From 959aafaf6aeee5db91e71d0850abd2b352ded2a4 Mon Sep 17 00:00:00 2001
From: Angelo Salese <angelosa@users.noreply.github.com>
Date: Mon, 27 Sep 2010 02:22:40 +0000
Subject: [PATCH] Fixed 8-bit dram (framebuffer) access for 32x [David Haywood]

---
 src/mame/drivers/megadriv.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/src/mame/drivers/megadriv.c b/src/mame/drivers/megadriv.c
index 3ea25c8dc49..373a6a2a73a 100644
--- a/src/mame/drivers/megadriv.c
+++ b/src/mame/drivers/megadriv.c
@@ -2314,7 +2314,32 @@ static READ16_HANDLER( _32x_68k_dram_r )
 
 static WRITE16_HANDLER( _32x_68k_dram_w )
 {
-	COMBINE_DATA(&_32x_access_dram[offset]);
+	if ((mem_mask&0xffff) == 0xffff)
+	{
+		// 16-bit accesses are normal
+		COMBINE_DATA(&_32x_access_dram[offset]);
+	}
+	else
+	{
+		// 8-bit writes act as if they were going to the overwrite region!
+		// bc-racers and world series baseball rely on this!
+		// (tested on real hw)
+
+		if ((mem_mask & 0xffff) == 0xff00)
+		{
+			if ((data & 0xff00) != 0x0000)
+			{
+				_32x_access_dram[offset] = (data & 0xff00) |  (_32x_access_dram[offset] & 0x00ff);
+			}
+		}
+		else if ((mem_mask & 0xffff) == 0x00ff)
+		{
+			if ((data & 0x00ff) != 0x0000)
+			{
+				_32x_access_dram[offset] = (data & 0x00ff) |  (_32x_access_dram[offset] & 0xff00);
+			}
+		}
+	}
 }
 
 static READ16_HANDLER( _32x_68k_dram_overwrite_r )