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//----------------------------------------------------------------------------

// Copyright (C) 2009 , Olivier Girard

//

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

// modification, are permitted provided that the following conditions

// are met:

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

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

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

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

//       documentation and/or other materials provided with the distribution.

//     * Neither the name of the authors nor the names of its contributors

//       may be used to endorse or promote products derived from this software

//       without specific prior written permission.

//

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE

// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,

// OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF

// THE POSSIBILITY OF SUCH DAMAGE

//

//----------------------------------------------------------------------------

//

// *File Name: omsp_frontend.v

// 

// *Module Description:

//                       openMSP430 Instruction fetch and decode unit

//

// *Author(s):

//              - Olivier Girard,    olgirard@gmail.com

//

//----------------------------------------------------------------------------

// $Rev: 117 $

// $LastChangedBy: olivier.girard $

// $LastChangedDate: 2011-06-23 21:30:51 +0200 (Thu, 23 Jun 2011) $

//----------------------------------------------------------------------------

`ifdef OMSP_NO_INCLUDE

`else

`include "openMSP430_defines.v"

`endif

module  omsp_frontend (

// OUTPUTs

    dbg_halt_st,                   // Halt/Run status from CPU

    decode_noirq,                  // Frontend decode instruction

    e_state,                       // Execution state

    exec_done,                     // Execution completed

    inst_ad,                       // Decoded Inst: destination addressing mode

    inst_as,                       // Decoded Inst: source addressing mode

    inst_alu,                      // ALU control signals

    inst_bw,                       // Decoded Inst: byte width

    inst_dest,                     // Decoded Inst: destination (one hot)

    inst_dext,                     // Decoded Inst: destination extended instruction word

    inst_irq_rst,                  // Decoded Inst: Reset interrupt

    inst_jmp,                      // Decoded Inst: Conditional jump

    inst_mov,                      // Decoded Inst: mov instruction

    inst_sext,                     // Decoded Inst: source extended instruction word

    inst_so,                       // Decoded Inst: Single-operand arithmetic

    inst_src,                      // Decoded Inst: source (one hot)

    inst_type,                     // Decoded Instruction type

    irq_acc,                       // Interrupt request accepted (one-hot signal)

    mab,                           // Frontend Memory address bus

    mb_en,                         // Frontend Memory bus enable

    nmi_acc,                       // Non-Maskable interrupt request accepted

    pc,                            // Program counter

    pc_nxt,                        // Next PC value (for CALL & IRQ)

// INPUTs

    cpu_en_s,                      // Enable CPU code execution (synchronous)

    cpuoff,                        // Turns off the CPU

    dbg_halt_cmd,                  // Halt CPU command

    dbg_reg_sel,                   // Debug selected register for rd/wr access

    fe_pmem_wait,                  // Frontend wait for Instruction fetch

    gie,                           // General interrupt enable

    irq,                           // Maskable interrupts

    mclk,                          // Main system clock

    mdb_in,                        // Frontend Memory data bus input

    nmi_evt,                       // Non-maskable interrupt event

    pc_sw,                         // Program counter software value

    pc_sw_wr,                      // Program counter software write

    puc_rst,                       // Main system reset

    wdt_irq                        // Watchdog-timer interrupt

);

// OUTPUTs

//=========

output              dbg_halt_st;   // Halt/Run status from CPU

output              decode_noirq;  // Frontend decode instruction

output        [3:0] e_state;       // Execution state

output              exec_done;     // Execution completed

output        [7:0] inst_ad;       // Decoded Inst: destination addressing mode

output        [7:0] inst_as;       // Decoded Inst: source addressing mode

output       [11:0] inst_alu;      // ALU control signals

output              inst_bw;       // Decoded Inst: byte width

output       [15:0] inst_dest;     // Decoded Inst: destination (one hot)

output       [15:0] inst_dext;     // Decoded Inst: destination extended instruction word

output              inst_irq_rst;  // Decoded Inst: Reset interrupt

output        [7:0] inst_jmp;      // Decoded Inst: Conditional jump

output              inst_mov;      // Decoded Inst: mov instruction

output       [15:0] inst_sext;     // Decoded Inst: source extended instruction word

output        [7:0] inst_so;       // Decoded Inst: Single-operand arithmetic

output       [15:0] inst_src;      // Decoded Inst: source (one hot)

output        [2:0] inst_type;     // Decoded Instruction type

output       [13:0] irq_acc;       // Interrupt request accepted (one-hot signal)

output       [15:0] mab;           // Frontend Memory address bus

output              mb_en;         // Frontend Memory bus enable

output              nmi_acc;       // Non-Maskable interrupt request accepted

output       [15:0] pc;            // Program counter

output       [15:0] pc_nxt;        // Next PC value (for CALL & IRQ)

// INPUTs

//=========

input               cpu_en_s;      // Enable CPU code execution (synchronous)

input               cpuoff;        // Turns off the CPU

input               dbg_halt_cmd;  // Halt CPU command

input         [3:0] dbg_reg_sel;   // Debug selected register for rd/wr access

input               fe_pmem_wait;  // Frontend wait for Instruction fetch

input               gie;           // General interrupt enable

input        [13:0] irq;           // Maskable interrupts

input               mclk;          // Main system clock

input        [15:0] mdb_in;        // Frontend Memory data bus input

input               nmi_evt;       // Non-maskable interrupt event

input        [15:0] pc_sw;         // Program counter software value

input               pc_sw_wr;      // Program counter software write

input               puc_rst;       // Main system reset

input               wdt_irq;       // Watchdog-timer interrupt

//=============================================================================

// 1)  UTILITY FUNCTIONS

//=============================================================================

// 16 bits one-hot decoder

function [15:0] one_hot16;

   input  [3:0] binary;

   begin

      one_hot16         = 16'h0000;

      one_hot16[binary] =  1'b1;

   end

endfunction

// 8 bits one-hot decoder

function [7:0] one_hot8;

   input  [2:0] binary;

   begin

      one_hot8         = 8'h00;

      one_hot8[binary] = 1'b1;

   end

endfunction

//=============================================================================

// 2)  Parameter definitions

//=============================================================================

//

// 2.1) Instruction State machine definitons

//-------------------------------------------

parameter I_IRQ_FETCH = `I_IRQ_FETCH;

parameter I_IRQ_DONE  = `I_IRQ_DONE;

parameter I_DEC       = `I_DEC;        // New instruction ready for decode

parameter I_EXT1      = `I_EXT1;       // 1st Extension word

parameter I_EXT2      = `I_EXT2;       // 2nd Extension word

parameter I_IDLE      = `I_IDLE;       // CPU is in IDLE mode

//

// 2.2) Execution State machine definitons

//-------------------------------------------

parameter E_IRQ_0     = `E_IRQ_0;

parameter E_IRQ_1     = `E_IRQ_1;

parameter E_IRQ_2     = `E_IRQ_2;

parameter E_IRQ_3     = `E_IRQ_3;

parameter E_IRQ_4     = `E_IRQ_4;

parameter E_SRC_AD    = `E_SRC_AD;

parameter E_SRC_RD    = `E_SRC_RD;

parameter E_SRC_WR    = `E_SRC_WR;

parameter E_DST_AD    = `E_DST_AD;

parameter E_DST_RD    = `E_DST_RD;

parameter E_DST_WR    = `E_DST_WR;

parameter E_EXEC      = `E_EXEC;

parameter E_JUMP      = `E_JUMP;

parameter E_IDLE      = `E_IDLE;

//=============================================================================

// 3)  FRONTEND STATE MACHINE

//=============================================================================

// The wire "conv" is used as state bits to calculate the next response

reg  [2:0] i_state;

reg  [2:0] i_state_nxt;

reg  [1:0] inst_sz;

wire [1:0] inst_sz_nxt;

wire       irq_detect;

wire [2:0] inst_type_nxt;

wire       is_const;

reg [15:0] sconst_nxt;

reg  [3:0] e_state_nxt;

// CPU on/off through the debug interface or cpu_en port

wire   cpu_halt_cmd = dbg_halt_cmd | ~cpu_en_s;

// States Transitions

always @(i_state    or inst_sz  or inst_sz_nxt  or pc_sw_wr or exec_done or

         irq_detect or cpuoff   or cpu_halt_cmd or e_state)

    case(i_state)

      I_IDLE     : i_state_nxt = (irq_detect & ~cpu_halt_cmd) ? I_IRQ_FETCH :

                                 (~cpuoff    & ~cpu_halt_cmd) ? I_DEC       : I_IDLE;

      I_IRQ_FETCH: i_state_nxt =  I_IRQ_DONE;

      I_IRQ_DONE : i_state_nxt =  I_DEC;

      I_DEC      : i_state_nxt =  irq_detect                  ? I_IRQ_FETCH :

                          (cpuoff | cpu_halt_cmd) & exec_done ? I_IDLE      :

                            cpu_halt_cmd & (e_state==E_IDLE)  ? I_IDLE      :

                                  pc_sw_wr                    ? I_DEC       :

                             ~exec_done & ~(e_state==E_IDLE)  ? I_DEC       :        // Wait in decode state

                                  (inst_sz_nxt!=2'b00)        ? I_EXT1      : I_DEC; // until execution is completed

      I_EXT1     : i_state_nxt =  irq_detect                  ? I_IRQ_FETCH :

                                  pc_sw_wr                    ? I_DEC       :

                                  (inst_sz!=2'b01)            ? I_EXT2      : I_DEC;

      I_EXT2     : i_state_nxt =  irq_detect                  ? I_IRQ_FETCH : I_DEC;

      default    : i_state_nxt =  I_IRQ_FETCH;

    endcase

// State machine

always @(posedge mclk or posedge puc_rst)

  if (puc_rst) i_state  <= I_IRQ_FETCH;

  else         i_state  <= i_state_nxt;

// Utility signals

wire   decode_noirq =  ((i_state==I_DEC) &  (exec_done | (e_state==E_IDLE)));

wire   decode       =  decode_noirq | irq_detect;

wire   fetch        = ~((i_state==I_DEC) & ~(exec_done | (e_state==E_IDLE))) & ~(e_state_nxt==E_IDLE);

// Debug interface cpu status

reg    dbg_halt_st;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)  dbg_halt_st <= 1'b0;

  else          dbg_halt_st <= cpu_halt_cmd & (i_state_nxt==I_IDLE);

//=============================================================================

// 4)  INTERRUPT HANDLING

//=============================================================================

// Detect nmi interrupt

reg         inst_nmi;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)                  inst_nmi <= 1'b0;

  else if (nmi_evt)             inst_nmi <= 1'b1;

  else if (i_state==I_IRQ_DONE) inst_nmi <= 1'b0;

// Detect reset interrupt

reg         inst_irq_rst;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)                  inst_irq_rst <= 1'b1;

  else if (exec_done)           inst_irq_rst <= 1'b0;

//  Detect other interrupts

assign  irq_detect = (inst_nmi | ((|irq | wdt_irq) & gie)) & ~cpu_halt_cmd & ~dbg_halt_st & (exec_done | (i_state==I_IDLE));

// Select interrupt vector

reg  [3:0] irq_num;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)         irq_num <= 4'hf;

  else if (irq_detect) irq_num <= inst_nmi           ?  4'he :

                                  irq[13]            ?  4'hd :

                                  irq[12]            ?  4'hc :

                                  irq[11]            ?  4'hb :

                                 (irq[10] | wdt_irq) ?  4'ha :

                                  irq[9]             ?  4'h9 :

                                  irq[8]             ?  4'h8 :

                                  irq[7]             ?  4'h7 :

                                  irq[6]             ?  4'h6 :

                                  irq[5]             ?  4'h5 :

                                  irq[4]             ?  4'h4 :

                                  irq[3]             ?  4'h3 :

                                  irq[2]             ?  4'h2 :

                                  irq[1]             ?  4'h1 :

                                  irq[0]             ?  4'h0 : 4'hf;

wire [15:0] irq_addr    = {11'h7ff, irq_num, 1'b0};

// Interrupt request accepted

wire [15:0] irq_acc_all = one_hot16(irq_num) & {16{(i_state==I_IRQ_FETCH)}};

wire [13:0] irq_acc     = irq_acc_all[13:0];

wire        nmi_acc     = irq_acc_all[14];

//=============================================================================

// 5)  FETCH INSTRUCTION

//=============================================================================

//

// 5.1) PROGRAM COUNTER & MEMORY INTERFACE

//-----------------------------------------

// Program counter

reg  [15:0] pc;

// Compute next PC value

wire [15:0] pc_incr = pc + {14'h0000, fetch, 1'b0};

wire [15:0] pc_nxt  = pc_sw_wr               ? pc_sw    :

                      (i_state==I_IRQ_FETCH) ? irq_addr :

                      (i_state==I_IRQ_DONE)  ? mdb_in   :  pc_incr;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)  pc <= 16'h0000;

  else          pc <= pc_nxt;

// Check if ROM has been busy in order to retry ROM access

reg pmem_busy;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)  pmem_busy <= 1'b0;

  else          pmem_busy <= fe_pmem_wait;

// Memory interface

wire [15:0] mab      = pc_nxt;

wire        mb_en    = fetch | pc_sw_wr | (i_state==I_IRQ_FETCH) | pmem_busy | (dbg_halt_st & ~cpu_halt_cmd);

//

// 5.2) INSTRUCTION REGISTER

//--------------------------------

// Instruction register

wire [15:0] ir  = mdb_in;

// Detect if source extension word is required

wire is_sext = (inst_as[`IDX] | inst_as[`SYMB] | inst_as[`ABS] | inst_as[`IMM]);

// Detect if destination extension word is required

wire is_dext = (inst_ad[`IDX] | inst_ad[`SYMB] | inst_ad[`ABS]);

// For the Symbolic addressing mode, add -2 to the extension word in order

// to make up for the PC address

wire [15:0] ext_incr = ((i_state==I_EXT1)     &  inst_as[`SYMB]) |

                       ((i_state==I_EXT2)     &  inst_ad[`SYMB]) |

                       ((i_state==I_EXT1)     & ~inst_as[`SYMB] &

                       ~(i_state_nxt==I_EXT2) &  inst_ad[`SYMB])   ? 16'hfffe : 16'h0000;

wire [15:0] ext_nxt  = ir + ext_incr;

// Store source extension word

reg [15:0] inst_sext;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)                                 inst_sext <= 16'h0000;

  else if (decode & is_const)                  inst_sext <= sconst_nxt;

  else if (decode & inst_type_nxt[`INST_JMP])  inst_sext <= {{5{ir[9]}},ir[9:0],1'b0};

  else if ((i_state==I_EXT1) & is_sext)        inst_sext <= ext_nxt;

// Source extension word is ready

wire inst_sext_rdy = (i_state==I_EXT1) & is_sext;

// Store destination extension word

reg [15:0] inst_dext;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)                           inst_dext <= 16'h0000;

  else if ((i_state==I_EXT1) & ~is_sext) inst_dext <= ext_nxt;

  else if  (i_state==I_EXT2)             inst_dext <= ext_nxt;

// Destination extension word is ready

wire inst_dext_rdy = (((i_state==I_EXT1) & ~is_sext) | (i_state==I_EXT2));

//=============================================================================

// 6)  DECODE INSTRUCTION

//=============================================================================

//

// 6.1) OPCODE: INSTRUCTION TYPE

//----------------------------------------

// Instructions type is encoded in a one hot fashion as following:

//

// 3'b001: Single-operand arithmetic

// 3'b010: Conditional jump

// 3'b100: Two-operand arithmetic

reg  [2:0] inst_type;

assign     inst_type_nxt = {(ir[15:14]!=2'b00),

                            (ir[15:13]==3'b001),

                            (ir[15:13]==3'b000)} & {3{~irq_detect}};

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)      inst_type <= 3'b000;

  else if (decode)  inst_type <= inst_type_nxt;

//

// 6.2) OPCODE: SINGLE-OPERAND ARITHMETIC

//----------------------------------------

// Instructions are encoded in a one hot fashion as following:

//

// 8'b00000001: RRC

// 8'b00000010: SWPB

// 8'b00000100: RRA

// 8'b00001000: SXT

// 8'b00010000: PUSH

// 8'b00100000: CALL

// 8'b01000000: RETI

// 8'b10000000: IRQ

reg   [7:0] inst_so;

wire  [7:0] inst_so_nxt = irq_detect ? 8'h80 : (one_hot8(ir[9:7]) & {8{inst_type_nxt[`INST_SO]}});

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)     inst_so <= 8'h00;

  else if (decode) inst_so <= inst_so_nxt;

//

// 6.3) OPCODE: CONDITIONAL JUMP

//--------------------------------

// Instructions are encoded in a one hot fashion as following:

//

// 8'b00000001: JNE/JNZ

// 8'b00000010: JEQ/JZ

// 8'b00000100: JNC/JLO

// 8'b00001000: JC/JHS

// 8'b00010000: JN

// 8'b00100000: JGE

// 8'b01000000: JL

// 8'b10000000: JMP

reg   [2:0] inst_jmp_bin;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)     inst_jmp_bin <= 3'h0;

  else if (decode) inst_jmp_bin <= ir[12:10];

wire [7:0] inst_jmp = one_hot8(inst_jmp_bin) & {8{inst_type[`INST_JMP]}};

//

// 6.4) OPCODE: TWO-OPERAND ARITHMETIC

//-------------------------------------

// Instructions are encoded in a one hot fashion as following:

//

// 12'b000000000001: MOV

// 12'b000000000010: ADD

// 12'b000000000100: ADDC

// 12'b000000001000: SUBC

// 12'b000000010000: SUB

// 12'b000000100000: CMP

// 12'b000001000000: DADD

// 12'b000010000000: BIT

// 12'b000100000000: BIC

// 12'b001000000000: BIS

// 12'b010000000000: XOR

// 12'b100000000000: AND

wire [15:0] inst_to_1hot = one_hot16(ir[15:12]) & {16{inst_type_nxt[`INST_TO]}};

wire [11:0] inst_to_nxt  = inst_to_1hot[15:4];

reg         inst_mov;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)     inst_mov <= 1'b0;

  else if (decode) inst_mov <= inst_to_nxt[`MOV];

//

// 6.5) SOURCE AND DESTINATION REGISTERS

//---------------------------------------

// Destination register

reg [3:0] inst_dest_bin;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)     inst_dest_bin <= 4'h0;

  else if (decode) inst_dest_bin <= ir[3:0];

wire  [15:0] inst_dest = dbg_halt_st          ? one_hot16(dbg_reg_sel) :

                         inst_type[`INST_JMP] ? 16'h0001               :

                         inst_so[`IRQ]  |

                         inst_so[`PUSH] |

                         inst_so[`CALL]       ? 16'h0002               :

                                                one_hot16(inst_dest_bin);

// Source register

reg [3:0] inst_src_bin;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)     inst_src_bin <= 4'h0;

  else if (decode) inst_src_bin <= ir[11:8];

wire  [15:0] inst_src = inst_type[`INST_TO] ? one_hot16(inst_src_bin)  :

                        inst_so[`RETI]      ? 16'h0002                 :

                        inst_so[`IRQ]       ? 16'h0001                 :

                        inst_type[`INST_SO] ? one_hot16(inst_dest_bin) : 16'h0000;

//

// 6.6) SOURCE ADDRESSING MODES

//--------------------------------

// Source addressing modes are encoded in a one hot fashion as following:

//

// 13'b0000000000001: Register direct.

// 13'b0000000000010: Register indexed.

// 13'b0000000000100: Register indirect.

// 13'b0000000001000: Register indirect autoincrement.

// 13'b0000000010000: Symbolic (operand is in memory at address PC+x).

// 13'b0000000100000: Immediate (operand is next word in the instruction stream).

// 13'b0000001000000: Absolute (operand is in memory at address x).

// 13'b0000010000000: Constant 4.

// 13'b0000100000000: Constant 8.

// 13'b0001000000000: Constant 0.

// 13'b0010000000000: Constant 1.

// 13'b0100000000000: Constant 2.

// 13'b1000000000000: Constant -1.

reg [12:0] inst_as_nxt;

wire [3:0] src_reg = inst_type_nxt[`INST_SO] ? ir[3:0] : ir[11:8];

always @(src_reg or ir or inst_type_nxt)

  begin

     if (inst_type_nxt[`INST_JMP])

       inst_as_nxt =  13'b0000000000001;

     else if (src_reg==4'h3) // Addressing mode using R3

       case (ir[5:4])

         2'b11  : inst_as_nxt =  13'b1000000000000;

         2'b10  : inst_as_nxt =  13'b0100000000000;

         2'b01  : inst_as_nxt =  13'b0010000000000;

         default: inst_as_nxt =  13'b0001000000000;

       endcase

     else if (src_reg==4'h2) // Addressing mode using R2

       case (ir[5:4])

         2'b11  : inst_as_nxt =  13'b0000100000000;

         2'b10  : inst_as_nxt =  13'b0000010000000;

         2'b01  : inst_as_nxt =  13'b0000001000000;

         default: inst_as_nxt =  13'b0000000000001;

       endcase

     else if (src_reg==4'h0) // Addressing mode using R0

       case (ir[5:4])

         2'b11  : inst_as_nxt =  13'b0000000100000;

         2'b10  : inst_as_nxt =  13'b0000000000100;

         2'b01  : inst_as_nxt =  13'b0000000010000;

         default: inst_as_nxt =  13'b0000000000001;

       endcase

     else                    // General Addressing mode

       case (ir[5:4])

         2'b11  : inst_as_nxt =  13'b0000000001000;

         2'b10  : inst_as_nxt =  13'b0000000000100;

         2'b01  : inst_as_nxt =  13'b0000000000010;

         default: inst_as_nxt =  13'b0000000000001;

       endcase

  end

assign    is_const = |inst_as_nxt[12:7];

reg [7:0] inst_as;

always @(posedge mclk or posedge puc_rst)

  if (puc_rst)     inst_as <= 8'h00;

  else if (decode) inst_as <= {is_const, inst_as_nxt[6:0]};

// 13'b0000010000000: Constant 4.

// 13'b0000100000000: Constant 8.

// 13'b0001000000000: Constant 0.

// 13'b0010000000000: Constant 1.

// 13'b0100000000000: Constant 2.

// 13'b1000000000000: Constant -1.

always @(inst_as_nxt)

  begin

     if (inst_as_nxt[7])        sconst_nxt = 16'h0004;

     else if (inst_as_nxt[8])   sconst_nxt = 16'h0008;

     else if (inst_as_nxt[9])   sconst_nxt = 16'h0000;

     else if (inst_as_nxt[10])  sconst_nxt = 16'h0001;

     else if (inst_as_nxt[11])  sconst_nxt = 16'h0002;

     else if (inst_as_nxt[12])  sconst_nxt = 16'hffff;

     else                       sconst_nxt = 16'h0000;

  end

//

// 6.7) DESTINATION ADDRESSING MODES

//-----------------------------------

// Destination addressing modes are encoded in a one hot fashion as following:

//

// 8'b00000001: Register direct.

// 8'b00000010: Register indexed.

// 8'b00010000: Symbolic (operand is in memory at address PC+x).

// 8'b01000000: Absolute (operand is in memory at address x).

reg  [7:0] inst_ad_nxt;

wire [3:0] dest_reg = ir[3:0];

always @(dest_reg or ir or inst_type_nxt)

  begin

     if (~inst_type_nxt[`INST_TO])

       inst_ad_nxt =  8'b00000000;

     else if (dest_reg==4'h2)   // Addressing mode using R2

       case (ir[7])

         1'b1   : inst_ad_nxt =  8'b01000000;

         default: inst_ad_nxt =  8'b00000001;