`default_nettype none

  module hex_print(input wire [3:0] halfbyte,
		   input wire [7:0]  x,
		   input wire [15:0] y,

		   output wire 	     pixel
		   );
   wire [0:7] 			     number_0 [15:0];
   wire [0:7] 			     number_1 [15:0];
   wire [0:7] 			     number_2 [15:0];
   wire [0:7] 			     number_3 [15:0];
   wire [0:7] 			     number_4 [15:0];
   wire [0:7] 			     number_5 [15:0];
   wire [0:7] 			     number_6 [15:0];
   wire [0:7] 			     number_7 [15:0];
   wire [0:7] 			     number_8 [15:0];
   wire [0:7] 			     number_9 [15:0];

   wire [0:7] 			     letter_A [15:0];
   wire [0:7] 			     letter_B [15:0];
   wire [0:7] 			     letter_C [15:0];
   wire [0:7] 			     letter_D [15:0];
   wire [0:7] 			     letter_E [15:0];
   wire [0:7] 			     letter_F [15:0];

   assign number_0[0]  = 8'b00000000;
   assign number_0[1]  = 8'b00011000;
   assign number_0[2]  = 8'b00100100;
   assign number_0[3]  = 8'b00100100;
   assign number_0[4]  = 8'b01000110;
   assign number_0[5]  = 8'b01001010;
   assign number_0[6]  = 8'b01001010;
   assign number_0[7]  = 8'b01001010;
   assign number_0[8]  = 8'b01010010;
   assign number_0[9]  = 8'b01010010;
   assign number_0[10] = 8'b01010010;
   assign number_0[11] = 8'b01100010;
   assign number_0[12] = 8'b00100100;
   assign number_0[13] = 8'b00100100;
   assign number_0[14] = 8'b00011000;
   assign number_0[15] = 8'b00000000;

   assign number_1[0]  = 8'b00000000;
   assign number_1[1]  = 8'b00001000;
   assign number_1[2]  = 8'b00011000;
   assign number_1[3]  = 8'b00111000;
   assign number_1[4]  = 8'b00001000;
   assign number_1[5]  = 8'b00001000;
   assign number_1[6]  = 8'b00001000;
   assign number_1[7]  = 8'b00001000;
   assign number_1[8]  = 8'b00001000;
   assign number_1[9]  = 8'b00001000;
   assign number_1[10] = 8'b00001000;
   assign number_1[11] = 8'b00001000;
   assign number_1[12] = 8'b00001000;
   assign number_1[13] = 8'b00001000;
   assign number_1[14] = 8'b00001000;
   assign number_1[15] = 8'b00000000;

   assign number_2[0]  = 8'b00000000;
   assign number_2[1]  = 8'b00111000;
   assign number_2[2]  = 8'b01000100;
   assign number_2[3]  = 8'b00000010;
   assign number_2[4]  = 8'b00000010;
   assign number_2[5]  = 8'b00000010;
   assign number_2[6]  = 8'b00000010;
   assign number_2[7]  = 8'b00000100;
   assign number_2[8]  = 8'b00001000;
   assign number_2[9]  = 8'b00010000;
   assign number_2[10] = 8'b00100000;
   assign number_2[11] = 8'b00100000;
   assign number_2[12] = 8'b01000000;
   assign number_2[13] = 8'b01000000;
   assign number_2[14] = 8'b01111110;
   assign number_2[15] = 8'b00000000;

   assign number_3[0]  = 8'b00000000;
   assign number_3[1]  = 8'b00111000;
   assign number_3[2]  = 8'b01000100;
   assign number_3[3]  = 8'b00000010;
   assign number_3[4]  = 8'b00000010;
   assign number_3[5]  = 8'b00000010;
   assign number_3[6]  = 8'b00000100;
   assign number_3[7]  = 8'b00011000;
   assign number_3[8]  = 8'b00000100;
   assign number_3[9]  = 8'b00000010;
   assign number_3[10] = 8'b00000010;
   assign number_3[11] = 8'b00000010;
   assign number_3[12] = 8'b00000010;
   assign number_3[13] = 8'b01000100;
   assign number_3[14] = 8'b00111000;
   assign number_3[15] = 8'b00000000;

   assign number_4[0]  = 8'b00000000;
   assign number_4[1]  = 8'b00001000;
   assign number_4[2]  = 8'b00001000;
   assign number_4[3]  = 8'b00010000;
   assign number_4[4]  = 8'b00010000;
   assign number_4[5]  = 8'b00100000;
   assign number_4[6]  = 8'b00100000;
   assign number_4[7]  = 8'b01000100;
   assign number_4[8]  = 8'b01111110;
   assign number_4[9]  = 8'b00000100;
   assign number_4[10] = 8'b00000100;
   assign number_4[11] = 8'b00000100;
   assign number_4[12] = 8'b00000100;
   assign number_4[13] = 8'b00000100;
   assign number_4[14] = 8'b00000100;
   assign number_4[15] = 8'b00000000;

   assign number_5[0]  = 8'b00000000;
   assign number_5[1]  = 8'b01111100;
   assign number_5[2]  = 8'b01000000;
   assign number_5[3]  = 8'b01000000;
   assign number_5[4]  = 8'b01000000;
   assign number_5[5]  = 8'b01000000;
   assign number_5[6]  = 8'b01111000;
   assign number_5[7]  = 8'b00000100;
   assign number_5[8]  = 8'b00000010;
   assign number_5[9]  = 8'b00000010;
   assign number_5[10] = 8'b00000010;
   assign number_5[11] = 8'b00000010;
   assign number_5[12] = 8'b00000010;
   assign number_5[13] = 8'b01000100;
   assign number_5[14] = 8'b00111000;
   assign number_5[15] = 8'b00000000;

   assign number_6[0]  = 8'b00000000;
   assign number_6[1]  = 8'b00001100;
   assign number_6[2]  = 8'b00010000;
   assign number_6[3]  = 8'b00100000;
   assign number_6[4]  = 8'b00100000;
   assign number_6[5]  = 8'b01000000;
   assign number_6[6]  = 8'b01000000;
   assign number_6[7]  = 8'b01011000;
   assign number_6[8]  = 8'b01100100;
   assign number_6[9]  = 8'b01000010;
   assign number_6[10] = 8'b01000010;
   assign number_6[11] = 8'b01000010;
   assign number_6[12] = 8'b01000010;
   assign number_6[13] = 8'b00100100;
   assign number_6[14] = 8'b00011000;
   assign number_6[15] = 8'b00000000;

   assign number_7[0]  = 8'b00000000;
   assign number_7[1]  = 8'b01111110;
   assign number_7[2]  = 8'b00000010;
   assign number_7[3]  = 8'b00000100;
   assign number_7[4]  = 8'b00000100;
   assign number_7[5]  = 8'b00001000;
   assign number_7[6]  = 8'b00001000;
   assign number_7[7]  = 8'b00111100;
   assign number_7[8]  = 8'b00010000;
   assign number_7[9]  = 8'b00010000;
   assign number_7[10] = 8'b00010000;
   assign number_7[11] = 8'b00100000;
   assign number_7[12] = 8'b00100000;
   assign number_7[13] = 8'b00100000;
   assign number_7[14] = 8'b00100000;
   assign number_7[15] = 8'b00000000;

   assign number_8[0]  = 8'b00000000;
   assign number_8[1]  = 8'b00011000;
   assign number_8[2]  = 8'b00100100;
   assign number_8[3]  = 8'b01000010;
   assign number_8[4]  = 8'b01000010;
   assign number_8[5]  = 8'b01000010;
   assign number_8[6]  = 8'b00100100;
   assign number_8[7]  = 8'b00011000;
   assign number_8[8]  = 8'b00100100;
   assign number_8[9]  = 8'b01000010;
   assign number_8[10] = 8'b01000010;
   assign number_8[11] = 8'b01000010;
   assign number_8[12] = 8'b01000010;
   assign number_8[13] = 8'b00100100;
   assign number_8[14] = 8'b00011000;
   assign number_8[15] = 8'b00000000;

   assign number_9[0]  = 8'b00000000;
   assign number_9[1]  = 8'b00011000;
   assign number_9[2]  = 8'b00100100;
   assign number_9[3]  = 8'b01000010;
   assign number_9[4]  = 8'b01000010;
   assign number_9[5]  = 8'b01000010;
   assign number_9[6]  = 8'b01000010;
   assign number_9[7]  = 8'b00100110;
   assign number_9[8]  = 8'b00011010;
   assign number_9[9]  = 8'b00000010;
   assign number_9[10] = 8'b00000010;
   assign number_9[11] = 8'b00000100;
   assign number_9[12] = 8'b00000100;
   assign number_9[13] = 8'b00001000;
   assign number_9[14] = 8'b00110000;
   assign number_9[15] = 8'b00000000;

   assign letter_A[0]  = 8'b00000000;
   assign letter_A[1]  = 8'b00011000;
   assign letter_A[2]  = 8'b00100100;
   assign letter_A[3]  = 8'b00100100;
   assign letter_A[4]  = 8'b00100100;
   assign letter_A[5]  = 8'b01000010;
   assign letter_A[6]  = 8'b01000010;
   assign letter_A[7]  = 8'b01000010;
   assign letter_A[8]  = 8'b01111110;
   assign letter_A[9]  = 8'b01000010;
   assign letter_A[10] = 8'b01000010;
   assign letter_A[11] = 8'b01000010;
   assign letter_A[12] = 8'b01000010;
   assign letter_A[13] = 8'b01000010;
   assign letter_A[14] = 8'b01000010;
   assign letter_A[15] = 8'b00000000;

   assign letter_B[0]  = 8'b00000000;
   assign letter_B[1]  = 8'b01111000;
   assign letter_B[2]  = 8'b01000100;
   assign letter_B[3]  = 8'b01000010;
   assign letter_B[4]  = 8'b01000010;
   assign letter_B[5]  = 8'b01000010;
   assign letter_B[6]  = 8'b01000100;
   assign letter_B[7]  = 8'b01111000;
   assign letter_B[8]  = 8'b01000100;
   assign letter_B[9]  = 8'b01000010;
   assign letter_B[10] = 8'b01000010;
   assign letter_B[11] = 8'b01000010;
   assign letter_B[12] = 8'b01000010;
   assign letter_B[13] = 8'b01000100;
   assign letter_B[14] = 8'b01111000;
   assign letter_B[15] = 8'b00000000;

   assign letter_C[0]  = 8'b00000000;
   assign letter_C[1]  = 8'b00011100;
   assign letter_C[2]  = 8'b00100010;
   assign letter_C[3]  = 8'b00100000;
   assign letter_C[4]  = 8'b01000000;
   assign letter_C[5]  = 8'b01000000;
   assign letter_C[6]  = 8'b01000000;
   assign letter_C[7]  = 8'b01000000;
   assign letter_C[8]  = 8'b01000000;
   assign letter_C[9]  = 8'b01000000;
   assign letter_C[10] = 8'b01000000;
   assign letter_C[11] = 8'b01000000;
   assign letter_C[12] = 8'b00100000;
   assign letter_C[13] = 8'b00100010;
   assign letter_C[14] = 8'b00011100;
   assign letter_C[15] = 8'b00000000;

   assign letter_D[0]  = 8'b00000000;
   assign letter_D[1]  = 8'b01110000;
   assign letter_D[2]  = 8'b01001000;
   assign letter_D[3]  = 8'b01000100;
   assign letter_D[4]  = 8'b01000100;
   assign letter_D[5]  = 8'b01000010;
   assign letter_D[6]  = 8'b01000010;
   assign letter_D[7]  = 8'b01000010;
   assign letter_D[8]  = 8'b01000010;
   assign letter_D[9]  = 8'b01000010;
   assign letter_D[10] = 8'b01000010;
   assign letter_D[11] = 8'b01000100;
   assign letter_D[12] = 8'b01000100;
   assign letter_D[13] = 8'b01001000;
   assign letter_D[14] = 8'b01110000;
   assign letter_D[15] = 8'b00000000;

   assign letter_E[0]  = 8'b00000000;
   assign letter_E[1]  = 8'b01111110;
   assign letter_E[2]  = 8'b01000000;
   assign letter_E[3]  = 8'b01000000;
   assign letter_E[4]  = 8'b01000000;
   assign letter_E[5]  = 8'b01000000;
   assign letter_E[6]  = 8'b01000000;
   assign letter_E[7]  = 8'b01111000;
   assign letter_E[8]  = 8'b01000000;
   assign letter_E[9]  = 8'b01000000;
   assign letter_E[10] = 8'b01000000;
   assign letter_E[11] = 8'b01000000;
   assign letter_E[12] = 8'b01000000;
   assign letter_E[13] = 8'b01000000;
   assign letter_E[14] = 8'b01111110;
   assign letter_E[15] = 8'b00000000;

   assign letter_F[0]  = 8'b00000000;
   assign letter_F[1]  = 8'b01111110;
   assign letter_F[2]  = 8'b01000000;
   assign letter_F[3]  = 8'b01000000;
   assign letter_F[4]  = 8'b01000000;
   assign letter_F[5]  = 8'b01000000;
   assign letter_F[6]  = 8'b01000000;
   assign letter_F[7]  = 8'b01111000;
   assign letter_F[8]  = 8'b01000000;
   assign letter_F[9]  = 8'b01000000;
   assign letter_F[10] = 8'b01000000;
   assign letter_F[11] = 8'b01000000;
   assign letter_F[12] = 8'b01000000;
   assign letter_F[13] = 8'b01000000;
   assign letter_F[14] = 8'b01000000;
   assign letter_F[15] = 8'b00000000;

   assign pixel = halfbyte == 0  ? number_0[y][x] :
		  halfbyte == 1  ? number_1[y][x] :
		  halfbyte == 2  ? number_2[y][x] :
		  halfbyte == 3  ? number_3[y][x] :
		  halfbyte == 4  ? number_4[y][x] :
		  halfbyte == 5  ? number_5[y][x] :
		  halfbyte == 6  ? number_6[y][x] :
		  halfbyte == 7  ? number_7[y][x] :
		  halfbyte == 8  ? number_8[y][x] :
		  halfbyte == 9  ? number_9[y][x] :
		  halfbyte == 10 ? letter_A[y][x] :
		  halfbyte == 11 ? letter_B[y][x] :
		  halfbyte == 12 ? letter_C[y][x] :
		  halfbyte == 13 ? letter_D[y][x] :
		  halfbyte == 14 ? letter_E[y][x] :
		  letter_F[y][x];
endmodule

module vga_timing(input wire       clock_50mhz,
		  input wire 	   reset,

		  output reg 	   h_sync,
		  output reg 	   v_sync,
		  output reg [9:0] col,
		  output reg [8:0] row,
		  output reg 	   display_on,
		  output wire 	   pixel_starting,
		  output wire 	   row_starting);

   parameter h_pixels = 640;
   parameter v_pixels = 480;
   parameter h_front_porch = 16;
   parameter v_front_porch = 10;
   parameter h_pulse = 96;
   parameter v_pulse = 2;
   parameter h_back_porch = 48;
   parameter v_back_porch = 33;
   parameter h_pol = 1'b0;
   parameter v_pol = 1'b1;

   parameter h_pulse_start = h_front_porch;
   parameter v_pulse_start = v_front_porch;
   parameter h_pulse_end = h_front_porch + h_pulse;
   parameter v_pulse_end = v_front_porch + v_pulse;
   parameter h_active_video_start = h_front_porch + h_pulse + h_back_porch;
   parameter v_active_video_start = v_front_porch + v_pulse + v_back_porch;
   parameter h_frame_end = h_active_video_start + h_pixels;
   parameter v_frame_end = v_active_video_start + v_pixels;

   reg [9:0] 			   h_counter;
   reg [9:0] 			   v_counter;

   reg 				   divider; // 25MHz

   wire 			   display_on_next_tick;
   assign display_on_next_tick =  (h_counter < h_frame_end - 1) &&
				  (h_counter >= h_active_video_start - 1) &&
				  (v_counter < v_frame_end) &&
				  (v_counter >= v_active_video_start);

   always @ (posedge clock_50mhz) begin
      if (reset) begin
	 divider <= 1'b0;

	 h_counter <= 0;
	 v_counter <= 0;

	 h_sync <= ~h_pol;
	 v_sync <= ~v_pol;
	 row <= 0;
	 col <= 0;
	 display_on <= 0;
      end // if (reset)
      else begin
	 divider <= divider + 1;

	 if (divider == 1'b1) begin
	    display_on <= display_on_next_tick;

	    if (h_counter < h_frame_end - 1) begin
	       h_counter <= h_counter + 1;

	       h_sync <= h_pol ^ (h_counter < h_pulse_start - 1 || h_counter >= h_pulse_end - 1);

	       if (h_counter >= h_active_video_start)
		 col <= col + 1;
	    end
	    else begin
	       h_counter <= 0;
	       col <= 0;

	       if (v_counter < v_frame_end - 1) begin
		  v_counter <= v_counter + 1;

		  v_sync <= v_pol ^ (v_counter < v_pulse_start - 1 || v_counter >= v_pulse_end - 1);

		  if (v_counter >= v_active_video_start)
		    row <= row + 1;
	       end
	       else begin
		  v_counter <= 0;
		  row <= 0;
	       end // else: !if(v_counter < v_frame_end - 1)
	    end // else: !if(h_counter < h_frame_end - 1)
	 end // if (divider == 1'b1)
      end // else: !if(reset)
   end // always @ (posedge clock_50mhz)

   assign pixel_starting = display_on && divider == 1'b0;
   assign row_starting = pixel_starting && col == 0;
endmodule // vga_timing

module vga_pass_colors(input wire       clock_50mhz,
		       input wire 	reset,

		       input wire 	h_sync_next_tick,
		       input wire 	v_sync_next_tick,
		       input wire 	display_on_next_tick,
		       input wire 	pixel_on_next_tick,

		       output reg 	h_sync,
		       output reg 	v_sync,
		       output reg [2:0] vga_red,
		       output reg [2:0] vga_green,
		       output reg [2:0] vga_blue
		       );

   wire [8:0] 				color_next_tick;
   assign color_next_tick = !display_on_next_tick ? 0 :
			    pixel_on_next_tick ? color1 : color2;

   parameter color1 = {3'b010, 3'b111, 3'b101};
   parameter color2 = {3'b000, 3'b000, 3'b111};

   always @ (posedge clock_50mhz) begin
      if (reset) begin
	 h_sync <= 0;
	 v_sync <= 0;
	 vga_red <= 0;
	 vga_green <= 0;
	 vga_blue <= 0;
      end
      else begin
	 h_sync <= h_sync_next_tick;
	 v_sync <= v_sync_next_tick;

	 vga_red <= color_next_tick[8:6];
	 vga_green <= color_next_tick[5:3];
	 vga_blue <= color_next_tick[2:0];
      end // else: !if(reset)
   end // always @ (posedge clock_50mhz)
endmodule // vga_pass_colors

module vga_hexmode(input wire         clock_50mhz,
		   input wire 	      reset,

		   input wire 	      can_read,
		   input wire [15:0]  read_data,

		   output wire 	      want_to_read,
		   output wire [17:0] read_addr,

		   input wire [17:0]  displayed_memory_base,

		   output wire 	      h_sync,
		   output wire 	      v_sync,
		   output wire [2:0]  vga_red,
		   output wire [2:0]  vga_green,
		   output wire [2:0]  vga_blue
		   );
   wire [9:0] 			      col_next_tick;
   wire [8:0] 			      row_next_tick;
   wire 			      h_sync_next_tick;
   wire 			      v_sync_next_tick;
   wire 			      display_on_next_tick;
   wire 			      pixel_starting_next_tick;
   wire 			      row_starting_next_tick;

   vga_timing timing(clock_50mhz, reset, h_sync_next_tick, v_sync_next_tick,
		     col_next_tick, row_next_tick, display_on_next_tick,
		     pixel_starting_next_tick, row_starting_next_tick);

   parameter queue_size = 4;

   /* will store 4 consecutive hex digits copied from read_data[15:0] */
   reg [15:0] 			      digits_queue;
   reg [1:0] 			      digits_in_queue;

   parameter words_per_line = 20;
   parameter lines = 30;
   parameter words_per_screen = words_per_line * lines;
   parameter digit_h_pixels = 8;
   parameter digit_v_pixels = 16;
   parameter word_digits = 4;

   reg [2:0] 			      digit_x;
   wire [2:0] 			      subsequent_digit_x;
   assign subsequent_digit_x = digit_x == digit_h_pixels - 1 ? 0 : digit_x + 1;

   reg [3:0] 			      digit_y;
   wire [3:0] 			      subsequent_digit_y;
   assign subsequent_digit_y = digit_y == digit_v_pixels - 1 ? 0 : digit_y + 1;

   wire 			      digit_starting_next_tick;
   assign digit_starting_next_tick = pixel_starting_next_tick &&
				     digit_x == digit_h_pixels - 1;

   wire 			      line_starting_next_tick;
   assign line_starting_next_tick = row_starting_next_tick &&
				    digit_y == digit_v_pixels - 1;

   reg [4:0] 			      word_in_line;
   reg [11:0] 			      word_idx;
   wire [11:0] 			      subsequent_word_idx;
   assign subsequent_word_idx = word_idx == words_per_screen - 1 ?
				0 : word_idx + 1;

   assign read_addr = displayed_memory_base + word_idx;

   reg [3:0] 			      current_digit;
   wire [3:0] 			      digit_next_tick;
   assign digit_next_tick = digit_starting_next_tick ?
			    digits_queue[15:12] : current_digit;

   wire 			      pixel_on_next_tick;

   hex_print hex_print(digit_next_tick,
		       pixel_starting_next_tick ? subsequent_digit_x : digit_x,
		       row_starting_next_tick ? subsequent_digit_y : digit_y,
		       pixel_on_next_tick);

   vga_pass_colors color_pass(clock_50mhz, reset, h_sync_next_tick,
			      v_sync_next_tick, display_on_next_tick,
			      pixel_on_next_tick, h_sync, v_sync,
			      vga_red, vga_green, vga_blue);

   /* 0 - waiting for read; 1 - reading; 2 - reading done */
   reg [1:0] 			      reading_state;

   assign want_to_read = reading_state == 0 ||
			 (digit_starting_next_tick &&
			  digits_in_queue - 1 == 0);

   wire 			      reading;
   /* If new read can start, the previous one is ignored */
   assign reading = reading_state == 1 && !(want_to_read);

   reg [11:0] 			      word_idx_at_some_point;
   reg [1:0] 			      digits_in_queue_at_some_point;
   reg [1:0] 			      reading_state_at_some_point;
   always @ (posedge clock_50mhz) begin
      if (reset) begin
	 digits_queue <= 16'bxxxxxxxxxxxxxxxx;
	 digits_in_queue <= 0;
	 reading_state <= 0;
	 digit_x <= digit_h_pixels - 1;
	 digit_y <= digit_v_pixels - 1;
	 word_in_line <= 0;
	 word_idx <= 0; word_idx_at_some_point <= 0; digits_in_queue_at_some_point <= 0; reading_state_at_some_point <= 0;
      end
      else begin
	 if (pixel_starting_next_tick && col_next_tick == 3 && row_next_tick == 1) begin
	    word_idx_at_some_point <= word_idx;
	    digits_in_queue_at_some_point <= digits_in_queue;
	    reading_state_at_some_point <= reading_state;
	 end

	 if (pixel_starting_next_tick)
	   digit_x <= subsequent_digit_x;

	 if (digit_starting_next_tick) begin
	    current_digit <= digit_next_tick;

	    /*
	     * Once there's only 1 digit in the queue, word_idx shall
	     * already be pointing at the next address.
	     */
	    if (digits_in_queue - 3 == 0) begin
	       if (word_in_line == words_per_line - 1) begin
		  word_in_line <= 0;
		  if (digit_y == digit_v_pixels - 1)
		    word_idx <= subsequent_word_idx;
		  else
		    word_idx <= word_idx - (words_per_line - 1);
	       end
	       else begin
		  word_in_line <= word_in_line + 1;
		  word_idx <= subsequent_word_idx;
	       end
	    end

	    /*
	     * Hopefully read will always finish before we pop the next digit
	     * from the queue. But we still need to avoid multiple drivers.
	     */
	    if (!reading)
	      digits_queue <= {digits_queue[11:0], 4'bxxxx};
	 end

	 if (row_starting_next_tick)
	   digit_y <= subsequent_digit_y;

	 digits_in_queue <= digits_in_queue + (reading ? queue_size : 0) -
			    (digit_starting_next_tick ? 1 : 0);

	 if (want_to_read) begin
	    if (can_read)
	      reading_state <= 1;
	    else
	      reading_state <= 0;
	 end
	 else if (reading_state == 1) begin
	    reading_state <= 2;
	    digits_queue <= read_data;
	 end
      end // else: !if(reset)
   end // always @ (posedge clock_50mhz)
endmodule

module on_button_write(input wire         clock_50mhz,
		       input wire 	  reset,

		       input wire 	  can_write,
		       output reg [15:0]  write_data,

		       output wire 	  want_to_write,
		       output wire [17:0] write_addr,

		       input wire [17:0]  written_memory_base,
		       input wire 	  but
		       );
   reg 					  button_pressed;

   /* 0 - waiting for write, 1 - writing, 2 - write complete */
   reg [1:0] 				  write_state;

   parameter cooldown_start = 25'b111111111111111111111111;
   reg [23:0] 				  but_cooldown;

   reg [4:0] 				  write_idx;
   assign write_addr = written_memory_base + write_idx;

   wire 				  perform_action;
   assign perform_action = button_pressed && !but_cooldown;
   assign want_to_write = perform_action || write_state == 0;

   always @ (posedge clock_50mhz) begin
      if (reset) begin
   	 but_cooldown <= cooldown_start;
   	 write_data <= 16'b000100000000000;
   	 write_state <= 2;
	 write_idx <= 0;
	 button_pressed <= 0;
      end
      else begin
	 button_pressed <= !but;

   	 if (perform_action)
   	   but_cooldown <= cooldown_start;
	 else if (but_cooldown)
	   but_cooldown <= but_cooldown - 1;

	 if (want_to_write) begin
   	    if (can_write)
   	      write_state <= 1;
   	    else
   	      write_state <= 0;
   	 end
   	 else if (write_state == 1) begin
   	    write_state <= 2;

	    write_data <= write_data + 1;
   	    write_idx <= write_idx >= 30 ? 0 : write_idx + 1;
   	 end
      end
   end // always @ (posedge clock_50mhz)
endmodule

module dumb_write(input wire         clock_50mhz,
		  input wire 	     reset,

		  input wire 	     can_write,
		  output wire [15:0] write_data,

		  output wire 	     want_to_write,
		  output wire [17:0] write_addr,

		  input wire [17:0]  written_memory_base,
		  input wire 	     start_operating
		  );
   reg [9:0] 			     write_idx;
   /* To each word we'll be writing its offset from written_memory_base */
   assign write_data = write_idx;
   assign write_addr = written_memory_base + write_idx;

   parameter max_idx = 599;
   reg 				     operating;
   assign want_to_write = operating;

   reg [1:0] 			     write_state;

   always @ (posedge clock_50mhz) begin
      if (reset) begin
	 write_idx <= 10'bxxxxxxxxxx;
	 operating <= 0;
      end
      else begin
	 if (start_operating && !operating) begin
	    operating <= 1;
	    write_state <= 0;
	    write_idx <= 0;
	 end
	 else if (operating) begin
	    if (can_write) begin
	       write_idx <= write_idx + 1;
	       if (write_idx == max_idx)
		 operating <= 0;
	    end
	 end
      end // else: !if(reset)
   end // always @ (posedge clock_50mhz)
endmodule // dumb_write

module top(input wire        clock_100mhz,
	   input wire 	     but1,
	   input wire 	     but2,

	   output reg [17:0] sram_addr,
	   inout wire [15:0] sram_io,
	   output reg 	     sram_cs_n,
	   output reg 	     sram_oe_n,
	   output reg 	     sram_we_n,

	   output wire 	     h_sync,
	   output wire 	     v_sync,
	   output wire [2:0] vga_red,
	   output wire [2:0] vga_green,
	   output wire [2:0] vga_blue,

	   output wire 	     sdo,
	   input wire 	     sdi,
	   output wire 	     sck,
	   output wire 	     ss_n
	   );
   reg 			     reset;
   reg 			     err_disabled;

   reg 			     clock_50mhz;
   always @ (posedge clock_100mhz)
     clock_50mhz <= clock_50mhz + 1;

   parameter memory_base = 18'b010101010101010101;

   wire 		     vga_io_available;
   assign vga_io_available = 1;
   wire 		     vga_wants_io;
   wire [17:0] 		     vga_io_addr;

   vga_hexmode vga(clock_50mhz, reset, vga_io_available, sram_io, vga_wants_io,
		   vga_io_addr, memory_base,
		   h_sync, v_sync, vga_red, vga_green, vga_blue);

   reg 			     last_reset;
   always @ (posedge clock_50mhz)
     last_reset <= reset;

   wire 		     dumbwrite_io_available;
   assign dumbwrite_io_available = vga_io_available && !vga_wants_io;

   wire 		     dumbwrite_wants_io;
   wire [17:0] 		     dumbwrite_io_addr;
   wire [15:0] 		     dumbwrite_write_data;

   dumb_write dumbwrite(clock_50mhz, reset, dumbwrite_io_available, dumbwrite_write_data,
			dumbwrite_wants_io, dumbwrite_io_addr, memory_base,
			!reset && last_reset);

   wire 		     butwrite_io_available;
   assign butwrite_io_available = dumbwrite_io_available && !dumbwrite_wants_io;

   wire 		     butwrite_wants_io;
   wire [17:0] 		     butwrite_io_addr;
   wire [15:0] 		     butwrite_write_data;

   on_button_write butwrite(clock_50mhz, reset, butwrite_io_available,
			    butwrite_write_data, butwrite_wants_io,
			    butwrite_io_addr, memory_base, but2);

   reg [15:0] 		     write_data;
   reg 			     write_in_progress;

   always @ (posedge clock_50mhz) begin
      reset <= !but1 || err_disabled;

      if (!but1)
	err_disabled <= 0;

      if (reset) begin
	 err_disabled <= 0;
	 write_in_progress <= 0;
   	 sram_oe_n <= 1'b1;
	 sram_cs_n <= 1'b1;
	 write_data <= 16'bxxxxxxxxxxxxxxxx;
      end
      else begin
	 if (vga_wants_io) begin
	    write_in_progress <= 0;
	    sram_oe_n <= 1'b0;
	    sram_cs_n <= 1'b0;
	    sram_addr <= vga_io_addr;
	 end
	 else if (dumbwrite_wants_io) begin
	    write_in_progress <= 1;
	    sram_oe_n <= 1'b1;
	    sram_cs_n <= 1'b0;
	    sram_addr <= dumbwrite_io_addr;
	    write_data <= dumbwrite_write_data;
	 end
	 else if (butwrite_wants_io) begin
	    write_in_progress <= 1;
	    sram_oe_n <= 1'b1;
	    sram_cs_n <= 1'b0;
	    sram_addr <= butwrite_io_addr;
	    write_data <= butwrite_write_data;
	    if (butwrite_io_addr > memory_base + 30)
	      err_disabled <= 1;
	 end
	 else begin
	    write_in_progress <= 0;
	    sram_oe_n <= 1'b1;
	    sram_cs_n <= 1'b1;
	 end
      end
   end // always @ (posedge clock_50mhz)

   always @ (negedge clock_100mhz) begin
      if (reset) begin
	 sram_we_n <= 1'b1;
      end
      else begin
	 if (write_in_progress)
	   sram_we_n <= !sram_we_n;
	 else
	   sram_we_n <= 1'b1;
      end
   end

   assign sram_io = sram_we_n ? 16'bzzzzzzzzzzzzzzzz : write_data;

   assign sdo = sdi;
   assign sck = 1;
   assign ss_n = 1;

endmodule // top