[SMS] Frameskip actually skip frames

BizHawk.Emulation/Consoles/Sega/SMS/VDP.Mode4.cs

@@ -224,6 +224,65 @@ namespace BizHawk.Emulation.Consoles.Sega
             }
         }
 
+        internal void ProcessSpriteCollisionForFrameskip()
+        {
+            if (DisplayOn == false) return;
+            int SpriteBase = SpriteAttributeTableBase;
+            int SpriteHeight = EnableLargeSprites ? 16 : 8;
+
+            // Clear the sprite collision buffer for this scanline
+            Array.Clear(SpriteCollisionBuffer, 0, 256);
+
+            // 208 is a special terminator sprite (in 192-line mode). Lets find it...
+            int TerminalSprite = 64;
+            if (FrameHeight == 192)
+                for (int i = 0; i < 64; i++)
+                {
+                    if (VRAM[SpriteBase + i] == 208)
+                    {
+                        TerminalSprite = i;
+                        break;
+                    }
+                }
+
+            // Loop through these sprites and render the current scanline
+            int SpritesDrawnThisScanline = 0;
+            for (int i = TerminalSprite - 1; i >= 0; i--)
+            {
+                if (SpritesDrawnThisScanline >= 8)
+                    StatusByte |= 0x40; // Set Overflow bit
+
+                int x = VRAM[SpriteBase + 0x80 + (i * 2)];
+                if (ShiftSpritesLeft8Pixels)
+                    x -= 8;
+
+                int y = VRAM[SpriteBase + i] + 1;
+                if (y >= (EnableLargeSprites ? 240 : 248)) y -= 256;
+
+                if (y + SpriteHeight <= ScanLine || y > ScanLine)
+                    continue;
+
+                int tileNo = VRAM[SpriteBase + 0x80 + (i * 2) + 1];
+                if (EnableLargeSprites)
+                    tileNo &= 0xFE;
+                tileNo += SpriteTileBase;
+
+                int ys = ScanLine - y;
+
+                for (int xs = 0; xs < 8 && x + xs < 256; xs++)
+                {
+                    byte color = PatternBuffer[(tileNo * 64) + (ys * 8) + xs];
+                    if (color != 0 && x + xs >= 0)
+                    {
+                        if (SpriteCollisionBuffer[x + xs] != 0)
+                            StatusByte |= 0x20; // Set Collision bit
+                        SpriteCollisionBuffer[x + xs] = 1;
+                    }
+                }
+                SpritesDrawnThisScanline++;
+            }
+        }
+
         /// <summary>
         /// Performs render buffer blanking. This includes the left-column blanking as well as Game Gear blanking if requested.
         /// Should be called at the end of the frame.

BizHawk.Emulation/Consoles/Sega/SMS/VDP.cs

@@ -364,17 +364,32 @@ namespace BizHawk.Emulation.Consoles.Sega
 
             if (TmsMode == 4)
             {
-                // TODO: make frameskip actually skip rendering
+                if (render == false)
+                {
+                    ProcessSpriteCollisionForFrameskip();
+                    return;
+                }
+
                 RenderBackgroundCurrentLine();
 
                 if (EnableDoubledSprites)
                     RenderSpritesCurrentLineDoubleSize();
                 else
                     RenderSpritesCurrentLine();
-            } else if (TmsMode == 2) {
+            }
+            else if (TmsMode == 2) 
+            {
+                if (render == false) 
+                    return;
+
                 RenderBackgroundM2();
                 RenderTmsSprites();
-            } else if (TmsMode == 0) {
+            } 
+            else if (TmsMode == 0) 
+            {
+                if (render == false)
+                    return;
+
                 RenderBackgroundM0();
                 RenderTmsSprites();
             }