`default_nettype none

module cpu
  (
    nrst,
    clk,
    nmi,
    irq,
   o_led);

`include "parameters.vh"

    localparam STATE_START_1 = 4'hc;
    localparam STATE_START_2 = 4'hd;
    localparam STATE_START_3 = 4'he;
    localparam STATE_START_4 = 4'hf;
    localparam STATE_START_0 = 4'hb;
    localparam STATE_FETCH = 0;
    localparam STATE_EXECUTE = 1;
    localparam STATE_WRITE = 2;
    localparam STATE_FETCH_DATA_1 = 3;
    localparam STATE_FETCH_DATA_2 = 4;
    localparam STATE_EXECUTE_DATA_2 = 5;
    localparam STATE_BRANCH = 7;

localparam
          READ_SOURCE_NONE = 2'h0,
          READ_SOURCE_MEM = 2'h1,
          READ_SOURCE_ALU = 2'h2;

    input wire nrst;
    input wire clk;
    input wire nmi;
    input wire irq;
    output wire [5:0] o_led;


    reg [7:0]  X;
    reg [7:0]  Y;
    reg [7:0]  A;
    reg [15:0] PC;
    reg [7:0]  S;
    reg [7:0]  P;

    reg [7:0]  tmp;

    reg [3:0]  state;

    reg [7:0]  instr;

    wire [4:0] address_code;
    wire [7:0] decoded_instr;

    wire       rst = !nrst;

    logic      mem_wr;
    wire [7:0] mem_data;
    wire [15:0] mem_addr_eff;
    logic [7:0] mem_data_wr;
    logic [2:0] mem_sel;
    logic [7:0] mem_aux_addr;

    logic [7:0]  alu_op_1;
    logic [7:0]  alu_op_2;
    logic        alu_op_c;
    logic [2:0]  alu_op_sel;
    wire [7:0]   alu_op_result;
    logic        alu_n;
    logic        alu_z;
    logic        alu_v;
    logic        alu_c;
    reg [7:0]    data;
    reg [15:0]   mem_addr;
    reg [2:0]    mem_addr_offset;
    logic [1:0]  pending_a_read;
    logic [1:0]  pending_x_read;
    logic        pending_rel_branch;

initial A = 0;
initial X = 0;
initial Y = 0;
initial P = 0;
initial S = 8'hff;
initial mem_data_wr = 0;
initial mem_wr = 0;
assign mem_addr_eff = mem_addr;
initial pending_a_read = 0;
initial pending_x_read = 0;
initial pending_rel_branch = 0;

alu alui(
         .A(alu_op_1),
         .B(alu_op_2),
         .C(alu_op_c),
         .sel(alu_op_sel),
         .result(alu_op_result),
         .r_n(alu_n),
         .r_z(alu_z),
         .r_v(alu_v),
         .r_c(alu_c));

decoder decoder (
                 .clk(clk),
                 .rst(rst),
                 .instr((state == STATE_EXECUTE ? mem_data : instr)),
                 .address_code(address_code),
                 .decoded(decoded_instr)
                 );

mio mioi(.clk(clk),
         .i_data(mem_data_wr),
         .i_addr(mem_addr_eff),
         .i_wr(mem_wr),
         .o_data(mem_data),
         .o_led(o_led));

dmux dmuxi(
           .sel(mem_sel),
           .i_pc(PC),
           .i_a(A),
           .i_x(X),
           .i_y(Y),
           .i_sp(S),
           .i_aux({mem_data, mem_aux_addr}),
           .o_mux(mem_addr));

initial state = STATE_FETCH;
initial PC = 0;
initial mem_sel = 0;

always @(posedge clk)
  if (state == STATE_START_0)
    begin
        mem_sel <= DMUX_AUX;
        mem_aux_addr <= 16'hfffc;
        state <= STATE_START_1;
    end
  else if (state == STATE_START_1)
    begin
        state <= STATE_START_2;
    end
  else if (state == STATE_START_2)
    begin
        PC[7:0] <= mem_data;
        mem_aux_addr <= 16'hfffd;
        state <= STATE_START_3;
    end
  else if(state == STATE_START_3)
    begin
        PC[15:8] <= mem_data;
        mem_sel <= DMUX_PC;
        state <= STATE_FETCH;
    end

always_comb
  begin
      alu_op_1 = 0;
      alu_op_2 = 0;
      alu_op_c = 0;
      alu_op_sel = 0;
      if (decoded_instr == I_DEX)
        begin
            alu_op_1 = X;
            alu_op_2 = 1'b1;
            alu_op_c = 1'b1;
            alu_op_sel = OP_SUB;
        end
      if (address_code == ADDR_MODE_ABSOLUTE)
        begin
            if (decoded_instr == I_EOR)
              begin
                  alu_op_1 = A;
                  alu_op_2 = mem_data;
                  alu_op_c = 0;
                  alu_op_sel = OP_EOR;
              end
        end
  end

always @(posedge clk)
  if (state == STATE_FETCH)
    begin
        if (pending_a_read == READ_SOURCE_MEM)
          begin
              A <= mem_data;
              P[P_Z] <= (mem_data == 0);
              P[P_N] <= mem_data[7];
          end
        else if (pending_a_read == READ_SOURCE_ALU)
          begin
              A <= alu_op_result;
              P[P_Z] <= alu_z;
              P[P_N] <= alu_n;
          end
        pending_a_read <= READ_SOURCE_NONE;

        if (pending_x_read == READ_SOURCE_MEM)
          begin
              X <= mem_data;
              P[P_Z] <= (mem_data == 0);
              P[P_N] <= mem_data[7];
          end
        else if (pending_x_read == READ_SOURCE_ALU)
          begin
              X <= alu_op_result;
              P[P_Z] <= alu_z;
              P[P_N] <= alu_n;
          end
        pending_x_read <= READ_SOURCE_NONE;
        PC <= PC + 1;
        state <= STATE_EXECUTE;
    end

always @(posedge clk)
  if (state == STATE_EXECUTE)
    begin
        instr <= mem_data;
        if (address_code == ADDR_MODE_ABSOLUTE)
          begin
              PC <= PC + 1;
              state <= STATE_FETCH_DATA_2;
          end
        else if (address_code == ADDR_MODE_IMMEDIATE)
          begin
              if (decoded_instr == I_LDX)
                begin
                    pending_x_read <= READ_SOURCE_MEM;

                    state <= STATE_FETCH;
                    PC <= PC + 1;
                end
              else if (decoded_instr == I_LDA)
                begin
                    pending_a_read <= READ_SOURCE_MEM;

                    state <= STATE_FETCH;
                    PC <= PC + 1;
                end
          end
        else if (address_code == ADDR_MODE_IMPLIED)
          begin
              if (decoded_instr == I_DEX)
                begin
                    pending_x_read <= READ_SOURCE_ALU;

                    state <= STATE_FETCH;
                end
          end
        else if (decoded_instr == I_BNE)
          begin
              if(!P[P_Z])
                begin
                    pending_rel_branch = 1'b1;
                    state <= STATE_BRANCH;
                end
              else
                begin
                    state <= STATE_FETCH;
                    PC <= PC + 1;
                end
          end
    end

always @(posedge clk)
  if (state == STATE_BRANCH)
    begin
        if (pending_rel_branch)
          begin
              PC <= PC + {{8{mem_data[7]}}, mem_data[7:0]} + 1'b1;
          end
        pending_rel_branch <= 0;
        state <= STATE_FETCH;
    end

always @(posedge clk)
  if (state == STATE_FETCH_DATA_2)
    begin
        if (address_code == ADDR_MODE_ABSOLUTE)
          begin
              mem_aux_addr[7:0] <= mem_data;
              mem_sel <= DMUX_AUX;

              state <= STATE_EXECUTE_DATA_2;
              PC <= PC + 1'b1;
          end
    end

always @(posedge clk)
  begin
      if (state == STATE_EXECUTE_DATA_2)
        begin
            if (address_code == ADDR_MODE_ABSOLUTE)
              begin
                  mem_sel <= DMUX_PC;
                  state <= STATE_FETCH;
                  if (decoded_instr == I_EOR)
                    begin
                        tmp <= mem_data;
                        pending_a_read <= READ_SOURCE_ALU;
                    end
                  else if(decoded_instr == I_JMP)
                    begin
                        PC <= mem_addr;
                    end
              end
        end
  end

always_comb
  begin
      mem_data_wr = 0;
      mem_wr = 0;
      if (state == STATE_EXECUTE_DATA_2)
        begin
            if (address_code == ADDR_MODE_ABSOLUTE)
              begin
                  if(decoded_instr == I_STA)
                    begin
                        mem_data_wr = A;
                        mem_wr = 1'b1;
                    end
              end
        end
  end

always_comb
  begin
      mem_addr_offset = 0;
      // if (state == STATE_FETCH)
      //   begin
      //       mem_addr_offset = 1;
      //   end
      // if (state == STATE_BRANCH)
      //   begin
      //       mem_addr_offset = 0;
      //   end
      // if (state == STATE_EXECUTE)
      //   begin
      //       if (address_code == ADDR_MODE_IMMEDIATE)
      //         begin
      //             mem_addr_offset = 1;
      //         end
      //       else if (address_code == ADDR_MODE_RELATIVE)
      //         begin
      //             if (decoded_instr == I_BNE )
      //               if (!P[P_Z])
      //                 mem_addr_offset = 0;
      //               else
      //                 mem_addr_offset = 1;
      //         end
      //       else if (address_code == ADDR_MODE_ABSOLUTE)
      //         begin
      //             mem_addr_offset = 1;
      //         end
      //   end
      // if (state == STATE_FETCH_DATA_2)
      //   begin
      //       if (address_code == ADDR_MODE_ABSOLUTE)
      //         begin
      //             mem_addr_offset = 1;
      //         end
      //   end
  end

endmodule