GBHawk: open bus behaviour on locked SRAM

src/BizHawk.Emulation.Cores/CPUs/LR35902/Execute.cs

@@ -7,6 +7,9 @@ namespace BizHawk.Emulation.Cores.Components.LR35902
 		private int EI_pending;
 		public bool interrupts_enabled;
 
+		// we need the last value on the bus for proper emulation of blocked SRAM
+		public byte bus_value;
+
 		// variables for executing instructions
 		public int instr_pntr = 0;
 		public ushort[] cur_instr = new ushort [60];

src/BizHawk.Emulation.Cores/CPUs/LR35902/LR35902.cs

@@ -815,6 +815,7 @@ namespace BizHawk.Emulation.Cores.Components.LR35902
 			ser.Sync(nameof(LY), ref LY);
 			ser.Sync(nameof(FlagI), ref FlagI);
 			ser.Sync(nameof(was_FlagI), ref was_FlagI);
+			ser.Sync(nameof(bus_value), ref bus_value);
 
 			ser.EndSection();
 		}

src/BizHawk.Emulation.Cores/CPUs/LR35902/Operations.cs

@@ -18,20 +18,21 @@ namespace BizHawk.Emulation.Cores.Components.LR35902
 				if (src_l == PCl) CDLCallback(addr, eCDLogMemFlags.FetchOperand);
 				else CDLCallback(addr, eCDLogMemFlags.Data);
 			}
-			Regs[dest] = ReadMemory(addr);
+			Regs[dest] = bus_value = ReadMemory(addr);
 		}
 
 		// special read for POP AF that always clears the lower 4 bits of F 
 		public void Read_Func_F(ushort dest, ushort src_l, ushort src_h)
 		{
-			Regs[dest] = (ushort)(ReadMemory((ushort)(Regs[src_l] | (Regs[src_h]) << 8)) & 0xF0);
+			Regs[dest] = bus_value = (byte)(ReadMemory((ushort)(Regs[src_l] | (Regs[src_h]) << 8)) & 0xF0);
 		}
 
 		public void Write_Func(ushort dest_l, ushort dest_h, ushort src)
 		{
 			ushort addr = (ushort)(Regs[dest_l] | (Regs[dest_h]) << 8);
 			CDLCallback?.Invoke(addr, eCDLogMemFlags.Write | eCDLogMemFlags.Data);
-			WriteMemory(addr, (byte)Regs[src]);
+			bus_value = (byte)Regs[src];
+			WriteMemory(addr, bus_value);
 		}
 
 		public void TR_Func(ushort dest, ushort src)

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.cs

@@ -11,6 +11,7 @@ using System.Runtime.InteropServices;
 // TODO: oam_dma_start.gb does not behave as expected but test still passes through lucky coincidences / test deficiency
 // TODO: LYC interrupt behaves differently in GBC and GB compat mode
 // TODO: Window Position A6 behaves differently
+// TODO: Verify open bus behaviour for bad SRAM accesses for other MBCs
 
 namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 {

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Mappers/Mapper_MBC1.cs

@@ -59,12 +59,12 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				}
 				else
 				{
-					return 0xFF;
+					return Core.cpu.bus_value;
 				}
 			}
 			else
 			{
-				return 0xFF;
+				return Core.cpu.bus_value;
 			}
 		}
 

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Mappers/Mapper_MBC1_Multi.cs

@@ -58,13 +58,13 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				}
 				else
 				{
-					return 0xFF;
+					return Core.cpu.bus_value;
 				}
 
 			}
 			else
 			{
-				return 0xFF;
+				return Core.cpu.bus_value;
 			}
 		}