Made some tweaks after experimenting with interlaced mode:

- VSync needs to be enabled for interlaced mode to work at all
- Y offset does not need to be doubled in interlaced mode

display.c

@@ -46,6 +46,7 @@ VECTOR aspect_scale = { SCREENWIDTH * ONE / 320, ONE, ONE };
 
 u_short polyCount;
 u_char enableTexturing = 1;
+u_char enableVsync = 0;
 
 void display_init()
 {
@@ -104,8 +105,8 @@ void display_reset(int mode, u_char interlaced, u_char widescreen, int *outScree
 	if (interlaced)
 	{
 		screenHeight <<= 1;
-		yOffset <<= 1;
 		aspect_scale.vy <<= 1;
+		enableVsync = 1;
 
 		// Define display environments, interlaced images cover the same area in VRAM
 		SetDefDispEnv(&disp[0], 0, 0, SCREENWIDTH, screenHeight);
@@ -119,6 +120,8 @@ void display_reset(int mode, u_char interlaced, u_char widescreen, int *outScree
 	}
 	else
 	{
+		enableVsync = 0;
+
 		// Define display environments, first on top and second on bottom
 		SetDefDispEnv(&disp[0], 0, 0, SCREENWIDTH, screenHeight);
 		SetDefDispEnv(&disp[1], 0, screenHeight, SCREENWIDTH, screenHeight);
@@ -184,9 +187,9 @@ void display_finish()
 	// Wait for all drawing to complete
 	DrawSync(0);
 	
-	// Wait for vertical sync to cap the logic to 60fps (or 50 in PAL mode)
-	// and prevent screen tearing
-	//VSync(0);
+	// Wait for vertical sync to cap the logic to 60fps (or 50 in PAL mode) and prevent screen tearing
+	if (enableVsync)
+		VSync(0);
 
 	// Switch pages	
 	PutDispEnv(&disp[db]);

input.c

@@ -98,8 +98,8 @@ void input_process()
     if (!(buttons & PAD_SELECT) && (prevButtons & PAD_SELECT))
     {
         // Switch display mode
-		//dispmode = (dispmode + 1) & 0x3;
-		//display_reset(!!(dispmode & 2), !!(dispmode & 1), widescreen, NULL);
+		// dispmode = (dispmode + 1) & 0x3;
+		// display_reset(!!(dispmode & 2), !!(dispmode & 1), widescreen, NULL);
 		display_reset(!GetVideoMode(), 0, widescreen, NULL);
     }
     if (!(buttons & PAD_START) && (prevButtons & PAD_START))