diff --git a/source/render.c b/source/render.c
index ad248bd..04fd5af 100644
--- a/source/render.c
+++ b/source/render.c
@@ -974,7 +974,6 @@ static void render_bg(uint32 line, uint32 width)
   uint32 pf_col_mask  = playfield_col_mask;
   uint32 pf_row_mask  = playfield_row_mask;
   uint32 pf_shift     = playfield_shift;
-  uint32 pf_y_mask    = y_mask;
 
   /* Plane B */
   uint8 *buf    = ntb_buf;
@@ -991,7 +990,7 @@ static void render_bg(uint32 line, uint32 width)
   uint32 v_line = (line + (yscroll & 0x3FF)) & pf_row_mask;
 #endif
 
-  uint32 *nt    = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+  uint32 *nt    = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & 0x1FC0)];
   v_line        = (v_line & 7) << 3;
 
   if(shift)
@@ -1044,7 +1043,7 @@ static void render_bg(uint32 line, uint32 width)
     v_line  = (line + ((yscroll >> 16) & 0x3FF)) & pf_row_mask;
 #endif
 
-    nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+    nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & 0x1FC0)];
     v_line  = (v_line & 7) << 3;
 
     if(shift)
@@ -1094,7 +1093,6 @@ static void render_bg_vs(uint32 line, uint32 width)
   uint32 pf_col_mask  = playfield_col_mask;
   uint32 pf_row_mask  = playfield_row_mask;
   uint32 pf_shift     = playfield_shift;
-  uint32 pf_y_mask    = y_mask;
   uint32 *vs          = (uint32 *)&vsram[0];
 
   /* Plane B */
@@ -1121,7 +1119,7 @@ static void render_bg_vs(uint32 line, uint32 width)
 #else
     v_line = (line + (vs[0] & 0x3FF)) & pf_row_mask;
 #endif
-    nt = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+    nt = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & 0x1FC0)];
     v_line = (v_line & 7) << 3;
 
     atbuf = nt[(index-1) & pf_col_mask];
@@ -1136,7 +1134,7 @@ static void render_bg_vs(uint32 line, uint32 width)
 #else
     v_line = (line + (vs[column] & 0x3FF)) & pf_row_mask;
 #endif
-    nt = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+    nt = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & 0x1FC0)];
     v_line = (v_line & 7) << 3;
 
     atbuf = nt[index & pf_col_mask];
@@ -1186,7 +1184,7 @@ static void render_bg_vs(uint32 line, uint32 width)
 #else
       v_line = (line + ((vs[start] >> 16) & 0x3FF)) & pf_row_mask;
 #endif
-      nt = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+      nt = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & 0x1FC0)];
       v_line = (v_line & 7) << 3;
 
       /* Window bug */
@@ -1204,7 +1202,7 @@ static void render_bg_vs(uint32 line, uint32 width)
 #else
       v_line = (line + ((vs[column] >> 16) & 0x3FF)) & pf_row_mask;
 #endif
-      nt = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+      nt = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & 0x1FC0)];
       v_line = (v_line & 7) << 3;
 
       atbuf = nt[index & pf_col_mask];
@@ -1240,7 +1238,6 @@ static void render_bg_im2(uint32 line, uint32 width, uint32 odd)
   uint32 pf_col_mask  = playfield_col_mask;
   uint32 pf_row_mask  = playfield_row_mask;
   uint32 pf_shift     = playfield_shift;
-  uint32 pf_y_mask    = y_mask;
 
   /* Plane B */
   uint8 *buf    = ntb_buf;
@@ -1257,7 +1254,7 @@ static void render_bg_im2(uint32 line, uint32 width, uint32 odd)
   uint32 v_line = (line + ((yscroll >> 1) & 0x3FF)) & pf_row_mask;
 #endif
 
-  uint32 *nt    = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+  uint32 *nt    = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & 0x1FC0)];
   v_line        = (((v_line & 7) << 1) | odd) << 3;
 
   if(shift)
@@ -1308,7 +1305,7 @@ static void render_bg_im2(uint32 line, uint32 width, uint32 odd)
     v_line  = (line + ((yscroll >> 17) & 0x3FF)) & pf_row_mask;
 #endif
 
-    nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+    nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & 0x1FC0)];
     v_line  = (((v_line & 7) << 1) | odd) << 3;
 
     if(shift)
@@ -1357,7 +1354,6 @@ static void render_bg_im2_vs(uint32 line, uint32 width, uint32 odd)
   uint32 pf_col_mask  = playfield_col_mask;
   uint32 pf_row_mask  = playfield_row_mask;
   uint32 pf_shift     = playfield_shift;
-  uint32 pf_y_mask    = y_mask;
   uint32 *vs          = (uint32 *)&vsram[0];
 
   /* Plane B */
@@ -1384,7 +1380,7 @@ static void render_bg_im2_vs(uint32 line, uint32 width, uint32 odd)
 #else
     v_line = (line + ((vs[0] >> 1) & 0x3FF)) & pf_row_mask;
 #endif
-    nt = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+    nt = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & 0x1FC0)];
     v_line = (((v_line & 7) << 1) | odd) << 3;
 
     atbuf = nt[(index-1) & pf_col_mask];
@@ -1399,7 +1395,7 @@ static void render_bg_im2_vs(uint32 line, uint32 width, uint32 odd)
 #else
     v_line  = (line + ((vs[column] >> 1) & 0x3FF)) & pf_row_mask;
 #endif
-    nt      = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+    nt      = (uint32 *)&vram[ntbb + (((v_line >> 3) << pf_shift) & 0x1FC0)];
     v_line  = (((v_line & 7) << 1) | odd) << 3;
 
     atbuf = nt[index & pf_col_mask];
@@ -1447,7 +1443,7 @@ static void render_bg_im2_vs(uint32 line, uint32 width, uint32 odd)
 #else
       v_line  = (line + ((vs[start] >> 17) & 0x3FF)) & pf_row_mask;
 #endif
-      nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+      nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & 0x1FC0)];
       v_line  = (((v_line & 7) << 1) | odd) << 3;
 
       /* Window bug */
@@ -1465,7 +1461,7 @@ static void render_bg_im2_vs(uint32 line, uint32 width, uint32 odd)
 #else
       v_line  = (line + ((vs[column] >> 17) & 0x3FF)) & pf_row_mask;
 #endif
-      nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & pf_y_mask)];
+      nt      = (uint32 *)&vram[ntab + (((v_line >> 3) << pf_shift) & 0x1FC0)];
       v_line  = (((v_line & 7) << 1) | odd) << 3;
 
       atbuf = nt[index & pf_col_mask];
diff --git a/source/vdp.c b/source/vdp.c
index 2a24bd6..4cf7202 100644
--- a/source/vdp.c
+++ b/source/vdp.c
@@ -82,10 +82,9 @@ uint16 lines_per_frame;           /* PAL: 313 lines, NTSC: 262 lines */
 
 
 /* Tables that define the playfield layout */
-static const uint8 shift_table[]      = { 6, 7, 0, 8 };
+static const uint8 shift_table[]      = { 6, 7, 0, 8 }; /* fixes Window Bug test program */
 static const uint8 col_mask_table[]   = { 0x0F, 0x1F, 0x0F, 0x3F };
 static const uint16 row_mask_table[]  = { 0x0FF, 0x1FF, 0x2FF, 0x3FF };
-static const uint32 y_mask_table[]    = { 0x1FC0, 0x1F80, 0x1FC0, 0x1F00 };
 
 static uint16 sat_base_mask;  /* Base bits of SAT */
 static uint16 sat_addr_mask;  /* Index bits of SAT */
@@ -194,7 +193,6 @@ void vdp_reset(void)
   playfield_shift = 6;
   playfield_col_mask = 0x0F;
   playfield_row_mask = 0x0FF;
-  y_mask = 0x1FC0;
 
   hint_pending = 0;
   vint_pending = 0;
@@ -906,7 +904,7 @@ static inline void reg_w(unsigned int r, unsigned int d)
       }
       break;
 
-    case 13: /* HSCB */
+    case 13: /* HScroll Base Address */
       hscb = (d << 10) & 0xFC00;
       break;
 
@@ -914,10 +912,9 @@ static inline void reg_w(unsigned int r, unsigned int d)
       playfield_shift = shift_table[(d & 3)];
       playfield_col_mask = col_mask_table[(d & 3)];
       playfield_row_mask = row_mask_table[(d >> 4) & 3];
-      y_mask = y_mask_table[(d & 3)];
       break;
 
-    case 17: /* Update clipping */
+    case 17: /* Window/Plane A clipping */
       reg[17] = d;
       window_clip();
       break;
diff --git a/source/vdp.h b/source/vdp.h
index f1b4f7c..e08c4db 100644
--- a/source/vdp.h
+++ b/source/vdp.h
@@ -54,7 +54,6 @@ extern uint8 bg_pattern_cache[0x80000];
 extern uint8 playfield_shift;
 extern uint8 playfield_col_mask;
 extern uint16 playfield_row_mask;
-extern uint32 y_mask;
 extern uint16 hc_latch;
 extern uint16 v_counter;
 extern uint32 dma_length;