b11001001,8'b11001010,8'b11001011,8'b11001100,8'b11001101,8'b11001111,
                  8'b11010000,8'b11010001,8'b11010010,8'b11010011,8'b11010100,8'b11010101,8'b11010111,
                  8'b11011000,8'b11011001,8'b11011010,8'b11011011,8'b11011100,8'b11011101,8'b11011111,
                  8'b11100000,8'b11100001,8'b11100010,8'b11100011,8'b11100100,8'b11100101,8'b11100111,
                  8'b11101000,8'b11101001,8'b11101010,8'b11101011,8'b11101100,8'b11101101,8'b11101111,
                  8'b11110000,8'b11110001,8'b11110010,8'b11110011,8'b11110100,8'b11110101,8'b11110111,
                  8'b11111000,8'b11111001,8'b11111010,8'b11111011,8'b11111100,8'b11111101,8'b11111111 :
                    begin
                      // SET b,r
                      if (MCycle[0] ) 
                        begin
                          ALU_Op = 4'b1010;
                          Read_To_Reg = 1'b1;
                          Save_ALU = 1'b1;
                        end
                    end // case: 8'b11000000,8'b11000001,8'b11000010,8'b11000011,8'b11000100,8'b11000101,8'b11000111,...
              
              8'b11000110,8'b11001110,8'b11010110,8'b11011110,8'b11100110,8'b11101110,8'b11110110,8'b11111110  :
                begin
                  // SET b,(HL)
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0], MCycle[6] :
                      Set_Addr_To = aXY;
                    MCycle[1] :
                      begin
                        ALU_Op = 4'b1010;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_Addr_To = aXY;
                        TStates = 3'b100;
                      end
                    MCycle[2] :
                      Write = 1'b1;
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b11000110,8'b11001110,8'b11010110,8'b11011110,8'b11100110,8'b11101110,8'b11110110,8'b11111110
              
              8'b10000000,8'b10000001,8'b10000010,8'b10000011,8'b10000100,8'b10000101,8'b10000111,
                  8'b10001000,8'b10001001,8'b10001010,8'b10001011,8'b10001100,8'b10001101,8'b10001111,
                  8'b10010000,8'b10010001,8'b10010010,8'b10010011,8'b10010100,8'b10010101,8'b10010111,
                  8'b10011000,8'b10011001,8'b10011010,8'b10011011,8'b10011100,8'b10011101,8'b10011111,
                  8'b10100000,8'b10100001,8'b10100010,8'b10100011,8'b10100100,8'b10100101,8'b10100111,
                  8'b10101000,8'b10101001,8'b10101010,8'b10101011,8'b10101100,8'b10101101,8'b10101111,
                  8'b10110000,8'b10110001,8'b10110010,8'b10110011,8'b10110100,8'b10110101,8'b10110111,
                  8'b10111000,8'b10111001,8'b10111010,8'b10111011,8'b10111100,8'b10111101,8'b10111111 :
                    begin
                      // RES b,r
                      if (MCycle[0] ) 
                        begin
                          ALU_Op = 4'b1011;
                          Read_To_Reg = 1'b1;
                          Save_ALU = 1'b1;
                        end
                    end // case: 8'b10000000,8'b10000001,8'b10000010,8'b10000011,8'b10000100,8'b10000101,8'b10000111,...
              
              8'b10000110,8'b10001110,8'b10010110,8'b10011110,8'b10100110,8'b10101110,8'b10110110,8'b10111110  :
                begin
                  // RES b,(HL)
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0], MCycle[6] :
                      Set_Addr_To = aXY;
                    MCycle[1] :
                      begin
                        ALU_Op = 4'b1011;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_Addr_To = aXY;
                        TStates = 3'b100;
                      end
                    
                    MCycle[2] :
                      Write = 1'b1;
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b10000110,8'b10001110,8'b10010110,8'b10011110,8'b10100110,8'b10101110,8'b10110110,8'b10111110
              
            endcase // case(IR)
          end // case: 2'b01
        
        
        default :
          begin : default_ed_block
            
            //----------------------------------------------------------------------------
            //
            //  ED prefixed instructions
            //
            //----------------------------------------------------------------------------

            casez (IR)
	      /*
	       * Undocumented NOP instructions commented out to reduce size of mcode
	       *
              8'b00000000,8'b00000001,8'b00000010,8'b00000011,8'b00000100,8'b00000101,8'b00000110,8'b00000111
                ,8'b00001000,8'b00001001,8'b00001010,8'b00001011,8'b00001100,8'b00001101,8'b00001110,8'b00001111
                  ,8'b00010000,8'b00010001,8'b00010010,8'b00010011,8'b00010100,8'b00010101,8'b00010110,8'b00010111
                    ,8'b00011000,8'b00011001,8'b00011010,8'b00011011,8'b00011100,8'b00011101,8'b00011110,8'b00011111
                      ,8'b00100000,8'b00100001,8'b00100010,8'b00100011,8'b00100100,8'b00100101,8'b00100110,8'b00100111
                        ,8'b00101000,8'b00101001,8'b00101010,8'b00101011,8'b00101100,8'b00101101,8'b00101110,8'b00101111
                          ,8'b00110000,8'b00110001,8'b00110010,8'b00110011,8'b00110100,8'b00110101,8'b00110110,8'b00110111
                            ,8'b00111000,8'b00111001,8'b00111010,8'b00111011,8'b00111100,8'b00111101,8'b00111110,8'b00111111
                

                              ,8'b10000000,8'b10000001,8'b10000010,8'b10000011,8'b10000100,8'b10000101,8'b10000110,8'b10000111
                                ,8'b10001000,8'b10001001,8'b10001010,8'b10001011,8'b10001100,8'b10001101,8'b10001110,8'b10001111
                                  ,8'b10010000,8'b10010001,8'b10010010,8'b10010011,8'b10010100,8'b10010101,8'b10010110,8'b10010111
                                    ,8'b10011000,8'b10011001,8'b10011010,8'b10011011,8'b10011100,8'b10011101,8'b10011110,8'b10011111
                                      ,                                            8'b10100100,8'b10100101,8'b10100110,8'b10100111
                                        ,                                            8'b10101100,8'b10101101,8'b10101110,8'b10101111
                                          ,                                            8'b10110100,8'b10110101,8'b10110110,8'b10110111
                                            ,                                            8'b10111100,8'b10111101,8'b10111110,8'b10111111
                                              ,8'b11000000,8'b11000001,8'b11000010,8'b11000011,8'b11000100,8'b11000101,8'b11000110,8'b11000111
                                                ,8'b11001000,8'b11001001,8'b11001010,8'b11001011,8'b11001100,8'b11001101,8'b11001110,8'b11001111
                                                  ,8'b11010000,8'b11010001,8'b11010010,8'b11010011,8'b11010100,8'b11010101,8'b11010110,8'b11010111
                                                    ,8'b11011000,8'b11011001,8'b11011010,8'b11011011,8'b11011100,8'b11011101,8'b11011110,8'b11011111
                                                      ,8'b11100000,8'b11100001,8'b11100010,8'b11100011,8'b11100100,8'b11100101,8'b11100110,8'b11100111
                                                        ,8'b11101000,8'b11101001,8'b11101010,8'b11101011,8'b11101100,8'b11101101,8'b11101110,8'b11101111
                                                          ,8'b11110000,8'b11110001,8'b11110010,8'b11110011,8'b11110100,8'b11110101,8'b11110110,8'b11110111
                                                            ,8'b11111000,8'b11111001,8'b11111010,8'b11111011,8'b11111100,8'b11111101,8'b11111110,8'b11111111 :
                                                              ; // NOP, undocumented
              
              8'b01111110,8'b01111111  :
                // NOP, undocumented
                ;
	       */
	      
              // 8 BIT LOAD GROUP
              8'b01010111  :
                begin
                  // LD A,I
                  Special_LD = 3'b100;
                  TStates = 3'b101;
                end
              
              8'b01011111  :
                begin
                  // LD A,R
                  Special_LD = 3'b101;
                  TStates = 3'b101;
                end
              
              8'b01000111  :
                begin
                  // LD I,A
                  Special_LD = 3'b110;
                  TStates = 3'b101;
                end
              
              8'b01001111  :
                begin
                  // LD R,A
                  Special_LD = 3'b111;
                  TStates = 3'b101;
                end
              
              // 16 BIT LOAD GROUP
              8'b01001011,8'b01011011,8'b01101011,8'b01111011  :
                begin
                  // LD dd,(nn)
                  MCycles = 3'b101;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        Inc_PC = 1'b1;
                        LDZ = 1'b1;
                      end
                    
                    MCycle[2] :
                      begin
                        Set_Addr_To = aZI;
                        Inc_PC = 1'b1;
                        LDW = 1'b1;
                      end
                    
                    MCycle[3] :
                      begin
                        Read_To_Reg = 1'b1;
                        if (IR[5:4] == 2'b11 ) 
                          begin
                            Set_BusA_To = 4'b1000;
                          end 
                        else 
                          begin
                            Set_BusA_To[2:1] = IR[5:4];
                            Set_BusA_To[0] = 1'b1;
                          end
                        Inc_WZ = 1'b1;
                        Set_Addr_To = aZI;
                      end // case: 4
                    
                    MCycle[4] :
                      begin
                        Read_To_Reg = 1'b1;
                        if (IR[5:4] == 2'b11 ) 
                          begin
                            Set_BusA_To = 4'b1001;
                          end 
                        else 
                          begin
                            Set_BusA_To[2:1] = IR[5:4];
                            Set_BusA_To[0] = 1'b0;
                          end
                      end // case: 5
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01001011,8'b01011011,8'b01101011,8'b01111011
              
              
              8'b01000011,8'b01010011,8'b01100011,8'b01110011  :
                begin
                  // LD (nn),dd
                  MCycles = 3'b101;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        Inc_PC = 1'b1;
                        LDZ = 1'b1;
                      end
                    
                    MCycle[2] :
                      begin
                        Set_Addr_To = aZI;
                        Inc_PC = 1'b1;
                        LDW = 1'b1;
                        if (IR[5:4] == 2'b11 ) 
                          begin
                            Set_BusB_To = 4'b1000;
                          end 
                        else 
                          begin
                            Set_BusB_To[2:1] = IR[5:4];
                            Set_BusB_To[0] = 1'b1;
                            Set_BusB_To[3] = 1'b0;
                          end
                      end // case: 3
                    
                    MCycle[3] :
                      begin
                        Inc_WZ = 1'b1;
                        Set_Addr_To = aZI;
                        Write = 1'b1;
                        if (IR[5:4] == 2'b11 ) 
                          begin
                            Set_BusB_To = 4'b1001;
                          end 
                        else 
                          begin
                            Set_BusB_To[2:1] = IR[5:4];
                            Set_BusB_To[0] = 1'b0;
                            Set_BusB_To[3] = 1'b0;
                          end
                      end // case: 4
                    
                    MCycle[4] :
                      begin
                        Write = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01000011,8'b01010011,8'b01100011,8'b01110011
              
              8'b10100000 , 8'b10101000 , 8'b10110000 , 8'b10111000  :
                begin
                  // LDI, LDD, LDIR, LDDR
                  MCycles = 3'b100;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        Set_Addr_To = aXY;
                        IncDec_16 = 4'b1100; // BC
                      end
                    
                    MCycle[1] :
                      begin
                        Set_BusB_To = 4'b0110;
                        Set_BusA_To[2:0] = 3'b111;
                        ALU_Op = 4'b0000;
                        Set_Addr_To = aDE;
                        if (IR[3] == 1'b0 ) 
                          begin
                            IncDec_16 = 4'b0110; // IX
                          end 
                        else 
                          begin
                            IncDec_16 = 4'b1110;
                          end
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        I_BT = 1'b1;
                        TStates = 3'b101;
                        Write = 1'b1;
                        if (IR[3] == 1'b0 ) 
                          begin
                            IncDec_16 = 4'b0101; // DE
                          end 
                        else 
                          begin
                            IncDec_16 = 4'b1101;
                          end
                      end // case: 3
                    
                    MCycle[3] :
                      begin
                        NoRead = 1'b1;
                        TStates = 3'b101;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b10100000 , 8'b10101000 , 8'b10110000 , 8'b10111000
              
              8'b10100001 , 8'b10101001 , 8'b10110001 , 8'b10111001  :
                begin
                  // CPI, CPD, CPIR, CPDR
                  MCycles = 3'b100;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        Set_Addr_To = aXY;
                        IncDec_16 = 4'b1100; // BC
                      end
                    
                    MCycle[1] :
                      begin
                        Set_BusB_To = 4'b0110;
                        Set_BusA_To[2:0] = 3'b111;
                        ALU_Op = 4'b0111;
                        Save_ALU = 1'b1;
                        PreserveC = 1'b1;
                        if (IR[3] == 1'b0 ) 
                          begin
                            IncDec_16 = 4'b0110;
                          end 
                        else 
                          begin
                            IncDec_16 = 4'b1110;
                          end
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        NoRead = 1'b1;
                        I_BC = 1'b1;
                        TStates = 3'b101;
                      end
                    
                    MCycle[3] :
                      begin
                        NoRead = 1'b1;
                        TStates = 3'b101;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b10100001 , 8'b10101001 , 8'b10110001 , 8'b10111001
              
              8'b01000100,8'b01001100,8'b01010100,8'b01011100,8'b01100100,8'b01101100,8'b01110100,8'b01111100  :
                begin
                  // NEG
                  ALU_Op = 4'b0010;
                  Set_BusB_To = 4'b0111;
                  Set_BusA_To = 4'b1010;
                  Read_To_Acc = 1'b1;
                  Save_ALU = 1'b1;
                end
              
              8'b01000110,8'b01001110,8'b01100110,8'b01101110  :
                begin
                  // IM 0
                  IMode = 2'b00;
                end
              
              8'b01010110,8'b01110110  :
                // IM 1
                IMode = 2'b01;
              
              8'b01011110,8'b01110111  :
                // IM 2
                IMode = 2'b10;
              
              // 16 bit arithmetic
              8'b01001010,8'b01011010,8'b01101010,8'b01111010  :
                begin
                  // ADC HL,ss
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        NoRead = 1'b1;
                        ALU_Op = 4'b0001;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_BusA_To[2:0] = 3'b101;
                        case (IR[5:4])
                          0,1,2  :
                            begin
                              Set_BusB_To[2:1] = IR[5:4];
                              Set_BusB_To[0] = 1'b1;
                            end
                          default :
                            Set_BusB_To = 4'b1000;
                        endcase
                        TStates = 3'b100;
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        NoRead = 1'b1;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        ALU_Op = 4'b0001;
                        Set_BusA_To[2:0] = 3'b100;
                        case (IR[5:4])
                          0,1,2  :
                            begin
                              Set_BusB_To[2:1] = IR[5:4];
                              Set_BusB_To[0] = 1'b0;
                            end
                          default :
                            Set_BusB_To = 4'b1001;
                        endcase // case(IR[5:4])
                      end // case: 3
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01001010,8'b01011010,8'b01101010,8'b01111010
              
              8'b01000010,8'b01010010,8'b01100010,8'b01110010  :
                begin
                  // SBC HL,ss
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        NoRead = 1'b1;
                        ALU_Op = 4'b0011;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_BusA_To[2:0] = 3'b101;
                        case (IR[5:4])
                          0,1,2  :
                            begin
                              Set_BusB_To[2:1] = IR[5:4];
                              Set_BusB_To[0] = 1'b1;
                            end
                          default :
                            Set_BusB_To = 4'b1000;
                        endcase
                        TStates = 3'b100;
                      end // case: 2
                    
                    MCycle[2] :
                      begin
                        NoRead = 1'b1;
                        ALU_Op = 4'b0011;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        Set_BusA_To[2:0] = 3'b100;
                        case (IR[5:4])
                          0,1,2  :
                            Set_BusB_To[2:1] = IR[5:4];
                          default :
                            Set_BusB_To = 4'b1001;
                        endcase
                      end // case: 3
                    
                    default :;
                    
                  endcase // case(MCycle)
                end // case: 8'b01000010,8'b01010010,8'b01100010,8'b01110010
              
              8'b01101111  :
                begin
                  // RLD
                  MCycles = 3'b100;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      begin
                        NoRead = 1'b1;
                        Set_Addr_To = aXY;
                      end
                    
                    MCycle[2] :
                      begin
                        Read_To_Reg = 1'b1;
                        Set_BusB_To[2:0] = 3'b110;
                        Set_BusA_To[2:0] = 3'b111;
                        ALU_Op = 4'b1101;
                        TStates = 3'b100;
                        Set_Addr_To = aXY;
                        Save_ALU = 1'b1;
                      end
                    
                    MCycle[3] :
                      begin
                        I_RLD = 1'b1;
                        Write = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01101111
              
              8'b01100111  :
                begin
                  // RRD
                  MCycles = 3'b100;
                  case (1'b1) // MCycle
                    MCycle[1] :
                      Set_Addr_To = aXY;
                    MCycle[2] :
                      begin
                        Read_To_Reg = 1'b1;
                        Set_BusB_To[2:0] = 3'b110;
                        Set_BusA_To[2:0] = 3'b111;
                        ALU_Op = 4'b1110;
                        TStates = 3'b100;
                        Set_Addr_To = aXY;
                        Save_ALU = 1'b1;
                      end
                    
                    MCycle[3] :
                      begin
                        I_RRD = 1'b1;
                        Write = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01100111
              
              8'b01000101,8'b01001101,8'b01010101,8'b01011101,8'b01100101,8'b01101101,8'b01110101,8'b01111101  :
                begin
                  // RETI, RETN
                  MCycles = 3'b011;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      Set_Addr_To = aSP;
                    
                    MCycle[1] :
                      begin
                        IncDec_16 = 4'b0111;
                        Set_Addr_To = aSP;
                        LDZ = 1'b1;
                      end
                    
                    MCycle[2] :
                      begin
                        Jump = 1'b1;
                        IncDec_16 = 4'b0111;
                        I_RETN = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01000101,8'b01001101,8'b01010101,8'b01011101,8'b01100101,8'b01101101,8'b01110101,8'b01111101
              
              8'b01000000,8'b01001000,8'b01010000,8'b01011000,8'b01100000,8'b01101000,8'b01110000,8'b01111000  :
                begin
                  // IN r,(C)
                  MCycles = 3'b010;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      Set_Addr_To = aBC;
                    
                    MCycle[1] :
                      begin
                        IORQ = 1'b1;
                        if (IR[5:3] != 3'b110 ) 
                          begin
                            Read_To_Reg = 1'b1;
                            Set_BusA_To[2:0] = IR[5:3];
                          end
                        I_INRC = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01000000,8'b01001000,8'b01010000,8'b01011000,8'b01100000,8'b01101000,8'b01110000,8'b01111000
              
              8'b01000001,8'b01001001,8'b01010001,8'b01011001,8'b01100001,8'b01101001,8'b01110001,8'b01111001  :
                begin
                  // OUT (C),r
                  // OUT (C),0
                  MCycles = 3'b010;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        Set_Addr_To = aBC;
                        Set_BusB_To[2:0]        = IR[5:3];
                        if (IR[5:3] == 3'b110 ) 
                          begin
                            Set_BusB_To[3] = 1'b1;
                          end
                      end
                    
                    MCycle[1] :
                      begin
                        Write = 1'b1;
                        IORQ = 1'b1;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b01000001,8'b01001001,8'b01010001,8'b01011001,8'b01100001,8'b01101001,8'b01110001,8'b01111001
              
              8'b10100010 , 8'b10101010 , 8'b10110010 , 8'b10111010  :
                begin
                  // INI, IND, INIR, INDR
                  MCycles = 3'b100;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        Set_Addr_To = aBC;
                        Set_BusB_To = 4'b1010;
                        Set_BusA_To = 4'b0000;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        ALU_Op = 4'b0010;
                      end
                    
                    MCycle[1] :
                      begin
                        IORQ = 1'b1;
                        Set_BusB_To = 4'b0110;
                        Set_Addr_To = aXY;
                      end
                    
                    MCycle[2] :
                      begin
                        if (IR[3] == 1'b0 ) 
                          begin
			    IncDec_16 = 4'b0110;
                          end 
                        else 
                          begin
			    IncDec_16 = 4'b1110;
                          end
                        TStates = 3'b100;
                        Write = 1'b1;
                        I_BTR = 1'b1;
                      end // case: 3
                    
                    MCycle[3] :
                      begin
                        NoRead = 1'b1;
                        TStates = 3'b101;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b10100010 , 8'b10101010 , 8'b10110010 , 8'b10111010
              
              8'b10100011 , 8'b10101011 , 8'b10110011 , 8'b10111011  :
                begin
                  // OUTI, OUTD, OTIR, OTDR
                  MCycles = 3'b100;
                  case (1'b1) // MCycle
                    MCycle[0] :
                      begin
                        TStates = 3'b101;
                        Set_Addr_To = aXY;
                        Set_BusB_To = 4'b1010;
                        Set_BusA_To = 4'b0000;
                        Read_To_Reg = 1'b1;
                        Save_ALU = 1'b1;
                        ALU_Op = 4'b0010;
                      end
                    
                    MCycle[1] :
                      begin
                        Set_BusB_To = 4'b0110;
                        Set_Addr_To = aBC;
                        if (IR[3] == 1'b0 ) 
                          begin
                            IncDec_16 = 4'b0110;
                          end 
                        else 
                          begin
                            IncDec_16 = 4'b1110;
                          end
                      end
                    
                    MCycle[2] :
                      begin
                        if (IR[3] == 1'b0 ) 
                          begin
                            IncDec_16 = 4'b0010;
                          end 
                        else 
                          begin
                            IncDec_16 = 4'b1010;
                          end
                        IORQ = 1'b1;
                        Write = 1'b1;
                        I_BTR = 1'b1;
                      end // case: 3
                    
                    MCycle[3] :
                      begin
                        NoRead = 1'b1;
                        TStates = 3'b101;
                      end
                    
                    default :;
                  endcase // case(MCycle)
                end // case: 8'b10100011 , 8'b10101011 , 8'b10110011 , 8'b10111011

              default : ;
              
            endcase // case(IR)                  
          end // block: default_ed_block        
      endcase // case(ISet)
      
      if (Mode == 1 ) 
        begin
          if (MCycle[0] ) 
            begin
              //TStates = 3'b100;
            end 
          else 
            begin
              TStates = 3'b011;
            end
        end

      if (Mode == 3 ) 
        begin
          if (MCycle[0] ) 
            begin
              //TStates = 3'b100;
            end 
          else 
            begin
              TStates = 3'b100;
            end
        end

      if (Mode < 2 ) 
        begin
          if (MCycle[5] ) 
            begin
              Inc_PC = 1'b1;
              if (Mode == 1 ) 
                begin
                  Set_Addr_To = aXY;
                  TStates = 3'b100;
                  Set_BusB_To[2:0] = SSS;
                  Set_BusB_To[3] = 1'b0;
                end
              if (IR == 8'b00110110 || IR == 8'b11001011 ) 
                begin
                  Set_Addr_To = aNone;
                end
            end
          if (MCycle[6] ) 
            begin
              if (Mode == 0 ) 
                begin
                  TStates = 3'b101;
                end
              if (ISet != 2'b01 ) 
                begin
                  Set_Addr_To = aXY;
                end
              Set_BusB_To[2:0] = SSS;
              Set_BusB_To[3] = 1'b0;
              if (IR == 8'b00110110 || ISet == 2'b01 ) 
                begin
                  // LD (HL),n
                  Inc_PC = 1'b1;
                end 
              else 
                begin
                  NoRead = 1'b1;
                end
            end
        end // if (Mode < 2 )      
      
    end // always @ (IR, ISet, MCycle, F, NMICycle, IntCycle)
endmodule // T80_MCode