module cpu (
    input wire system_ready,
    output bit [7:0] memory[0:4095],
    input wire clk_in,
    input wire keyboard[15:0],
    input wire [7:0] random_number,
    output int cycle_counter,
    output wire [15:0] program_counter,
    output wire vram[0:2047],
    output wire [7:0] sound_timer
);

  bit halt;
  int watch_key;

  logic [15:0] stack[0:15];
  logic [15:0] index_reg;
  logic [3:0] stack_pointer;
  logic [7:0] registers[0:15];
  logic [15:0] opcode;
  logic [7:0] delay_timer;

  logic [15:0] scratch;
  logic [15:0] scratch2;

  logic [7:0] scratch_8;
  logic [7:0] scratch_82;

  logic [31:0] x_cord;
  logic [31:0] y_cord;
  logic [7:0] size;
  logic screen_pixel;
  logic [7:0] sprite_pixel;

  always_ff @(negedge clk_in) begin
    if (system_ready) begin
    opcode = {memory[program_counter+0], memory[program_counter+1]};
    $display("HW     : opcode is 0x%h (%b)", opcode, opcode);
    $display("HW     : PC %0d 0x%h", program_counter, program_counter);

    if (cycle_counter % 20 == 0) begin
      if (delay_timer > 0) delay_timer--;
      if (sound_timer > 0) sound_timer--;
    end

    casez (opcode)
      'h00E0: begin
        $display("HW     : INSTR CLS");
        for (int i = 0; i < 2048; i++) begin
          vram[i] = 0;
        end
      end
      'h00EE: begin
        $display("HW     : INSTR RET");
        stack_pointer--;
        program_counter = stack[stack_pointer];
      end
      'h0???:  $display("HW     : INSTR SYS addr (Treating as NOP)");
      'h1???: begin
        $display("HW     : INSTR JP addr");
        program_counter = (opcode & 'h0FFF) - 2;
      end
      'h2???: begin
        $display("HW     : INSTR CALL addr");
        stack[stack_pointer] = program_counter;
        stack_pointer++;
        program_counter = (opcode & 'h0FFF) - 2;
      end
      'h3???: begin
        $display("HW     : INSTR SE Vx, byte");
        scratch = (opcode & 'h00FF);
        if (scratch[7:0] == registers[(opcode&'h0F00)>>8]) begin
          program_counter += 2;
        end
      end
      'h4???: begin
        $display("HW     : INSTR SNE Vx, byte");
        scratch = (opcode & 'h00FF);
        if (scratch[7:0] != registers[(opcode&'h0F00)>>8]) begin
          program_counter += 2;
        end
      end
      'h5??0: begin
        $display("HW     : INSTR SE Vx, Vy");
        if (registers[(opcode&'h00F0)>>4] == registers[(opcode&'h0F00)>>8]) begin
          program_counter += 2;
        end
      end
      'h6???: begin
        $display("HW     : INSTR LD Vx, byte");
        scratch = (opcode & 'h00FF);
        registers[(opcode&'h0F00)>>8] = scratch[7:0];
      end
      'h7???: begin
        $display("HW     : INSTR ADD Vx, byte");
        scratch = (opcode & 'h00FF);
        registers[(opcode&'h0F00)>>8] += scratch[7:0];
      end
      'h8??0: begin
        $display("HW     : INSTR LD Vx, Vy");
        registers[(opcode&'h0F00)>>8] = registers[(opcode&'h00F0)>>4];
      end
      'h8??1: begin
        $display("HW     : INSTR OR Vx, Vy");
        registers[(opcode&'h0F00)>>8] |= registers[(opcode&'h00F0)>>4];
        registers[15] = 0;
      end
      'h8??2: begin
        $display("HW     : INSTR AND Vx, Vy");
        registers[(opcode&'h0F00)>>8] &= registers[(opcode&'h00F0)>>4];
        registers[15] = 0;
      end
      'h8??3: begin
        $display("HW     : INSTR XOR Vx, Vy");
        registers[(opcode&'h0F00)>>8] ^= registers[(opcode&'h00F0)>>4];
        registers[15] = 0;
      end
      'h8??4: begin
        $display("HW     : INSTR ADD Vx, Vy");
        scratch_8 = registers[(opcode&'h0F00)>>8];
        registers[(opcode&'h0F00)>>8] += registers[(opcode&'h00F0)>>4];
        registers[15] = {7'b0000000, scratch_8 > registers[(opcode&'h0F00)>>8]};
      end
      'h8??5: begin
        $display("HW     : INSTR SUB Vx, Vy");
        scratch_8 = registers[(opcode&'h0F00)>>8];
        registers[(opcode&'h0F00)>>8] -= registers[(opcode&'h00F0)>>4];
        registers[15] = {7'b0000000, scratch_8 >= registers[(opcode&'h0F00)>>8]};
      end
      'h8??6: begin
        $display("HW     : INSTR SHR Vx {, Vy}");
        scratch_8 = registers[(opcode&'h0F00)>>8];
        registers[(opcode&'h0F00)>>8] = registers[(opcode&'h00F0)>>4] >> 1;
        registers[15] = {7'b0000000, ((scratch_8 & 8'h01) == 8'h01)};
      end
      'h8??7: begin
        $display("HW     : INSTR SUBN Vx, Vy");
        scratch_8 = registers[(opcode&'h00F0)>>4];
        scratch_82 = registers[(opcode&'h0F00)>>8];
        registers[(opcode & 'h0F00) >> 8] = registers[(opcode & 'h00F0) >> 4] - registers[(opcode & 'h0F00) >> 8];
        registers[15] = {7'b0000000, (scratch_8 >= scratch_82)};
      end
      'h8??E: begin
        $display("HW     : INSTR SHL Vx {, Vy}");
        scratch_8 = registers[(opcode&'h0F00)>>8];
        registers[(opcode&'h0F00)>>8] = registers[(opcode&'h00F0)>>4] << 1;

        registers[15] = {7'b0000000, (scratch_8[7])};
      end
      'h9??0: begin
        $display("HW     : INSTR SNE Vx, Vy");
        if (registers[(opcode&'h00F0)>>4] != registers[(opcode&'h0F00)>>8]) begin
          program_counter += 2;
        end
      end
      'hA???: begin
        $display("HW     : INSTR LD I, addr");
        index_reg = (opcode & 'h0FFF);
      end
      'hb???: begin
        $display("HW     : INSTR JP V0, addr");
        program_counter = {8'h00, registers[0]} + (opcode & 'h0FFF) - 2;
      end
      'hc???: begin
        $display("HW     : RND Vx, addr");
        // TODO: use a real RNG module, this is not synthesizeable
        scratch = {8'h00, random_number} % 16'h0100;
        scratch2 = (opcode & 'h00FF);
        registers[(opcode&'h0F00)>>8] = scratch[7:0] & scratch2[7:0];
      end
      'hD???: begin
        $display("HW     : INSTR DRW Vx, Vy, nibble");
        if (cycle_counter % 20 != 0) begin
          halt = 1;
        end else begin
          halt   = 0;
          x_cord = {24'h000000, registers[(opcode&'h0F00)>>8]};
          y_cord = {24'h000000, registers[(opcode&'h00F0)>>4]};

          x_cord %= 64;
          y_cord %= 32;

          scratch = (opcode & 'h000F);
          size = scratch[7:0];
          registers[15] = 0;

          for (int r = 0; r < size; r++) begin
            for (int c = 0; c < 8; c++) begin
              if (r + y_cord >= 32 || x_cord + c >= 64) continue;
              screen_pixel = vram[((r+y_cord)*64)+(x_cord+c)];
              sprite_pixel = memory[{16'h0000, index_reg}+r] & ('h80 >> c);

              if (|sprite_pixel) begin
                if (screen_pixel == 1) begin
                  registers[15] = 1;
                end
                vram[((r+y_cord)*64)+(x_cord+c)] ^= 1;
              end
            end
          end
        end
      end
      'hE?9E: begin
        $display("HW     : INSTR SKP Vx");
        scratch_8 = registers[(opcode&'h0F00)>>8];
        if (keyboard[scratch_8[3:0]] == 1) begin
          program_counter += 2;
        end
      end
      'hE?A1: begin
        $display("HW     : INSTR SNE Vx");
        scratch_8 = registers[(opcode&'h0F00)>>8];
        if (keyboard[scratch_8[3:0]] != 1) begin
          program_counter += 2;
        end
      end
      'hF?07: begin
        $display("HW     : INSTR LD Vx, DT");
        registers[(opcode&'h0F00)>>8] = delay_timer;
      end
      'hF?0A: begin
        $display("HW     : INSTR LD Vx, K");
        halt = 1;
        for (int i = 0; i < 16; i++) begin
          if (watch_key == 255) begin
            if (keyboard[i]) begin
              watch_key = i;
            end
          end else begin
            if (!keyboard[watch_key]) begin
              halt = 0;
              watch_key = 255;
            end
          end
        end
      end
      'hF?15: begin
        $display("HW     : INSTR LD DT, Vx");
        delay_timer = registers[(opcode&'h0F00)>>8];
      end
      'hF?18: begin
        $display("HW     : INSTR LD ST, Vx");
        sound_timer = registers[(opcode&'h0F00)>>8];
      end
      'hF?1E: begin
        $display("HW     : INSTR ADD I, Vx");
        index_reg = index_reg + {8'h00, registers[(opcode&'h0F00)>>8]};
      end
      'hF?29: begin
        $display("HW     : INSTR LDL F, Vx");
        index_reg = registers[(opcode&'h0F00)>>8] * 5;
      end
      'hF?33: begin
        $display("HW     : INSTR LD B, Vx");
        scratch = {8'h00, registers[(opcode&'h0F00)>>8]};
        scratch2 = scratch % 10;
        memory[index_reg+2] = scratch2[7:0];
        scratch /= 10;
        scratch2 = scratch % 10;
        memory[index_reg+1] = scratch2[7:0];
        scratch /= 10;
        scratch2 = scratch % 10;
        memory[index_reg+0] = scratch2[7:0];
      end
      'hF?55: begin
        $display("HW     : INSTR LD [I], Vx");
        scratch = (opcode & 'h0F00) >> 8;
        for (bit [7:0] i8 = 0; i8 <= scratch[7:0]; i8++) begin
          scratch2 = index_reg + {8'h00, i8};
          memory[scratch2[11:0]] = registers[i8[3:0]];
        end
        index_reg++;
      end
      'hF?65: begin
        $display("HW     : INSTR LD Vx, [I]");
        scratch = (opcode & 'h0F00) >> 8;
        for (bit [7:0] i8 = 0; i8 <= scratch[7:0]; i8++) begin
          scratch2 = index_reg + {8'h00, i8};
          registers[i8[3:0]] = memory[scratch2[11:0]];
        end
        index_reg++;
      end
      default: $display("HW     : ILLEGAL INSTRUCTION");
    endcase

    if (!halt) program_counter += 2;

    cycle_counter++;
end
  end
endmodule