`default_nettype none

module decoder (clk, rst, instr, address_code, decoded);

`include "parameters.vh"

    input wire clk;
    input wire rst;
    input wire [7:0] instr;
    output logic [4:0] address_code;
    output logic [7:0] decoded;

    logic [2:0]      row;
    logic [2:0]      column;
    logic [1:0]      sector;

initial address_code = 0;
always_comb
  begin
      row = instr[7:5];
      column = instr[4:2];
      sector = instr[1:0];
  end


    wire            sec_2_stp = (sector == 2) && ((column == 0 && row <= 3) || (column == 4));
    wire            sec_2_nop = (sector == 2) && ((column == 0 && (row == 4 || row == 6 || row == 7))
                                                  || (column == 6 && (row != 4 && row != 5)));

always_comb begin
    address_code = 5'h1f;
    decoded = 8'hff;
    if (!rst) begin
        if (sector == 1) begin
            case (row)
              0: decoded = I_ORA;
              1: decoded = I_AND;
              2: decoded = I_EOR;
              3: decoded = I_ADC;
              4: decoded = (column == 2 ? I_NOP : I_STA);
              5: decoded = I_LDA;
              6: decoded = I_CMP;
              7: decoded = I_SBC;
            endcase

            address_code = column;
        end
        else if (sector == 2) begin
            case (row)
              0: decoded = (sec_2_stp ? I_STP : (sec_2_nop ? I_NOP : I_ASL));
              1: decoded = (sec_2_stp ? I_STP : (sec_2_nop ? I_NOP : I_ROL));
              2: decoded = (sec_2_stp ? I_STP : (sec_2_nop ? I_NOP : I_LSR));
              3: decoded = (sec_2_stp ? I_STP : (sec_2_nop ? I_NOP : I_ROR));
              4: decoded = (sec_2_stp ? I_STP : (sec_2_nop ? I_NOP : (column[0] ? I_STX : I_TXA)));
              5: decoded = (sec_2_stp ? I_STP : (column == 2 ? I_TAX : (column == 6 ? I_TSX : I_LDX)));
              6: decoded = (sec_2_stp ? I_STP : (sec_2_nop ? I_NOP : (column == 2 ? I_DEX : I_DEC)));
              7: decoded = (sec_2_stp ? I_STP : ((sec_2_nop || column == 2) ? I_NOP : I_INC));
            endcase
        end
        else if (sector == 0) begin
            if (column == 0) begin
                case (row)
                  0: decoded = I_BRK;
                  1: decoded = I_JSR;
                  2: decoded = I_RTI;
                  3: decoded = I_RTS;
                  4: decoded = I_NOP;
                  5: decoded = I_LDY;
                  6: decoded = I_CPY;
                  7: decoded = I_CPX;
                endcase
            end
            else if (column == 1) begin
                case (row)
                  0: decoded = I_NOP;
                  1: decoded = I_BIT;
                  2: decoded = I_NOP;
                  3: decoded = I_NOP;
                  4: decoded = I_STY;
                  5: decoded = I_LDY;
                  6: decoded = I_CPY;
                  7: decoded = I_CPX;
                endcase
            end
            else if (column == 2) begin
                case (row)
                  0: decoded = I_PHP;
                  1: decoded = I_PLP;
                  2: decoded = I_PHA;
                  3: decoded = I_PLA;
                  4: decoded = I_DEY;
                  5: decoded = I_TAY;
                  6: decoded = I_INY;
                  7: decoded = I_INX;
                endcase
            end
            else if (column == 3) begin
                case (row)
                  0: decoded = I_NOP;
                  1: decoded = I_BIT;
                  2: decoded = I_JMP;
                  3: decoded = I_JMP;
                  4: decoded = I_STY;
                  5: decoded = I_LDY;
                  6: decoded = I_CPY;
                  7: decoded = I_CPX;
                endcase
            end
            else if (column == 4) begin
                case (row)
                  0: decoded = I_BPL;
                  1: decoded = I_BMI;
                  2: decoded = I_BVC;
                  3: decoded = I_BVS;
                  4: decoded = I_BCC;
                  5: decoded = I_BCS;
                  6: decoded = I_BNE;
                  7: decoded = I_BEQ;
                endcase
            end
            else if (column == 5) begin
                case (row)
                  0: decoded = I_NOP;
                  1: decoded = I_NOP;
                  2: decoded = I_NOP;
                  3: decoded = I_NOP;
                  4: decoded = I_STY;
                  5: decoded = I_LDY;
                  6: decoded = I_NOP;
                  7: decoded = I_NOP;
                endcase
            end
            else if (column == 6) begin
                case (row)
                  0: decoded = I_CLC;
                  1: decoded = I_SEC;
                  2: decoded = I_CLI;
                  3: decoded = I_SEI;
                  4: decoded = I_TYA;
                  5: decoded = I_CLV;
                  6: decoded = I_CLD;
                  7: decoded = I_SED;
                endcase
            end
            else if (column == 7) begin
                case (row)
                  0: decoded = I_NOP;
                  1: decoded = I_NOP;
                  2: decoded = I_NOP;
                  3: decoded = I_NOP;
                  4: decoded = I_SHY;
                  5: decoded = I_LDY;
                  6: decoded = I_NOP;
                  7: decoded = I_NOP;
                endcase
            end
        end

        if (sector == 0) begin
            if (column == 0) begin
                if (row == 0 || (row >= 2 && row <= 4)) begin
                    address_code = ADDR_MODE_IMPLIED;
                end
                else if (row == 1) begin
                    address_code = ADDR_MODE_ABSOLUTE;
                end
                else begin
                    address_code = ADDR_MODE_IMMEDIATE;
                end
            end
            else if(column == 1) begin
                address_code = ADDR_MODE_ZP;
            end
            else if (column == 2) begin
                address_code = ADDR_MODE_IMPLIED;
            end
            else if (column == 3) begin
                if (row == 3) begin
                    address_code = ADDR_MODE_INDIRECT;
                end
                else begin
                    address_code = ADDR_MODE_ABSOLUTE;
                end
            end
            else if (column == 4) begin
                address_code = ADDR_MODE_RELATIVE;
            end
            else if (column == 5) begin
                address_code = ADDR_MODE_INDEXED_ZP;
            end
            else if (column == 6) begin
                address_code = ADDR_MODE_IMPLIED;
            end
            else if (column == 7) begin
                address_code = ADDR_MODE_INDEXED_ABSOLUTE_X;
            end
        end
        else if(sector == 1) begin
            if (column == 0) begin
                address_code = ADDR_MODE_PREINDEXED_INDIRECT;
            end
            else if (column == 1) begin
                address_code = ADDR_MODE_ZP;
            end
            else if (column == 2) begin
                address_code = ADDR_MODE_IMMEDIATE;
            end
            else if (column == 3) begin
                address_code = ADDR_MODE_ABSOLUTE;
            end
            else if (column == 4) begin
                address_code = ADDR_MODE_POSTINDEXED_INDIRECT;
            end
            else if (column == 5) begin
                address_code = ADDR_MODE_INDEXED_ZP;
            end
            else if (column == 6) begin
                address_code = ADDR_MODE_INDEXED_ABSOLUTE_Y;
            end
            else if (column == 7) begin
                address_code = ADDR_MODE_INDEXED_ABSOLUTE_X;
            end
        end
        else if(sector == 2) begin
            if (column == 0) begin
                address_code = ADDR_MODE_IMMEDIATE;
            end
            else if (column == 1) begin
                address_code = ADDR_MODE_ZP;
            end
            else if (column == 2 || column == 6) begin
                address_code = ADDR_MODE_IMPLIED;
            end
            else if (column == 3) begin
                address_code = ADDR_MODE_ABSOLUTE;
            end
            // column == 4 are  invalid opcodes
            else if (column == 5) begin
                address_code = ADDR_MODE_INDEXED_ZP;
            end
            else if (column == 7) begin
                if (row == 5)
                  address_code = ADDR_MODE_INDEXED_ABSOLUTE_Y;
                else
                  address_code = ADDR_MODE_INDEXED_ABSOLUTE_X;
            end
        end
    end
end

`ifdef FORMAL

    reg f_past_valid;
initial f_past_valid = 0;

always_comb
  assume(rst == 0);

always @(posedge clk)
  f_past_valid <= 1;

always_comb
  if (instr == 8'h00) // BRK
    begin
        assert (decoded == I_BRK);
    end;

always_comb
  if (instr == 8'h4C) // JMP abs
    begin
        assert (decoded == I_JMP);
        assert (address_code == ADDR_MODE_ABSOLUTE);
    end;

always_comb
  if (instr == 8'h8D) // STA abs
    begin
        assert (decoded == I_STA);
        assert (address_code == ADDR_MODE_ABSOLUTE);
    end;

always_comb
  if (instr == 8'hA2) // LDX imm
    begin
        assert (decoded == I_LDX);
        assert (address_code == ADDR_MODE_IMMEDIATE);
    end;

always_comb
  if (instr == 8'hA9) // LDA imm
    begin
        assert (decoded == I_LDA);
        assert (address_code == ADDR_MODE_IMMEDIATE);
    end;

always_comb
  if (instr == 8'hA0) // LDY imm
    begin
        assert (decoded == I_LDY);
        assert (address_code == ADDR_MODE_IMMEDIATE);
    end;

always_comb
  if (instr == 8'hA4) // LDY zp
    begin
        assert (decoded == I_LDY);
        assert (address_code == ADDR_MODE_ZP);
    end;

always_comb
  if (instr == 8'hB4) // LDY zp,X
    begin
        assert (decoded == I_LDY);
        assert (address_code == ADDR_MODE_INDEXED_ZP);
    end;

always_comb
  if (instr == 8'hAC) // LDY abs
    begin
        assert (decoded == I_LDY);
        assert (address_code == ADDR_MODE_ABSOLUTE);
    end;

always_comb
  if (instr == 8'hBC) // LDY abs,X
    begin
        assert (decoded == I_LDY);
        assert (address_code == ADDR_MODE_INDEXED_ABSOLUTE_X);
    end;

always_comb
  if (instr == 8'hCA) // DEX
    begin
        assert (decoded == I_DEX);
        assert (address_code == ADDR_MODE_IMPLIED);
    end;

always_comb
  if (instr == 8'hE0) // CPX
    begin
        assert (decoded == I_CPX);
    end;
always_comb
  if (instr == 8'hE0) // CPX
    begin
        assert (decoded == I_CPX);
    end;

`endif

endmodule