From 2e0e89c494e6318e414e4e8079981b699b6e41bb Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 24 Oct 2025 16:43:57 -0400
Subject: [PATCH] Implement fast modify path; fix more NOPs.

---
 Processors/6502Mk2/Decoder.hpp             | 22 ++++++++++++++--------
 Processors/6502Mk2/Implementation/6502.hpp | 21 +++++++++++++++++++++
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/Processors/6502Mk2/Decoder.hpp b/Processors/6502Mk2/Decoder.hpp
index 1f4892b38..fd07c4c85 100644
--- a/Processors/6502Mk2/Decoder.hpp
+++ b/Processors/6502Mk2/Decoder.hpp
@@ -49,6 +49,7 @@ enum class AddressingMode {
 
 	Absolute,
 	AbsoluteIndexed,
+	FastAbsoluteIndexedModify,
 	Zero,
 	ZeroIndexed,
 	ZeroIndirect,
@@ -556,14 +557,19 @@ struct Decoder<model, std::enable_if_t<model == Model::Synertek65C02>> {
 			case 0x3c:	return {AbsoluteIndexed, Operation::BIT};
 			case 0x9c:	return {Absolute, Operation::STZ};
 
-			case 0x1f:	return {AbsoluteIndexed, Operation::NOP};
-			case 0x3f:	return {AbsoluteIndexed, Operation::NOP};
-			case 0x5f:	return {AbsoluteIndexed, Operation::NOP};
-			case 0x7f:	return {AbsoluteIndexed, Operation::NOP};
-			case 0x9f:	return {AbsoluteIndexed, Operation::NOP};
-			case 0xbf:	return {AbsoluteIndexed, Operation::NOP};
-			case 0xdf:	return {AbsoluteIndexed, Operation::NOP};
-			case 0xff:	return {AbsoluteIndexed, Operation::NOP};
+			case 0x1e: return {FastAbsoluteIndexedModify, Operation::ASL};
+			case 0x3e: return {FastAbsoluteIndexedModify, Operation::ROL};
+			case 0x5e: return {FastAbsoluteIndexedModify, Operation::LSR};
+			case 0x7e: return {FastAbsoluteIndexedModify, Operation::ROR};
+
+			case 0x1f:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0x3f:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0x5f:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0x7f:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0x9f:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0xbf:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0xdf:	return {AbsoluteIndexed, Operation::FastNOP};
+			case 0xff:	return {AbsoluteIndexed, Operation::FastNOP};
 		}
 	}
 };
diff --git a/Processors/6502Mk2/Implementation/6502.hpp b/Processors/6502Mk2/Implementation/6502.hpp
index 8002de9a4..8ec4d0280 100644
--- a/Processors/6502Mk2/Implementation/6502.hpp
+++ b/Processors/6502Mk2/Implementation/6502.hpp
@@ -323,6 +323,27 @@ void Processor<model, Traits>::run_for(const Cycles cycles) {
 			access(BusOperation::Read, Literal(registers.pc.full), throwaway);
 			goto access_absolute;
 
+		// MARK: - Fast absolute indexed modify, which is a 65c02 improvement but not applied universally.
+
+		case access_program(FastAbsoluteIndexedModify):
+			++registers.pc.full;
+
+			// Read top half of address.
+			Storage::address_.halves.low = Storage::operand_;
+			check_interrupt();
+			access(BusOperation::Read, Literal(registers.pc.full), Storage::address_.halves.high);
+
+			// If this is a read and the top byte doesn't need adjusting, skip that cycle.
+			Storage::operand_ = Storage::address_.halves.high;
+			Storage::address_.full += index();
+			if(Storage::address_.halves.high == Storage::operand_) {
+				goto access_absolute;
+			}
+
+			check_interrupt();
+			access(BusOperation::Read, Literal(registers.pc.full), throwaway);
+			goto access_absolute;
+
 		// MARK: - Indexed indirect.
 
 		case access_program(IndexedIndirect):