From 86ef3406503e7b4d4b71889abca570322e458d8b Mon Sep 17 00:00:00 2001
From: Adrian Conlon <98398945+AdrianConlon@users.noreply.github.com>
Date: Sun, 7 Jan 2024 14:35:12 +0000
Subject: [PATCH] Tidy arithmetic overflow handling in M6502 core

---
 M6502/inc/mos6502.h   | 8 ++++++++
 M6502/src/mos6502.cpp | 6 +++---
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/M6502/inc/mos6502.h b/M6502/inc/mos6502.h
index 9e53c5e..89e4d74 100644
--- a/M6502/inc/mos6502.h
+++ b/M6502/inc/mos6502.h
@@ -145,6 +145,14 @@ namespace EightBit {
 			adjustNegative(datum);
 		}
 
+		constexpr void adjustOverflow_add(uint8_t operand, uint8_t data, uint8_t intermediate) noexcept {
+			set_flag(VF, negative(~(operand ^ data) & (operand ^ intermediate)));
+		}
+
+		constexpr void adjustOverflow_subtract(uint8_t operand, uint8_t data, uint8_t intermediate) noexcept {
+			set_flag(VF, negative((operand ^ data) & (operand ^ intermediate)));
+		}
+
 		// Miscellaneous
 
 		void branch(int condition) noexcept;
diff --git a/M6502/src/mos6502.cpp b/M6502/src/mos6502.cpp
index 6fa613b..d9325f4 100644
--- a/M6502/src/mos6502.cpp
+++ b/M6502/src/mos6502.cpp
@@ -549,7 +549,7 @@ uint8_t EightBit::MOS6502::sbc(const uint8_t operand, const uint8_t data) noexce
 
 	const auto difference = m_intermediate;
 	adjustNZ(difference.low);
-	set_flag(VF, negative((operand ^ data) & (operand ^ difference.low)));
+	adjustOverflow_subtract(operand, data, difference.low);
 	reset_flag(CF, difference.high);
 
 	return returned;
@@ -591,7 +591,7 @@ uint8_t EightBit::MOS6502::add(uint8_t operand, uint8_t data, int carrying) noex
 uint8_t EightBit::MOS6502::add_b(uint8_t operand, uint8_t data, int carrying) noexcept {
 	m_intermediate.word = operand + data + carrying;
 
-	set_flag(VF, negative(~(operand ^ data) & (operand ^ m_intermediate.low)));
+	adjustOverflow_add(operand, data, m_intermediate.low);
 	set_flag(CF, carry(m_intermediate.high));
 
 	adjustNZ(m_intermediate.low);
@@ -612,7 +612,7 @@ uint8_t EightBit::MOS6502::add_d(uint8_t operand, uint8_t data, int carry) noexc
 	}
 
 	adjustNegative(high.low);
-	set_flag(VF, negative(~(operand ^ data) & (operand ^ high.low)));
+	adjustOverflow_add(operand, data, high.low);
 
 	if (high.word > 0x90)
 		high += 0x60;