From e6320fd823b82c06c032e66266807ade47b8cdba Mon Sep 17 00:00:00 2001
From: Aaron Giles <aaron@aarongiles.com>
Date: Tue, 2 Mar 2021 17:59:41 -0800
Subject: [PATCH] ymfm: Also explicitly sync writes to the mode register to
 ensure timer control bits are handled in sync.

---
 src/devices/sound/ymfm.cpp | 47 ++++++++++++++++++++++++++------------
 src/devices/sound/ymfm.h   |  3 +++
 2 files changed, 35 insertions(+), 15 deletions(-)

diff --git a/src/devices/sound/ymfm.cpp b/src/devices/sound/ymfm.cpp
index c57986856b3..35fff69c3a0 100644
--- a/src/devices/sound/ymfm.cpp
+++ b/src/devices/sound/ymfm.cpp
@@ -1388,25 +1388,19 @@ void ymfm_engine_base<RegisterType>::output(s32 &lsum, s32 &rsum, u8 rshift, s16
 template<class RegisterType>
 void ymfm_engine_base<RegisterType>::write(u16 regnum, u8 data)
 {
+	// special case: writes to the mode register can impact IRQs;
+	// schedule these writes to ensure ordering with timers
+	if (regnum == RegisterType::REG_MODE)
+	{
+		m_device.machine().scheduler().synchronize(timer_expired_delegate(FUNC(ymfm_engine_base<RegisterType>::synced_mode_w), this), data);
+		return;
+	}
+
 	// most writes are passive, consumed only when needed
 	m_regs.write(regnum, data);
 
-	// handle writes to the mode register
-	if (regnum == RegisterType::REG_MODE)
-	{
-		// reset timer status
-		if (m_regs.reset_timer_b())
-			set_reset_status(0, STATUS_TIMERB);
-		if (m_regs.reset_timer_a())
-			set_reset_status(0, STATUS_TIMERA);
-
-		// load timers
-		update_timer(1, m_regs.load_timer_b());
-		update_timer(0, m_regs.load_timer_a());
-	}
-
 	// handle writes to the keyon registers
-	else if (regnum == RegisterType::REG_KEYON)
+	if (regnum == RegisterType::REG_KEYON)
 	{
 		u8 chnum = m_regs.keyon_channel();
 		if (chnum < RegisterType::CHANNELS)
@@ -1654,6 +1648,29 @@ TIMER_CALLBACK_MEMBER(ymfm_engine_base<RegisterType>::check_interrupts)
 }
 
 
+//-------------------------------------------------
+//  synced_mode_w - handle a mode register write
+//  via timer callback
+//-------------------------------------------------
+
+template<class RegisterType>
+TIMER_CALLBACK_MEMBER(ymfm_engine_base<RegisterType>::synced_mode_w)
+{
+	// actually write the mode register now
+	m_regs.write(RegisterType::REG_MODE, param);
+
+	// reset timer status
+	if (m_regs.reset_timer_b())
+		set_reset_status(0, STATUS_TIMERB);
+	if (m_regs.reset_timer_a())
+		set_reset_status(0, STATUS_TIMERA);
+
+	// load timers
+	update_timer(1, m_regs.load_timer_b());
+	update_timer(0, m_regs.load_timer_a());
+}
+
+
 // Explicit template instantiation
 template class ymfm_engine_base<ymopm_registers>;
 template class ymfm_engine_base<ymopn_registers>;
diff --git a/src/devices/sound/ymfm.h b/src/devices/sound/ymfm.h
index 8cf7e854f7d..9d5c4197dad 100644
--- a/src/devices/sound/ymfm.h
+++ b/src/devices/sound/ymfm.h
@@ -790,6 +790,9 @@ private:
 	// check interrupts
 	TIMER_CALLBACK_MEMBER(check_interrupts);
 
+	// handle a mode register write
+	TIMER_CALLBACK_MEMBER(synced_mode_w);
+
 	// internal state
 	device_t &m_device;              // reference to the owning device
 	u32 m_env_counter;               // envelope counter; low 2 bits are sub-counter