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//////////////////////////////////////////////////////////////////

//                                                              //

//  Decode stage of Amber 2 Core                                //

//                                                              //

//  This file is part of the Amber project                      //

//  http://www.opencores.org/project,amber                      //

//                                                              //

//  Description                                                 //

//  This module is the most complex part of the Amber core      //

//  It decodes and sequences all instructions and handles all   //

//  interrupts                                                  //

//                                                              //

//  Author(s):                                                  //

//      - Conor Santifort, csantifort.amber@gmail.com           //

//                                                              //

//////////////////////////////////////////////////////////////////

//                                                              //

// Copyright (C) 2010 Authors and OPENCORES.ORG                 //

//                                                              //

// This source file may be used and distributed without         //

// restriction provided that this copyright statement is not    //

// removed from the file and that any derivative work contains  //

// the original copyright notice and the associated disclaimer. //

//                                                              //

// This source file is free software; you can redistribute it   //

// and/or modify it under the terms of the GNU Lesser General   //

// Public License as published by the Free Software Foundation; //

// either version 2.1 of the License, or (at your option) any   //

// later version.                                               //

//                                                              //

// This source is distributed in the hope that it will be       //

// useful, but WITHOUT ANY WARRANTY; without even the implied   //

// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR      //

// PURPOSE.  See the GNU Lesser General Public License for more //

// details.                                                     //

//                                                              //

// You should have received a copy of the GNU Lesser General    //

// Public License along with this source; if not, download it   //

// from http://www.opencores.org/lgpl.shtml                     //

//                                                              //

//////////////////////////////////////////////////////////////////

`include "global_defines.vh"

module a23_decode

(

input                       i_clk,

input       [31:0]          i_read_data,

input                       i_fetch_stall,                  // stall all stages of the cpu at the same time

input                       i_irq,                          // interrupt request

input                       i_firq,                         // Fast interrupt request

input                       i_dabt,                         // data abort interrupt request

input                       i_iabt,                         // instruction pre-fetch abort flag

input                       i_adex,                         // Address Exception

input       [31:0]          i_execute_address,              // Registered address output by execute stage

                                                            // 2 LSBs of read address used for calculating

                                                            // shift in LDRB ops

input       [7:0]           i_abt_status,                   // Abort status

input       [31:0]          i_execute_status_bits,          // current status bits values in execute stage

input                       i_multiply_done,                // multiply unit is nearly done

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

// Control signals to execute stage

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

output reg  [31:0]          o_read_data = 1'd0,

output reg  [4:0]           o_read_data_alignment = 1'd0,  // 2 LSBs of read address used for calculating shift in LDRB ops

output reg  [31:0]          o_imm32 = 'd0,

output reg  [4:0]           o_imm_shift_amount = 'd0,

output reg                  o_shift_imm_zero = 'd0,

output wire [3:0]           o_condition,

output reg                  o_exclusive_exec = 'd0,         // exclusive access request ( swap instruction )

output reg                  o_data_access_exec = 'd0,       // high means the memory access is a read

                                                            // read or write, low for instruction

output wire [1:0]           o_status_bits_mode,

output wire                 o_status_bits_irq_mask,

output wire                 o_status_bits_firq_mask,

output reg  [3:0]           o_rm_sel = 'd0,

output reg  [3:0]           o_rds_sel = 'd0,

output reg  [3:0]           o_rn_sel = 'd0,

output      [3:0]           o_rm_sel_nxt,

output      [3:0]           o_rds_sel_nxt,

output      [3:0]           o_rn_sel_nxt,

output reg  [1:0]           o_barrel_shift_amount_sel = 'd0,

output reg  [1:0]           o_barrel_shift_data_sel = 'd0,

output reg  [1:0]           o_barrel_shift_function = 'd0,

output reg  [8:0]           o_alu_function = 'd0,

output reg                  o_use_carry_in = 'd0,

output reg  [1:0]           o_multiply_function = 'd0,

output reg  [2:0]           o_interrupt_vector_sel = 'd0,

output wire [3:0]           o_address_sel,

output wire [1:0]           o_pc_sel,

output reg  [1:0]           o_byte_enable_sel = 'd0,        // byte, halfword or word write

output reg  [2:0]           o_status_bits_sel = 'd0,

output reg  [2:0]           o_reg_write_sel,

output reg                  o_user_mode_regs_load,

output reg                  o_user_mode_regs_store_nxt,

output reg                  o_firq_not_user_mode,

output reg                  o_write_data_wen = 'd0,

output reg                  o_base_address_wen = 'd0,       // save LDM base address register

                                                            // in case of data abort

output wire                 o_pc_wen,

output reg  [14:0]          o_reg_bank_wen = 'd0,

output reg  [3:0]           o_reg_bank_wsel = 'd0,

output reg                  o_status_bits_flags_wen = 'd0,

output reg                  o_status_bits_mode_wen = 'd0,

output reg                  o_status_bits_irq_mask_wen = 'd0,

output reg                  o_status_bits_firq_mask_wen = 'd0,

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

// Co-Processor interface

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

output reg  [2:0]           o_copro_opcode1 = 'd0,

output reg  [2:0]           o_copro_opcode2 = 'd0,

output reg  [3:0]           o_copro_crn = 'd0,

output reg  [3:0]           o_copro_crm = 'd0,

output reg  [3:0]           o_copro_num = 'd0,

output reg  [1:0]           o_copro_operation = 'd0, // 0 = no operation,

                                                     // 1 = Move to Amber Core Register from Coprocessor

                                                     // 2 = Move to Coprocessor from Amber Core Register

output reg                  o_copro_write_data_wen = 'd0,

output                      o_iabt_trigger,

output      [31:0]          o_iabt_address,

output      [7:0]           o_iabt_status,

output                      o_dabt_trigger,

output      [31:0]          o_dabt_address,

output      [7:0]           o_dabt_status

);

`include "a23_localparams.vh"

`include "a23_functions.vh"

localparam [4:0] RST_WAIT1      = 5'd0,

                 RST_WAIT2      = 5'd1,

                 INT_WAIT1      = 5'd2,

                 INT_WAIT2      = 5'd3,

                 EXECUTE        = 5'd4,

                 PRE_FETCH_EXEC = 5'd5,  // Execute the Pre-Fetched Instruction

                 MEM_WAIT1      = 5'd6,  // conditionally decode current instruction, in case

                                         // previous instruction does not execute in S2

                 MEM_WAIT2      = 5'd7,

                 PC_STALL1      = 5'd8,  // Program Counter altered

                                         // conditionally decude current instruction, in case

                                         // previous instruction does not execute in S2

                 PC_STALL2      = 5'd9,

                 MTRANS_EXEC1   = 5'd10,

                 MTRANS_EXEC2   = 5'd11,

                 MTRANS_EXEC3   = 5'd12,

                 MTRANS_EXEC3B  = 5'd13,

                 MTRANS_EXEC4   = 5'd14,

                 MTRANS5_ABORT  = 5'd15,

                 MULT_PROC1     = 5'd16,  // first cycle, save pre fetch instruction

                 MULT_PROC2     = 5'd17,  // do multiplication

                 MULT_STORE     = 5'd19,  // save RdLo

                 MULT_ACCUMU    = 5'd20,  // Accumulate add lower 32 bits

                 SWAP_WRITE     = 5'd22,

                 SWAP_WAIT1     = 5'd23,

                 SWAP_WAIT2     = 5'd24,

                 COPRO_WAIT     = 5'd25;

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

// Internal signals

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

wire    [31:0]         instruction;

wire                   instruction_iabt;        // abort flag, follows the instruction

wire                   instruction_adex;        // address exception flag, follows the instruction

wire    [31:0]         instruction_address;     // instruction virtual address, follows

                                                // the instruction

wire    [7:0]          instruction_iabt_status; // abort status, follows the instruction

wire    [1:0]          instruction_sel;

reg     [3:0]          itype;

wire    [3:0]          opcode;

wire    [7:0]          imm8;

wire    [31:0]         offset12;

wire    [31:0]         offset24;

wire    [4:0]          shift_imm;

wire                   opcode_compare;

wire                   mem_op;

wire                   load_op;

wire                   store_op;

wire                   write_pc;

wire                   immediate_shifter_operand;

wire                   rds_use_rs;

wire                   branch;

wire                   mem_op_pre_indexed;

wire                   mem_op_post_indexed;

// Flop inputs

wire    [31:0]         imm32_nxt;

wire    [4:0]          imm_shift_amount_nxt;

wire                   shift_imm_zero_nxt;

wire    [3:0]          condition_nxt;

reg                    exclusive_exec_nxt;

reg                    data_access_exec_nxt;

wire                   shift_extend;

reg     [1:0]          barrel_shift_function_nxt;

wire    [8:0]          alu_function_nxt;

reg                    use_carry_in_nxt;

reg     [1:0]          multiply_function_nxt;

reg     [1:0]          status_bits_mode_nxt;

reg                    status_bits_irq_mask_nxt;

reg                    status_bits_firq_mask_nxt;

reg     [1:0]          barrel_shift_amount_sel_nxt;

reg     [1:0]          barrel_shift_data_sel_nxt;

reg     [3:0]          address_sel_nxt;

reg     [1:0]          pc_sel_nxt;

reg     [1:0]          byte_enable_sel_nxt;

reg     [2:0]          status_bits_sel_nxt;

reg     [2:0]          reg_write_sel_nxt;

reg                    user_mode_regs_load_nxt;

wire                   firq_not_user_mode_nxt;

// ALU Function signals

reg                    alu_swap_sel_nxt;

reg                    alu_not_sel_nxt;

reg     [1:0]          alu_cin_sel_nxt;

reg                    alu_cout_sel_nxt;

reg     [3:0]          alu_out_sel_nxt;

reg                    write_data_wen_nxt;

reg                    copro_write_data_wen_nxt;

reg                    base_address_wen_nxt;

reg                    pc_wen_nxt;

reg     [3:0]          reg_bank_wsel_nxt;

reg                    status_bits_flags_wen_nxt;

reg                    status_bits_mode_wen_nxt;

reg                    status_bits_irq_mask_wen_nxt;

reg                    status_bits_firq_mask_wen_nxt;

reg                    saved_current_instruction_wen;   // saved load instruction

reg                    pre_fetch_instruction_wen;       // pre-fetch instruction

reg     [4:0]          control_state = RST_WAIT1;

reg     [4:0]          control_state_nxt;

wire                   dabt;

reg                    dabt_reg = 'd0;

reg                    dabt_reg_d1;

reg                    iabt_reg = 'd0;

reg                    adex_reg = 'd0;

reg     [31:0]         abt_address_reg = 'd0;

reg     [7:0]          abt_status_reg = 'd0;

reg     [31:0]         saved_current_instruction = 'd0;

reg                    saved_current_instruction_iabt = 'd0;          // access abort flag

reg                    saved_current_instruction_adex = 'd0;          // address exception

reg     [31:0]         saved_current_instruction_address = 'd0;       // virtual address of abort instruction

reg     [7:0]          saved_current_instruction_iabt_status = 'd0;   // status of abort instruction

reg     [31:0]         pre_fetch_instruction = 'd0;

reg                    pre_fetch_instruction_iabt = 'd0;              // access abort flag

reg                    pre_fetch_instruction_adex = 'd0;              // address exception

reg     [31:0]         pre_fetch_instruction_address = 'd0;           // virtual address of abort instruction

reg     [7:0]          pre_fetch_instruction_iabt_status = 'd0;       // status of abort instruction

wire                   instruction_valid;

wire                   instruction_execute;

reg     [3:0]          mtrans_reg;              // the current register being accessed as part of STM/LDM

reg     [3:0]          mtrans_reg_d1 = 'd0;     // delayed by 1 period

reg     [3:0]          mtrans_reg_d2 = 'd0;     // delayed by 2 periods

reg     [31:0]         mtrans_instruction_nxt;

wire   [31:0]          mtrans_base_reg_change;

wire   [4:0]           mtrans_num_registers;

wire                   use_saved_current_instruction;

wire                   use_pre_fetch_instruction;

wire                   interrupt;

wire   [1:0]           interrupt_mode;

wire   [2:0]           next_interrupt;

reg                    irq = 'd0;

reg                    firq = 'd0;

wire                   firq_request;

wire                   irq_request;

wire                   swi_request;

wire                   und_request;

wire                   dabt_request;

reg    [1:0]           copro_operation_nxt;

reg                    mtrans_r15 = 'd0;

reg                    mtrans_r15_nxt;

reg                    restore_base_address = 'd0;

reg                    restore_base_address_nxt;

wire                   regop_set_flags;

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

// registers for output ports with non-zero initial values

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

reg  [3:0]           condition_r = 4'he;             // 4'he = al

reg  [1:0]           status_bits_mode_r = 2'b11;     // SVC

reg                  status_bits_irq_mask_r = 1'd1;

reg                  status_bits_firq_mask_r = 1'd1;

reg  [3:0]           address_sel_r = 4'd2;

reg  [1:0]           pc_sel_r = 2'd2;

reg                  pc_wen_r = 1'd1;

assign o_condition              = condition_r;

assign o_status_bits_mode       = status_bits_mode_r;

assign o_status_bits_irq_mask   = status_bits_irq_mask_r;

assign o_status_bits_firq_mask  = status_bits_firq_mask_r;

assign o_address_sel            = address_sel_r;

assign o_pc_sel                 = pc_sel_r;

assign o_pc_wen                 = pc_wen_r;

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

// Instruction Abort and Data Abort outputs

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

assign o_iabt_trigger     = instruction_iabt && status_bits_mode_r == SVC && control_state == INT_WAIT1;

assign o_iabt_address     = instruction_address;

assign o_iabt_status      = instruction_iabt_status;

assign o_dabt_trigger     = dabt_reg && !dabt_reg_d1;

assign o_dabt_address     = abt_address_reg;

assign o_dabt_status      = abt_status_reg;

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

// Instruction Decode

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

// for instructions that take more than one cycle

// the instruction is saved in the 'saved_mem_instruction'

// register and then that register is used for the rest of

// the execution of the instruction.

// But if the instruction does not execute because of the

// condition, then need to select the next instruction to

// decode

assign use_saved_current_instruction =  instruction_execute &&

                          ( control_state == MEM_WAIT1     ||

                            control_state == MEM_WAIT2     ||

                            control_state == MTRANS_EXEC1  ||

                            control_state == MTRANS_EXEC2  ||

                            control_state == MTRANS_EXEC3  ||

                            control_state == MTRANS_EXEC3B ||

                            control_state == MTRANS_EXEC4  ||

                            control_state == MTRANS5_ABORT ||

                            control_state == MULT_PROC1    ||

                            control_state == MULT_PROC2    ||

                            control_state == MULT_ACCUMU   ||

                            control_state == MULT_STORE    ||

                            control_state == INT_WAIT1     ||

                            control_state == INT_WAIT2     ||

                            control_state == SWAP_WRITE    ||

                            control_state == SWAP_WAIT1    ||

                            control_state == SWAP_WAIT2    ||

                            control_state == COPRO_WAIT     );

assign use_pre_fetch_instruction = control_state == PRE_FETCH_EXEC;

assign instruction_sel  =         use_saved_current_instruction  ? 2'd1 :  // saved_current_instruction

                                  use_pre_fetch_instruction      ? 2'd2 :  // pre_fetch_instruction

                                                                   2'd0 ;  // o_read_data

assign instruction      =         instruction_sel == 2'd0 ? o_read_data               :

                                  instruction_sel == 2'd1 ? saved_current_instruction :

                                                            pre_fetch_instruction     ;

// abort flag

assign instruction_iabt =         instruction_sel == 2'd0 ? iabt_reg                       :

                                  instruction_sel == 2'd1 ? saved_current_instruction_iabt :

                                                            pre_fetch_instruction_iabt     ;

assign instruction_address =      instruction_sel == 2'd0 ? abt_address_reg                   :

                                  instruction_sel == 2'd1 ? saved_current_instruction_address :

                                                            pre_fetch_instruction_address     ;

assign instruction_iabt_status =  instruction_sel == 2'd0 ? abt_status_reg                        :

                                  instruction_sel == 2'd1 ? saved_current_instruction_iabt_status :

                                                            pre_fetch_instruction_iabt_status     ;

// instruction address exception

assign instruction_adex =         instruction_sel == 2'd0 ? adex_reg                       :

                                  instruction_sel == 2'd1 ? saved_current_instruction_adex :

                                                            pre_fetch_instruction_adex     ;

// Instruction Decode - Order is important!

always @*

    casez ({instruction[27:20], instruction[7:4]})

        12'b00010?001001 : itype = SWAP;

        12'b000000??1001 : itype = MULT;

        12'b00?????????? : itype = REGOP;

        12'b01?????????? : itype = TRANS;

        12'b100????????? : itype = MTRANS;

        12'b101????????? : itype = BRANCH;

        12'b110????????? : itype = CODTRANS;

        12'b1110???????0 : itype = COREGOP;

        12'b1110???????1 : itype = CORTRANS;

        default:           itype = SWI;

    endcase

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

// Fixed fields within the instruction

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

assign opcode        = instruction[24:21];

assign condition_nxt = instruction[31:28];

assign o_rm_sel_nxt    = instruction[3:0];

assign o_rn_sel_nxt    = branch  ? 4'd15              : // Use PC to calculate branch destination

                                   instruction[19:16] ;

assign o_rds_sel_nxt   = control_state == SWAP_WRITE  ? instruction[3:0]   : // Rm gets written out to memory

                         itype == MTRANS              ? mtrans_reg         :

                         branch                       ? 4'd15              : // Update the PC

                         rds_use_rs                   ? instruction[11:8]  :

                                                        instruction[15:12] ;

assign shift_imm     = instruction[11:7];

// this is used for RRX

assign shift_extend  = !instruction[25] && !instruction[4] && !(|instruction[11:7]) && instruction[6:5] == 2'b11;

assign offset12      = { 20'h0, instruction[11:0]};

assign offset24      = {{6{instruction[23]}}, instruction[23:0], 2'd0 }; // sign extend

assign imm8          = instruction[7:0];

assign immediate_shifter_operand = instruction[25];

assign rds_use_rs                = (itype == REGOP && !instruction[25] && instruction[4]) ||

                                   (itype == MULT &&

                                    (control_state == MULT_PROC1  ||

                                     control_state == MULT_PROC2  ||

                                     instruction_valid && !interrupt )) ;

assign branch                    = itype == BRANCH;

assign opcode_compare =

            opcode == CMP ||

            opcode == CMN ||

            opcode == TEQ ||

            opcode == TST ;

assign mem_op               = itype == TRANS;

assign load_op              = mem_op && instruction[20];

assign store_op             = mem_op && !instruction[20];

assign write_pc             = pc_wen_nxt && pc_sel_nxt != 2'd0;

assign regop_set_flags      = itype == REGOP && instruction[20];

assign mem_op_pre_indexed   =  instruction[24] && instruction[21];

assign mem_op_post_indexed  = !instruction[24];

assign imm32_nxt            =  // add 0 to Rm

                               itype == MULT               ? {  32'd0                      } :

                               // 4 x number of registers

                               itype == MTRANS             ? {  mtrans_base_reg_change     } :

                               itype == BRANCH             ? {  offset24                   } :

                               itype == TRANS              ? {  offset12                   } :

                               instruction[11:8] == 4'h0  ? {            24'h0, imm8[7:0] } :

                               instruction[11:8] == 4'h1  ? { imm8[1:0], 24'h0, imm8[7:2] } :

                               instruction[11:8] == 4'h2  ? { imm8[3:0], 24'h0, imm8[7:4] } :

                               instruction[11:8] == 4'h3  ? { imm8[5:0], 24'h0, imm8[7:6] } :

                               instruction[11:8] == 4'h4  ? { imm8[7:0], 24'h0            } :

                               instruction[11:8] == 4'h5  ? { 2'h0,  imm8[7:0], 22'h0     } :

                               instruction[11:8] == 4'h6  ? { 4'h0,  imm8[7:0], 20'h0     } :

                               instruction[11:8] == 4'h7  ? { 6'h0,  imm8[7:0], 18'h0     } :

                               instruction[11:8] == 4'h8  ? { 8'h0,  imm8[7:0], 16'h0     } :

                               instruction[11:8] == 4'h9  ? { 10'h0, imm8[7:0], 14'h0     } :

                               instruction[11:8] == 4'ha  ? { 12'h0, imm8[7:0], 12'h0     } :

                               instruction[11:8] == 4'hb  ? { 14'h0, imm8[7:0], 10'h0     } :

                               instruction[11:8] == 4'hc  ? { 16'h0, imm8[7:0], 8'h0      } :

                               instruction[11:8] == 4'hd  ? { 18'h0, imm8[7:0], 6'h0      } :

                               instruction[11:8] == 4'he  ? { 20'h0, imm8[7:0], 4'h0      } :

                                                            { 22'h0, imm8[7:0], 2'h0      } ;

assign imm_shift_amount_nxt = shift_imm ;

       // This signal is encoded in the decode stage because

       // it is on the critical path in the execute stage

assign shift_imm_zero_nxt   = imm_shift_amount_nxt == 5'd0 &&       // immediate amount = 0

                              barrel_shift_amount_sel_nxt == 2'd2;  // shift immediate amount

assign alu_function_nxt     = { alu_swap_sel_nxt,

                                alu_not_sel_nxt,

                                alu_cin_sel_nxt,

                                alu_cout_sel_nxt,

                                alu_out_sel_nxt  };

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

// MTRANS Operations

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

   // Bit 15 = r15

   // Bit 0  = R0

   // In LDM and STM instructions R0 is loaded or stored first

always @*

    casez (instruction[15:0])

    16'b???????????????1 : mtrans_reg = 4'h0 ;

    16'b??????????????10 : mtrans_reg = 4'h1 ;

    16'b?????????????100 : mtrans_reg = 4'h2 ;

    16'b????????????1000 : mtrans_reg = 4'h3 ;

    16'b???????????10000 : mtrans_reg = 4'h4 ;

    16'b??????????100000 : mtrans_reg = 4'h5 ;

    16'b?????????1000000 : mtrans_reg = 4'h6 ;

    16'b????????10000000 : mtrans_reg = 4'h7 ;

    16'b???????100000000 : mtrans_reg = 4'h8 ;

    16'b??????1000000000 : mtrans_reg = 4'h9 ;

    16'b?????10000000000 : mtrans_reg = 4'ha ;

    16'b????100000000000 : mtrans_reg = 4'hb ;

    16'b???1000000000000 : mtrans_reg = 4'hc ;

    16'b??10000000000000 : mtrans_reg = 4'hd ;

    16'b?100000000000000 : mtrans_reg = 4'he ;

    default              : mtrans_reg = 4'hf ;

    endcase

always @*

    casez (instruction[15:0])

    16'b???????????????1 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 1],  1'd0};

    16'b??????????????10 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 2],  2'd0};

    16'b?????????????100 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 3],  3'd0};

    16'b????????????1000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 4],  4'd0};

    16'b???????????10000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 5],  5'd0};

    16'b??????????100000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 6],  6'd0};

    16'b?????????1000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 7],  7'd0};

    16'b????????10000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 8],  8'd0};

    16'b???????100000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15: 9],  9'd0};

    16'b??????1000000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15:10], 10'd0};

    16'b?????10000000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15:11], 11'd0};

    16'b????100000000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15:12], 12'd0};

    16'b???1000000000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15:13], 13'd0};

    16'b??10000000000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15:14], 14'd0};

    16'b?100000000000000 : mtrans_instruction_nxt = {instruction[31:16], instruction[15   ], 15'd0};

    default              : mtrans_instruction_nxt = {instruction[31:16],                     16'd0};

    endcase

// number of registers to be stored

assign mtrans_num_registers =   {4'd0, instruction[15]} +

                                {4'd0, instruction[14]} +

                                {4'd0, instruction[13]} +