slon/src/decoder.verilog

module decoder
  (
   input            clk,
   input            rst,
   input [7:0]      instr,
   output logic [4:0] address_code,
   output logic [7:0] decoded
   );

`include "parameters.vh"

    wire [2:0]      row = instr[7:5];
    wire [2:0]      column = instr[4:2];
    wire [1:0]      sector = instr[1:0];

    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 = 0;
    decoded = 0;
    if (rst) begin
        address_code = 8'h0;
    end
    else 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_STA : I_NOP);
              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
            if (column > 0) begin
                address_code = column;
                end
            else begin
                address_code = ADDR_MODE_IMMEDIATE;
            end
        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_ABSOLUTE;
                end
                else begin
                    address_code = ADDR_MODE_INDEXED_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;
            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 || column == 7) begin
                address_code = ADDR_MODE_INDEXED_ABSOLUTE;
            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
                address_code = ADDR_MODE_INDEXED_ABSOLUTE;
            end
        end
    end
end

endmodule