From 68dc12dab7796840ac6c806124b92b70be983cda Mon Sep 17 00:00:00 2001
From: nattthebear <goyuken@gmail.com>
Date: Thu, 14 May 2020 19:51:19 -0400
Subject: [PATCH] Prohibit U+D/L+R at the core level in SGB

fixes #1308
---
 .../Consoles/Nintendo/Gameboy/Sameboy.cs           | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/Gameboy/Sameboy.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/Gameboy/Sameboy.cs
index 24835942af..98bedef11f 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/Gameboy/Sameboy.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/Gameboy/Sameboy.cs
@@ -150,16 +150,22 @@ namespace BizHawk.Emulation.Cores.Consoles.Nintendo.Gameboy
 					b |= LibSameboy.Buttons.SELECT;
 				if (c.IsPressed($"P{i} Start"))
 					b |= LibSameboy.Buttons.START;
+				if (_sgb)
+				{
+					// The SGB SNES side code enforces U+D/L+R prohibitions
+					if (((uint)b & 0x30) == 0x30)
+						b &= unchecked((LibSameboy.Buttons)~0x30);
+					if (((uint)b & 0xc0) == 0xc0)
+						b &= unchecked((LibSameboy.Buttons)~0xc0);
+				}
 				if (i != 1)
+				{
 					b = (LibSameboy.Buttons)((uint)b << 8);
+				}
 			}
 			return b;
 		}
 
-
-
-
-
 		public new bool SaveRamModified => _core.HasSaveRam();
 
 		public new byte[] CloneSaveRam()