SNES: Fixed CPU 6502 emulation mode edge case regarding wrapping beha…

…vior for (direct, x) addressing mode
SourMesen · Oct 29, 2023 · c0ea124 · c0ea124
1 parent 020b29f
commit c0ea124
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Showing 4 changed files with 21 additions and 1 deletion.
diff --git a/Core/SNES/Coprocessors/SA1/Sa1Cpu.h b/Core/SNES/Coprocessors/SA1/Sa1Cpu.h
@@ -38,6 +38,7 @@ class Sa1Cpu : public ISerializable
 	uint16_t GetDirectAddress(uint16_t offset, bool allowEmulationMode = true);
 
 	uint16_t GetDirectAddressIndirectWord(uint16_t offset);
+	uint16_t GetDirectAddressIndirectWordWithPageWrap(uint16_t offset);
 	uint32_t GetDirectAddressIndirectLong(uint16_t offset);
 
 	uint8_t GetOpCode();

diff --git a/Core/SNES/SnesCpu.Instructions.h b/Core/SNES/SnesCpu.Instructions.h
@@ -1280,7 +1280,7 @@ void SnesCpu::AddrMode_DirIdxIndX()
 {
 	uint8_t operandByte = ReadDirectOperandByte();
 	Idle();
-	_operand = GetDataAddress(GetDirectAddressIndirectWord(operandByte + _state.X));
+	_operand = GetDataAddress(GetDirectAddressIndirectWordWithPageWrap(operandByte + _state.X));
 }
 
 void SnesCpu::AddrMode_DirIndIdxY(bool isWrite)

diff --git a/Core/SNES/SnesCpu.Shared.h b/Core/SNES/SnesCpu.Shared.h
@@ -500,6 +500,24 @@ uint16_t SnesCpu::GetDirectAddress(uint16_t offset, bool allowEmulationMode)
 	}
 }
 
+uint16_t SnesCpu::GetDirectAddressIndirectWordWithPageWrap(uint16_t offset)
+{
+	uint8_t lsb = ReadData(GetDirectAddress(offset + 0));
+	if(!_state.EmulationMode || (_state.D & 0xFF) == 0) {
+		return (ReadData(GetDirectAddress(offset + 1)) << 8) | lsb;
+	} else {
+		uint16_t addr = GetDirectAddress(offset + 1);
+		uint8_t msb;
+		if((addr & 0xFF) == 0) {
+			//Crossed to next page, wrap around to start of previous page instead (cpu bug?)
+			msb = ReadData((uint16_t)(addr - 0x100));
+		} else {
+			msb = ReadData(addr);
+		}
+		return (msb << 8) | lsb;
+	}
+}
+
 uint16_t SnesCpu::GetDirectAddressIndirectWord(uint16_t offset)
 {
 	uint8_t lsb = ReadData(GetDirectAddress(offset + 0));

diff --git a/Core/SNES/SnesCpu.h b/Core/SNES/SnesCpu.h
@@ -49,6 +49,7 @@ class SnesCpu : public ISerializable
 	uint16_t GetDirectAddress(uint16_t offset, bool allowEmulationMode = true);
 
 	uint16_t GetDirectAddressIndirectWord(uint16_t offset);
+	uint16_t GetDirectAddressIndirectWordWithPageWrap(uint16_t offset);
 	uint32_t GetDirectAddressIndirectLong(uint16_t offset);
 
 	uint8_t GetOpCode();