From c397b4c41b78b57ec1437484551d967481b4f5b5 Mon Sep 17 00:00:00 2001
From: Adrian Conlon <98398945+AdrianConlon@users.noreply.github.com>
Date: Mon, 18 Mar 2024 21:04:50 +0000
Subject: [PATCH] Swap fixed/unfixed page usage

---
 M6502/inc/mos6502.h   | 33 ++++++++++++++++++++-------------
 M6502/src/mos6502.cpp |  7 ++++---
 2 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/M6502/inc/mos6502.h b/M6502/inc/mos6502.h
index 10bacdb..04c1df9 100644
--- a/M6502/inc/mos6502.h
+++ b/M6502/inc/mos6502.h
@@ -105,7 +105,7 @@ namespace EightBit {
 
 		// Addressing modes
 
-		constexpr void noteUnfixedPage() noexcept { m_unfixed_page = BUS().ADDRESS().high; }
+		constexpr void noteFixedPage(uint8_t page) noexcept { m_fixed_page = page; }
 
 		constexpr void Address_Immediate() noexcept { BUS().ADDRESS() = PC()++; }
 		void Address_Absolute() noexcept { BUS().ADDRESS() = fetchWord(); }
@@ -115,11 +115,23 @@ namespace EightBit {
 		void Address_ZeroPageWithIndex(uint8_t index) noexcept { AM_ZeroPage(); BUS().ADDRESS().low += index; }
 		void Address_ZeroPageX() noexcept { Address_ZeroPageWithIndex(X()); }
 		void Address_ZeroPageY() noexcept { Address_ZeroPageWithIndex(Y()); }
-		void Address_AbsoluteWithIndex(uint8_t index) noexcept { Address_Absolute(); noteUnfixedPage(); BUS().ADDRESS() += index; }
 		void Address_AbsoluteX() noexcept { Address_AbsoluteWithIndex(X()); }
 		void Address_AbsoluteY() noexcept { Address_AbsoluteWithIndex(Y()); }
 		void Address_IndexedIndirectX() noexcept { Address_ZeroPageX(); BUS().ADDRESS() = getWordPaged(); }
-		void Address_IndirectIndexedY() noexcept { Address_ZeroPageIndirect(); noteUnfixedPage(); BUS().ADDRESS() += Y(); }
+
+		void Address_AbsoluteWithIndex(uint8_t index) noexcept {
+			Address_Absolute();
+			const auto address = BUS().ADDRESS() + index;
+			noteFixedPage(address.high);
+			BUS().ADDRESS().low = address.low;
+		}
+
+		void Address_IndirectIndexedY() noexcept {
+			Address_ZeroPageIndirect();
+			const auto address = BUS().ADDRESS() + Y();
+			noteFixedPage(address.high);
+			BUS().ADDRESS().low = address.low;
+		}
 
 		// Addressing modes, with read
 
@@ -184,19 +196,13 @@ namespace EightBit {
 		}
 
 		void maybe_fixup() noexcept {
-			const auto fixed_page = BUS().ADDRESS().high;
-			BUS().ADDRESS().high = m_unfixed_page;
-			if (m_unfixed_page != fixed_page) {
-				memoryRead();
-				BUS().ADDRESS().high = fixed_page;
-			}
+			if (BUS().ADDRESS().high != m_fixed_page)
+				fixup();
 		}
 
 		void fixup() noexcept {
-			const auto fixed_page = BUS().ADDRESS().high;
-			BUS().ADDRESS().high = m_unfixed_page;
 			memoryRead();
-			BUS().ADDRESS().high = fixed_page;
+			BUS().ADDRESS().high = m_fixed_page;
 		}
 
 		void maybe_fixupR() noexcept { maybe_fixup(); memoryRead(); }
@@ -288,6 +294,7 @@ namespace EightBit {
 		bool m_handlingNMI = false;
 		bool m_handlingINT = false;
 
-		uint8_t m_unfixed_page = 0;
+		//uint8_t m_unfixed_page = 0;
+		uint8_t m_fixed_page = 0;
 	};
 }
\ No newline at end of file
diff --git a/M6502/src/mos6502.cpp b/M6502/src/mos6502.cpp
index 557cb4c..a6041c9 100644
--- a/M6502/src/mos6502.cpp
+++ b/M6502/src/mos6502.cpp
@@ -427,9 +427,10 @@ void EightBit::MOS6502::branch(const int condition) noexcept {
 	const auto relative = int8_t(fetchByte());
 	if (condition) {
 		swallow();
-		m_unfixed_page = PC().high;
-		jump(PC() + relative);
-		BUS().ADDRESS() = PC();
+		const auto address = PC() + relative;
+		noteFixedPage(address.high);
+		jump(address);
+		BUS().ADDRESS().low = address.low;
 		maybe_fixup();
 	}
 }