diff --git a/codemirror b/codemirror
index ebf81921..4334727f 160000
--- a/codemirror
+++ b/codemirror
@@ -1 +1 @@
-Subproject commit ebf81921f80024e3e94c7a8e1235ab59464fa5ef
+Subproject commit 4334727f0e07acd4541b0a7b8f81a5984cd4aafe
diff --git a/demo.html b/demo.html
index 6da52dd3..865e2d57 100644
--- a/demo.html
+++ b/demo.html
@@ -34,9 +34,10 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
     <nav class="navbar navbar-inverse navbar-fixed-top">
       <div class="container">
         <div class="navbar-header">
-          <a class="navbar-brand" href="https://twitter.com/8bitworkshop">@8bitworkshop</a>
+          <span class="navbar-brand"><a target="_new" href="https://twitter.com/8bitworkshop" class="twitter-follow-button" data-show-count="false">Follow @8bitworkshop</a></span>
+          <script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script>
           <a class="navbar-brand" href="./blog/">Blog</a>
-          <a class="navbar-brand" href="https://www.amazon.com/default/e/B01N7J10NF/ref=dp_byline_cont_pop_book_1">Books</a>
+          <a class="navbar-brand" href="https://www.amazon.com/default/e/B01N7J10NF/ref=dp_byline_cont_pop_book_1" target="_blank" onclick="ga('send', 'event', 'books', 'click');">Books</a>
         </div>
         <div id="navbar" class="hidden-sm hidden-xs">
          <form class="navbar-form navbar-right">
@@ -80,11 +81,12 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
           We've got debugging tools too; single step through your code and use our CPU Cycle Analyzer to
           develop that perfect Stella kernel.
           </p>
+          <img class="img-responsive" src="images/atarivcs.jpg">
         </div>
         <div class="col-md-4">
           <h2>Arcade Games</h2>
           <p>
-          You can develop your own games on classic arcade game hardware, using our in-browser C compiler.
+          You can develop your own games on classic arcade game hardware, using our in-browser C compiler targeting the Z80 CPU.
           Platforms include
           <a href="redir.html?platform=vicdual">VIC Dual</a>,
           <a href="redir.html?platform=mw8080bw">Midway 8080</a>,
@@ -92,8 +94,54 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
           <a href="redir.html?platform=vector-z80color">Atari Vector</a>,
           and
           <a href="redir.html?platform=williams-z80">Williams</a>.
+          <img class="img-responsive" src="images/arcadegames.jpg">
           </p>
        </div>
+       <div class="col-md-4">
+<p>
+We're always adding new things here, so subscribe to our mailing list for updates!
+</p>
+<style> .gumroad-follow-form-embed { zoom: 1; } .gumroad-follow-form-embed:before, .gumroad-follow-form-embed:after { display: table; line-height: 0; content: ""; } .gumroad-follow-form-embed:after { clear: both; } .gumroad-follow-form-embed * { margin: 0; border: 0; padding: 0; outline: 0; box-sizing: border-box !important; float: left !important; } .gumroad-follow-form-embed input { border-radius: 4px; border-top-right-radius: 0; border-bottom-right-radius: 0; font-family: -apple-system, ".SFNSDisplay-Regular", "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 15px; line-height: 20px; background: #fff; border: 1px solid #ddd; border-right: 0; color: #aaa; padding: 10px; box-shadow: inset 0 1px 0 rgba(0, 0, 0, 0.02); background-position: top right; background-repeat: no-repeat; text-rendering: optimizeLegibility; font-smoothing: antialiased; -webkit-appearance: none; -moz-appearance: caret; width: 65% !important; height: 40px !important; } .gumroad-follow-form-embed button { border-radius: 4px; border-top-left-radius: 0; border-bottom-left-radius: 0; box-shadow: 0 1px 1px rgba(0, 0, 0, 0.12); -webkit-transition: all .05s ease-in-out; transition: all .05s ease-in-out; display: inline-block; padding: 11px 15px 12px; cursor: pointer; color: #fff; font-size: 15px; line-height: 100%; font-family: -apple-system, ".SFNSDisplay-Regular", "Helvetica Neue", Helvetica, Arial, sans-serif; background: #36a9ae; border: 1px solid #31989d; filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#5ccfd4, endColorstr=#329ca1, GradientType=0)"; background: -webkit-linear-gradient(#5ccfd4, #329ca1); background: linear-gradient(to bottom, #5ccfd4, #329ca1); height: 40px !important; width: 35% !important; } </style> <form action="https://gumroad.com/follow_from_embed_form" class="form gumroad-follow-form-embed" method="post"> <input name="seller_id" type="hidden" value="1507538753888"> <input name="email" placeholder="Your email address" type="email"> <button data-custom-highlight-color="" type="submit">Subscribe</button> </form>
+<p></p>
+<!-- Begin MailChimp Signup Form -->
+<!--
+<link href="//cdn-images.mailchimp.com/embedcode/classic-10_7.css" rel="stylesheet" type="text/css">
+<style type="text/css">
+	#mc_embed_signup{background:#fff; clear:left; font:14px Helvetica,Arial,sans-serif; }
+	/* Add your own MailChimp form style overrides in your site stylesheet or in this style block.
+	   We recommend moving this block and the preceding CSS link to the HEAD of your HTML file. */
+</style>
+<div id="mc_embed_signup">
+<form action="https://8bitworkshop.us19.list-manage.com/subscribe/post?u=819ee5274c8c9c1afe2005023&amp;id=75411a270f" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate>
+    <div id="mc_embed_signup_scroll">
+	<h2>We're always adding new things here, so subscribe to our mailing list for updates!</h2>
+<div class="indicates-required"><span class="asterisk">*</span> indicates required</div>
+<div class="mc-field-group">
+	<label for="mce-EMAIL">Email Address  <span class="asterisk">*</span>
+</label>
+	<input type="email" value="" name="EMAIL" class="required email" id="mce-EMAIL">
+</div>
+<div class="mc-field-group">
+	<label for="mce-FNAME">First Name (optional)</label>
+	<input type="text" value="" name="FNAME" class="" id="mce-FNAME">
+</div>
+<div class="mc-field-group">
+	<label for="mce-LNAME">Last Name (optional)</label>
+	<input type="text" value="" name="LNAME" class="" id="mce-LNAME">
+</div>
+	<div id="mce-responses" class="clear">
+		<div class="response" id="mce-error-response" style="display:none"></div>
+		<div class="response" id="mce-success-response" style="display:none"></div>
+	</div>
+    <div style="position: absolute; left: -5000px;" aria-hidden="true"><input type="text" name="b_819ee5274c8c9c1afe2005023_75411a270f" tabindex="-1" value=""></div>
+    <div class="clear"><input type="submit" value="Subscribe" name="subscribe" id="mc-embedded-subscribe" class="button"></div>
+    </div>
+</form>
+</div>
+<script type='text/javascript' src='//s3.amazonaws.com/downloads.mailchimp.com/js/mc-validate.js'></script><script type='text/javascript'>(function($) {window.fnames = new Array(); window.ftypes = new Array();fnames[0]='EMAIL';ftypes[0]='email';fnames[1]='FNAME';ftypes[1]='text';fnames[2]='LNAME';ftypes[2]='text';fnames[3]='ADDRESS';ftypes[3]='address';fnames[4]='PHONE';ftypes[4]='phone';}(jQuery));var $mcj = jQuery.noConflict(true);</script>
+-->
+<!--End mc_embed_signup-->
+       </div>
        <!--
        <div class="col-md-4">
           <h2>Hardware Design</h2>
@@ -113,12 +161,12 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
     <div class="container">
       <div class="row">
         <div class="col-md-3">
-        <a target="_blank"  href="https://www.amazon.com/gp/product/1541021304/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1541021304&linkCode=as2&tag=pzp-20&linkId=c149f6365c0a676065eb6d7c5f8dd6ae">
+        <a target="_blank"  href="https://www.amazon.com/gp/product/1541021304/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1541021304&linkCode=as2&tag=pzp-20&linkId=c149f6365c0a676065eb6d7c5f8dd6ae" onclick="ga('send', 'event', 'books', 'click', 'vcs');">
 <img border="0" src="//ws-na.amazon-adsystem.com/widgets/q?_encoding=UTF8&MarketPlace=US&ASIN=1541021304&ServiceVersion=20070822&ID=AsinImage&WS=1&Format=_SL250_&tag=pzp-20" ></a>
 <img src="//ir-na.amazon-adsystem.com/e/ir?t=pzp-20&l=am2&o=1&a=1541021304" />
         </div>
         <div class="col-md-8">
-        <a target="_blank"  href="https://www.amazon.com/gp/product/1541021304/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1541021304&linkCode=as2&tag=pzp-20&linkId=c149f6365c0a676065eb6d7c5f8dd6ae">
+        <a target="_blank"  href="https://www.amazon.com/gp/product/1541021304/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1541021304&linkCode=as2&tag=pzp-20&linkId=c149f6365c0a676065eb6d7c5f8dd6ae" onclick="ga('send', 'event', 'books', 'click', 'vcs');">
           <h3>Making Games For The Atari 2600</h3>
         </a>
         <p>The Atari 2600 was released in 1977, and now there's finally a book about how to write games for it! You'll learn about the 6502 CPU, NTSC frames, scanlines, cycle counting, players, missiles, collisions, procedural generation, pseudo-3D, and more. While using the manual, take advantage of our Web-based IDE to write 6502 assembly code, and see your code run instantly in the browser. We'll cover the same programming tricks that master programmers used to make classic games. Create your own graphics and sound, and share your games with friends!</p>
@@ -126,18 +174,31 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
       </div>
       <div class="row">
         <div class="col-md-3">
-<a target="_blank"  href="https://www.amazon.com/gp/product/1545484759/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1545484759&linkCode=as2&tag=pzp-20&linkId=2f3c42111bffe3466830939fff4053fc">
+        <a target="_blank"  href="https://www.amazon.com/gp/product/1545484759/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1545484759&linkCode=as2&tag=pzp-20&linkId=2f3c42111bffe3466830939fff4053fc" onclick="ga('send', 'event', 'books', 'click', 'arcade');">
 <img border="0" src="//ws-na.amazon-adsystem.com/widgets/q?_encoding=UTF8&MarketPlace=US&ASIN=1545484759&ServiceVersion=20070822&ID=AsinImage&WS=1&Format=_SL250_&tag=pzp-20" ></a>
 <img src="//ir-na.amazon-adsystem.com/e/ir?t=pzp-20&l=am2&o=1&a=1545484759" />
         </div>
         <div class="col-md-8">
-        <a target="_blank"  href="https://www.amazon.com/gp/product/1545484759/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1545484759&linkCode=as2&tag=pzp-20&linkId=2f3c42111bffe3466830939fff4053fc">
+        <a target="_blank"  href="https://www.amazon.com/gp/product/1545484759/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1545484759&linkCode=as2&tag=pzp-20&linkId=2f3c42111bffe3466830939fff4053fc" onclick="ga('send', 'event', 'books', 'click', 'arcade');">
           <h3>Making 8-Bit Arcade Games in C</h3>
         </a>
         <p>With this book, you'll learn all about the hardware of Golden Age 8-bit arcade games produced in the late 1970s to early 1980s. We'll learn how to use the C programming language to write code for the Z80 CPU. The following arcade platforms are covered: * Midway 8080 (Space Invaders) * VIC Dual (Carnival) * Galaxian/Scramble (Namco) * Atari Color Vector * Williams (Defender, Robotron) We'll describe how to create video and sound for each platform. Use the online 8bitworkshop IDE to compile your C programs and play them right in the browser!</p>
         </div>
       </div>
     </div>
+
+
+    <div class="container">
+      <div class="row">
+        <div class="col-md-3">
+        </div>
+        <div class="col-md-6">
+        </div>
+        <div class="col-md-3">
+        </div>
+      </div>
+    </div>
+
     
       <hr>
 
diff --git a/doc/notes.txt b/doc/notes.txt
index 30e744fe..5ffa5f30 100644
--- a/doc/notes.txt
+++ b/doc/notes.txt
@@ -48,6 +48,9 @@ TODO:
 - compile stuck when errors unchanged
 - sound mute?
 - $error updates source editor
+- Revert loads cached files?
+- Verilog compile spins forever?
+- how to revert included files?
 
 WEB WORKER FORMAT
 
diff --git a/presets/verilog/ball_absolute.v b/presets/verilog/ball_absolute.v
index 0de14ae6..b5dc5df7 100644
--- a/presets/verilog/ball_absolute.v
+++ b/presets/verilog/ball_absolute.v
@@ -10,16 +10,18 @@ module ball_absolute_top(clk, reset, hsync, vsync, rgb);
   wire [8:0] hpos;
   wire [8:0] vpos;
   
-  reg [8:0] ball_hpos;
-  reg [8:0] ball_vpos;
+  reg [8:0] ball_hpos;	// ball current X position
+  reg [8:0] ball_vpos;	// ball current Y position
   
-  reg [8:0] ball_horiz_initial = 128;
-  reg [8:0] ball_horiz_move = -2;
-  reg [8:0] ball_vert_initial = 128;
-  reg [8:0] ball_vert_move = 2;
+  reg [8:0] ball_horiz_move = -2;	// ball current X velocity
+  reg [8:0] ball_vert_move = 2;		// ball current Y velocity
   
-  localparam BALL_SIZE = 4;
+  localparam ball_horiz_initial = 128;	// ball initial X position
+  localparam ball_vert_initial = 128;	// ball initial Y position
   
+  localparam BALL_SIZE = 4;		// ball size (in pixels)
+
+  // video sync generator
   hvsync_generator hvsync_gen(
     .clk(clk),
     .reset(reset),
@@ -34,9 +36,11 @@ module ball_absolute_top(clk, reset, hsync, vsync, rgb);
   always @(posedge vsync or posedge reset)
   begin
     if (reset) begin
+      // reset ball position to center
       ball_hpos <= ball_horiz_initial;
       ball_vpos <= ball_vert_initial;
     end else begin
+      // add velocity vector to ball position
       ball_hpos <= ball_hpos + ball_horiz_move;
       ball_vpos <= ball_vpos + ball_vert_move;
     end
@@ -54,19 +58,22 @@ module ball_absolute_top(clk, reset, hsync, vsync, rgb);
     ball_horiz_move <= -ball_horiz_move;
   end
   
+  // offset of ball position from video beam
   wire [8:0] ball_hdiff = hpos - ball_hpos;
   wire [8:0] ball_vdiff = vpos - ball_vpos;
 
+  // ball graphics output
   wire ball_hgfx = ball_hdiff < BALL_SIZE;
   wire ball_vgfx = ball_vdiff < BALL_SIZE;
   wire ball_gfx = ball_hgfx && ball_vgfx;
 
   // collide with vertical and horizontal boundaries
+  // these are set when the ball touches a border
   wire ball_vert_collide = ball_vpos >= 240 - BALL_SIZE;
   wire ball_horiz_collide = ball_hpos >= 256 - BALL_SIZE;
-  
-  wire grid_gfx = (((hpos&7)==0) && ((vpos&7)==0));
 
+  // combine signals to RGB output
+  wire grid_gfx = (((hpos&7)==0) && ((vpos&7)==0));
   wire r = display_on && (ball_hgfx | ball_gfx);
   wire g = display_on && (grid_gfx | ball_gfx);
   wire b = display_on && (ball_vgfx | ball_gfx);
diff --git a/presets/verilog/ball_paddle.v b/presets/verilog/ball_paddle.v
index 668fec3f..03d6a7ce 100644
--- a/presets/verilog/ball_paddle.v
+++ b/presets/verilog/ball_paddle.v
@@ -13,34 +13,34 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
   wire [8:0] hpos;
   wire [8:0] vpos;
   
-  reg [8:0] paddle_pos;
+  reg [8:0] paddle_pos;	// paddle X position
   
-  reg [8:0] ball_x;
-  reg [8:0] ball_y;
-  reg ball_dir_x;
-  reg ball_speed_x;
-  reg ball_dir_y;
+  reg [8:0] ball_x;	// ball X position
+  reg [8:0] ball_y;	// ball Y position
+  reg ball_dir_x;	// ball X direction (0=left, 1=right)
+  reg ball_speed_x;	// ball speed (0=1 pixel/frame, 1=2 pixels/frame)
+  reg ball_dir_y;	// ball Y direction (0=up, 1=down)
   
   reg brick_array [0:BRICKS_H*BRICKS_V-1]; // 16*8 = 128 bits
 
-  wire [3:0] score0;
-  wire [3:0] score1;
-  wire [3:0] lives;
-  reg incscore;
+  wire [3:0] score0;	// score right digit
+  wire [3:0] score1;	// score left digit
+  wire [3:0] lives;	// # lives remaining
+  reg incscore;		// incscore signal 
   reg declives = 0; // TODO
   
-  localparam BRICKS_H = 16;
-  localparam BRICKS_V = 8;
+  localparam BRICKS_H = 16;	// # of bricks across
+  localparam BRICKS_V = 8;	// # of bricks down
 
   localparam BALL_DIR_LEFT = 0;
   localparam BALL_DIR_RIGHT = 1;
-
   localparam BALL_DIR_DOWN = 1;
   localparam BALL_DIR_UP = 0;
   
-  localparam PADDLE_WIDTH = 31;
-  localparam BALL_SIZE = 6;
-  
+  localparam PADDLE_WIDTH = 31;	// horizontal paddle size
+  localparam BALL_SIZE = 6;	// square ball size
+
+  // video sync generator  
   hvsync_generator hvsync_gen(
     .clk(clk),
     .reset(reset),
@@ -52,35 +52,37 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
   );
   
   // scoreboard
-
+  wire score_gfx; // output from score generator
   player_stats stats(.reset(reset), 
                      .score0(score0), .score1(score1), .incscore(incscore),
                      .lives(lives), .declives(declives));
-
-  wire score_gfx;
-  
   scoreboard_generator score_gen(
     .score0(score0), .score1(score1), .lives(lives),
     .vpos(vpos), .hpos(hpos), 
     .board_gfx(score_gfx));
 
-  wire [5:0] hcell = hpos[8:3];
-  wire [5:0] vcell = vpos[8:3];
-  wire lr_border = hcell==0 || hcell==31;
+  wire [5:0] hcell = hpos[8:3];		// horizontal brick index
+  wire [5:0] vcell = vpos[8:3];		// vertical brick index
+  wire lr_border = hcell==0 || hcell==31; // along horizontal border?
 
   // TODO: unsigned compare doesn't work in JS
   wire [8:0] paddle_rel_x = ((hpos-paddle_pos) & 9'h1ff);
+
+  // player paddle graphics signal
   wire paddle_gfx = (vcell == 28) && (paddle_rel_x < PADDLE_WIDTH);
 
-  wire [8:0] ball_rel_x = (hpos-ball_x);
-  wire [8:0] ball_rel_y = (vpos-ball_y);
-  
+  // difference between ball position and video beam
+  wire [8:0] ball_rel_x = (hpos - ball_x);
+  wire [8:0] ball_rel_y = (vpos - ball_y);
+
+  // ball graphics signal
   wire ball_gfx = ball_rel_x < BALL_SIZE
   	       && ball_rel_y < BALL_SIZE;
 
-  reg main_gfx;
-  reg brick_present;
-  reg [6:0] brick_index;
+  reg main_gfx;		// main graphics signal (bricks and borders)
+  reg brick_present;	// 1 when we are drawing a brick
+  reg [6:0] brick_index;// index into array of current brick
+  // brick graphics signal
   wire brick_gfx = lr_border || (brick_present && vpos[2:0] != 0 && hpos[3:1] != 4);
   
   // scan bricks: compute brick_index and brick_present flag
@@ -92,14 +94,11 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
       if (hpos[3:0] == 8) begin
         // compute brick index
         brick_index <= {vpos[5:3], hpos[7:4]};
-        //main_gfx <= 0; // 2 pixel horiz spacing between bricks
       end
       // every 17th pixel
       else if (hpos[3:0] == 9) begin
         // load brick bit from array
         brick_present <= brick_array[brick_index];
-      end else begin
-        //main_gfx <= brick_present && vpos[2:0] != 0; // 1 pixel vert. spacing
       end
     end else begin
       brick_present <= 0;
@@ -111,6 +110,7 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
     if (!hpaddle)
       paddle_pos <= vpos;
 
+  // 1 when ball signal intersects main (brick + border) signal
   wire ball_pixel_collide = main_gfx & ball_gfx;
   
   /* verilator lint_off MULTIDRIVEN */
@@ -141,6 +141,7 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
       incscore <= 0; // reset incscore
     end
 
+  // computes position of ball in relation to center of paddle
   wire signed [8:0] ball_paddle_dx = ball_x - paddle_pos + 8;
 
   // ball bounce: determine new velocity/direction
@@ -189,9 +190,11 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
   always @(negedge vsync or posedge reset)
     begin
       if (reset) begin
+        // reset ball position to top center
         ball_x <= 128;
         ball_y <= 180;
       end else begin
+        // move ball horizontal and vertical position
         if (ball_dir_x == BALL_DIR_RIGHT)
           ball_x <= ball_x + (ball_speed_x?1:0) + 1;
         else
@@ -223,8 +226,8 @@ module ball_paddle_top(clk, reset, hpaddle, hsync, vsync, rgb);
       endcase
     end
 
+  // combine signals to RGB output
   wire grid_gfx = (((hpos&7)==0) || ((vpos&7)==0));
-  
   wire r = display_on && (ball_gfx | paddle_gfx);
   wire g = display_on && (main_gfx | ball_gfx);
   wire b = display_on && (grid_gfx | ball_gfx | brick_present);
diff --git a/presets/verilog/ball_slip_counter.v b/presets/verilog/ball_slip_counter.v
index aa6d078c..be499a9c 100644
--- a/presets/verilog/ball_slip_counter.v
+++ b/presets/verilog/ball_slip_counter.v
@@ -9,7 +9,6 @@ module ball_slip_counter_top(clk, reset, hsync, vsync, rgb);
   wire display_on;
   wire [8:0] hpos;
   wire [8:0] vpos;
-  reg ball_reset;
   
   // 9-bit ball timers
   reg [8:0] ball_htimer;
@@ -26,7 +25,11 @@ module ball_slip_counter_top(clk, reset, hsync, vsync, rgb);
   // 5-bit constants to load into counters
   localparam ball_horiz_prefix = 5'b01100; // 192
   localparam ball_vert_prefix = 5'b01111; // 240
+
+  // reset ball; will be unset when video beam reaches center position
+  reg ball_reset;
   
+  // video sync generator
   hvsync_generator hvsync_gen(
     .clk(clk),
     .reset(reset),
@@ -64,10 +67,10 @@ module ball_slip_counter_top(clk, reset, hsync, vsync, rgb);
       if (reset)
         ball_reset <= 1;
       else if (hpos == 128 && vpos == 128)
-        ball_reset <= 0;
+        ball_reset <= 0;	// un-reset when beam reaches center position
     end
   
-  // collide with vertical and horizontal boundaries
+  // collide with vertical and horizontal boundaries?
   wire ball_vert_collide = ball_vgfx && vpos >= 240;
   wire ball_horiz_collide = ball_hgfx && hpos >= 256 && vpos == 255;
   
@@ -75,7 +78,7 @@ module ball_slip_counter_top(clk, reset, hsync, vsync, rgb);
   always @(posedge ball_vert_collide or posedge reset)
   begin
     if (reset)
-      ball_vert_move <= 4'd9;
+      ball_vert_move <= 4'd9; // initial vertical velocity
     else
       ball_vert_move <= (4'd9 ^ 4'd11) ^ ball_vert_move; // change dir.
   end
@@ -84,13 +87,13 @@ module ball_slip_counter_top(clk, reset, hsync, vsync, rgb);
   always @(posedge ball_horiz_collide or posedge reset)
   begin
     if (reset)
-      ball_horiz_move <= 4'd10;
+      ball_horiz_move <= 4'd10;	// initial horizontal velocity
     else
       ball_horiz_move <= (4'd10 ^ 4'd12) ^ ball_horiz_move; // change dir.
   end
 
   // compute ball display
-  wire ball_hgfx = ball_htimer >= 508;
+  wire ball_hgfx = ball_htimer >= 508;	// 512-508 = 4 pixel ball
   wire ball_vgfx = ball_vtimer >= 508;
   wire ball_gfx = ball_hgfx && ball_vgfx;
 
diff --git a/presets/verilog/digits10.v b/presets/verilog/digits10.v
index e0f0f544..21818b46 100644
--- a/presets/verilog/digits10.v
+++ b/presets/verilog/digits10.v
@@ -3,12 +3,14 @@
 
 `include "hvsync_generator.v"
 
+// module for 10-digit bitmap ROM
 module digits10_case(digit, yofs, bits);
   
-  input [3:0] digit;
-  input [2:0] yofs;
-  output reg [4:0] bits;
+  input [3:0] digit;		// digit 0-9
+  input [2:0] yofs;		// vertical offset (0-4)
+  output reg [4:0] bits;	// output (5 bits)
 
+  // combine {digit,yofs} into single ROM address
   wire [6:0] caseexpr = {digit,yofs};
   
   always @(*)
@@ -79,13 +81,13 @@ endmodule
 
 module digits10_array(digit, yofs, bits);
   
-  input [3:0] digit;
-  input [2:0] yofs;
-  output [4:0] bits;
+  input [3:0] digit;		// digit 0-9
+  input [2:0] yofs;		// vertical offset (0-4)
+  output [4:0] bits;		// output (5 bits)
 
-  reg [4:0] bitarray[0:15][0:4];
+  reg [4:0] bitarray[0:15][0:4];	// ROM array
 
-  assign bits = bitarray[digit][yofs];
+  assign bits = bitarray[digit][yofs];	// assign module output
   
   integer i,j;
   
@@ -157,6 +159,7 @@ module digits10_array(digit, yofs, bits);
   end
 endmodule
 
+// test module
 module test_numbers_top(clk, reset, hsync, vsync, rgb);
   
   input clk, reset;
diff --git a/presets/verilog/hvsync_generator.v b/presets/verilog/hvsync_generator.v
index 5c1f32bc..b8d6c465 100644
--- a/presets/verilog/hvsync_generator.v
+++ b/presets/verilog/hvsync_generator.v
@@ -10,18 +10,18 @@ module hvsync_generator(clk, reset, hsync, vsync, display_on, hpos, vpos);
   output reg [8:0] hpos;
   output reg [8:0] vpos;
 
-  // constant declarations for TV-simulator sync parameters
-  // horizontal
+  // declarations for TV-simulator sync parameters
+  // horizontal constants
   parameter H_DISPLAY       = 256; // horizontal display width
   parameter H_BACK          =  23; // horizontal left border (back porch)
   parameter H_FRONT         =   7; // horizontal right border (front porch)
   parameter H_SYNC          =  23; // horizontal sync width
-  // vertical
+  // vertical constants
   parameter V_DISPLAY       = 240; // vertical display height
   parameter V_TOP           =   5; // vertical top border
   parameter V_BOTTOM        =  14; // vertical bottom border
   parameter V_SYNC          =   3; // vertical sync # lines
-  // derived
+  // derived constants
   parameter H_SYNC_START    = H_DISPLAY + H_FRONT;
   parameter H_SYNC_END      = H_DISPLAY + H_FRONT + H_SYNC - 1;
   parameter H_MAX           = H_DISPLAY + H_BACK + H_FRONT + H_SYNC - 1;
@@ -29,8 +29,8 @@ module hvsync_generator(clk, reset, hsync, vsync, display_on, hpos, vpos);
   parameter V_SYNC_END      = V_DISPLAY + V_BOTTOM + V_SYNC - 1;
   parameter V_MAX           = V_DISPLAY + V_TOP + V_BOTTOM + V_SYNC - 1;
 
-  wire hmaxxed = (hpos == H_MAX) || reset;
-  wire vmaxxed = (vpos == V_MAX) || reset;
+  wire hmaxxed = (hpos == H_MAX) || reset;	// set when hpos is maximum
+  wire vmaxxed = (vpos == V_MAX) || reset;	// set when vpos is maximum
   
   // horizontal position counter
   always @(posedge clk)
@@ -53,6 +53,7 @@ module hvsync_generator(clk, reset, hsync, vsync, display_on, hpos, vpos);
         vpos <= vpos + 1;
   end
   
+  // display_on is set when beam is in "safe" visible frame
   assign display_on = (hpos<H_DISPLAY) && (vpos<V_DISPLAY);
 
 endmodule
diff --git a/presets/verilog/lfsr.v b/presets/verilog/lfsr.v
index b9fa6193..63661a4c 100644
--- a/presets/verilog/lfsr.v
+++ b/presets/verilog/lfsr.v
@@ -1,15 +1,15 @@
 `ifndef LFSR_V
 `define LFSR_V
 
-module LFSR(clk,reset,enable,lfsr);
+module LFSR(clk, reset, enable, lfsr);
   
-  parameter TAPS   = 8'b11101;
-  parameter INVERT = 0;
-  localparam NBITS  = $size(TAPS);
+  parameter TAPS   = 8'b11101;	// bitmask for taps
+  parameter INVERT = 0;		// invert feedback bit?
+  localparam NBITS  = $size(TAPS); // bit width (derived from TAPS)
   
   input clk, reset;
-  input enable;
-  output reg [NBITS-1:0] lfsr;
+  input enable;			// only perform shift when enable=1
+  output reg [NBITS-1:0] lfsr;  // shift register
 
   wire feedback = lfsr[NBITS-1] ^ INVERT;
 
diff --git a/presets/verilog/paddles.v b/presets/verilog/paddles.v
index cbdd1b90..922b6960 100644
--- a/presets/verilog/paddles.v
+++ b/presets/verilog/paddles.v
@@ -10,9 +10,11 @@ module paddles_top(clk, reset, hsync, vsync, hpaddle, vpaddle, rgb);
   wire [8:0] hpos;
   wire [8:0] vpos;
 
+  // player position (only set at VSYNC)
   reg [7:0] player_x;
   reg [7:0] player_y;
   
+  // paddle position (set continuously during frame)
   reg [7:0] paddle_x;
   reg [7:0] paddle_y;
 
diff --git a/presets/verilog/racing_game.v b/presets/verilog/racing_game.v
index 98c66683..7e435d43 100644
--- a/presets/verilog/racing_game.v
+++ b/presets/verilog/racing_game.v
@@ -12,17 +12,22 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
   wire [8:0] hpos;
   wire [8:0] vpos;
 
+  // player car position (set at VSYNC)
   reg [7:0] player_x;
   reg [7:0] player_y;  
+  // paddle position (set continuously during frame)
   reg [7:0] paddle_x;
   reg [7:0] paddle_y;
+  // enemy car position
   reg [7:0] enemy_x = 128;
   reg [7:0] enemy_y = 128;
-  reg enemy_dir = 0;
+  // enemy car direction, 1=right, 0=left
+  reg enemy_dir = 0;	
   
-  reg [15:0] track_pos = 0;
-  reg [7:0] speed = 31;
+  reg [15:0] track_pos = 0;	// player position along track (16 bits)
+  reg [7:0] speed = 31;		// player velocity along track (8 bits)
   
+  // video sync generator
   hvsync_generator hvsync_gen(
     .clk(clk),
     .reset(0),
@@ -33,6 +38,14 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
     .vpos(vpos)
   );
   
+  // set paddle registers
+  always @(posedge hsync)
+    begin
+      if (!hpaddle) paddle_x <= vpos[7:0];
+      if (!vpaddle) paddle_y <= vpos[7:0];
+    end
+  
+  // wire up car sprite ROM
   wire [3:0] car_sprite_yofs;
   wire [7:0] car_sprite_bits;
   
@@ -40,16 +53,19 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
     .yofs(car_sprite_yofs), 
     .bits(car_sprite_bits));
   
+  // signals for player sprite generator
   wire player_vstart = {1'd0,player_y} == vpos;
   wire player_hstart = {1'd0,player_x} == hpos;
   wire player_gfx;
   wire player_is_drawing;
 
+  // signals for enemy sprite generator
   wire enemy_vstart = {1'd0,enemy_y} == vpos;
   wire enemy_hstart = {1'd0,enemy_x} == hpos;
   wire enemy_gfx;
   wire enemy_is_drawing;
   
+  // player sprite generator
   sprite_renderer player_renderer(
     .clk(clk),
     .vstart(player_vstart),
@@ -60,6 +76,7 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
     .gfx(player_gfx),
     .in_progress(player_is_drawing));
 
+  // enemy sprite generator
   sprite_renderer enemy_renderer(
     .clk(clk),
     .vstart(enemy_vstart),
@@ -69,17 +86,14 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
     .rom_bits(car_sprite_bits),
     .gfx(enemy_gfx),
     .in_progress(player_is_drawing));
-  
-  always @(posedge hsync)
-    begin
-      if (!hpaddle) paddle_x <= vpos[7:0];
-      if (!vpaddle) paddle_y <= vpos[7:0];
-    end
-  
+
+  // signals for enemy bouncing off left/right borders  
   wire enemy_hit_left = (enemy_x == 64);
   wire enemy_hit_right = (enemy_x == 192);
   wire enemy_hit_edge = enemy_hit_left || enemy_hit_right;
   
+  // update player, enemy, track counters
+  // runs once per frame
   always @(posedge vsync)
     begin
       player_x <= paddle_x;
@@ -101,6 +115,7 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
         speed <= speed - 1;
     end
   
+  // set to 1 when player collides with enemy or track
   reg frame_collision;
   
   always @(posedge clk)
@@ -109,10 +124,12 @@ module racing_game_top(clk, hsync, vsync, rgb, hpaddle, vpaddle);
     else if (vsync)
       frame_collision <= 0;
   
+  // track graphics signals
   wire track_offside = (hpos[7:5]==0) || (hpos[7:5]==7);
   wire track_shoulder = (hpos[7:3]==3) || (hpos[7:3]==28);
   wire track_gfx = (vpos[5:1]!=track_pos[5:1]) && track_offside;
   
+  // combine signals for RGB output
   wire r = display_on && (player_gfx || enemy_gfx || track_shoulder);
   wire g = display_on && (player_gfx || track_gfx);
   wire b = display_on && (enemy_gfx || track_shoulder);
diff --git a/presets/verilog/sprite_scanline_renderer.v b/presets/verilog/sprite_scanline_renderer.v
index da5302bd..944de44a 100644
--- a/presets/verilog/sprite_scanline_renderer.v
+++ b/presets/verilog/sprite_scanline_renderer.v
@@ -1,3 +1,4 @@
+
 `include "hvsync_generator.v"
 `include "ram.v"
 
@@ -53,14 +54,12 @@ module sprite_scanline_renderer(clk, reset, hpos, vpos, rgb,
   output [15:0] rom_addr; // sprite ROM address
   input [15:0] rom_data;  // sprite ROM data
   
-  // copy of sprite data from RAM
+  // copy of sprite data from RAM (N entries)
   reg [7:0] sprite_xpos[0:N-1];
   reg [7:0] sprite_ypos[0:N-1];
   reg [7:0] sprite_attr[0:N-1];
-  
-  // mapping of N sprites to M line slots
-  reg [NB-1:0] sprite_to_line[0:M-1];
 
+  // M sprite slots
   reg [7:0] line_xpos[0:M-1]; // X pos for M slots
   reg [7:0] line_yofs[0:M-1]; // Y pos for M slots
   reg [7:0] line_attr[0:M-1]; // attr for M slots
@@ -74,6 +73,7 @@ module sprite_scanline_renderer(clk, reset, hpos, vpos, rgb,
   reg [8:0] write_ofs;
   reg [15:0] out_bitmap;
   reg [7:0] out_attr;
+  reg romload;
 
   // which sprite are we currently reading?
   wire [NB-1:0] load_index = hpos[NB:1];
@@ -122,43 +122,49 @@ module sprite_scanline_renderer(clk, reset, hpos, vpos, rgb,
         endcase
       end
     end else if (hpos < N*2) begin
+      // setup vars for next phase
       k <= 0;
+      romload <= 0;
       // select the sprites that will appear in this scanline
       case (hpos[0])
         // compute Y offset of sprite relative to scanline
         0: z <= 8'(vpos - sprite_ypos[i]);
-        // sprite is active if Y offset is 0..15
         1: begin
+          // sprite is active if Y offset is 0..15
           if (z < 16) begin
+            line_xpos[j] <= sprite_xpos[i]; // save X pos
             line_yofs[j] <= z; // save Y offset
-            sprite_to_line[j] <= i; // save main array index
+            line_attr[j] <= sprite_attr[i]; // save attr
             line_active[j] <= 1; // mark sprite active
             j <= j + 1; // inc counter
           end
           i <= i + 1; // inc main array counter
         end
       endcase
-    end else if (hpos < N*2+M*24) begin
+    end else if (hpos < N*2+M*18) begin
+      // setup vars for next phase
       j <= 0;
-      // divide hpos by 24 (8 setup + 16 render)
-      if ((hpos[3:0] < 8) ^^ hpos[4]) begin
-        // render sprites into write buffer
-        case (hpos[3:0])
-          // grab index into main sprite array
-          0: i <= sprite_to_line[k];
-          // load scanline buffer offset to write
-          1: write_ofs <= {~vpos[0], sprite_xpos[i]};
-          // set ROM address and fetch bitmap
-          2: rom_addr <= {4'b0, sprite_attr[i][7:4], line_yofs[k]};
-          // fetch 0 if sprite is inactive
-          3: out_bitmap <= line_active[k] ? rom_data : 0;
-          // load attribute for sprite
-          4: out_attr <= sprite_attr[i];
-          // disable sprite for next scanline
-          6: line_active[k] <= 0;
-          // go to next sprite in 2ndary buffer
-          7: k <= k + 1;
+      // if sprite shift register is empty, load new sprite
+      if (out_bitmap == 0) begin
+        case (romload)
+          0: begin
+            // set ROM address and fetch bitmap
+            rom_addr <= {4'b0, line_attr[k][7:4], line_yofs[k]};
+          end
+          1: begin
+            // load scanline buffer offset to write
+            write_ofs <= {~vpos[0], line_xpos[k]};
+            // fetch 0 if sprite is inactive
+            out_bitmap <= line_active[k] ? rom_data : 0;
+            // load attribute for sprite
+            out_attr <= line_attr[k];
+            // disable sprite for next scanline
+            line_active[k] <= 0;
+            // go to next sprite in 2ndary buffer
+            k <= k + 1;
+          end
         endcase
+        romload <= !romload;
       end else begin
         // write color to scanline buffer if low bit == 1
         if (out_bitmap[0])
diff --git a/presets/verilog/test2.asm b/presets/verilog/test2.asm
index b7224061..41b22bc2 100644
--- a/presets/verilog/test2.asm
+++ b/presets/verilog/test2.asm
@@ -1,4 +1,4 @@
-
+
 .include "hvsync_generator.v"
 .include "font_cp437_8x8.v"
 .include "ram.v"
@@ -140,11 +140,15 @@ MoveSprites:
 	mov	dx,@SpriteTable
         mov	cx,#5
 MoveSLoop:
-	mov	ax,[dx]	; x/y pos
-        mov	bx,[dx+1] ; attributes/dir
-        and	bx,#3	; bx = direction (0-3)
+	mov	ax,[dx]    ; x/y pos
+        mov	bx,[dx+1]  ; direction
+        rol	bx
+        rol	bx
+        rol	bx
+        rol	bx
+        and	bx,#3	   ; bx = direction (0-3)
         add	bx,@DirectionXY
-	add	ax,[bx] ; lookup & add to x/y
+	add	ax,[bx]    ; lookup & add to x/y
         ; make sure we don't collide with wall
         mov	ex,ax
         add	ex,#$08
@@ -170,19 +174,18 @@ MoveSLoop:
         bz	DoNotUpdatePos
         ; update x/y position
         mov	[dx],ax	; store x/y pos
-	inc	dx
-	inc	dx
 NextSLoop:
+        add	dx,#2
         dec	cx
         bnz	MoveSLoop
         rts
 ; Choose a new direction
 DoNotUpdatePos:
-        mov	bx,[dx+1] ; attributes/dir
-        inc	bx
-        inc	dx
+        add	dx,#1
+        mov	bx,[dx] ; direction
+        add	bx,@$1000
         mov	[dx],bx
-        inc	dx
+        sub	dx,#1
 	jmp	NextSLoop
 
 HelloWorld:
@@ -190,6 +193,7 @@ HelloWorld:
 .data 0
 
 SpriteInitData:
+.data $b373 $1233
 .data $8363 $3456
 .data $8373 $4567
 .data $8383 $5678
diff --git a/src/platform/verilog.js b/src/platform/verilog.js
index c4145b84..05f0b37e 100644
--- a/src/platform/verilog.js
+++ b/src/platform/verilog.js
@@ -592,27 +592,28 @@ var VerilogPlatform = function(mainElement, options) {
   this.loadROM = function(title, output) {
     var mod;
     if (output.code) {
-      try {
-        mod = new Function('base', output.code);
-      } catch (e) {
-        this.printErrorCodeContext(e, output.code);
-        throw e;
+      // is code identical?
+      if (current_output && current_output.code == output.code) {
+      } else {
+        try {
+          mod = new Function('base', output.code);
+        } catch (e) {
+          this.printErrorCodeContext(e, output.code);
+          throw e;
+        }
+        // compile Verilog code
+        var base = new VerilatorBase();
+        gen = new mod(base);
+        //$.extend(gen, base);
+        gen.__proto__ = base;
+        current_output = output;
+        module_name = output.name ? output.name.substr(1) : "top";
+        trace_ports = current_output.ports;
+        trace_signals = current_output.ports.concat(current_output.signals);
+        trace_index = 0;
+        // power on module
+        this.poweron();
       }
-      // compile Verilog code
-      var base = new VerilatorBase();
-      gen = new mod(base);
-      //$.extend(gen, base);
-      gen.__proto__ = base;
-      current_output = output;
-      module_name = output.name ? output.name.substr(1) : "top";
-      trace_ports = current_output.ports;
-      trace_signals = current_output.ports.concat(current_output.signals);
-      trace_index = 0;
-      // power on module
-      this.poweron();
-    } else {
-      // TODO: :^P
-      output = {program_rom_variable: title, program_rom: output};
     }
     // replace program ROM, if using the assembler
     if (output.program_rom && output.program_rom_variable) {
@@ -624,6 +625,7 @@ var VerilogPlatform = function(mainElement, options) {
       } else {
         alert("No program_rom variable found (" + output.program_rom_variable + ")");
       }
+      this.reset();
     }
     // restart audio
     restartAudio();
diff --git a/src/project.ts b/src/project.ts
index 97c6a1a8..9e5a0a26 100644
--- a/src/project.ts
+++ b/src/project.ts
@@ -60,7 +60,7 @@ export class CodeProject {
       var m;
       while (m = re.exec(text)) {
         files.push(m[2]);
-        files.push('local/'+m[2]); // TODO?
+        //files.push('local/'+m[2]); // TODO: shows up 2x in interface
       }
     } else {
       var re = /^\s*([;#]|[/][/][#])link\s+"(.+?)"/gm;
diff --git a/src/worker/workermain.js b/src/worker/workermain.js
index 3e78824f..5c50d15c 100644
--- a/src/worker/workermain.js
+++ b/src/worker/workermain.js
@@ -1120,6 +1120,8 @@ function compileJSASM(asmcode, platform, options, is_inline) {
     result.output.program_rom = asmout;
     // cpu_platform__DOT__program_rom
     result.output.program_rom_variable = jsasm_module_top + "__DOT__program_rom";
+    result.listings = {};
+    result.listings[options.path] = {lines:result.lines};
   }
   return result;
 }