From 76767110b7c49e78dd0b718f642b28a15bc635e6 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Wed, 22 Jun 2022 15:12:08 -0400
Subject: [PATCH] Fix overflow for 8-bit calculations; essentially a revert for
 ADC.

---
 .../Implementation/65816Implementation.hpp    | 46 ++++++-------------
 1 file changed, 15 insertions(+), 31 deletions(-)

diff --git a/Processors/65816/Implementation/65816Implementation.hpp b/Processors/65816/Implementation/65816Implementation.hpp
index 9f77567c6..d5161e64a 100644
--- a/Processors/65816/Implementation/65816Implementation.hpp
+++ b/Processors/65816/Implementation/65816Implementation.hpp
@@ -910,33 +910,25 @@ template <typename BusHandler, bool uses_ready_line> void Processor<BusHandler,
 						case SBC:
 							if(registers_.flags.decimal) {
 								const uint16_t a = registers_.a.full & registers_.m_masks[1];
-								const uint16_t decimal_result = uint16_t(a - data_buffer_.value - (1 ^ registers_.flags.carry));
 								data_buffer_.value = ~data_buffer_.value & registers_.m_masks[1];
 
-#define begin_nibble(mask)						\
-	result += (a & mask) + (data_buffer_.value & mask);
+								unsigned int result = registers_.flags.carry;
+								uint16_t partials = 0;
 
-#define end_nibble(adjustment, carry)			\
-	if(result < carry) result -= adjustment;	\
+#define nibble(mask, adjustment, carry)					\
+	result += (a & mask) + (data_buffer_.value & mask);	\
+	partials += result & mask;							\
+	if(result < carry) result -= adjustment;			\
 	result &= (carry | (carry - 1));
 
-#define nibble(mask, adjustment, carry)			\
-	begin_nibble(mask);							\
-	end_nibble(adjustment, carry)
-
-								unsigned int result = registers_.flags.carry;
 								nibble(0x000f, 0x0006, 0x00010);
 								nibble(0x00f0, 0x0060, 0x00100);
 								nibble(0x0f00, 0x0600, 0x01000);
-
-								begin_nibble(0xf000);
-								registers_.flags.overflow = uint8_t((( (decimal_result ^ result) & (~decimal_result ^ result) ) >> (1 + registers_.m_shift))&0x40);
-								end_nibble(0x6000, 0x10000);
+								nibble(0xf000, 0x6000, 0x10000);
 
 #undef nibble
-#undef begin_nibble
-#undef end_nibble
 
+								registers_.flags.overflow = (( (partials ^ registers_.a.full) & (partials ^ data_buffer_.value) ) >> (1 + registers_.m_shift))&0x40;
 								registers_.flags.set_nz(uint16_t(result), registers_.m_shift);
 								registers_.flags.carry = (result >> (8 + registers_.m_shift))&1;
 								LDA(result);
@@ -945,7 +937,6 @@ template <typename BusHandler, bool uses_ready_line> void Processor<BusHandler,
 							}
 
 							data_buffer_.value = ~data_buffer_.value & registers_.m_masks[1];
-
 						[[fallthrough]];
 
 						case ADC: {
@@ -953,29 +944,22 @@ template <typename BusHandler, bool uses_ready_line> void Processor<BusHandler,
 							const uint16_t a = registers_.a.full & registers_.m_masks[1];
 
 							if(registers_.flags.decimal) {
-#define begin_nibble(mask)						\
-	result += (a & mask) + (data_buffer_.value & mask);
+								uint16_t partials = 0;
+								result = registers_.flags.carry;
 
-#define end_nibble(limit, adjustment, carry)	\
+#define nibble(mask, limit, adjustment, carry)									\
+	result += (a & mask) + (data_buffer_.value & mask);							\
+	partials += result & mask;													\
 	if(result >= limit) result = ((result + adjustment) & (carry - 1)) + carry;
 
-#define nibble(mask, limit, adjustment, carry)	\
-	begin_nibble(mask);	\
-	end_nibble(limit, adjustment, carry)
-
-								result = registers_.flags.carry;
 								nibble(0x000f, 0x000a, 0x0006, 0x00010);
 								nibble(0x00f0, 0x00a0, 0x0060, 0x00100);
 								nibble(0x0f00, 0x0a00, 0x0600, 0x01000);
-
-								begin_nibble(0xf000);
-								registers_.flags.overflow = (( (uint16_t(result) ^ registers_.a.full) & (uint16_t(result) ^ data_buffer_.value) ) >> (1 + registers_.m_shift))&0x40;
-								end_nibble(0xa000, 0x6000, 0x10000);
+								nibble(0xf000, 0xa000, 0x6000, 0x10000);
 
 #undef nibble
-#undef begin_nibble
-#undef end_nibble
 
+								registers_.flags.overflow = (( (partials ^ registers_.a.full) & (partials ^ data_buffer_.value) ) >> (1 + registers_.m_shift))&0x40;
 							} else {
 								result = int(a + data_buffer_.value + registers_.flags.carry);
 								registers_.flags.overflow = (( (uint16_t(result) ^ registers_.a.full) & (uint16_t(result) ^ data_buffer_.value) ) >> (1 + registers_.m_shift))&0x40;