From b1ff22c9e6445c4846914879b2deec7f3ae0d310 Mon Sep 17 00:00:00 2001
From: Marc Parsons <marc@parsons.cc>
Date: Sat, 16 May 2026 20:18:07 +0100
Subject: [PATCH] Tried to fix Sonic 1 demo loop

---
 Core/Video/SmsVdp.cs | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/Core/Video/SmsVdp.cs b/Core/Video/SmsVdp.cs
index 7a67b33..278db60 100644
--- a/Core/Video/SmsVdp.cs
+++ b/Core/Video/SmsVdp.cs
@@ -104,10 +104,20 @@ namespace Core.Video
 
         public byte ReadVCounter()
         {
-            // Note: On real NTSC hardware, the V-Counter jumps slightly around 
-            // the VBlank period to keep the math 8-bit, but simply returning 
-            // the raw current scanline is perfectly fine to get us booting!
-            return (byte)_currentScanline;
+            // NTSC Hardware Quirk: The Master System outputs 262 lines, 
+            // but an 8-bit register can only hold a maximum value of 255!
+            // To prevent a hardware overflow to 0, the silicon jumps backward during VBlank.
+
+            if (_currentScanline <= 218) // 0xDA
+            {
+                return (byte)_currentScanline;
+            }
+            else
+            {
+                // At scanline 219, the counter jumps back to 213 (0xD5).
+                // It then counts up to exactly 255 (0xFF) at the end of the frame.
+                return (byte)(_currentScanline - 6);
+            }
         }
 
         public void Update(int tStates)