From c8ef579b8fbc111ce6e596009450c4b8f5e0afc9 Mon Sep 17 00:00:00 2001
From: alyosha-tas <alexei.f.k@gmail.com>
Date: Wed, 8 Dec 2021 16:29:36 -0500
Subject: [PATCH] NESHawk: Implement new findings related to cpu register
 accesses, fix nanjing board

---
 .../Consoles/Nintendo/NES/Boards/NanJing.cs   |  3 +-
 .../Consoles/Nintendo/NES/NES.Core.cs         | 30 +++++++++++++++++--
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/NanJing.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/NanJing.cs
index a76db41c51..d60d34165a 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/NanJing.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/NanJing.cs
@@ -140,7 +140,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 		{
 			if (addr < 0x2000)
 			{
-				if ((reg1 & 0x80) != 0 && NES.ppu.ppur.status.rendering)
+				
+				if ((reg1 & 0x80) != 0 && NES.ppu.ppur.status.rendering && NES.ppu.PPUON)
 				{
 					if (NES.ppu.ppur.status.sl <= 128)
 						Vram[addr & 0xfff] = value;
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/NES.Core.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/NES.Core.cs
index 187fbde945..a899f49183 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/NES.Core.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/NES.Core.cs
@@ -1016,11 +1016,35 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 			}
 			else if (addr < 0x4020)
 			{
-				ret = ReadReg(addr); // we're not rebasing the register just to keep register names canonical
+				// oam dma access board memory if cpu is not accessing registers
+				// this means that OAM DMA can actually access memory that the cpu cannot
+				if (oam_dma_exec)
+				{
+					if ((cpu.PC >= 0x4000) && (cpu.PC < 0x4020))
+					{
+						ret = ReadReg(addr);
+					}
+					else
+					{
+						ret = Board.ReadExp(addr - 0x4000);
+					}
+				}
+				else
+				{
+					ret = ReadReg(addr);
+				}
 			}
 			else if (addr < 0x6000)
 			{
-				ret = Board.ReadExp(addr - 0x4000);
+				// oam dma will access registers if cpu is accessing them
+				if (oam_dma_exec && ((oam_dma_addr & 0xFF00) == 0x4000) && (cpu.PC >= 0x4000) && (cpu.PC < 0x4020))
+				{
+					ret = ReadReg(addr & 0x401F);
+				}
+				else
+				{
+					ret = Board.ReadExp(addr - 0x4000);
+				}
 			}
 			else
 			{
@@ -1082,7 +1106,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 			}
 			else if (addr < 0x4020)
 			{
-				WriteReg(addr, value);  //we're not rebasing the register just to keep register names canonical
+				WriteReg(addr, value);
 			}
 			else if (addr < 0x6000)
 			{