From 047babbe7c0ac4ae793516cd035c99d6e97b3777 Mon Sep 17 00:00:00 2001
From: Adrian Conlon <Adrian.conlon@gmail.com>
Date: Tue, 8 Jan 2019 23:09:52 +0000
Subject: [PATCH] Rearrange the RESET handler for cycle accuracy.  Use more of
 the general interrupt handler, but with "dummy" stack write access.

Signed-off-by: Adrian Conlon <Adrian.conlon@gmail.com>
---
 M6502/inc/mos6502.h   |  3 +++
 M6502/src/mos6502.cpp | 26 ++++++++++++++++++--------
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/M6502/inc/mos6502.h b/M6502/inc/mos6502.h
index 97d82a1..34f598d 100644
--- a/M6502/inc/mos6502.h
+++ b/M6502/inc/mos6502.h
@@ -76,6 +76,9 @@ namespace EightBit {
 		virtual void push(uint8_t value) final;
 		virtual uint8_t pop() final;
 
+		// Dummy stack push, used during RESET
+		void dummyPush(uint8_t value);
+
 		// Addressing modes
 
 		register16_t Address_Absolute();
diff --git a/M6502/src/mos6502.cpp b/M6502/src/mos6502.cpp
index d41c0de..54adc3b 100644
--- a/M6502/src/mos6502.cpp
+++ b/M6502/src/mos6502.cpp
@@ -72,17 +72,21 @@ void EightBit::MOS6502::handleIRQ() {
 }
 
 void EightBit::MOS6502::interrupt() {
-	uint8_t vector = RSTvector;
-	if (!m_handlingRESET) {
-		const bool nmi = m_handlingNMI;
-		const bool irq = m_handlingIRQ;
-		const bool hardware = nmi || irq;
-		const bool software = !hardware;
+	const bool reset = m_handlingRESET;
+	const bool nmi = m_handlingNMI;
+	const bool irq = m_handlingIRQ;
+	const bool hardware = nmi || irq || reset;
+	const bool software = !hardware;
+	if (reset) {
+		dummyPush(PC().high);
+		dummyPush(PC().low);
+		dummyPush(P());
+	} else {
 		pushWord(PC());
 		push(P() | (software ? BF : 0));
-		setFlag(P(), IF);	// Disable IRQ
-		vector = nmi ? NMIvector : IRQvector;
 	}
+	setFlag(P(), IF);	// Disable IRQ
+	const uint8_t vector = reset ? RSTvector : (nmi ? NMIvector : IRQvector);
 	jump(getWordPaged(0xff, vector));
 	m_handlingRESET = m_handlingNMI = m_handlingIRQ = false;
 }
@@ -392,6 +396,12 @@ uint8_t EightBit::MOS6502::pop() {
 	return getBytePaged(1, ++S());
 }
 
+void EightBit::MOS6502::dummyPush(const uint8_t value) {
+	addCycle();
+	BUS().DATA() = value;
+	BUS().ADDRESS() = register16_t(S()--, 1);
+}
+
 ////
 
 EightBit::register16_t EightBit::MOS6502::Address_Absolute() {