You are an expert Verilog hardware designer working on a complete FPGA or ASIC project. You are given the full source code of a Verilog project. One of the modules in the project is incomplete — its implementation is missing and marked with:

// >>> Module Implementation Begin

// <<< Module Implementation End

Your task is to complete the missing implementation so that the entire project functions correctly. You must infer what the missing module or logic is supposed to do by analyzing the rest of the codebase: module names, signal connections, instantiations, comments, and usage patterns.

Follow these rules:

1. Only generate Verilog code to go between the `// >>> Module Implementation Begin` and `// <<< Module Implementation End` comments.
2. Use Verilog-2001 syntax. Ensure the code is synthesizable.
3. Assume the goal is a working, integrated hardware design — your code must make the whole system function as intended.
4. Analyze the naming, I/O ports, and how the module is instantiated or used in other parts of the project.
5. Implement correct behavior based on design context. Do not guess randomly — reason it out.
6. Avoid vendor-specific features unless explicitly required.
7. Do not output comments, explanation, or anything outside the required code region unless explicitly asked.

Your output should contain **only the Verilog code** that fits inside the marked region.

/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

module tt_um_zec_square1 (
    input  wire [7:0] ui_in,    // Dedicated inputs
    output wire [7:0] uo_out,   // Dedicated outputs
    input  wire [7:0] uio_in,   // IOs: Input path
    output wire [7:0] uio_out,  // IOs: Output path
    output wire [7:0] uio_oe,   // IOs: Enable path (active high: 0=input, 1=output)
    input  wire       ena,      // always 1 when the design is powered, so you can ignore it
    input  wire       clk,      // clock
    input  wire       rst_n     // reset_n - low to reset
);

  // All output pins must be assigned. If not used, assign to 0.
  assign uio_out[6:0] = 7'b0;
  assign uio_oe  = 8'b1000_0000;

  // List all unused inputs to prevent warnings
  wire _unused = &{ena, ui_in, uio_in, in_frame};

  reg  [1:0] R; // red component           the components are `reg`s to
  reg  [1:0] G; // green component         match the number of reg layers from
  reg  [1:0] B; // blue component          [hv]pos to [hv]sync for inter-signal alignment
  wire vsync;   // VSync
  wire hsync;   // HSync

  wire [9:0] hpos; // X coordinate in frame
  wire [9:0] vpos; // Y coordinate in frame
  wire in_frame;

  hvsync_generator sync_gen(
      .clk(clk),
      .reset(~rst_n),
      .vsync(vsync),
      .hsync(hsync),
      .hpos(hpos),
      .vpos(vpos),
      .display_on(in_frame)
  );


  // Pinout of Tiny VGA Pmod:
  assign uo_out[0] = R[1];
  assign uo_out[1] = G[1];
  assign uo_out[2] = B[1];
  assign uo_out[3] = vsync;
  assign uo_out[4] = R[0];
  assign uo_out[5] = G[0];
  assign uo_out[6] = B[0];
  assign uo_out[7] = hsync;


  // frame counter
  reg [8:0] frame_no;
  // last clock cycle's value of VSync
  reg prev_vsync;

  always @(posedge clk) begin
    if (~rst_n) begin
      frame_no <= 9'd0;
      prev_vsync <= 1;
    end
    else begin
      if (vsync & ~prev_vsync) begin // positive edge
        frame_no <= frame_no + 9'd1;
      end
      prev_vsync <= vsync;
    end
  end


  // number of frames we emulate phosphor persistence for
  `define N_LAG 15

  // color_controls[i] and color are 3-bit color indexes where
  // 3'b0_00 is black, 3'b0_11 is bright cyan, and 3'b1_xx is shades of yellow

  wire [2:0] color_controls [(`N_LAG-1):0];
  assign color_controls[0]  = 3'b0_11; // bright cyan for lag 0...
  assign color_controls[1]  = 3'b1_11; // ...changing to bright yellow...
  assign color_controls[2]  = 3'b1_11;
  assign color_controls[3]  = 3'b1_10; // ...fading...
  assign color_controls[4]  = 3'b1_10;
  assign color_controls[5]  = 3'b1_10;
  assign color_controls[6]  = 3'b1_10;
  assign color_controls[7]  = 3'b1_01; // ...to dim yellow...
  assign color_controls[8]  = 3'b1_01;
  assign color_controls[9]  = 3'b1_01;
  assign color_controls[10] = 3'b1_01;
  assign color_controls[11] = 3'b1_01;
  assign color_controls[12] = 3'b1_01;
  assign color_controls[13] = 3'b1_01;
  assign color_controls[14] = 3'b1_01; // ...before going black after lag 14.

  wire [2:0] color_maybe [(`N_LAG-1):0];

  genvar i, j;
  generate
    for (i = 0; i < `N_LAG; i = i + 1) begin
      wire [8:0] delay = i;
      // here's the actual munching-squares algorithm:
      assign color_maybe[i] = (hpos[8:0] == (vpos[8:0] ^ (frame_no - delay))) ? color_controls[i] : 3'b0_00;
    end
  endgenerate

  // we can only do reducing operations over the least-significant dimension,
  // so we need to exchange dimensions in color_maybe before calculating
  // color
  wire [(`N_LAG-1):0] cm_transposed [2:0];

  generate
    for (i = 0; i < `N_LAG; i = i + 1) begin
      for (j = 0; j < 3; j = j + 1) begin
        assign cm_transposed[j][i] = color_maybe[i][j];
      end
    end
  endgenerate

  wire [2:0] color;

  generate
    for (i = 0; i < 3; i = i + 1) begin
      assign color[i] = rst_n & |cm_transposed[i];
    end
  endgenerate

  always @(posedge clk) begin
    if (!rst_n) begin
      {R, G, B} <= 6'd0;
    end
    else begin
      // map from 3-bit color indexes to color components, and clip to
      // a 512x480 window
      R <= ((vpos < 480) & (hpos < 512)) ? color[1:0] & {2{color[2]}} : 2'b00;
      G <= ((vpos < 480) & (hpos < 512)) ? color[1:0] : 2'b00;
      B <= ((vpos < 480) & (hpos < 512)) ? color[1:0] & {2{~color[2]}} : 2'b00;
    end
  end


  // our sound, neatly contained in a module:

  logistic_snd #(
    .N_OSC(8),
    .ITER_LEN(15_361),
    .R_INC(2),
    .FRAC(16),
    .PHASE_BITS(16),
    .FREQ_RES(0)
  ) project_audio(
    .clk(clk),
    .reset(~rst_n),
    .snd(uio_out[7])
  );

endmodule


/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// This intentionally has the same interface as the "hvsync_generator"
// module in the VGA Playground (https://tinytapeout.github.io/vga-playground/)
// to make it easy to copy-and-paste code for _in silico_ prototyping.
module hvsync_generator(
  input  wire       clk,   // clock, assumed to be ~25.175 or 25.2 MHz
                           // (for 640x480 pixel, 59.94 or 60 Hz video)
  input  wire       reset, // reset (active HIGH)
  output reg        vsync, // VSync
  output reg        hsync, // HSync
  output reg  [9:0] hpos,  // horizontal position in frame
  output reg  [9:0] vpos,  // vertical position in frame
  output wire       display_on  // are we in the "addressable" part of the frame
                                // (i.e., the part which actually contains image data)?
);
// >>> Module Implementation Begin

// <<< Module Implementation End
endmodule




/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// A sonification of the logistic map (https://en.wikipedia.org/wiki/Logistic_map):
//
//         x_(n+1) := r * x_n * (1 - x_n)     (0 < x_n < 1, 1 < r < 4)
//
// as PWM'ed audio.

module logistic_snd #(
  parameter N_OSC = 4,       // number of square-wave generators running
  parameter ITER_LEN = 100,  // number of clock cycles for an iteration of the map
  parameter R_INC = 1000,    // the number of iterations of the map after which
                             // we should update 'r'

  parameter FREQ  = 30'd25_200_000, // frequency of clk (Hz)
  parameter LO_F  = 200,     // frequency corresponding to x_n = 0 (Hz)
  parameter HI_F  = 1200,    // frequency corresponding to x_n = 1 (Hz)

  parameter FRAC  = 8,       // fractional part of r, x

  parameter PHASE_BITS = 12, // number of bits in phase accumulators
  parameter FREQ_RES   = 0   // approximate frequency resolution
                             // of square-wave generators is 2^FREQ_RES Hz
) (
  input  wire clk,   // clock
  input  wire reset, // reset (active HIGH)
  output wire snd    // PWM audio
);

  // this section implements the logistic map,
  // changing r every R_INC iterations of the map

  wire [(FRAC-1):0] x;  // the 'x' variable in 0.FRAC fixed-point
  wire next_x_ready;    // this is 1 when a new value of 'x' is produced by the
                        // logs_iterate_map block

  reg [(2+FRAC-1):0] r; // the 'r' variable in 2.FRAC fixed-point

  parameter R_COUNTER_BITS = (R_INC >= 2) ? $clog2(R_INC) : 1;

  reg [(R_COUNTER_BITS-1):0] r_counter;

  parameter INITIAL_R = (1 << FRAC) | (1 << (FRAC - 4)); // 1.0625

  // increment more in the area r < 3; this makes us spend more time in 3 <= r < 4
  wire [1:0] r_top = r[(2+FRAC-1):FRAC];
  wire [(2+FRAC-1):0] r_increment = (r_top < 2'b11) ? 4 : 1;

  always @(posedge clk) begin
    if (reset) begin
      r <= INITIAL_R; // initialize 'r' to 1.0625
      r_counter <= 0;
    end
    else begin
      if (next_x_ready & (r_counter >= (R_INC-1))) begin
        r <= (&r) ? INITIAL_R : r + r_increment;  // increment, wrapping from 4.0 to INITIAL_R
      end

      if (next_x_ready) begin
        r_counter <= (r_counter >= (R_INC-1)) ? 0 : r_counter + 1;
      end
    end
  end

  logs_iterate_map #(FRAC, ITER_LEN) iter(
    .clk(clk),
    .reset(reset),

    .r(r),
    .x(x),

    .next_ready(next_x_ready)
  );


  // this section implements the square-wave generators, the frequencies
  // being derived from values of 'x'

  // we make use of the fact that 25_200_000 is divisible by 128
  parameter LOW_FREQUENCY  = (LO_F << (PHASE_BITS + PHASE_DEC - 7)) / (FREQ >> 7);
  parameter HIGH_FREQUENCY = (HI_F << (PHASE_BITS + PHASE_DEC - 7)) / (FREQ >> 7);
  parameter FREQUENCY_INC  = HIGH_FREQUENCY - LOW_FREQUENCY;

  wire [(PHASE_BITS + FRAC - 1):0] low_frequency_w = LOW_FREQUENCY;
  wire [(PHASE_BITS + FRAC - 1):0] frequency_inc_w = FREQUENCY_INC;
  wire [(PHASE_BITS + FRAC - 1):0] x_scaled_product
    = low_frequency_w + (frequency_inc_w * {{(PHASE_BITS){1'b0}}, x});

  // the 'x' value, appropriately scaled to a frequency for the NCOs
  wire [(PHASE_BITS-2):0] scaled_x = x_scaled_product[(PHASE_BITS + FRAC - 2):FRAC];

  // the frequency registers for the NCOs
  reg [(PHASE_BITS-2):0] freq [(N_OSC-1):0];

  parameter FC_LEN = $clog2(N_OSC);

  // which square wave's frequency should we update now?
  reg [(FC_LEN-1):0] f_counter;

  // the number of oscillators we care about at any given time, minus one
  wire [(FC_LEN-1):0] max_n_osc;
  // which oscillators we care about at any given time
  wire [(N_OSC-1):0] osc_mask;

  wire [(FC_LEN-1):0] max_osc_default = N_OSC - 1;
  parameter N_OSC_6 = (N_OSC < 6) ? N_OSC : 6 * (N_OSC / 6);
  wire [(FC_LEN-1):0] max_osc_6 = N_OSC_6 - 1;
  parameter N_OSC_5 = (N_OSC < 5) ? N_OSC : 5 * (N_OSC / 5);
  wire [(FC_LEN-1):0] max_osc_5 = N_OSC_5 - 1;

  // so, why change the number of used oscillators?
  // because, in non-chaotic regions, it sounds better when
  // you use a multiple of stable cycle length.

  // quick and dirty selection of some regions for said special treatment:

  assign {max_n_osc, osc_mask}
    = (r[(FRAC+1):(FRAC-6)] == 8'b11_101000) ? {max_osc_6, {(N_OSC - N_OSC_6){1'b0}}, {N_OSC_6{1'b1}}} :
      ((r[(FRAC+1):(FRAC-4)] == 6'b11_1101) & ~&r[(FRAC-5):(FRAC-6)]) ? {max_osc_6, {(N_OSC - N_OSC_6){1'b0}}, {N_OSC_6{1'b1}}} :
      (r[(FRAC+1):(FRAC-5)] == 7'b11_10111) ? {max_osc_5, {(N_OSC - N_OSC_5){1'b0}}, {N_OSC_5{1'b1}}} :
      {max_osc_default, {N_OSC{1'b1}}};

  integer j;

  always @(posedge clk) begin
    if (reset) begin
      f_counter <= 0;

      for (j = 0; j < N_OSC; j = j + 1) begin
        freq[j] <= 0;
      end
    end
    else if (next_x_ready) begin
      freq[f_counter] <= scaled_x;
      f_counter <= (f_counter >= max_n_osc) ? 0 : f_counter + 1;
    end
  end

  // the output of the square-wave generators
  wire [(N_OSC-1):0] osc;

  // log2(slowdown of phase accumulators from clk)
  parameter PHASE_DEC = $clog2(FREQ) - PHASE_BITS - FREQ_RES;

  wire nco_increment;
  logs_divider #(1 << PHASE_DEC) nco_increment_gen(
    .clk(clk),
    .reset(reset),
    .mod_n(nco_increment)
  );

  genvar i;
  generate
    for (i = 0; i < N_OSC; i = i + 1) begin
      logs_nco #(PHASE_BITS) n_c_oh_my(
        .clk(clk),
        .reset(reset),
        .step(nco_increment),
        .freq_in(freq[i]),
        .snd(osc[i])
      );
    end
  endgenerate


  // this section mixes the square waves and output the result!

  logs_mixer #(N_OSC, $clog2(N_OSC + 1)) mixer(
    .clk(clk),
    .reset(reset),
    .audio_in(osc),
    .audio_mask(osc_mask),
    .audio_out(snd)
  );

endmodule


/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// performs iterations of the logistic map: x <- r * x * (1 - x)

module logs_iterate_map #(
  parameter FRAC = 4,     // number of fraction bits in numbers
  parameter ITER_LEN = 20 // length of an iteration in clock cycles
) (
  input  wire                clk,   // clock
  input  wire                reset, // reset (active HIGH)

  input  wire [(2+FRAC-1):0] r,  // the map parameter 'r' (2.FRAC fixed-point)

  output reg  [(FRAC-1):0]   x,         // 'x', the value we iteratively apply the map to (0.FRAC fixed-point)
  output reg                 next_ready // set to 1 when we have finished calculating a new value of 'x'
);
  // initial value of 'x'; 0.0625
  parameter INITIAL_X = 1 << (FRAC - 4);

  // size of multiplier output
  parameter MULT_SZ = FRAC + (FRAC + 2);

  // we multiply using an iterative shift-and-add algorithm

  reg [(MULT_SZ-1):0] mult1_shift; // shifted version of first multiplicand for multiplier
  reg [(FRAC-1):0]    mult2_shift; // shifted version of second multiplicand for multiplier
  reg [(MULT_SZ-1):0] mult_accum;  // the multiplier's accumulator

  // this module operates on a cycle lasting at least (2 * FRAC + 3) clock cycles, to wit:
  //
  //            cycle(s)     :        action(s)
  //              0          : mult1 := x;  mult2 := "1 - x";  accum := 0
  //        1 ..= FRAC       : if (mult2[0]) { accum += mult1 }; mult1 <<= 1; mult2 <<= 1
  //              FRAC + 1   : mult1 := r;  mult2 := accum >> [suitable shift]; accum := 0
  // (FRAC+2) ..= (2*FRAC+1) : if (mult2[0]) { accum += mult1 }; mult1 <<= 1; mult2 <<= 1
  //            2 * FRAC + 2 : x := accum >> [suitable shift]
  //      remainder (if any) : idle

  parameter CYCLE_LEN = (ITER_LEN >= (2 * FRAC + 3)) ? ITER_LEN : 2 * FRAC + 3;

  reg [($clog2(CYCLE_LEN)-1):0] counter;

  always @(posedge clk) begin
    if (reset) begin
      x <= INITIAL_X;
      next_ready <= 0;
      counter <= 0;
    end
    else begin
      next_ready <= 0;

      if (counter == 0) begin
        mult_accum <= 0;
        mult1_shift <= {{(MULT_SZ - FRAC){1'b0}}, x};
        mult2_shift <= ~x;  // in fixed-point, essentially 1-x
      end
      else if ( ((counter > 0       ) & (counter <= FRAC         ))
              | ((counter > (FRAC+1)) & (counter <= (FRAC+FRAC+1)))) begin

        if (mult2_shift[0]) begin
          mult_accum <= mult_accum + mult1_shift;
        end
        mult1_shift <= {mult1_shift[(MULT_SZ-2):0], 1'b0};
        mult2_shift <= {1'b0, mult2_shift[(FRAC-1):1]};

      end
      else if (counter == (FRAC + 1)) begin
        mult1_shift <= {{(MULT_SZ - FRAC - 2){1'b0}}, r};
        mult2_shift <= mult_accum[(MULT_SZ - 3):(MULT_SZ - FRAC - 2)]; // x * (1 - x)
        mult_accum <= 0;
      end
      else if (counter == (FRAC+FRAC+2)) begin
        x <= mult_accum[(MULT_SZ - 3):(MULT_SZ - FRAC - 2)]; // r * x * (1 - x)
        next_ready <= 1;
      end

      counter <= (counter >= (CYCLE_LEN - 1)) ? 0 : counter + 1;
    end
  end
endmodule


/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// mixer: sums the input lines, then uses that to drive a PWM

module logs_mixer #(
  parameter N = 1,   // number of audio inputs
  parameter K = 2    // PWM effectively divides the output by 2^K
) (
  input  wire clk,      // clock
  input  wire reset,    // reset (active HIGH)
  input  wire [(N-1):0] audio_in, // input audio lines
  input  wire [(N-1):0] audio_mask, // which lines to actually mix
  output reg  audio_out // output audio line
);
  parameter SUM_WIDTH = $clog2(N + 1);

  // PWM counter
  reg [(K-1):0] counter;

  wire [(K-1):0] sum;

  generate
    if (K > SUM_WIDTH) begin : gen_zero_pad
      assign sum[(K-1):SUM_WIDTH] = 0;
    end
  endgenerate

  // sum the input lines...
  logs_popcount #(N) popcount(
    .word(audio_in & audio_mask),
    .sum(sum[(SUM_WIDTH-1):0])
  );

  always @(posedge clk) begin
    if (reset) begin
      audio_out <= 0;
      counter <= 0;
    end
    else begin
      audio_out <= (counter < sum); // ...and use the sum as the PWM duty cycle
      counter <= counter + 1;
    end
  end
endmodule


/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// sets `sum` to the sum of the NBITS bits of `word`

module logs_popcount #(
  parameter NBITS = 3
) (
  input  wire [(NBITS-1):0]           word,  // the word whose bits we're going to sum
  output wire [($clog2(NBITS+1)-1):0] sum    // the resulting population count
);

  parameter HALF = NBITS / 2;

  // widths
  parameter WHALF = $clog2(HALF + 1);
  parameter WREST = $clog2(NBITS - HALF + 1);
  parameter W_ALL = $clog2(NBITS + 1);

  generate
    if (NBITS == 1) begin : gen_1bit
      assign sum = word[0];
    end
    else if (NBITS == 2) begin : gen_2bits
      assign sum = {1'b0,word[0]} + {1'b0,word[1]};
    end
    else if (NBITS == 3) begin : gen_3bits
      assign sum = {1'b0,word[0]} + {1'b0,word[1]} + {1'b0,word[2]};
    end
    else begin : gen_recurse
      wire [(WHALF-1):0] lo_sum;
      wire [(WREST-1):0] hi_sum;

      logs_popcount #(HALF) low_bits(
        .word(word[(HALF-1):0]),
        .sum(lo_sum)
      );

      logs_popcount #(NBITS - HALF) high_bits(
        .word(word[(NBITS-1):HALF]),
        .sum(hi_sum)
      );

      assign sum
        = { {(W_ALL-WHALF){1'b0}}, lo_sum } + { {(W_ALL-WREST){1'b0}}, hi_sum };
    end
  endgenerate

endmodule


/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// numerically-controlled oscillator: generates a square wave
// of frequency f(step) * (freq_in / 2^N)

module logs_nco #(
  parameter N = 5   // number of bits in phase accumulator
) (
  input  wire clk,     // clock
  input  wire reset,   // reset (active HIGH)
  input  wire step,    // whether to step our logic
  input  wire [(N-2):0] freq_in, // frequency (in units of [frequency of clock] / 2^N)
  output reg  snd      // square wave out
);

  // our phase accumulator
  reg [(N-1):0] phase;

  always @(posedge clk) begin
    if (reset) begin
      phase <= 0;
      snd <= 0;
    end
    else if (step) begin
      snd <= phase[N-1];
      phase <= phase + {1'b0,freq_in};
    end
  end
endmodule


/*
 * Copyright (c) 2024 Zachary Catlin
 * SPDX-License-Identifier: Apache-2.0
 */

`default_nettype none

// generates a pulse once every N clocks

module logs_divider #(
  parameter N = 2
) (
  input  wire clk,    // clock
  input  wire reset,  // reset (active HIGH)
  output reg  mod_n   // output (HIGH once every N clocks)
);

  parameter NBITS = $clog2(N);
  reg [(NBITS-1):0] counter;

  always @(posedge clk) begin
    if (reset) begin
      counter <= 0;
      mod_n <= 0;
    end
    else begin
      mod_n   <= ~|counter; // "counter == 0"
      counter <= (counter >= (N-1)) ? 0 : counter + 1;
    end
  end
endmodule


Reference solution:
rtl/generated.v: 
// Like other VGA timing modules, we rotate the parts of the line and frame
// from the depiction in, e.g., VESA standards
// so as to have (hpos, vpos) == (0, 0) be the first pixel of image data.

// length of the addressable portion of a line (all H lengths in pixels)
`define H_ADDR 640
// length of the right border and front porch of a line
`define H_FRONT 16
// length of the HSync pulse
`define H_SYNC 96
// length of the back porch and left porder of a line
`define H_BACK 48

// height of the addressible portion of a frame (all V heights in lines)
`define V_ADDR 480
// height of the bottom border and front porch of a frame
`define V_FRONT 10
// height of the VSync pulse
`define V_SYNC 2
// height of the back porch and top border of a frame
`define V_BACK 33

// For VGA video, VSync and HSync are active-low.

always @(posedge clk) begin
  if (reset) begin
    {vsync, hsync} <= 2'b11;
    hpos <= 10'd0;
    vpos <= 10'd0;
  end
  else begin
    hsync <= ~((hpos >= (`H_ADDR + `H_FRONT)) & (hpos < (`H_ADDR + `H_FRONT + `H_SYNC)));
    vsync <= ~((vpos >= (`V_ADDR + `V_FRONT)) & (vpos < (`V_ADDR + `V_FRONT + `V_SYNC)));
    hpos <= (hpos >= (`H_ADDR + `H_FRONT + `H_SYNC + `H_BACK - 1)) ? 10'd0 : hpos + 10'd1;
    if (hpos >= (`H_ADDR + `H_FRONT + `H_SYNC + `H_BACK - 1)) begin
      vpos <= (vpos >= (`V_ADDR + `V_FRONT + `V_SYNC + `V_BACK - 1)) ? 10'd0 : vpos + 10'd1;
    end
  end
end

wire out_of_h_addr = hpos[9] & |hpos[8:7]; // "hpos >= 640"
wire out_of_v_addr = vpos[9] | &vpos[8:5]; // "vpos >= 480"

assign display_on = ~(out_of_h_addr | out_of_v_addr);